WO2008065373A1 - Dispositif d'émission de lumière cristalline photonique pyramidale. - Google Patents

Dispositif d'émission de lumière cristalline photonique pyramidale. Download PDF

Info

Publication number
WO2008065373A1
WO2008065373A1 PCT/GB2007/004521 GB2007004521W WO2008065373A1 WO 2008065373 A1 WO2008065373 A1 WO 2008065373A1 GB 2007004521 W GB2007004521 W GB 2007004521W WO 2008065373 A1 WO2008065373 A1 WO 2008065373A1
Authority
WO
WIPO (PCT)
Prior art keywords
layer
light
led
pyramidal
semiconductor material
Prior art date
Application number
PCT/GB2007/004521
Other languages
English (en)
Inventor
James Mckenzie
Tom Lee
Majd Zoorob
Original Assignee
Luxtaltek Corporation
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Priority claimed from US11/564,213 external-priority patent/US7700962B2/en
Priority claimed from US11/564,207 external-priority patent/US7615398B2/en
Application filed by Luxtaltek Corporation filed Critical Luxtaltek Corporation
Priority to KR1020097013252A priority Critical patent/KR101281504B1/ko
Priority to CN2007800495892A priority patent/CN101606248B/zh
Publication of WO2008065373A1 publication Critical patent/WO2008065373A1/fr
Priority to HK10103622.8A priority patent/HK1138432A1/xx

Links

Classifications

    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L33/00—Semiconductor devices having potential barriers specially adapted for light emission; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L33/02—Semiconductor devices having potential barriers specially adapted for light emission; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by the semiconductor bodies
    • H01L33/20—Semiconductor devices having potential barriers specially adapted for light emission; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by the semiconductor bodies with a particular shape, e.g. curved or truncated substrate
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L2933/00—Details relating to devices covered by the group H01L33/00 but not provided for in its subgroups
    • H01L2933/0083—Periodic patterns for optical field-shaping in or on the semiconductor body or semiconductor body package, e.g. photonic bandgap structures
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L33/00—Semiconductor devices having potential barriers specially adapted for light emission; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L33/02—Semiconductor devices having potential barriers specially adapted for light emission; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by the semiconductor bodies
    • H01L33/10—Semiconductor devices having potential barriers specially adapted for light emission; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by the semiconductor bodies with a light reflecting structure, e.g. semiconductor Bragg reflector
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L33/00—Semiconductor devices having potential barriers specially adapted for light emission; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L33/44—Semiconductor devices having potential barriers specially adapted for light emission; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by the coatings, e.g. passivation layer or anti-reflective coating
    • H01L33/46—Reflective coating, e.g. dielectric Bragg reflector

Definitions

  • the present invention relates to light-emitting diodes with improved light extraction and directionality, and in particular to devices employing photonic crystal structures.
  • LEDs Light emitting diodes
  • LEDs are based on a forward biased p-n junction and have recently reached sufficiently high brightness levels that they are now suitable for new solid-state lighting applications as well as replacements for projector light sources. Entry into these markets has also been enabled by the economical gains achieved through the high efficiencies of LEDs, as well as reliability, long lifetime and environmental benefits. In particular, applications in solid-state lighting require that LEDs exceed efficiencies currently achievable by alternative fluorescent lighting technologies.
  • Back Light Units (BLU) for liquid crystal display (LCD) panels are key elements in the performance of an LCD panel.
  • LCD panels employ compact cathode fluorescent light (ccfl) sources.
  • ccfl compact cathode fluorescent light
  • these sources suffer from several problems, including poor colour gamut, environmental recycling and manufacture issues, thickness and profile, high voltage requirements, poor thermal management, weight and high power consumption.
  • LED BLU units offer benefits in many areas, including colour gamut, lower power consumption, thin profiles, low voltage requirements, good thermal management and low weight.
  • US2006/0181899 and US2006/0181903 disclose an optical light guide or waveguide arranged to evenly distribute diffused light across the complete LCD panel surface.
  • the light is coupled into the light guide by using side-mounted LED optical sources.
  • Side mounting is beneficial as the number of LEDs required is dramatically reduced per unit area of backlit LCD, when compared with direct LED backlighting. It is proposed that a 32 inch LCD panel can be side lit with as few as 12 LED sources. However, to maximise the light diffused onto the LCD panel the most optimal coupling of the LED light into the light guide is required.
  • High Intensity Discharge (HID) type projector light engines have always been hindered by low efficiency and short lifetime, resulting in slow adoption into consumer markets.
  • the Etendue value of the light source needs to be smaller or matched to the microdisplay Etendue value. This compatibility is very important for improving the overall system efficiency of the complete light projection engine.
  • high total luminous outputs and low power consumption are also very important, especially in applications where large rear projection screens (bigger than 50 inch) and front projection systems are required. Low power consumption is desirable in order to minimise thermal management issues.
  • Etendue value E is calculated according to the following equation:
  • E ;rx Ax n 2 xsin( ⁇ ) (1 )
  • E the Etendue of the light source
  • A the surface area of the light emission device
  • a the half-angle of the light source.
  • the overall efficiency of an LED can be quantified by three main factors, namely internal quantum efficiency, injection efficiency, and extraction efficiency.
  • One of the main limiting factors reducing the efficiency of light extraction from an LED is total internal reflection of the emitted photons and their trapping in the high refractive index of the epi-material forming the LED. These trapped waveguide modes propagate in the LED structure until they are scattered or reabsorbed. The thickness of the LED structure determines the number of modes that can be supported.
  • US5779924 and US5955749 both describe the use of photonic crystal patterns defined in the semiconductor layers of the LED to affect the way light propagates through the epi-structure.
  • the photonic band structure formed allows the trapped modes to be extracted out and hence increase the extraction efficiency and ultimately the total external efficiency of the LED.
  • the use of photonic crystal structures in an LED is advantageous over other light extraction techniques as they scale with the active surface area of the LED and hence provide an ideal means for improving the light extraction for a large-area high-brightness LED die.
  • the scaling of the LED size is important for solid state lighting applications where absolute luminous output is required.
  • the total light extraction for many of these photonic crystal LEDs is not as high as for more conventional surface-roughened LEDs.
  • US6831302 describes a fabrication process that involves the following steps: growing an n-GaN layer on a lattice matched single crystal wafer, an active QW region, and a p-GaN layer, followed by eutectic bonding of a sub-mount or substrate on the top surface, wafer flipping, lift-off from the growth wafer (using a technique such as laser lift-off), surface polishing to provide an optically-smooth surface (using a process such as chemical mechanical polishing), definition of a photonic crystal pattern on the surface (by a process such as nano-imprinting, lithography, or holography), and finally photonic crystal pattern transfer into the GaN material using a suitable dry (e.g. RIE or ICP) or wet etch of the GaN.
  • RIE reactive ion etching
  • One of the complex processing steps involved is the polishing of the wafers, which can adversely affect the yield due to the difficulty in controlling the surface quality across the complete wafer. Small scratches across the surface may affect the current spreading across the LED wafer and ultimately provide a path to short circuiting the complete LED or adversely affecting the forward voltage.
  • the thickness of the GaN epi-structure is important for the effective design of the photonic crystal extraction pattern.
  • the high refractive index acts like a highly-multimode waveguide, whereby the thickness determines the number of modes residing in the LED heterostructure.
  • Another complex fabrication step is the definition of small-scale first-order photonic crystal features in the range of 300nm to 500nm for the pitch of the features and in the range of 200nm to 400nm for the diameter of the hole features.
  • Such patterns are currently defined using either nano-imprinting or holography.
  • the former technique is currently not a proven technique for such small-scale features on LED wafers and only low yields are achieved.
  • the technique suffers from a higher cost for small manufacturing volumes.
  • the latter lithography technique suffers from complex alignment and stability as well as low volumes.
  • a light-emitting device comprises: a first layer comprising a first semiconductor material having a first type of doping; a second layer comprising a second semiconductor material having a second type of doping; and, a light-generating layer disposed between the first and second layers, wherein the first layer has an upper surface distal the light-generating layer and a lower surface proximate the light-generating layer and wherein light generated in the light-generating layer emerges from the LED structure through the upper surface of the first layer, the first layer further comprising a tiling arrangement of pyramidal or frustro-pyramidal protrusions of the first semiconductor material from the upper surface surrounded by a material of different refractive index to the first semiconductor material, wherein the tiling arrangement of protrusions and surrounding material comprise a photonic band structure, and wherein the protrusions and their tiling arrangement are configured such that light emerging from the LED structure through the upper surface is substantially more directional than from a Lambert
  • a light-emitting device comprises: a first layer comprising a first semiconductor material having a first type of doping; a second layer comprising a second semiconductor material having a second type of doping; and, a light-generating layer disposed between the first and second layers, wherein the first layer has an upper surface distal the light-generating layer and a lower surface proximate the light-generating layer and wherein light generated in the light-generating layer emerges from the LED structure through the upper surface of the first layer, the first layer further comprising a tiling arrangement of inverted pyramidal or inverted frustro-pyramidal indentations in the first semiconductor material extending from the upper surface towards the light- generating layer and comprising a material of different refractive index to the first semiconductor material, wherein the tiling arrangement of indentations and surrounding first semiconductor material comprise a photonic band structure, and wherein the indentations and their tiling arrangement are configured such that light emerging from the LED
  • a Lambertian source emits a luminous flux per unit solid angle that in any direction is proportional to the cosine of the angle between that direction and the normal to the surface from which it is emitted. This leads to a uniform radiance with a spherical distribution.
  • An LED comprising a structure according to the present invention combines greater directionality of emitted light with an improvement in the efficiency with which light generated by the device is coupled into the emitted beam. This is achieved through the innovative tiling arrangement of pyramidal or frustro-pyramidal surface projections, or inverted pyramidal or inverted frustro- pyramidal indentations.
  • photonic tiling arrangements Three particular classes of photonic tiling arrangements are preferred, namely photonic crystals having both short range and long range order, photonic quasicrystals having short range translation disorder but long range order, amorphous tiling patterns having short range separation order but long range disorder.
  • photonic crystals having both short range and long range order
  • photonic quasicrystals having short range translation disorder but long range order
  • amorphous tiling patterns having short range separation order but long range disorder.
  • the separation between neighbouring pyramidal regions is fixed while the rotational symmetry is randomised.
  • a high-order pyramidal or inverted pyramidal photonic crystal or quasicrystal pattern is proposed that provides increased light extraction when compared to more conventional first-order photonic crystal patterns.
  • Careful design of the photonic crystal also allows tailoring of the far-field light pattern produced by the device.
  • the pyramidal shape of the protrusions or the inverted pyramidal shape of the indentations, with angled side walls, and their well- defined tiling arrangement can extract light from the LED in a beam that is more collimated than a from Lambertian source, even for patterns with large lattice constants (>1 ⁇ m).
  • the pyramidal protrusions or inverted pyramidal indentations have a size greater than 1.0 ⁇ m. However, they may be greater than 1.5 ⁇ m or 2.0 ⁇ m in size or even greater than 2.5 ⁇ m in size.
  • the relatively large pyramid or inverted pyramid sizes relaxes the tolerances for fabrication and also means that surface polishing is not required prior to formation of the pyramidal protrusions or inverted pyramidal indentations, as their dimensions are sufficiently greater than the residual surface roughness.
  • the pitch of the tiling arrangement is greater than 1.5 ⁇ m, although it may be greater than 2.0 ⁇ m or 2.5 ⁇ m in size or even greater than 3.0 ⁇ m in size.
  • the protrusions or indentations are configured such that a significant proportion (>35%) of the light is extracted in a central cone having a 30° half-angle to the vertical axis. Preferably, greater than 37%, 38% or even 40% is extracted in the central cone. This enables efficient and uniform coupling of light into narrow elongated light guides that are often used with light sources for projection applications.
  • the pyramids or inverted pyramids and the tiling pattern may be configured such that light is predominantly extracted at angles greater than this, in a side-emitting fashion.
  • light may be emitted in a collimated ring or donut-like distribution around the vertical axis, rather than within a narrow central cone.
  • the distribution may be centred at angles greater than or equal to 30°, 40°, 50° or 60° to the vertical, or equivalents at angles less than or equal to 60°, 50°, 40° or 30° to the surface.
  • the first semiconductor material having the first type of doping may be either n- doped or p-doped, in which case the second semiconductor material having the second type of doping will be either p-doped or n-doped, respectively.
  • the first layer may comprise a layer of etch stop material embedded at a predetermined depth in the first semiconductor material, in which case the projections formed of first semiconductor material will extend from a surface of the etch stop material layer and the inverted pyramidal indentations will extend down to the etch stop material layer.
  • the first semiconductor material comprises n-doped GaN or InGaN and the second semiconductor material comprises p-doped GaN or InGaN. It is preferred that the light-generating layer comprises a multiple quantum well structure of GaN-InGaN.
  • the higher order photonic crystal dimensions are typically in the range 1.0 ⁇ m to 3.0 ⁇ m, although this depends on a range of factors, including wavelength, far-field pattern, LED thickness and location of the multiple quantum well structure within the overall GaN heterostructure.
  • Suitable etch stop materials include AIGaN and InGaN.
  • the LED further comprises an optical reflector to reflect light propagating away from the upper extraction surface of the first layer and which would otherwise not be extracted.
  • the optical reflector is disposed adjacent the second layer of the second semiconductor material such that the second layer is located between the light-generating layer and the reflector.
  • the optical reflector comprises a simple layer of a metallic material or it may comprise a multi-layer dielectric structure.
  • the optical reflector may comprise a distributed Bragg reflector (DBR) or an omni-directional reflector (ODR).
  • DBR distributed Bragg reflector
  • ODR omni-directional reflector
  • a separation distance between the light-generating layer and the optical reflector is such as to comprise a microcavity which enhances the amount of generated light propagating towards the upper extraction surface of the first layer.
  • the microcavity effect gives an even greater enhancement in extraction efficiency over and above that provided by simple reflection at the optical reflector.
  • An optimal separation is between 0.5 to 0.7 times the wavelength of light generated in the light-generating layer.
  • the pyramidal protrusions or the inverted pyramidal indentations and their tiling arrangement are configured to cooperate optimally with the microcavity effect to further enhance the efficiency of light extraction from the LED.
  • a light engine for an optical projector unit comprises a plurality of light emitting devices according to the first or second aspect.
  • the LED described above is particularly suitable for application in solid-state lighting sources, including those for front and rear projectors.
  • a method for fabricating a light-emitting device comprises the steps of: providing a light-emitting device heterostructure comprising a first layer comprising a first semiconductor material having a first type of doping, a second layer comprising a second semiconductor material having a second type of doping, and a light-generating layer disposed between the first and second layers, wherein the first layer has an upper surface distal the light-generating layer and a lower surface proximate the light-generating layer and wherein light generated in the light-generating layer emerges from the LED structure through the upper surface of the first layer, forming an etch mask on the first layer, the mask comprising islands of mask material at locations corresponding to a predetermined tiling arrangement, wherein the step of forming the mask comprises the steps of: depositing a layer of photoresist over the first layer; patterning the photoresist by exposure according to the predetermined tiling arrangement; and, removing unexposed photore
  • a method for fabricating a light- emitting device comprises the steps of: providing a light-emitting device heterostructure comprising a first layer comprising a first semiconductor material having a first type of doping, a second layer comprising a second semiconductor material having a second type of doping, and a light-generating layer disposed between the first and second layers, wherein the first layer has an upper surface distal the light-generating layer and a lower surface proximate the light-generating layer and wherein light generated in the light-generating layer emerges from the LED structure through the upper surface of the first layer, forming an etch mask on the first layer, the mask comprising islands of missing mask material at locations corresponding to a predetermined tiling arrangement, wherein the step of forming the mask comprises the steps of: depositing a layer of photoresist over the first layer; patterning the photoresist by exposure according to the predetermined tiling arrangement; and, removing unexposed photore
  • the light-emitting device heterostructure itself may be fabricated by any suitable known process, including flip-chip processes.
  • the layer of photoresist may itself be the mask layer, in which case the islands of mask material are the islands of photoresist. Any suitable process for patterning the photoresist by exposure may be used, including UV lithography.
  • the unexposed photoresist is removed using a suitable developer and the remaining exposed islands of photoresist removed by stripping.
  • a variety of etchants are available for the anisotropic wet etch, including solutions of KOH, NaOH or H 3 PO 4 .
  • a harder mask material may be employed, which is etch-resistant to the anisotropic wet etch. In this case, further process steps are required.
  • the step of forming an etch mask further comprises the steps of: depositing a layer of hard mask material over the first layer before the step of depositing the layer of photoresist; removing hard mask material after the step of removing photoresist to leave islands of hard mask material beneath the islands of photoresist; and, removing the remaining islands of photoresist to leave behind the etch mask comprising the islands of hard mask material at locations corresponding to the predetermined tiling arrangement.
  • the step of forming an etch mask further comprises the steps of: depositing a layer of hard mask material over the first layer before the step of depositing the layer of photoresist; removing hard mask material after the step of removing photoresist to leave islands of missing hard mask material beneath the islands of missing photoresist; and, removing the remaining photoresist to leave behind the etch mask comprising the islands of missing 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 a metal deposited by sputtering or evaporation. Once the pyramidal protrusions have been formed, the remaining islands of hard mask material are removed by a suitable wet or dry etch process. Similarly, once the inverted pyramidal indentations have been formed, the remaining hard mask material surrounding the islands is removed by a suitable wet or dry etch process.
  • the depth of the anisotropic etch will determine whether frustro-pyramidal or pyramidal projections are formed and also their size.
  • precise control of the anisotropic etch depth may be difficult, if based on speed of etch and etch time. Therefore, an etch stop may be employed to ensure etching is terminated at a particular depth.
  • the depth of the anisotropic etch will determine the size of the indentations formed. Again, precise control of the anisotropic etch depth may be difficult, if based on speed of etch and etch time.
  • the etch stop is required in order to form inverted frustro-pyramidal indentations.
  • the first layer of the light-emitting device heterostructure comprises a layer of etch stop material embedded at the predetermined depth in the first semiconductor material.
  • the anisotropic etch of the first semiconductor material will then continue until the layer of etch stop material is reached, thus providing greater uniformity and repeatability of the fabrication method.
  • it also permits the formation of inverted flat-top or truncated pyramidal indentations.
  • the present invention also provides a simplified method of manufacture of a photonic crystal type LED structure, whereby the definition of a large feature photonic crystal is transferred onto an LED by the use of low cost photolithography or similar process and without the need for complex polishing processes.
  • Figure 1 shows a cross section of the proposed device
  • Figure 2A shows pyramids arranged in a regular square lattice
  • Figure 2B shows pyramids arranged in a 12-fold quasicrystal
  • Figure 3 shows normalised light intensity of a GaN LED as a function of quantum well to mirror separation distance
  • Figures 4A to 4D illustrate a Flip Chip-based manufacturing process for fabricating a light emitting device
  • Figures 4E to 4I illustrate the additional manufacturing steps for fabricating a pyramidal photonic crystal structure in the upper layer of the device shown in
  • Figure 5A shows the enhancement in light extraction in a 30 ° cone for a pyramidal PC-LED as compared to an un-patterned device
  • Figure 5B shows the enhancement in total light extraction for a pyramidal PC-LED as compared to an un-patterned device
  • Figure 5C shows the percentage of light in the 30 ° cone for different pyramidal photonic crystal light emitting devices
  • Figure 6A shows the far-field pattern for a PC-LED structure according to a preferred embodiment of the invention
  • Figure 6B shows the far-field pattern for a PC-LED structure according to another preferred embodiment of the invention.
  • Figure 7A shows an SEM micrograph of an isolated pyramid formed using the fabrication method of the invention
  • Figure 7B shows an SEM micrograph of a photonic quasicrystal arrangement of pyramids formed using the fabrication method of the invention
  • Figure 8A is a plot of light extraction enhancement versus photonic crystal fill fraction for a conventional PC-LED and for a pyramidal PC-LED with micro-cavity compared to an un-patterned LED;
  • Figure 8B is a plot of light extraction enhancement versus photonic crystal fill fraction for a pyramidal PC-LED compared to an un-patterned LED with and without micro-cavity;
  • Figure 9 is a plot of light extraction enhancement versus LED heterostructure core thickness for a pyramidal PC-LED compared to an un-patterned LED.
  • Figure 10 shows a cross section of the inverted pyramidal device
  • FIGS 11A to 1D illustrate a Flip Chip-based manufacturing process for fabricating a light emitting device
  • Figures 11 E to 111 or 11J illustrate the additional manufacturing steps for fabricating an inverted pyramidal or truncated inverted pyramidal photonic crystal structure in the upper layer of the device shown in Figure 11 D;
  • Figure 12A shows the enhancement in light extraction in a 30 ° cone for a pyramidal
  • PC-LED as compared to an un-patterned device
  • Figure 12B shows the enhancement in total light extraction for a pyramidal PC-LED as compared to an un-patterned device
  • Figure 12C shows the percentage of light in the 30° cone for different pyramidal photonic crystal light emitting devices
  • Figure 13 shows the far-field pattern for a PC-LED structure according to a preferred embodiment of the invention
  • Figure 14 shows an SEM micrograph of an isolated inverted pyramid formed using the fabrication method of the invention
  • Figure 15A is a plot of light extraction enhancement versus photonic crystal fill fraction for a conventional PC-LED and for an inverted pyramidal PC-LED with micro-cavity compared to an un-patterned LED;
  • Figure 15B is a plot of light extraction enhancement versus photonic crystal fill fraction for an inverted pyramidal PC-LED compared to an un-patterned LED with and without micro-cavity.
  • An aim of the present invention is to provide improved light extraction as well as tailored far-field emission from light emitting devices.
  • These devices can employ a wide range of light emitting semiconductor material system including, but not restricted to, InGaN, InGaP, InGaAs, InP, or ZnO.
  • the description will focus on the implementation of the directional light extraction technique implemented in green InGaN light emitting devices.
  • the design can be equally optimised and implemented for other emission wavelengths (such as blue or UV) using this material as well as for other material systems, such as InGaP which is suitable for red and yellow wavelengths.
  • a novel high-order pyramidal photonic crystal (PC) or quasicrystal pattern is proposed that provides increased light extraction, as compared to first-order photonic crystal patterns.
  • the PC design also allows for the tailoring of the far-field light distribution emitted by the devices.
  • the pyramidal shape of the PC sub-regions and their well defined tiling arrangement permits extraction of light from the device in a beam which is more collimated than from a Lambertian source. A simplified process for manufacturing the device will also be described.
  • the higher-order PC dimensions are greater than 1.0 ⁇ m in size and may be greater than 1.5 ⁇ m or 2.0 ⁇ m, and can be up to, but not limited to, 3.0 ⁇ m, 3.5 ⁇ m and 4.0 ⁇ m in size. This varies depending on the far-field pattern, the LED thickness and the location of the quantum wells within the light- generating region of the GaN heterostructure.
  • Figure 1 demonstrates a cross section of the proposed light emitting device, which comprises pyramids 101 pyramids protruding out of the n-doped GaN or InGaN layer in a periodic, quasicrystalline, amorphous or other complex ordered or repeating tiling arrangement.
  • the pyramidal tiling arrangement is designed to provide dispersion bands that allow trapped light to couple into the Bloch modes of the photonic crystal. Once the light is coupled into the Bloch modes, the photonic crystal provides a means of coupling the light out into free space.
  • the device shown in Figure 1 comprises a light-emitting heterostructure, which includes an n-GaN or InGaN top layer 102 and a lower p-type GaN or InGaN layer 104 with a multiple quantum well (MQW) structure 103 residing between these layers.
  • MQW multiple quantum well
  • the light emitting structure is supported by a carrier substrate or mount, 106.
  • the carrier substrate comprises an electrically conductive material with a high thermal conductivity such as metal or metal alloy or alternatively Silicon or Silicon Carbide.
  • an etch stop layer, 107 formed from materials such as AIGaN, InGaN, although not limited to these, which allows precise control of the depth of the pyramids.
  • This layer is embedded in between n-type layers 101 and 102.
  • photonic tiling arrangements which are preferred in the present invention, and in which the arrangements possess the following properties: short range and long range order, namely photonic crystals; short range translation disorder but possess long range order, namely photonic quasicrystals; and short range separation order and long range disorder, namely amorphous tiling arrangements.
  • short range and long range order namely photonic crystals
  • short range translation disorder but possess long range order namely photonic quasicrystals
  • short range separation order and long range disorder namely amorphous tiling arrangements.
  • amorphous tiling arrangements the separation between neighbouring pyramidal and inverted pyramidal regions is fixed while the rotational symmetry is randomised.
  • These classes of patterning can also comprise of repeating cells of the above- mentioned tiling arrangements. Additionally, they can comprise of regions with defects, that is to say regions where pyramids and inverted pyramids are removed or the shape or size of the pyramids is modified. The sub-regions may also comprise of pyramids with unetched sharp apex regions, giving rise to flat-top (frustro) pyramids.
  • the patterning can be characterised by a number of parameters, including the pitch a of the lattice defined as the distance separating the centres of two neighbouring pyramids or inverted pyramids, and the angle ⁇ formed between the pyramid crystallographic exposed face and the horizontal crystal plane of the GaN lattice.
  • an etch stop layer, 107 is used to control the etch depth. This allows the accurate control of the pyramid diameter at the base. For a given etch depth, d, the diameter ⁇ of the base of the pyramids is:
  • One of the main aspects of the present invention is the use of large features that are crystallographically aligned to provide maximum pattern reproduction accuracy as well as relaxed positional accuracy.
  • the hexagonal pyramids or inverted pyramids can be arranged in a regular pattern, quasicrystalline pattern, amorphous pattern or other suitable arrangement.
  • An example is shown in Figure 2A where the hexagonal pyramids are arranged in a regular square lattice to form a photonic crystal.
  • Figure 2B shows pyramids or inverted pyramids arranged in a 12-fold symmetric square-triangle quasicrystal tiling to form a photonic quasicrsytal.
  • An enhanced version of the invention employs an optical reflector placed somewhere below the light emitting region to reflect downward propagating light in the upward direction.
  • the separation distance between the light emitting region and the reflector is designed to enhance upward emitting light due to the so-called microcavity effect.
  • the pyramidal or inverted pyramidal photonic crystal is then optimised in conjunction with the microcavity effect to further enhance light extraction.
  • W 0 amplitude of emitted light
  • w r amplitude of reflected light
  • ⁇ phase shift upon reflection on the mirror
  • Figure 3 shows a plot of the intensity of the light obtained from an GaN LED device with a mirror as a function of the quantum well to mirror distance.
  • the intensity is normalized to that obtained from a GaN LED without a mirror, thereby illustrating the enhancement.
  • the mirror is assumed to have 100% reflectivity. It is evident that a maximum of approximately 3.5 times more light is extracted when compared to a bare flat GaN light emitting device without a reflector. With respect to the contribution from the reflector alone this amounts to approximately 1.75 more light extracted due to the microcavity effect. Placing the quantum wells at the right distance from the mirror is critical to obtain maximum extraction efficiency in an LED.
  • the microcavity effect also introduces deviations in the far-field radiation shapes of the LED when compared to that of a Lambertian source.
  • the increased lobe emission can be optimised in conjunction with the top surface photonic crystal pattern, thereby enabling greater light extraction as well as directionality from the LED than would be expected from employing the two extraction techniques separately.
  • the microcavity effect reduces the isotropy of the emission inside the heterostructure, thereby allowing light incident internally onto the photonic crystal to be more collimated.
  • the photonic bandstructure Due to the high aspect ratio features and large dielectric contrast that the photonic bandstructure offer, it is possible to efficiently overlap the LED waveguide modes with the dispersion bands of the photonic bandstructure to allow strong coupling between them.
  • many waveguide modes are set up.
  • the dispersion bands of the photonic crystal cannot be designed to overlap with all the trapped modes.
  • the photonic bandstructure can be optimally designed to extract efficiently the more closely spaced and lesser number of modes trapped in the LED.
  • a further aspect of the present invention is a simplified method of manufacture of a photonic bandstructure LED of the pyramidal structure type described above.
  • a possible manufacturing process is shown in figures 4A to 41.
  • an n-doped GaN or InGaN layer 401 is grown by metal-organic chemical vapor deposition (MOCVD) or other similar techniques (such as MBE) on a lattice matched substrate 410. Common substrates used are Sapphire, GaN and SiC.
  • an etch stop layer 409 formed from a material such as InGaN or AIGaN is embedded in the n-GaN layer.
  • the n-GaN or InGaN layer growth is continued as layer 408 above the etch stop.
  • Multiple GaN-InGaN quantum wells 402 are grown followed by a p-type GaN layer 403.
  • the complete LED heterostructure stack produced at this stage is shown in figure 4A.
  • a mirror 404 is then deposited on top of the p-GaN layer 403 as shown in Figure 4B.
  • the reflector can be metallic comprising a layer of suitable metal, such as silver or gold, deposited by sputtering or evaporation.
  • the reflector may comprise of a dielectric multi-layer stack in the form of a distributed feedback reflector (DBR) or omni-directional reflector (ODR).
  • DBR distributed feedback reflector
  • ODR omni-directional reflector
  • the heterostructure of figure 4B is then bonded to a substrate 405.
  • the substrate 405 is preferably a metal alloy as it allows good thermal and electrical conductivity, but can comprise of other materials such as SiC or Si.
  • additional layers may be deposited on the mirror 404 to aid with the bonding process.
  • the Sapphire substrate 410 is then removed using laser lift off or other similar technique to give the heterostructure shown in figure 4D.
  • Such a structure might form a final complete conventional device, although the etch stop layer would typically be absent.
  • the laser lift-off process leaves the surface of the n-GaN layer 401 rough (typically of the order of 50nm to 300nm). In a more conventional device, this surface might be roughened further to improve light extraction.
  • the dimensions of the photonic bandstructure pyramids are large compared to the surface roughness and hence there is no need for polishing the surface prior to patterning.
  • Figures 4E to 4I show the additional process steps required to fabricate a device according to the present invention, as shown in figure 4I.
  • Figure 4E shows the deposition of the masking layers.
  • a hard mask layer 406 is deposited for subsequently transferring the desired pattern into the n-GaN layer401.
  • This can comprise of SiO 2 or Si 3 N 4 deposited by PECVD or it can be a metal deposited by sputtering or evaporation.
  • a layer of photo-resist 407 is then deposited on the hard mask 406.
  • the hard mask layer could be dispensed with and the layer of photo-resist 407 deposited directly onto the n-GaN layer 401.
  • the photo-resist 407 may be exposed using standard UV lithography to pattern it with the desired tiling arrangement.
  • the transverse shape of the exposed regions might correspond to the cross-sectional shape of the desired pyramids, or else may be simpler shapes, such as square.
  • the exposed photoresist is then developed leaving behind isolated islands of material corresponding to the desired positions of the vertices of the pyramids, as shown in figure 4F. If the hard mask 406 is present, it is then dry etched using RIE, ICP or a similar process. This step transfers the patterning from the photoresist 407 to the hard mask 406, as shown in figure 4G. The remaining photoresist 407 is then stripped.
  • the n-GaN layer 401 is crystallographically wet etched using a anisotropic wet etch, as shown in figure 4H.
  • a preferred method of wet etching GaN is by using KOH solution at concentrations ranging from 1 M to 8M with a bath temperature ranging from room temperature to 100 0 C. Etching times range around approximately 45minutes.
  • Alternative wet etchants comprise NaOH or H 3 PO 4 .
  • the crystal faces of the hexagonal pyramids that are formed by the etching process are the ⁇ 10-1-1 ⁇ planes of the GaN crystal. They form an angle of 58.4° with the base of the pyramids.
  • the presence of the etch stop layer 409 allows accurate control of the height and therefore also the base diameter of the pyramids.
  • a hard mask 406 is utilised, it is then removed using a suitable wet or dry etching process, leaving the final structure shown in figure 4I.
  • Figure 7A shows an SEM micrograph of one of the pyramids produced by this process.
  • the pyramid is isolated and is formed using the preferred fabrication embodiment, with the crystal ⁇ 10-1-1 ⁇ facet shown at 701.
  • Figure 7B shows an SEM micrograph of a photonic quasicrystal tiling of such pyramids, which have been etched in the top surface of the n-GaN using the preferred fabrication technique.
  • the pyramids are arranged in a square-triangle tiling and the constructions lines shown in figure 7B demonstrate the underlying tiling squares and triangles, while the circles highlight the vertices of the quasicrystal pattern.
  • the composite n-GaN upper region comprising layers 401 , 409 and 408 is located above the light emitting structure.
  • light is emitting from layer 402 and experiences multiple internal reflections before finally escaping through region 401.
  • a thick n-GaN growth region is necessary to reduce the defect density for the formation of high-quality quantum well (QW) layers and hence improve internal quantum efficiency of the LED.
  • the upper region acts as a protection layer for the fragile QW region 402, preventing damage during wet etching of the pyramids and minimising surface recombination in the QW region. Etching into the QW region also adversely affects the total luminous output of the LED by reducing the maximum active light emission area.
  • the pyramid dimensions required are of the order of 1.75 ⁇ m in diameter centred on a pitch of 2.5 ⁇ m.
  • the dimensions limit the minimum thickness of the layer 401 and hence it is preferred for the pyramids to reside in the thick n-GaN layer.
  • reducing the total thickness of the waveguiding region in the LED by etching reduces the number of trapped modes residing in the heterostructure. This allows the photonic bandstructures to overlap a larger percentage of the trapped modes thereby yielding improved light extraction as shown in figure 8.
  • n-GaN is also highly conductive and this property minimises the need for separate electrical current spreading layers to be deposited on the top surface of the photonic bandstructure, which would adversely affect light extraction from the device.
  • FIGS. 5A, 5B and 5C The results of numerical simulations are shown Figures 5A, 5B and 5C, demonstrating the performance of typical pyramidal photonic bandstructures devices.
  • the z-axis shows the total extraction enhancement factor compared to an unpattemed LED with a reflector.
  • the results are plotted along the x-axis 501 as a function of the fill fraction (in %) and along the y-axis 502 as a function of the pitch of the photonic bandstructure (in nm).
  • the fill fraction is defined as diameter/pitch * 100.
  • Figure 5A shows the extraction enhancement in a central 30 ° cone for a pyramidal photonic crystal, as compared to an un-patterned LED with a bottom reflector, as a function of lattice constant a and fill fraction.
  • Figure 5B shows the total extraction enhancement compared to an un-patterned LED with a bottom reflector as a function of lattice constant and fill fraction.
  • the results highlight that, as in figure 5A, the optimal operation range appears at a pitch of 2500nm and a fill fraction of 75%, with an optimized microcavity design located beneath the QWs.
  • figure 5C demonstrates the percentage of light in the 30 ° cone for a device with a pyramidal photonic crystal structure. As can be seen, up to 45% of the light emitted by the device can be directed in the central cone having a half-angle of 30° perpendicular to the surface of the device. This amounts to 84% more light in a directional cone as compared to a Lambertian light emitting device.
  • the increased directionality is attributed to the ordered arrangement of pyramids as well as the well defined angle sidewalls of the pyramids.
  • the angled sidewalls provide approximately 30% more light in the 30 degree cone compared to regular straight sidewall, etched, air rod photonic crystal LEDs.
  • Figures 6A and 6B are cross-sections through the light distribution in a plane, which shows representative far-field patterns. The result is plotted as a function of far field angle 601 along the x-axis and indicates and the light intensity 602 is normalized to that from an un-patterned LED with a bottom reflector. The far-field pattern is referenced with respect to the perpendicular to the LED surface.
  • Figure 6A shows the far-field pattern for an LED with a lattice constant of 1500nm and pyramid diameter of 1120nm, which gives a total extraction enhancement of x2.67 over and above a light emitting device with a reflector and an optimized microcavity.
  • the enhancement in the 30° cone is x4.57 and the 30 ° cone contains 40.5% of the total light extracted light.
  • Figure 6B shows the far-field pattern for an LED with a lattice constant of 2500nm and pyramid diameter of 1870nm, which gives a total extraction enhancement of x3.61 over and above a light emitting device with a reflector and an optimized microcavity.
  • the enhancement in the 30 ° cone is x5.45 and the 30° cone contains 35.8% of the total light. This amounts to 46% more light in a directional cone, as compared to a Lambertian light-emitting device.
  • Table 1 below shows a comparison of the light emitted in a narrow 30° cone angle as a percentage of the total light emitted, for the same green GaN LED configured as a simple Lambertian emitter, as a first-order photonic crystal LED comprising etched air rods, and as a pyramidal photonic crystal LED comprising etched pyramids.
  • the dimensions were optimized to extract the maximum percentage of light in the 30° cone.
  • the first- order photonic crystal dimensions comprised a lattice air rods with pitch 350nm, air-rod diameter approximately 210nm and an etch depth of around 120nm, while the pyramidal photonic crystal dimensions were as described above.
  • Figures 8A and 8B demonstrate the increased light extraction achievable when a photonic bandstructure is optimized with a microcavity light emitting device.
  • a regular photonic crystal with a pitch of 500nm and a simple reflector is compared with a pyramidal photonic crystal having the same pitch, but also possessing a microcavity reflector.
  • the total light enhancement 802 is plotted as a function of photonic crystal fill fraction 801.
  • solid line 803 indicates the total extraction enhancement for a photonic crystal normalized to the output from an unpatterned LED possessing a reflector.
  • dashed line 804 indicates the total light extraction enhancement for a photonic crystal with both a microcavity and reflector when compared to an unpatterned LED possessing a reflector.
  • Figure 8B highlights the increased extraction obtained as a result of the microcavity effect.
  • Dashed line 805 shows the increased extraction effect of the photonic crystal when the microcavity is incorporated, normalized to an unpatterned LED possessing a reflector and a microcavity, as compared to solid line 803, which shows the results for the same device when normalized to those from an unpatterned LED possessing only a simple reflector. Hence the differential increase due to the combined effects is clearly visible.
  • Figure 9 illustrates the effect of reducing the thickness of a photonic bandstructure light-emitting heterostructure region of both the pyramidal and inverted pyramidal type .
  • the enhancement in light extraction 902 as compared to a bare flat LED with a reflector is plotted against varying LED heterostructure core thicknesses 901 in nanometers. It is clearly visible that, as the thickness of the heterostructure is reduced, the amount of light extraction increases.
  • the photonic crystal pattern dimensions and geometry was fixed for all heterostructure thicknesses and the microcavity effect was not employed.
  • a novel high-order inverted pyramidal photonic crystal (PC) or quasicrystal pattern is proposed that provides increased light extraction, as compared to first-order photonic crystal patterns.
  • the PC design also allows for the tailoring of the far-field light distribution emitted by the devices.
  • the inverted pyramidal shape of the PC sub-regions and their well defined tiling arrangement permits extraction of light from the device in a beam which is more collimated than from a Lambertian source. A simplified process for manufacturing the device will also be described.
  • the higher-order PC dimensions are greater than 1.O ⁇ m in size and may be greater than 1.5 ⁇ m or 2.0 ⁇ m, and can be up to, but not limited to, 3.0 ⁇ m, 3.5 ⁇ m and 4.0 ⁇ m in size. This varies depending on the far- field pattern, the LED thickness and the location of the quantum wells within the light-generating region of the GaN heterostructure.
  • Figure 10 demonstrates a cross section of the proposed light emitting device, which comprises pyramids 1001 etched in the n-doped GaN or InGaN layer in a periodic, quasicrystalline, amorphous or other complex ordered or repeating tiling arrangement.
  • the inverted pyramidal tiling arrangement is designed to provide dispersion bands that allow trapped light to couple into the Bloch modes of the photonic crystal. Once the light is coupled into the Bloch modes, the photonic crystal provides a means of coupling the light out into free space.
  • the device shown in Figure 10 comprises a light-emitting heterostructure, which includes an n-GaN or InGaN top layer 1002 and a lower p-type GaN or InGaN layer 1004 with a multiple quantum well (MQW) structure 1003 residing between these layers.
  • MQW multiple quantum well
  • the light emitting structure is supported by a carrier substrate or mount, 1006.
  • the carrier substrate comprises an electrically conductive material with a high thermal conductivity such as metal or metal alloy or alternatively Silicon or Silicon Carbide.
  • the photonic crystal can comprise of regions with defects, that is to say regions where inverted pyramids are removed or the shape or size of the inverted pyramids is modified.
  • the sub-regions may also comprise of inverted pyramids with unetched sharp apex regions, giving rise to flat-top (frustro) inverted pyramids.
  • the patterning can be characterised by a number of parameters, including the pitch a of the lattice defined as the distance separating the centres of two neighbouring inverted pyramids, and the angle ⁇ formed between the inverted pyramid crystallographic exposed face and the horizontal crystal plane of the GaN lattice.
  • an etch stop layer, 1007 is used to control the etch depth. This allows the accurate control of the inverted pyramid diameter at the base. For a given etch depth, d, the diameter ⁇ of the base of the pyramids is:
  • a further aspect of the present invention is a simplified method of manufacture of a photonic bandstructure LED of the inverted pyramid type described above.
  • Two variations of a possible manufacturing process are shown in figures 11A to 11J.
  • an n-doped GaN or InGaN layer 1101 is grown by metal-organic chemical vapor deposition (MOCVD) or other similar techniques (such as MBE) on a lattice matched substrate 1110.
  • MOCVD metal-organic chemical vapor deposition
  • Common substrates used are Sapphire, GaN and SiC.
  • an etch stop layer 409, formed from a material such as InGaN or AIGaN is embedded in the n-GaN layer.
  • the n-GaN or InGaN layer growth is continued as layer 1108 above the etch stop.
  • the complete LED heterostructure stack produced at this stage is shown in figure 11 A.
  • a mirror 1104 is then deposited on top of the p-GaN layer 1103 as shown in Figure 11 B.
  • the reflector can be metallic comprising a layer of suitable metal, such as silver or gold, deposited by sputtering or evaporation.
  • the reflector may comprise of a dielectric multi-layer stack in the form of a distributed feedback reflector (DBR) or omni-directional reflector (ODR).
  • DBR distributed feedback reflector
  • ODR omni-directional reflector
  • the heterostructure of figure 11B is then bonded to a substrate 1105.
  • the substrate 1105 is preferably a metal alloy as it allows good thermal and electrical conductivity, but can comprise of other materials such as SiC or Si.
  • additional layers may be deposited on the mirror 1104 to aid with the bonding process.
  • the Sapphire substrate 1110 is then removed using laser lift off or other similar technique to give the heterostructure shown in figure 11 D.
  • Such a structure might form a final complete conventional device, although the etch stop layer would typically be absent.
  • the laser lift-off process leaves the surface of the n-GaN layer 1101 rough (typically of the order of
  • the dimensions of the photonic bandstructure pyramids are large compared to the surface roughness and hence there is no need for polishing the surface prior to patterning.
  • Figures 11 E to 111 or 1 U show the additional process steps required to fabricate a final device according to the present invention, as shown in figures 111 and 11J.
  • the etch stop layer 1109 is absent in the preceding steps 11 A to 11 H, but is present for the embodiment of the process leading to the device shown in figure 11 J.
  • Figure 11 E shows the deposition of the masking layers.
  • a hard mask layer 1106 is deposited for subsequently transferring the desired pattern into the n-GaN layer 1101.
  • This can comprise of SiO 2 or Si 3 N 4 deposited by PECVD or it can be a metal deposited by sputtering or evaporation.
  • a layer of photo-resist 1107 is then deposited on the hard mask 1106.
  • the hard mask layer could be dispensed with and the layer of photo-resist 1107 deposited directly onto the n- GaN layer 1101.
  • the photo-resist 1107 may be exposed using standard UV lithography to pattern it with the desired tiling arrangement.
  • the transverse shape of the exposed regions might correspond to the cross-sectional shape of the desired inverted pyramids, or else may be simpler shapes, such as square.
  • the exposed photoresist is then developed leaving behind isolated islands of material corresponding to the desired positions of the vertices of the inverted pyramids, as shown in figure 11 F. If the hard mask 1106 is present, it is then dry etched using RIE, ICP or a similar process. This step transfers the patterning from the photoresist 1107 to the hard mask 1106, as shown in figure 11G. The remaining photoresist 1107 is then stripped.
  • the n-GaN layer 1101 is crystallographically wet etched using a anisotropic wet etch, as shown in figure 11 H.
  • a preferred method of wet etching GaN is by using KOH solution at concentrations ranging from 1 M to 8M with a bath temperature ranging from room temperature to 100 0 C. Etching times range around approximately 45minut.es.
  • Alternative wet etchants comprise NaOH or H 3 PO 4 .
  • the crystal faces of the hexagonal inverted pyramids that are formed by the etching process are the ⁇ 10-1-1 ⁇ planes of the GaN crystal. They form an angle of 58.4 ° with the base of the pyramids.
  • a hard mask 1106 is utilised, it is then removed using a suitable wet or dry etching process, leaving the final structure shown in figure 111.
  • the absolute dimensions of the inverted pyramids will be determined primarily by the dimensions of the etched hard mask regions.
  • the perimeters of the hard mask will provide an etch stop barrier and allow the top surface (the c-plane) of the GaN to etch downwards forming inverted pyramids.
  • the nominal diameter of the inverted pyramid will be equal to the largest distance subtended between any two points on the perimeter of the etched hard mask region.
  • the diameter of the inverted pyramid is also determined by the selective etch rate of the different crystal planes and consequently the total etch time.
  • an etch stop layer, 1109 is present embedded in the n-doped material between layers 1101 and 1108.
  • the etch stop layer comprises a material such as AIGaN, InGaN, although other suitable materials may be used.
  • the presence of the etch stop layer 1109 allows the formation of truncated inverted pyramids (inverted frustro-pyramids) and also allows accurate control of the height of the pyramidal structure while the etch time determines the absolute diameter of the inverted pyramids.
  • Figure 11J shows the final structure after etching and hard mask removal.
  • the inset in Figure 11J shows an enlarged top view of the truncated inverted pyramid structure.
  • Figure 14 shows an SEM micrograph of one of the inverted pyramids produced by this process.
  • the pyramid is isolated and is formed using the preferred fabrication embodiment, with the crystal ⁇ 10-1-1 ⁇ facet shown at 701.
  • the composite n-GaN upper region comprising layers
  • a thick n-GaN growth region is necessary to reduce the defect density for the formation of high-quality quantum well (QW) layers and hence improve internal quantum efficiency of the LED.
  • the upper region acts as a protection layer for the fragile QW region
  • the inverted pyramid dimensions required are of the order of 1.75 ⁇ m in diameter centred on a pitch of
  • n-GaN is also highly conductive and this property minimises the need for separate electrical current spreading layers to be deposited on the top surface of the photonic bandstructure, which would adversely affect light extraction from the device.
  • Figures 12A, 12B and 12C The results of numerical simulations are shown Figures 12A, 12B and 12C, demonstrating the performance of typical inverted pyramidal photonic bandstructures devices.
  • the z-axis shows the total extraction enhancement factor compared to an unpattemed LED with a reflector.
  • the results are plotted along the y-axis 1202 as a function of the pitch of the photonic bandstructure (in nm).
  • the fill fraction is defined as diameter/pitch * 100.
  • Figure 12A shows the extraction enhancement in a central 30 ° cone for an inverted pyramidal photonic crystal, as compared to an un-patterned LED with a bottom reflector, as a function of lattice constant a and fill fraction.
  • Figure 12B shows the total extraction enhancement compared to an un-patterned LED with a bottom reflector as a function of lattice constant and fill fraction. The results highlight that, as in figure 12A, the optimal operation range appears at a pitch of 1500nm and a fill fraction of 100%, with an optimized microcavity design located beneath the QWs.
  • figure 12C demonstrates the percentage of light in the 30 ° cone for a device with an inverted pyramidal photonic crystal structure. As can be seen, up to
  • 39% of the light emitted by the device can be directed in the central cone having a half-angle of 30° perpendicular to the surface of the device. This amounts to 57% more light in a directional cone as compared to a Lambertian light emitting device.
  • the increased directionality is attributed to the ordered arrangement of inverted pyramids as well as the well defined angle sidewalls of the inverted pyramids.
  • the angled sidewalls provide approximately 15% more light in the 30 degree cone compared to regular straight sidewall, etched, air rod photonic crystal LEDs.
  • Figure 13 is a cross-section through the light distribution in a plane for an inverted pyramidal structure, which shows a representative far-field pattern. The result is plotted as a function of far field angle 1301 along the x-axis and indicates and the light intensity 1302 is normalized to that from an un-patterned LED with a bottom reflector. The far-field pattern is referenced with respect to the perpendicular to the LED surface.
  • Figure 13 shows the far-field pattern for an LED with a lattice constant of 1500nm and pyramid diameter of 1500nm, which gives a total extraction enhancement of x2.98 over and above a light emitting device with a reflector and an optimized microcavity.
  • the enhancement in the 30° cone is x4.85 and the 30° cone contains 38.6% of the total light extracted light.
  • Table 2 below shows a comparison of the light emitted in a narrow 30° cone angle as a percentage of the total light emitted, for the same green GaN LED configured as a simple Lambertian emitter, as a first-order photonic crystal LED comprising etched air rods, and as an inverted pyramidal photonic crystal LED comprising etched inverted pyramids.
  • the dimensions were optimized to extract the maximum percentage of light in the 30° cone.
  • the first-order photonic crystal dimensions comprised a lattice air rods with pitch 350nm, air-rod diameter approximately 210nm and an etch depth of around 120nm, while the inverted pyramidal photonic crystal dimensions were as described above.
  • Figures 15A and 15B demonstrate the increased light extraction achievable when a photonic bandstructure is optimized with a microcavity light emitting device.
  • a regular photonic crystal with a pitch of 500nm and a simple reflector is compared with an inverted pyramidal photonic crystal having the same pitch, but also possessing a microcavity reflector.
  • the total light enhancement 1502 is plotted as a function of photonic crystal fill fraction 1501.
  • solid line 1503 indicates the total extraction enhancement for a photonic crystal normalized to the output from an unpatterned LED possessing a reflector.
  • dashed line 1504 indicates the total light extraction enhancement for a photonic crystal with both a microcavity and reflector when compared to an unpatterned LED possessing a reflector.
  • Figure 15B highlights the increased extraction obtained as a result of the microcavity effect.
  • Dashed line 1505 shows the increased extraction effect of the photonic crystal when the microcavity is incorporated, normalized to an unpatterned LED possessing a reflector and a microcavity, as compared to solid line 1503, which shows the results for the same device when normalized to those from an unpatterned LED possessing only a simple reflector. Hence the differential increase due to the combined effects is clearly visible.
  • the present invention allows highly efficient and directional light emitting devices to be realized, thereby making them practical as alternative (solid-state) light sources to may existing sources.
  • the invention lies in the careful design of pyramidal protrusions and inverted pyramidal etches and their tiling arrangement, which gives rise to a photonic band structure that can be optimized for efficient light coupling, whilst allowing control over the propagation and far-field properties of the light emitted by the device.
  • the practicality of the device is further enhanced by the provision of a simple patterning and etching process for fabricating the devices and which can readily be used to augment existing techniques for fabricating more conventional devices.

Landscapes

  • Engineering & Computer Science (AREA)
  • Manufacturing & Machinery (AREA)
  • Computer Hardware Design (AREA)
  • Microelectronics & Electronic Packaging (AREA)
  • Power Engineering (AREA)
  • Led Devices (AREA)

Abstract

La présente invention concerne un dispositif d'émission de lumière (LED) comprenant une couche de génération de lumière disposée entre des première et seconde couches de matériau semi-conducteur, comprenant chacune un type de dopage différent. Une surface supérieure de la première couche comprend soit un agencement en pavage de protubérances pyramidales ou troncopyramidales de matériau semi-conducteur entouré par un matériau d'indice de réfraction différent, soit un agencement en pavage d'indentations pyramidales ou troncopyramidales inversées dans le matériau semi-conducteur rempli par un matériau d'indice de réfraction différent, chaque agencement étant doté d'une structure de bande photonique. Les protubérances ou indentations et leur agencement en pavage sont configurées pour extraire efficacement la lumière du dispositif par l'intermédiaire de la surface supérieure de la première couche et dans un faisceau avec un profil d'émission qui est sensiblement plus directionnel que depuis une source de Lambert. Un dispositif amélioré utilise un réflecteur sous la seconde couche pour utiliser un effet de microcavité. Les procédés de fabrication du dispositif sont également décrits, lesquels utilisent une gravure par voie humide anisotrope pour produire les protubérances pyramidales ou les indentations pyramidales inversées.
PCT/GB2007/004521 2006-11-28 2007-11-27 Dispositif d'émission de lumière cristalline photonique pyramidale. WO2008065373A1 (fr)

Priority Applications (3)

Application Number Priority Date Filing Date Title
KR1020097013252A KR101281504B1 (ko) 2006-11-28 2007-11-27 피라미드형 광 결정 발광 소자
CN2007800495892A CN101606248B (zh) 2006-11-28 2007-11-27 锥形光子晶体发光器件
HK10103622.8A HK1138432A1 (en) 2006-11-28 2010-04-14 Pyramidal photonic crystal light emitting device

Applications Claiming Priority (4)

Application Number Priority Date Filing Date Title
US11/564,213 US7700962B2 (en) 2006-11-28 2006-11-28 Inverted-pyramidal photonic crystal light emitting device
US11/564,207 US7615398B2 (en) 2006-11-28 2006-11-28 Pyramidal photonic crystal light emitting device
US11/564,207 2006-11-28
US11/564,213 2006-11-28

Publications (1)

Publication Number Publication Date
WO2008065373A1 true WO2008065373A1 (fr) 2008-06-05

Family

ID=39126629

Family Applications (1)

Application Number Title Priority Date Filing Date
PCT/GB2007/004521 WO2008065373A1 (fr) 2006-11-28 2007-11-27 Dispositif d'émission de lumière cristalline photonique pyramidale.

Country Status (4)

Country Link
KR (1) KR101281504B1 (fr)
HK (1) HK1138432A1 (fr)
TW (1) TWI342629B (fr)
WO (1) WO2008065373A1 (fr)

Cited By (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN102299243A (zh) * 2011-09-14 2011-12-28 青岛理工大学 一种薄膜倒装光子晶体led芯片及其制造方法
WO2012107759A1 (fr) * 2011-02-08 2012-08-16 Seren Photonics Limited Procédé de fabrication d'une diode électroluminescente
CN104103727A (zh) * 2013-04-09 2014-10-15 江苏稳润光电有限公司 一种提高量子效率的led芯片及其制备方法
US9190560B2 (en) 2010-05-18 2015-11-17 Agency For Science Technology And Research Method of forming a light emitting diode structure and a light diode structure
EP2597687B1 (fr) * 2011-11-23 2016-02-03 Imec Procédé de fabrication d'un dispositif GaN DEL

Families Citing this family (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
TWI459593B (zh) * 2009-06-19 2014-11-01 Fu Der Lai 發光二極體增加發光效率之結構
KR102200075B1 (ko) * 2014-12-22 2021-01-11 엘지이노텍 주식회사 자외선 발광소자 및 조명시스템

Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US20030218179A1 (en) * 2002-04-23 2003-11-27 Sharp Kabushiki Kaisha Nitride-based semiconductor light-emitting device and manufacturing method thereof
EP1526583A2 (fr) * 2003-10-21 2005-04-27 LumiLeds Lighting U.S., LLC Elément luminescent à crystal photonique
US20050285132A1 (en) * 2004-06-28 2005-12-29 Matsushita Electric Industrial Co., Ltd. Semiconductor light emitting element, semiconductor light emitting device, and method for fabricating semiconductor light emitting element
US20050285136A1 (en) * 2004-03-24 2005-12-29 Chen Ou Light-emitting device
WO2006032865A1 (fr) * 2004-09-22 2006-03-30 Mesophotonics Limited Structures a diode electroluminescente

Patent Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US20030218179A1 (en) * 2002-04-23 2003-11-27 Sharp Kabushiki Kaisha Nitride-based semiconductor light-emitting device and manufacturing method thereof
EP1526583A2 (fr) * 2003-10-21 2005-04-27 LumiLeds Lighting U.S., LLC Elément luminescent à crystal photonique
US20050285136A1 (en) * 2004-03-24 2005-12-29 Chen Ou Light-emitting device
US20050285132A1 (en) * 2004-06-28 2005-12-29 Matsushita Electric Industrial Co., Ltd. Semiconductor light emitting element, semiconductor light emitting device, and method for fabricating semiconductor light emitting element
WO2006032865A1 (fr) * 2004-09-22 2006-03-30 Mesophotonics Limited Structures a diode electroluminescente

Non-Patent Citations (2)

* Cited by examiner, † Cited by third party
Title
NG H ET AL: "GaN nanotip pyramids formed by anisotropic etching in KOH solution", JOURNAL OF APPLIED PHYSICS, vol. 94, no. 1, 2003, pages 650 - 653, XP012059700, ISSN: 0021-8979 *
ORITA K ET AL: "High-extraction-efficiency blue LED using extended-pitch photonic crystal", JAPANESE J. APPLIED PHYSICS, PART 1, vol. 43, no. 8B, August 2004 (2004-08-01), pages 5809 - 5813, XP002450042, ISSN: 0021-4922 *

Cited By (7)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US9190560B2 (en) 2010-05-18 2015-11-17 Agency For Science Technology And Research Method of forming a light emitting diode structure and a light diode structure
US10186635B2 (en) 2010-05-18 2019-01-22 Agency For Science Technology And Research Method of forming a light emitting diode structure and a light diode structure
WO2012107759A1 (fr) * 2011-02-08 2012-08-16 Seren Photonics Limited Procédé de fabrication d'une diode électroluminescente
US9070839B2 (en) 2011-02-08 2015-06-30 Seren Photonics Limited Method of manufacturing a light emitting diode
CN102299243A (zh) * 2011-09-14 2011-12-28 青岛理工大学 一种薄膜倒装光子晶体led芯片及其制造方法
EP2597687B1 (fr) * 2011-11-23 2016-02-03 Imec Procédé de fabrication d'un dispositif GaN DEL
CN104103727A (zh) * 2013-04-09 2014-10-15 江苏稳润光电有限公司 一种提高量子效率的led芯片及其制备方法

Also Published As

Publication number Publication date
TWI342629B (en) 2011-05-21
TW200847482A (en) 2008-12-01
KR101281504B1 (ko) 2013-07-03
KR20090099537A (ko) 2009-09-22
HK1138432A1 (en) 2010-08-20

Similar Documents

Publication Publication Date Title
US7700962B2 (en) Inverted-pyramidal photonic crystal light emitting device
US7615398B2 (en) Pyramidal photonic crystal light emitting device
US8946741B2 (en) LED with enhanced light extraction
EP1526583B1 (fr) Elément luminescent à crystal photonique
US8283690B2 (en) Light emitting device having light extraction structure and method for manufacturing the same
US8080480B2 (en) Method of forming fine patterns and manufacturing semiconductor light emitting device using the same
KR101898680B1 (ko) 나노구조 발광 소자
EP1624499A2 (fr) Elément semiconducteur luminescent à crystal photonique avec resaux mutiples
US9406839B2 (en) Nanostructure semiconductor light emitting device
KR101281504B1 (ko) 피라미드형 광 결정 발광 소자
JP2012513681A (ja) オプトエレクトロニクス半導体チップおよびオプトエレクトロニクス半導体チップの製造方法
TWI476957B (zh) 發光二極體的製備方法
KR20100041138A (ko) 광자 결정과 경사진 측벽이 집적된 발광 다이오드 및 그 제조방법
WO2008035932A1 (fr) Structure de diode électroluminescente à base de iii-nitrure comportant des déflecteurs monolitiques formés dans la paroi latérale, et procédés associés
TW202404069A (zh) 反射器光電子器件
TWI496322B (zh) 發光二極體

Legal Events

Date Code Title Description
WWE Wipo information: entry into national phase

Ref document number: 200780049589.2

Country of ref document: CN

121 Ep: the epo has been informed by wipo that ep was designated in this application

Ref document number: 07824715

Country of ref document: EP

Kind code of ref document: A1

NENP Non-entry into the national phase

Ref country code: DE

WWE Wipo information: entry into national phase

Ref document number: 1020097013252

Country of ref document: KR

122 Ep: pct application non-entry in european phase

Ref document number: 07824715

Country of ref document: EP

Kind code of ref document: A1