CN118043978A

CN118043978A - Variable wavelength light emitting diode, display device having the same, method of controlling the same, and method of manufacturing variable wavelength LED

Info

Publication number: CN118043978A
Application number: CN202280066094.5A
Authority: CN
Inventors: 朱彤彤; 穆罕默德·阿里; 西蒙·汉默斯利
Original assignee: Bora Technology Co ltd
Current assignee: Bora Technology Co ltd
Priority date: 2021-07-28
Filing date: 2022-07-28
Publication date: 2024-05-14
Also published as: CN118043971A; CN118103985A; GB202209054D0

Abstract

A variable wavelength Light Emitting Diode (LED) is provided that includes an n-doped portion, a p-doped portion, and a light emitting region between the n-doped portion and the p-doped portion. The light emitting region includes a light emitting layer that emits light at a peak emission wavelength under an electrical bias thereacross. The LED is configured to receive a power source and by varying the power source, the peak emission wavelength of the LED is continuously controllable over an emission wavelength range of at least 40 nm. Methods of controlling and methods of manufacturing variable wavelength LEDs are also provided.

Description

Variable wavelength light emitting diode, display device having the same, method of controlling the same, and method of manufacturing variable wavelength LED

Technical Field

The present invention relates to a variable wavelength light emitting Diode (LIGHT EMITTING Diode, LED), a method of controlling a variable wavelength LED, and a method of manufacturing a variable wavelength LED.

Background

III-V semiconductor materials are particularly interesting for semiconductor device design, especially III-nitride semiconductor materials.

"III-V" semiconductors include binary, ternary, and quaternary alloys of group III elements (such as Ga, al, and In) with group V elements (such as N, P, as and Sb), and are of great significance for many applications, including optoelectronics.

Of particular interest are a class of semiconductor materials known as "group III nitride" materials, which include gallium nitride (GaN), indium nitride (InN), and aluminum nitride (AlN) and ternary and quaternary alloys thereof. (Al, in) GaN is a term covering AlGaN, inGaN, and GaN. Group III nitride materials have not only achieved commercial success in solid state lighting and power electronics, but also have shown particular advantages in quantum light source and light species interactions.

Doping In into GaN semiconductor materials is significant for optoelectronic semiconductor devices because changing the In content of the semiconductor changes the electronic bandgap of the material, thereby changing the wavelength of the semiconductor's luminescence. However, changing the In content of the material also affects the In-plane lattice constant of the semiconductor. For example, the in-plane lattice constant of InN is about 11% larger than that of GaN, and the lattice size of the intermediate component varies according to the indium content. This creates a problem in device design where active semiconductor layers need to be deposited on substrate layers having different lattice dimensions. The reason for this is that the lattice mismatch at the layer boundaries introduces strain into the lattice, which results in the formation of defects in the material that act as non-radiative recombination centers. This greatly compromises device performance.

There is a great need for light emitting diodes that emit at all visible wavelengths, especially at longer wavelengths towards green, yellow and red, but manufacturers have encountered more problems in manufacturing light emitting diodes that emit at longer wavelengths.

For example, one challenge faced by growing longer wavelength LEDs, such as green, yellow and red LEDs, on GaN-based platforms is the need to use high indium (In) content to reduce the bandgap In the active region to a level suitable for long wavelength emission. The desired InGaN active region has a larger lattice parameter than the underlying GaN and the resulting strain causes defects to form in the material that act as non-radiative recombination centers, degrading device performance.

High quality InGaN (high indium content > 20%) is difficult to achieve due to the large lattice mismatch between InN and GaN. The mismatch strain also causes a reduction in the indium composition through the composition pulling effect.

Shorter wavelength LEDs are easier to manufacture because they can be manufactured using InGaN light emitting regions where the proportion of indium is lower than that required for long wavelength light emission.

Disclosure of Invention

The present application relates to a variable wavelength Light Emitting Diode (LED), a method of controlling an LED, and a method of manufacturing an LED.

The invention is defined in the independent claims, to which reference should now be made. Preferred or advantageous features of the invention are defined in the appended dependent claims.

The light emitting diode or LED described in the present application is preferably formed of a group III-V semiconductor material, particularly preferably formed of a group III nitride semiconductor material.

"III-V" semiconductors include binary, ternary, and quaternary alloys of group III elements (such as Ga, al, and In) with group V elements (such as N, P, as and Sb), and are of great interest for many applications, including optoelectronics.

Of particular interest are a class of semiconductor materials known as "group III nitride" materials, which include gallium nitride (GaN), indium nitride (InN), and aluminum nitride (AlN), as well as their ternary and quaternary alloys (Al, in) GaN. Different crystal orientations may be used in the present invention, such as polar c-plane, nonpolar, and semipolar orientations. There are two principal nonpolar orientations, the a-plane (11-20) and the m-plane (1-100). For semi-polar, there is (11-22), {2021}, which is a family of crystal planes. Group III nitride materials have not only achieved commercial success in solid state lighting and power electronics, but also have shown particular advantages in quantum light source and light species interactions.

While various group III nitride materials are commercially interesting, gallium nitride (GaN) is widely recognized as one of the most important new semiconductor materials and is particularly interesting for many applications.

It is well known that the introduction of voids in bulk group III nitrides, such as GaN, can profoundly affect their material properties (optical, mechanical, electrical, thermal, etc.). Thus, the possibility to tailor various material properties of GaN and group III nitride semiconductors by varying the porosity of porous GaN makes porous GaN of great interest in optoelectronic applications.

The invention will be described primarily with reference to GaN and InGaN, but may be advantageously applied to alternative group III nitride material combinations.

In the following description, the substrate or template for overgrowth is a semiconductor structure on which additional semiconductor layers are grown in order to produce an LED semiconductor device. An exemplary substrate template for overgrowth in the present invention may be a GaN semiconductor structure comprising multiple layers of doped and undoped GaN.

Such as International patent application PCT/GB2017/052895 (publication No. WO 2019/063957) and

As described in PCT/GB2019/050213 (publication No. WO 2019/145728), regions or layers of a semiconductor structure may be porous by electrochemical etching.

Problems to be solved

In conventional multi-color (RGB) LED displays, each subpixel comes from a separate LED die and must therefore be combined by mass transfer.

The red and blue-green LEDs typically must be made of different semiconductor materials. To provide an RGB LED display, LEDs emitting in three colors need to be grown separately, transferred onto a common substrate and individually re-pitched to form a multicolor display device.

The current mass transfer process needs to overcome the following problems:

complicated COT flow: punching, taking and placing;

Unstable yield: assembling jet flow;

Higher cost: laser transfer printing

As the resolution requirements of mass transfer systems increase, this becomes particularly difficult as the size of individual sub-pixels becomes smaller.

This is also problematic for high resolution displays because the reliability of the mass transfer system needs to be extremely high, placing millions of individual devices in order to produce a single working display.

The complexity of the mass transfer process is also very complex when combining sub-pixels created using multiple material systems.

Most LED display technologies require the use of multiple material systems to produce red, green and blue light emitting sub-pixels, such as InGaN for the blue and green light emitting sub-pixels and InAlGaP for the red light emitting sub-pixels.

To enable mass production and commercialization of multi-wavelength LED microdisplays, the present invention uses variable wavelength LEDs with chip-on-Chip (COW) designs based on InGaN materials. It uses a variable wavelength LED to obtain multiple emission wavelengths from one diode structure, which can simplify the process of manufacturing a multicolor display device, improve production efficiency, and reduce cost.

Variable wavelength LED

In a first aspect of the present invention, there is provided a Light Emitting Diode (LED) comprising:

An n-doped portion;

A p-doped portion;

A light emitting region between the n-doped portion and the p-doped portion, the light emitting region comprising a light emitting layer that emits light at a peak emission wavelength under an electrical bias thereacross;

Wherein the LED is configured to receive a power source, wherein the peak emission wavelength of the LED is continuously controllable over the emission wavelength range by varying or controlling the power source. The peak emission wavelength of the variable wavelength LED is preferably continuously controllable or continuously variable over an emission wavelength range of at least 40nm by varying or controlling the power supply.

Since the peak emission wavelength of a variable wavelength LED is preferably continuously controllable or continuously variable over the emission wavelength range, the LED may be described as a variable wavelength LED.

The variable wavelength LED is configured to receive a power supply or drive current from a power supply or LED driver. The term "power supply" as used herein refers to the power or current supplied to drive an LED during use.

By varying or controlling the magnitude of the drive current provided to the variable wavelength LED, the peak emission wavelength of the LED may preferably be continuously controllable or continuously variable over the emission wavelength range.

In conventional LED devices, variations in the drive current provided to the LEDs produce very small emission wavelength shifts, but the inventors have found that the wavelength shift can be widened and controlled to a greater extent than in conventional LED materials. Unlike the emission range of the prior art devices of a few nm, the LED of the present invention can be controlled to emit over a wider emission range, for example over a range of at least 40 nm. Since the present LED is tunable to emit over a wide wavelength range, it may be referred to as a variable wavelength LED.

The LEDs may be dynamic color tunable LEDs in which the peak emission wavelength of the LED is tunable by varying the driving conditions that the power supply provides to the LED.

The LEDs are preferably drivable to emit at a single peak emission wavelength in response to a steady power supply, but at different peak emission wavelengths in response to a change in power supply. Thus, LEDs may be used to emit a particular color for a long period of time, or alternatively, LEDs may be driven to emit a variety of different wavelengths by providing varying driving conditions.

Preferably, the n-doped portion, the p-doped portion and the light emitting region each comprise or consist of a group III nitride material, preferably GaN, inGaN, alGaN or AlInGaN

The variable wavelength LED preferably contains a single epitaxially grown diode structure containing an n-doped portion, a p-doped portion, and a light emitting region. Thus, the variable peak emission wavelengths of the LEDs are all emitted by the same LED diode structure and composition.

The LED preferably comprises a porous region of a group III nitride material. The light emitting region of the LED is preferably formed over a porous region of group III nitride material. In some embodiments, one of the n-doped or p-doped portions may contain a porous region of group III nitride material. In other embodiments, the n-doped portion; a p-doped portion; and the light emitting region is disposed on a substrate comprising a porous region of a group III nitride material. During epitaxial growth of the LED, the light emitting region is preferably overgrown after the porous region is formed.

The inventors have found that the porous region of the group III nitride material enables the same LED to emit in the range of peak emission wavelengths, rather than at one particular wavelength. By varying the power supplied to the LEDs, the peak emission wavelength of the LEDs can be varied over a range of emission wavelengths. Accordingly, the present invention provides a variable wavelength LED that can be controlled to emit at any wavelength within a continuous emission wavelength range. By varying the driving conditions that the power supply provides to the LEDs, the LEDs can emit at any wavelength within the emission wavelength range of the LEDs, rather than just at discrete peak emission wavelengths.

The inventors have found that by incorporating a porous region of a group III nitride semiconductor material into an LED structure, or forming an LED diode structure over a porous region of a group III nitride semiconductor material, the ability of an LED to emit at tunable wavelengths over a wide emission range can be imparted. The porous region provides benefits to the LED including strain relaxation, lattice parameter increase, wafer bow reduction, and beneficial mechanical and thermal effects during growth of the light emitting region at high temperatures.

The light emitting region of the LED is preferably formed over a porous region of a group III nitride material during fabrication such that the porous region affects the structural and mechanical properties of the semiconductor layer epitaxially deposited over the porous region. The semiconductor material layer deposited over the porous region during growth experiences benefits such as reduced strain, increased lattice parameter, and reduced wafer bow, which are imparted to the LED light emitting region and affect its structure and light emitting behavior.

Once the LED light emitting (active) region has been epitaxially grown over the porous region and the quality of the active region has been improved by the effect of the porous region, the beneficial effect of the porous region on the emission properties is permanently imparted to the LED active region. Thus, the LED diode structure may remain on the porous region, in which case the variable wavelength LED comprises a porous region of group III nitride material, or alternatively, the porous region may be removed from the LED structure during the process of processing the LED into a device after epitaxial growth.

The width of the emission wavelength range may vary depending on the structure and composition of the LED structure (n-doped portion, light emitting region, and p-doped portion) as well as the structure and porosity of the porous region. The width of the emission wavelength range may also vary depending on the size and shape of the LED (pixel size and shape).

The invention is not limited to a particular LED structure, as the variable wavelength behavior of the LEDs may be achieved using a variety of conventional LED structures, preferably provided over a porous template. A variety of LED structures are known in the art and LEDs having different emission wavelength ranges can be obtained by providing different conventional LED structures on a template comprising porous regions.

In a preferred embodiment, the peak emission wavelength is controllable over an emission wavelength range of at least 40nm, or at least 50nm, or at least 60nm, or at least 70nm, or at least 80nm by varying the power supply. Preferably, the peak emission wavelength is controllable over an emission wavelength range up to 100nm, or 110nm, or 120nm, or 130nm, or 140nm, or 150nm, or 160nm, or 170nm, or 180nm, or 190nm, or 200nm, or 400nm, or 450 nm. Thus, the size of the emission wavelength range achievable by the LED of the present invention is much larger than that achievable by the LED of the prior art.

The LED may advantageously be controlled to emit at any peak emission wavelength within its emission wavelength range. By varying the characteristics of the power supply and the size and shape of the LED pixels, the LEDs can thus be controlled to emit light at any selected peak emission wavelength within this range.

The emission wavelength of the variable wavelength LED is preferably continuously variable over its range of emission wavelengths in response to a driving condition provided by a power supply that continuously varies over the range of driving conditions.

The location of the emission wavelength range in the electromagnetic spectrum may also vary depending on the design of the LED structure (n-doped part, light emitting region and p-doped part). For example, the wavelengths contained within the emission wavelength range may depend on the number and composition of the light emitting layers in the LED. A variety of LED active regions are known in the art for emitting at different wavelengths in the visible spectrum, so by forming the light emitting regions of the inventive LED with different light emitting regions, an emission wavelength range covering different parts of the spectrum can be obtained.

The LED emission wavelength range may be between 400nm and 850nm, or between 400nm and 800nm, or between 400nm and 690nm, or between 400nm and 675 nm. The emission wavelength range may be a subrange within the range of 400nm to 750 nm. By selecting different LED active areas and controlling the size and shape of the LED pixels, the emission wavelength range can be adjusted to cover any portion of that range.

Preferably, the emission wavelength range of the LED extends from a lower end below 410nm or 430nm or 450nm or 470nm or 500nm or 520nm or 540nm or 560nm to an upper end above 570nm or 580nm or 600nm or 610nm or 630nm or 650nm or 675 nm. As described above, the first and second ends of the emission wavelength range may be adjusted depending on the LED structure and the choice of LED shape and size.

For example, in a preferred embodiment, the lower end of the emission wavelength may be between 400nm and 450nm (violet) or between 450nm and 500nm (blue) or between 500nm and 570nm (green), and the upper end of the emission wavelength may be between 570nm and 590nm (yellow), or between 590nm and 610nm (orange), or between 610nm and 700nm (red).

In a preferred embodiment, the LED emission wavelength range may extend from a low end below 500nm to a high end above 610nm, such that the peak emission wavelength of the LED may be varied to emit from any wavelength of blue (below 500 nm) to red (above 610 nm) by changing the power supply. It would be highly advantageous to provide a single LED design that can be controlled to emit at blue wavelengths (450 nm-500 nm), green wavelengths (500 nm-570 nm), and yellow wavelengths (570 nm-590 nm), orange wavelengths (590 nm-610 nm), and red wavelengths (610 nm-760 nm), and that can provide significant advantages for LED displays.

In other preferred embodiments, the LED emission wavelength range may extend between 520nm and 660nm, or between 550nm and 650nm, by varying the power supply to the LEDs.

In a particularly preferred embodiment, the peak emission wavelength can be controlled between 540nm and 680nm, or between 560nm and 675nm, by varying the power supply. Thus, the same LED may be controllable to emit at any peak emission wavelength between 540nm green and 680nm red. Green and red LEDs have historically been more difficult to manufacture than shorter wavelength blue LEDs due to issues such as difficulty in incorporating the required indium content into the light emitting region. It would therefore be highly advantageous to provide a single LED design that can be controlled to emit at green wavelengths (500 nm-570 nm) as well as yellow wavelengths (570 nm-590 nm), orange wavelengths (590 nm-610 nm) and red wavelengths (610 nm-760 nm), and that can provide significant advantages for LED displays.

In another preferred embodiment, the peak emission wavelength can be controlled between 520nm and 675nm, or between 550nm and 650nm, by varying the power supply.

Although variable wavelength LEDs may emit over a continuous emission wavelength range, in some embodiments it may be desirable to control the LEDs to operate in multiple discrete emission modes, for example in response to a power supply having multiple drive modes. For example, by driving the LEDs in a plurality of different modes corresponding to discrete emission colors, which are mixed in a known manner to give the desired visual effect, a simplified color display can be provided.

The LEDs may preferably be controllable to emit at least two discrete peak emission wavelengths by changing the driving conditions provided by the power supply between two discrete driving conditions, such as two discrete magnitudes of driving current. The LEDs may be controllable to emit at a first peak emission wavelength in response to a first driving condition provided by the power supply (which may be a driving current having a first magnitude) and at a second peak emission wavelength in response to a second driving condition provided by the power supply (which may be a driving current having a second magnitude different from the first magnitude).

The LEDs are preferably controllable to emit at least three discrete peak emission wavelengths by varying the driving conditions provided by the power supply. The peak emission wavelength of a variable wavelength LED may thus be variable over at least three "colors" in the EM spectrum.

The LEDs may be controllable to emit at a first peak emission wavelength in response to a first driving condition provided by the power supply, at a second peak emission wavelength in response to a second driving condition provided by the power supply, and at a third peak emission wavelength in response to a third driving condition provided by the power supply.

The LEDs are preferably controllable to emit a blue peak emission wavelength in response to a first driving condition provided by the power supply, a green peak emission wavelength in response to a second driving condition provided by the power supply, and a red peak emission wavelength in response to a third driving condition provided by the power supply.

The LED may be controllable to emit a first peak emission wavelength in the range of 400nm-500nm in response to a first driving condition provided by the power supply, a second peak emission wavelength in the range of 500nm-550nm in response to a second driving condition provided by the power supply, and a third peak emission wavelength greater than 600nm in response to a third driving condition provided by the power supply.

Preferably, the LEDs are controllable to emit a first peak emission wavelength in the range of 430nm-460nm in response to a first driving condition provided by the power supply, a second peak emission wavelength in the range of 510nm-560nm in response to a second driving condition provided by the power supply, and a third peak emission wavelength in the range of 600nm-660nm in response to a third driving condition provided by the power supply.

The first driving condition, the second driving condition, and the third driving condition may be a first current density, a second current density, and a third current density, or the first driving condition, the second driving condition, and the third driving condition may be a first power density, a second power density, and a third power density.

In a preferred embodiment of the invention, the variable wavelength emission behavior of the LED structure is achieved by growing the LED structure (n-doped part, light emitting region and p-doped part) on a template containing porous regions. The inventors have found that the presence of porous regions of group III nitride material in the template structure results in higher quality crystal growth before the LED structure overgrows and thus brings significant benefits, including the possibility of changing the emission wavelength of the LED light emitting region. The mechanism by which the porous region enables variable wavelength emission of the LED is the subject of ongoing research. The porous region provides benefits to the LED including strain relaxation, lattice parameter increase, wafer bow reduction, and mechanical and thermal effects during growth of the light emitting region at high temperatures.

The inventors have recognized that electrochemical porosification of a group III nitride material advantageously results in strain in the group III nitride lattice and a reduction in overall wafer bow or curvature. Without wishing to be bound by theory, it is believed that the process of making porous regions of group III nitride material porous also etches away structural defects such as threading dislocations formed during growth of the layer on top of the substrate.

Removing dislocations from the semiconductor material of the porous region during porosification greatly reduces strain in the porous region, especially if the lattice size of the porous region does not match the lattice size of the underlying substrate material. Thus, during epitaxial growth of the LED structure, the porous material is better suited to match the lattice of the overlying non-porous layer when a layer of group III-nitride material is deposited over the porous region. This results in the layer above the porous region being subjected to significantly lower strain than would be the case without the porous region. Since the light emitting region is formed over the porous region, the light emitting region of the LED is formed such that strain in the semiconductor lattice is reduced, and the porous region imparts unusual properties to the structure and light emitting characteristics of the light emitting region.

Since the nonporous semiconductor material layers formed over the porous regions are subject to lower strain, there are fewer structural defects in these layers that act as non-radiative recombination centers that impair device performance.

Component traction effect: kawaguchi et al report a so-called InGaN composition pulling effect in which the indium fraction is smaller in the initial stages of growth but increases with increasing growth thickness. This observation is, to a first extent, independent of the underlying GaN or AlGaN layer. The authors believe that this effect is caused by strain due to interface lattice mismatch. They found that a larger lattice mismatch between inGaN and the bottom epitaxial layer was accompanied by a larger variation In content.

In theoretical studies of the component pulling effect in Inatomi et al (J.App., volume 56, 7 (Japanese Journal of APPLIED PHYSICS, volume 56, number 7)) on InGaN metal organic vapor phase epitaxial growth, compressive strain was found to inhibit the incorporation of InN. On the other hand, tensile strain promotes InN incorporation compared to the relaxed bulk growth case.

The inventors have found that the presence of porous regions in the semiconductor structure during epitaxial growth results in "strain relaxation", which reduces the strain in the semiconductor structure layer, and which may lead to an improvement in the component pulling effect. The porosification reduces the strain In the group III nitride layer and makes the strain of the semiconductor structure smaller, so higher In-bonding conditions can be obtained. Thus, the present invention can help incorporate higher indium into the LED layer grown on top of the porous region, which is highly desirable for longer wavelength emission.

The n-doped region, the light emitting region and the p-doped region are preferably formed on the porous region during LED fabrication. The porous region may then remain permanently in the LED and be incorporated into the device, or alternatively, the porous region may be removed after the formation of the LED diode structure. Even if the porous region is removed by cleaving the native structure, the mechanical and structural benefits of the porous region to the growth of the light emitting region remain in the light emitting region.

The n-doped region, the light emitting region and the p-doped region are preferably arranged over the porous region. In other words, the porous region may be located below the n-doped region, the light emitting region, and the p-doped region in the LED structure.

The light emitting layer may preferably be an indium gallium nitride (InGaN) layer.

By providing a porous region of group III-nitride material, the n-doped region, the light emitting region, and the p-doped region may thus be grown over the porous region (possibly with an intermediate layer of group III-nitride material between the porous region and the LED structure) with a lower strain than would be possible without the porous region. Thus, the reduced strain level in the layered semiconductor structure may facilitate incorporation of higher indium into the light emitting layer(s) of the LED, so that high quality InGaN light emitting layers with high indium content may be grown.

As mentioned in the background section above, despite the great demand for LEDs emitting light between 400nm and 750nm, in particular between 500nm and 750nm, the technical difficulty of incorporating sufficient indium into the light-emitting layer(s) means that longer wavelength LEDs are difficult to realize.

The inventors have found that growing an LED structure over a porous region of a group III nitride material results in a significant shift in emission wavelength to longer wavelengths compared to the same LED structure grown on a non-porous substrate.

The inventors demonstrate this by growing conventional green/yellow (emission wavelength between 500nm-570nm or 570nm-590 nm) InGaN LED structures on non-porous GaN wafers, and demonstrate that the LEDs emit green/yellow light as expected. The same "green/yellow" InGaN LED structure was then grown on a template containing porous regions, the LED emitting light in the 600nm to 750nm red range when an electrical bias was applied across the LED.

The present invention advantageously allows conventional, easily manufactured LED structures to be converted to longer wavelength emissions and to emit in a range of different peak emission wavelengths by controlling the power supply of the LED. While a variety of different emission ranges can be achieved, in one particularly preferred embodiment, known LED structures previously referred to as yellow or green LEDs can be made as variable wavelength green-red LEDs.

The LED light emitting region may be an LED light emitting region for emission at a peak wavelength of 500nm-600nm, or 500nm-550nm, or 550nm-600nm, or 510nm-570nm, or 530nm-560nm, or 540nm-600 nm. The LED light emitting region may be an LED light emitting region that emits at a peak wavelength of 500nm to 600nm, or 510nm to 570nm, or 530nm to 560nm, or 540nm to 600nm, or 590nm to 640nm when not overgrown on the porous group III nitride layer. However, growing the LED light emitting region over the porous region of group III nitride material may shift the emission wavelength of the light emitting region to longer wavelengths (e.g., between 600nm and 750 nm), and also enable the LED to emit a continuous range of different emission wavelengths.

Conventionally, in order to grow InGaN quantum wells incorporating higher amounts of indium required for longer wavelength emission, lower growth temperatures are required during epitaxial deposition of InGaN materials. Different growth pressures and growth rates have also been tried as a method to increase indium incorporation. Disadvantages of lower growth temperatures include more defects in the crystal structure and lower NH3 cleavage efficiency.

However, in the present invention, the presence of porous regions In the LED template during growth reduces strain In the crystal structure, increases the lattice parameter, and enables more In to be incorporated into the active region than was previously possible at a given growth temperature. By incorporating porous regions into the structure, it is therefore no longer necessary to reduce the growth temperature of InGaN to increase In incorporation, as a greater amount of In is incorporated at higher temperatures. This allows the LED to use higher InGaN growth temperatures, resulting in higher crystal quality, fewer defects, and improved performance and LED characteristics compared to prior art LEDs.

The crystal quality of the LED structure grown over the porous region is improved, which also enables the LED to emit light over a wider emission wavelength range than in the prior art.

In some previous attempts to incorporate porous materials into LEDs, it was found that the porous materials resulted in a high degree of spectral broadening such that the Full width at half maximum (Full WIDTH HALF Max, FWHM) of the spectral emission peak became undesirably broad. This is undesirable for most LED applications where a narrow emission peak is preferred so that the light emitted by the LED is at or near the desired wavelength.

Advantageously, in the present invention, the LED preferably emits light having a FWHM of 50nm or less, or 40nm or less, or 30nm or less, preferably wherein the FWHM of the LED is < 20nm. Thus, while the peak emission wavelength of a variable wavelength LED may be varied by varying the driving conditions supplied to the LED, the LED preferably emits light having a FWHM of 50nm or less, or 40nm or less, or 30nm or less, or 20nm or less, under any given driving condition.

In a preferred embodiment, the light emitting layer is a light emitting indium gallium nitride layer. The LED preferably further comprises a GaN material region. The stress relaxation effect created by the porous region is particularly advantageous due to the lattice mismatch between GaN and InGaN.

The light emitting diode may comprise at least one feature selected from the group consisting of:

(a) The light emitting region comprises one or two or three or four or five or six or seven or eight quantum wells (or at least one quantum well); or alternatively

(B) The iii-nitride layer comprises an aluminum gallium nitride layer having a composition Al _yGa_(I-y) N, wherein y is in the range of 0.1 to

1.0; Or alternatively

(C) An InGaN/GaN or InGaN/InGaN superlattice or InGaN layer emitting UV or blue light is located at n

Between the doped portion and the light emitting region.

The n-doped portion, p-doped portion and light emitting region are preferably designed according to conventional LED designs known in the art. For example, the thickness, composition, and number of these layers may be selected according to conventional principles of LED design. The LEDs may contain other layers that are conventional in the art of LED design and well understood by those skilled in the art. For example, an LED may include a group III nitride layer on a light emitting layer and a group III nitride barrier layer on the group III nitride layer. Such structural features are well known and may be used in the LEDs of the present invention.

Control of power supply

The peak emission wavelength of the variable wavelength LED is controlled by controlling the power supply of the LED.

To achieve the variable wavelength properties of the present invention, the power supply may be controlled in a variety of ways. For example, in Continuous Wave (CW) or pulsed modes, a voltage or current drive scheme may be used.

In a preferred embodiment, the power source may be a pulsed power source. Alternatively, the power source may be a Continuous Wave (CW) or near continuous wave power source.

The power supply may be a constant voltage power supply or a constant current power supply.

The parameters that are varied to control the power supply may vary depending on the driving scheme used to drive the LEDs. For example, the LEDs may be controlled by varying the power, power density, current density, or voltage of the LED power supply. Since power, current and voltage are related by p=iv, the control of these parameters is well understood by those skilled in the art.

The peak emission wavelength of the variable wavelength LED emitted during use is determined by the current density (which may alternatively be described in terms of power density) through the LED diode structure during use. The current density (in a/cm ²) experienced by a variable wavelength LED is determined by the magnitude of the current supplied by the power supply (in amperes) and the cross-sectional area (in cm ²) of the LED diode structure through which the current passes. Once the variable wavelength LED is fabricated, the cross-sectional area (in cm ²) of the LED diode structure through which current flows is fixed, thus changing the current density through the LED by varying the magnitude of the current supplied to the LED in use.

For a given LED of fixed size, the driving conditions may be discussed in terms of the driving current magnitude. However, since LEDs can be manufactured in a variety of shapes and sizes, it may be more appropriate to define general driving conditions depending on current density or power density.

The emission wavelength range is a continuous wavelength range, so by varying the power supply, the LED can advantageously emit at any wavelength within this range.

The power supply may vary within a power supply range corresponding to the range of LED emission wavelengths. For example, the power supply range may be a range of power (in watts) or power density (in W/cm ²), defined by a lower power emitted by the LED at the longer wavelength limit of the emission wavelength range and a higher power emitted by the LED at the shorter wavelength limit of the emission wavelength range. Alternatively, the power supply range may be defined by upper and lower limits of current (in amperes) or current density (in a/cm ²) or upper and lower limits of voltage.

The power supply of the LEDs is preferably regulated or controlled by an LED driver. The light emitting diode may be configured to be connected to or connected to an LED driver configured to provide power to the LED. The LED driver is preferably configured to provide a variable power supply to the LEDs. For example, the LED driver is preferably capable of changing the magnitude of the driving current supplied to the LEDs by continuously changing the magnitude of the driving current within a range, or by providing a plurality of discrete driving current patterns having different fixed magnitudes.

A variety of conventional LED drivers may be used to regulate the power supply to the LEDs. The LED driver may be an integrated circuit (INTEGRATED CIRCUIT, IC), for example the LED driver may be a CMOS driver or a TFT driver. The driver may be a discrete element such as a back-plane IC driver or an on-chip IC driver made from the same GaN epitaxial wafer.

The duty cycle for each driving condition may be at least 0.001%, 0.01%, 0.1%, 1%, 5%, 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90% or 100%.

Power control

The peak emission wavelength of an LED may advantageously be controlled by varying the power or power density supplied to the LED.

The peak emission wavelength may decrease in response to an increase in power provided by the power source to the LED, and the peak emission wavelength may increase in response to a decrease in power provided by the power source.

For example, when the power supply provides a first power to the LED, the LED may emit at a first peak emission wavelength, and when the power supply provides a second power lower than the first power, the LED may emit at a second peak emission wavelength longer than the first emission wavelength.

In a particularly preferred embodiment, the first peak emission wavelength is below 570nm and the second peak emission wavelength is above 610nm, such that the LED emits green light in response to the first power and red light in response to the second power. Thus, by controlling the power supplied to the LEDs, the LEDs can be controlled to emit light at any wavelength between green and red.

When the power supply provides a third power density different from the first power and the second power, the LED preferably emits at a third peak emission wavelength.

The peak emission wavelength of the LED of the present invention advantageously varies in a consistent manner with the power or power density of the power supply, which can be calibrated into a display device containing the LED. In a preferred embodiment, the peak emission wavelength of the LEDs of the present invention may vary in a calibratable relationship to the logarithm of the power or power density of the power supply. This is very advantageous for device design because the predictable relationship between power supply and peak emission wavelength enables precise control of LED emission. The power supply can be easily calibrated so that when a particular emission wavelength is desired, the exact power required to produce that emission wavelength can be directly calculated and provided.

Current control

The peak emission wavelength of an LED may be advantageously controllable by varying the current or current density of the power supplied to the LED.

The drive current density dependent offset of the peak emission wavelength of the variable wavelength LED may preferably be greater than 10 nm/decade or greater than 20 nm/decade.

The peak emission wavelength may decrease in response to an increase in the current density provided by the power source to the LED, and the peak emission wavelength may increase in response to a decrease in the current density provided by the power source.

For example, when the power supply provides a first current density to the LED, the LED may emit at a first peak emission wavelength, and when the power supply provides a second current density lower than the first current density, the LED may emit at a second peak emission wavelength longer than the first emission wavelength.

When the power supply has a third current density different from the first current density and the second current density, the LED preferably emits at a third peak emission wavelength.

In a particularly preferred embodiment, the first peak emission wavelength is below 570nm and the second peak emission wavelength is above 610nm, such that the LED emits green light in response to a first current density and red light in response to a second current density. Thus, by controlling the current density supplied to the LEDs, the LEDs can be controlled to emit light at any wavelength between green and red.

The peak emission wavelength of the LED of the present invention advantageously varies with the current or current density of the power supply in a consistent manner that can be calibrated into the display device. In a preferred embodiment, the peak emission wavelength of the LED of the present invention may have a calibratable dependence on the logarithm of the current or current density of the power supply. This is very advantageous for device design because the predictable relationship between power supply characteristics and peak emission wavelength enables precise control of LED emission. The power supply can be easily calibrated so that when a particular emission wavelength is desired, the exact current density required to produce that emission wavelength can be directly calculated and provided.

The LEDs may be driven by a current density of 0.001A/cm ² to 1000A/cm ², or 0.01A/cm ² to 500A/cm ², or 0.1A/cm ² to 250A/cm ².

In a preferred embodiment of the invention, the LEDs emit green light at a wavelength of 570nm or less in response to a current density of greater than 5A/cm ², or greater than 7A/cm ², or greater than 9A/cm ², or greater than 10A/cm ², or greater than 11A/cm ². The same LED emits red light having a wavelength greater than 610nm in response to a current density of less than 4A/cm ², or less than 3A/cm ², or less than 2A/cm ².

In a preferred embodiment, the LEDs emit light having a wavelength between 430nm and 500nm in response to a current density greater than 19A/cm ², or greater than 20A/cm ², or greater than 21A/cm ².

Porous template

The n-type region, light emitting region, and p-type region (which may be referred to as an LED structure or LED diode structure) are preferably grown on a semiconductor template containing porous regions. The semiconductor template may also contain a plurality of layers of semiconductor material arranged to provide a suitable substrate for overgrowth of the LED structure. However, once the n-type region, light emitting region, and p-type region have been grown on the template, both the LED structure and the template form part of the LED.

The porous region and the template may optionally be removed from the LED structure during subsequent chip processing.

The thickness of the porous region may be at least 1nm, preferably at least 10nm, particularly preferably at least 50nm. For example, the porous region may have a thickness of between 1nm and 10000 nm.

The porosity of the porous region may be between 1% and 99% porosity, or between 10% and 80% porosity, or between 20% and 70% porosity, or between 30% and 60% porosity. The porosity of a porous region can be measured as the volume of all pores relative to the volume of the entire porous region.

Porosity has been found to have an effect on the magnitude of the wavelength shift induced by the porous region. In general, the higher the% porosity, the greater the wavelength shift of the LED compared to the same LED structure on a non-porous template.

The porous region is preferably formed of one of GaN, inGaN, alGaN, alInGaN or AlN.

The porous region may be under or below the n-type region, the light emitting region, and the p-type region of the LED. Preferably, the n-type region, the light emitting region, and the p-type region (LED structure) are located on or over the porous region, as defined by the growth sequence of the layers in the LED. The LED structure is preferably overgrown over the porous region such that the LED structure benefits from strain relaxation in the porous group III nitride layer.

LED layer

The LED may include a connection layer of group III nitride material between the n-doped portion and the porous region. Preferably, the thickness of the tie layer is at least 100nm, although smaller or larger thicknesses may also be employed. The connection layer may preferably be one of GaN, inGaN, alGaN, alInGaN or AlN.

The LED preferably comprises a non-porous intermediate layer of a porous region of a group III nitride material between the porous region and the light emitting region. Since the porous region is preferably formed by electrochemical porosification using the methods of PCT/GB2017/052895 (publication No. WO 2019/063957) and PCT/GB2019/050213 (publication No. WO 2019/145728) by a non-porous layer of group III nitride material, the non-porous layer of group III nitride material typically forms a non-porous intermediate layer that remains on top of the porous region. The non-porous intermediate layer advantageously provides a smooth surface for overgrowth of other layers during fabrication.

Preferably, the LED comprises a non-porous intermediate layer of group III nitride material between the porous region and the connection layer. This may preferably be a non-porous layer through which electrochemical etching of the porous region is performed.

The non-porous intermediate layer may preferably be one of GaN, inGaN, alGaN, alInGaN or AlN.

The porous region may be a porous layer such that the light emitting diode comprises a porous layer of a group III nitride material. Preferably, the porous region may be a continuous porous layer, for example formed from a continuous layer of porous group III nitride material.

The porous region may comprise a plurality of porous layers, and optionally a plurality of non-porous layers. In a preferred embodiment of the invention, the porous region is a stack of alternating porous and non-porous layers, the top surface of the stack defining the top of the porous region and the bottom surface of the stack defining the bottom of the porous region. The light emitting region may be formed over a porous region of the stack comprising a porous layer of a group III nitride material.

In some embodiments, the light emitting region is located over a stack of multiple porous layers of group III nitride material. Thus, the porous region may be a stack of layers of group III-nitride material, with at least some of the layers being porous, rather than a single porous layer of group III-nitride material. The stack of porous layers may preferably be a stack of alternating porous layers and non-porous layers.

Alternatively, the porous region may be a layer of group III nitride material containing one or more porous regions, such as one or more porous regions in a non-porous layer of group III nitride material. In other words, the porous region need not be a continuous layer of porous material.

In preferred embodiments, the lateral dimension (width or length) of the porous region or porous layer may be equal to the lateral dimension of the porous layer or substrate on which the porous region is grown. For example, conventional substrate wafer sizes may have various sizes, such as 1cm ² or 2 inches, 4 inches, 6 inches, 8 inches, 12 inches, or 16 inches in diameter. However, by patterning one or more layers and/or depositing regions of different charge carrier concentrations in the same layer, smaller porous regions may be formed that do not span the entire substrate. Thus, the lateral dimensions of the porous layer or region may vary from about 1/10 pixel (e.g., 0.1 μm) to the lateral dimensions of the substrate itself.

The n-doped portion preferably comprises an n-doped group III nitride layer.

Preferably, the n-doped portions and/or n-doped layers comprise n-GaN or n-InGaN, or stacks of alternating layers of n-GaN/n-InGaN, or stacks of alternating layers of n-InGaN/n-InGaN containing different indium concentrations.

The n-doped portion may comprise a monocrystalline n-doped group III nitride portion, preferably wherein the n-doped portion comprises a monocrystalline n-doped group III nitride layer having a planar top surface.

Each of the porous region and the single crystal n-doped III-nitride layer may be a planar layer having a respective top surface and a respective bottom surface parallel to a planar top surface of the single crystal n-doped III-nitride layer.

The light emitting layer preferably comprises one or more InGaN quantum wells, preferably 1 to 10 quantum wells.

The light emitting layer may be a nanostructure layer of InGaN, comprising quantum structures such as quantum dots, fragmented quantum wells, or discontinuous quantum wells.

The light-emitting layer and/or the quantum well preferably has a composition of InxGa ₁ -xN, wherein 0.07.ltoreq.x.ltoreq.0.40, preferably 0.12.ltoreq.x.ltoreq.0.30 or 0.22.ltoreq.x.ltoreq.0.30 or 0.30.ltoreq.x.ltoreq.0.40, particularly preferably 0.22.ltoreq.x.ltoreq.0.27 and 0.27.ltoreq.x.ltoreq.0.40.

The LED preferably comprises a group III nitride layer on the light emitting layer; and a group III nitride barrier layer on the group III nitride layer.

The group III nitride layer on the light emitting layer may be referred to as a "cap layer". The cap layer serves to 1) increase the quantum confinement stark effect of the band bending, thereby red shifting and achieving longer wavelength emission, and 2) protect the high In% In InGaN to ensure that sufficient In% is incorporated to achieve long wavelength and provide a larger barrier.

The LED preferably comprises a cap layer of group III nitride material between the quantum well and the p-doped region. The cap layer may be GaN, inGaN, alGaN or AlN.

The LED preferably comprises a barrier layer of group III nitride material between the quantum well and the p-doped region. The barrier layer may be GaN, inGaN, alGaN or AlN.

The p-doped region may include a p-doped group III nitride layer and a p-doped aluminum gallium nitride layer located between the p-doped group III nitride layer and the light emitting region. The p-doped aluminum nitride layer is preferably an electron-blocking-layer (EBL) layer located between the cap layer and the p-type layer, wherein the electron-blocking layer contains 5-25at% aluminum, preferably wherein the electron-blocking layer has a thickness between 10nm and 50 nm.

In a preferred embodiment, the porous region is not part of a distributed Bragg reflector (distributed Bragg reflector, DBR). However, in other embodiments, the porous region may be formed and act as an optical reflector or mirror or filter having different reflectivity/transmission bands over a range of wavelengths.

The morphology of the Quantum Well (QW) in the active light emitting region may vary. For example, the light emitting region may contain uniform QWs with well defined interfaces or fragmented QWs, fragments or QW well width/composition fluctuations or similarly centered quantum dots with less well defined interfaces. Such control of QW morphology may determine the range of variable emission wavelengths to be controlled and manipulated.

The light emitting region preferably comprises a plurality of Quantum Wells (QWs). The quantum wells may be continuous. The quantum wells may be fragmented or discontinuous.

Current confinement layer

The LED may include a current confinement layer or current confinement layer that is a dielectric layer configured to limit the lateral area of the LED that conducts current. The use of a current confinement layer may advantageously allow for additional control of the current density in order to better control the peak emission wavelength of the LED.

The current confinement layer may advantageously enable control of the power density provided to the variable wavelength LED to control the peak emission wavelength.

The current confinement layer is preferably a layer of dielectric material. For example, the current confinement layer may be any dielectric, such as SiO ₂, siN, or SiNx.

The current confinement layer may be located in various locations in the LED as long as it limits the lateral area of the LED through which current is conducted. The current confinement layer may be located in the LED between the electrical n-contact and the electrical p-contact.

The current confinement layer may be positioned adjacent to an n-doped portion or a p-doped portion of the LED. For example, the current confinement layer may be located between the n-doped portion and the light emitting region. Alternatively, the current confinement layer may be located between the light emitting region and the p-doped portion. The current confinement layer may be located between the electrical contact and the LED structure (n-doped portion, p-doped portion, and light emitting region).

The current confinement layer preferably comprises a hole extending through the current confinement layer, or one or more holes extending through the current confinement layer. The aperture may preferably be located in the centre of the current confinement layer. For example, the current confinement layer may comprise a circular opening in the center of the LED structure.

The LED may be configured such that the electrical contact is in contact with the LED structure via an aperture in the current confinement layer such that an area of the aperture defines a contact area over which the contact and the LED structure are in contact.

The or each aperture preferably has a lateral dimension which is substantially smaller than the lateral dimension of the LED. By providing holes through the dielectric current confinement layer, a high local current density may be achieved, which may advantageously enable improved control of power through the LED.

For example, the lateral width (or diameter) of the aperture may be equal to or less than 50% of the lateral width of the LED structure (LED mesa). The width of the aperture may be equal to or less than 45%, 40%, 35%, 30%, 25% or 20% of the width of the LED structure.

The relative area of the holes may be varied compared to the total area of the current confinement layer (blocking region) to change the local current density.

Pixel size

The lateral dimensions (width and length when viewed from above) of the light emitting region and/or the LED may be greater than 50nm, 100nm, 200nm, 300nm, 500nm, 600nm, 700nm, 800nm, 900nm, 1 μm, 2 μm, 3 μm, 4 μm, 5 μm or 10 μm, 20 μm, 30 μm, 40 μm, 50 μm, 60 μm, 70 μm, 80 μm, 90 μm or greater than 100 μm or 200 μm, 300 μm and less than 1000 μm

The lateral dimensions (width and length) of the light emitting region and/or the LED may be greater than 100 μm and less than 300 μm. In this case, the LED may be referred to as a "mini-LED". In a preferred embodiment, the mini-LEDs may be square or round or square with rounded corners and have dimensions such as 300 μm, 200 μm, 100 μm.

The lateral dimensions (width and length) of the light emitting area and/or the LED may alternatively be less than 100 μm. In this case, the LED may be referred to as a "micro-LED". The micro-LEDs may have a lateral dimension preferably smaller than 80 μm, or 70 μm, or 60 μm, or 50 μm or 30 μm, or 25 μm, or 20 μm, or 15 μm or 10 μm, or 5 μm or 3 μm or 1 μm or 500nm, or 200nm, or 100nm or 50nm.

In a preferred embodiment, the micro-LEDs may be square or round or square with rounded corners and have dimensions such as 75 μm×75 μm, 50 μm×50 μm, 40 μm×40 μm, 30 μm×30 μm, 25 μm×25 μm, 20 μm×20 μm or 10 μm×10 μm, or 5 μm×5 μm, or 2 μm×2 μm, or 1 μm×1 μm, or 500nm×500nm or less.

The LEDs may be circular, triangular, rectangular, square, oval, diamond, hexagonal, pentagonal, and any combination of these shapes. In the case of irregularly shaped pixel designs, at least one dimension should be within the above-mentioned dimension range in order to classify the light emitting diode as a mini-or micro-LED. For example, the width or diameter of the LEDs is preferably less than 100 μm, so LEDs are classified as micro-LEDs.

Light emitting region

The light emitting region preferably comprises a Multiple Quantum Well (MQW) comprising a plurality of Quantum Wells (QWs), or quantum dots, quantum wires or other quantum nanostructures.

In some embodiments, the light emitting region comprises a plurality of Quantum Wells (QWs), and the quantum wells are continuous.

The present inventors have found that non-uniformity in the light emitting region has a significant effect on widening the emission wavelength range in which the light emitting region is capable of emitting light in response to a change in power supplied to the LED. In the prior art, non-uniformities in the light emitting area are typically considered problematic defects, which are unwanted and should be avoided in any possible way, as the target is typically a high quality, low defect semiconductor wafer. The inventors have circumvented this prejudice in the art and found that deliberately creating non-uniformities in the light emitting area can advantageously widen the emission wavelength range and result in variable wavelength LEDs that can emit in a wider wavelength range than is possible in the prior art.

In alternative embodiments of the invention, the light emitting region is non-uniform, fragmented or discontinuous. The light emitting region may be intentionally introduced to achieve effects of localized centers of carriers in InGaN quantum wells, such as multiple types of QW regions with different indium compositions and well widths and quantum barriers, non-uniform, or fragmented, or broken, or bandgap, or discontinuous quantum wells (which would result in fluctuations in well width), inGaN quantum dots or nanostructures, quantum wells formed on polar, semi-polar, or nonpolar planes.

In a preferred embodiment, the light emitting region comprises a plurality of Quantum Wells (QWs), and the quantum wells are non-uniform, fragmented or discontinuous.

The plurality of QWs may contain fluctuations in the well width. For example, the well width of the QW may fluctuate by at least 2%, 5%, 10%, 20%, 25% or 50% or 75%. The well width fluctuation may be a variation between quantum wells (vertical direction) and within one quantum well (lateral direction).

The plurality of QWs may comprise fluctuations in alloy composition. For example, the indium composition of the QW may vary by at least 2%, 5%, 10%, 20%, 25% or 50% or 75% across the light emitting area.

The inventors have found that fluctuations in well width and/or alloy composition may induce carrier localization centers in the upper or lower interface of the QW. Any carrier localization center will induce a variable wavelength in the variable wavelength LED of the present invention. The greater the density of these carrier localization centers, the greater the range of variable wavelengths that can be achieved.

The LED may comprise v-shaped pits extending or propagating through the light emitting active area. Preferably, the LED comprises a plurality of v-shaped pits extending through the light emitting area.

Preferably, the LEDs may comprise a v-shaped pit density of at least 1 x 10 ⁷/cm² (measured from above looking down on the LED structure), for example a v-shaped pit density of at least 5 x 10 ⁷/cm² or at least 1 x 10 ⁸/cm², for example 1 x 10 ⁷/cm² to 5 x 10 ⁹/cm².

The LED may include a v-shaped pit density of less than 5 x 10 ⁹/cm², such as a v-shaped pit density of less than 1 x 10 ⁹/cm² or less than 5 x 10 ⁸/cm².

V-pits are a phenomenon known in the field of epitaxial semiconductor growth, and methods of growing V-pits in semiconductor structures are known in the art. For example, in the prior art, V-pits and their growth are described in the impact of nano-scale V-pits on the electronic and optical properties and efficiency degradation of GaN-based green light emitting diodes; zhou et al; scientific report (SCIENTIFIC REPORTS) 8:11053I DOI 10.1038/s41598-018-29440-4.

These v-shaped pits are v-shaped in cross-section, but in practice form conical or funnel-shaped voids in the semiconductor structure grown from the bottom up using conventional epitaxial growth methods. Although the cross-section of the pit is v-shaped, the pit is typically hexagonal when viewed from above. The points of the v-shaped pits are always directed downward toward the early deposited layers of the semiconductor structure, widening the pits as subsequent epitaxially grown layers are deposited on top of the structure.

While v-pits are known in the art, they are typically considered problematic defects in semiconductor structures, which are undesirable because the target is typically a high quality, low defect semiconductor wafer.

In the unusual case of incorporating v-pits into semiconductor structures in the past, v-pits were used as a shielding mechanism to create higher band gap regions, preventing carriers from penetrating dislocations as leakage paths.

However, in a preferred embodiment of the invention, v-shaped dimples are intentionally incorporated into the LED structure. The v-shaped pits extend far enough down into the semiconductor structure that they terminate in a layer below the active light emitting area. This means that the v-shaped pits must extend through the thickness of the active light emitting area.

The inventors have found that v-shaped pits extending through the light emitting area of the LED structure can advantageously widen the emission wavelength range that can be emitted by the variable wavelength LED.

As the v-shaped pits extend through the active area of the LED, a Quantum Well (QW) layer is grown on the sloped sidewalls of the v-shaped pits flush with the rest of the structure during epitaxial growth from bottom up. The QWs deposited on the pit sidewalls are distorted and stretched around the sides of the pit, and thus ultimately have a different thickness and composition than the planar QWs on the structural body.

Around the v-shaped pits, a QW layer of semiconductor material is grown as a planar layer. Thus, the active light emitting region is planar around the v-shaped pit. However, at the location of the v-shaped pits, the active layer is distorted and extends down the sidewalls into the v-shaped pits. This stretching effect changes the thickness of the QWs on the pit sidewalls so that they are different compared to the planar QW layer formed on the rest of the LED structure.

The inventors have found that v-pits can create localized strain relaxation and that MQWs deposited on the sidewalls of these v-pits will have different thicknesses and compositions than the rest of the MQWs, so MQWs in the v-pit regions will produce different emission wavelengths.

The quantum wells grown on the sidewalls of the v-shaped pits are thinner than the bulk plane QW elsewhere in the structure, which can affect the QW bandgap and allow QW in this region to emit a different wavelength than the planar QW elsewhere in the structure. In addition, the QW on the pit sidewalls may ultimately have a higher indium (In) content than the surrounding planar QW, because the sidewalls expose the semi-polar face of the QW-which incorporates more indium during epitaxial growth-the indium content of the QW In the v-shaped pit region may be higher than the planar QW surrounding the pit. Higher indium incorporation typically results in longer peak emission wavelengths. Both QW thickness and indium content affect the emission wavelength produced by the light emitting region. Thus, the presence of v-shaped pits in the LED structure may advantageously change the composition and thickness of the QW in the light emitting region, thereby extending the emission wavelength range over which the LED may be driven to emit light.

V-shaped pits are typically grown from threading dislocations in the semiconductor structure. As the additional layer grows over the layer containing threading dislocations, the threading dislocations continue upward through the structure and dislocation broaden into V-shaped pits at a point. Typically, the goal of the technician is to keep threading dislocation concentration low in order to produce "high quality" low defect wafers.

Alternatively, v-shaped pits may be grown using a three-dimensional epitaxial growth mode. 3D epitaxial deposition techniques are known in the art and are typically used to grow "islands" or "pyramids" of semiconductor material on templates. By controlling the deposition of the LED structure using 3D epitaxial deposition techniques, v-pits can be grown artificially at desired locations without the need for threading dislocations to "seed" the formation of v-pits. By using such deposition control, the bottom (nadir) of the pit can be created at a desired location in the structure-at both a desired lateral position and a desired height in the structure, for example in a particular layer of the semiconductor structure below the active light emitting region.

The bottom of the v-shaped recess may be located in a connection layer of the semiconductor structure. The connection layer may be located between the porous region and the n-doped portion.

The bottom of the v-shaped pit may be located in a pre-strained layer of the semiconductor structure. The pre-strained layer may be located above the n-doped portion and below the light emitting region.

Preferably, the LED comprises a plurality of v-shaped pits extending through the active light emitting area.

Preferably, the LED comprises a v-shaped pit density (measured looking down at the LED structure from above) of at least 1 x 10 ⁷/cm², such as at least 5 x 10 ⁷/cm² or at least 1 x 10 ⁸/cm². The LED may include a v-shaped pit density of less than 5 x 10 ⁹/cm², such as a v-shaped pit density of less than 1 x 10 ⁹/cm² or less than 5 x 10 ⁸/cm².

Such as a v-shaped pit density of 1 x 10 ⁷/cm² to 5 x 10 ⁹/cm², or 5 x 10 ⁷/cm² to 5 x 10 ⁹/cm², or 1 x 10 ⁸/cm² to 5 x 10 ⁸/cm².

The LED may contain more than 0.1 v-shaped pits per square micron, or more than 1 v-shaped pit per square micron, or more than 2 v-shaped pits per square micron.

The concentration of v-shaped pits in an LED is preferably controlled, as too many v-shaped pits can negatively affect the light emission of the LED by disrupting the radiative recombination. For example, an LED may contain less than 10 v-shaped dimples per square micron, or less than 8 v-shaped dimples per square micron, or less than 6 v-shaped dimples per square micron.

In a preferred embodiment, the LED structure may include no more than 10-9 threading dislocations per square centimeter. Preferably, the semiconductor structure (typically the substrate, porous region and connection layer) under the active light emitting region contains no more than 10 a 9 threading dislocations per square centimeter. The threading dislocation density is preferably limited to this level so that the further epitaxial growth does not create too many v-shaped pits in the light emitting area.

The density and size (depth) of the v-shaped pits can be controlled. The size of the V-pits can be controlled by the location and growth conditions of the pre-strained layer and the low temperature n-gan layer at the beginning of the pits.

The morphology of the Quantum Well (QW) in the active light emitting region may vary. For example, the light emitting region may contain a uniform QW with a well-defined interface or a fragmented QW, fragment or QW well width/composition fluctuation or quantum dot-like positioning center with a less well-defined interface. Such control of QW morphology may determine the range of variable emission wavelengths to be controlled and manipulated.

If the quantum well is continuous and of very uniform thickness and composition, the recombination of charge carriers can only take place in a regular, well-defined manner. On the other hand, if the QW is fragmented or discontinuous, a large number of nanostructures are created, which in turn create different bandgaps, resulting in different color emissions.

Method for controlling variable wavelength LED

In another aspect of the invention, there is provided a method of controlling a variable wavelength LED, the method comprising the steps of:

Providing a power supply to the variable wavelength LED according to the first aspect of the invention; and

The power supply is controlled to vary the peak emission wavelength of the variable wavelength LED over the emission wavelength range. The method may comprise the step of varying the power supply to vary the peak emission wavelength of the variable wavelength LED over the emission wavelength range.

The method may include dynamically tuning the power to the LEDs during a single display frame.

The variable wavelength LED may be provided with a power source or supply of power from a power source, optionally via an LED driver.

The method may comprise the steps of: providing a driving current to the variable wavelength LED; and varying the magnitude of the drive current to vary the peak emission wavelength of the variable wavelength LED over the emission wavelength range.

Preferably, the peak emission wavelength of a variable wavelength LED is varied by varying the drive current supplied to the LED during operation.

The LED may be controlled to emit light at a plurality of discrete peak emission wavelengths by varying the drive current provided to the subpixel between a plurality of discrete non-zero values. This may advantageously provide a tunable monochrome display.

The LEDs may be controlled to emit light at a plurality of discrete peak emission wavelengths by varying the drive current supplied to the subpixel between a plurality of discrete non-zero values during a display frame. This may advantageously enable dynamic pixel tuning, wherein the color emitted by the individual LEDs during a display frame is tunable.

The power supply may be controlled to vary the peak emission wavelength over an emission wavelength range of at least 40nm, or at least 50nm, or at least 60nm, or at least 70nm, or at least 80nm, preferably over a range of up to 100nm or 110nm or 120nm or 140nm, or 160nm, or 180nm or 200 nm.

In a particularly preferred embodiment, the power supply may be controlled to vary the peak emission wavelength between 400nm and 680nm, 430nm and 670nm, 450nm and 650nm, 500nm and 680nm, or 520nm and 675 nm.

The power source may be a pulsed power source, or the power source may be a Continuous Wave (CW) or near continuous wave power source.

The control of the power supply may be current control or voltage control.

The power supply can be a constant voltage power supply or a constant current power supply. AC or DC power sources are also available.

The power supply may operate in a pulse width modulation (Pulse Width Modulation, PWM) mode or a Pulse Amplitude Modulation (PAM) mode, or both.

The magnitude or amplitude of the power supply may vary between at least two non-zero values during one display frame.

The magnitude or amplitude of the power supply may vary between a plurality of discrete non-zero values during a display frame, for example, between three non-zero values during a display frame.

The power supply may be controlled to provide a current density to the LEDs of between 0.001A/cm ² and 1000A/cm ², or between 0.01A/cm ² and 500A/cm ², or between 0.1A/cm ² and 250A/cm ².

The power supply may be controlled to provide a drive current to the LEDs with a duty cycle of at least 0.001%, 0.01%, 0.1%, 1%, 5%, 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90% or 100%.

The method may comprise the steps of: providing a first drive current such that the LED emits at a first peak emission wavelength; and providing a second drive current having a magnitude or amplitude different from the first drive current such that the LED emits at a second peak emission wavelength.

The first drive current may be provided to the LED at a first duty cycle and the second drive current may be provided to the LED at a second duty cycle. The method may comprise the step of controlling the duration of the first duty cycle and/or the second duty cycle in order to control the observed luminance and/or chrominance produced by the display device.

The method may include the step of providing a third drive current having a magnitude or amplitude different from the first drive current and the second drive current such that the LED emits at a third peak emission wavelength.

The method may comprise the steps of: a third drive current is provided to the LEDs at a third duty cycle, and the duration of the third duty cycle is preferably controlled in order to control the observed luminance and/or chromaticity produced by the display device.

The drive current provided to the LEDs may be varied between the first drive current, the second drive current and/or the third drive current in order to control the observed chromaticity produced by the display device.

The or each LED may be operable in a plurality of modes in response to a plurality of different drive current magnitudes or amplitudes, the LED being configured to emit at a discrete peak emission wavelength in each of the plurality of modes. In each mode, the subpixels are driven with different magnitude current densities so that the subpixels emit different peak wavelengths of light in different magnitude modes. The magnitude of the drive current supplied to the variable wavelength LED may be varied during operation of the display device, preferably within a single display frame.

Power control

The power supply may be controlled to vary the peak emission wavelength of the LED by varying the power (in watts) or power density (in watts/cm ²) provided to the LED.

The power provided by the power supply may be increased to reduce the peak emission wavelength, or the power provided by the power supply may be decreased to increase the peak emission wavelength.

The power supply may be controlled to supply a first power to the LEDs that emit at a first peak emission wavelength, and the power supply may be controlled to supply a second power that is lower than the first power, such that the LEDs emit at a second peak emission wavelength that is longer than the first emission wavelength.

In a particularly preferred embodiment of the green-red LED, the first peak emission wavelength may be below 570nm and the second peak emission wavelength may be above 610nm, such that the LED emits green light in response to the first power and red light in response to the second power. By varying the power between the first power and the second power, the LED may also emit light in a continuous spectrum between the first peak emission wavelength and the second peak emission wavelength.

Current control

The power supply may be controlled to vary the peak emission wavelength of the LED by varying the current (in amperes) or current density (in a/cm ²) supplied to the LED.

Similar to power, the current density provided by the power supply may be increased to decrease the peak emission wavelength, or decreased to increase the peak emission wavelength.

The power supply may be controlled to supply a first current density at which the LED emits at a first peak emission wavelength and to supply a second current density lower than the first current density at which the LED emits at a second peak emission wavelength longer than the first emission wavelength.

In a particularly preferred embodiment of the green-red LED, the first peak emission wavelength may be below 570nm and the second peak emission wavelength may be above 610nm, such that the LED emits green light in response to a first current density and red light in response to a second current density.

Voltage control

The power supply may be controlled to vary the peak emission wavelength of the LED by varying the voltage supplied to the LED.

The voltage provided by the power supply may be increased to decrease the peak emission wavelength, or the voltage provided by the power supply may be decreased to increase the peak emission wavelength.

The power supply may be controlled to supply a first voltage at which the LED emits at a first peak emission wavelength, and the power supply may be controlled to supply a second voltage lower than the first voltage such that the LED emits at a second peak emission wavelength longer than the first emission wavelength.

In a particularly preferred embodiment of the green-red LED, the first peak emission wavelength may be below 570nm and the second peak emission wavelength may be above 610nm, such that the LED emits green light in response to a first voltage and red light in response to a second voltage. By varying the voltage between the first voltage and the second voltage, the LED may also emit light in a continuous spectrum between the first peak emission wavelength and the second peak emission wavelength.

Manufacturing method of variable wavelength LED

In another aspect, there is provided a method of manufacturing a variable wavelength LED comprising the steps of growing: an n-doped portion;

A p-doped portion; and

A light emitting region between the n-doped portion and the p-doped portion, the light emitting region comprising a light emitting layer that emits light at a peak emission wavelength under an electrical bias thereacross.

The method may include the step of overgrowing the n-doped portion, the p-doped portion, and the light emitting region over a porous region of the group III-nitride material.

The method may include the steps of forming a porous region of group III nitride material in at least one of the n-doped portion or the p-doped portion, and forming a light emitting region over the porous region of group III nitride material.

The method may optionally comprise the step of removing the porous region from the LED structure (n-doped portion, p-doped portion and light emitting region) after the n-doped portion, p-doped portion and light emitting region have been formed.

The light emitting layer may emit light at a peak emission wavelength under an electrical bias between 400nm and 800nm, or between 450nm and 800nm, or between 500nm and 800nm, or between 550nm and 800nm, or between 610nm and 800 nm.

The method may comprise the step of connecting the LED to a variable power supply.

The method may include the step of connecting the LED to an LED driver configured to provide a variable power supply to the LED. The LED driver may be configured to control the power or current or voltage of the LED power supply. The LED driver may be configured to provide pulsed or CW or near CW power to the LEDs.

The LED structure including the n-doped portion, the p-doped portion, and the light emitting region may be an LED structure for emitting at a wavelength below the peak emission wavelength of the LED such that the porous region of group III nitride material red shifts the emission wavelength of the light emitting region to the peak emission wavelength.

The n-doped portion, the p-doped portion, and the light emitting region are preferably formed of a group III nitride semiconductor material.

In a preferred embodiment, the light emitting region may comprise a light emitting indium gallium nitride layer for emission at a peak wavelength of 500nm-550nm or 550nm-600nm, wherein overgrowth on the porous region of group III nitride material shifts the emission wavelength of the light emitting region to a peak wavelength between 600nm and 750nm under an electrical bias.

The light emitting region may comprise a luminescent indium gallium nitride layer for emission at peak wavelengths of 500nm-550nm, or 500nm-580nm, or 510nm-570nm, or 530nm-560nm, or 550nm-600 nm. The luminescent indium gallium nitride layer may be one or more layers known to emit light at these wavelengths when grown in conventional LEDs, for example on a non-porous GaN substrate. However, the inventors have found that growing a conventional yellow or green LED structure on a porous group III nitride layer results in the LED emitting light at a peak wavelength between 600nm and 750nm under an electrical bias.

The method may include the step of growing a yellow or green LED structure over the porous region of the group III-nitride material.

The method may comprise the step of forming a light emitting active region having a carrier localized centre in a quantum well, preferably an InGaN QW. Such as various types of QW regions with different indium compositions and well widths, as well as quantum barriers, non-uniform, or fragmented, or broken, or bandgap, or discontinuous quantum wells (which would result in fluctuations in well width), inGaN quantum dots or nanostructures, quantum wells formed on polar, semi-polar, or nonpolar faces.

The method may comprise the step of forming a plurality of Quantum Wells (QWs), wherein the quantum wells are non-uniform, fragmented or discontinuous.

The plurality of QWs may contain fluctuations in indium composition and/or fluctuations in well width.

The method may include the step of forming one or more v-shaped dimples in the LED structure such that the v-shaped dimples extend through the thickness of the light emitting area. Preferably, the method comprises the step of forming at least 0.1 v-shaped pits per square micron, or at least 1 v-shaped pit per square micron, or at least 2 v-shaped pits per square micron. Preferably, the method comprises the step of forming a v-shaped pit density of at least 1 x 10 ⁷/cm², such as at least 5 x 10 ⁷/cm² or at least 1 x 10 ⁸/cm², such as 1 x 10 ⁷/cm² to 5 x 10 ⁹/cm², in the light emitting region. Preferably, the method comprises the step of forming a v-shaped pit density in the light emitting area of less than 5 x 10 ⁹/cm², for example less than 1 x 10 ⁹/cm² or less than 5 x 10 ⁸/cm².

V-pits are a phenomenon known in the field of epitaxial semiconductor growth, and methods of growing V-pits in semiconductor structures are known in the art. For example, in the prior art, V-pits and their growth are described in the impact of nano-scale V-pits on the electronic and optical properties and efficiency degradation of GaN-based green light emitting diodes; zhou et al; scientific report (2018) 8:11053. Sub.DOI:10.1038/s 41598-018-29440-4.

V-shaped pits may be grown in the semiconductor structure such that they terminate in a layer below the active light emitting region. This means that the v-shaped pits must extend through the thickness of the active light emitting area.

V-shaped pits can be grown from threading dislocations in a semiconductor structure by controlling growth conditions during epitaxial deposition of layers above the threading dislocation-containing layer. As the additional layer grows over the layer containing threading dislocations, the threading dislocations continue upward through the structure and by controlling the growth conditions, the dislocations are widened into V-shaped pits.

Quantum Wells (QWs) in the active light emitting region may be deposited such that the quantum wells are continuous and/or uniform in thickness. Alternatively, quantum Wells (QWs) in the active light emitting region may be deposited such that the quantum wells are fragmented or discontinuous.

Manufacturing procedure

The n-type region, light emitting region, and p-type region (which may be referred to as an LED structure) are preferably grown over a semiconductor template that includes porous regions. The semiconductor template may also contain a plurality of layers of semiconductor material arranged to provide a suitable substrate for overgrowth of the LED structure.

The method may include a first step of electrochemically porosifying a layer of group III-nitride material to form a porous region of group III-nitride material. This can be achieved using the wafer level porosification process set forth in international patent applications PCT/GB2017/052895 (publication No. WO 2019/063957) and PCT/GB2019/050213 (publication No. WO 2019/145728).

The method may preferably comprise the step of forming a porous region of the group III nitride material by electrochemical porosification of the non-porous layer of the group III nitride material such that the non-porous layer of the group III nitride material forms a non-porous intermediate layer. The non-porous intermediate layer may advantageously provide a smooth surface for overgrowth of other layers, such as one or more tie layers of group III nitride material.

The porous region may be formed by porosifying one or more layers or regions of group III-nitride material on the substrate. The substrate may be silicon, sapphire, siC, β -Ga2O3. The crystal orientation of the substrate may be a polar, semi-polar or nonpolar orientation. The substrate thickness typically can vary between 100 μm and 1500 μm.

The porous region may be a porous layer such that the method comprises the step of overgrowing over the porous layer of group III nitride material: an n-doped portion; a p-doped portion; and an LED light emitting area. Preferably, the porous region may be a continuous porous layer, for example formed from a continuous layer of porous group III nitride material.

The porous region may comprise a plurality of porous layers, and optionally a plurality of non-porous layers. In a preferred embodiment of the invention, the porous region is a stack of alternating porous and non-porous layers, the top surface of the stack defining the top of the porous region and the bottom surface of the stack defining the bottom of the porous region.

Alternatively, the porous region may be a layer of group III nitride material containing one or more porous regions, such as one or more porous regions in a non-porous layer of group III nitride material.

A doped region of n-doped group III nitride semiconductor material, preferably containing a layer or stack of layers, may be deposited on the substrate prior to the porosification step. The group III nitride layer(s) may contain one or a combination of these elements: al, ga, in (ternary or quaternary layers). The thickness of the group III nitride stack is preferably between 10nm and 4000 nm. The doping concentration of the group III nitride region may be between 1 x 10 ¹⁷cm^-3–5×10²⁰cm^-3.

Preferably, an intermediate layer of undoped group III nitride material is deposited on the doped material before the doped material is porous. The thickness of the intermediate layer is preferably between 1nm and 3000nm, preferably between 5nm and 2000 nm. Because the intermediate layer is undoped, it remains non-porous after the pore-forming step, which advantageously provides a good surface for epitaxial overgrowth of additional layers of the semiconductor.

In a preferred embodiment, the doped region consists of an alternating stack of doped and undoped layers. In a preferred embodiment, the stack contains 5-50 pairs of layers. The thickness of each highly doped layer may vary between 10nm and 200nm, and the thickness of the low doped or undoped layer may be between 5 and 180 nm.

Electrochemical porosification removes material from n-doped regions of group III nitride material and creates voids in the semiconductor material, as is known in the art.

In a preferred embodiment, the LED structure is formed over a stack of multiple porous layers of group III nitride material. Thus, the porous region may be a stack of layers of group III-nitride material, with at least some of the layers being porous, rather than a single porous layer of group III-nitride material. The stack of porous layers may preferably be a stack of alternating porous layers and non-porous layers.

The method may preferably comprise the step of depositing one or more connection layers of group III-nitride material on the surface of the intermediate layer of group III-nitride material, prior to overgrowing the n-doped region, the LED light emitting region and the p-doped region on the connection layer.

Alternatively, without a non-porous intermediate layer over the porous region, the method may include the step of depositing a tie layer of group III-nitride material onto the surface of the porous region of group III-nitride material.

The method may comprise the further step of overgrowing the n-doped region, the LED light emitting region and the p-doped region on the connection layer.

The LED produced by the manufacturing method is preferably a variable wavelength LED according to the first aspect of the invention.

Display device

A first aspect of the invention provides a variable wavelength LED whose peak emission wavelength depends on the drive current density or drive power density supplied to the LED during use.

Such variable wavelength LEDs may be incorporated into display devices in a variety of ways to give a range of desired device characteristics.

According to a second aspect of the present invention there may be provided a display device comprising a variable wavelength LED according to the first aspect of the present invention, the LED being configured to receive a power supply.

The display device may include a variable wavelength Light Emitting Diode (LED) including:

An n-doped portion;

A p-doped portion;

wherein the LED is configured to receive a power source, wherein by varying the power source, the peak emission wavelength of the LED is continuously controllable over an emission wavelength range of at least 40 nm.

The display device preferably comprises a plurality of variable wavelength LEDs according to the first aspect of the invention, each variable wavelength LED being configured to receive its own power supply, wherein each of the plurality of variable wavelength LEDs is controllable such that the peak emission wavelength of each variable wavelength LED is controllable by varying the power supply of that LED.

The display device preferably comprises a plurality of LED pixels. Each pixel may comprise a single variable wavelength LED or a plurality of LED sub-pixels. In these sub-pixels, some or all may be variable wavelength LEDs as described above.

Various embodiments of a display device are described and illustrated in conjunction with the accompanying drawings.

In the display device according to the invention, the display device preferably comprises a plurality of LED pixels, and preferably each device pixel comprises at least one variable wavelength LED according to the first aspect of the invention. Preferably, at least one sub-pixel of each pixel is a variable wavelength LED according to the first aspect of the invention. The display device may comprise a plurality of LED pixels and each device pixel may comprise one, two, three or four variable wavelength LEDs according to the first aspect of the invention.

In a preferred embodiment of the invention, some or all of the variable wavelength LEDs are preferably configured to receive a variable magnitude drive current supply from the power source such that the magnitude of the drive current to each variable wavelength LED is variable. By varying the magnitude of the drive current of the variable wavelength LED, the peak emission wavelength of the LED may be varied with the use of the display device. The drive current provided to each variable wavelength LED may be individually controllable such that the peak emission wavelength of each variable wavelength LED in the display may be individually controlled and varied. Alternatively, the device may be configured such that the same driving conditions are provided to a group of variable wavelength LEDs at the same time, such that when the driving current is on, all of the variable wavelength LEDs in the group emit light at the same peak emission wavelength, and the peak emission wavelength of the entire group may be changed by changing the magnitude of the driving current.

In alternative embodiments, some or all of the variable wavelength LEDs in the display device may be configured to receive a fixed magnitude (i.e., non-variable) drive current, which is either on or off. When the fixed drive current is on, these variable wavelength LEDs will behave as conventional LEDs and emit at a single peak emission wavelength determined by the drive conditions provided to the LEDs. Thus, a variable wavelength LED configured to receive a fixed magnitude drive current may be used as a fixed emission wavelength LED in a display device.

Preferably, at least one sub-pixel of each pixel is a variable wavelength LED according to the first aspect of the invention. Other sub-pixels may also be incorporated into the display device. For example, each pixel in a display device may contain one or more fixed emission wavelength LED sub-pixels. The fixed emission wavelength LED sub-pixels are preferably configured to receive a fixed magnitude of drive current.

The device may be configured to individually control the drive current provided to each of the plurality of LEDs such that each of the plurality of LEDs is individually drivable. The device may be configured to provide a plurality of different drive currents to the plurality of LEDs such that individual LEDs are drivable to emit at different peak emission wavelengths in response to the different drive currents.

Alternatively, the device may be configured to control groups of two or more LEDs individually such that each LED in the group emits at the same peak emission wavelength. The display device may be configured to provide different drive currents to different groups of LEDs such that the different groups of LEDs may be driven to emit at different peak emission wavelengths in response to the different drive currents.

The display device preferably comprises a power supply configured to provide a driving current to the plurality of LEDs. The display device may contain a power source or may be connected to a power source.

The power source may be a pulsed power source. The power supply may be configured to operate in a Pulse Width Modulation (PWM) mode or a Pulse Amplitude Modulation (PAM) mode. Preferably, the power supply is configured to operate in a pulse amplitude modulation (PAM mode) and a Pulse Width Modulation (PWM) mode.

The power source may be a Continuous Wave (CW) or near continuous wave power source. The power supply can be a constant voltage power supply or a constant current power supply.

Each light emitting diode is preferably connected to an LED driver configured to provide power to the LEDs.

The power supply of the LEDs is preferably regulated or controlled by an LED driver. The LED driver is preferably configured to provide a variable magnitude power supply to the variable wavelength LED(s) in the display device. For example, the LED driver is preferably capable of changing the magnitude of the driving current supplied to each variable wavelength LED by continuously changing the magnitude of the driving current within a range or by providing a plurality of discrete driving current patterns having different fixed magnitudes.

A variety of conventional LED drivers may be used to regulate the power supply to the LEDs. The LED driver may be an Integrated Circuit (IC), for example the LED driver may be a CMOS driver or a TFT driver. The driver may be a discrete element such as a back-plane IC driver or an on-chip IC driver made from the same GaN epitaxial wafer.

The LED driver may be connected to a power source, such as an external power source (mains) or a battery.

The display device may include a controller configured to control the power supply of the LEDs in the device. The controller may be programmed to control the power or drive current provided to the LEDs by the LED driver. For example, the controller may be programmed or programmable to control the magnitude, duration, and phase of power provided to each LED in the display.

The controller may be programmable to control the device in different control modes. For example, the controller may be responsive to user input such that in response to user input, the controller controls the LEDs in the device to operate in a selected mode.

The controller may be programmable to control the LEDs in the display device in a fixed wavelength emission mode by providing a fixed magnitude of drive current to the LEDs, the magnitude of the fixed drive current corresponding to the wavelength to be emitted. And/or the controller may be programmable to control the LEDs in the display device in a dynamically variable wavelength emission mode by providing a variable magnitude of drive current to the LEDs, the magnitude of the fixed drive current varying corresponding to the wavelength emitted at a given time.

The display device may comprise an array of LED pixels, wherein each pixel comprises two or more sub-pixels, and wherein at least one sub-pixel of each pixel is a variable wavelength LED.

The display device may comprise an array of LEDs, wherein each LED has the same diode structure, and wherein each LED is tunable to emit at a peak wavelength within the same emission wavelength range.

The display device may be an Augmented Reality (AR), mixed Reality (MR), or Virtual Reality (VR) device, or the device may be a smart wearable device, a smart display, or a direct view display.

At least one of the LED or LEDs is preferably a dynamic color tunable LED.

The display device preferably comprises a porous region of semiconductor material, preferably a porous region of group III nitride semiconductor material. As described above with respect to the first aspect, the first sub-pixel and/or the second sub-pixel is preferably formed over the porous region.

Herein, the term "display device (DISPLAY DEVICE)" may be replaced with "LED device (LED DEVICE)".

In the prior art, arrays of red, green and blue (RGB) LED pixels are closely arranged together such that operating the pixels individually or in combination emits a series of secondary color effects to the viewer. However, the present invention provides a better choice by providing a single LED pixel that can be controlled to emit light at a continuous range of different wavelengths. Thus, the LEDs can be controlled to emit "true" colors of various different wavelengths without the need to superimpose the different emission wavelengths of discrete LEDs (such as RGB pixels) to obtain a composite emission wavelength.

The display device may be a monolithic full color display.

A display device may be provided comprising an array comprising a plurality of identical variable wavelength LEDs according to the invention, wherein each LED in the controllable array may be controlled to emit at a different wavelength within the same wavelength or emission wavelength range. The array may also be controlled to turn individual pixels on or off in order to control the intensity of the emitted light.

The display device of the present invention may comprise a variable wavelength LED as described herein and a further LED: or the further variable wavelength LEDs have different emission wavelength ranges; or other conventional single wavelength light emitting diodes. In a preferred embodiment, the variable wavelength light emitting diodes of the present invention may be provided in an array, wherein the further light emitting diodes are configured to emit at a further peak emission wavelength outside the emission wavelength range of the variable wavelength light emitting diodes. If the other peak emission wavelengths are outside the emission wavelength range, these wavelengths are not available using variable wavelength LEDs. Similar to conventional RGB arrays, the variable wavelength LEDs and additional LEDs can thus be used in combination to provide a wider range of possible emission wavelengths.

The display device of the present invention may comprise an LED array comprising a plurality of LED sub-pixels according to any of the preceding aspects of the present invention. The array may be a mini-LED or micro-LED array, depending on the size of the LED pixels.

In another aspect, an LED display device may be provided that includes a plurality of variable wavelength LEDs (or mini-LEDs or micro-LEDs) that function as sub-pixels as described above. In an LED display, each of the plurality of LEDs is preferably individually controllable such that the peak emission wavelength of each LED is controllable by individually varying the power supply of each LED. By controlling the emission wavelength of each LED individually, a color display can be provided. However, since each variable wavelength LED can emit light over a range of wavelengths, rather than a standard single wavelength, fewer LEDs than are required in conventional RGB multicolor displays of the prior art can be used to provide a wide range of possible colors.

In an LED display, a group of multiple variable wavelength LEDs may be configured to receive the same driving conditions from a power source such that the peak emission wavelength of all the variable wavelength LEDs in the group is simultaneously controllable by varying the power source to the group.

The LED display may comprise a first variable wavelength LED having a peak emission wavelength controllable within a first emission wavelength range and a second variable wavelength LED having a peak emission wavelength controllable within the first emission wavelength range or within a second emission wavelength range. The second emission wavelength range may be a separate wavelength range, or it may partially overlap the first emission wavelength range. Thus, the first variable wavelength LED and the second variable wavelength LED may be controlled to emit at different peak emission wavelengths. For example, a pixel of an LED display device may include a first variable wavelength LED sub-pixel having a peak emission wavelength controllable within a first emission wavelength range, and a second variable wavelength LED sub-pixel having a peak emission wavelength controllable within the first emission wavelength range or a second emission wavelength range.

In another preferred embodiment, the LED display may comprise a plurality of variable wavelength LEDs controllable within a first emission wavelength range, and at least one further LED configured to emit at a peak emission wavelength outside the first emission wavelength range. For example, the additional LEDs may be configured to emit at wavelengths outside the wavelength range of the variable wavelength LEDs in order to increase the range of colors available from the display. For example, a pixel of an LED display device may include a plurality of variable wavelength LED sub-pixels controllable within a first emission wavelength range, and at least one additional LED sub-pixel configured to emit at a peak emission wavelength outside of the first emission wavelength range.

The plurality of variable wavelength light emitting diodes are preferably operable at different peak emission wavelengths within the first range of emission wavelengths. For example, one of the plurality of variable wavelength LEDs may emit at a first wavelength within the range, while another of the plurality of variable wavelength LEDs emits at a second wavelength within the first emission wavelength range. Thus a wide variety of secondary and complementary colors are available.

In a preferred embodiment, some or all of the plurality of variable wavelength LEDs are operable at the same peak emission wavelength to provide a desired emission intensity at that wavelength. Since longer wavelengths are achieved by reducing the magnitude supplied to the LEDs, longer wavelength LEDs emit at lower intensities than shorter wavelength ones. Providing a plurality of variable wavelength LEDs may advantageously enable the display to compensate for this intensity difference by controlling the number of LEDs emitting at the desired wavelength. For example, two or more LEDs may be controlled to emit red wavelengths in order to increase the brightness of the emitted red light to a level suitable for the display.

In a preferred embodiment, the LED display comprises a plurality of variable wavelength LEDs which are controllable to emit at a peak emission wavelength between 500nm and 680nm or between 520nm and 675nm, and the LEDs comprise at least one further LED configured to emit at a peak emission wavelength below 560nm, preferably below 500 nm. Thus, the variable wavelength LEDs are controllable to emit light in a color ranging from green, yellow and orange to red, while the other LEDs are configured to emit blue light. Thus, these LEDs enable the display to emit a wide range of wavelengths between the blue and red spectra.

In a particularly preferred embodiment, the LED display comprises a plurality of variable wavelength LEDs which are controllable to emit at peak emission wavelengths between 560nm and 680nm or between 570nm and 675nm, and at least one further LED configured to emit at peak emission wavelengths below 560nm, preferably below 500 nm. Thus, the variable wavelength LEDs are controllable to emit light in a color ranging from green, yellow and orange to red, while the other LEDs are configured to emit blue light.

In another preferred embodiment, the LED display comprises a plurality of variable wavelength LEDs that are controllable to emit at peak emission wavelengths between 400nm and 680nm, or between 450nm and 630nm, or between 470nm and 610 nm. Thus, the variable wavelength LEDs are controllable to emit light from violet or blue to green, yellow and orange to red.

In another aspect, a display device may be provided that includes a variable wavelength LED configured to receive a drive current and to emit light in response to the drive current, wherein a peak emission wavelength of the variable wavelength LED is dependent on a magnitude of the drive current supplied to the variable wavelength LED; and wherein the amplitude of the drive current of the variable wavelength LED is variable between at least two non-zero values during one display frame.

The variable wavelength LED is preferably a variable wavelength LED according to the first aspect of the invention described above, the LED being configured to receive a power supply. Thus, any of the features of the variable wavelength LED described above are applicable to the present display device.

The variable wavelength LED is preferably a pixel of the display device and the peak emission wavelength of the pixel is preferably variable by varying the drive current supplied to the pixel during operation.

The variable wavelength LED is preferably controllable to emit a desired spectral output in response to an analog drive current pulse, wherein the amplitude of the analog drive current pulse varies between non-zero values within a single display frame.

The variable wavelength LED is preferably controllable to emit light at a plurality of discrete peak emission wavelengths by varying the drive current supplied to the variable wavelength LED between a plurality of discrete non-zero values during one display frame. For example, the variable wavelength LED is controllable to emit light at a plurality of discrete peak emission wavelengths by varying the drive current provided to the variable wavelength LED between at least 3, or at least 4, or at least 5, or at least 6, or at least 7 discrete non-zero values during one display frame. Each discrete drive current applied to the variable wavelength LED causes the LED to emit light at a corresponding discrete peak emission wavelength. By applying three drive current pulses at discrete current magnitudes, the variable wavelength LED will thus emit light at three discrete peak emission wavelengths, each emission wavelength corresponding to a different input drive current.

The variable wavelength LED is controllable to emit a desired spectral output in response to a drive current comprising a sequence of discrete current pulses within a single display frame, the discrete current pulses having a plurality of different amplitudes. The discrete current pulse sequence preferably comprises at least 3, or at least 5, or at least 7 current pulses having different amplitudes.

The amplitude of the drive current of the variable wavelength LED preferably varies between at least three, or at least four, or at least five, or at least six, or at least seven non-zero values during one display frame.

The perceived spectral output of a variable wavelength LED in a display frame is a temporal combination of two or more peak emission wavelengths that the variable wavelength LED emits in response to two or more non-zero drive currents supplied to the LED during the display frame.

The duration of each drive current pulse in the sequence will determine the brightness corresponding to the peak emission wavelength of that drive current and thus the sum of the emission wavelengths that make up the emission spectrum perceived by the detector (e.g., the human eye viewing the display device).

By selecting which of the available discrete drive currents are provided to the variable wavelength LED during a display frame, the time-averaged emission spectrum perceived by the viewer can be varied within a wide color gamut.

The duration of the display frame may be 50 milliseconds or less, or 45 milliseconds or less, or 40 milliseconds or less. The duration of the display frame is preferably less than or equal to the response time of the detector. The duration of the display frame is preferably less than or equal to the response time of the human eye as a detector of about 50 milliseconds.

Each variable wavelength LED in the multi-LED display device is preferably controllable to reproduce a predetermined spectral output corresponding to a temporal combination of the plurality of peak emission wavelengths emitted during the display frame.

The display device may include an array of variable wavelength LEDs, wherein each variable wavelength LED is controllable to reproduce a predetermined spectral output corresponding to a temporal combination of a plurality of peak emission wavelengths emitted during a display frame.

Preferably, the plurality of variable wavelength LED sub-pixels are individually tunable over the display area of the display device.

The display device may be a large area illuminator for reproducing a programmed illumination spectrum or sequence of illumination spectra.

The display device may be configured to generate an emission spectrum, wherein a shape of the emission spectrum is controllable by applying a plurality of fixed drive currents.

The display device may be configured to generate an emission spectrum, wherein the shape of the emission spectrum is controllable by applying a continuous driving current. In other words, the drive current may be provided in the form of continuous pulses, such as an analog signal of the drive current that varies in magnitude during a display frame, rather than driving the variable wavelength LED by applying one of a plurality of available discrete drive current levels.

The display device may include a pulsed current source configured to generate drive current pulses that vary between a plurality of non-zero values during a display frame. For example, the pulsed current source may be configured to generate drive current pulses that vary between at least two, or at least three, or at least four, or at least five, or at least six, or at least seven non-zero values during one display frame.

The display device may include a pulse current source configured to generate an analog drive current pulse that varies in amplitude during a display frame.

The display device may include a multiplexer configured to provide individual drive currents to the plurality of LEDs.

The device may be an illumination device, such as a hyperspectral light source.

A plurality of variable wavelength light emitting diodes ("tunable" LEDs) may be provided in an array to form a display device. Each variable wavelength LED is preferably arranged to form a pixel (or sub-pixel) of the display device. Each pixel of the display device may be configured to receive its own drive current from a pulse current source configured to generate a current pulse (a series of digital pulses having discrete amplitudes or an analog pulse of varying amplitude). Preferably, the drive current supplied to each pixel is controllable independently of the drive current applied to the other pixels. The pulse current source may be configured to provide driving current pulses to a multiplexer connected to the respective pixels in the display device.

Current pulses can be designed for two main applications:

Accurately representing the spectrum of the reconstructed image of the display device; or alternatively

By adjusting the current pulses of each LED pixel or sub-pixel so that they emit the same emission wavelength and intensity across the display, the LED performance non-uniformities caused during manufacturing are corrected. The tuned display is then driven with the adjusted current pulses, where the emissions from two or more sub-pixels combine to produce a perceived color for each pixel.

An aspect of the present invention may provide a display device formed of a light emitting diode, preferably comprising a porous semiconductor material, wherein each pixel is comprised of a plurality of sub-pixels; wherein the peak emission wavelength of each sub-pixel is dependent on the drive current density; and wherein the peak emission wavelength of at least one sub-pixel is intentionally not fixed during operation.

The peak emission wavelength of at least one sub-pixel having an unfixed peak emission during operation is preferably controlled by varying the drive current density supplied to that sub-pixel during operation.

A display device formed of a light emitting diode including a porous semiconductor material may be provided, wherein each pixel is composed of a plurality of sub-pixels;

wherein each sub-pixel has the same diode structure; and wherein the display device is configured to provide a drive current density to each sub-pixel separately; and wherein the peak emission wavelength of each sub-pixel is independently controllable by varying the drive current density supplied to the sub-pixel.

Each sub-pixel may be comprised of an LED formed over a porous region of semiconductor material.

Each LED sub-pixel is configured to receive a power supply and at least one sub-pixel is a variable wavelength sub-pixel whose peak emission wavelength is controllable within an emission wavelength range by varying the power supply.

The peak emission wavelength of the variable wavelength sub-pixel is preferably controllable within the emission wavelength range by varying the current density of the power supply or by varying the power density of the power supply.

The peak emission wavelength of the variable wavelength sub-pixel is preferably controllable over an emission wavelength range of at least 40nm, or at least 50nm, or at least 60nm, or at least 70nm, or at least 80nm, preferably over a range of up to 100nm or 110nm or 120nm or 140nm, or 160nm, or 180nm or 200nm, by varying the power supply.

In a preferred embodiment, each sub-pixel preferably has the same diode structure and only the drive current density is used to control the emission peak wavelength of each sub-pixel.

Preferably, at least one of the sub-pixels is operable in a plurality of modes, the sub-pixel being configured to emit at a discrete peak emission wavelength in each of the plurality of modes. Preferably, in each mode, the subpixels are driven at different current densities so that the subpixels emit light at different peak wavelengths in different modes.

The pattern of sub-pixels is preferably dynamically variable during operation of the display device.

Each of the sub-pixels in the display device is preferably a variable wavelength sub-pixel.

The display device may be configured to vary the emission peak wavelength of the or each variable wavelength sub-pixel at regular time intervals. For example, the display device may be configured to rotate or swap the emission wavelengths of the sub-pixels at regular time intervals in order to distribute the heating of the sub-pixels caused by the high current density emission wavelengths. The time interval may be one display frame or less.

The display device may be configured to change the emission wavelength of the sub-pixels by changing the driving current applied to the sub-pixels at regular intervals.

The display device may include a plurality of subpixels configured to emit at peak wavelengths within a same color, wherein the plurality of subpixels are configured to emit at different peak wavelengths within the color. In a preferred embodiment, the device may comprise four sub-pixels, two of which are pixels configured to emit at peak wavelengths within the same color. The plurality of sub-pixels configured to emit at peak wavelengths within the same color preferably have the same diode structure and the device is configured to drive them at different current densities to produce different peak emission wavelengths.

The display device may include a plurality of variable wavelength sub-pixels configured to emit at different peak emission wavelengths in response to different applied current densities.

Preferably, at least one sub-pixel is controllable to emit at a peak wavelength in the range 450nm to 530 nm. Preferably, the peak emission wavelength of the variable wavelength sub-pixel is controllable in the range of 450nm to 630nm or more.

Optionally, at least one subpixel is configured to be driven at a single current density and thus emit at a single peak wavelength. The sub-pixel may preferably be configured to emit red light of a single peak wavelength.

The display device preferably comprises a plurality of variable wavelength sub-pixels configured to emit at different peak emission wavelengths in response to different applied current densities, wherein each of the plurality of variable wavelength sub-pixels is controllable to emit at a peak wavelength within the wavelength range.

Each variable wavelength sub-pixel may preferably have a controllable peak emission wavelength in the range 450nm to 630nm or more. The display device may be configured to vary the current density provided to each of the variable wavelength sub-pixels such that varying the current density over a continuous current density range varies the peak emission wavelength over a continuous wavelength range of 450nm to 630nm or more.

Each of the variable wavelength sub-pixels may be configured to emit over a wavelength range above or below the center wavelength of the sub-pixel, preferably over a wavelength range 20nm or more below or above the center wavelength. Each of the variable wavelength sub-pixels in a pixel of the display device may optionally have a different center wavelength.

The display device may include: a first sub-pixel configured to emit at a peak emission wavelength of 430nm to 480 nm; and/or a second sub-pixel configured to emit at a peak emission wavelength of 500nm to 540 nm; and/or a third sub-pixel configured to emit at a peak emission wavelength of 580nm to 620 nm.

By incorporating variable wavelength light emitting diodes into the display device, the effective color gamut of the display may advantageously be greater than what can be achieved by operating each sub-pixel at a fixed peak emission wavelength.

The number of sub-pixels in a pixel of the display device may be less than three, preferably less than two. One or more of the subpixels may have a fixed peak emission wavelength.

The drive current density dependent shift of the peak emission wavelength of the variable wavelength sub-pixel is preferably greater than 20 nm/decade.

The peak emission wavelengths used to display a particular chromaticity point may be selected to those that give the most efficient overall operation of the display.

Preferably, each sub-pixel has the same diode structure and the peak emission wavelength of the sub-pixel is controlled by providing different current densities to different sub-pixels.

Contact pad ratio

The variable wavelength LED of the first aspect enables the same LED diode structure to emit light at different peak emission wavelength ranges at different current densities. While the primary advantage is that a single variable wavelength LED can be controlled to emit at a variety of different emission wavelengths, the inventors have recognized that the present invention can also utilize the same semiconductor materials and diode structures to provide a variety of fixed emission wavelength LEDs. Thus, rather than varying the current density provided to an LED by varying the drive current during use, the current density experienced by a given LED may be set by fixing the magnitude of the drive current for that LED and controlling the area that the LED is subjected to the drive current.

Thus, some or all of the variable wavelength LEDs in a display device may be configured to receive a fixed magnitude (i.e., non-variable) drive current that is either on or off. When the fixed drive current is on, these variable wavelength LEDs will behave as conventional LEDs and emit at a single peak emission wavelength determined by the current density produced by the fixed drive current and the LED area experiencing the drive current. Thus, the variable wavelength LED may be used as a fixed emission wavelength LED in a display device. The drive current supplied to some or all of the subpixels may be the same, with only the difference in LED area and electrical contact area creating the difference in drive current density experienced by the different subpixels. By providing separate LEDs that experience different current densities, multiple LEDs with the same diode structure can be used to emit at different peak emission wavelengths. This may advantageously eliminate the need to combine separate semiconductor material systems to obtain different emission colors, thereby significantly simplifying the display device manufacturing process.

The display device may include a plurality of variable wavelength LED sub-pixels configured to emit at different peak emission wavelengths. The device may include a first subpixel including a first light emitting layer having a first light emitting area a ₁ and a first electrical contact contacting the first subpixel over a first contact area, the first electrical contact configured to apply a drive current to the first subpixel. The first subpixel may have a first contact ratio defined by a ratio of the first contact area to the first light emitting area a ₁. The device may additionally include a second subpixel including a second light emitting layer having a second light emitting area a ₂ and a second electrical contact contacting the second subpixel over the second contact area, the second electrical contact configured to apply a drive current to the second subpixel. The second subpixel may have a second contact ratio defined by a ratio of the second contact area to the second light emitting area a ₂. The first contact ratio is preferably different from the second contact ratio such that the two sub-pixels are configured to emit light at different peak wavelengths in response to the same drive current.

Each of the variable wavelength LEDs is a variable wavelength LED as described above with respect to the first aspect of the invention.

The display device preferably comprises a porous region of semiconductor material, preferably a porous region of group III nitride semiconductor material. The first sub-pixel and/or the second sub-pixel is preferably formed over the porous region.

The display device may comprise a plurality of pixels, each pixel of the display device comprising a first sub-pixel and a second sub-pixel as defined above.

The preferred embodiments may provide a display device comprising at least two LED sub-pixels, each sub-pixel having a light emitting layer with a light emitting area and electrical contacts in contact with the sub-pixel over a contact area,

Wherein each sub-pixel has a contact ratio determined by the ratio of the contact area to the light emitting area of the sub-pixel, and wherein at least two sub-pixels do not have the same contact ratio.

At least two sub-pixels having different contact ratios are preferably configured to emit at different peak emission wavelengths at the same drive current.

The light emitting area Ai of a subpixel is the effective light emitting area of the subpixel if viewed from above. This may be referred to as the mesa area of the sub-pixel because the LED sub-pixel is typically formed as a mesa with vertical sidewalls, as is known in the art. The light emitting area is an area of the light emitting layer that is electrically activated when a driving current is applied to the sub-pixel.

In conventional LED devices, the variation of the emission wavelength with drive current variation produced by the LED device is minimal, but by providing LEDs on a porous group III nitride template, the inventors have found that wavelength shifts can be broadened and controlled to a greater extent than in conventional LED materials. This will be discussed in more detail in connection with the first aspect of the invention.

This will allow multiple sub-pixels to be combined together on the same wafer, thereby reducing the number of mass transfer operations required to produce a display. In particular, multiple sub-pixels may be formed by etching a single LED diode structure into discrete sub-pixel mesas. When this is done, each discrete mesa has the same diode structure. However, by using the present invention to process LED sub-pixel mesas into display devices, the sub-pixel mesas can be made to emit at different peak emission wavelengths, although they are formed of the same semiconductor material and the same diode structure.

Since the drive current density experienced by a given LED or LED sub-pixel is determined by both the absolute drive current (the magnitude of the drive current) and the LED area through which the current flows, the inventors have found that the drive current density experienced by an LED can be manipulated by controlling the contact area at which the electrical contacts provide the drive current to the diode structure of the LED.

In particular, the inventors have found that by controlling the "contact ratio" -the ratio of the contact area to the light emitting area of a given subpixel-the peak emission wavelength of a subpixel that responds to any given drive current can be varied.

By varying the contact ratio, individual LED sub-pixels having different areas but otherwise identical diode structures (the same layered LED structure of n-type layer(s), active layer and p-type layer formed of the same material) will therefore emit at different peak emission wavelengths in response to the same absolute drive current.

The contact ratio of each sub-pixel determines the peak emission wavelength of the emission in response to the drive current. Thus, when the first and second sub-pixels have different contact ratios, this means that the first and second sub-pixels are configured to emit at different peak emission wavelengths in response to the same drive current.

The first subpixel may be configured to emit at a first peak emission wavelength in response to the drive current having a first magnitude, and the second subpixel may be configured to emit at a second peak emission wavelength different from the first peak emission wavelength in response to the drive current having the first magnitude. The difference in emission wavelength may be entirely due to the difference between the first contact ratio and the second contact ratio.

The first LED sub-pixel has a diode structure and preferably the second LED sub-pixel has the same diode structure.

The light emitting area Ai is the area of the light emitting region when viewed from above. The light emitting area is preferably arranged to extend over or across the entire sub-pixel diode structure such that the light emitting area is the same as the sub-pixel area when viewed from above.

The first sub-pixel and the second sub-pixel are preferably formed of the same semiconductor material. It is particularly preferred that the first sub-pixel and the second sub-pixel are formed from the same LED structure, e.g. by etching the LED structure to form a first sub-pixel and a second sub-pixel mesa. In this case the layer structure and materials of the different sub-pixels are identical, so that any difference in emission wavelength is controlled only by the difference in drive current and/or contact ratio between the electrical contacts and the respective sub-pixels.

The two sub-pixels preferably comprise light emitting regions comprising InGaN semiconductor material.

The first contact area and the second contact area are surface areas where the first contact and the second contact are in contact with the first subpixel and the second subpixel, respectively.

In some preferred embodiments, the first light emitting area a ₁ is the same size as the second light emitting area a ₂. In order to have the first subpixel and the second subpixel have different contact ratios, the first contact area is different from the second contact area.

In an alternative embodiment, the first light emitting area a ₁ is different in size from the second light emitting area a ₂. The first contact area may be the same size as the second contact area, or alternatively, the contact areas may be different. As long as the first contact ratio and the second contact ratio are not the same, the two sub-pixels will emit light at different peak wavelengths in response to the same drive current.

The different first and second contact areas are configured to convert drive currents having the same magnitude to different current densities at the first and second sub-pixels.

The first subpixel may receive a first current density through a first contact area in response to the drive current having a first magnitude, and the second subpixel may receive a second current density through a second contact area in response to the drive current having the first magnitude. If the first contact area and the second contact area are different, the second current density will be different from the first current density.

The contact ratio is defined by the contact area: the ratio of the light emitting area (contact area/light emitting area) is defined.

The lower the contact ratio (the smaller the contact area relative to the light emitting area), the shorter the peak emission wavelength of the subpixel in response to a given drive current.

The higher the contact ratio (the larger the contact area relative to the light emitting area), the longer the peak emission wavelength of the subpixel in response to a given drive current.

In some embodiments, the first contact area is smaller than the second contact area such that a drive current of a first magnitude applied to the first subpixel via the first electrical contact is driven through a smaller contact area than the same drive current applied to the second subpixel via the second electrical contact such that the first subpixel receives a first current density higher than a second current density received by the second subpixel.

The first subpixel may be configured to emit at a first peak emission wavelength shorter than a second peak emission wavelength emitted by the second subpixel in response to the drive current having the first magnitude. Thus, the same absolute drive current may be applied to both sub-pixels, but different contact ratios of the two sub-pixels will result in the two sub-pixels emitting light at different peak wavelengths.

The contact area of the electrical contacts on the sub-pixels can be controlled by varying the size of the electrical contacts. Alternatively, a masking (passivation) layer may be provided over the sub-pixels, the masking layer comprising apertures through which electrical contacts contact the sub-pixels. Thus, the size of the aperture in the mask layer may determine the contact area between the electrical contact and its sub-pixel. The mask layer advantageously serves as a current limiting layer limiting the current density.

The first subpixel may include a first mask layer including a first hole through which the first electrical contact contacts the first subpixel such that the first hole defines a first contact area.

The second subpixel may comprise a second mask layer comprising a second aperture through which the second electrical contact is in contact with the second subpixel such that the second aperture defines a second contact area.

The first electrical contact and/or the second electrical contact may be a p-type electrical contact or an n-type electrical contact.

In some embodiments, the first electrical contact may be in contact with the p-doped portion of the first sub-pixel diode structure and the second electrical contact may be in contact with the p-doped portion of the second sub-pixel diode structure.

In other embodiments, the first electrical contact may be in contact with an n-doped portion of the first sub-pixel diode structure and the second electrical contact may be in contact with an n-doped portion of the second sub-pixel diode structure.

The invention is not limited to two sub-pixels. The display device may include a plurality of sub-pixels, each having a different contact ratio.

The display device may include a plurality of sub-pixels having a first contact ratio, a plurality of sub-pixels having a second contact ratio, and optionally a plurality of sub-pixels having a third contact ratio. In response to a standard drive current having a set magnitude, the different contact ratios of the sub-pixels may thus cause the sub-pixels to emit light at different wavelengths corresponding to their contact ratios.

The display device may include a third subpixel including a third light emitting layer having a third light emitting area a ₁;

A third electrical contact contacting the third subpixel over the third contact area, the third electrical contact configured to apply a drive current to the third subpixel;

Wherein the third subpixel has a third contact ratio defined by the ratio of the third contact area to the third light emitting area a ₁. The third contact ratio is preferably different from the first contact ratio and the second contact ratio.

The third sub-pixel may thus emit light at a third peak emission wavelength different from the first and second peak wavelengths in response to the same drive current having the first magnitude.

In a particularly preferred embodiment, the first, second and third sub-pixels are configured to emit at red, green and blue wavelengths in response to the same drive current. The display device may thus be a red-green-blue (RGB) display device.

The display device may include a plurality of pixels. Each pixel may comprise two or more sub-pixels, preferably three or more sub-pixels.

The first sub-pixel and the second sub-pixel may be located on a shared n-type conductive layer of semiconductor material.

A display device according to the invention may comprise a driver circuit configured to control the drive current supplied to each sub-pixel in the display device.

The first light emitting area and the second light emitting area are preferably the coverage area of the first light emitting layer and the second light emitting layer over the porous region. In a preferred embodiment, the first sub-pixel and the second sub-pixel may be formed entirely over the porous region of the display device.

The first and/or second and/or third electrical contacts may comprise titanium, platinum, chromium, aluminum, nickel, gold, or Indium Tin Oxide (ITO) or any other material known in the art to provide suitable electrical contacts on the semiconductor device.

Contact ratio between different subpixel colors

Depending on the size (area) of the sub-pixel compared to the contact area, the inventors have found that the distance that the current travels outwards from the electrical contacts is more dependent on the contact area than the total LED mesa area. For example, for LEDs having a much smaller contact area than the LED area, current spreading may be limited to a relatively small area around the electrical contacts, regardless of the overall size of the LED mesa, so otherwise increasing the mesa area does not affect the peak emission wavelength. Accordingly, the inventors have realized that by referencing the ratio of the contact areas between different LEDs or different LED sub-pixels, the difference in applied current density can be more properly defined.

In one aspect of the invention, a display device may be provided that includes a plurality of LED sub-pixels configured to emit at different peak emission wavelengths. The device may comprise:

A first subpixel and a first electrical contact contacting the first subpixel over a first contact area AC ₁, the first electrical contact configured to apply a drive current to the first subpixel;

A second subpixel and a second electrical contact contacting the second subpixel over a second contact area AC ₂, the second electrical contact configured to apply a drive current to the second subpixel.

The LED sub-pixels configured to emit at different peak emission wavelengths are preferably variable wavelength LEDs according to the first aspect described above.

The first contact area AC ₁ may be different in size than the second contact area AC ₂. The device may have a color contact ratio defined by the ratio of the first contact area AC ₁ to the second contact area AC ₂.

When the same driving current I is supplied to both sub-pixels, the color contact ratio determines the relationship between the different current densities J experienced by both sub-pixels. A sub-pixel with a smaller contact area will experience a higher current density J from the same drive current I than a sub-pixel with a larger contact area.

The current density experienced by a subpixel is defined by: j=i/AC (contact area)

Thus, if the fixed driving current I is supplied to the two sub-pixels through two electrical contacts having different contact areas AC ₁ and AC ₂, the difference in contact area is Δac=ac ₁–AC₂, and the difference in current density experienced by the two sub-pixels is Δj=i/Δac.

The desired current density at which the sub-pixel emits at the desired peak emission wavelength is known or can be readily found by conventional calibration techniques.

In order to provide two sub-pixels emitting at two desired peak emission wavelengths, the two required current densities J are thus known and Δj can be calculated. Thus, Δac between the first contact area and the second contact area can be found.

The ratio of contact areas required to produce the different colors is also fixed.

In a preferred embodiment, the display device comprises a first subpixel and a first electrical contact contacting the first subpixel over a first contact area AC ₁, the first electrical contact configured to apply a drive current to the first subpixel; and

A second subpixel and a second electrical contact contacting the second subpixel over a second contact area AC ₂, the second electrical contact configured to apply a drive current to the second subpixel. The first contact area AC ₁ is different in size from the second contact area AC ₂. The size of the first contact area AC ₁ is selected so that the first subpixel emits red light when the driving current I is applied through the first contact. The size of the second contact area AC ₂ is selected such that the second sub-pixel emits green light when the driving current I is applied through the second contact.

The ratio of the two contact areas can be expressed as R _{Red color / Green colour}＝AC₁ (red)/AC ₂ (green).

Since the current density is inversely proportional to the contact area, i.e., j=i/AC, the ratio R _{Red color / Green colour}＝AC₁ (red)/AC ₂ (green) =j ₂ (green)/J ₁ (red).

When I is fixed, the peak wavelength W _{Peak value} -J-1/AC

Because J has a specific range of visible wavelengths for which sub-pixels emit is desired, the range of contact areas AC is also limited.

	R	Y	G
				J	1～2	3～7	>20

However, since I cannot be limited, the wavelength of the emission is not determined by AC alone.

To control J within a certain range, AC will vary proportionally with drive current I.

To reduce the I-score, the ratio of the contact areas between different colors at the same I can be used to fix the color relationship between the individual subpixels. R=red. Y=yellow. G=green.

In a preferred embodiment, a display device is provided that includes a plurality of LED sub-pixels configured to emit at different peak emission wavelengths. The device may comprise:

A second subpixel and a second electrical contact contacting the second subpixel over a second contact area AC ₂, the second electrical contact configured to apply a drive current to the second subpixel; and

The third subpixel and a third electrical contact contacting the third subpixel over the third contact area AC ₃, the third electrical contact configured to apply a drive current to the third subpixel.

The first contact area AC ₁ may be different in size from the second contact area AC ₂ and the third contact area AC ₃. The second contact area AC ₂ may be different in size from the first contact area AC ₁ and the third contact area AC ₃. The device may have a color contact ratio defined by a ratio of the first contact area AC ₁ to the second contact area AC ₂ and a ratio of the first contact area AC ₁ to the third contact area AC ₃.

The first subpixel is preferably a red subpixel. The second sub-pixel is preferably a green sub-pixel. The third subpixel is preferably a blue subpixel. The device may have a red/green color ratio defined by the ratio of the first contact area AC ₁ to the second contact area AC ₂ and a red/blue color ratio defined by the ratio of the first contact area AC ₁ to the third contact area AC ₃.

Alternatively, the first subpixel may be a red subpixel, the second subpixel may be a yellow subpixel, and the third subpixel may be a green subpixel. The device may have a red/yellow color ratio defined by the ratio of the first contact area AC ₁ to the second contact area AC ₂ and a red/green color ratio defined by the ratio of the first contact area AC ₁ to the third contact area AC ₃. The red/yellow color ratio is preferably between 1.5:1 and 7:1. The red/green color ratio is preferably at least 10:1.

A) I/j=i/ac→r _1/2＝AC₁/AC₂ =j2/J1 (under the same current injection)

In use, the display device is preferably controlled by providing a fixed drive current I to all sub-pixels in the display device. Since all sub-pixels receive the same magnitude of drive current, the drive current density experienced by different sub-pixels will differ primarily based on the respective contact areas. When the same driving current I is supplied to both sub-pixels, the color contact ratio determines the relationship between the different current densities J experienced by both sub-pixels.

The method of manufacturing such a display may comprise the steps of: forming a first electrical contact on the first subpixel, the first electrical contact contacting the first subpixel over the first contact area AC ₁; and forming a second electrical contact on the second subpixel, the second electrical contact contacting the second subpixel over the second contact area AC ₂. The first contact area AC ₁ is preferably different in size from the second contact area AC ₂.

The method may comprise the step of forming a first variable wavelength LED sub-pixel and a second variable wavelength LED sub-pixel according to the first aspect described above.

The ratio of the first contact area AC ₁ to the second contact area AC ₂ defines the color contact ratio of the device.

The current density required for the sub-pixels to emit at the desired peak emission wavelength is known or can be readily found by conventional calibration techniques.

In order to provide two sub-pixels emitting at two desired peak emission wavelengths, the two required current densities J are thus known and the required Δj between the two sub-pixels can be calculated. Thus, Δac between the first contact area and the second contact area can be found.

The ratio of the contact areas required to produce the different colors is also fixed to fix the color relationship between the individual subpixels. R=red. Y=yellow. G=green.

As described above, the contact area of the red pixel is at least 10 times larger than the contact area of the green pixel. The contact area of the red pixel is formed to be about 1.5-7 times the contact area of the yellow pixel.

In a preferred embodiment, the first contact area AC ₁ is formed to a different size than the second contact area AC ₂. The size of the first contact area AC ₁ is selected so that the first subpixel emits red light when the driving current I is applied through the first contact. The size of the second contact area AC ₂ is selected such that the second sub-pixel emits green light when the driving current I is applied through the second contact. In this embodiment, AC ₁ is formed at least 10 times the area of AC ₂.

In a preferred embodiment, a method of manufacturing a display device comprises the steps of:

All of the sub-pixels in the device are preferably formed of the same LED diode structure.

The first contact area AC ₁ is preferably different in size from the second contact area AC ₂ and the third contact area AC ₃. The second contact area AC ₂ is preferably different in size from the first contact area AC ₁ and the third contact area AC ₃. The device may have a color contact ratio defined by a ratio of the first contact area AC ₁ to the second contact area AC ₂ and a ratio of the first contact area AC ₁ to the third contact area AC ₃. If the three contact areas are of different sizes and the subpixels have the same variable wavelength LED diode structure, the three subpixels will emit different peak wavelengths in response to the same magnitude of drive current.

Pixel size and geometry

In one aspect of the invention, a display device is provided that includes a Light Emitting Diode (LED), which preferably includes a porous semiconductor material. The device may include a pixel including a plurality of sub-pixels, each sub-pixel having a light emitting layer.

The first subpixel has a first light emitting layer having a first area a ₁ and the second subpixel has a second light emitting layer having a second area a ₂ different from the first area a ₁.

The first subpixel is configured to emit at a first peak wavelength and the second subpixel is configured to emit at a second peak wavelength different from the first peak wavelength.

Throughout this document, the term "display device (DISPLAY DEVICE)" may be replaced with "LED device".

The area a of the sub-pixel is the effective light emitting area of the sub-pixel if viewed from above. This is referred to as the mesa area of the sub-pixel because the LED sub-pixel is formed as a mesa with vertical sidewalls, as is known in the art. The area is an area of the light emitting layer that is electrically activated when a driving current is applied to the sub-pixel.

In the present invention, the display device may thus comprise a plurality of LED sub-pixels having different relative areas, which emit at different emission wavelengths in response to the same driving current.

The invention may relate to a display in which the emission color of a sub-pixel is controlled by the total area of the sub-pixel.

In conventional LED devices, the variation of the emission wavelength with drive current variation produced by the LED device is minimal, but by providing LEDs on a porous group III nitride template, the inventors have found that wavelength shifts can be broadened and controlled to a greater extent than in conventional LED materials.

This will allow a plurality of sub-pixels to be combined together, reducing the number of mass transfer operations required to produce the display.

The geometry of the LEDs may be selected to achieve a variety of different results, such as a specified peak wavelength for a plurality of LEDs at a given current, a specified emission intensity at a given wavelength, a specified luminosity at a given wavelength.

As described above, by providing LEDs on a porous template of group III nitride material, a variable wavelength LED with a continuous correlation between peak emission wavelength and drive current can be provided.

Since the drive current density experienced by a given LED or LED sub-pixel is determined by both the absolute drive current and the LED area through which the current flows, the drive current density experienced by the LED can be manipulated by controlling the area or size of the LED.

Thus, individual LEDs having diode structures of different areas but otherwise identical (the same layered LED structure of the n-type layer(s), active layer and p-type layer) will emit at different peak emission wavelengths in response to the same magnitude of drive current.

In the present display device, at least one of the sub-pixels may be a variable wavelength sub-pixel, and the display device may be configured to provide a variable drive current to the variable wavelength sub-pixel such that the variable wavelength sub-pixel emits at a different peak emission wavelength in response to a different drive current.

In a preferred embodiment, the first subpixel is a variable wavelength subpixel configured to emit at a first peak emission wavelength in response to a first drive current applied to the first subpixel and to emit at a third peak emission wavelength in response to a third drive current applied to the first subpixel.

Also, the second subpixel may be a variable wavelength subpixel configured to emit at a second peak emission wavelength in response to a second drive current applied to the second subpixel and at a fourth peak emission wavelength in response to a fourth drive current applied to the second subpixel.

The inventors have found that growing the n-doped, light emitting and p-doped portions of an LED over a porous region of a group III nitride material advantageously enables the same LED to emit at a range of peak emission wavelengths, rather than at one particular wavelength. By varying the power supplied to the LEDs, the peak emission wavelength of the LEDs can be varied over a range of emission wavelengths. Thus, the present invention may comprise a variable wavelength LED that may be controlled to emit at any wavelength within a continuous emission wavelength range.

Individual variable wavelength LED sub-pixels may be provided as part of a pixel comprising a plurality of sub-pixels. The LED sub-pixels correspond to individual LED devices, as each sub-pixel can be controlled individually.

The structure and properties of the variable wavelength LED sub-pixel are described above in relation to the first aspect.

The display device may be configured to provide the same fixed driving current I to the first sub-pixel and the second sub-pixel.

The display device may be configured to provide a first driving current I ₁ to the first subpixel and a second driving current I ₂ to the second subpixel, the second driving current having a different magnitude than the first driving current.

The first sub-pixel may have a first geometry or shape and the second sub-pixel may have a second geometry or shape. The first geometry or shape may be different from the second geometry or shape, or the first geometry or shape and the second geometry or shape may be the same.

In a preferred embodiment, the first sub-pixel is circular and the second sub-pixel is formed as a ring arranged concentrically around the circular first sub-pixel.

Preferably, the first subpixel and the second subpixel have the same diode structure. Accordingly, the emission characteristics of the sub-pixels may vary, which is not a result of the diode layer structure, but a result of differences in the shape and size of the sub-pixels and differences in the driving current applied to the sub-pixels.

The first subpixel may be configured to emit at a first peak wavelength at a first emission intensity in response to a first drive current applied to the first subpixel and/or the second subpixel may be configured to emit at a second peak wavelength at a second emission intensity in response to a second drive current applied to the second subpixel.

The first subpixel may be configured to emit light at a first peak wavelength at a first luminance in response to a first drive current applied to the first subpixel, and/or the second subpixel may be configured to emit light at a second peak wavelength at a second luminance in response to a second drive current applied to the second subpixel.

The first area a ₁ of the first light-emitting layer may be greater than the second area a ₂ of the second light-emitting area, or the first area a ₁ of the first light-emitting layer may be less than the second area a ₂ of the second light-emitting area.

The first area a ₁ of the first light-emitting layer may have a different shape from the second area A2 of the second light-emitting area.

The first area and the second area are preferably the coverage of the first light emitting layer and the second light emitting layer over the porous region. In a preferred embodiment, the first sub-pixel and the second sub-pixel may be formed entirely over the porous region of the display device.

Method for controlling LED display device

In another aspect, a method of controlling an LED display device is provided.

The method may comprise the steps of: the power supply to each of a plurality of LEDs (which may be LED sub-pixels) in an LED display is controlled individually such that the peak emission wavelength of each LED is controlled to emit at a desired peak emission wavelength within its emission wavelength range.

Each LED or LED sub-pixel in the display may preferably be controlled as described above in relation to the previous aspects.

The method may comprise the steps of: providing a first driving condition to the first group of LEDs such that the first group of LEDs each emit light at a first wavelength; and providing a second driving condition to the second set of LEDs such that the second set of LEDs each emit light at a second wavelength.

The method may comprise the steps of: controlling a power supply of a first variable wavelength LED (e.g., LED sub-pixel) such that the first variable wavelength LED emits at a first peak emission wavelength within a first emission wavelength range; and controlling the power supply of the second variable wavelength LED such that the second variable wavelength LED emits at a second peak emission wavelength. The second peak emission wavelength may be a wavelength within the first emission wavelength range or within the second emission wavelength range.

The method may comprise the steps of: controlling a power supply of a first plurality of variable wavelength LEDs in the LED display such that the first plurality of LEDs emit at peak emission wavelengths within a first emission wavelength range, and controlling a power supply of at least one additional LED to emit at peak emission wavelengths outside the first emission wavelength range. For example, the plurality of variable wavelength LEDs may be green-red LEDs capable of emitting at wavelengths between green and red, while the additional LEDs may be blue LEDs for emitting blue light that cannot be realized by the variable wavelength LEDs alone.

The method may include the step of providing different power sources to the plurality of variable wavelength LEDs such that the plurality of variable wavelength LEDs emit at different peak emission wavelengths within the first emission wavelength range.

The method may include the step of providing power to some or all of the first plurality of variable wavelength LEDs to emit at a first peak emission wavelength to provide a desired emission intensity at the first peak emission wavelength. Since the intensity of the emitted light depends on the power supplied to the LED, the emission intensity of longer wavelengths is lower than the emission intensity of shorter wavelengths. By operating some or all of the plurality of LEDs at the same wavelength, the overall intensity of the emitted light can be controlled.

In a particularly preferred embodiment, the method may comprise the steps of: the plurality of variable wavelength LEDs are powered such that they emit at a peak emission wavelength between 560nm and 680nm or between 570nm and 675nm, and the at least one further LED is powered such that it emits at a peak emission wavelength below 560nm, preferably below 500 nm.

The method may include changing the emission wavelength of the LED by changing the driving current applied to the LED at regular intervals.

The method may comprise the step of rotating or swapping the emission wavelengths of the LEDs or LED sub-pixels at regular time intervals in order to spread the heating of the sub-pixels caused by the emission wavelengths due to the high current density.

In another aspect of the present invention, there is provided a method of controlling a display device including a variable wavelength LED, the method including the steps of:

providing a driving current to the variable wavelength LED; and varying the magnitude of the drive current between a plurality of non-zero values during a single display frame such that the variable wavelength LED produces a plurality of peak emission wavelengths within the single display frame.

Varying the amplitude of the drive current between a plurality of non-zero values during a single display frame produces a plurality of peak emission wavelengths from the variable wavelength LED over the duration of the display frame such that a temporal combination of the plurality of peak emission wavelengths reproduces the spectrum.

This may advantageously allow the viewer to perceive that the LED or LEDs are emitting a particular spectrum, wherein the spectrum is not normally emitted by the LEDs. For example, a temporal combination of multiple peak emission wavelengths may produce light of a particular color that does not correspond to the emission band gap of known semiconductor materials. Using this approach, the perceived color range of the LED emission is thus greatly expanded.

The method may include providing a first drive current to the variable wavelength LED; and varying the magnitude of the first drive current between a plurality of non-zero values during a single display frame such that the variable wavelength LED produces a plurality of peak emission wavelengths within the single display frame and providing a second drive current to a second variable wavelength LED; and varying the amplitude of the second drive current between a plurality of non-zero values during a single display frame such that the second variable wavelength LED produces a plurality of peak emission wavelengths within the single display frame.

The display device may comprise a plurality of variable wavelength LEDs and the method may preferably comprise providing a separate drive current to each LED. The magnitude and duration of the drive current provided to the individual LEDs may be different such that the individual LEDs emit separate emission spectra during the same display frame.

The variable wavelength LED is preferably a variable wavelength LED as described above according to the previous aspect of the invention.

The method may comprise the step of varying the drive current amplitude during a single display frame by providing a sequence of discrete drive current pulses of discrete amplitude. The method may comprise the step of varying the drive current between at least two, or at least three, or at least four, or at least five, or at least six, or at least seven non-zero amplitudes during the display frame. The method may comprise the step of controlling the brightness of each emitted peak emission wavelength by controlling the duration of each drive current pulse. The method may comprise the step of repeating the sequence of drive current pulses in a subsequent display frame.

The method may comprise the steps of generating a sequence of drive current pulses having discrete magnitude and duration, and providing the sequence of drive current pulses to the variable wavelength LED(s) in the display device.

Alternatively, the method may comprise the step of varying the drive current amplitude over a continuous range of amplitudes during a single display frame.

The method may comprise the steps of generating an analogue drive current pulse having a magnitude that varies over the duration of the display frame and providing the drive current pulse to the variable wavelength LED(s) in the display device.

The method may include controlling a plurality of variable wavelength LEDs in the display device to reproduce the spectrum.

According to another aspect of the present invention, there is provided a method of controlling a display device comprising a first variable wavelength LED sub-pixel and a second variable wavelength LED sub-pixel configured to emit at different peak emission wavelengths. As described above, the device may include a first subpixel including a first light emitting layer having a first light emitting area a ₁ and a first electrical contact contacting the first subpixel over the first contact area, the first electrical contact configured to apply a drive current to the first subpixel. The first subpixel may have a first contact ratio defined by a ratio of the first contact area to the first light emitting area a ₁. The device may additionally comprise a second sub-pixel comprising a second light emitting layer having a second light emitting area A ₂, and

A second electrical contact contacting the second subpixel over the second contact area, the second electrical contact configured to apply a drive current to the second subpixel. The second subpixel may have a second contact ratio defined by a ratio of the second contact area to the second light emitting area a ₂. The first contact ratio is preferably different from the second contact ratio such that the two sub-pixels are configured to emit light at different peak wavelengths in response to the same drive current.

The method of controlling such a display device comprises the steps of:

providing a first driving current I ₁ to the first sub-pixel, an

A second drive current I ₂ is provided to the second subpixel.

The first driving current I ₁ may have the same magnitude as the second driving current I ₂. In this case, the different first contact ratio and second contact ratio may provide different current densities to the first subpixel and the second subpixel. j=i/AC.

The method may include the step of providing a third drive current I ₃ to a third subpixel having a third contact ratio different from the first contact ratio and the second contact ratio. The third driving current I ₃ may have the same magnitude as the first driving current and the second driving current. The different first, second and third contact ratios may thus provide different current densities to the first, second and third sub-pixels.

Variable current injection may be used to control the device. The current density J can be controlled as follows:

1.J＝ΔI/A

2.J＝I/ΔA

3.J＝ΔI/ΔA

No specific contact area is required.

According to another aspect of the present invention, there is provided a method of controlling a display device including a pixel including a plurality of sub-pixels, each sub-pixel having a light emitting layer, a first sub-pixel having a first light emitting layer, the first light emitting layer having a first area a ₁, a second sub-pixel having a second light emitting layer, and the second light emitting layer having a second area a ₂ different from the first area a ₁. The first subpixel is configured to emit at a first peak wavelength and the second subpixel is configured to emit at a second peak wavelength different from the first peak wavelength.

The method may comprise the steps of:

Providing a first drive current I ₁ to the first subpixel, an

The second driving current I ₂ is supplied to the second subpixel.

The magnitude of the first driving current I ₁ may be the same as the magnitude of the second driving current I ₂. Alternatively, the magnitude of the first drive current I ₁ may be different from the magnitude of the second drive current I ₂.

Preferably, the magnitudes of the first and second drive currents are fixed (i.e. constant) during use of the display device. The peak emission wavelengths of the first and second sub-pixels will also be fixed during use. The peak emission wavelength emitted by the first and second sub-pixels will be determined by the current density experienced by the sub-pixels as a result of the magnitude of their respective drive currents and the size of the sub-pixel mesas and/or electrical contacts through which current is supplied to the sub-pixels.

All of the steps of controlling the LEDs or a display device comprising one or more LEDs may be performed by a controller configured to control the power supply of the LEDs in the device.

The controller may control the power supply of the LEDs in the device in response to user input. In response to a user input, the controller may control the LEDs in the device to operate in a selected mode. For example, the controller may control one or more LEDs in the display device to operate in a fixed wavelength emission mode by providing a fixed magnitude of drive current to the LEDs, the magnitude of the fixed drive current corresponding to the wavelength to be emitted. And/or the controller may control one or more LEDs in the display device to operate in a dynamically variable wavelength emission mode by providing a variable magnitude of drive current to the LEDs, the magnitude of the fixed drive current varying corresponding to the wavelength emitted at a given time.

Method of manufacturing display device

In another aspect of the present disclosure, there is provided a method of manufacturing a display device, including the steps of:

Forming an LED diode structure into a plurality of discrete LED mesas;

And connecting the LED mesas to a power supply configured to provide a variable power supply to at least some of the plurality of discrete LED mesas.

Preferably, the LED diode structure is a variable wavelength LED manufactured according to the method of the previous aspect.

For example, a variable wavelength LED diode structure may be fabricated at the wafer level and then divided into a plurality of discrete LED mesas. The discrete LED mesas may then form individual LED sub-pixels of the display device.

This allows multiple sub-pixels to be combined together on the same wafer, reducing the number of mass transfer operations required to produce a display. In particular, multiple sub-pixels may be formed by etching a single LED diode structure into discrete sub-pixel mesas. When this is done, each discrete mesa has the same diode structure. However, by using the present invention to process LED sub-pixel mesas into display devices, the sub-pixel mesas can be made to emit at different peak emission wavelengths, although they are formed of the same semiconductor material and the same diode structure.

The method may include forming individual LED mesas having different surface areas.

The method may be a method of manufacturing a display device comprising a plurality of LED sub-pixels configured to emit at different peak emission wavelengths. The method may comprise the steps of: a first electrical contact is deposited over a first LED sub-pixel comprising a first light emitting layer having a first light emitting area a ₁, the first electrical contact being in contact with the first sub-pixel over the first contact area such that the first sub-pixel has a first contact ratio defined by a ratio of the first contact area to the first light emitting area a ₁. The method additionally comprises the steps of: a second electrical contact is deposited over the second LED sub-pixel, the second LED sub-pixel comprising a second light emitting layer having a second light emitting area a ₂, the second electrical contact being in contact with the second sub-pixel over the second contact area such that the second sub-pixel has a second contact ratio defined by a ratio of the second contact area to the second light emitting area a ₂. The first contact ratio may be different from the second contact ratio.

The method may comprise the steps of: depositing a first mask layer over the first sub-pixel before depositing the first electrical contact, forming a first aperture in the first mask layer, and depositing the first electrical contact to contact the first sub-pixel through the first aperture, and/or depositing a second mask layer over the second sub-pixel before depositing the second electrical contact, forming a second aperture in the second mask layer, and depositing the second electrical contact to contact the second sub-pixel through the second aperture. In this embodiment, the area of the first hole is a first contact area and/or the area of the second hole is a second contact area.

The method may comprise the steps of: the LED structure having the diode structure including the light emitting layer is etched prior to depositing the electrical contact to form a first sub-pixel mesa having a first light emitting layer of a first area a ₁ and a second sub-pixel mesa having a second light emitting layer of a second area a ₂, the first sub-pixel and the second sub-pixel having the diode structure of the LED structure. Thus, the first subpixel and the second subpixel may have the same diode structure and composition, as both are formed from the same starting LED structure.

The LED structure may comprise an n-type conductive layer and a porous region under the diode structure, and the etching step preferably does not etch through the n-type conductive layer.

In some embodiments, the first subpixel mesa has the same area as the second subpixel mesa. In other embodiments, the first subpixel mesa does not have the same area as the second subpixel mesa.

In another aspect, there is provided a method of manufacturing an LED display device, comprising:

The LED structure having a diode structure including a light emitting layer is etched to form a first subpixel mesa having a first light emitting layer of a first area a ₁ and a second subpixel mesa having a second light emitting layer of a second area a ₂, the first subpixel and the second subpixel having a diode structure of an LED structure. Since the first sub-pixel and the second sub-pixel are etched from the same LED structure, the resulting two sub-pixels will have the same diode structure and the same composition.

The LED structure preferably comprises an n-type conductive layer and a porous region under the diode structure. The etching step preferably does not etch through the n-type conductivity layer.

The first area is preferably selected to produce a first current density at the first light emitting layer when a first drive current I ₁ is applied to the first subpixel and the second area is preferably selected to produce a second current density at the second light emitting layer when a second drive current I ₂ is applied to the second subpixel.

When the first driving current I ₁ is applied to the first subpixel, the first area may be selected such that the first subpixel emits at the first emission intensity. When a second drive current I ₂ is applied to the second subpixel, the second area may be selected such that the second subpixel emits at a second emission intensity.

When the first driving current I ₁ is applied to the first subpixel, the first area may be selected such that the first subpixel emits light at the first emission level. When the second driving current I ₂ is applied to the second subpixel, the second area may be selected such that the second subpixel emits light at the second luminance.

The method may comprise the step of coupling the first sub-pixel and the second sub-pixel to a driver circuit configured to control a drive current provided to each sub-pixel in the display device.

The method may comprise the additional step of applying a first electrical contact to the first sub-pixel, the first electrical contact being in contact with the first sub-pixel over the first contact area. The method may comprise the additional step of applying a second electrical contact to the second sub-pixel, the second electrical contact being in contact with the second sub-pixel over the second contact area. As described above, the mask layer may be used to control the size of the first contact area and/or the second contact area.

Method for reproducing spectrum

In many applications, it is desirable to be able to reproduce a particular spectrum.

In the prior art, this is achieved by:

a) The intensity of the one or more illumination sources is modulated with a fixed broadband emission spectrum. This is inherently inefficient due to the nature of the subtraction.

B) Emissions from multiple narrowband emission sources are combined. This provides limited tunability due to the fixed number of emission sources.

C) The light from the high power broad spectrum light source is modified using a filter. This is affected by inherent inefficiency and limited tunability due to the fixed number of tuning elements.

In another aspect of the invention, there is provided a method of reproducing a spectrum with a variable wavelength LED, the method comprising the steps of: providing a driving current to the variable wavelength LED; and varying the amplitude of the drive current between the plurality of non-zero values during a single display frame, wherein varying the amplitude of the drive current between the plurality of non-zero values during a single display frame produces a plurality of peak emission wavelengths such that a temporal combination of the plurality of peak emission wavelengths reproduces the spectrum.

The method may comprise the steps of generating a sequence of drive current pulses having discrete magnitudes and durations, and providing the sequence of drive current pulses to the variable wavelength LED(s).

The method may comprise the steps of generating an analogue drive current pulse having a magnitude that varies over the duration of the display frame and providing the drive current pulse to the variable wavelength LED(s).

Features described herein in relation to one aspect of the invention are equally applicable to all other aspects of the invention.

Drawings

Embodiments of the present invention will now be described with reference to the accompanying drawings, in which:

FIG. 1 illustrates a porous template suitable for use with an LED in accordance with the present invention;

FIGS. 2-18 illustrate steps for fabricating LEDs according to a preferred embodiment of the invention;

FIG. 19 is a graph of normalized Electroluminescent (EL) intensity versus wavelength for an InGaN LED over a porous region;

FIG. 20 is a graph of normalized Electroluminescent (EL) intensity versus wavelength for InGaN LEDs on non-porous substrates at different current injections;

FIG. 21 is a graph of normalized Electroluminescent (EL) intensity versus wavelength at different current injection for the same InGaN LED as FIG. 20 grown over a porous region;

FIG. 22 is an I-V plot of InGaN micro-LEDs of different pixel sizes measured on a non-porous substrate, with the inset showing yellow emission;

FIG. 23 is an I-V plot of InGaN micro-LEDs of different pixel sizes measured on a porous substrate, with the inset showing red emission;

FIG. 24 is an I-V curve of an InGaN micro-LED of different pixel sizes measured on a template with subsurface porous regions;

FIG. 25 is a low current I-V curve of an InGaN micro-LED of different pixel sizes measured on a template with subsurface porous regions;

FIG. 26 is a series of five EL images of the same MicroLED pixels driven at different currents in constant wave mode (CW), showing five different emission colors;

FIG. 27A is a graph of emission wavelength versus current density for a 25 μm 100 variable wavelength LED pixel array driven in a pulsed mode, with a duty cycle of the pulses of 1% and a pulse time of 100 microseconds;

FIG. 27B is a graph of emission wavelength versus current density for a 30 μm by 30 μm 100 by 100 variable wavelength LED pixel array driven in a pulsed mode, with a duty cycle of the pulses of 1% and a pulse time of 100 microseconds;

FIG. 28 is a plot of intensity versus wavelength for a single variable wavelength LED driven at different currents in a pulsed drive mode of 100 microseconds with a duty cycle of 1%;

FIG. 29A is a schematic relationship diagram of an LED structure including a current-constraining layer in accordance with a preferred embodiment of the present invention;

FIG. 29B is a schematic relationship diagram of an LED structure including a current confinement layer in accordance with another preferred embodiment of the present invention;

FIG. 30 is a schematic cross-section of an LED according to a preferred embodiment of the present invention;

FIGS. 31-34 are schematic cross-sections of the LED of FIG. 30, including two v-shaped dimples;

FIG. 35 is a TEM image of a cross section of an LED with v-shaped pits according to a preferred embodiment of the present invention;

FIG. 36 is a graph of normalized intensity versus peak emission wavelength for a variable wavelength LED according to the present invention;

FIG. 37 is a graph of peak emission wavelength versus drive current for a variable wavelength LED according to the present invention; and

FIG. 38 is a graph of peak emission wavelengths of variable wavelength LEDs of different pixel sizes;

FIG. 39 is a schematic side cross-section of a variable wavelength LED structure according to a preferred embodiment of the present invention;

FIG. 40 is a schematic side cross-section of a variable wavelength LED structure according to another preferred embodiment of the present invention;

FIG. 41 is a schematic illustration of the contact area between the contact layer and the semiconductor layer in an LED diode;

FIG. 42 is a schematic plan view of electrical contacts over an LED sub-pixel mesa;

FIG. 43A is a schematic illustration of three subpixel mesas having the same subpixel mesa area, but different contact areas, and therefore different contact ratios;

FIG. 43B is a schematic illustration of three subpixel mesas having the same subpixel mesa area and the same contact pad size, but different contact areas, and therefore different contact ratios;

FIG. 43C is a schematic illustration of three sub-pixel mesas with different sub-pixel mesa areas, different contact areas and different contact ratios;

FIG. 44 is a schematic side cross-section of three subpixels on a shared semiconductor template, with different contact areas and thus different contact ratios;

fig. 45 is a schematic side cross-section of three sub-pixels on a shared semiconductor template, the three sub-pixels having different contact areas and thus different contact ratios.

46A-46G illustrate alternative embodiments of non-uniform, fragmented, or discontinuous light emitting regions of a variable wavelength LED according to the present invention;

FIG. 47A is a TEM image of a cross section of a conventional non-variable wavelength LED;

FIG. 47B is a TEM image of the light emitting region of a variable wavelength LED containing v-pits, according to an embodiment of the invention;

FIG. 47C is a TEM image of the variable wavelength LED of FIG. 47B showing a porous region and a light emitting region comprising a plurality of v-shaped pits, in accordance with a preferred embodiment of the present invention;

FIG. 48A is a graph of peak emission wavelength versus drive current density for a conventional non-variable wavelength LED;

FIG. 48B is a graph of peak emission wavelength versus drive current density for a variable wavelength LED according to an embodiment of the present invention;

FIG. 48C is a graph of peak emission wavelength versus drive current density for a variable wavelength LED according to another embodiment of the present invention;

FIG. 49A is a graph of peak emission wavelength versus drive current density for another variable wavelength LED according to the present invention;

49B-49D are photographs of the variable wavelength LED of FIG. 49A, with the interpolated emission spectra showing different peak emission wavelengths at different drive current densities;

FIG. 50A is a schematic illustration of spatial combination of monochrome subpixels in a conventional display pixel;

FIGS. 50B-50G are schematic illustrations of LED display device pixels containing at least one variable wavelength LED in accordance with the present invention;

51A-51C illustrate the color gamut obtainable with prior art displays and devices of the present invention;

52A-52D illustrate a display pixel and a color gamut obtainable with the pixel in accordance with an embodiment of the invention;

FIG. 53 illustrates a method of controlling a display pixel including a variable wavelength LED in accordance with the present invention;

FIGS. 54A and 54B illustrate a display pixel and the color gamut achievable by the pixel in accordance with an embodiment of the invention;

55A-55D illustrate a display pixel and the color gamut achievable by the pixel according to another embodiment of the invention;

FIG. 56 illustrates a display pixel and the color gamut achievable by the pixel in accordance with another embodiment of the invention;

FIG. 57 illustrates a display pixel and the color gamut achievable by the pixel in accordance with another embodiment of the invention;

FIG. 58 illustrates a display pixel and the color gamut achievable by the pixel in accordance with another embodiment of the invention;

FIG. 59A illustrates a variable wavelength display pixel according to an embodiment of the invention, and FIGS. 59B-60D illustrate a drive scheme for controlling the pixel of FIG. 59A;

FIG. 61A illustrates a display pixel according to an embodiment of the invention, and FIGS. 61B-60C illustrate a drive scheme for controlling the pixel of FIG. 61A;

FIG. 62 illustrates a display pixel and a drive scheme for controlling the pixel in accordance with an embodiment of the invention;

FIG. 63 illustrates a display pixel and a drive scheme for controlling the pixel in accordance with an embodiment of the invention;

FIG. 64A is a schematic illustration of the drive current conditions of a variable wavelength LED according to the present invention

FIG. 64B illustrates the peak emission wavelength of a variable wavelength LED according to the present invention emitted in response to different drive currents;

FIG. 65 is a schematic illustration of the drive current conditions of a variable wavelength LED according to embodiment 1 of the present invention;

FIG. 66 is a schematic illustration of the drive current conditions of a variable wavelength LED according to embodiment 2 of the present invention;

FIG. 67 is a schematic illustration of the drive current conditions of a variable wavelength LED according to embodiment 3 of the present invention;

FIG. 68 is a schematic illustration of the drive current conditions of a variable wavelength LED according to embodiment 4 of the present invention;

FIG. 69 is a schematic illustration of the drive current conditions of a variable wavelength LED according to embodiment 5 of the present invention;

FIG. 70 illustrates the peak emission wavelength emitted by a variable wavelength LED according to the present invention, covered with five different drive currents;

FIG. 71 illustrates digital pulse patterns for five different drive currents;

FIG. 72 illustrates the resulting emission spectrum emitted by the variable wavelength LED in response to the pulse pattern of FIG. 71;

FIGS. 73A-73D illustrate spectral reconstruction using multiple digital pulses of drive current according to embodiment A of the present invention;

FIG. 74 illustrates an exemplary target emission spectrum;

FIG. 75 illustrates the peak emission wavelength of a variable wavelength LED according to the present invention emitted in response to different drive currents;

FIG. 76 illustrates an exemplary analog drive current pulse that may be used with embodiment B of the present invention;

FIG. 77 illustrates the perceived output spectrum of a variable wavelength LED in response to the analog current pulses of FIG. 63;

FIG. 78 illustrates a large area spectrally tunable illumination source according to preferred embodiment C of the present invention;

FIG. 79 illustrates a spectrally corrected display device comprising spectrally tunable pixels in accordance with a preferred embodiment D of the present invention;

FIG. 80A is a schematic plan view of a display pixel having two variable wavelength sub-pixels with the same diode structure but different light emitting areas;

FIG. 80B shows the red-green (RG) pixel package array of FIG. 80A mounted on a backplane driver integrated circuit having a blue LED sub-pixel array;

FIG. 81 is a graph of drive current density versus emission wavelength for a variable wavelength LED sub-pixel in accordance with an aspect of the present invention;

Fig. 82A and 82B illustrate an exemplary device pixel including two sub-pixels having the same diode structure but different areas;

FIG. 83 illustrates a relationship between emission efficiency and emission wavelength for an exemplary variable wavelength LED sub-pixel in accordance with an aspect of the present invention;

FIG. 84 illustrates photopic luminance functions of different wavelength emissions of an exemplary variable wavelength LED sub-pixel in accordance with an aspect of the present invention;

FIG. 85A is a schematic plan view of a pixel of a two sub-pixel device in accordance with a preferred embodiment of the invention; and

Fig. 85B is a schematic side cross-sectional view of the pixel of fig. 85A taken along line A-A.

Detailed Description

Fig. 1 illustrates a porous template suitable for use in an LED according to the present invention.

The porous template comprises a porous region of a group III nitride material on a substrate, a non-porous layer of the group III nitride material being disposed over a top surface of the porous region. Alternatively, there may be more layers of group III-nitride material between the substrate and the porous region.

As described in more detail below, the porous region may be provided by epitaxially growing an n-doped region of a group III nitride material and then growing an undoped layer of the group III nitride material, and pore forming the n-doped region using pore forming processes set forth in international patent applications PCT/GB2017/052895 (publication No. WO 2019/063957) and PCT/GB2019/050213 (publication No. WO 2019/145728).

As mentioned above, such porosification results in strain relaxation in the lattice, which means that the subsequent overgrowth of the further semiconductor layer benefits from a reduction of the compressive strain in its lattice.

The porous region may comprise one or more layers of one or more group III nitride materials and may have a range of thicknesses while still providing strain relaxation benefits that shift the wavelength of the InGaN light emitting layer grown over the porous region. In a preferred embodiment, the porous region may for example comprise GaN and/or InGaN.

Various LED structures may be overgrown over the template shown in fig. 1.

In particular, LED structures containing InGaN light emitting layers can be overgrown on porous templates using standard LED fabrication steps, as is known in the art for fabricating yellow or green LEDs. However, when grown on a porous template, an LED structure that normally emits at a first wavelength will emit at a red-shifted longer wavelength.

In this way, the use of porous regions of group III nitride material as templates or pseudo-substrates for overgrowth of known InGaN LED structures allows longer wavelength LEDs to be fabricated in a simple manner.

In a preferred embodiment, the LED according to the present invention comprises the following layers and can be manufactured using the following stepwise procedure.

The following description of the LED structure relates to a top-emitting architecture described from bottom up, but the invention is equally applicable to a bottom-emitting architecture.

FIG. 2-substrate and group III nitride layer for porosification

A compatible substrate is used as a starting surface for epitaxial growth. The substrate may be silicon, sapphire, siC, beta-Ga 2O3, gaN, glass or metal. The crystal orientation of the substrate may be a polar, semi-polar or nonpolar orientation. The substrate size may vary from 1cm ², 2 inches, 4 inches, 6 inches, 8 inches, 12 inches, 16 inches in diameter, and the thickness of the substrate may be greater than 1 μm, for example between 1 μm and 15000 μm.

A layer or stack of group III-nitride material is epitaxially grown on a substrate. The group III nitride layer may comprise one or a combination of these elements: al, ga, in (binary, ternary or quaternary layers).

The thickness T of the group III nitride stack is preferably at least 10nm, or at least 50nm, or at least 100nm, for example between 10nm and 10000 nm.

The group III nitride layer includes a doped region having an n-type doping concentration of between 1 x 10 ¹⁷cm^-3–5×10²⁰cm^-3. The group III-nitride layer may also include an undoped "capping" layer of group III-nitride material over the doped region.

The doped region may terminate at the exposed upper surface of the group III nitride layer, in which case the surface of the layer will be porous during the electrochemical etch.

Alternatively, the doped region of group III-nitride material may be covered by an undoped "cap" layer of group III-nitride material, such that the doped region is a subsurface in the semiconductor structure. The subsurface starting depth (d) of the doped region may be, for example, between 1-2000 nm.

FIG. 3-porosification of porous regions

After the group III nitride layer (or stack of layers) is deposited on the substrate, it is porosified using the wafer level porosification process set forth in international patent applications PCT/GB2017/052895 (publication No. WO 2019/063957) and PCT/GB2019/050213 (publication No. WO 2019/145728). During this process, the doped regions of the group III-nitride material become porous, while any undoped regions of the group III-nitride material do not become porous.

After the porosification step, the structure thus contains porous regions that remain where the n-doped group III nitride material is pre-existing, and optionally contains a non-porous intermediate layer covering the porous regions.

The porosity of the porous region is controlled by the electrochemical etching process and may be between 1% and 99%, preferably between 20% and 90% or between 30% and 80%, although smaller or larger porosities may also be employed.

The thickness of the porous region after porosification is preferably greater than 1nm, more preferably greater than 10nm, particularly preferably at least 40nm or 50nm or 100nm. However, the thickness of material required to obtain the strain relief benefit provided by the porous region may vary depending on the type of group III nitride material from which the porous region is fabricated.

The porous region created by the porosification process may be a bulk layer of group III nitride material having a uniform composition and uniform porosity. Alternatively, the porous region may comprise multiple layers of porous material of different composition and/or porosity, forming a porous stack of group III nitride material. For example, the porous region may be a continuous layer of porous GaN, or a continuous layer of porous InGaN, or a stack comprising one or more layers of porous GaN and/or one or more layers of porous InGaN. The inventors have found that strain relaxation benefits of porous regions for overgrowth can be obtained over a wide range of porous regions having different thicknesses, compositions and layered stacks.

In the embodiment shown in the drawings, the porous region is a single porous layer.

In the case of an undoped group III nitride material cap layer over the doped region, the undoped region remains non-porous after the underlying through-surface of the doped region has been made porous. The thickness D of the non-porous cap layer may preferably be at least 2nm, or at least 5nm, or at least 10nm, preferably 5-3000nm. Providing an undoped cap layer over the doped region advantageously results in a non-porous layer of group III nitride material covering the porous region after porosification. The non-porous covering layer may advantageously allow for better overgrowth of additional material over the porous region.

Since the porosification methods of PCT/GB2017/052895 (publication No. WO 2019/063957) and PCT/GB2019/050213 (publication No. WO 2019/145728) can be performed on the entire semiconductor wafer, no processing/patterning/handling is required to prepare templates for porosification.

FIG. 4-connecting layer

After forming the porous layer, the group III nitride LED epitaxial structure may be grown on a porous template/pseudo substrate provided by the porous layer and the non-porous cap layer.

The first layer used to grow the LED structure on the template may be referred to as the connection layer 1.

Although the LED epitaxial structure may be grown directly on the non-porous cap layer, it is preferred to provide the connection layer 1 over the cap layer before the LED structure is overgrown. The inventors have found that the use of a group III nitride connection layer 1 between the porous region and the LED epitaxial structure can advantageously ensure a good epitaxial relationship between the LED and the porous template/substrate. The growth of this layer ensures that the subsequent overgrowth on top of the connection layer is smooth, epitaxial and of a suitably high quality.

The connection layer 1 is formed of a group III nitride material, and may contain one or a combination of these elements: al, ga, in (binary, ternary or quaternary layers).

The connection layer may be a doped or undoped layer. The connection layer may optionally be doped with a suitable n-type doping material, such as Si, ge, C, O. The group III nitride layer may have a doping concentration between 1 x 10 ¹⁷cm^-3–5×10²⁰cm^-3.

The thickness of the connection layer is preferably at least 100nm and may be, for example, between 100nm and 10000 nm.

FIG. 5-N-doped region

After the growth of the connection layer, the growth body n-dopes the group III nitride region 2.

The n-doped region 2 may comprise or consist of a III-nitride layer containing indium, or a stack of thin III-nitride layers with or without indium, or a bulk layer or stack of grown III-nitride layers with varying atomic percent of indium across layers or stacks. For example, the n-doped region may be an n-GaN layer or an n-InGaN layer, or the n-doped region may be a stack of n-GaN/n-InGaN alternating layers, or a stack of n-InGaN/n-InGaN alternating layers with different amounts of indium in the alternating layers.

Preferably, the n-doped region 2 comprises indium such that the lattice of the n-doped region has a lattice parameter similar to the lattice of the InGaN light emitting layer in the LED. For example, the atomic percent of indium in the n-doped region may vary between 0.1-25%.

In a preferred embodiment, the indium content of the n-doped region is within 20at%, or within 15at%, or within 10at%, or within 5at% of the indium content of the InGaN light emitting layer. This may advantageously ensure that the lattice parameter of the n-doped region is sufficiently similar to that of the InGaN light emitting layer to avoid excessive strain between these layers.

The total thickness of the n-doped regions may be at least 2nm, or at least 5nm, or at least 10nm, or at least 20nm. For example, the thickness of the n-doped region may vary from 2nm to 5000nm, or even thicker. If the n-doped region comprises a stack, the thickness of each individual layer in the stack is preferably between 1-40 nm.

The n-type doping concentration of the n-type doped region is preferably between 1×10 ¹⁷cm^-3–5×10²⁰cm^-3, preferably between 1×10 ¹⁸cm^-3–5×10²⁰cm^-3, particularly preferably greater than 1×10 ¹⁸cm^-3.

FIG. 6-light emitting region

After growing the n-doped region 2, an underlayer or pre-layer or pre-well (not labeled in fig. 6) may be grown in order to relieve the strain in the light emitting layer(s). The underlayer may be a monolayer or stack/multilayer of GaN, inGaN, or GaN/InGaN or InGaN/InGaN. Alternatively, the underlayer may have a structure similar to an InGaN QW/GaN quantum barrier, but with a lower proportion of indium. For example, an underlayer composed of a bulk InGaN layer having a lower indium fraction than the light emitting layer may be grown before depositing the light emitting layer having a relatively high indium fraction. Alternatively, the underlayer may take the form of an InGaN "pseudo" QW with a lower indium fraction than the light emitting layer, and one or more GaN quantum barriers.

After growing the n-doped region 2 and the optional underlying layer, a light emitting region 3 containing an InGaN light emitting layer is grown.

The light emitting region 3 may contain at least one InGaN light emitting layer. Each InGaN light emitting layer may be an InGaN Quantum Well (QW). Preferably, the light emitting region may comprise 1-7 quantum wells. Adjacent quantum wells are separated by barrier layers of group III nitride material having a different composition than the quantum wells.

Throughout this document, the light emitting layer(s) may be referred to as "quantum wells," but may take a variety of forms. For example, the light emitting layer may be a continuous layer of InGaN, or the layers may be continuous, fragmented, broken layers containing gaps or nanostructures such that the quantum well effectively contains multiple 3D nanostructures that appear as quantum dots.

The quantum wells and barriers are grown at a temperature in the range of 600-800 ℃.

Each quantum well is composed of an InGaN layer with an indium atomic percentage between 15-40%. Preferably, the luminescent indium gallium nitride layer(s) and/or the quantum well have a composition of In _xGa_1-x N, wherein 0.05.ltoreq.x.ltoreq.0.40, preferably 0.12.ltoreq.x.ltoreq.0.35 or 0.22.ltoreq.x.ltoreq.0.30, particularly preferably 0.22.ltoreq.x.ltoreq.0.27.

The thickness of each quantum well layer may be between 1.5-8nm, preferably between 1.5nm and 6nm, or between 1.5nm and 4nm. The quantum wells may be covered with a thin (0.5-3 nm) group III nitride QW cap layer, which may contain one or a combination of the following elements: al, ga, in (ternary or quaternary layer)

The QW cap layer added immediately after QW growth may be any of AlN, alGaN, gaN, inGaN with Al%0.01-99.9%, and any of In% 0.01-30%.

The group III nitride QW barrier separating the light emitting layers (quantum wells) may contain one or a combination of these elements: al, ga, in (ternary of quaternary layers). The QW barrier may be any of AlN, alGaN, and InGaN with Al% of 0.01-99.9% and In% of 0.01-15%. Preferably, the QW barrier layer contains AlN and/or AlGaN.

The QW cover layer(s) and QW barrier layer are not indicated with separate reference numerals in the figures, as these layers form part of the light emitting region 3.

The QW cladding layer may be grown after each QW but before barrier growth. For example, if an LED contains 3 QWs, each of these QWs may be grown with a QW cover layer and QW barrier layer such that the light emitting region contains 3 such QW cover layers and 3 such QW barrier layers.

1. The cap may be grown under the same conditions as the QW.

2. The lid can be grown ramp-grown to a higher temperature without growth, and in practice this is an annealing step, and here the ramp-growth can be performed in a different gas mixture.

3. Can rise and grow during the temperature rise.

The design of the light emitting area may vary according to parameters known in the art and conventional in LED design. For example, the composition, thickness, and number of light emitting layers and barrier layers can vary depending on the target EL emission wavelength of the LED. As described in the present application, the indium content of the InGaN light emitting layer may be increased when longer wavelength emission is desired.

As described above, the present invention may be provided by growing a known LED structure over a template containing porous regions, the LED structure known to emit at a first wavelength under a voltage bias. The strain relaxation caused by the porous region under the LED structure is able to incorporate more indium into the light emitting layer(s) under the same growth conditions, thus the wavelength of the resulting LED is red shifted when compared to the same LED structure grown over a non-porous substrate under the same conditions. Thus, a greater variety of emission wavelengths can be achieved using the present invention than in the prior art, and in particular, longer wavelengths can be achieved at higher InGaN growth temperatures. This results in a higher quality of crystal structure in the LED, resulting in a higher performance LED.

For the manufacture of longer wavelength LEDs, the large amount of indium in the luminescent layer(s) makes the cover layer more important, as previous attempts to manufacture longer wavelength yellow, orange or red LEDs failed by not introducing enough indium. Thus, the coverage is very important to ensure that enough indium is captured in the light emitting region.

FIG. 7-cover layer

After the growth of the light-emitting layer(s), an undoped cap layer 4 is grown. The undoped cap layer 4 may be referred to as a light emitting region cap layer because the layer is formed after the entire light emitting region has been grown, for example after the growth of a stack of QW, QW cap layer and QW barrier layer.

The cap layer (light emitting region cap layer) 4 is a standard layer well known in the growth scheme of group III nitride LEDs.

The thickness of the cap layer may be between 5nm and 30nm, preferably between 5nm and 25nm or between 5nm and 20 nm.

The purpose of the light emitting region cap layer 4 is to protect the indium in the light emitting region (QW stack) and prevent it from desorbing/evaporating during subsequent processing. Because InGaN QWs are normally grown at lower temperatures, which is disadvantageous for GaN/AlGaN, a temperature ramp-up step is typically required before growing more layers over the light emitting region. The capping layer is used to ensure that the InGaN light emitting layer(s) are properly capped and protected, thus giving opportunities and time windows to change the p-doped layer growth conditions for better material quality. The light emitting region cap layer 4 also ensures that no Mg dopant enters the QW region during the growth of the p-type layer.

Electron Blocking Layer (EBL)

After the growth of the quantum well, cap layer and barrier layer, an electron blocking group III nitride layer (EBL) 5 containing aluminum is grown. For example, the Al% may be between 5-25%, although higher Al contents are also possible.

The EBL is doped with a suitable p-type dopant material. The p-type doping concentration of the EBL is preferably between 5 x 0 ¹⁸cm^-3-3–8×10²⁰cm^-3.

The thickness of the EBL may be between 10nm and 50nm, preferably 20nm.

FIG. 8P-type doped layer

A p-doped layer 6 is grown over an Electron Blocking Layer (EBL) 5.

The p-type region is preferably doped with Mg and the p-type doping concentration of the p-type layer is preferably between 5 x 10 ¹⁸cm^-3–8×10²⁰cm^-3.

The p-doped group III nitride layer may contain In and Ga.

The thickness of the doped layer is preferably between 20nm and 200nm, particularly preferably between 50nm and 100 nm. The doping concentration may vary over the p-type layer and may have a peak in doping level in the last 10nm-30nm of the layer towards the LED surface to allow for a better p-contact.

The structure may be annealed in a MOCVD reactor or in an annealing furnace in order to activate the Mg acceptor in the p-doped layer. In an N ₂ or N ₂/O₂ environment, the annealing temperature may be in the range 700 ℃ -850 ℃.

Because both the EBL and the p-doped layers are p-type doped, these layers may be referred to as p-doped regions.

FIG. 9-transparent conductive layer

The stack of active semiconductor layers is covered with a transparent conductive layer 7. The transparent conductive layer may be made of Ni/Au, indium tin oxide, indium zinc oxide, graphene, pd, rh, silver, znO, etc., or a combination of these materials.

The transparent conductive layer may have a thickness between 10nm and 250 nm.

Transparent conductive layers are well known in the art and any suitable material and thickness may be used.

An annealing step may be required to ohmic the p-contact.

FIG. 10

Depending on the manufactured LED structure, the semiconductor structure may be processed into an LED, mini-LED or micro-LED device.

Common LEDs are typically greater than 200 μm (referring to the lateral dimensions of the width and length of the LED structure. Mini-LEDs typically have a lateral dimension of 100 μm-200 μm, while Micro-LEDs typically have a size of less than 100 μm.

Fig. 10 illustrates the semiconductor structure after etching layers 2-7 of the semiconductor structure into a plurality of discrete LED stacks or mesas, each having the same structure.

The steps of LED fabrication are conventional and well known to those skilled in the art. The order of the following fabrication steps is not specific to the present invention, and those skilled in the art will appreciate that LED devices within the scope of the present invention may be prepared using alternative fabrication steps to those shown below. However, for illustrative purposes only, one preferred manufacturing route for preparing LEDs in accordance with the present invention is described below.

In a next step, the transparent conductive layer 7 is structured to cover only the top surface of the active emissive element. Structuring can be accomplished using standard semiconductor processing methods including resist coating and lithography. The transparent conductive layer is etched by a sputter etching process using wet chemistry or using argon. This step is followed by wet or dry etching of the group III nitride structure. Inductively coupled plasma reactive ion etching, reactive ion etching only or neutral beam etching is used to create mesas in the group III nitride layer. The dry etching process may include one or more of Cl, ar, BCl ₃、SiCl₄ gases.

The purpose of this step is to isolate the individual emissive elements and to approximate the buried n-doped layer of the p-n junction.

After the dry etching process, a wet etching process is performed to remove dry etching damage from the sidewalls of the mesa. Wet chemistry may involve KOH (1-20%), TMAH, or other base chemistry.

FIG. 11 passivation

The next step is to deposit the passivation layer 8 or a combination of passivation layers. The initial passivation layer may be Al2O3 (10 nm-100 nm) (deposited by atomic layer deposition) followed by sputtering or plasma enhanced chemical vapor deposited SiO ₂, siN, or SiON (50 nm-300 nm).

Al2O3 may be deposited at a temperature between 50℃and 150 ℃.

SiO2, siN, and SiON may be deposited at temperatures between 250 ℃ and 350 ℃.

The sputtering process may be performed at room temperature.

FIGS. 12-13

The next step is to form an opening in the oxide passivation layer 8 to expose the top of the LED structure. This may be achieved by wet or dry etching or a combination of both.

For wet etch buffered oxide etching, dilute hydrofluoric acid, phosphoric acid, or mixtures thereof may be used.

The channels are also etched through the connection layer 1 between the LED structures, and subsequently the LED structures are electrically insulated from each other by depositing a dielectric mask material 8 into the channels, so that the LEDs can operate independently of each other.

The next step in device fabrication is to cover the transparent conductive layer 7 on the p-doped layer 6 with a metal layer to act as an electrical p-contact 9. The covering may be accomplished in a single step or in multiple steps. The metal may completely or partially cover the pixels. In this example, one step is used to simplify the details.

The metal contact 9 may contain Ti, pt, pd, rh, ni, au. The thickness of the stack of complete metal layers may be between 200nm and 2000 nm.

FIG. 14-exposed tie layer

Openings may be created in the second mask layer 8 using standard photolithographic techniques to expose regions of the connection layer 1. The size of the openings may vary between 200nm and 50000 nm. The distance between the openings may be between 500nm and 30000 nm. The openings are created only in areas of the wafer not occupied by the LED structures.

The dry etching is preferably used to etch the second mask layer 8 using a fluorine-based gas.

FIG. 15-N contact

The next step in the device fabrication is to cover the opening in the oxide 8 with a metal contact 10 to access the connection layer1, the connection layer1 being in electrical contact with the n-doped layer of the LED structure. The covering may be accomplished in a single step or in multiple steps. The metal may completely or partially cover the pixels. In this example, one step is used to simplify the details.

The metal may contain Ti, pt, pd, rh, ni, au. The thickness of the complete metal stack may be between 200nm and 2000 nm.

FIGS. 16-18

After this treatment, the substrate may be thinned, and/or the porous region may be removed, thereby exposing the connection layer 1.

Surface structuring or texturing may be performed on the substrate, at the porous region or on layer 1 to enhance light output and control emission angle, as well as other optical engineering and design.

Finally, the wafer/device may be flipped and bonded to another carrier substrate, which may be silicon/sapphire or any type of passive device. Alternatively, the device may be bonded to a CMOS silicon back plate for an active matrix micro-LED display panel.

As shown in fig. 16, the top surface of the device may be bonded to another carrier wafer/substrate/backplate 11, or to a micro-driver circuit board to form an array of pixels.

The substrate may then be removed from the device and the bottom side of the device may be bonded to the cover glass or transparent material 12, as shown in fig. 17.

As shown in fig. 18, the substrate, as well as the porous and non-porous regions, may be removed from the device. The top surface of the device may be bonded to another carrier wafer/substrate/backplate 11 or to a micro-driver circuit board to form an array of pixels. The bottom side of the device may be bonded to cover glass or transparent material 12.

The skilled person will understand that the emission wavelength of the individual LED structures can be controlled by varying the composition and layer structure of the LED structures according to known principles of LED construction. Thus, various variable wavelength LED devices emitting in different emission wavelength ranges can be provided using the present invention, and color combinations other than green to red can be provided.

FIGS. 19-23

Fig. 19 shows an example of an InGaN LED over a porous layer that emits at a peak wavelength of about 625nm due to the wavelength red shift caused by the porous region.

Fig. 20 and 21 compare the emission characteristics of InGaN LEDs on a non-porous substrate (fig. 20) and the same InGaN LEDs grown on a template comprising a porous layer of group III nitride material. A comparison of the two figures demonstrates the shift to longer emission wavelengths caused by the porous underlayer, since the emission of the LEDs on the porous template is always 21nm to 45nm longer than the emission of the same LEDs on the non-porous template.

Fig. 22 and 23 compare I-V characteristics of a yellow InGaN micro-LED (fig. 22) on a non-porous substrate with the same InGaN micro-LED grown on a template containing a porous layer. On the porous template, the InGaN micro-LED emits red light as shown in the inset.

FIG. 24 is an I-V curve of an InGaN micro LED of different pixel sizes (10 μm;200 μm 20 μm;30 μm;50 μm) measured on a porous substrate. Fig. 25 shows the I-V characteristics of the same pixel, with the axis changed to focus on low currents from 1 x 10 ^-6 mua to slightly above 100 mua.

Fig. 26 is a series of five EL images of the same variable wavelength microLED InGaN pixels driven at different currents in constant wave mode (CW), showing five different emission colors. In the left image, the micro-LED emission color is red at a drive current of 50 μΑ. In the second image on the left, the micro-LED emission color is red-orange at a drive current of 100 μΑ. In the third image on the left, the micro-LED emission color is orange at a drive current of 1 mA. In the fourth image from the left, the emission color of the micro-LED is yellowish green at a driving current of 10 mA. In the right graph, the emission color of the micro-LED is green at a driving current of 20 mA.

By varying the drive current between 50 μa and 20mA, the same micro-LED is thus able to emit light in the wavelength range from red to green. The spectral width of this emission wavelength range is about 90nm (from about 570nm to about 660 nm). This is a much larger emission wavelength than the emission wavelength range achievable by a single LED in the prior art.

Fig. 27A is a graph of emission wavelength versus current density for a 25 μm x 25 μ M INGAN LED pixel array (100 x 100 array containing 10,000 pixels) driven in a pulsed mode, where the pulses are 100 microseconds with a duty cycle of 1%. Fig. 27B is a graph of emission wavelength versus current density for a 30 μm x 30 μ M INGAN LED pixel array (100 x 100 array containing 10,000 pixels) driven in a pulsed mode, where the pulses are 100 microseconds with a duty cycle of 1%.

Both of these graphs show the controllability of the peak emission wavelength of the pulsed drive power supply. In particular, the wavelength is linearly dependent on the current density (plotted on a logarithmic scale). This linearity can also be controlled when driven with a pulsed voltage power supply. Thus, the variable emission wavelength of the LED can be controlled with a voltage or current drive scheme in CW or pulsed mode, all of which are standard ways of displaying driver ICs.

This linear relationship between the driving current density and the resulting emission wavelength is very advantageous for LED display designs, as it enables accurate control of the emission wavelength by varying the current density of the power supply.

Fig. 28 is a graph of intensity versus wavelength for variable wavelength InGaN LEDs driven at different DC currents. The power supply operates in a pulsed drive mode, where the pulses are 100 microseconds with a duty cycle of 1%.

Fig. 28 again reflects a gradual continuous transition of the LED peak emission wavelength as the supply current varies. At a drive current of 200mA, the peak emission wavelength was about 575nm and the intensity was about 10. Mu.W/nm. However, as the drive current decreases, the peak emission wavelength gradually shifts to longer wavelengths and lower emission intensities. When the drive current reaches 7mA, the peak emission wavelength is about 675nm and the intensity is about 0.1. Mu.W/nm.

Fig. 29A is a schematic diagram of the relationship of LED structures on a porous template comprising porous regions of group III nitride material. The LED comprises a current confinement layer 100, which current confinement layer 100 is located between an n-doped part of the LED (in this example denoted n-GaN) and the light emitting region. The light emitting region is labeled as the MQW (multiple quantum well) region of the LED. Although SiN current-constraining layer 100 is shown in the figures, current-constraining layer 100 (which may also be referred to as a current-constraining layer) may be formed from another dielectric material.

A circular aperture 110 is provided through the center of the dielectric current confined layer 100. The hole extends through the thickness of the current confinement layer, providing a conductive path between the n-doped region and the light emitting region of the LED. In the illustrated embodiment, the diameter of the aperture is approximately 33% of the lateral width of the LED structure, but the width of the aperture may be varied to vary the local current density through the aperture.

Fig. 29B is a schematic diagram of an LED structure containing a current confinement layer 100 in an alternative location, the current confinement layer 100 being located between the p-doped portion of the LED (labeled p-GaN in this example) and the light emitting region (MQW region). The holes 110 extend through the thickness of the current confinement layer 100, providing a conductive path between the p-doped region and the light emitting region of the LED.

FIGS. 30-35

Fig. 30-34 are schematic cross-sections of LED structures formed over a porous template in accordance with a preferred embodiment of the present invention.

The LED structure includes a substrate, which as described above may be silicon, sapphire, siC, β -Ga2O3, gaN. The substrate size may be as small as 1X 1cm-2, 50mm, 100mm, 150mm, 200mm, 300mm or larger diameter.

A porous region is formed over the substrate, and a connection layer (layer 1) of (Al, in) GaN is formed over the porous region. An n-type layer (layer 2) of n (Al, in) GaN is located over the connection layer and forms an n-type portion of the LED device. A pre-strained layer (layer 3) is formed over the n-type layer and an active region (layer 4) containing Multiple Quantum Wells (MQW) is located over the pre-strained layer and under the p-type layer (layer 5) of p- (Al, in) GaN.

This is a typical LED structure formed on a porous semiconductor template. The porous region may be a uniform porous layer or region, or may be a partially patterned porous region.

The porous region may be any porous region as described above—a variety of thicknesses, compositions, and configurations are possible within the scope of the invention.

In a preferred embodiment of the invention, schematically illustrated in fig. 31, two v-shaped pits are formed in the LED structure. The pits are v-shaped in cross section and create v-shaped voids in the upper layers of the LED structure.

As shown, the first v-shaped recess 311 extends from the connection layer (layer 1) to the upper surface of the LED device, which is formed by the outer surface of the p-type layer (layer 5). The narrow point of the v-shaped pit is located in the connection layer and the width of the pit expands with each layer epitaxially grown over the connection layer, reaching its widest point at the surface of the p-type layer.

The second v-shaped pits 312 extend from the pre-strained layer (layer 3) to the upper surface of the LED device, which is formed by the outer surface of the p-type layer (layer 5).

As shown in fig. 32, the first v-shaped pits 311 and the second v-shaped pits 312 extend through the active area MQW of the LED structure. Thus, the v-shaped pits create gaps or voids in the semiconductor structure. The V-shaped pits may be created in layer 1 or layer 3, but must pass through layer 4, the MQW region.

The active light emitting Multiple Quantum Well (MQW) and Quantum Barrier (QB) may be made of (Al, in) GaN and (Al, in) GaN, respectively, and are virtually any combination of materials, compositions, and thicknesses. The MQW may have any period, 1xQW, 2xQW, 3xQW, 4xQW, 5xQW, up to 10xQW or longer.

In some preferred embodiments, the QW is continuous. In some preferred embodiments, the QW is fragmented.

The V-shaped pits may originate or be caused by the presence of threading dislocations. Alternatively, v-shaped pits may be formed by different growth modes during epitaxy, i.e. three-dimensional growth.

For example, V-pits and their growth are described in the influence of nanoscale V-pits on the electronic and optical properties and efficiency degradation of GaN-based green light emitting diodes; zhou et al; science report (2018) 8:11053

DOI:10.1038/s41598-018-29440-4。

As shown in fig. 33, the second v-shaped pits 312 originate from threading dislocations 330. Threading dislocations originate in the porous region or connecting layer (layer 1) and extend upward through the sequentially deposited layers of semiconductor material (layers 1 and 2). At the pre-strained layer (layer 3), threading dislocations 330 begin to widen into v-pits 312, and v-pits 312 become wider as more semiconductor layers are epitaxially grown over layer 3.

As the v-pits pass through the MQW active region during epitaxial growth, MQWs may also be grown on the sidewalls 340 of the v-pits. The deposited MQW will have a different thickness and composition compared to a planar MQW without such pits.

Since v-shaped pits have been started to form before depositing the active area including the MQW, semiconductor material epitaxially deposited over the underlying layer is deposited into the v-shaped pits. At the v-shaped pits, the material layers forming the MQW are thus distorted as the layers are stretched down the sidewalls 340 of the v-shaped pits. This is shown in the Transmission Electron Microscope (TEM) image of fig. 35.

As shown in fig. 35, the semiconductor structure layer is grown as a flat planar layer on either side of v-shaped pits 312. Thus, the active MQW region is flat (on either side of the cross-section shown) around the v-shaped pits. However, at the location of the v-shaped pits, the MQW layer is twisted and extends down the sidewalls 340 into the v-shaped pits. This stretching effect changes the thickness of the QWs on the pit sidewalls 340 such that they are different in thickness compared to the planar QW layer formed over the rest of the LED structure.

Fig. 36 and 37

Fig. 36 is a graph of normalized intensity versus peak emission wavelength for the variable wavelength LED containing v-shaped pits shown in fig. 31-35. As the driving conditions applied to the LEDs change, the LEDs emit at different peak emission wavelengths. As shown in fig. 36, the peak emission wavelength of the LED may vary over a continuous wavelength range from about 530nm to about 640nm,

Fig. 37 is a graph of peak emission wavelength versus drive current for a variable wavelength LED according to the present invention. Fig. 37 shows that as the drive current applied to the LED increases, the peak emission wavelength smoothly changes from about 550nm to about 635nm, with higher drive currents resulting in the LED emitting at shorter wavelengths and lower drive currents resulting in the LED emitting at lower wavelengths. The variation of the peak emission wavelength is continuous and consistent with the variation of the drive current, resulting in a simple calibration of the LED device.

FIG. 38

Fig. 38 is a graph of peak emission wavelengths of variable wavelength LEDs of different pixel sizes. As shown in fig. 38, changing the size of the LED pixel affects the peak emission wavelength of the pixel emission. Under the same driving conditions, pixels of different sizes (otherwise identical) will emit at different peak wavelengths.

Fig. 39 and 40

Fig. 39 and 40 are schematic side cross-sections of variable wavelength LED structures according to two alternative embodiments.

In the following examples, the LEDs and LED sub-pixels are preferably variable wavelength LEDs as described above.

The LED structure in fig. 39 and 40 is shown as a simplified diode structure in which a light emitting region containing Multiple Quantum Wells (MQWs) is located between a first semiconductor layer and a second semiconductor layer. The LED structure is provided on a substrate, which preferably contains porous regions of semiconductor material.

Fig. 39 and 40 illustrate alternative arrangements of electrical contacts to a light emitting diode. In both embodiments, the first electrical contact is shown above the first semiconductor layer. An electrically insulating mask layer 390 (which may be referred to as a passivation layer) is partially located between the first semiconductor layer and the first contact layer such that the first contact layer contacts the first semiconductor layer only through the holes in the mask layer 390. The size of the hole determines the contact area where the first semiconductor layer and the first contact layer contact each other. When a drive current is applied through the electrical contacts, the contact area limits the area through which the drive current can enter the LED structure. The size of the contact area and the magnitude of the drive current determine the current density experienced by the LED structure. The current density experienced by the LED structure in turn determines the peak emission wavelength of the light emitted by the MQW.

In fig. 39 and 40, the arrangement of the second contact layer is different.

In fig. 39, the second contact layer is arranged to contact the second semiconductor layer through holes in the mask layer 390. As for the first contact layer, the size of the hole determines the contact area shared by the second contact layer and the second semiconductor layer.

In fig. 40, the second contact layer is located on the substrate, not the first semiconductor layer.

The LED structures of fig. 39 and 40 may be formed as described above with respect to fig. 1-18, with the first and second semiconductor layers acting as p-type and n-type layers such that the MQW emits at the peak wavelength when a drive current is applied between the two electrical contacts.

The first semiconductor layer may include, but is not limited to, p-Gan and n-Gan.

The first contact layer may include, but is not limited to, titanium, platinum, chromium, aluminum, nickel, gold, and some compounds such as ITO (indium tin oxide).

Since the emission wavelength will shift blue with increasing injection current density, there will be different contact areas for controlling the current density, as shown in fig. 41.

Fig. 41 illustrates three different contact areas between the contact layer and the semiconductor layer in the LED diode. The first electrical contact layer is shown above the first semiconductor layer. An electrically insulating mask layer 390 is partially located between the first semiconductor layer and the first contact layer such that the first contact layer contacts the first semiconductor layer only through the holes in the mask layer 390. The size of the hole determines the contact area where the first semiconductor layer and the first contact layer contact each other. When a drive current is applied through the electrical contacts, the contact area limits the area through which the drive current can enter the LED structure. The size of the contact area and the magnitude of the drive current determine the current density experienced by the LED structure. The current density experienced by the LED structure in turn determines the peak emission wavelength of the light emitted by the MQW.

Fig. 42 is a plan view of electrical contacts on an LED sub-pixel mesa. The contact area a _{Contact point} must be smaller than the area of the sub-pixel mesa a _{Table top} because the largest possible contact will contact the LED sub-pixel mesa over its entire area, giving a contact ratio of 1:1.

Fig. 43A-43C illustrate three alternative ways of obtaining RGB sub-pixels using three sub-pixels with the same diode structure.

In fig. 43A, the three sub-pixel mesas have the same sub-pixel mesa area, but the electrical contact pads are different in size. The different sizes of the electrical contact pads means that each of the three sub-pixels has a different contact area and thus each sub-pixel has a different contact ratio (the ratio of contact area to light emitting area to the ratio of contact area to sub-pixel mesa area is the same as the light emitting area spans the entire sub-pixel mesa). The left sub-pixel has the largest contact area and thus the largest contact ratio. The right sub-pixel has the smallest contact area and thus the smallest contact ratio. And the center sub-pixel has a contact area and a contact ratio between the other two sub-pixels.

The smaller the contact ratio, the smaller the contact area through which the drive current is supplied to the sub-pixel, and thus the higher the current density experienced by the sub-pixel. As shown in fig. 27A and 27B, a higher current density results in a shorter peak emission wavelength, and thus, among the three sub-pixels of fig. 43A, the left sub-pixel emits at the longest peak wavelength and the right sub-pixel emits at the shortest peak wavelength.

In a particularly preferred embodiment, the contact ratios and drive currents of the three sub-pixels are selected such that the three sub-pixels emit red (left sub-pixel), green (middle sub-pixel) and blue (right sub-pixel) light in response to a single drive current.

Fig. 43B shows an alternative way of obtaining the same three contact ratios as fig. 43A. In fig. 43B, the three sub-pixels also have the same mesa area, and thus the same light emitting area. In fig. 43B, the areas of the three electrical contact pads are also the same as viewed from above. However, the mask layer 390 is located between the sub-pixels and the electrical contact layer, the mask layer on the three sub-pixels containing holes of different sizes. The contact layer can only contact the sub-pixel diode structure through the aperture, so the size of the aperture controls the contact area. The left sub-pixel has the largest aperture in the mask layer and thus the largest contact area, which results in the largest contact ratio. The right sub-pixel has the smallest aperture through the mask layer and therefore has the smallest contact area and the smallest contact ratio. The center subpixel has a mask layer aperture sized between the other two subpixels, which results in a contact area and contact ratio between the other two subpixels.

The size of the holes in the mask layer is selected to create the same contact area as the sub-pixels in fig. 43A. Thus, in response to the same drive current, the three sub-pixels in fig. 43A and 43B will emit at three corresponding peak emission wavelengths, preferably RGB wavelengths.

Fig. 43C shows a third way of creating three sub-pixels with the same contact ratio as shown in fig. 43A and 43B.

In fig. 43C, the three subpixel mesas each have a different subpixel mesa area and each have a different size of electrical contact, creating a different contact area on the three subpixels. However, in fig. 43C, the relative sizes of the sub-pixels and the contact areas are the same as those shown in fig. 43A and 43B. Therefore, the red subpixel shown in the left side of fig. 43C has the same contact ratio as the red subpixel shown in the left side of fig. 43A and 43B; the green sub-pixel shown in the upper right corner of fig. 43C has the same contact ratio as the green sub-pixel in the middle of fig. 43A and 43B; the blue sub-pixel shown in the lower right of fig. 43C has the same contact ratio as the blue sub-pixel on the right of fig. 43A and 43B. Since the peak emission wavelength at a given drive current is determined by the contact ratio (the ratio of contact area to light emitting area to the ratio of contact area to subpixel mesa area is the same because the light emitting area spans the entire subpixel mesa), all subpixels in fig. 43A-43C will emit at the same three wavelengths in response to the same drive current.

Similar to the embodiment shown in fig. 13, fig. 44 and 45 illustrate three LED sub-pixels formed from a single LED structure. However, in the embodiment shown in fig. 44 and 45, not each LED sub-pixel has a uniform metal contact layer 9, but rather the three sub-pixels each have different sized electrical contacts 9A, 9B, 9C, which creates different contact areas between the contacts and the three sub-pixel diode structures.

The embodiment of fig. 44 corresponds to the example of fig. 43A, wherein the three sub-pixels each have the same mesa size and thus the same light emitting area, but wherein the three electrical contacts are formed in holes of different sizes in the electrically insulating mask layer 8. The left sub-pixel has the largest electrical contact 9A and thus the largest contact area and the largest contact ratio. The right sub-pixel has the smallest electrical contact 9C and therefore the smallest contact area and the smallest contact ratio. The central sub-pixel has an electrical contact 9B sized between the sizes of the other contacts 9A, 9C, so the contact area and contact ratio of the central sub-pixel is between the other two sub-pixels. The three sub-pixels will emit at three different peak wavelengths in response to the same drive current. The left sub-pixel will emit at the longest wavelength because of its highest contact ratio and the right sub-pixel will emit at the shortest wavelength because of its lowest contact ratio.

Fig. 45 is similar to fig. 43B, wherein the electrical contacts 9A, 9B, 9C have the same size when viewed from above, but the contact area between the electrical contacts and the three sub-pixels is controlled by the size of the apertures through the mask layer 8. The contact area between the electrical contacts 9A, 9B, 9C and the three sub-pixels is the same as in fig. 44, so the three sub-pixels will emit at the respective wavelengths in response to the same drive current.

Fig. 46A-46G illustrate alternative embodiments of the light emitting area of a variable wavelength LED according to the present invention.

Examples of MQWs:

1. continuous MQW

2.V pit

3. Broken QW, bandgap QW, fragmented QW

4.QD

5. Well width fluctuation

6. Alloy composition

Different combinations of MQW and underlayer

These structural features can be identified and inspected by standard material characterization techniques such as cross-sectional Transmission Electron Microscopy (TEM), X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDX or EDS), 3D atom probe (3 DAP).

Fig. 46A shows a continuous MQW light-emitting region of an LED, in which three identical Quantum Barriers (QBs) are provided between four identical QWs.

FIG. 46B shows the continuous MQW of FIG. 46B, where the V-shaped pits propagate through the light emitting area. The V-shaped pits terminate in threading dislocations and have QWs on their semi-polar faces.

Fig. 46C shows an MQW in which the QW layer contains discontinuities or gaps in the semiconductor material.

Fig. 46D shows Quantum Dots (QDs) creating non-uniform MQWs in the MQW. QDs may be provided on or in the QB or QW layers, for example in the gaps of the QW structure.

Fig. 46E shows MQW with well width fluctuation, in which the thickness of the QW layer is non-uniform over the entire light-emitting region. QWs may have different widths from each other and may also have varying widths within a single QW.

Fig. 46F shows MQWs in which the alloy composition of the light-emitting region fluctuates. The composition of QB and QW varies from layer to layer. In particular, the indium In% composition varies within the same QW, i.e. In% varies between 10% and 12% or 10% and 15% or 10% and 25% or 10% and 35% In QW 2.

FIG. 46G shows a MQW containing different combinations of MQWs and liners. The In% composition varies between QWs. For example, the In% In QW1 is 15%, the In% In QW2 is 25%, and the In% In QW3 is 30%. In embodiments of the present invention, the lower In% QW is preferably located at the bottom of the MQW and the higher In% QW is preferably located on top due to its strain and thermal effects. For example, in a preferred embodiment, QW1 is a blue-emitting QW, QW2 is a green-emitting QW, and QW3 is a red-emitting QW.

Fig. 47A is a TEM image of a cross section of a conventional non-variable wavelength LED. The non-variable LED-MQW is uniformly smooth at both the upper and lower interfaces (5 MQWs are shown here clearly)

Fig. 47B and 47C are TEM images of variable wavelength LEDs comprising v-pits according to an embodiment of the invention. Variable wavelength LED-MQW is non-uniform, caused by various methods, one example being v-pits and semi-polar facets, which will contain more indium and thinner QWs. Another example is also evident in fig. 47B, where MQW is not uniform in terms of broken QWs, discontinuous QWs, fragmented QWs, QWs with well width, or composition fluctuations

Fig. 47C shows a cross-section of the variable wavelength LED of fig. 47B showing a porous region and a light emitting region comprising a plurality of v-shaped pits, in accordance with a preferred embodiment of the present invention.

In this structure, the light emitting region contains intentionally introduced multiple emission wavelength regions, such as multiple types of QW regions with v-shaped pits extending through the light emitting region.

The V-shaped pits (V-pits) are actually hexagonal pits seen from above, and the V-shape is a shape when seen in cross section. During growth of InGaN, gaN, inGaN/InGaN superlattice or InGaN/GaN superlattice structures underneath the MQW, v-pits may be initialized at each site of the dislocation under special epitaxial growth conditions, such as low growth temperatures (e.g., <1000 ℃, or <900 ℃, or <800 ℃, or <700 ℃) and nitrogen environments.

Fig. 48A is a graph of peak emission wavelength versus drive current density for a conventional non-variable wavelength LED. By varying the drive current density applied to the LED, the emission wavelength can be slightly varied over an emission wavelength range of about 15 nm.

Fig. 48B is a graph of peak emission wavelength versus drive current density for a variable wavelength LED according to an embodiment of the present invention. In variable wavelength LEDs, varying the current density of the driving power supply creates a larger variation in the peak emission Wavelength (WLP) of the LED emission. In this embodiment, varying the drive current density between about 0.1A/cm ² to 100A/cm ² changes the peak emission wavelength from about 635nm to an emission wavelength range of about 550nm to about 85 nm.

Fig. 48C is a graph of peak emission wavelength versus drive current density for a variable wavelength LED according to another embodiment of the present invention. In this embodiment, varying the drive current density will vary the peak emission wavelength from about 720nm to about 580nm, an emission wavelength range of about 140nm.

Fig. 49A is a graph of peak emission wavelength versus drive current density for another variable wavelength LED according to the present invention. In this embodiment, varying the drive current density between about 0.1A/cm ² to 200A/cm ² changes the peak emission wavelength from 615nm to 508nm, an emission wavelength range of about 100nm. The data for this plot is only 514.5nm due to the limitations of the test capability. Thus, a current density of 508nm was estimated. However, the available emission wavelength range can be significantly pushed to either way.

Fig. 49B-D are photographs of the variable wavelength LED of fig. 49A showing the same variable wavelength LED emitting at four different wavelengths within its emission wavelength range. The interpolated emission spectra show different peak emission wavelengths at different drive current densities. This shows that the same variable wavelength LED emits at peak emission wavelengths of orange (615 nm), yellow (556 nm), green (534 nm) and blue (508 nm) in response to different drive current densities.

Display device

Conventionally, a color display pixel is composed of a plurality of single color sub-pixels: blue, green, and red subpixels. The observed pixel chromaticity is a spatial combination of the light emitted by the three sub-pixels, as shown in fig. 50B.

With the variable wavelength LED of the present invention, a single LED sub-pixel can display colors over a wide spectral range, e.g., from blue to red. This allows different colors to be achieved by using a single LED chip and different driving time frames. The resulting observed pixel color is the temporal combination of the light emitted by the LED sub-pixels.

In accordance with the present invention, a variety of display device pixels may be configured to incorporate one or more variable wavelength LEDs. In all pixel embodiments below, the overall emission color perceived by the viewer is the spatial and temporal combination of the light emitted by the sub-pixels in any given device pixel.

Fig. 51B illustrates a device pixel composed of a single variable wavelength LED. By varying the driving conditions supplied to the LEDs, the peak emission wavelength from the pixel can be varied over a range of emission wavelengths, with the width and absolute wavelength being determined by the driving conditions and the size and configuration of the LED diodes.

Fig. 50C illustrates a device pixel comprising two sub-pixels, both of which are variable wavelength LEDs according to the present invention. The two sub-pixels may be controlled individually by controlling the drive current provided to each sub-pixel individually, so that the sub-pixels may be controlled to emit at different wavelengths.

Fig. 50D illustrates a device pixel comprising three variable wavelength LED sub-pixels. By controlling the drive current supplied to the three individual sub-pixels, the peak emission wavelength of each sub-pixel can be varied individually.

Fig. 50E illustrates a device pixel including one variable wavelength subpixel and one fixed emission wavelength subpixel.

Fig. 50F illustrates a device pixel including one variable wavelength subpixel and two fixed emission wavelength subpixels.

Fig. 50G illustrates a device pixel comprising two fixed emission wavelength blue and red subpixels and two fixed emission wavelength green subpixels configured to emit at different peak wavelengths in the green range of the spectrum. The green subpixel may be a variable wavelength subpixel configured to receive two different fixed drive current densities corresponding to different peak emission wavelengths in green.

Display device with expanded color gamut

As shown in fig. 50A, conventional LED displays typically display color by combining light from sub-pixels having different primary colors. The conventional pixels include red, green, and blue sub-pixels.

The combination of light from the three primary color subpixels allows any color within the triangle (defined by the primary colors) shown in fig. 51A to be displayed. The triangle shown in fig. 51A defines the color gamut achievable with this type of display. Conventional three sub-pixel pixels cannot access color spaces outside the triangle, and therefore the color gamut achievable by the pixel is limited.

Solution to improve color gamut:

The o includes additional sub-pixels having different colors, and the color gamut is defined by a quadrangle as shown in fig. 51B. This requires additional cost and complexity.

Extending the color gamut further to 5 or more sub-pixels would extend the color gamut further, but would increase complexity and cost.

By incorporating the variable wavelength LED of the present invention into an LED display device, there is preferably provided a display device having controllable chromaticity, the display device comprising: the emission spectrum of an LED structure is strongly dependent on the drive current density, wherein the shift of the peak emission wavelength is greater than 20 nm/decade, wherein the peak emission wavelength is controllable in the range of 450nm to 630nm or more.

The display comprises sub-pixels formed by LED devices with controllable chromaticity (variable emission wavelength).

Preferably, the subpixels have the same diode structure and the peak emission wavelength from any given subpixel is controlled only by the current density supplied to that subpixel during use.

The sub-pixels may have a constant chromaticity (e.g., in response to a fixed drive current) or may change chromaticity (in response to a changing drive current) so that a larger color gamut may be achieved. The achievable color gamut is equal to or larger than the color gamut defined by the sRGB primary colors.

In some embodiments, all sub-pixels may dynamically change chromaticity from frame to frame. In other embodiments, only some of the subpixels may dynamically change chromaticity from frame to frame.

Thus, according to the present invention, a wide color gamut can be achieved with a display device comprising one or more variable wavelength LEDs.

FIG. 51C illustrates the extended color gamut achievable by a variable wavelength LED device when the peak emission wavelength is controlled from 450nm to 620nm

Example 1 (fig. 52A-52D):

Fig. 52A illustrates a display pixel including three sub-pixels. Each sub-pixel may be an LED device with a controllable emission peak wavelength.

When viewed from a distance, the chromaticity of the observed emitted light is the spatial and temporal combination of sub-pixel light emissions.

The blue (B) and red (R) sub-pixels operate with fixed emissions corresponding to the fixed observed chromaticity. The peak emission wavelengths of the B and R sub-pixels may be fixed by providing a drive current having a fixed magnitude, which corresponds to the current density required to emit at the desired blue and red wavelengths.

By providing two independent drive current modes, the green (G) subpixel can operate in two modes with different peak wavelengths, the two drive current modes having different magnitudes corresponding to the current densities required to emit at the two desired green wavelengths. The green sub-pixel may be operated in any one of the green modes by switching to a desired drive current mode.

In combination with the B and R sub-pixels, each of the two G sub-modes is capable of displaying a different color gamut.

The effective color gamut of the display is the color gamut achievable with the G sub-pixels in either mode

By extension, when variable wavelength LEDs are used for the R or B sub-pixels, these sub-pixels can also be switched between two or more modes. All pixels can be switched between two or more modes, allowing the color gamut to be set once or dynamically controlled during normal operation.

In another embodiment, conventional blue and red LEDs may be incorporated into the pixel as blue and red subpixels, with the variable wavelength LEDs forming the green subpixels.

Example 2 (fig. 53):

a display formed from each display pixel comprising three sub-pixels. As described above, each sub-pixel is preferably a variable wavelength LED device with a controllable peak emission wavelength.

During operation of the display device, the sub-pixels switch from emitting one peak wavelength to a different peak wavelength per unit time, as shown in fig. 53. The peak emission wavelength of any given subpixel is changed by changing the magnitude of the drive current supplied to that subpixel. The time units may be one display frame or less such that multiple time units occur in a single display frame.

The peak wavelength at which high current densities are required for emission can lead to significant localized heating, which can affect the performance and reliability of the device and the overall display.

The advantage of this method of controlling the sub-pixels is that the heat generation of the sub-pixels is more evenly distributed over the display pixels and hot spots on specific sub-pixels are avoided.

In the four time units shown in fig. 53, the B and R sub-pixels are shown as being swapped, but if the G sub-pixel is a variable wavelength LED, the G sub-pixel may also switch the emission wavelength. Any switching combination of the invention is possible simply by varying the drive current supplied to the individual sub-pixels.

Example 3 (fig. 54A and 54B):

Fig. 54A schematically illustrates a pixel of a display device formed of pixels including four sub-pixels. As described above, each subpixel is preferably a variable wavelength LED device that emits light with a controllable peak wavelength, although in the illustrated embodiment, the B and R subpixels may alternatively be provided by conventional blue and red LEDs.

The display comprises a plurality of sub-pixels of the same color (e.g. red, green or blue, as each of these "colors" is often considered to extend over a range of wavelengths of the visible spectrum), the plurality of sub-pixels being configured to emit at different peak wavelengths within that color.

For example, in a preferred embodiment, the pixel "color" may be: 400nm-450nm (purple); 450nm-500nm (blue); 500nm-570nm (green); 570nm to 590nm (yellow), 590nm to 610nm (orange) or 610nm to 700nm (red). Where there are multiple sub-pixels in a given "color," these sub-pixels may all be configured to emit at a wavelength in one of these ranges.

In the preferred embodiment shown in fig. 54A, the display pixel comprises two green sub-pixels having different peak wavelengths.

The advantage of this pixel design is that it expands the color gamut of the display without the need to switch individual LED sub-pixels between G sub-pixel modes of operation (as required in fig. 52A-52D)

In the embodiment shown in fig. 54A and 54B, the peak emission wavelength of the green (G) subpixel G1 is different from the peak emission wavelength of the green subpixel G2.

By extension, there may (additionally or alternatively) be two R or two B pixels with different peak wavelengths. For example, the pixel may have two red subpixels R1 and R2 with different peak emission wavelengths in red. And/or the pixel may contain two blue sub-pixels B1 and B2 with different peak emission wavelengths in the blue.

In all cases, the LED devices may preferably have the same diode structure (N, active area, P) and only the driving current density is used to control the peak wavelength of the emission. This is applicable to all embodiments. Thus, the two green sub-pixels G1 and G2 may be identical to each other in structure, for example, but driven at different driving current densities. The difference in drive current density causes G1 to emit at a different peak emission wavelength than G2.

Example 4 (fig. 55A-55D):

fig. 55A schematically illustrates a pixel of a display device comprising two sub-pixels, wherein each sub-pixel is a variable wavelength LED device with a controllable peak wavelength of emitted light.

Both sub-pixels operate with controllable emissions corresponding to a particular observed chromaticity.

The two sub-pixels may preferably be controllable to emit at a peak emission wavelength of 450nm to 630nm by varying the drive current supplied to the sub-pixels. Fig. 55B illustrates the chromaticity achievable by a pixel when the peak emission wavelength of two variable wavelength sub-pixels varies from 450nm to 620 nm.

For any set of observed chromaticity points that may be displayed by two subpixels, all chromaticity that exists on a straight line between the points may be displayed, as shown in fig. 55C.

The effective color gamut of the display is defined by a line corresponding to the chromaticity of the LED device when the peak emission wavelength is controlled between 450nm and 630nm and a straight line between the highest peak emission wavelength and the lowest peak emission wavelength, as shown in fig. 55D.

Example 5 (fig. 56):

Fig. 56 illustrates a pixel of a display formed of pixels including two sub-pixels. As described above, each subpixel is preferably a variable wavelength LED device having a controllable peak emission wavelength, although in the illustrated embodiment, the R subpixel may alternatively be provided by a conventional red LED.

In fig. 56, one sub-pixel has controllable emission between a range of wavelengths, for example 450nm to 530nm (B to G). The other sub-pixel operating at a fixed emission wavelength corresponding to a fixed observed chromaticity, e.g. Even though the R sub-pixel is a variable wavelength LED, the peak emission wavelength of the R sub-pixel may be fixed by providing a drive current having a fixed magnitude, which corresponds to emitting a desired current density at a desired red wavelength.

This is particularly effective because a higher drive current is required to achieve blue light emission, which results in an increased radiation flux, whereas the human eye is less sensitive to blue light, so as the peak wavelength moves between about 530nm to about 450nm, the change in luminous flux from the green-blue pixels will be less than expected, thereby achieving a more efficient display, while reducing control complexity.

Example 6 (fig. 57):

Fig. 57 illustrates a pixel of a display formed from a pixel comprising three sub-pixels, wherein each sub-pixel is a variable wavelength LED device that emits light with a controllable peak wavelength.

The subpixel SP1 is configured to emit light having a wavelength between 440nm and 480nm by changing the magnitude of a driving current supplied to the SP1 during use. The sub-pixel SP2 is configured to emit light having a wavelength between 500nm and 540nm by varying the magnitude of the driving current supplied to the SP2 during use. The sub-pixel SP3 is configured to emit light having a wavelength between 580nm and 620nm by changing the magnitude of the driving current supplied to the SP3 during use. Accordingly, SP1, SP2, and SP3 operate as blue (B), green (G), and red (R) sub-pixels, respectively, and by changing the magnitude of the driving current of each sub-pixel, the peak emission wavelength of each sub-pixel can be varied within an emission wavelength range of 40 nm.

R, G and B can each be operated in multiple modes, where the peak wavelength can be switched 20nm below or 20nm above the center wavelength.

In combination, each mode can display a different color gamut

As shown in fig. 57, the effective color gamut of the display is larger than the effective color gamuts of R, G and B sub-pixels with fixed emission wavelengths.

Summarizing example 6 (fig. 58):

fig. 58 illustrates a pixel of a display formed from a pixel comprising three sub-pixels, wherein each sub-pixel is a variable wavelength LED device that emits light with a controllable peak wavelength.

Each variable wavelength sub-pixel is controllable to emit over a wide emission wavelength range such that each sub-pixel can operate as a red, green or blue sub-pixel depending on the drive current provided to the sub-pixel. The subpixels are controllable to operate in multiple modes with different peak wavelengths.

In combination, each mode can display a different color gamut.

As shown in FIG. 58, in any mode, R, G and B sub-pixels can achieve the effective color gamut of the display.

For each chromaticity within the achievable color space, there is a continuous range of primary color combinations that can display that chromaticity.

When controlling the display device, it is preferable to select which combination is selected by calculating the efficiency of each combination and selecting the highest efficiency.

Calculation of efficiency may advantageously take into account:

-light emission efficiency from each sub-pixel at a specific peak emission wavelength;

-the amount of light emission required for each sub-pixel to achieve the selected chromaticity and luminance;

-light extraction efficiency from the active area of the LED device to the observer;

-efficiency of delivering power from the display device driver to the LED device.

Time color control

In a typical display, different colors cannot be shown at the same location. Thus, each light emitting area (pixel) is divided into individually addressable monochromatic areas (sub-pixels). When viewed from a distance, the color seen is a spatial combination of sub-pixel colors. The sub-pixels emit a fixed chromaticity that is different from the other sub-pixels.

By adjusting the proportional amount of light emitted from each sub-pixel, the observed chromaticity of the pixel is set, as shown in fig. 50A.

Three or more sub-pixels are typically required to enable the display to display a wide range of colors (large gamut). Reducing the number of sub-pixels is beneficial for reducing cost and complexity, but doing so affects the achievable color gamut.

In some prior art display technologies not all color sub-pixels may be created from the same material, thus requiring significant cost and complexity to combine and arrange sub-pixels formed from different semiconductor materials and form the pixels into a display.

Sub-pixels with different emission properties will have different efficiencies, resulting in non-uniform local heating.

To achieve a particular observed chromaticity, a subpixel with higher efficiency will have a shorter drive time ("on time") than a subpixel with lower efficiency. Systems with large numbers of subpixels that do not emit light most of the time are inefficient.

By incorporating the variable wavelength LED of the present invention into a display device, a display is provided in which each pixel can emit a wide range of colors.

Each pixel is composed of a plurality of sub-pixels. However, in some embodiments, a single variable wavelength LED may be used to form a pixel having only one sub-pixel, as shown in fig. 59A.

The peak wavelength and chromaticity of the light emitted from the sub-pixels depend on the drive current. By selecting the drive signal at which the drive current varies during a display frame, the observed chromaticity of the pixel is determined by the temporal combination of colors sequentially emitted by one or more sub-pixels.

Changing the duty cycle of the sub-pixels is used to achieve gray scale control for each chromaticity.

The present invention may advantageously provide a display with less than three sub-pixels that may display a wide color gamut, which has significant advantages over prior art displays.

Benefits and all advantages over existing solutions of the present invention:

fewer subpixels reduces complexity and cost

The pixels can always emit light, increasing the system efficiency and thus the observed brightness

The pixels are heated during high current operation, and operating with a low drive current cools the pixels

Example 7:

Fig. 59A illustrates a preferred embodiment in which the pixels of the display device consist of a single variable wavelength LED. As described above, by varying the drive current supplied to the pixel, the peak emission wavelength of the LED can be varied over a wide emission wavelength range, so that the device pixel can emit a color from blue to red.

In the case where there is a single pixel, the duty cycle control is used to access different gray levels. The duration of the current pulse applied to the pixel can be varied to control the observed brightness of the pixel.

In the preferred embodiment shown in fig. 59B and 59C, the display device is configured to provide three discrete drive current modes to the LED pixels. The power supply may be operated to provide either of I _{Blue color}、I_{Green colour} or I _{Red color} to the LED at any time.

The color of a pixel observed by a viewer is the temporal combination of light emitted by the pixel during a display frame. In fig. 59B, each of the three drive modes supplies one third of the display frame, so the observed pixel color will be an equal mix of blue, green and red wavelengths.

Fig. 59C illustrates an alternative control mode in which each of the three drive modes supplies one sixth of the display frames. Since the duty cycles of each drive current are still equal to each other, the observed pixel color will be an equal mix of blue, green and red wavelengths, exactly as in fig. 59B. However, since the sub-pixels have a lower total on-time in fig. 59C, the brightness of the pixel will be lower when using the control mode of fig. 59B.

Fig. 60A-60D show three drive current patterns I _{Blue color}、I_{Green colour} or I _{Red color} provided to the LED at the same overall duty cycle, but in a different order. In the presence of a single pixel, the four drive scheme variations shown in fig. 60A-60D give equivalent results, as the observer experiences a time average.

Example 8:

in some preferred embodiments, the pixel may include a plurality of variable wavelength sub-pixels that may be configured to emit at different peak emission wavelengths in response to different drive currents. For example, in response to a change in the drive current provided to a single subpixel, the single subpixel may be driven to emit from a blue to red wavelength.

Fig. 61A illustrates a display device pixel having two sub-pixels, each of which is a variable wavelength LED that emits in a wavelength range encompassing blue to red wavelengths.

The driving scheme a in fig. 61B illustrates a driving scheme of an upper sub-pixel and a different driving scheme of a lower sub-pixel. The upper and lower sub-pixels are each driven in a blue driving mode for a quarter of a display frame, in a green driving mode for a quarter of a display frame, and in a red driving mode for half of the display frame. Since the pixel color observed by the viewer is the temporal combination of light emitted by the pixel during the display frame, both the upper and lower sub-pixels will appear to emit the same color of light.

The drive scheme B in fig. 61C illustrates an alternative drive scheme that uses shorter current pulses to achieve the same result as drive scheme a. Although the current pulses are shorter, at the same total duty cycle, the upper and lower subpixels are still driven in blue, green, and red modes, so the observed colors are the same.

In all of these driving schemes, the pixel color produced is a spatial and temporal combination of the emission color.

Example 9:

In some preferred embodiments, the pixel may comprise a plurality of sub-pixels, at least one of which is a variable wavelength sub-pixel that is configurable to emit at different peak emission wavelengths in response to different drive currents. For example, in response to a change in the drive current provided to a single subpixel, the single subpixel may be driven to emit from a blue to red wavelength.

In some particularly preferred embodiments, each pixel may comprise two sub-pixels, wherein:

One subpixel operating in a fixed color

O one subpixel changes color

An advantage of this arrangement is that the less efficient sub-pixels may have a longer "on-time".

Fig. 62 illustrates a display device pixel having two sub-pixels. The upper sub-pixel is a variable wavelength LED with an emission wavelength range covering the blue to green wavelength, while the lower sub-pixel is a red sub-pixel. The red subpixel may be a conventional red LED subpixel incorporated into a display device, but preferably the red subpixel is a variable wavelength LED with an emission wavelength range encompassing the red wavelength.

Fig. 62 shows that during a display frame, the upper sub-pixel may be driven by sequentially applying driving current pulses in the blue mode I _{Blue color} and the green mode I _{Green colour}, and the red lower sub-pixel may be driven by applying continuous driving current in the red driving mode I _{Red color}.

This arrangement may advantageously provide simplified RGB pixels.

The indicated emission colors are by way of example only, as sub-pixels having a fixed emission wavelength other than red may likewise be provided, and sub-pixels having emission wavelengths controllable in a range other than blue to green may be provided.

Example 10:

in some preferred embodiments, the pixel may comprise a plurality of sub-pixels, which are variable wavelength sub-pixels configured to emit at different peak emission wavelengths in response to different drive currents.

For example, in the embodiment shown in fig. 63, a single upper subpixel is drivable to emit at wavelengths from blue to red in response to a change in drive current provided to the subpixel. The other lower subpixel is drivable to emit at a wavelength from green to red in response to a change in drive current provided to the subpixel.

In some particularly preferred embodiments, each pixel may comprise two sub-pixels, wherein a first variable wavelength sub-pixel is drivable to emit wavelengths within a first range in response to a change in drive current provided to the sub-pixel (e.g., drive current is controlled within the first drive current range); and the second variable wavelength sub-pixel is drivable to emit wavelengths within a second range in response to a change in drive current provided to the sub-pixel (e.g., drive current is controlled within the second drive current range). Preferably, the first wavelength range covers a different wavelength range than the second wavelength range, although the first wavelength range may overlap with the second wavelength range.

In this embodiment, both sub-pixels change color.

An advantage of this arrangement is that a higher drive current that generates significant local heating is followed by a lower drive current that does not generate local heating, allowing the sub-pixels to maintain a more stable temperature.

Fig. 64A is a schematic illustration of the drive current conditions of a variable wavelength LED according to the present invention. Within a single display frame, the drive current is activated in three different non-zero modes. The duty cycle (drive current pulse relative to the duration of the display frame) of each mode is different and is individually controllable.

The current gain of the drive current determines the wavelength produced by the variable wavelength sub-pixel, while the duty cycle of the drive current determines the gray level produced by the sub-pixel.

The length of the display frames may be varied to correspond to any predetermined frame rate. The length of the display frame may be controlled by controlling the LED driving conditions provided by the power supply.

In the case where a single LED forms a pixel, the duty cycle control is used to access different gray levels.

The following embodiments illustrate various possible driving conditions that may be used to control a display device incorporating one or more variable wavelength LED sub-pixels according to the present invention.

These control methods advantageously allow dynamic pixel tuning of the LEDs in the display device.

Example 11 (fig. 65):

The single pixel may be one or more sub-pixels, and each sub-pixel may change color according to image information (the sub-pixel may display a color from blue to red)

Each pixel can determine its own color and generate image information by modulating the signal

The pixel color and brightness are combinations of the duty cycle and amplitude of the signal pulses

Example 12 (fig. 66):

A single pixel may be three or more sub-pixels (colors from blue to red may be displayed).

The sub-pixel sizes will be the same.

Each sub-pixel may determine its own color by modulating the signal, and the image information of each pixel may be determined by combining a plurality of sub-pixels.

Pixel color and brightness are combinations of signal pulse duty cycle and amplitude.

In the presence of a single pixel, these drive scheme variations are equivalent because the observer experiences a time average

Example 13 (fig. 67):

The chip size is changed to adjust the magnitude of the current and pulse width.

Example 14 (fig. 68):

The chip size is optimized to achieve the same drive pattern for each subpixel.

Example 15 (fig. 69):

Some of the subpixels may be dedicated to generating a particular color by signal modulation, while other subpixels may change color (may display colors from blue to red) as needed for image information.

The sub-pixels may be the same or different in size.

Spectral reconstruction

In the prior art, this is achieved by:

Modulating the intensity of one or more illumination sources with a fixed broadband emission spectrum. This is inherently inefficient due to the nature of the subtraction.

Combining emissions from multiple narrowband emission sources. This provides limited tunability due to the fixed number of emission sources.

Modifying light from a high power broad spectrum light source using a filter. This is affected by inherent inefficiency and limited tunability due to the fixed number of tuning elements.

As shown in fig. 70-79, one aspect of the present invention relates to the use of a variable wavelength LED emission source in which the emission wavelength can be continuously tuned by controlling the applied current.

The variable wavelength LED source may be sequentially driven with a plurality of different current pulses. Different current pulses may have different magnitudes or amplitudes. The time at which a particular current pulse is applied may be different from the time at which other current pulses are applied to the variable wavelength LED.

Because the emission wavelength emitted by a variable wavelength LED is related to the driving conditions applied to the LED at any given time, each applied current pulse having a different amplitude will produce a different peak emission wavelength.

In a preferred embodiment, the total length of pulses applied at different current amplitudes is faster than the response time of the detector (the response time of the human eye as detector is about 50 milliseconds). In this case, the emission spectrum perceived by the detector (preferably the human eye) will be a time average of the spectrum emitted by the variable wavelength LED, i.e. the time average of the emission spectrum created by each current pulse.

The current control may be performed using analog or digital pulse forms.

Fig. 70-72 illustrate examples of "digital pulses" comprising a sequence of discrete current pulses.

Fig. 70 illustrates the emission spectrum of a variable wavelength LED covered with lines indicating five discrete drive currents I ₁-I₅ according to a preferred aspect of the present invention. Each of the drive currents I ₁-I₅ is different in magnitude from the other drive currents. The intersection between the overlay line I ₁-I₅ and the LED emission spectrum indicates the peak emission wavelength of the variable wavelength LED emitted in response to each of the discrete drive currents I ₁-I₅.

Fig. 71 illustrates an exemplary sequence of current pulses applied to a variable wavelength LED. Each of I ₁-I₅ has its own discrete magnitude (amplitude) and therefore produces its own discrete peak emission wavelength when applied to a variable wavelength LED. Thus, the order in which the pulses are applied to the LEDs, as well as the duration of each current pulse, will determine the total emission spectrum produced by the LEDs over the time of a given display frame. The sequence and duration of the current pulses may be controlled to obtain a variety of different perceived emission spectra.

Fig. 72 illustrates a combination of five discrete emission spectra having different peak emission wavelengths, which corresponds to the five spectra produced by the five current pulses of fig. 70. The total output spectrum seen by the observer is a combination of these five discrete spectra, as indicated by the "output" line in fig. 71.

Example a:

Fig. 73A-73D illustrate spectral reconstruction using multiple current set points (digital pulses) using the variable wavelength LEDs described above.

N current set points (n=5 in the example shown in fig. 73A) are selected over the tuning range of the LEDs (the range of peak emission wavelengths that can be emitted by the variable wavelength LEDs).

The target spectrum (as shown in fig. 73B) is reconstructed as a linear combination of the LED emission spectra at the selected current set point (as shown in fig. 73C). The intensity of each component peak wavelength is converted to a time (pulse duration) for each of the n set currents to account for the LED emission brightness at each set current, resulting in a digital pulse pattern completed over the duration of the display frame (as shown in fig. 73D). In use, the pulse pattern is repeated as long as the target emission spectrum is to be displayed until a change in the desired output spectrum occurs. At this point, the LEDs may be driven with different pulse patterns to produce different perceived spectra.

Example B (fig. 74-77):

In case the number of current setpoints is large (→infinity), spectral reconstruction using analog current pulses can be considered.

To reconstruct a particular desired emission spectrum, the total current pulses may be calculated such that the total emitted light for each wavelength within the tuning range of the variable wavelength LED matches the desired target emission spectrum.

Fig. 74 shows an exemplary target emission spectrum.

To produce the target emission spectrum of fig. 74, an analog current pulse may be used to drive a variable wavelength LED having the emission characteristics shown in fig. 75, where the amplitude of the current pulse varies over time. Fig. 76 shows an illustrative example of an "analog pulse" of drive current whose amplitude varies during a display frame. When such analog pulses are used to drive a variable wavelength LED, the LED will produce different peak emission wavelengths as the amplitude of the drive current pulses changes within the display frame, resulting in an output spectrum as shown in fig. 77.

Similar to the digital pulses in embodiment a described above, the target spectrum shown in fig. 74 is reconstructed as a temporal combination of the LED emission spectra, with the analog drive pulses exhibiting a very high number (n→infinity) of constituent drive currents.

Example C (fig. 78):

One or more variable wavelength light emitting diodes may provide a large area spectrally tunable illumination source for use as a hyperspectral light source or as general illumination. This can be achieved using the concepts of embodiments a and B described above.

The generated spectrum may be a narrow spectrum or a broad spectrum depending on the driving conditions applied to the one or more light emitting diodes.

The illumination spectrum may be fixed between illumination frames (by keeping the driving conditions the same, or repeating the same driving pulse during each display frame) or modified between frames (by modifying the pulse pattern/sequence or pulse shape between frames), where the time of the illumination frames is shorter than the response time of the detector used.

This may be particularly useful for applications requiring a specific controlled spectrum, including medical imaging, phototherapy, professional lighting, agricultural techniques.

Example D (fig. 79):

in one aspect of the invention, one or more spectrally tunable pixels may be provided for a spectrally corrected display, wherein each individual pixel (or each individual sub-pixel) behaves as described in embodiments A or B

As shown in fig. 79, a plurality of wavelength-variable light emitting diodes ("tunable" LEDs) may be arranged in an array to form a display device. Each variable wavelength LED is preferably arranged to form a pixel (or sub-pixel) of the display device. Each pixel of the display device may be configured to receive its own drive current from a pulse current source configured to generate a current pulse (either a digital pulse train as described with respect to embodiment a or an analog pulse as described with respect to embodiment B). Preferably, the drive current supplied to each pixel is controllable independently of the drive current applied to the other pixels. The pulse current source may be configured to provide driving current pulses to a multiplexer connected to the respective pixels in the display device.

Current pulses can be designed for two main applications:

Accurately representing the spectrum of the image being reconstructed; or alternatively

The non-uniformities that occur during the manufacturing process are corrected by adjusting the current pulses of each sub-pixel so that they emit the same emission wavelength and intensity across the display. The tuned display is then driven with the adjusted current pulses, where the emissions from two or more sub-pixels combine to produce a perceived color for each pixel.

Pixel size and geometry

The embodiments set forth below illustrate aspects of the invention in terms of pixels comprising two sub-pixels having different areas and different emission wavelengths. However, those skilled in the art will appreciate that pixels having other numbers of sub-pixels may likewise be provided using the same principles.

Examples E to H use the same variable wavelength LED structure described in the present application. By controlling the light emitting area of the LED mesa during fabrication, the current density experienced by the mesa can be controlled. As described above, since the current density determines the emission wavelength of a given LED, by controlling the light emitting area of the LED mesa, different sub-pixels having the same diode structure can be caused to emit at different wavelengths in response to the same driving current.

Fig. 80A is a plan view of a display pixel having two variable wavelength sub-pixels with the same diode structure but different sizes that emit at different peak emission wavelengths in response to the same fixed magnitude drive current. Thus, independently variable wavelength LEDs having diode structures of different areas but otherwise identical (identical layered LED structure of n-type layer(s), active layer and p-type layer) will emit at different peak emission wavelengths in response to the same absolute drive current. In the illustrated embodiment, the larger sub-pixels emit at a red peak emission wavelength λ ₁, while the smaller sub-pixels experience a higher current density and emit at a shorter green peak emission wavelength λ ₂. The red and green sub-pixels together form a combined red-green (RG) pixel package that can be integrated onto the device backplane driver, as shown in fig. 80B.

In fig. 80B, a combined red-green (RG) pixel package array is mounted on the backplane driver integrated circuit with a blue LED sub-pixel array. Each red-green (RG) pixel package is combined with a blue subpixel to form an RGB pixel of the display device.

Example E:

The LED geometry is designed to achieve multiple wavelengths at a specified current density.

For a given absolute current applied to an LED (or LED sub-pixel), the current density experienced by the LED will depend on the area of the LED. The peak emission wavelength of a variable wavelength LED may be varied by controlling the current density applied to the LED. Thus, the size, area, and geometry of the LED will affect the current density produced by any given absolute current, and thus the peak emission wavelength of the emission.

For example, for a display device configured to apply a fixed drive current I, providing multiple LEDs with light emitting layers of different areas will result in the LEDs emitting at different peak emission wavelengths. Although the absolute drive current applied to different LEDs may be the same, different surface areas of the LEDs will mean that the different LEDs experience different drive current densities, which will result in emissions at different peak emission wavelengths.

Therefore, even when different LED sub-pixels have the same diode structure (the same arrangement and composition of semiconductor layers), by changing the light emitting areas of the different sub-pixels, the sub-pixels can be configured to emit at different peak emission wavelengths even when all of the sub-pixels receive the same absolute drive current.

By providing a display device configured to apply more than one fixed drive current to a plurality of LEDs, the diversity of peak emission wavelengths that the LEDs can emit can be increased. For example, the drive current is switchable between two different drive current modes such that each LED sub-pixel is drivable to emit at two different peak emission wavelengths. The absolute value of the peak emission wavelength is determined by the light emitting area of each sub-pixel and the magnitude of the driving current in the driving current mode.

Using the drive current density versus wavelength curve shown in fig. 81 (for an exemplary variable wavelength LED), the current density ρ required for a given peak emission wavelength λ can be derived.

Thus, the pixel surface area for each wavelength can be calculated using the following formula:

LED mesas having the corresponding area required for the desired peak emission wavelength may then be etched into the LED wafer, connecting a plurality of LEDs having different emission wavelengths to a common substrate.

Fig. 82A and 82B illustrate an exemplary device pixel comprising two sub-pixels having the same diode structure but different areas that emit at different peak emission wavelengths in response to the same drive current.

In the example shown in fig. 82A and 82B, the first sub-pixel mesa (mesa 1) forms a first sub-pixel having a first area a ₁, and the second sub-pixel mesa (mesa 2) forms a second sub-pixel having a second area a ₂. The same drive current may be applied to both sub-pixels, but the difference in sub-pixel area means that both sub-pixels will experience different current densities. These different current densities will drive the two sub-pixels to emit light of different wavelengths, even though the first sub-pixel and the second sub-pixel have the same diode structure.

The pixels of fig. 82A and 82B are preferably fabricated by growing LED structures on a semiconductor template containing porous regions of group III nitride material. The LED structure may be grown as a uniform diode structure across the semiconductor wafer.

The wafer level LED structure may then be processed into a multicolor display device, or into a plurality of multicolor LED pixels, to be separated and integrated into the display device, as described below.

To provide LED pixels, each LED pixel contains two LED sub-pixels that emit at mutually different peak emission wavelengths in response to a fixed drive current I ₁, two desired peak emission wavelengths lambda ₁、λ₂ are selected. The drive current densities ρ ₁ and ρ ₂ required for each subpixel to produce the desired peak emission wavelength are calculated based on the characteristic relationship between the peak emission wavelength and the drive current density of the grown LED structure. Based on the fixed drive current I ₁ to be used to drive the display, the above listed formula is used to calculate the LED area a ₁、A₂, which will result in two sub-pixels experiencing the drive current densities ρ ₁ and ρ ₂.

Conventional semiconductor etching techniques are used to etch the LED structure into a plurality of pixels, where each pixel contains two mesas of the LED diode structure that form two discrete LED sub-pixels: a first mesa having an area a ₁ that will experience a drive current density ρ ₁ when a drive current I ₁ is applied, and will therefore emit at a desired peak emission wavelength λ ₁; and a second mesa having an area a ₂ that will experience a drive current density ρ ₂ when a drive current I ₂ is applied, and will therefore emit at a desired peak emission wavelength λ ₂.

The drive currents I ₁ and I ₂ provided to the two sub-pixels may be the same drive current, or they may be different in magnitude. The drive currents I ₁ and I ₂ may be drive currents of fixed magnitude such that the sub-pixels act as fixed wavelength LEDs in use. Alternatively, the drive currents I ₁ and _I2 may be variable-magnitude drive currents, such that the variable-wavelength sub-pixels may be driven to emit in different wavelength ranges.

Since the two LED sub-pixel mesas are grown on the same substrate, both are on a shared n-type substrate. The n-contact and p-contact are applied according to conventional methods so that the two sub-pixels can be driven separately.

Although the above description relates to a pixel comprising two sub-pixels, the invention is applicable to a pixel comprising any number of sub-pixels and the same process can be used to form an LED pixel comprising a different number of sub-pixels.

Example F:

the LED geometry is designed to produce a given emission intensity at a given emission wavelength.

Using the drive current density versus wavelength curve shown in fig. 81 (for the exemplary variable wavelength LED), the required drive current density (ρ ₁、ρ₂) is selected to achieve the two desired emission wavelengths (λ ₁、λ₂).

The relationship between the emission efficiency and the emission wavelength is known for a given LED diode structure, or its characteristics can be easily determined, as shown in fig. 83.

The emission efficiency at a given wavelength is given by ηi.

In this embodiment:

1. selecting a wavelength λ and for determining a drive current density ρ;

2. The emission efficiency η at λ (or ρ) is used to scale the emission area a of the sub-pixels to achieve a desired output density (emission intensity);

3. the required drive current can be found from the p x area.

Thus, once the emission wavelength is selected, the current density is fixed and then the subpixel area is scaled according to the efficiency at that wavelength in order to achieve the total flux (intensity) required for the subpixel.

Using this approach, it can be ensured that the sub-pixels emit not only at the desired wavelength, but also at the desired emission intensity.

At the desired emission wavelength (lambda ₁、λ₂) of the two sub-pixel pixels, the required drive current density (p ₁、ρ₂) for emission at the desired emission wavelength (lambda ₁、λ₂) is known. The emission efficiency at these wavelengths is given by η _i. By considering the emission efficiency, the required light emitting area (a _i) for the two sub-pixels to emit at the desired emission intensity is calculated.

From the subpixel area (a) and the drive current density, the total drive current (I _i) required to give the desired emission intensity is calculated.

As described above with respect to embodiment E, the mesa may be etched into the LED structure to give a mesa with the correct light emitting area (a ₁、A₂) that will emit at the desired peak emission wavelength (lambda ₁、λ₂). Since the two sub-pixels are etched from the same LED structure, the two sub-pixels have the same diode structure, but differ in area (footprint-area of the sub-pixel is the surface area of the mesa when viewed from above).

Since the drive current (Ii) required to give the desired emission intensity has been calculated for each sub-pixel separately, the sub-pixel will emit at the desired emission intensity whenever the drive current is applied.

The drive current (Ii) applied to each individual subpixel may optionally be different. This can advantageously compensate for differences in emission efficiency at different emission wavelengths. Thus, a first subpixel (e.g., mesa 1 in fig. 82A and 82B) may be driven by a first drive current I ₁ to produce the desired emission intensity for the first subpixel, while a second subpixel (e.g., mesa 2 in fig. 82A and 82B) may be driven by a second drive current I ₂ to produce the desired emission intensity for the second subpixel.

Example G:

LED geometry design to produce a given luminosity at a given emission wavelength

Using the drive current density versus wavelength curve shown in fig. 81 (for the exemplary variable wavelength LED), the required drive current density (ρ ₁、ρ₂) is selected to obtain the two desired emission wavelengths (λ ₁、λ₂) from the two individual LED sub-pixels.

The emission efficiency is given by η _i at the design drive current density required to emit at the desired emission wavelength.

This can then be combined with a photopic luminosity function. The photopic luminance functions of the different wavelength emissions are known for a given LED diode structure or can be readily characterized as shown in the exemplary pixel of fig. 84.

From this, the total current (I _i) required for the LED sub-pixel to achieve the desired emission luminosity can be calculated.

Finally, from this required current (I _i) and current density, the above formula can be used to calculate the mesa area of the LED sub-pixel so that the requirements are met and the sub-pixel will emit light at the desired peak emission wavelength and the desired luminosity when the drive current is applied.

Similar to the above-described embodiments, by etching the wafer into a plurality of sub-pixel mesas having mesa areas corresponding to the desired sub-pixel areas, a plurality of LED sub-pixels having different areas (footprints) can be formed in the same semiconductor wafer.

Example H:

LED geometry optimized for concentric emission at two or more wavelengths.

Fig. 85A and 85B illustrate an exemplary embodiment of a two-subpixel device pixel, wherein the two subpixels are provided in the form of a ring-shaped subpixel 1, the ring-shaped subpixel 1 being concentrically located around a circular subpixel 2. Both sub-pixels are arranged on a shared conductive substrate and connected to a common n-contact, but sub-pixels 1 and 2 are connected to separate p-contacts, through which the sub-pixels are bonded to the driver backplane IC.

Fig. 85A and 85B show two concentric sub-pixels, however the number of emission wavelengths may be as desired, with each subsequent sub-pixel forming a ring centered on the smallest sub-pixel.

The additional degrees of freedom in controlling the device layout in a single layer rather than mass transfer allows:

1. Correction of chromatic aberration of the imaging optical system; and

2. And generating multicolor pixels with the same apparent center.

The above principle can be used to calculate the area required for different sub-pixels to emit light of the desired wavelength. Once these sub-pixel areas are known, the sub-pixels can be formed in a variety of shapes, with a variety of sub-pixel geometries. For example, in the above example, the subpixels may be formed as concentric rings around one another. This concentric arrangement can be converted into a display device by etching concentric circles with the desired footprint into an LED wafer containing a shared n-type conductive layer, and then flipping the wafer before bonding the p-contacts to connect each individual subpixel to the driver back plate IC. The driver backplane IC is configured to apply a drive current or a plurality of different drive currents to each sub-pixel such that the discrete sub-pixels can be controlled independently of each other.

Claim (modification according to treaty 19)

1. A variable wavelength Light Emitting Diode (LED), comprising:

An n-doped portion;

A p-doped portion;

Wherein the LED is configured to receive a power source, wherein the peak emission wavelength of the LED is continuously controllable over an emission wavelength range of at least 40nm by varying the power source.

2. The LED of claim 1, wherein the LED is a dynamic color tunable LED, wherein the peak emission wavelength of the LED is tunable by varying a driving condition provided to the LED by the power supply.

3. The LED of claim 1 or 2, wherein the LED is drivable to emit at a single peak emission wavelength in response to a stable power supply and to emit at different peak emission wavelengths in response to a change in the power supply.

4. An LED according to any preceding claim, wherein the n-doped portion, the p-doped portion and the light emitting region each comprise or consist of a group III nitride material, preferably GaN, inGaN, alGaN or AlInGaN.

5. An LED according to any preceding claim, wherein the variable wavelength LED comprises a single epitaxially grown diode structure comprising an n-doped portion, the p-doped portion and the light emitting region.

6. An LED according to any preceding claim wherein the light emitting diode comprises a porous region of group III nitride material.

7. The LED according to claim 6, wherein one of the n-doped portion or the p-doped portion contains a porous region of the group III nitride material.

8. The LED of claim 6, wherein the n-doped portion; the p-doped portion; and the light emitting region is disposed on a substrate comprising a porous region of the group III nitride material.

9. An LED according to any preceding claim wherein the light emitting region comprises a Multiple Quantum Well (MQW) containing a plurality of Quantum Wells (QWs) or quantum dots, quantum wires and other quantum nanostructures.

10. An LED according to any preceding claim wherein the light emitting region comprises a plurality of Quantum Wells (QWs), and wherein the quantum wells are continuous.

11. The LED of any of claims 1 to 10, wherein the light emitting region is configured to contain carrier localization centers in quantum wells, such as a plurality of QWs with different indium compositions, and/or quantum barriers with different compositions, and/or QWs with fluctuations in well width, and/or InGaN quantum dots or nanostructures contained in the light emitting region, and/or quantum wells formed on polar, semi-polar or non-polar faces.

12. An LED according to any preceding claim wherein the light emitting region comprises a plurality of Quantum Wells (QWs), and wherein the quantum wells are non-uniform, fragmented or discontinuous.

13. The LED of claim 11 or 12, wherein the plurality of QWs comprises fluctuations in well width or alloy composition.

14. The LED of claim 13, wherein the well width of the QW fluctuates by at least 2%, 5%, 10%, 20%, 25% or 50% or 75%, or wherein the indium content of the QW varies by at least 2%, 5%, 10%, 20%, 25% or 50% or 75% throughout the light emitting region.

15. The LED according to any one of claims 11 to 14, wherein the LED comprises v-shaped pits extending or propagating through the light emitting area, preferably wherein the LED comprises a plurality of v-shaped pits extending through the light emitting area.

16. The LED of claim 15, wherein the LED comprises a v-shaped pit density of at least 1 x 10 ⁷/cm², such as at least 5 x 10 ⁷/cm² or at least 1 x 10 ⁸/cm², such as 1 x 10 ⁷/cm² to 5 x 10 ⁹/cm².

17. The LED of claim 15 or 16, wherein the LED comprises a v-shaped pit density of less than 5 x 10 ⁹/cm², such as a v-shaped pit density of less than 1 x 10 ⁹/cm² or less than 5 x 10 ⁸/cm².

18. An LED according to any preceding claim wherein the emission wavelength of the LED is continuously variable over the emission wavelength range in response to a drive condition provided by the power supply being continuously variable over the drive range.

19. An LED according to any preceding claim, wherein the peak emission wavelength is controllable over an emission wavelength range of at least 50nm, or at least 60nm, or at least 70nm, or at least 80nm, preferably over a range up to 100nm or 110nm or 120nm or 140nm, or 160nm, or 180nm, or 200nm, or 400nm or 450nm, by varying the power supply.

20. An LED according to any preceding claim, wherein the peak emission wavelength is controllable by varying the power supply between 400nm and 850nm, or between 400nm and 800nm, preferably between 430nm and 675nm, or between 520nm and 660nm, or between 550nm and 650 nm.

21. An LED according to any preceding claim, wherein the LED is controllable to emit at least three discrete peak emission wavelengths by varying the driving conditions provided by the power supply.

22. An LED according to any preceding claim, wherein the LED is controllable to emit at a first peak emission wavelength in response to a first driving condition provided by the power supply, at a second peak emission wavelength in response to a second driving condition provided by the power supply, and at a third peak emission wavelength in response to a third driving condition provided by the power supply.

23. An LED according to any preceding claim, wherein the LED is controllable to emit a blue peak emission wavelength in response to a first driving condition provided by the power supply, a green peak emission wavelength in response to a second driving condition provided by the power supply, and a red peak emission wavelength in response to a third driving condition provided by the power supply.

24. An LED according to any preceding claim, wherein the LED is controllable to emit a first peak emission wavelength in the range 400nm-500nm in response to a first driving condition provided by the power supply, a second peak emission wavelength in the range 500nm-550nm in response to a second driving condition provided by the power supply, and a third peak emission wavelength greater than 600nm in response to a third driving condition provided by the power supply.

25. An LED according to any preceding claim, wherein the LED is controllable to emit a first peak emission wavelength in the range 430nm-460nm in response to a first driving condition provided by the power supply, a second peak emission wavelength in the range 510nm-560nm in response to a second driving condition provided by the power supply, and a third peak emission wavelength in the range 600nm-660nm in response to a third driving condition provided by the power supply.

26. The LED of any preceding claim, wherein the LED is configured to receive a drive current, and wherein the peak emission wavelength of the LED is continuously controllable over an emission wavelength range of at least 40nm by varying the magnitude of the drive current provided to the LED.

27. The LED according to any one of claims 23-25, wherein the first, second, and third driving conditions are a first current density, a second current density, and a third current density.

28. An LED according to any preceding claim wherein the peak emission wavelength of the LED is controllable by varying the current density of the power supplied to the LED.

29. The LED of claim 28, wherein the peak emission wavelength decreases in response to an increase in the current density of the power supply.

30. The LED of claim 28 or 29, wherein the LED emits at a first peak emission wavelength when the power supply has a first current density, and emits at a second peak emission wavelength longer than the first emission wavelength when the power supply has a second current density lower than the first current density.

31. The LED of claim 28, 29 or 30, wherein the first peak emission wavelength is below 570nm and the second peak emission wavelength is above 610nm, such that the LED emits green light in response to the first current density and red light in response to the second current density.

32. The LED of claim 30 or 31, wherein the LED emits at a third peak emission wavelength when the power supply has a third current density.

33. An LED according to any preceding claim, wherein the LED is drivable by a current density of from 0.001A/cm ² to 1000A/cm ².

34. The LED of any preceding claim, wherein each driving condition provided to the LED by the power supply has a duty cycle of at least 0.001%, 0.01%, 0.1%, 1%, 5%, 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90% or 100%.

35. The LED of any one of claims 29 to 34, wherein the LED emits light having a wavelength of 570nm or less in response to a current density of greater than 3A/cm ², or greater than 5A/cm ², or greater than 7A/cm ², or greater than 9A/cm ², or greater than 10A/cm ², or greater than 11A/cm ².

36. The LED of any one of claims 29 to 35, wherein the LED emits light having a wavelength greater than 610nm in response to a current density of less than 4A/cm ², or less than 3A/cm ², or less than 2A/cm ².

37. The LED of any one of claims 29 to 36, wherein the LED emits light having a wavelength between 430nm and 500nm in response to a current density of greater than 19A/cm ², or greater than 20A/cm ², or greater than 21A/cm ².

38. An LED according to any preceding claim, wherein the light emitting layer is an indium gallium nitride layer.

39. An LED according to any one of claims 6 to 38 when dependent on claim 6, wherein the porous region has a thickness of at least 1nm, preferably at least 10nm, particularly preferably at least 50nm.

40. An LED according to any one of claims 6 to 39 when dependent on claim 6, wherein the LED comprises a connection layer of group III nitride material between the n-doped portion and the porous region, preferably wherein the connection layer has a thickness of at least 10nm.

41. The LED of claim 40, comprising a non-porous intermediate layer of group III nitride material between the porous region and the connection layer.

42. The LED of any preceding claim, wherein the light emitting region is formed over a porous region comprising a stack of alternating porous and non-porous layers of group III nitride material.

43. An LED according to any preceding claim, wherein the light emitting layer comprises one or more quantum wells, preferably 1 to 7 quantum wells.

44. An LED according to any preceding claim, wherein the light emitting layer is a nanostructure layer comprising quantum structures such as quantum dots, fragmented or discontinuous quantum wells.

45. An LED according to claim 43 or 44 wherein the light emitting layer is a light emitting indium gallium nitride layer and the light emitting indium gallium nitride layer and/or the quantum well has a composition of In _xGa_1-x N, wherein 0.05 +.x +.0.40, preferably 0.12 +.x +.0.35 or 0.20 +.x +.0.30 or 0.22 +.x +.0.30.

46. An LED according to any of claims 6 to 45 when dependent on claim 6, wherein the porous region is a continuous porous region, or an alternating porous region with non-porous regions, or is part of a distributed bragg reflector, or is not part of a Distributed Bragg Reflector (DBR).

47. The LED of any preceding claim, wherein the lateral dimension of the light emitting region is greater than 50nm, 100nm, 200nm, 300nm, 500nm, 600nm, 700nm, 800nm, 900nm, 1 μιη,2 μιη,3 μιη,4 μιη,5 μιη or 10 μιη, 20 μιη,30 μιη, 40 μιη, 50 μιη, 60 μιη, 70 μιη, 80 μιη, 90 μιη or greater than 100 μιη or 200 μιη, 300 μιη and less than 1000 μιη.

48. The LED according to any preceding claim, wherein the lateral dimension of the light emitting region is between 100 μιη to 200 μιη, or wherein the lateral dimension of the light emitting region is less than 100 μιη, or less than 75 μιη, or less than 50 μιη, or less than 30 μιη, or less than 25 μιη, or less than 10 μιη, or less than 5 μιη, or less than 2 μιη.

49. The LED of any preceding claim, wherein the shape of the LED is selected from: circular, triangular, rectangular, square, oval, diamond, hexagonal, pentagonal, and any combination of these shapes.

50. An LED array comprising a plurality of LEDs according to any preceding claim.

51. A display device comprising the variable wavelength LED of any one of claims 1 to 49 configured to receive a drive current from a power supply.

52. The display device of claim 51, comprising a plurality of variable wavelength LEDs according to any one of claims 1 to 49, each variable wavelength LED configured to receive its own power supply, wherein each of the plurality of LEDs is controllable such that the peak emission wavelength of each LED is controllable by varying the power supply to the LED.

53. The display device of claim 51 or 52, comprising a power supply configured to provide a drive current to the variable wavelength LED or the plurality of variable wavelength LEDs.

54. A method of controlling a variable wavelength LED, the method comprising the steps of:

providing power to the variable wavelength LED of any one of claims 1 to 49; and

The power supply is controlled to vary the peak emission wavelength of the variable wavelength LED over the emission wavelength range.

55. The method of claim 54, wherein the power supply is controlled to vary the peak emission wavelength over an emission wavelength range of at least 40nm, or at least 50nm, or at least 60nm, or at least 70nm, or at least 80nm, preferably over a range up to 100nm or 110nm or 120nm or 140nm, or 160nm, or 180nm or 200 nm.

56. The method of claim 54 or 55, wherein the power supply is controlled to vary the peak emission wavelength between 400nm and 800nm, preferably between 520nm and 660nm, or between 550nm and 650nm, or between 560nm and 680nm by varying the power supply.

57. The method of any one of claims 54 to 56, comprising the steps of providing a drive current to the variable wavelength LED and varying the magnitude of the drive current to vary the peak emission wavelength of the variable wavelength LED over the emission wavelength range.

58. The method of any of claims 54 to 57, wherein the power supply is controlled to vary the peak emission wavelength of an LED by varying the current density provided to the LED.

59. The method of claim 58, wherein the current density provided by the power supply is increased so as to decrease the peak emission wavelength of the LED, or wherein the current density provided by the power supply is decreased so as to increase the peak emission wavelength of the LED.

60. The method of claim 58 or 59, wherein the power supply is controlled to supply a first current density at which the LEDs emit at a first peak emission wavelength, and the power supply is controlled to supply a second current density lower than the first current density such that the LEDs emit at a second peak emission wavelength longer than the first emission wavelength.

61. The method of claim 60, wherein the first peak emission wavelength is below 570nm and the second peak emission wavelength is above 610nm such that the LED emits green light in response to the first current density and red light in response to the second current density.

62. The method of any one of claims 54 to 61, wherein the power supply operates in a Pulse Width Modulation (PWM) mode and/or a Pulse Amplitude Modulation (PAM) mode.

63. The method of any one of claims 54-62, wherein the power source is a pulsed power source, or wherein the power source is a Continuous Wave (CW) or near-continuous wave power source.

64. The method of any one of claims 54 to 63, wherein the power supply is a constant voltage power supply, or wherein the power supply is a constant current power supply.

65. The method of any of claims 54 to 64, wherein the amplitude of the power supply varies between at least two non-zero values during one display frame.

66. The method of any of claims 54 to 65, wherein the amplitude of the power supply varies between a plurality of discrete non-zero values during a display frame.

67. The method of any one of claims 54 to 66, wherein the power supply is controlled to provide a current density of 0.001A/cm ² to 1000A/cm ² to the LED.

68. The method of any one of claims 54 to 67, wherein the power supply is controlled to provide a drive current to the LED with a duty cycle of at least 0.001%, 0.01%, 0.1%, 1%, 5%, 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90% or 100%.

69. The method of any one of claims 54 to 68, comprising the steps of: providing a first drive current such that the LED emits at a first peak emission wavelength; and providing a second drive current having a different magnitude than the first drive current such that the LED emits at a second peak emission wavelength.

70. The method of claim 69, wherein the first drive current is provided to the LED at a first duty cycle and the second drive current is provided to the LED at a second duty cycle.

71. A method according to claim 70, comprising the step of controlling the duration of the first and/or second duty cycles so as to control the observed luminance and/or chrominance produced by the display device.

72. The method of any one of claims 69 to 71 including the step of providing a third drive current having a different amplitude to the first and second drive currents such that the LED emits at a third peak emission wavelength.

73. The method of claim 72, comprising the step of providing the third drive current to the LED at a third duty cycle.

74. A method according to claim 73, comprising the step of controlling the duration of the third duty cycle so as to control the observed luminance and/or chrominance produced by the display device.

75. A method of fabricating a variable wavelength LED, comprising the steps of growing:

An n-doped portion;

A p-doped portion; and

76. The method of claim 75, comprising the step of overgrowing the n-doped portion, the p-doped portion, and the light emitting region over a porous region of a group III nitride material.

77. A method as claimed in claim 75 comprising the steps of forming a porous region of group III nitride material in at least one of the n-doped portion or the p-doped portion, and forming the light emitting region over the porous region of group III nitride material.

78. A method according to claim 75, 76 or 77 comprising the step of forming the light emitting region with a carrier localised centre in a quantum well such as a plurality of types of QW regions with different indium composition and well width and quantum barriers, non-uniform or fragmented or broken or bandgap or discontinuous quantum wells, quantum dots or nanostructures, quantum wells formed on polar, semi-polar or non-polar faces etc. which will cause fluctuations in well width.

79. The method of claim 78, wherein the light emitting region comprises a plurality of Quantum Wells (QWs), and wherein the quantum wells are non-uniform, fragmented, or discontinuous.

80. The method of claim 78 wherein the plurality of QWs comprises fluctuations in the indium composition and/or fluctuations in trap width.

81. A method as claimed in claim 78 comprising the step of forming one or more v-shaped dimples in the LED structure such that the v-shaped dimples extend through the light emitting area, preferably forming a plurality of v-shaped dimples extending through the light emitting area.

82. The method of claim 81, comprising the step of forming at least 0.1 v-shaped pits per square micron, or at least 1 v-shaped pit per square micron, or at least 2 v-shaped pits per square micron.

83. The method of claim 81, comprising the step of forming a v-shaped pit density of at least 1 x 10 ⁷/cm², such as at least 5 x 10 ⁷/cm² or at least 1 x 10 ⁸/cm², such as 1 x 10 ⁷/cm² to 5 x 10 ⁹/cm², in the light emitting region.

84. The method of claim 81, comprising the step of forming a v-shaped pit density in the light emitting area of less than 5 x 10 ⁹/cm², such as less than 1 x 10 ⁹/cm² or less than 5 x 10 ⁸/cm².

Claims

1. A variable wavelength Light Emitting Diode (LED), comprising:

An n-doped portion;

A p-doped portion;

A light emitting region between the n-doped and p-doped portions, the light emitting region comprising a light emitting layer that emits light at a peak emission wavelength under an electrical bias thereacross;

2. The LED according to claim 1, wherein the LED is a dynamic color tunable LED, wherein the peak emission wavelength of the LED is tunable by varying a driving condition provided to the LED by the power supply.

3. The LED according to claim 1 or 2, wherein the LED is drivable to emit at a single peak emission wavelength in response to a stable power supply and to emit at different peak emission wavelengths in response to a change in the power supply.

8. The LED according to claim 6, wherein the n-doped portion; the p-doped portion; and the light emitting region is disposed on a substrate comprising a porous region of the group III nitride material.

11. The LED according to any of claims 1 to 10, wherein the light emitting region is configured to contain carrier localization centers in quantum wells, such as a plurality of QWs with different indium compositions, and/or quantum barriers with different compositions, and/or QWs with fluctuations in the well width, and/or InGaN quantum dots or nanostructures in the light emitting region, and/or quantum wells formed on polar, semi-polar or non-polar faces.

13. An LED according to claim 11 or 12 wherein the plurality of QWs comprises fluctuations in well width or alloy composition.

14. An LED according to claim 13 wherein the well width of the QW fluctuates by at least 2%, 5%, 10%, 20%, 25% or 50% or 75%, or wherein the indium content of the QW varies by at least 2%, 5%, 10%, 20%, 25% or 50% or 75% throughout the light emitting region.

15. The LED according to any of claims 11 to 14, wherein the LED comprises v-shaped pits extending or propagating through the light emitting area, preferably wherein the LED comprises a plurality of v-shaped pits extending through the light emitting area.

16. The LED according to claim 15, wherein the LED comprises a v-shaped pit density of at least 1 x 10 ⁷/cm², such as at least 5 x 10 ⁷/cm² or at least 1 x 10 ⁸/cm², such as 1 x 10 ⁷/cm² to 5 x 10 ⁹/cm².

17. The LED according to claim 15 or 16, wherein the LED comprises a v-shaped pit density of less than 5 x 10 ⁹/cm², such as a v-shaped pit density of less than 1 x 10 ⁹/cm² or less than 5 x 10 ⁸/cm².

20. An LED according to any preceding claim, wherein by varying the power supply the peak emission wavelength is controllable between 400nm and 850nm, or between 400nm and 800nm, preferably between 430nm and 675nm, or between 520nm and 660nm, or between 550nm and 650 nm.

24. An LED according to any preceding claim, wherein the LED is controllable to emit a first peak emission wavelength in the range 400nm to 500nm in response to a first driving condition provided by the power supply, a second peak emission wavelength in the range 500nm to 550nm in response to a second driving condition provided by the power supply, and a third peak emission wavelength greater than 600nm in response to a third driving condition provided by the power supply.

25. An LED according to any preceding claim, wherein the LED is controllable to emit a first peak emission wavelength in the range 430nm to 460nm in response to a first driving condition provided by the power supply, a second peak emission wavelength in the range 510nm to 560nm in response to a second driving condition provided by the power supply, and a third peak emission wavelength in the range 600nm to 660nm in response to a third driving condition provided by the power supply.

26. An LED according to any preceding claim, wherein the LED is configured to receive a drive current, and wherein the peak emission wavelength of the LED is continuously controllable over an emission wavelength range of at least 40nm by varying the magnitude of the drive current provided to the LED.

27. The LED according to any one of claims 23 to 26, wherein the first driving condition, the second driving condition, and the third driving condition are a first current density, a second current density, and a third current density.

28. An LED according to any preceding claim, wherein the peak emission wavelength of the LED is controllable by varying the current density of the power supply provided to the LED.

29. The LED according to claim 28, wherein the peak emission wavelength decreases in response to an increase in the current density of the power supply.

30. The LED according to claim 28 or 29, wherein the LED emits at a first peak emission wavelength when the power supply has a first current density, and emits at a second peak emission wavelength longer than the first emission wavelength when the power supply has a second current density lower than the first current density.

31. An LED according to claim 28, 29 or 30 wherein the first peak emission wavelength is below 570nm and the second peak emission wavelength is above 610nm such that the LED emits green light in response to the first current density and red light in response to the second current density.

32. The LED according to claim 30 or 31, wherein the LED emits at a third peak emission wavelength when the power supply has a third current density.

34. An LED according to any preceding claim, wherein the duty cycle of each driving condition may be at least 0.001%, 0.01%, 0.1%, 1%, 5%, 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90% or 100%.

35. An LED according to any one of claims 29 to 34 wherein the LED emits light having a wavelength of 570nm or less in response to a current density of greater than 3A/cm ², or greater than 5A/cm ², or greater than 7A/cm ², or greater than 9A/cm ², or greater than 10A/cm ², or greater than 11A/cm ².

36. An LED according to any one of claims 29 to 35 wherein the LED emits light having a wavelength greater than 610nm in response to a current density of less than 4A/cm ², or less than 3A/cm ², or less than 2A/cm ².

37. An LED according to any one of claims 29 to 36 wherein the LED emits light having a wavelength between 430nm and 500nm in response to a current density of greater than 19A/cm ², or greater than 20A/cm ², or greater than 21A/cm ².

38. An LED according to any preceding claim, wherein the light emitting layer is a light emitting indium gallium nitride layer.

39. An LED according to any preceding claim, wherein the porous region has a thickness of at least 1nm, preferably at least 10nm, particularly preferably at least 50nm.

40. An LED according to any preceding claim, wherein the LED comprises a connection layer of group III nitride material between the n-doped portion and the porous region, preferably wherein the connection layer has a thickness of at least 10nm.

41. The LED according to claim 40, comprising a non-porous intermediate layer of group III nitride material between the porous region and the connection layer.

42. An LED according to any preceding claim, wherein the light emitting region is formed over a porous region comprising a stack of alternating porous and non-porous layers of group III nitride material.

45. The LED according to claim 43 or 44, wherein the light emitting layer is a light emitting indium gallium nitride layer and the light emitting indium gallium nitride layer and/or the quantum well has a composition of In _xGa_1-x N, wherein 0.05.ltoreq.x.ltoreq.0.40, preferably 0.12.ltoreq.x.ltoreq.0.35 or 0.20.ltoreq.x.ltoreq.0.30 or 0.22.ltoreq.x.ltoreq.0.30.

46. An LED according to any preceding claim wherein the porous region is a continuous porous region, or is an alternating porous region with non-porous regions, or is part of a distributed bragg reflector, or is not part of a Distributed Bragg Reflector (DBR).

47. An LED according to any preceding claim, wherein the lateral dimensions of the light emitting region are greater than 50nm, 100nm, 200nm, 300nm, 500nm, 600nm, 700nm, 800nm, 900nm, 1 μm, 2 μm, 3 μm, 4 μm, 5 μm or 10 μm, 20 μm, 30 μm, 40 μm, 50 μm, 60 μm, 70 μm, 80 μm, 90 μm or greater than 100 μm or 200 μm, 300 μm and less than 1000 μm.

48. An LED according to any preceding claim, wherein the lateral dimension of the light emitting region is between 100 μm and 200 μm, or wherein the lateral dimension of the light emitting region is less than 100 μm, or less than 75 μm, or less than 50 μm, or less than 30 μm, or less than 25 μm, or less than 10 μm, or less than 5 μm, or less than 2 μm.

49. An LED according to any preceding claim, wherein the shape of the LED is selected from: circular, triangular, rectangular, square, oval, diamond, hexagonal, pentagonal, and any combination of these shapes.

51. A display device comprising a variable wavelength LED according to any one of claims 1 to 49, the variable wavelength LED being configured to receive a drive current from a power supply.

52. The display device of claim 51 comprising a plurality of variable wavelength LEDs as claimed in claims 1 to 49, each variable wavelength LED configured to receive its own power supply, wherein each of the plurality of LEDs is controllable such that the peak emission wavelength of each LED is controllable by varying the power supply to the LED.

53. The display device of claim 51 or 52, comprising a power supply configured to provide a drive current to the plurality of variable wavelength LEDs.

Providing power to a variable wavelength LED according to any one of claims 1 to 49; and

55. The method according to claim 54, wherein the power supply is controlled to vary the peak emission wavelength over an emission wavelength range of at least 40nm, or at least 50nm, or at least 60nm, or at least 70nm, or at least 80nm, preferably over a range of up to 100nm or 110nm or 120nm or 140nm, or 160nm, or 180nm or 200 nm.

56. The method according to claim 54 or 55, wherein the power supply is controlled to vary the peak emission wavelength between 400nm and 800nm, preferably between 520nm and 660nm, or between 550nm and 650nm, or between 560nm and 680nm by varying the power supply.

57. The method of any one of claims 54 to 56, comprising the steps of providing a drive current to said variable wavelength LED, and varying the magnitude of said drive current to vary said peak emission wavelength of said variable wavelength LED over said emission wavelength range.

58. The method of any one of claims 54 to 57, wherein the power supply is controlled to vary the peak emission wavelength of an LED by varying the current density provided to the LED.

59. The method of claim 58, wherein the current density provided by the power supply is increased to decrease the peak emission wavelength of the LED, or wherein the current density provided by the power supply is decreased to increase the peak emission wavelength of the LED.

62. The method according to any one of claims 54 to 61, wherein the power supply operates in a Pulse Width Modulation (PWM) mode and/or a Pulse Amplitude Modulation (PAM) mode.

63. The method of any one of claims 54 to 62, wherein the power source is a pulsed power source, or wherein the power source is a Continuous Wave (CW) or near continuous wave power source.

65. The method of any of claims 54 to 64, wherein during a display frame, the amplitude of the power supply varies between at least two non-zero values.

66. The method of any of claims 54 to 65, wherein during a display frame, the amplitude of the power supply varies between a plurality of discrete non-zero values.

69. The method according to any one of claims 54 to 68, comprising the steps of: providing a first drive current such that the LED emits at a first peak emission wavelength; and providing a second drive current having a different magnitude than the first drive current such that the LED emits at a second peak emission wavelength.

71. The method of claim 70, comprising the step of controlling the duration of said first duty cycle and/or said second duty cycle so as to control the observed luminance and/or chrominance produced by said display device.

72. A method as in any of claims 54-71, comprising the step of providing a third drive current having a different magnitude than the first drive current and the second drive current such that the LED emits at a third peak emission wavelength.

73. The method of claim 72, comprising the step of providing said third drive current to said LED at a third duty cycle.

74. A method according to claim 72 or 73, comprising the step of controlling the duration of said third duty cycle so as to control the observed luminance and/or chrominance produced by said display device.

75. A method of manufacturing a variable wavelength LED comprising the steps of growing:

An n-doped portion;

A p-doped portion; and

A light emitting region between the n-doped and p-doped portions, the light emitting region comprising a light emitting layer that emits light at a peak emission wavelength under an electrical bias thereacross.

76. The method of claim 75, comprising the step of overgrowing said n-doped portion, said p-doped portion, and said light emitting region over a porous region of group III nitride material.

77. The method of claim 75, comprising the steps of forming a porous region of group III nitride material in at least one of the n-doped portion or the p-doped portion, and forming the light emitting region over the porous region of group III nitride material.

78. A method according to claim 75, 76 or 77 comprising the step of forming said light emitting active region having a carrier localised centre in a quantum well such as a plurality of types of QW regions having different indium composition and well width and quantum barriers, non-uniform or fragmented or broken or bandgap or discontinuous quantum wells, quantum dots or nanostructures, quantum wells formed on polar, semi-polar or non-polar faces etc. which will result in fluctuations in well width.

79. The method of claim 178 wherein said light emitting region comprises a plurality of Quantum Wells (QWs), and wherein said quantum wells are non-uniform, fragmented, or discontinuous.

80. The method of claim 78 wherein said plurality of QWs comprises fluctuations in said indium composition and/or fluctuations in well width.

81. A method according to claim 78, comprising the step of forming one or more v-shaped dimples in said LED structure such that said v-shaped dimples extend through said light emitting area, preferably forming a plurality of v-shaped dimples extending through said light emitting area.

83. A method according to claim 81, comprising the step of forming a v-shaped pit density of at least 1 x 10 ⁷/cm², such as at least 5 x 10 ⁷/cm² or at least 1 x 10 ⁸/cm², such as 1 x 10 ⁷/cm² to 5 x 10 ⁹/cm², in said light emitting region.

84. A method according to claim 81, comprising the step of forming a v-shaped pit density in said light emitting area of less than 5 x 10 ⁹/cm², such as a v-shaped pit density of less than 1 x 10 ⁹/cm² or less than 5 x 10 ⁸/cm².