KR20090099537A - Pyramidal photonic crystal light emitting device - Google Patents

Abstract

A light-emitting device (LED) is described which comprises a light generating layer disposed between first and second layers of semiconductor material, each having a different type of doping. An upper surface of the first layer has either a tiling arrangement of pyramidal or frustro-pyramidal protrusions of semiconductor material surrounded by a material of different refractive index, or it has a tiling arrangement of inverted pyramidal or inverted frustro-pyramidal indentations in the semiconductor material filled by a material of different refractive index. Each comprises a photonic band structure. The protrusions or indentations and their tiling arrangement are configured for efficient extraction of light from the device via the upper surface of the first layer and in a beam with an emission profile that is substantially more directional than from a Lambertian source. An enhanced device employs a reflector beneath the second layer to utilise the microcavity effect. Methods for fabricating the device are also described, which employs anisotropic wet etching to produce the pyramidal protrusions or the inverted pyramidal indentations.

Description

Pyramidal Photonic Crystal Light Emitting Device

The present invention relates to light emitting diodes having improved light extraction and directivity, and more particularly to devices using photonic crystal structures.

Light emitting diodes (hereinafter referred to as' LED's) are based on forward-biased pn junctions, which have recently reached sufficiently high luminance levels, and now light emitting diodes are the only new solid state lighting applications. It is also suitable as a substitute for projector light sources. In addition to the economic benefits obtained through the high efficiencies of the LEDs, the entry into these markets has also been made possible by the reliability, long life and environmental advantages. Specifically, applications of solid state lighting require LEDs to surpass the efficiencies currently obtainable by alternative fluorescent lighting techniques.

Back Light Units for Liquid Crystal Display (hereinafter referred to as "LCD") panels are important factors in the performance of LCD panels. Currently, most LCD panels use compact cathode fluorescent light (CCFL) light sources. However, these light sources are problematic due to several problems, including poor color gamut, environmental recycling and manufacturing issues, thickness and profile, high voltage requirements, inferior thermal management, weight and high power consumption. Have To alleviate these problems, LCD manufacturers are implementing LED BLU units. These offer advantages in many areas, including color range, low power consumption, thin profiles, low voltage requirements, good thermal management and low weight.

Current LED BLU systems place LEDs across the back of the LED as disclosed in US Pat. No. 7,052,152. In the case of small displays, these are generally low cost designs. However, in the case of large LCD panels, eg, 32 inch or larger LCD panels, the number of LEDs required to distribute light evenly across the back of the LCD panel no longer makes this approach cost effective. can not do it. In US Pat. No. 7,052,152, it is proposed that 124 LEDs be used for a 32 inch LCD panel display.

For some applications, a more isotropic or Lambertian light distribution from the LED is required, one of the techniques used to achieve this is to roughen the surface where light is collected. Some roughness is inherent in the LED manufacturing process. However, techniques such as etching can be used to obtain a controlled degree of increased surface roughness to improve scrambling and uniformity of the light coming from the LEDs.

US Patent Application Publication Nos. 2006/0181899 and 2006/0181903 disclose optical light guides or waveguides arranged such that diffused light is evenly distributed across the entire LCD panel surface. Light is coupled into the light path using side-mounted LED light sources. Side mounting is advantageous because the number of LEDs required per unit area of the backlight LCD is significantly reduced compared to direct LED backlighting. It is proposed that the 32-inch LCD panel can be side illuminated with as few as 12 LED light sources. However, in order to maximize the light diffused onto the LCD panel, it is desired to optically couple the LED light into the light path as much as possible.

Another application area for high brightness LEDs is light engines for front projectors and rear projectors. Conventional High Intensity Discharge (HID) type projector light engines have always been hampered by low efficiency and short lifespan, which has led to sluggish adoption in consumer markets. In this particular application, the etendue value of the light source is required to be less than or equal to the etendue value of the microdisplay. This compatibility is very important to improve the overall system efficiency of the entire light projection engine. In addition, high total luminous outputs and low power consumption are also very important, especially in applications where large rear projection screens (greater than 50 inches) and front projection systems are required. Low power consumption is desirable to minimize thermal management issues. Light from a set of red, green and blue monochromatic LED light sources with small etendue values is multiplexed to produce a predetermined color in the projection system. This eliminates the need for color wheels and associated additional costs. The etendue value E is calculated according to Equation 1 below.

[Equation 1]

Here, E is etendue of the light source, A is the surface of the light emitting element, and α is the half angle of the light source. Therefore, in the case of projection applications, the degree of collimation of the light source is a key factor, and it can be seen that reducing the half angle of the light source greatly improves the overall efficiency of the light engine.

The overall efficiency of an LED can be quantified by three main factors: internal quantum efficiency, implantation efficiency and extraction efficiency. One of the major limitations in reducing light extraction efficiency from LEDs is the total internal reflection of the emitted photons and the trapping of photons in the high refractive index epi material forming the LED. These trapped waveguide modes propagate within the LED structure until they are scattered or reabsorbed. The thickness of the LED structure determines the number of modes that can be supported.

US Patent Nos. 5,779,924 and 5,955,749 disclose the use of photonic crystal patterns defined on semiconductor layers of LEDs that affect the path of light propagation through the epi structure. The formed optical band structure allows the trapped modes to be extracted outward, thereby improving the extraction efficiency of the LED, and ultimately the total external efficiency. The use of photonic crystal structures in LEDs is beneficial over other light extraction techniques, where they can be applied proportionally depending on the active surface area of the LED, and thus light extraction for large area high brightness LED dies. This is because it provides an ideal means to improve. In solid state lighting applications where absolute light output is required, the scaling characteristics of the LED size are important. However, total light extraction for many of these photonic crystal LEDs is not as high as many conventional surface-roughened LEDs.

US Patent No. 6,831,302 and US Patent Application Publication No. 2005/0285132 disclose processes for manufacturing light emitting diodes with photonic crystal structures using gallium nitride (GaN) based materials. In both cases, the process includes many complex and expensive steps that ultimately affect the yield and cost of LED wafers. Specifically, US Pat. No. 6,831,302 discloses growing an n-GaN layer, an active quantum well (QW) region and a p-GaN layer on a lattice matched single crystal wafer, followed by Eutectic bonding of the sub-mount or substrate on the surface, flipping the wafer, lifting off from the growth wafer (using techniques such as laser lift-off), optical Polishing the surface (using a process such as chemical mechanical polishing) to provide a smooth surface, by a process such as nano-imprinting, lithography, or holography. ) Defining the photonic crystal pattern on the surface, and finally transferring the photonic crystal pattern to the GaN material using a suitable dry etching (eg RIE or ICP) or wet etching of GaN. It discloses a manufacturing process including the step of.

One of the complex process steps involved is the polishing of the wafers, which can negatively affect the yield since it is difficult to control the surface quality over the entire wafer. Small scratches all over the surface can affect the current spreading across the LED wafer, which in turn can provide a path that shorts the finished LED or negatively affects the forward voltage. In addition, the thickness of the GaN epi structure is important for the effective design of the photonic crystal extraction pattern. The high index of refraction acts like a highly multimode waveguide, whereby the thickness determines the number of modes present in the LED heterostructure. Using a polishing process results in insufficient control over the absolute thickness of the LED structure, which in turn affects the overall output of the LED wafers from one process batch to another.

Another complex manufacturing step involves small scale first-order photonic crystal features in the range of 300 nm to 500 nm with respect to the pitch of the features, and of the hole features. It is a step of defining in the range of 200nm to 400nm with respect to the diameter. Such patterns are currently defined using nano-imprinting or holography. The former technique is not currently proven for such small scale features on LED wafers, only low yields are achieved. In addition, this technique has a problem due to the high cost for low production quantities. The latter lithography technique has problems due to complex alignment and stability as well as small amounts.

Therefore, there is a need for a new type of surface patterned LED that works better than conventional roughened LED devices or photonic crystal LED devices and can be manufactured in a simple and cost effective manner.

According to a first aspect of the invention, a light emitting device (LED) comprises a first layer comprising a first semiconductor material with a doping of a first type; A second layer comprising a second semiconductor material with a doping of a second type; And a light generating layer disposed between the first layer and the second layer, the first layer having a top surface distal from the light generating layer and a bottom surface close to the light generating layer, Light generated in the light generating layer emerges from the LED structure through the top surface of the first layer, and the first layer is surrounded by a material having a refractive index different from that of the first semiconductor material and from the top surface. And further comprising a tiling arrangement of protruding pyramidal or frustro pyramidal protrusions made of the first semiconductor material, wherein the tiling arrangement of the protrusions and the surrounding material comprise a light band structure; The protrusions and their tiling arrangement are substantially more directional than light exiting the LED structure through the top surface from the Lambertian light source. It is configured to have.

According to a second aspect of the invention, a light emitting device (LED) comprises: a first layer comprising a first semiconductor material with a doping of a first type; A second layer comprising a second semiconductor material with a doping of a second type; And a light generating layer disposed between the first layer and the second layer, the first layer having a top surface far from the light generating layer and a bottom surface close to the light generating layer, wherein the light generating layer Light emitted from the LED structure through the top surface of the first layer, wherein the first layer extends from the top surface toward the light generating layer and has a different refractive index than the first semiconductor material And further comprising a tiling arrangement of inverted pyramidal or truncated inverted pyramidal indentations in the first semiconductor material, wherein the tiling arrangement of the recesses and the surrounding first semiconductor material comprise a light band structure. Wherein the recesses and their tiling arrangement are substantially more directional than light exiting from the LED structure through the top surface than light exiting from a Lambertian light source. Have to be configured.

Lambertian light sources emit luminous flux per unit solid angle that is proportional to the cosine of the angle between that direction and the normal to the surface from which light is emitted. This leads to uniform light emission with a spherical distribution. The light emitting device comprising the structure according to the invention combines the enhanced directionality of the emitted light with the improvement in the efficiency with which the light generated by the device is coupled to the emitted beam. This is achieved through an innovative tiling arrangement of pyramidal or truncated pyramidal surface protrusions, or inverted pyramidal or truncated inverted pyramidal recesses.

Three main types of light tiling arrangements are photonic crystals with short range and long range order, photonic crystals with short range transitional disorder but long range order, and amorphous tiling arrangements with short range spacing order but long range disorder. In the case of amorphous, the spacing between adjacent pyramidal regions is fixed, while rotational symmetry is randomized.

In the present invention, higher order pyramidal or inverted pyramidal photonic crystals or quasi-crystalline patterns are proposed that provide improved light extraction as compared to conventional primary photonic crystal patterns. Careful design of the photonic crystal allows customization of the far-field light pattern produced by the device. Specifically, pyramidal protrusions or inverted pyramid-shaped recesses and their well-defined tiling arrangement with inclined angular sidewalls are obtained from the Lambertian light source, even for patterns with large lattice constants (> 1 μm). Light can be extracted from the LED in the form of a beam that is more parallel than the light.

Preferably, the pyramidal protrusions or inverted pyramidal recesses have a size greater than 1.0 μm. However, they may have a size larger than 1.5 μm or 2.0 μm or larger than 2.5 μm. Relatively large pyramid or inverted pyramid sizes alleviate manufacturing tolerances and also mean that no surface polishing is required prior to the formation of the pyramidal protrusions or inverted pyramidal indents, which are dimensions This is because they are larger than the remaining surface roughness.

Moreover, the pitch of the tiling arrangement is greater than 1.5 μm, but the pitch may be larger than 2.0 μm or 2.5 μm, or larger than 3.0 μm.

In many applications, the protrusions or recesses are preferably configured such that a significant proportion (> 35%) of light is extracted in the central cone with a 30 ° half angle with respect to the vertical axis. Preferably at least 37%, 38% or 40% is extracted from the central cone. This makes it possible to efficiently and uniformly couple light into narrowly extending light paths which are often used as light sources for projection applications.

In another embodiment, the pyramids or inverted pyramids and the tiling pattern can be configured such that the light is prominently extracted at angles greater than this, in a lateral emission manner. For example, light may be emitted in a ring or donut-shaped distribution that becomes parallel around the vertical axis, rather than inside a narrow central cone. In this case, the distribution may be centered at angles greater than or equal to 30 °, 40 °, 50 °, or 60 ° with respect to the vertical, or 60 °, 50 °, 40 °, or 30 with respect to the surface. The same is true for angles less than or equal to °.

The first semiconductor material with the doping of the first type may be doped with n-type or p-type, in which case the second semiconductor material with the doping of the second type is each doped with p-type or will be doped n-type.

The first layer may comprise a layer of etch stop material interposed at a predetermined depth in the first semiconductor material, in which case the protrusions formed of the first semiconductor material may extend from a surface of the etch stop material layer. And the inverted pyramidal recesses will extend down to the etch stop material layer.

In a particularly preferred embodiment, the first semiconductor material comprises GaN or InGaN doped with n-type, and the second semiconductor material includes GaN or InGaN doped with p-type. The light generating layer preferably comprises a multi-quantum well structure of GaN-InGaN. In terms of pyramid diameters, higher order photonic crystal dimensions generally range from 1.0 μm to 3.0 μm, but this is due to factors including wavelength, far field pattern, LED thickness, and location of the multiple quantum well structure within the entire GaN heterostructure. Depends on the scope Suitable etch stop materials include AlGaN and InGaN.

In order to improve light extraction, the LED further comprises an optical reflector for reflecting light propagating away from the top extraction surface of the first layer, which otherwise would not be extracted. will be. The optical reflector is disposed adjacent to the second layer of the second semiconductor material such that the second layer is positioned between the light generating layer and the reflector.

Preferably, the optical reflector comprises a single layer of metallic material, or it may comprise a multilayer dielectric structure. In another embodiment, the optical reflector may comprise a distributed Bragg reflector (DBR) or an omni-directional reflector (ODR).

Preferably, the separation distance between the light generating layer and the optical reflector is to include a micro cavity which enhances the amount of generated light propagating towards the top surface of the first layer. The micro cavity effect results in a much greater improvement in extraction efficiency than provided by simple reflections in the optical reflector. The optimum spacing is between 0.5 and 0.7 times the wavelength of light generated in the light generating layer. If there is a micro cavity, the pyramidal protrusions or the inverted pyramidal recesses and their tiling arrangement are configured to optimally cooperate with the micro cavity effect, to improve the efficiency of light extraction from the LED.

According to a third aspect of the invention, an optical engine for an optical projector unit comprises a plurality of light emitting elements according to the first or second aspect. The above-described light emitting element is particularly suitable for the application of solid state light sources, including light engines for front projectors and rear projectors.

According to a fourth aspect of the present invention, a method of manufacturing a light emitting device according to the first aspect comprises: a first layer comprising a first semiconductor material having a doping of a first type, a second semiconductor having a doping of a second type A second layer comprising a material, and a light generating layer disposed between the first layer and the second layer, the first layer being a top surface far from the light generating layer and a lower portion near the light generating layer Providing a light emitting element heterostructure having a plane, wherein light generated in the light generating layer exits the LED structure through the top surface of the first layer; Forming an etch mask on the first layer, wherein the etching mask comprises islands of mask material at locations corresponding to a predetermined tiling arrangement, depositing a photoresist layer on top of the first layer, Patterning the captive resist layer by exposing according to the predetermined tiling arrangement, and removing the unexposed photo resist to leave islands of photoresist at locations corresponding to the predetermined tiling arrangement. Forming the etch mask on the first layer; Anisotropic wet etch the first semiconductor material to a predetermined depth along predetermined crystal planes, such that a pyramidal or truncated pyramid of the first semiconductor material of the first layer at locations immediately below the islands of the mask material Forming shaped protrusions; And removing islands of the mask material to leave the predetermined tiling arrangement of the pyramidal or truncated pyramidal protrusions, including a predetermined optical band structure in combination with a peripheral material having a different refractive index. do.

According to a fifth aspect of the present invention, a method of manufacturing a light emitting device according to the second aspect includes: a first layer comprising a first semiconductor material having a doping of a first type, a second semiconductor having a doping of a second type A second layer comprising a material, and a light generating layer disposed between the first layer and the second layer, the first layer being a top surface far from the light generating layer and a lower portion near the light generating layer Providing a light emitting device heterostructure having a surface, wherein light generated in the light generating layer exits the LED structure through the top surface of the first layer; Forming an etch mask on the first layer, the etching mask comprising islands of mask material lost at locations corresponding to a predetermined tiling arrangement, depositing a photoresist layer on top of the first layer; Patterning the photoresist by exposing according to a tiling arrangement of the substrate, and removing the unexposed photoresist to leave islands of missing photoresist at locations corresponding to the predetermined tiling arrangement. Forming the etch mask on the first layer; Anisotropically wet etch the first semiconductor material to a predetermined depth along predetermined crystal planes to pyramid or cut the first semiconductor material of the first layer at locations just below the islands of the lost mask material. Forming two pyramidal recesses; And removing the remaining mask material to leave the predetermined tiling arrangement of inverted pyramidal or truncated inverted pyramidal recesses in the first semiconductor material, the recesses being in the peripheral first portion. It includes a material having a refractive index different from that of the semiconductor material, and includes an optical band structure together.

The light emitting device heterostructure itself may be manufactured by any suitable known process, including flip chip processes.

The photoresist layer may itself be the mask layer, in which case the islands of the mask material are the islands of the photoresist. Any suitable process for patterning the photoresist by exposure, including UV lithography, will be used. The exposed photoresist is removed using a suitable developer, and the islands of the remaining exposed photoresist are removed by stripping. Various kinds of etchants, including solutions of KOH, NaOH or H ₃ PO ₄ , can be used for anisotropic wet etching.

In other embodiments, thicker mask materials that are etch-resistant to the anisotropic wet etch may be used. In this case, additional process steps are needed.

In the method of the fourth aspect, forming the etch mask comprises: depositing a hard mask material layer on top of the first layer prior to depositing the photoresist layer; Removing the hard mask material after removing the photo resist to leave islands of hard mask material directly below the islands of the photo resist; And removing the remaining islands of photoresist so as to leave the etch mask comprising islands of the hard mask material at locations corresponding to the predetermined tiling arrangement.

In the method of the fifth aspect, the forming of the etch mask comprises: depositing a hard mask material layer on top of the first layer prior to depositing the photoresist layer; Removing the hard mask material after removing the photo resist to leave islands of missing hard mask material directly below the islands of the missing photo resist; And removing the remaining photoresist islands so as to leave the etch mask including the islands of lost hard mask material at locations corresponding to the predetermined tiling arrangement.

Suitable hard mask materials include SiO ₂ or Si ₃ N ₄ deposited by PECVD or metal deposited by sputtering or evaporation. Once the pyramidal protrusions are formed, the islands of remaining hard mask material are removed by a suitable wet or dry etching process. Similarly, once the inverted pyramidal recesses are formed, the remaining hard mask material surrounding the islands is removed by a suitable wet or dry etching process.

In the method of the fourth aspect, the depth of the anisotropic wet will determine whether cut pyramidal or pyramidal protrusions are formed and their size. However, accurate control of the anisotropic wet depth will be difficult when based on the etching rate and the etching time. Similarly, in the method of the fifth aspect, the depth of the anisotropic etching will determine the size of the formed recesses. In addition, accurate control of the anisotropic etching depth will be difficult when based on the etching rate and the etching time. Moreover, in order to form cut inverted pyramidal recesses, an etch stop is required.

Advantageously, said first layer of said light emitting device heterostructure comprises an etch stop material layer interposed at said predetermined depth in said first semiconductor material. The anisotropic etching of the first semiconductor material will then continue until reaching the etch stop material layer, which provides greater uniformity and repeatability of the manufacturing method. In the method of the fifth aspect, it also allows the formation of inverted pyramidal recesses or truncated pyramidal recesses with flat tops.

Accordingly, the present invention provides a simplified method of manufacturing a photonic crystal type LED structure, wherein the use of low cost photolithography or similar processes, and for large feature photonic crystals, without the need for complex polishing processes Definitions are transferred on the LEDs.

Embodiments of the present invention will be described in more detail below with reference to the accompanying drawings.

1 shows a cross section of the proposed device.

2A shows pyramids arranged in a square grid.

2B shows the pyramids arranged in a twelve fold quasicrystal.

3 shows the normalized luminosity of GaN LEDs as a function of quantum wells for mirror spacing.

4A illustrates a flip chip based manufacturing process for manufacturing a light emitting device.

4I in FIG. 4E shows additional fabrication steps for fabricating the pyramidal photonic crystal structure of the upper layer of the device shown in FIG. 4D.

5A shows the improvement in light extraction at the 30 ° cone for pyramidal photonic crystal LEDs compared to the unpatterned device.

5B shows the improvement in total light extraction for pyramidal photonic crystal LEDs compared to the unpatterned device.

5C shows the ratio of light at a 30 ° cone for different pyramidal photonic crystal light emitting devices.

6A shows a far-field pattern for a photonic crystal LED structure according to a preferred embodiment of the present invention.

Figure 6B shows a far field pattern for a photonic crystal LED structure according to another preferred embodiment of the present invention.

7A shows SEM micrographs of isolated pyramids formed using the manufacturing method of the present invention.

7B shows SEM micrographs of photo quasicrystalline arrangements of pyramids formed using the manufacturing method of the present invention.

FIG. 8A shows the light crystal fill fraction versus light extraction enhancement of conventional photonic crystal LEDs and pyramidal photonic crystal LEDs with micro-cavities when compared to non-patterned LEDs. FIG. to be.

FIG. 8B shows the fraction of photonic crystal charges versus light extraction enhancement of pyramidal photonic crystal LEDs when compared to unpatterned LEDs with microcavities and unpatterned LEDs without microcavities.

FIG. 9 shows the thickness of the LED heterostructure core compared to the light extraction enhancement of the pyramidal photonic crystal LED when compared to the unpatterned LED.

10 shows a cross section of an inverted pyramidal element.

11A illustrates a flip chip based manufacturing process for manufacturing a light emitting device.

11I or 11J in FIG. 11E represent additional fabrication steps for fabricating the inverted pyramid or truncated inverted pyramidal photonic crystal structure of the top layer of the device shown in FIG. 11D.

FIG. 12A shows the improvement in light extraction at a 30 ° cone for a pyramidal photonic crystal LED when compared to an unpatterned device.

12B shows the improvement in total light extraction for pyramidal photonic crystal LEDs as compared to the unpatterned device.

12C shows the ratio of light at a 30 ° cone for different pyramidal photonic crystal light emitting devices.

13 shows a far-field pattern for a photonic crystal LED structure according to a preferred embodiment of the present invention.

14 shows SEM micrographs of isolated inverted pyramids formed using the preparation method of the present invention.

FIG. 15A is a plot of photonic crystal charge fraction vs. light extraction enhancement of conventional photonic crystal LEDs and inverse pyramid type photonic crystal LEDs with microcavities when compared to unpatterned LEDs.

FIG. 15B is a plot of photonic crystal charge fraction versus light extraction enhancement of an inverse pyramid type photonic crystal LED when compared to an unpatterned LED with a micro cavity and an unpatterned LED without a micro cavity. FIG.

It is an object of the present invention to provide tailored far-field emission from light emitting devices as well as improved light extraction. Such devices may use a wide range of light emitting semiconductor materials, including but not limited to InGaN, InGaP, InGaAs, InP or ZnO. The specification will focus on the implementation of the directional light extraction technology implemented in green InGaN light emitting devices. However, the design can be equally optimized or implemented for other emission wavelengths (such as blue or UV), as well as other material systems such as InGaP that are suitable for the red and yellow wavelengths as well.

In a preferred embodiment of the present invention, a novel high-order pyramidal photonic crystal (PC) or quasicrystalline pattern is proposed that provides increased light extraction compared to primary photonic crystal patterns. The photonic crystal design also allows customization of the far field light distribution emitted by the devices. The shape of the pyramid of the photonic crystal sub-regions and their well-defined tiling arrangement enable light to be extracted from the device in the form of a beam that is more parallel than the light from the Lambertian light source. . Simplified processes for manufacturing the device will also be described above.

In terms of pyramid diameter, higher order photonic crystal dimensions may be larger than 1.0 μm in size, but may be larger than 1.5 μm or 2.0 μm and further up to 3.0 μm, 3.5 μm and 4.0 μm, but are not limited thereto. . This size varies with the far field pattern, the LED thickness, and the position of the quantum wells within the light generating region of the GaN heterostructure.

1 illustrates a cross-section of a proposed light emitting device, which is a periodic, quasi-crystalline, amorphous or other complexly aligned or repeated tiling arrangement, with pyramids protruding out of an n-doped GaN or InGaN layer. 101). The pyramidal tiling arrangement is designed to provide scattering bands that allow the trapped light to couple into the Bloch modes of the photonic crystal. If the light is coupled in bleach modes, the photonic crystal provides a means for coupling the light into free space.

The device shown in FIG. 1 comprises a light emitting heterostructure, which includes an n-GaN or InGaN top layer 102 and a bottom p-type GaN or InGaN layer 104 and a multiple quantum well. well, MQW) structure 103 is present between the layers. There is a reflector layer 105 directly below the p-type layer 104, which may be in the form of a metal reflector such as silver or in the form of a DBR or omni-directional reflector (ODR). The light emitting structure is supported by a carrier substrate, i. In a preferred embodiment, the carrier substrate comprises a highly thermally conductive electrically conductive material, such as a metal or metal alloy, or instead silicon or silicon carbide. In another preferred embodiment, there is an etch stop layer 107 formed of, but not limited to, materials such as AlGaN, InGaN, which allows precise control of the depth of the pyramids. Let's do it. The layer is sandwiched between n-type layers 101 and 102.

There are three main kinds of optical tiling arrangements preferred in the present invention, in which the arrangements comprise photonic crystal tiling arrangements having the following characteristics: short range and long range order; Quasi-crystals tiling placement, with short-range translation disorder but long-range order; And an amorphous tiling arrangement, with short-range spacing order and long-range disorder. In the case of an amorphous arrangement, the spacing between adjacent pyramidal and inverted pyramidal regions is fixed, while rotational symmetry is randomized.

This kind of patterning may also include repeated cells, composed of the tiling arrangements described above. They may also include areas with defects, ie areas where pyramids and inverted pyramids have been removed or where the shape or size of the pyramids has been modified. Sub-regions may also include pyramids with sharp apex regions that are not etched, resulting in flat-top (frustro) pyramids.

This patterning can be characterized by a number of parameters, the parameters of which are defined by the distance (a) of the lattice defined by the distance between the centers of two adjacent pyramids or inverted pyramids, and the crystallographically exposed surface of the pyramid. An angle θ formed between the horizontal crystal planes of the GaN lattice. In one aspect of the invention, etch stop layer 107 is used to control the etch depth. This allows precise control of the pyramid diameter at the base. The diameter? Of the bottom of the pyramids for the predetermined etch depth d is given by Equation 2 below.

[Equation 2]

One of the main aspects of the present invention is to provide relaxed position accuracy as well as maximum pattern reproduction accuracy using crystallographically aligned large features.

Hexagonal pyramids or inverted pyramids (formed in c-plane GaN) may be arranged in a constant pattern, semicrystalline pattern, amorphous pattern or other suitable arrangement. 2A shows an example where hexagonal pyramids are arranged in a square grating to form a photonic crystal. FIG. 2B shows pyramids or inverted pyramids arranged in twelve ply symmetric square-triangular quasicrystalline tiling to form a quasi-crystal.

An improved version of the present invention utilizes an optical reflector located somewhere below the light emitting region and reflecting light propagating downwards upwards. Furthermore, the separation distance between the light emitting area and the reflector is designed to improve the upward light emission due to the so-called micro cavity effect. Pyramidal or inverted pyramidal photonic crystals are then optimized with microcavity effects to improve light extraction. As described by Shen in April 7, 2003, Appl. Phys. Lett. 82, 14, 2221, the following equations appear as a function of the separation distance between the quantum well regions and the reflector: , Represents the relative power intensity at the top of the LED.

[Equation 3]

[Equation 4]

Here, related parameters are as follows.

w _o = amplitude of emitted light,

w _r = amplitude of reflected light,

φ = phase change due to reflection in the mirror,

φ ' = Phase change due to the difference in path distance between emitted and reflected light, which varies with the angle of incidence and wavelength in the LED material as well as the separation distance between the microcavity and the quantum wells,

θ = emission angle with respect to the vertical.

To illustrate the micro cavity effect, FIG. 3 shows a plot of the intensity of light obtained from a GaN LED element with a mirror as a function of the distance from the quantum well to the mirror. Luminance is normalized based on the luminosity obtained from GaN LEDs without a mirror, thereby indicating the degree of improvement. For simplicity, it is assumed that the mirror has a reflectance of 100%. When compared to a flat bare GaN light emitting device having no reflector, it is evident that approximately up to about 3.5 times more light is extracted. For the contribution to the reflector alone, this corresponds to about 1.75 times more light extracted due to the microcavity effect. Placing quantum wells at a suitable distance from the mirror is important for obtaining maximum extraction efficiency in the LED.

This microcavity effect also introduces deviations in the far field radiation forms of the LED as compared to the far field radiation forms of the Lambertian light source. In the context of the present invention, increased lobe emission can be optimized in combination with the top surface photonic crystal pattern, thereby improving light extraction and directivity from such LEDs as would be expected by using two extraction techniques separately. To make it possible. This microcavity effect reduces the isotropy of the emission in the heterostructure, thereby causing the light incident internally to the photonic crystal to be more parallel.

Due to the high aspect ratio features provided by the optical band structure and the large dielectric constant, it is possible to effectively overlap the LED waveguide modes with the dispersion bands of the optical band structure and thereby allow strong coupling between them. However, many waveguide modes are set in the case of isotropic emission in thick core LEDs. In this case, the dispersion bands of the photonic crystal cannot be designed to overlap with all trap modes. However, by parallelly aligning light using a micro cavity, the number of set waveguide modes is reduced. As a result, the optical band structure can be optimally designed to effectively extract more dense and fewer modes trapped in such LEDs.

Another aspect of the invention is a simplified method of manufacturing a wide band structure LED of the pyramidal structure type described above. In Figure 4A a possible manufacturing process for 4I is shown. First, an n-type doped GaN or InGaN layer 401 on the lattice matched substrate 410 by metal-organic chemical vapor deposition (MOCVD) or other similar techniques (such as MBE). Grow). Common substrates used are sapphire, GaN and SiC. In a preferred embodiment of the present invention, an etch stop layer 409 formed of a material such as InGaN or AlGaN is interposed in the n-GaN layer. Growth of the n-GaN or InGaN layer continues to layer 408 over the etch stop layer. Multiple GaN-InGaN quantum wells 402 are grown followed by a p-type GaN layer 403. The complete LED heterostructure stack created at this stage is shown in FIG. 4A.

As shown in FIG. 4B, a mirror 404 is then deposited on top of the p-GaN layer 403. The reflector may be a metal comprising a suitable metal layer, such as silver or gold, deposited by sputtering or evaporation. In another embodiment, the reflector may comprise a dielectric multilayer stack in the form of a distributed feedback reflector (DBR) or an omnidirectional reflector (ODR). Such structures can be deposited by sputtering using techniques such as PECVD.

As shown in FIG. 4C, the heterostructure of FIG. 4B is then bonded to the substrate 405. The substrate 405 is preferably a metal alloy because the metal alloy allows for good thermal electrical conductivity. However, the substrate 405 may include other materials such as SiC or Si. Prior to the bonding operation, additional layers may be deposited on the mirror 404 to aid in the bonding process.

In a conventional final fabrication step, the sapphire substrate 410 is then removed using laser lift off or other similar technique to provide the heterostructure shown in FIG. 4D. Although generally lacking an etch stop layer, this structure can form a conventional final finished device. The laser lift off process leaves the surface of n-GaN layer 401 rough (typically at a level of approximately 50 nm to 300 nm). In another conventional device, such surfaces can be made rougher to improve light extraction. In one aspect of the invention, the dimensions of the light band structure pyramids are large compared to the surface roughness, and thus, it is not required to polish the surface prior to patterning.

4I in FIG. 4E shows the additional process steps required to manufacture the device according to the invention, as shown in FIG. 4I.

4E shows the deposition of masking layers. Specifically, optional method steps are shown whereby a hard mask layer 406 is deposited to subsequently transfer a predetermined pattern into the n-GaN layer 401. This may include SiO 2 or Si ₃ N ₄ deposited by PECVD, or it may be a metal deposited by sputtering or evaporation. A photoresist layer 407 is then deposited on the hard mask 406. However, in some circumstances, the hard mask layer may be omitted and the photoresist layer 407 is deposited directly onto the n-GaN layer 401.

Due to the large size of the pyramidal features, photoresist 407 can be exposed using standard UV lithography to pattern it into a predetermined tiling arrangement. The transverse shape of the exposed areas may correspond to the shape of the cross section of certain pyramids, or else may be simpler shapes such as squares. The exposed photoresist is then developed, leaving isolated island-like material corresponding to predetermined locations of the vertices of the pyramids, as shown in FIG. 4F. If hard mask 406 is present, then hard mask 406 is dry etched using RIE, ICP or similar processes. This step transfers the patterning from the photoresist 407 to the hard mask 406, as shown in FIG. 4G. The remaining photoresist 407 is subsequently stripped.

Subsequently, as shown in FIG. 4H, the n-GaN layer 401 is crystallographically wet etched using anisotropic wet etching. A preferred method of wet etching GaN is to use a KOH solution in a concentration range from 1M to 8M with a bath temperature range from room temperature to 100 ° C. The etching time is in the range of about 45 minutes. In another embodiment the wet etchant comprises NaOH or H ₃ PO ₄ .

The crystal planes of the hexagonal pyramids formed by the etching process are {10-1-1} planes of the GaN crystal. They form an angle of 58.4 ° with the bottom of the pyramids. The presence of the etch stop layer 409 allows for accurate adjustment of the heights of the pyramids, and consequently also for accurate adjustment of the bottom diameter of the pyramids. Finally, if hard mask 406 is used, hard mask 406 is then removed using a suitable wet or dry etching process, leaving the final structure shown in FIG. 4I.

7A shows an SEM micrograph of one of the pyramids produced by the process. In this case, the pyramid is in an isolated state and is formed to have the crystal {10-1-1} face shown in 701 using the preferred manufacturing example. FIG. 7B shows an SEM micrograph of the photo quasicrystalline tiling of these pyramids, which were etched at the top surface of n-GaN using preferred fabrication techniques. The pyramids are arranged in square-triangle tiling, and the construction lines shown in FIG. 7B represent the underlying tiling squares and triangles, while the circles highlight the vertices of the quasi-crystalline pattern.

In a preferred embodiment, a composite n-GaN top region comprising layers 401, 409, 408 is positioned over the light emitting structure. Therefore, light is emitted at layer 402 to experience internal multiple reflections before finally escaping through region 401.

Thick n-GaN growth regions are needed to reduce the defect density for the formation of high quality quantum well (QW) layers, thus improving the internal quantum efficiency of the LED. For manufacturing advantages, the upper region (layers 401, 409, 408) acts as a protective layer for the fragile quantum well region 402, preventing damage during wet etching of the pyramids and quantum wells. Minimize surface recombination in the region. In addition, etching into the quantum well region negatively affects the total light output of the LED by reducing the maximum active light emitting area.

In addition, in terms of optical extraction enhancement, the required pyramid dimensions are on the order of about 1.75 μm in diameter centered on a pitch of 2.5 μm. These dimensions limit the minimum thickness of layer 401, so that the pyramids are preferably present in the thick n-GaN layer. In addition, for improved light extraction, reducing the total thickness of the waveguide region of the LED by etching reduces the number of trap modes present in the heterostructure. This allows the optical band structures to overlap with a higher proportion of trap modes, thereby resulting in improved light extraction as shown in FIG. 8. In addition, n-GaN has high conductivity and this property minimizes the need for individual electric current spreading layers to be deposited on the top surface of the light band structure, which can negatively affect light extraction from the device.

5A, 5B, and 5C show the results of numerical simulations, showing the performance of devices with typical pyramidal optical band structures. The z-axis shows the total extraction enhancement factor against unpatterned LEDs with reflectors. These results are shown along the x-axis 501 (in%) as a function of fill fraction and along the y-axis 502 (in nm) as a function of the pitch of the optical band structure. The filling fraction is defined as diameter / pitch * 100.

FIG. 5A shows the extraction enhancement at the central 30 ° cone for pyramidal photonic crystals versus unpatterned LEDs with bottom reflectors, expressed as a function of lattice constant (a) and charge fraction. These results are at a maximum of 5.45 for a pitch of 2500 nm and a filling fraction of 75%, and with an optimized microcavity design located at d = 0.6 / λn to 131 nm and located just below the quantum well region. Shows an improvement. These parameters correspond to devices with pyramid diameters of about 1.9 μm with spacing on a 2.5 μm pitch.

Simulations were performed using the 2D Finite Difference Time Domain method. Since these simulations do not include spatial numerical inconsistencies in the conversion from two-dimensional to three-dimensional simulations, it is important that the experimental results are expected to provide much larger extraction values.

FIG. 5B shows the improvement in total extraction versus unpatterned LEDs with bottom reflectors, represented as a function of lattice constant and charge fraction. These results emphasize that, as shown in FIG. 5A, the optimum operating range appears with an optimized microcavity design located just below the quantum wells, at a pitch of 2500 nm and a fill fraction of 75%.

Finally, Figure 5C shows the ratio of light at a 30 ° cone for a device with a pyramidal photonic crystal structure. As shown, up to 45% of the light emitted from the device can be directed to fall within a central cone with a half angle perpendicular to the surface of the device. This corresponds to 84% more light in the directional cones compared to Lambertian light emitting devices. This increased directionality is due not only to the orderly arrangement of the pyramids but also to the well defined inclined angle sidewalls of the pyramids. Inclined angle sidewalls provide approximately 30% more light at a 30 degree cone compared to etched, air rod photonic crystal LEDs of a constant vertical sidewall.

6A and 6B are cross-sections through light distribution in the plane and show representative far field patterns. This result is shown as a function of the far field angle 601 along the x axis, and the luminance 602 is normalized to the luminance from an unpatterned LED with a bottom reflector. The far field pattern is referenced to the vertical plane of the LED surface. 6A shows a far field pattern for an LED with a lattice constant of 1500 nm and a pyramid diameter of 1120 nm, which provides a 2.67 times and higher total extraction improvement over light emitting devices with reflectors and optimized microcavities. . The improvement in the 30 ° cone is x4.57 and the 30 ° cone contains 40.5% of the total extracted light.

6B shows a far field pattern for an LED with a lattice constant of 2500 nm and a pyramid diameter of 1870 nm, providing 3.61 times and higher total extraction enhancement compared to light emitting devices with reflectors and optimized microcavities. The improvement in the 30 ° cone is x5.45 and the 30 ° cone contains 35.8% of the total light. This corresponds to 46% more light entering the directional cones than Lambertian light emitting devices.

Table 1 below shows 30 ° when the same green GaN LED is configured as a simple Lambertian emitter, as a primary photonic crystal LED with etched air rods, and as a pyramidal photonic crystal LED with etched pyramids. Shows the result of comparing the light emitted at a narrow cone angle with the ratio of total emitted light. In the case of photonic crystal elements, the dimensions are optimized to extract the maximum proportion of light within the 30 ° cone. The dimensions of the primary photonic crystals included lattice air rods with a pitch of 350 nm, an air rod diameter of approximately 210 nm and an etch depth of about 120 nm, while the pyramidal photonic crystal dimensions were as described above.

TABLE 1

Device type Lambertian LED Photonic Crystal LED Pyramid Photonic Crystal LED Proportion of light within a 30 ° cone 24.9 34.9 45.0

As shown in Table 1, the more optical structures are used, the more directional increase is obtained.

8A and 8B show the increased light extraction obtainable when the optical band structure is optimized with the micro cavity light emitting device. In this case, a typical photonic crystal with a pitch of 500 nm and a simple reflector is compared with a pyramidal photonic crystal with the same pitch but further with a micro cavity reflector. In both FIGS. 8A and 8B, the total light enhancement 802 is plotted as a function of the photonic crystal charge fraction 801.

In FIG. 8A, the solid line 803 represents the total extraction enhancement for light crystals that have a reflector and are normalized based on the output from the unpatterned LED. Dotted line 804 represents the total light extraction enhancement for light crystals with both microcavities and reflectors as compared to LEDs with reflectors and not patterned. 8B highlights the increased extraction obtained as a result of the micro cavity effect. Dotted line 805 indicates the increased extraction effect of the photonic crystal when the microcavity is integrated, normalized based on the non-patterned LED with reflectors and microcavities, patterning with only reflectors for the same device. This is shown in comparison with a solid line 803 showing the results when normalized based on the output from an unlit LED. Thus, the increase in difference due to the combined effects is clearly seen.

Fig. 9 shows the effect of reducing the thickness of light emitting heterostructures of pyramidal and inverse pyramid type optical band structures. The improvement in light extraction 902 as compared to flat bare LEDs with reflectors is plotted in nanometers for changes in LED heterostructure core thicknesses 901. It is clearly seen that as the thickness of the heterostructure decreases, the amount of light extraction increases. In the case of this example, the dimensions and geometry of the photonic crystal pattern are invariant for all heterostructure thickness values, and the microcavity effect was not used. Thus, in the case of optical extraction, it is evident that the etching of the pyramidal structures into the heterostructure is beneficial because the effective thickness of the LED results in a decrease.

In another preferred embodiment of the present invention, a novel high order inverse pyramid type photonic crystal or quasicrystalline pattern is proposed, which provides increased light extraction compared to primary photonic crystal patterns. Photonic crystal design also allows customization of the far-field light distribution emitted by the devices. The inverted pyramid form of the photonic crystal subregions and their well-defined tiling arrangement enable light to be extracted from the device in the form of a beam that is more parallel than the light coming from the Lambertian light source. In addition, a simplified process for manufacturing the device will be described in detail.

In terms of inverse pyramid diameter, higher order photonic crystal dimensions may be larger than 1.0 μm in size, which may be greater than 1.5 μm or 2.0 μm, and further up to 3.0 μm, 3.5 μm and 4.0 μm, but not limited thereto. It doesn't work. This varies with the far field pattern, the LED thickness and the position of the quantum wells within the light generating region of the GaN heterostructure.

Fig. 10 shows a cross section of the proposed light emitting device, which is a pyramid etched in an n-type doped GaN or InGaN layer in a periodic, quasi-crystalline, amorphous or other complexly aligned or repeated tiling arrangement. 1001). The inverted pyramidal tiling arrangement is designed to provide dispersion bands that allow the trapped light to couple into the bleach modes of the photonic crystal. If the light is coupled in bleach modes, the photonic crystal provides a means for coupling the light into free space.

The device shown in FIG. 10 includes a light emitting heterostructure, which includes an n-GaN or InGaN top layer 1002 and a bottom p-type GaN or InGaN layer 1004, and a multi quantum well (MQW). Structure 1003 is present between the layers. There is a reflector layer 1005 directly below the p-type layer 1004, which may be in the form of a metal reflector such as silver or a distributed feedback reflector (DBR) or omnidirectional reflector (ODR). It may be in the form of. The light emitting structure is supported by a carrier substrate, that is, mount 1006. In a preferred embodiment, the carrier substrate comprises an electrically conductive material having high thermal conductivity, such as a metal or metal alloy, or instead silicon or silicon carbide.

In addition, the photonic crystal may include regions having defects, that is, regions in which the reverse pyramids are removed or the shape or size of the reverse pyramids is modified. Sub-regions may also include inverted pyramids with sharp etched regions that are not etched, resulting in flattened (frustrated) inverted pyramids.

This patterning can be characterized by a number of parameters, the parameters of which are defined by the distance (a) of the lattice defined by the distance between the centers of two adjacent inverse pyramids, and the crystallographic exposed surface of the inverse pyramid and the GaN lattice And an angle θ formed between the horizontal crystal planes of. In one aspect of the invention, an etch stop layer 1007 is used to control the etch depth. This allows precise control of the reverse pyramid diameter at the bottom. The diameter φ of the bottom of the pyramids for the predetermined etch depth d is given by Equation 5 below.

[Equation 5]

Another aspect of the present invention is a simplified method of manufacturing the inverted pyramid type optical band structure LED described above. In Fig. 11A two variations of the manufacturing process possible for 11J are shown. First, an n-doped GaN or InGaN layer 1101 is grown on the lattice matched substrate 1110 by metal organic chemical vapor deposition (MOCVD) or other similar techniques (such as MBE). Common substrates used are sapphire, GaN and SiC. In certain embodiments of the invention, an etch stop layer 409 formed from a material such as InGaN or AlGaN is interposed in the n-GaN layer. Growth of the n-GaN or InGaN layer continues to layer 1108 over the etch stop layer. Multiple GaN-InGaN quantum wells 1102 are grown followed by a p-type GaN layer 1103. The completed LED heterostructure stack created at this stage is shown in FIG. 11A.

As shown in FIG. 11B, a mirror 1104 is then deposited on top of the p-GaN layer 1103. The reflector may be a metal comprising a suitable metal layer, such as silver or gold, deposited by sputtering or evaporation. In another embodiment, the reflector may comprise a dielectric multilayer stack in the form of a distributed feedback reflector (DBR) or an omnidirectional reflector (ODR). Such structures can be deposited by sputtering using techniques such as PECVD.

As shown in FIG. 11C, the heterostructure of FIG. 11B is then bonded to the substrate 1105. The substrate 1105 is preferably a metal alloy because the metal alloy allows for good thermal electrical conductivity. However, the substrate 1105 may include other materials such as SiC or Si. Prior to the bonding operation, additional layers may be deposited on the mirror 1104 to aid in the bonding process.

In a conventional final fabrication step, the sapphire substrate 1110 is then removed using laser lift off or other similar technique to provide the heterostructure shown in FIG. 11D. Although generally lacking an etch stop layer, this structure can form a conventional final finished device. The laser lift off process leaves the surface of n-GaN layer 1101 rough (at a level of approximately 50 nm to 300 nm in general). In another conventional device, such surfaces can be made rougher to improve light extraction. In one aspect of the present invention, the dimensions of the light band structure pyramids are large compared to the surface roughness, and thus it is not required to polish the surface prior to patterning.

11I or 11J in FIG. 11E illustrate additional processing steps required to fabricate the final device according to the present invention, as shown in FIGS. 11I and 11J. Although the final step is not an etch stop layer 1109 in the preceding steps, in the case of the embodiment of the process shown in FIG. 11I, the etch stop layer in the embodiment of the process leading the device shown in FIG. 11J. (1109) is present.

11E shows deposition of masking layers. Specifically, an optional method step is shown whereby a hard mask layer 1106 is deposited to subsequently transfer a predetermined pattern into the n-GaN layer 1101. This may include SiO 2 or Si 3 N 4 deposited by PECVD, or it may be a metal deposited by sputtering or evaporation. A photoresist layer 1107 is then deposited on the hard mask 1106. However, in some circumstances, the hard mask layer can be omitted and the photoresist layer 1107 is deposited directly onto the n-GaN layer 1101.

Due to the large size of the inverted pyramidal features, photoresist 1107 can be exposed using standard UV lithography to pattern it into a predetermined tiling arrangement. The transverse shape of the exposed areas may correspond to the shape of the cross section of certain inverted pyramids, or else may be simpler shapes such as squares. The exposed photoresist is then developed, leaving isolated island-like material corresponding to predetermined locations of the vertices of the inverse pyramids, as shown in FIG. 11F. If the hard mask 1106 is present, then the hard mask 1106 is dry etched using RIE, ICP or similar processes. This step transfers the patterning from the photoresist 1107 to the hard mask 1106, as shown in FIG. 11G. The remaining photoresist 1107 is then removed.

Subsequently, as shown in FIG. 11H, n-GaN layer 1101 is crystallographically wet etched using anisotropic wet etching. A preferred method of wet etching GaN is to use a KOH solution in a concentration range of 1M to 8M with bath temperature ranging from room temperature to 100 ° C. The etching time is in the range of about 45 minutes. Alternative wet etchants include NaOH or H ₃ PO ₄ . Crystal planes of hexagonal inverted pyramids formed by an etching process are {10-1-1} planes of GaN crystals. They form an angle of 58.4 ° with the bottom of the pyramids. Finally, if hard mask 1106 is used, hard mask 1106 is then removed using a suitable wet or dry etching process, leaving the final structure shown in FIG. 11I.

The absolute dimensions of the inverted pyramids will mainly be determined by the dimensions of the etched hard mask regions. Peripherals of the hard mask will provide an etch stop barrier and allow the top surface (c-plane) of GaN to be etched downward to form inverse pyramids. The nominal diameter of the inverse pyramid will be equal to the maximum of the corresponding distances between any two points on the periphery of the etched hard mask area. In addition, the diameter of the inverted pyramid is also determined by the selective etching rate of the different crystal planes, and consequently the total etching time.

In another embodiment, the etch stop layer 1109 is interposed in the form of an n-type doped material between the layers 1101 and 1108. The etch stop layer includes materials such as AlGaN, InGaN, although other suitable materials may be used. The presence of the etch stop layer 1109 allows the formation of truncated inverted pyramids (inverted truncated pyramids), and also allows precise control of the height of the pyramidal structure, while the etch time is the absolute of the inverted pyramids. Determine the diameter. 11J shows the final structure after etching and hard mask removal. The figure inserted in FIG. 11J shows an enlarged plan view of the truncated inverted pyramid structure.

14 shows an SEM micrograph of one of the inverted pyramids produced by the process. In this case, the pyramids are in an isolated state and are formed with the crystal {10-1-1} face shown in 701 using the preferred manufacturing example.

In a preferred embodiment, a complex n-GaN top region comprising layers 1101, 1109, 1108 is positioned over the light emitting structure. Therefore, light is emitted at layer 1102 to experience internal multiple reflections before finally escaping through region 1101.

Thick n-GaN growth regions are needed to reduce the defect density for the formation of high quality quantum well (QW) layers, thus improving the internal quantum efficiency of the LED. For manufacturing advantages, the upper region (layers 1101, 1109, 1108) acts as a protective layer for vulnerable quantum well regions 1102 to prevent damage during wet etching of the inverted pyramids and to prevent quantum well regions. Minimize surface recombination at In addition, etching into the quantum well negatively affects the total light output of the LED by reducing the maximum active light emitting area.

In addition, with respect to optical extraction enhancement, the required reverse pyramid dimensions are on the order of a diameter of about 1.75 μm centered on a pitch of 2.5 μm. These dimensions limit the minimum thickness of layer 1101, so that inverse pyramids are preferably present in the thick n-GaN layer. In addition, for improved light extraction, reducing the total thickness of the waveguide region in the LED by etching reduces the number of trap modes present in the heterostructure. This allows the optical band structures to overlap with a higher proportion of trap modes, thereby resulting in improved light extraction as shown in FIG. 15. In addition, n-GaN has high conductivity and this property minimizes the need for individual electric current spreading layers to be deposited on the top surface of the light band structure, which can negatively affect light extraction from the device.

12A, 12B and 12C show the results of the numerical simulations, showing the performance of typical inverted pyramid type optical band structure elements. The z-axis shows the total extraction enhancement factor against unpatterned LEDs with reflectors. These results are shown along the y axis 1202 (in nm) as a function of the pitch of the optical band structure. The filling fraction is defined as diameter / pitch * 100.

FIG. 12A shows the extraction enhancement at the central 30 ° cone for the inverted pyramidal photonic crystals versus the unpatterned LED with bottom reflector, expressed as a function of lattice constant (a) and charge fraction. These results are 4.90 for a pitch of 1500 nm and a fill fraction of 100%, and with an optimized microcavity design located at d = 0.6 / λn to 131 nm and located just below the quantum well region. Shows maximum improvement. These parameters correspond to devices with inverse pyramid diameters of about 1.5 μm with spacing on a 1.5 μm pitch.

Simulations were performed using the two-dimensional time domain finite difference method, as in the case of the pyramidal form, and the experimental results in this case are expected to provide much larger extraction values.

12B shows the improvement in total extraction versus unpatterned LEDs with bottom reflectors, represented as a function of lattice constant and charge fraction. These results emphasize that, as shown in FIG. 12A, the optimum operating range appears with an optimized microcavity design located just below the quantum wells, at a pitch of 1500 nm and a fill fraction of 100%.

Finally, FIG. 12C shows the light ratio at the 30 ° cone for the device with inverse pyramid type photonic crystal structure. As shown, up to 39% of the light emitted from the device can be directed to enter the central cone with a half angle perpendicular to the surface of the device. This corresponds to 57% more light in the directional cones than Lambertian light emitting devices. This increased directionality is due not only to the ordered arrangement of the inverted pyramids but also to the well defined inclined angle sidewalls of the inverted pyramids. Tilt angle sidewalls provide approximately 15% more light at the 30 degree cone compared to the etched, air loaded photonic crystal LEDs of constant vertical sidewalls.

FIG. 13 is a cross section through light distribution in plane for an inverted pyramidal structure and illustrates a representative far-field pattern. FIG. This result is shown and represented as a function of the far field angle 1301 along the x axis, and the luminance 1302 is normalized to the luminance from the unpatterned LED with the bottom reflector. The far field pattern is referenced to the vertical plane of the LED surface. FIG. 13 shows a far field pattern for an LED with a lattice constant of 1500 nm and a pyramid diameter of 1500 nm, which provides a 2.98 times and higher total extraction enhancement compared to light emitting devices with reflectors and optimized microcavities. . The improvement in the 30 ° cone is x4.85 and the 30 ° cone contains 38.6% of the total extracted light.

Table 2 below shows that when the same green GaN LED is configured as a simple Lambertian emitter, as a primary photonic crystal LED including etched air rods, and as an inverted pyramid type photonic crystal LED including etched inverse pyramids, The results show a comparison of the light emitted at a narrow cone angle of 30 ° as a percentage of the total emitted light. In the case of photonic crystal elements, the dimensions have been optimized to extract the maximum proportion of light within the 30 ° cone. The dimensions of the primary photonic crystals included lattice air rods with a pitch of 350 nm, an air rod diameter of approximately 210 nm and an etch depth of about 120 nm, while the reverse pyramid type photonic crystal dimensions were as described above.

TABLE 2

Device type Lambertian LED Photonic Crystal LED Pyramid Photonic Crystal LED The proportion of light within a 30 ° cone 24.9 34.9 38.6

As shown in Table 2, the more the optical structure is used, the more the directional increase is obtained.

15A and 15B show the increased light extraction obtainable when the optical band structure is optimized with the micro cavity light emitting device. In this case, a typical photonic crystal with a pitch of 500 nm and a simple reflector is compared with an inverted pyramidal photonic crystal with the same pitch but further microcavity reflectors. In both FIGS. 15A and 15B, the total light enhancement 1502 is plotted as a function of the photonic crystal charge fraction 1501.

In FIG. 15A, solid line 1503 shows the total extraction improvement for the photonic crystal with reflectors and normalized based on output from the unpatterned LED. Dotted line 1504 represents the total light extraction enhancement for light crystals with both microcavities and reflectors as compared to LEDs with reflectors and not patterned. 15B highlights the increased extraction obtained as a result of the micro cavity effect. Dotted line 1505 indicates the increased extraction effect of the photonic crystal when the microcavity is integrated, normalized based on the unpatterned LED with the reflector and microcavity, patterning with only the reflector for the same device. This is shown in comparison with a solid line 1503 showing the results when normalized based on the output from an unlit LED. Thus, the increase in difference due to the combined effects is clearly seen.

One of ordinary skill in the art can make the light emitting elements practical as alternative (solid state) light sources for existing light sources by allowing the present invention to be realized with highly efficient and directional light emitting elements. I will understand. The present invention is directed to the careful design of pyramidal protrusions and inverted pyramidal etches and their tiling arrangements, which lead to optical band structures that can be optimized for efficient optical coupling, and the propagation and distance of light emitted from the device. Allows you to control chapter properties. The practicality of the device is further enhanced by the provision of simple patterning and etching processes for manufacturing the devices, which can be easily used to augment existing techniques for manufacturing more conventional devices.

Claims (34)

As a light-emitting device (LED), A first layer comprising a first semiconductor material with a doping of a first type; A second layer comprising a second semiconductor material with a doping of a second type; And A light generating layer disposed between the first layer and the second layer, The first layer has a top surface distant from the light generating layer and a bottom surface close to the light generating layer, Light generated in the light generating layer exits the LED structure through the top surface of the first layer, The first layer is surrounded by a material having a refractive index different from that of the first semiconductor material, and protrudes from the top surface and consists of the first semiconductor material and pyramidal or frustro pyramidal protrusions. Further comprises a tiling arrangement of The tiling arrangement of the protrusions and the surrounding material comprises a photonic band structure, And the protrusions and their tiling arrangement are configured such that light exiting the LED structure through the top surface is substantially more directional than light exiting from a Lambertian light source. As a light emitting element (LED), A first layer comprising a first semiconductor material with a doping of a first type; A second layer comprising a second semiconductor material with a doping of a second type; And A light generating layer disposed between the first layer and the second layer, The first layer has a top surface far from the light generating layer and a bottom surface close to the light generating layer, Light generated in the light generating layer exits the LED structure through the top surface of the first layer, The first layer is inverted pyramidal or truncated inverted pyramidal in the first semiconductor material, the material extending from the top surface toward the light generating layer and having a different refractive index than the first semiconductor material. Further comprising a tiling arrangement of indentations, The tiling arrangement of the recesses and the surrounding first semiconductor material comprise an optical band structure, Wherein the recesses and their tiling arrangement are configured such that light exiting the LED structure through the top surface is substantially more directional than light exiting a Lambertian light source. The method according to claim 1 or 2, Wherein said tiling arrangement comprises a photonic crystal, a photonic quasicrystal, or an amorphous tiling pattern. The method of claim 3, And wherein said tiling arrangement comprises repeating cells composed of a photonic crystal, a photoquasitic crystal, or an amorphous tiling pattern. The method according to any one of claims 1 to 4, And wherein said tiling arrangement comprises a defect. The method according to any one of claims 1 to 5, LEDs, characterized in that the protrusions or recesses have a size greater than 1.0 μm. The method according to any one of claims 1 to 5, LEDs, characterized in that the protrusions or recesses have a size greater than 1.5 μm. The method according to any one of claims 1 to 5, Wherein the protrusions or recesses have a size greater than 2.0 μm. The method according to any one of claims 1 to 5, Wherein the protrusions or recesses have a size greater than 2.5 μm. The method according to any one of claims 1 to 9, Wherein the pitch of the tiling arrangement is greater than 1.5 μm. The method according to any one of claims 1 to 9, And the pitch of the tiling arrangement is greater than 2.0 μm. The method according to any one of claims 1 to 9, Wherein the pitch of the tiling arrangement is greater than 2.5 μm. The method according to any one of claims 1 to 9, Wherein the pitch of the tiling arrangement is greater than 3.0 μm. The method according to any one of claims 1 to 13, And an optical reflector disposed adjacent said second layer of said second semiconductor material, said second layer being located between said light generating layer and said reflector. The method of claim 14, And the separation distance between the light generating layer and the optical reflector is to include a micro cavity which enhances the amount of generated light propagating towards the top surface of the first layer. The method of claim 15, And the protrusions or the recesses and their tiling arrangement are configured to optimally cooperate with the microcavity effect to improve light extraction efficiency from the LED. The method according to any one of claims 1 to 16, At least 35% of the light exiting the LED structure through the top surface is in a cone having a 30 ° half angle with respect to the surface normal. The method according to any one of claims 1 to 16, At least 37% of the light exiting the LED structure through the top surface is in a cone having a 30 ° half angle to the surface normal. The method according to any one of claims 1 to 16, Wherein at least 38% of the light exiting the LED structure through the top surface is in a cone having a 30 ° half angle to the surface normal. The method according to any one of claims 1 to 16, Wherein at least 40% of the light exiting the LED structure through the top surface is in a cone having a 30 ° half angle to the surface normal. The method according to any one of claims 1 to 16, And wherein the distribution of light exiting the LED structure through the top surface is centered at an angle less than or equal to 60 ° with respect to the top surface. The method according to any one of claims 1 to 16, And wherein the distribution of light exiting the LED structure through the top surface is centered at an angle less than or equal to 50 ° with respect to the top surface. The method according to any one of claims 1 to 16, And wherein the distribution of light exiting the LED structure through the top surface is centered at an angle less than 40 ° or equal to 40 ° with respect to the top surface. The method according to any one of claims 1 to 16, And wherein the distribution of light exiting the LED structure through the top surface is centered at an angle less than or equal to 30 ° with respect to the top surface. The method according to any one of claims 1 to 24, The first layer includes an etch stop material layer interposed at a predetermined depth in the first semiconductor material, such that protrusions formed of the first semiconductor material extend from a surface of the etch stop material layer, or The recesses of which extend only to the surface of the etch stop material layer. The method according to any one of claims 1 to 25, Wherein the first semiconductor material comprises GaN doped with n-type and the second semiconductor material comprises GaN doped with p-type. A method of manufacturing a light emitting device (LED) comprising a first layer having projections according to claim 1, A first layer comprising a first semiconductor material having a doping of a first type, a second layer comprising a second semiconductor material having a doping of a second type, and disposed between the first layer and the second layer A light generating layer, said first layer having a top surface far from said light generating layer and a bottom surface close to said light generating layer, wherein light generated in said light generating layer passes through the top surface of said first layer Providing a light emitting device heterostructure emerging from the LED structure; Forming an etch mask on the first layer, the etching mask comprising islands of mask material at locations corresponding to a predetermined tiling arrangement, Depositing a photoresist layer on top of the first layer, Patterning the captive resist layer by exposing according to the predetermined tiling arrangement, and Forming the etch mask on the first layer, including removing the unexposed photoresist so as to leave islands of photoresist at locations corresponding to the predetermined tiling arrangement; Anisotropic wet etch the first semiconductor material to a predetermined depth along predetermined crystal planes, such that a pyramidal or truncated pyramid of the first semiconductor material of the first layer at locations immediately below the islands of the mask material Forming shaped protrusions; And Removing islands of the mask material to leave the desired tiling arrangement of the pyramidal or truncated pyramidal protrusions, including a predetermined optical band structure in combination with a peripheral material having a different refractive index; Method of manufacturing LEDs. The method of claim 27, The forming of the etching mask may include: Depositing a layer of hard mask material on top of the first layer prior to depositing the photoresist layer; Removing the hard mask material after removing the photo resist to leave islands of hard mask material directly below the islands of the photo resist; And Removing the islands of the remaining photoresist so as to leave the etch mask comprising islands of the hard mask material at locations corresponding to the predetermined tiling arrangement. Way. A manufacturing method of a light emitting device comprising a first layer having recesses according to claim 2, A first layer comprising a first semiconductor material with a doping of a first type, a second layer comprising a second semiconductor material with a doping of a second type, and disposed between the first layer and the second layer A light generating layer, said first layer having a top surface far from said light generating layer and a bottom surface close to said light generating layer, wherein light generated in said light generating layer is directed to said top surface of said first layer; Providing a light emitting device heterostructure, emerging from the LED structure through; Forming an etch mask on the first layer, the etching mask comprising islands of mask material missing at locations corresponding to a predetermined tiling arrangement, Depositing a photoresist layer on top of the first layer, Patterning the photoresist by exposing according to the predetermined tiling arrangement, and Forming the etch mask on the first layer, including removing the unexposed photoresist to leave islands of missing photoresist at locations corresponding to the predetermined tiling arrangement; Anisotropically wet etch the first semiconductor material to a predetermined depth along predetermined crystal planes to pyramid or cut the first semiconductor material of the first layer at locations just below the islands of the lost mask material. Forming two pyramidal recesses; And Removing the remaining mask material to leave the predetermined tiling arrangement of inverted pyramidal or truncated inverted pyramidal recesses in the first semiconductor material, The concave portion includes a material having a refractive index different from that of the surrounding first semiconductor material, and includes a light band structure together. The method of claim 29, Forming the etching mask, Depositing a layer of hard mask material on top of the first layer prior to depositing the photoresist layer; Removing the hard mask material after removing the photo resist to leave islands of missing hard mask material directly below the islands of the missing photo resist; And Removing the remaining photoresist islands to leave the etch mask comprising islands of the lost hard mask material at locations corresponding to the predetermined tiling arrangement. Manufacturing method. The method according to any one of claims 27 to 30, And wherein said first layer of said light emitting element heterostructure comprises an etch stop material layer interposed at said predetermined depth of said first semiconductor material. The method of claim 31, wherein And said predetermined depth corresponds to the thickness required for said light emitting element. The method of claim 31, wherein The predetermined depth corresponds to the required shape of the truncated pyramidal protrusion or the truncated inverted pyramid-shaped recess. The method according to any one of claims 27 to 33, The light emitting device heterostructure is manufactured using a flip chip process.

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