Strain relief layer

Abstract

1. A method for forming a strain-relieved layer in an epitaxial crystalline structure, the method comprising: providing a crystalline template layer comprising a material having a first naturally relaxed in-plane lattice parameter; forming a first epitaxial crystalline layer on the crystalline template layer, the first epitaxial crystalline layer having an initial conductivity higher than that of the crystalline template layer; and forming a second epitaxial crystalline layer on the first epitaxial crystalline layer, the second epitaxial crystalline layer having an electrical conductivity lower than that of the first epitaxial crystalline layer and a second naturally relaxed in-plane lattice parameter different from the first naturally relaxed in-plane lattice parameter of the crystalline template layer. forming a first epitaxial crystalline layer comprising a material having a molecular parameter; electrochemically etching the first epitaxial crystalline layer to form pores in the first epitaxial crystalline layer, allowing strain relaxation in the first epitaxial crystalline layer and / or in the second epitaxial crystalline layer by plastic deformation of bonds at an interface between the first epitaxial crystalline layer and the second epitaxial crystalline layer; and forming one or more channels comprising a conductive material through at least the first epitaxial crystalline layer and the second epitaxial crystalline layer, thereby allowing electrical connection to a crystalline template layer through the first epitaxial crystalline layer and the second epitaxial crystalline layer.

Description

[Technical Field]

[0001] TECHNICAL FIELD The present invention relates to templates and methods for forming templates, particularly, but not exclusively, the present invention relates to templates and methods for forming templates that enable the growth of strain-relaxed crystalline materials for the formation of practical devices. [Background technology]

[0002] Epitaxially grown crystalline devices, such as light emitting diode (LED) devices, are typically formed using techniques such as metal organic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE) to provide crystalline structures with layers of different compositions. The different compositions of the layers, which may be necessitated by the desire to achieve specific radiative recombination, can result in, for example, a mismatch between the crystal lattice parameters of the layers. Such a mismatch can typically result in high strain, which in turn can result in reduced internal quantum efficiency (IQE). This is a known problem in the formation of indium gallium nitride (InGaN)-based LED structures, particularly when high indium content in InGaN quantum wells (QWs) is used to provide longer wavelength light emission, e.g., red light emission.

[0003] In such cases, attempts to reduce strain in the underlying layers to improve device performance often pose additional challenges. For example, in InGaN-based devices, growing QWs on a relaxed InGaN layer, which has a larger in-plane lattice constant than gallium nitride (GaN), reduces strain and therefore improves IQE. However, achieving a high-quality relaxed InGaN layer while also enabling carrier injection within the QW to form a functional device is problematic because high-quality n-type layers with desirable doping levels are typically and advantageously formed from n-doped GaN and are not easily achieved for materials with the composition required to reduce strain and thus improve IQE in red-emitting InGaN-based LEDs.

[0004] Porous GaN layers have been proposed as a mechanism for reducing strain in subsequently grown layers because they are flexible and contain many broken bonds, which allow the propagation of misfit dislocations at the porous GaN / InGaN interface. However, such porous GaN is highly resistive and therefore generally unsuitable for the formation of functional devices. Summary of the Invention [Means for solving the problem]

[0005] To alleviate at least some of the above-mentioned problems, a method for forming a strain-relieved layer in an epitaxial crystalline structure includes providing a crystalline template layer including a material having a first naturally relaxed in-plane lattice parameter; forming a first epitaxial crystalline layer on the crystalline template layer, the first epitaxial crystalline layer having an initial conductivity higher than that of the crystalline template layer; and forming a second epitaxial crystalline layer on the first epitaxial crystalline layer, the second epitaxial crystalline layer having an electrical conductivity lower than that of the first epitaxial crystalline layer and a second naturally relaxed in-plane lattice parameter different from the first naturally relaxed in-plane lattice parameter of the crystalline template layer. a first epitaxial crystalline layer including a material having an in-plane lattice parameter; electrochemically etching the first epitaxial crystalline layer to form pores in the first epitaxial crystalline layer, allowing strain relaxation in the first epitaxial crystalline layer and / or in the second epitaxial crystalline layer by plastic deformation of bonds at an interface between the first epitaxial crystalline layer and the second epitaxial crystalline layer; and forming one or more channels including a conductive material through at least the first epitaxial crystalline layer and the second epitaxial crystalline layer, thereby allowing electrical connection to a crystalline template layer through the first epitaxial crystalline layer and the second epitaxial crystalline layer.

[0006] Also disclosed is an epitaxial crystalline structure comprising a crystalline template layer including a material having a first naturally relaxed in-plane lattice parameter; a first epitaxial crystalline layer formed on the crystalline template layer; a second epitaxial crystalline layer formed on the first epitaxial crystalline layer, the second epitaxial crystalline layer including a material having a second naturally relaxed in-plane lattice parameter different from the first naturally relaxed in-plane lattice parameter of the crystalline template layer; and one or more pores formed in the first epitaxial crystalline layer, thereby forming a second epitaxial crystalline layer. An epitaxial crystal structure is provided that includes one or more pores in which any strain in the epitaxial crystal layer is relieved by plastic deformation of bonds in the first epitaxial crystal layer and / or at the interface between the first epitaxial crystal layer and the second epitaxial crystal layer, and one or more channels containing a conductive material formed through at least the first epitaxial crystal layer and the second epitaxial crystal layer, thereby enabling electrical connection to a crystalline template layer through the first epitaxial crystal layer and the second epitaxial crystal layer.

[0007] Advantageously, porosification of the initially highly conductive layer in combination with the formation of one or more channels allows relaxation of the crystalline layer having a different in-plane relaxed lattice parameter compared to the intrinsic relaxed in-plane lattice parameter of the underlying template layer, which allows for the formation of devices that would otherwise be difficult to achieve, and further allows for electrical connection to the underlying template layer in a manner that improves device formation.

[0008] Preferably, the pores are formed to have a density of more than 50%, preferably more than 60%, and more preferably more than 70% of the volume of the first epitaxial layer between the crystalline template layer and the second epitaxial crystalline layer. Advantageously, the density of voids in the first epitaxial crystalline layer is controlled to promote plastic deformation of bonds at the interface within and / or between the first epitaxial crystalline layer.

[0009] Preferably, the second epitaxial crystalline layer includes at least one V-pit, and preferably the depth of the at least one V-pit is substantially the thickness of the combined first and second epitaxial crystalline layers, and advantageously, a V-pit of such depth allows electrical connection to the underlying crystalline template layer.

[0010] Preferably, the method includes enlarging the at least one V-pit, and preferably enlarging the at least one V-pit includes etching material from a sidewall of the at least one V-pit, thereby exposing at least a portion of the crystalline template layer. Advantageously, the V-pit facilitates electrical connection between the crystalline template layer and a layer subsequently formed on the second epitaxial crystalline layer through the porosified, electrically highly resistive porous first epitaxial crystalline layer.

[0011] Preferably, the method includes etching through at least the second epitaxial crystalline layer, thereby forming one or more islands in the second epitaxial crystalline layer. Advantageously, the formation of the islands controls relaxation of the second epitaxial crystalline layer.

[0012] Preferably, the method includes patterning the second epitaxial crystalline layer, the patterning comprising at least one of lithographic techniques and a self-assembled Ni hard mask. Advantageously, patterning the second epitaxial crystalline layer with a self-assembled Ni hard mask enables strain relief and the formation of narrow channels for electrical connection with the underlying crystalline template layer.

[0013] Preferably, the method includes forming a crystalline template layer on the patterned template layer. Advantageously, controlling the underlying morphology of the crystalline template layer facilitates improved device formation. Advantageously, formation of LED devices and arrays of LED devices, such as high-resolution micro LED arrays, with improved light distortion management, more efficient light generation, and more efficient light extraction may be achieved.

[0014] Preferably, the patterned template layer is patterned to provide trenches, and preferably, forming the crystalline template layer comprises forming overgrowth material in the trenches to provide v-grooves, thereby enabling the formation of two-dimensional regions defined at least in part by valleys in the second epitaxial crystalline layer. Advantageously, the formation of the trenches enables the size of the valleys to be tailored by the overgrowth of material in the trenches.

[0015] Preferably, the patterned template layer is patterned to provide one or more overgrowth structures, and preferably, forming the patterned crystalline template layer comprises at least partially masking the crystalline template layer to provide one or more holes, and forming an overgrowth material in the one or more holes, thereby providing one or more overgrowth structures. Advantageously, overgrowth on a patterned substrate can be used to provide structures that protrude from an initially substantially flat, planar substrate. Such morphology can be used to enable improved light-emitting characteristics from LED devices in combination with improved strain management in the underlying crystalline material on which the LED device is formed.

[0016] Preferably, when the patterned template layer is patterned to provide one or more overgrowth structures, forming one or more channels comprising conductive material through at least the first and second epitaxial crystalline layers comprises removing material from one or more sidewalls of the one or more overgrowth structures. Advantageously, removing material to form channels through at least the first and second epitaxial crystalline layers enables electrical connection to the crystalline template layer through the first and second epitaxial crystalline layers.

[0017] Preferably, the method includes planarizing at least a portion of the second epitaxial crystalline layer, thereby forming a third epitaxial crystalline layer pseudomorphic to the second epitaxial crystalline layer. Advantageously, the third epitaxial crystalline layer provides a surface for the formation of a functional device.

[0018] Preferably, the method includes forming a material in the one or more channels, preferably the material forming part of a third epitaxial crystalline layer. Preferably, the third epitaxial crystalline layer is a conductive layer, preferably the third epitaxial crystalline layer is in electrical communication with the crystalline template layer, and / or the third epitaxial layer forms part of an active region configured to emit light in response to carrier recombination. Advantageously, devices formed on the third epitaxial crystalline layer may be in contact with and electrically connected to an underlying crystalline template later. Advantageously, the third epitaxial layer may be in electrical communication with the crystalline template layer. When the third epitaxial layer forms a portion of an active region configured to emit light in response to coupling, light generation efficiency is improved. Advantageously, when the third epitaxial layer forms a portion of the active region on the sidewalls of features of the patterned crystalline template layer, improved electrical connection and carrier injection into the third epitaxial crystalline layer is provided, which, combined with the strain-relaxed second epitaxial crystalline layer, provides improved light generation from devices formed at least in part on the second epitaxial crystalline layer.

[0019] Preferably, at least one of the crystalline template layer, the first epitaxial crystalline layer, the second epitaxial crystalline layer, and the third epitaxial crystalline layer comprises a semiconductor material, and preferably the semiconductor material is a III-V based material, and more preferably the III-V material is a nitride based material. Advantageously, techniques for providing such materials can provide high quality crystalline materials with low defect densities.

[0020] Preferably, the epitaxial crystalline structure forms part of a light emitting device. Advantageously, strain relaxation is managed in combination with electrical conductivity to form a high quality practical device.

[0021] Preferably, the light emitting device forms part of an array of light emitting pixels. Advantageously, the array of light emitting pixels may be formed on a strain relieved layer, whereby the in-plane lattice parameter is matched to the materials requirements, thus resulting in higher quality devices which can be electrically connected on both sides, thereby increasing the density of pixels in the array.

[0022] Further aspects of the invention will become apparent from the description and the appended claims. [Brief explanation of the drawings]

[0023] A detailed description of embodiments of the present invention will now be given, by way of example only, with reference to the accompanying drawings, in which:

[0024] [Figure 1A] FIG. 1A shows a cross-sectional view of the epitaxial crystal structure. [Figure 1B] FIG. 1B shows a cross-sectional view of the epitaxial crystalline structure of FIG. 1A after further processing. [Figure 2A] FIG. 2A shows a cross-sectional view of an epitaxial crystal structure containing a V-pit. [Figure 2B] FIG. 2B shows a cross-sectional view of the epitaxial crystalline structure of FIG. 2A after further processing. [Figure 2C] FIG. 2C shows a cross-sectional view of the structure of FIG. 2B after further processing. [Figure 3A] FIG. 3A shows a cross-sectional view of an epitaxial structure including a V-pit and a V-groove. [Figure 3B] FIG. 3B shows a cross-sectional view of the structure of FIG. 3A after further processing. [Figure 3C] FIG. 3C shows a top view of the structure of FIG. 3B. [Figure 3D] FIG. 3D shows a cross-sectional view of the structure of FIG. 3B after further processing. [Figure 4A] FIG. 4A shows a cross-sectional view of the structure of FIG. 1B after further processing. [Figure 4B] FIG. 4B shows a cross-sectional view of the structure of FIG. 4A after further processing. [Figure 5A] FIG. 5A shows a cross-sectional view of an epitaxial structure formed on a template. [Figure 5B] FIG. 5B shows a cross-sectional view of the structure of FIG. 5A after further processing. DETAILED DESCRIPTION OF THE INVENTION

[0025] The formation of strain-relaxed epitaxial crystalline layers upon which functional solid-state devices may be formed is described. The structures and methods for forming the structures address at least some of the problems discussed above. Figures 1-5 illustrate the formation of strain-relaxed layers within epitaxial crystalline structures, where the strain-relaxed layers have a native lattice parameter that differs from the native lattice parameter of the substrate upon which the strain-relaxed layers are formed. The formation of such strain-relaxed layers results in subsequently formed layers having native crystalline lattice parameters that are more closely aligned with the strain-relaxed layers. This means that the crystals can be formed with higher crystalline quality and reduced defects.

[0026] The methods and structures described herein are based on III-V crystalline materials, particularly nitride-based semiconductor materials. However, in further examples, those skilled in the art will understand that the techniques described herein can be applied to different crystalline structures and semiconductor materials, such as other III-V crystalline materials or II-VI crystalline materials. Advantageously, the structure not only provides a template layer for forming a material having a native lattice parameter more closely aligned with the template than the underlying bulk substrate layer, but also allows for materials having a native lattice parameter more closely aligned with the template to be formed on the conductive template, thus enabling the formation of practical functional devices of high crystalline quality, such as high-resolution arrays of light-emitting diode (LED) devices.

[0027] FIG. 1A shows a cross-sectional view of an epitaxial structure 100A. The epitaxial structure 100A provides an initial template for device formation and is formed by metalorganic chemical vapor deposition (MOCVD). In further examples, alternative and / or additional growth and / or deposition techniques are used to provide the epitaxial layers described herein. In one example, molecular beam epitaxy (MBE) is used. Light-emitting diode (LED) devices are typically formed by providing a growth substrate on which multiple epitaxial layers of semiconductor crystalline materials are grown and / or deposited to form functional devices. For gallium nitride (GaN)-based LED devices, n-type doped n-GaN is typically provided as a base layer for the formation of p-n junctions. The epitaxial structure 100A enables the formation of devices, such as LED devices, with high-quality crystalline materials and provides a structure that can be further processed to enable devices to be formed on conductive layers.

[0028] 1A shows a growth substrate 102. The growth substrate 102 is formed from silicon. In further examples, alternative and / or additional materials are used to form the substrate, such as sapphire, silicon carbide, or any other suitable substrate material. On the growth substrate 102, n-type doped n-GaN is provided, effectively serving as a crystalline template layer 104. While the crystalline template layer 104 is shown as an n-type GaN layer, in further examples, alternative and / or additional layers, such as buffer layers, are included to control the properties (such as crystalline quality, doping level, thickness, etc.) of the n-GaN layer. The crystalline template layer 104 is approximately 1000 nm thick, with a thickness of 5×10 18 at / cm 3 In a further example, crystalline template layer 104 is formed to different thicknesses and has different doping concentrations, while still enabling the functionality described herein. In a further example, crystalline template layer 104 is a p-type doped layer formed to provide the functionality described herein, and the composition of subsequently formed layers is adjusted accordingly.

[0029] A first epitaxial crystalline layer 106 is formed on the n-type crystalline template layer 104, and the first epitaxial crystalline layer 106 is formed from gallium nitride (GaN). Therefore, the first epitaxial crystalline layer 106 has the same natural relaxed in-plane lattice constant as the crystalline template layer 104. In a further example, the first epitaxial crystalline layer 106 is formed from a different material and is pseudomorphic to the crystalline template layer 104. The first epitaxial crystalline layer 106 is formed as a heavily doped layer, where the doping is such that there is an initial doping contrast between the heavily doped first epitaxial crystalline layer 106 and the n-type crystalline template layer 104. This contrast allows for the porosity of the first epitaxial crystalline layer 106 in a subsequent step, as described below with reference to FIG. 1B. The layer has a thickness of approximately 30 nm and is initially 1x10 20at / cm 3 The first epitaxial crystal layer 106 is doped to a concentration on the order of 1000 Å. Such a doping concentration is achieved using Si. In a further example, co-doping with Si and Al is used to achieve such a doping concentration while preventing excessive surface roughening. In a further example, different techniques, concentrations, and thicknesses are alternatively and / or additionally used to form the first epitaxial crystal layer 106. Advantageously, the use of a thin first epitaxial crystal layer 106 on the order of 30 nm results in a highly defective layer after the porosification process, with broken bonds allowing the movement of misfit dislocations. In a further example, the first epitaxial crystal layer 106 has a thickness of 1 nm to 300 nm, preferably 20 nm to 50 nm. In a further example, the first epitaxial crystal layer 106 has a thickness of 8×10 Å. 18 at / cm 3 ~5x10 20 at / cm 3 , and preferably 3x10 19 at / cm 3 ~1x10 20 at / cm 3 In a further example, the first epitaxial crystalline layer 106 is co-doped with Al and Si having a mole fraction of 0.5% to 10%, and preferably 1% to 3%.

[0030] A second epitaxial crystalline layer having an undoped layer 108 is provided on the first epitaxial crystalline layer 106, the undoped second epitaxial crystalline layer 108 being a layer having a lattice constant that is uniquely different from the crystalline template layer 104 and the first epitaxial crystalline layer 106. In the example of FIG. 1A, the second epitaxial crystalline layer 108 is an undoped layer of indium gallium nitride (InGaN). The undoped second epitaxial crystalline layer 108 has a thickness of approximately 100 nm to 150 nm. Advantageously, the second epitaxial crystalline layer 108 has a thickness that allows for elastic relaxation. Thus, the growth of the second epitaxial crystalline layer 108 is initially strained by the underlying materials of the crystalline template layer 104 and the first epitaxial crystalline layer 106, resulting in the second epitaxial crystalline layer 108 having a different intrinsic natural in-plane lattice parameter that is pseudomorphically aligned with the intrinsic natural in-plane lattice parameter of the first epitaxial crystalline layer 106. In a further example, the second epitaxial crystalline layer 108 is formed to have different thicknesses while still enabling the functionality described herein. In one example, the second epitaxial crystalline layer 108 has a thickness between 100 nm and 500 nm. In a further example, the second epitaxial crystalline layer 108 preferably has a thickness between 150 nm and 200 nm. The second epitaxial crystalline layer 108 is a bulk layer of InGaN having an In composition of 0% to 25%, and preferably 6% to 15%. While the second epitaxial crystalline layer 108 is a bulk InGaN layer, in a further example, the second epitaxial crystalline layer 108 is a superlattice structure of multiple layers having alternating chemical compositions that result in an average indium composition in the InGaN of 0% to 25%, and preferably 6% to 15%. In one example, when the second epitaxial crystalline layer of InGaN is a superlattice structure, the inherently different lattice constant relative to the crystalline template layer 104 is related to the average composition throughout the superlattice layers that form the second epitaxial crystalline layer.

[0031] Providing such an epitaxial structure 100A with a high doping contrast between the first epitaxial crystalline layer 106, the crystalline template layer 104, and the second epitaxial crystalline layer 108 allows for the porosification of the initially highly doped first epitaxial crystalline layer 106. The porosification of the first epitaxial crystalline layer 106 is achieved by an electrochemical process. An electrochemical etching process can be used to selectively etch the epitaxial layers so that the most conductive layers are etched first. Porosification of buried layers within an epitaxial structure, such as the epitaxial structure 100A of FIG. 1A, can be achieved through threading dislocation cores in the second epitaxial crystalline layer 108. Advantageously, the top second epitaxial crystalline layer 108 does not need to be attacked, and there is no need to pattern the second epitaxial crystalline layer 108 to porosify subsurface layers. Although the second epitaxial crystalline layer 108 is described as an undoped layer 108, in further examples, The undoped second epitaxial crystalline layer 108 has a doping level that provides sufficient contrast in conductivity to enable porosification of the subsurface layer according to the methods described herein.

[0032] Electrochemical etching of the epitaxial structure 100A results in porosification of the first epitaxial crystalline layer 106 through threading dislocations or other openings in the undoped InGaN of the second epitaxial crystalline layer 108, which allow such electrochemical treatment of the subsurface first epitaxial crystalline layer 106 to occur. This is facilitated by the high doping contrast between the n-type crystalline template layer 104 and the first epitaxial crystalline layer 106, which means that only the first epitaxial crystalline layer 106 is porosified during the electrochemical etching of this layer. Advantageously, the doping contrast allows for control of the porosification process, because as the first epitaxial crystalline layer 106 has pores formed therein, the porosified first epitaxial crystalline layer 106' becomes highly resistive, and therefore the electrochemical process used to porosify the first epitaxial crystalline layer 106 stops the formation of pores.

[0033] Therefore, when the epitaxial structure 100A is processed by electrochemical etching to provide the porous first epitaxial crystalline layer 106', the first epitaxial crystalline layer 106 becomes a porous and flexible epitaxial crystalline layer 106'.

[0034] FIG. 1B shows a processed version of the epitaxial structure 100A. The epitaxial structure 100B is a processed version of the epitaxial structure 100A in which the first epitaxial crystal layer 106 has been porosified to create pores throughout the porous first epitaxial crystal layer 106′. Such porosification allows for strain relaxation of the undoped InGaN second epitaxial crystal layer 108. Such strain relaxation requires the second epitaxial crystal layer 108 to expand. As described below, the lateral expansion of the second epitaxial crystal layer 108 is controlled to provide advantageous structures. The extent to which the porosified first epitaxial crystal layer 106′ is pore- and GaN-material-rich by volume depends on the electrochemical etching process. The electrochemical etching process advantageously results in a soft material in the subsurface layer while providing a porous first epitaxial crystal layer 106' having reduced conductivity compared to the heavily doped conductivity of the first epitaxial crystal layer 106 as initially formed.

[0035] As described above, the second epitaxial crystalline layer 108, which is initially strained relative to the first epitaxial crystalline layer 106, must expand to relax. The expansion of the second epitaxial crystalline layer 108 is controlled by the formation of appropriate gaps and / or channels, as described with reference to FIGS.

[0036] FIG. 2A shows a cross-sectional view of an epitaxial structure 200A provided in accordance with the processed epitaxial structure 100B described with reference to FIG. 1B, whereby a crystalline template layer 104, a porous first epitaxial crystalline layer 106′, and a second epitaxial crystalline layer 108 are present. The epitaxial structure 200A further shows a V-pit 202 in the surface of the second epitaxial crystalline layer 108. Such a V-pit 202 is created during the formation of the n-type crystalline template layer 104. FIG. 2A also shows the top layer 105 of the crystalline template layer 104. The top layer 105 is also formed from GaN, but the formation of a V-pit in the top layer 105 is enabled by modifying the growth conditions during the formation of the crystalline template layer 104. For example, a V-pit in the top layer 105 of the crystalline template layer 104 can be initiated by lowering the growth temperature. Such a top layer 105 is on the order of 50 nm thick. Advantageously, such thickness allows for V-pins in subsequent layers of appropriate thickness to allow channels to be formed through the epitaxial structure 200A. In a further example, top layer 105 has a configuration designed to provide the functionality described herein. The cross-sectional view of epitaxial structure 200A shows one V-pit. However, in a further example, additional V-pits are formed across the planar surface of top layer 105 of the crystalline template layer. In one example, such V-pits are formed randomly across top layer 105 of the crystalline template layer.

[0037] The formation of the V-pit in the top layer 105 of the crystalline template layer 104 results in the V-pit propagating through subsequently grown layers. Thus, a V-pit 202 is formed in the surface of the second epitaxial crystalline layer 108 according to the V-pit formed in the top layer 105 of the crystalline template layer 104. In further examples, additional and / or alternative techniques are used to form the V-pit 202 in the second epitaxial crystalline layer 108.

[0038] Once one or more V-pits 202 are formed in second epitaxial crystalline layer 108, epitaxial structure 200A is processed to enlarge V-pits 202, thereby opening one or more channels and exposing crystalline template layer 104 and / or at least a portion of top layer 105 of crystalline template layer 104. This is shown in Figure 2B, where the sidewalls of V-pits 202 are shown as being etched.

[0039] FIG. 2B shows a cross-sectional view of epitaxial structure 200B, which is the epitaxial structure 200A of FIG. 2A after further processing. FIG. 2B illustrates the widening of V-pit 202 by etching the sidewalls of V-pit 202. Treating epitaxial structure 200A of FIG. 2A with wet etching allows for anisotropic etching of epitaxial structure 200A. Wet etching is performed using potassium hydroxide (KOH) at high temperatures. In further examples, additional and / or alternative techniques are used to etch and widen V-pit 202. For example, tetramethylammonium hydroxide (TMAh) is used.

[0040] As shown in FIG. 2B, the sidewalls 208 of the undoped InGaN second epitaxial crystalline layer 108 are etched to provide an open path to the conductive GaN. The sidewalls 206 of the porosified first epitaxial crystalline layer 106 and the sidewalls 205 of the V-pit-opening top layer 105 are then widened. The resulting etched V-pit 202 has a depth substantially equal to the thickness of the porosified first epitaxial crystalline layer 106' combined with the thickness of the undoped InGaN second epitaxial crystalline layer 108. Advantageously, such widening provides a route for electrical connection to the n-type crystalline template layer 104 through the epitaxial structure 200B. Once the sidewalls 205, 206, 208 of the V-pit 202 have been etched to provide one or more channels penetrating at least the porosified first epitaxial crystal layer 106' and the second epitaxial crystal layer 108, the epitaxial structure 200B is further processed as shown in FIG. 2C.

[0041] FIG. 2C shows a cross-sectional view of epitaxial structure 200C, which is the epitaxial structure 200B of FIG. 2B after further processing. FIG. 2C shows conductive material, n-type material 210, formed in the channel created by the enlarged V-pit 202 described with reference to FIGS. 2A and 2D. The n-type material 210 also forms a third epitaxial crystalline layer, which is a thin pseudomorphic layer of n-type material 210 on the second epitaxial crystalline layer 108 of undoped InGaN. The n-type material 210 makes electrical contact with the n-type material of the crystalline template layer 104, advantageously allowing for current spreading and ease of making contacts through the n-doped side of epitaxial structure 200C in subsequently formed devices. The n-type material 210 planarizes the structure. If formed of GaN, the planarizing layer of n-type material 210 will be tensile when the second epitaxial crystalline layer 108 of undoped InGaN is relaxed. The conductive n-type material 210 is formed of GaN, although in further examples, the conductive material 210 is formed of a different material. In one example, the conductive n-type material 210 is formed of InGaN having an In composition of 0% to 6%. The conductive material 210 is formed as a layer having a thickness of 5 nm to 300 nm, and preferably 20 nm to 100 nm.

[0042] The conductive material formed in the one or more channels provided by the V-pit 202 is n-type material 210 shown to planarize the structure, however, in further examples, alternatively or in addition, the conductive n-type material 210 may partially planarize the second epitaxial crystal layer 108, or may not planarize the second epitaxial crystal layer 108 while still at least partially filling the one or more channels provided by the V-pit 202 with conductive material, thereby providing conductivity to the relatively strain-relaxed regions formed following relaxation of the second epitaxial crystal layer 108.

[0043] Formed on the n-type material 210 is a further layer 212. The further layer 212, in one example, is a multi-layer epitaxial structure having an active region associated with providing red light from an InGaN-based quantum well (QW). Advantageously, the further layer 212 benefits from being formed on a conductive layer pseudomorphic to the second epitaxial crystalline layer 108. While the further layer 212 is shown as being provided on the n-type material 210, in a further example, the further layer 212 is provided on both the n-type material 210 and the second epitaxial crystalline layer 108. In a further example, the further layer 212 includes one or more LED devices, which, in a further example, form an array of devices, such as a high-resolution array of micro LED devices. In a further example, further layer 212 may alternatively and / or additionally form a portion of and / or replace n-type material 210, thereby advantageously enabling direct carrier injection into portions of strain-relieved material formed on second epitaxial crystalline layer 108 through further layer 212 formed at least partially on sidewalls of n-type crystalline template layer 104, such as V-pitted top layer 105 of crystalline template layer 104.

[0044] 2A-2C to provide a route for allowing conductive material to form on the second epitaxial crystalline layer 108 and to facilitate strain relaxation of the second epitaxial crystalline layer 108, the location and density of the V-pits 202 in the top layer 105 of the crystalline template layer 104 are somewhat random. The density of the V-pits 202 in the top layer 105 is controlled using appropriate growth techniques to enable electrical connection in a controlled manner.

[0045] 3A-3D show additional structures incorporated within the epitaxial structures 200A-C of FIGS. 2A-2C to provide further control over the resulting template structure.

[0046] Figure 3A shows a cross-sectional view of epitaxial structure 300A, which is structure 200A of Figure 2A that has been processed using additional steps.

[0047] 3A shows trenches 304 formed in crystalline template layer 104. Trench 304 is formed using lithographic techniques that use patterning and etching steps to form trench 304. In a further example, crystalline template layer 104 is processed using a different technique to provide trench 304. Trench 304 is formed according to any beneficial structure in crystalline template layer 104 that will be subsequently processed.

[0048] Once trench 304 is formed in crystalline template layer 104, subsequent layers 105, 106 6, 108 are epitaxially formed. Such formation results in a V-groove 302 in epitaxial structure 300A corresponding to trench 304. A V-pit is additionally formed in top layer 105 of crystalline template layer 104 and propagates through the layer to provide V-pit 202 in addition to V-groove 302. In a further example, V-groove 302 is provided without V-pit 202.

[0049] While the trenches 304 are shown formed in the crystalline template layer 104 prior to forming subsequent epitaxial layers to enable the formation of channels through at least the porosified first epitaxial crystalline layer 106′ and the second epitaxial crystalline layer 108, in a further example, the trenches 304 are formed by patterning the crystalline template layer 104 with a mask and forming an overgrowth material on the mask-patterned crystalline template layer 104. Such techniques are used to provide an array of pillars, each having a regular trapezoidal cross-section perpendicular to the substrate, as described, for example, in WO 2020 / 008200 A1. In a further example, the overgrowth on such a mask-patterned crystalline template layer 104 is used to provide an underlayer for the formation of an array of light-emitting devices, as described in further detail with respect to FIGS. 5A and 5B. In a further example, the morphology of crystalline template layer 104 is controlled by patterning and / or other techniques to provide a suitable underlying substrate for improved device formation.

[0050] Once V-groove 302 is formed, processing proceeds similarly to that shown with respect to Figure 2. Figure 3B shows a cross-sectional view of epitaxial structure 300B, which is the epitaxial structure 300A of Figure 3A that has been further processed.

[0051] FIG. 3B illustrates the widening of the V-pit 202 by etching its sidewalls and the widening of the V-groove 302 by etching its sidewalls. Treating the epitaxial structure 300A of FIG. 3A with a wet etch allows for anisotropic etching of the epitaxial structure 300A. As shown in FIG. 3B, the sidewalls 308 of the second epitaxial crystalline layer 108 are etched. Then, the sidewalls 306 of the porosified first epitaxial crystalline layer 106′ and the sidewalls 305 of the V-pit-opened top layer 105 are widened. Advantageously, such widening provides room for the undoped InGaN second epitaxial crystalline layer 108 to expand and thus relax strain, as well as providing a route for electrical connection to the n-type crystalline template layer 104 through the epitaxial structure 300B. Advantageously, forming the V-groove 302 in this manner provides a method for tailoring the width of the gap into which material will subsequently be formed. This is useful, for example, when lithographic techniques for forming V-grooves in an already formed epitaxial structure to provide a conductive path to n-type material are not practical for forming the V-grooves on a sufficiently small scale.

[0052] Strain relaxation is facilitated by annealing the structure 300B of FIG. 3B. In further examples, strain relaxation is facilitated using additional and / or alternative techniques. Once the sidewalls 305, 306, and 308 of the V-groove 302 are etched, the epitaxial structure 300B is further processed, as shown in FIG. 3D. FIG. 3C shows a top view 300C of the structure of FIG. 3B. Randomly distributed V-pits 202 are shown within the surface of the second epitaxial crystal layer 108. Also shown are the V-grooves 302 forming a hexagonal pattern. Thus, the V-grooves 302 isolate portions of the InGaN second epitaxial crystal layer 108 to form islands or regions with valleys that at least partially form the perimeter of one or more islands within the second epitaxial crystal layer 108, thereby at least partially surrounding one or more islands within the second epitaxial crystal layer 108. Although the V-grooves 302 are shown in a hexagonal configuration, in further examples, the V-grooves 302 may alternatively and / or additionally be formed in any suitable configuration. Formation of two-dimensional islands in the second epitaxial crystalline layer 108 based on such lithographic patterning and etching of the substrate layer 104 results in strain-relieved islands having lateral dimensions on the order of 1 μm to 10 μm. Such formation of islands is highly controllable, resulting in relaxed island regions comparable in size to devices that can be formed thereon, such as micro LED devices having pixel sizes on the order of 1 μm to 10 μm. In a further example, the lateral dimensions of the islands are controlled to result in strain-relieved islands of dimensions suitable for the formation of devices based on subsequently formed crystalline layers.

[0053] Once the epitaxial structure 300B of FIG. 3B is provided, the epitaxial structure 300B is then planarized. This is shown in FIG. 3D. FIG. 3D shows the further processed epitaxial structure 300B of FIG. 3B, resulting in structure 300D. The n-type material 210 is shown formed within the channels provided by the V-grooves 302 and V-pits 202. The random V-pits 202 filled with n-type material 210 provide uniform electron injection into subsequent layers. Similar to the epitaxial structure 200C of FIG. 2C, a subsequent pseudomorphic layer 212 is formed on the planarized layer of n-type material 210. When formed from n-GaN, the planarized layer of n-type material 210 is under tensile stress when the second epitaxial crystalline layer 108 beneath it is relaxed. The conductive material formed in the V-groove 302 and / or one or more channels provided by the etched V-groove 302 is the conductive n-type material 210 shown to planarize the structure, however, in further examples, alternatively or in addition, the conductive n-type material 210 may partially planarize the second epitaxial crystal layer 108 or may not planarize the second epitaxial crystal layer 108 while still at least partially filling the one or more channels provided by the V-pit 202 and / or V-groove 302 with conductive material, thereby providing conductivity in the relatively strain-relaxed region formed following relaxation of the second epitaxial crystal layer 108.

[0054] While the further layer 212 is shown as being provided on the n-type material 210, in further examples, the further layer 212 is provided on both the n-type material 210 and the second epitaxial crystalline layer 108. In further examples, the further layer 212 includes one or more LED devices, which in further examples form an array of devices, such as a high-resolution array of micro LED devices. In further examples, the further layer 212 may alternatively and / or additionally form a portion of the n-type material 210 and / or replace the n-type material, thereby advantageously enabling carrier injection directly into the portion of the strain-relieved material formed on the second epitaxial crystalline layer 108 through the further layer 212 formed at least partially on the sidewalls of the n-type crystalline template layer 104, such as the sidewalls of the enlarged V-groove 302.

[0055] Photolithography and etching techniques are used to form trenches 304 in crystalline template layer 104, although different patterning techniques are used in further examples. Additionally, while the relative cross-sectional sizes of V-pit 202 and V-groove 302 are shown in Figures 3A-3D as V-groove 302 being significantly deeper than V-pit 202, in further examples, V-groove 302 and V-pit 202 have different relative sizes. V-pit 202 and V-groove 302 advantageously effectively provide channels for electrical connection of crystalline template layer 104 with layers formed on insulating second epitaxial crystalline layer 108. Advantageously, V-pit 202 assists in uniform electron injection into subsequently grown layers.

[0056] Advantageously, V-grooves 302 aid in relaxation of second epitaxial crystalline layer 108, and V-pits 202 provide channels for electrical connection with crystalline template layer 104. If a subsequently formed device, such as an LED device providing pixels with defined light-emitting surfaces, has a light-emitting area greater than the density of V-grooves 302, then the structure can be provided without V-pits 202, as sufficient current spreading can be enabled through V-grooves 302. The V-pits 202 are particularly beneficial for providing sufficient current spreading within the LED device associated with the pixel when the optical area of ​​the pixel is smaller than, for example, the density of the V-grooves 302.

[0057] Figure 4A shows a cross-sectional view of an epitaxial structure 400A that is a processed version of the epitaxial structure 100B of Figure 1B. Channels are formed through the epitaxial structure 400A to allow lateral relaxation of the second epitaxial crystalline layer 108 after porosifying the first epitaxial crystalline layer 106, resulting in a porosified first epitaxial crystalline layer 106'.

[0058] The epitaxial structure 400A is patterned on the surface of the second epitaxial crystalline layer 108 using a self-assembled hard mask. A thin film of nickel (Ni) is then deposited on the surface of the second epitaxial crystalline layer 108. The nickel thin film is then annealed to form random droplets. A dry etch is then used to form channels 402 through the epitaxial structure 400A to provide channels to the conductive n-type crystalline template layer 104. The nickel mask is then removed using a wet cleaning technique. While the first epitaxial crystalline layer 106 is porosified prior to the formation of the channels 402, in a further example, the channels 402 are alternatively or additionally formed before the first epitaxial crystalline layer 106 is porosified. Advantageously, the channels 402 formed through the first epitaxial crystal layer 106 (or the porous first epitaxial crystal layer 106') and the second epitaxial crystal layer 108 at least partially define the periphery of a two-dimensional region or island within the second epitaxial crystal layer 108.

[0059] Once the channel 402 is formed, the process proceeds to FIG. 4B , which shows a cross-sectional view of an epitaxial structure 400B, which is the epitaxial structure 400A of FIG. 4A that has been further processed to form a planarized layer 404 of material within the channel 402. While the conductive material 404 formed within one or more channels is an n-type material 404, such as n-type GaN, shown to planarize the structure, in further examples, the conductive n-type material 404 may alternatively or additionally partially planarize the second epitaxial crystalline layer 108, or may not planarize the second epitaxial crystalline layer 108, while still at least partially filling one or more channels provided by the channel 402 with the conductive material 404, thereby providing electrical conductivity in the relatively strain-relaxed regions formed following relaxation of the second epitaxial crystalline layer 108.

[0060] Next, a further layer 412, an exemplary InGaN red-light-emitting active region, is formed on the material 404 in a manner similar to the further layer 212 described with reference to FIGS. 2 and 3. Such further layer 412, in a further example, forms one or more LED structures. In a further example, such LED structure forms part of an array of LED devices, such as a high-resolution array of micro LED devices. While the further layer 412 is shown provided on the n-type material 404, in a further example, the further layer 412 is provided on both the n-type material 404 and the second epitaxial crystalline layer 108. In a further example, the further layer 412 includes one or more LED devices, which, in a further example, form an array of devices, such as a high-resolution array of micro LED devices. In a further example, the further layer 412 may alternatively and / or additionally form a portion of and / or replace the n-type material 404, thereby advantageously enabling direct carrier injection into the portion of the strain-relieved material formed on the second epitaxial crystalline layer 108 through the further layer 412 formed at least partially on the sidewalls of the n-type crystalline template layer 104, such as the sidewalls of the channel 402 formed through to the crystalline template layer 104.

[0061] Advantageously, the self-assembled nickel droplets are small and dense, creating very narrow gaps. This allows for easier strain relief and planarization. Planarization can be achieved over short distances due to the narrow gaps. Further filling of this dense distribution of gaps with n-type material 210 provides for good current flow through epitaxial structure 400B.

[0062] Advantageously, the use of a hard mask, such as the self-assembled nickel hard mask described with respect to Figures 4A and 4B, enables the formation of densely packed two-dimensional islands in the second epitaxial crystalline layer 108 having lateral dimensions on the order of 250 nm to 1000 nm. Advantageously, islands formed at such a scale not only promote planarization, but they also provide a uniform strain-relieved layer in which the lateral dimensions of the islands are smaller than the lateral dimensions of devices formed in the layer above the relaxed islands, such as micro LED devices, where pixel sizes are typically larger than 1000 nm. This effectively means that a size-independent strain-relieved layer can be provided in which the scale of the relaxed islands is smaller than the scale of the functional devices formed above them.

[0063] 5A and 5B show the formation of a porosified first epitaxial crystalline layer 106′ and a second epitaxial crystalline layer 108 on a crystalline template layer 104 having a morphology provided by patterning the crystalline template layer 104.

[0064] FIG. 5A shows a cross-sectional view of an epitaxial structure 500A formed on a patterned substrate. The crystalline template layer 104 is shown as described with reference to FIGS. 1-4. The crystalline template layer 104 is patterned using well-known techniques for depositing a mask 502. While the mask 502 is shown in cross section, those skilled in the art will understand that the mask 502 can be formed two-dimensionally to provide an array of holes on the crystalline template layer. The mask 502 is formed using silicon dioxide. In further examples, the mask 502 is alternatively and / or additionally formed using additional materials. Overgrowth 504 of n-type GaN on the masked crystalline template layer results in the overgrowth 504 forming a GaN structure in electrical communication with the n-type GaN crystalline template layer 104. In FIG. 5A, the overgrowth 504 is shown forming a trapezoidal cross-sectional structure. However, one skilled in the art will appreciate that the morphology of such overgrowth 504 extends into three dimensions and is related to the underlying crystalline structure of the material used for the structure of overgrowth 504. When grown, the structure of overgrowth 504 forms part of crystalline template layer 104.

[0065] The dimensions of the trapezoidal overgrowth 504 structure described with respect to FIGS. 5A and 5B vary as a function of the height of the trapezoidal cone. The lateral dimensions of the trapezoidal feature are on the order of 4 μm at the base of the trapezoidal feature and on the order of 3 μm at the top of the trapezoidal feature, for a height of approximately 1 μm. In a further example, the lateral dimensions of the trapezoidal feature are controlled to provide alternative or additional features of different sizes. In a further example, the dimensions of the overgrowth 504 features are controlled to provide laterally sized islands in the second epitaxial crystalline layer 108 suitable for forming micro LED devices having pixel dimensions on the order of 1 μm to 10 μm. Alternatively, or in addition, island regions in the second epitaxial crystalline layer 108 having lateral dimensions smaller than 1 μm are provided. Advantageously, the formation of two-dimensional islands in the second epitaxial crystalline layer 108 based on such lithographic patterning and overgrowth results in strain-relieved islands having lateral dimensions on the order of 1 μm to 10 μm or less. Such formation of islands is highly controllable, resulting in relaxed island regions on the same order of size as devices that can be formed thereon, such as micro LED devices having pixel sizes on the order of 1 μm to 10 μm.

[0066] Once the overgrowth 504 structure is formed, an initially heavily doped first epitaxial crystalline layer 106, such as that described with reference to FIGS. 1-4, is formed. The first epitaxial crystalline layer 106 is formed on the top and sidewalls of the overgrowth 504 formed within the holes in the mask 502. The thickness of the first epitaxial crystalline layer 106 on the top of the overgrowth 504 feature is approximately 30 nm. The growth rate of crystalline material on different crystalline planes, such as those provided by the top and sidewalls of the overgrowth 504 structure, may be different and can be controlled, for example, by varying the growth rate and composition of the crystalline material. Therefore, the thicknesses of the crystalline layers deposited on different planes of the overgrowth 504 structure may be different. The thicknesses of the first epitaxial crystalline layer 106 and the second epitaxial crystalline layer 108 have been described above in terms of layer thicknesses generally perpendicular to the underlying planar substrate (and thus parallel to the primary growth direction). Where the thickness of the first epitaxial crystalline layer 106 on the top of the overgrowth 504 feature is approximately 30 nm, the thickness of the first epitaxial crystalline layer 106 on the sidewalls of the overgrowth 504 feature is on the order of 1-3 nm. In a further example, the thickness of the first epitaxial crystalline layer 106 on the sidewalls is a different thickness. Advantageously, the relatively thinner first epitaxial crystalline layer 106 on the sidewalls of the overgrowth 504 feature means that the first epitaxial crystalline layer 106 on the sidewalls of the overgrowth 504 feature can be simply etched away, as described below, to provide a channel to the underlying n-GaN.

[0067] A second epitaxial crystalline layer 108, such as that described with reference to FIGS. 1-4, is then formed on the first epitaxial crystalline layer 106. As described above with reference to FIGS. 1-4, the second epitaxial crystalline layer 108 is initially strained by the crystalline template layer 104 and the underlying material of the first epitaxial crystalline layer 106, such that the different intrinsic natural in-plane lattice parameters of the second epitaxial crystalline layer 108 are pseudomorphically aligned with the intrinsic natural in-plane lattice parameters of the first epitaxial crystalline layer 106. The second epitaxial crystalline layer 108 has a thickness of 100 nm to 150 nm over the overgrowth 504 features. As described above with respect to the first epitaxial crystalline layer 106 in FIG. 5A, the growth rates of crystalline material on different crystal planes, such as those provided by the top and sidewalls of the overgrowth 504 structure, may be different. Where the thickness of the second epitaxial crystalline layer 108 on the top of the overgrowth 504 feature is approximately 100-150 nm, the thickness of the second epitaxial crystalline layer 108 on the sidewalls of the overgrowth 504 feature is on the order of 5-20 nm. In a further example, the thickness of the second epitaxial crystalline layer 108 on the sidewalls is a different thickness.

[0068] Electrochemical etching of the epitaxial structure 500A results in porosification of the first epitaxial crystalline layer 106 through threading dislocations or other openings in the undoped InGaN of the second epitaxial crystalline layer 108, which allow such electrochemical treatment of the subsurface first epitaxial crystalline layer 106 to occur. This is facilitated by the high doping contrast between the n-type crystalline template layer 104 and the first epitaxial crystalline layer 106, which means that only the first epitaxial crystalline layer 106 is porosified during the electrochemical etching of this layer. Advantageously, the doping contrast allows control of the porosification process, because as the first epitaxial crystalline layer 106 has pores formed therein, the porosified first epitaxial crystalline layer 106′ becomes highly resistive, and therefore the electrochemical process used to porosify the first epitaxial crystalline layer 106 stops the formation of pores. FIG. 5B shows the first epitaxial crystalline layer 106' having been made porous.

[0069] FIG. 5B illustrates the structure 500A of FIG. 5A further processed to provide an array of light-emitting structures. 1 shows an epitaxial structure 500B in which a porosified first epitaxial crystalline layer 106′ is formed on top of the overgrowth 504 structure, and a second epitaxial crystalline layer 108 is formed on the porosified first epitaxial crystalline layer 106′. The porosification of the first epitaxial crystalline layer 106 results in the removal of material from the sidewalls of the overgrowth 504 structure in channels between the overgrowth 504 structure. In a further example, material from the sidewalls of the overgrowth 504 structure is removed by different means to form channels through the porosified first epitaxial crystalline layer 106′ and the second epitaxial crystalline layer 108, forming a path through the layers 106′, 108 to the n-type GaN of the GaN overgrowth 504 structure formed on the crystalline template layer 104. Advantageously, in GaN-based structures, such as the epitaxial structures 500A and 500B described with reference to Figures 5A and 5B, the growth direction is typically perpendicular to the c-plane, and the etching rate of inclined facets, such as the sidewalls of the overgrowth 504 structure, is faster than that of c-plane material, such as that formed on top of the overgrowth 504 structure. This means that the porosified first epitaxial crystalline layer 106' and the second epitaxial crystalline layer 108 can be etched from the sidewalls of the overgrowth 504 structure at a faster rate than the material formed on top of the overgrowth 504 structure is etched. Therefore, material removal to form a channel through the porosified first epitaxial crystalline layer 106' and the second epitaxial crystalline layer 108 to the n-GaN is facilitated.

[0070] The removal of the porosified first epitaxial crystalline layer 106' and second epitaxial crystalline layer 108 effectively creates two-dimensional regions or islands within the second epitaxial crystalline layer 108. Porosification of the first epitaxial crystalline layer 106 allows for strain relaxation within the undoped InGaN second epitaxial crystalline layer 108. Such strain relaxation requires the second epitaxial crystalline layer 108 to expand. As described herein, the lateral expansion of the second epitaxial crystalline layer 108 is controlled to provide advantageous structures. The extent to which the porosified first epitaxial crystalline layer 106' is pore and GaN material by volume density depends on the electrochemical etching process. The electrochemical etching process advantageously results in a soft material in the subsurface layer while providing a porosified first epitaxial crystalline layer 106' having a reduced electrical conductivity compared to the heavily doped electrical conductivity of the as-formed first epitaxial crystalline layer 106. Advantageously, the removal of the porosified first epitaxial crystalline layer 106' and the second epitaxial crystalline layer 108 to effectively create two-dimensional regions or islands in the second epitaxial crystalline layer 108 also creates channels for electrical connection of subsequently grown layers grown on the second epitaxial crystalline layer 108 with the crystalline template layer 104.

[0071] A further layer 506, similar to the further layers 212, 414 described above with respect to FIGS. 2-4, is conformally deposited and formed on the relaxed second epitaxial crystalline layer 108. The further layer 506 is an active region. The further layer 506 includes one or more quantum wells, such as InGaN-based quantum wells, designed to emit light having a dominant wavelength corresponding to red light upon carrier injection and recombination. In a further example, the further layer 506 includes additional and / or alternative layers that benefit from formation on the relatively relaxed second epitaxial crystalline layer 108. The further layer 506 is a third epitaxial crystalline layer that provides a channel for electrical conductivity from the n-GaN overgrowth 504, which forms part of the crystalline template layer 104, to the portion of the active region formed on the relaxed second epitaxial crystalline layer 108.

[0072] On the further layer 506, a p-type region 508 is formed. The p-type region 508 is formed on top of the overgrowth 504 structure and on the sidewalls of the overgrowth 504 structure. The p-type region 508 is formed from p-type doped GaN and conformally deposited on the layer 506. In further examples, additional and / or alternative layers are used to form the p-type region 508.

[0073] Advantageously, structure 500B of FIG. 5B provides an array of light emitting diode structures having active regions formed on relatively strain-relaxed layers having lattice constants that more closely match the lattice constant of the material forming the active region, thereby providing a high-quality light-emitting region with improved efficiency. Formation of such active regions on overgrowth 504 structures improves light extraction from devices based on such high-quality light-emitting regions and improves collimation.

[0074] Advantageously, when a third epitaxial layer forming an electrical connection through the porosified first epitaxial crystalline layer 106′ and the second epitaxial crystalline layer 108 forms part of the active region on one or more sidewalls of one or more of the structures of the overgrowth 504, a direct current path is provided for electrons between the crystalline template layer 104 and the active region of the further layer 506 on the islands of the second epitaxial crystalline layer 108. Advantageously, such injection is controlled, and the sidewall injection allows for a reduction in the forward voltage for multiple quantum well (MQW) structures forming part of the further layer 506 where high operating currents exist.

[0075] 5B shows mask 502, but in a further example, mask 502 is removed after the overgrowth 504 step. Further, further layers 506 and p-type regions 508 are conformally formed on the overgrowth 504 structures, although the individual structures are optionally separated, thereby enabling the formation of multiple individually addressable light emitting diode structures having a common n-type electrode formed by crystalline template layer 104.

[0076] Advantageously, the techniques and structures described herein provide a way to form a relatively strain-relaxed layer on a conductive template layer while enabling electrical connection between the conductive template layer and a layer subsequently grown on the relatively strain-relaxed layer. Advantageously, an epitaxial crystalline layer having an intrinsic in-plane lattice parameter different from that of the crystalline template layer is formed on an intervening layer, such that when the epitaxial crystalline layer is formed pseudomorphically to the intervening layer, the porosity of the intervening layer in conjunction with the porosified intervening layer and the formation of channels through the epitaxial crystalline layer enables strain relaxation in the epitaxial crystalline layer and a route for current injection in devices formed on the relaxed epitaxial crystalline layer.

[0077] Advantageously, the channels formed and filled with material to enable electrical connection overcome the difficulties associated with the use of high resistivity layers used for strain relief and thus providing functional devices. Advantageously, the use of channels having faces that are angled relative to a direction perpendicular to the planar growth direction of the epitaxial layer provides an inclined surface onto which connections can be made to the underlying crystalline template layer, while simultaneously providing strain-relaxed light-emitting structures on isolated islands of relatively strain-relaxed material having an intrinsic in-plane lattice parameter that differs from the intrinsic in-plane lattice parameter of the crystalline template layer over which the relatively strain-relaxed material is formed.

[0078] Advantageously, light emitting diode (LED) devices can be formed on the structures described with reference to Figures 1-5. Advantageously, arrays of LEDs with associated pixels can be formed on the strain-relieved layer, while the strain-relief allows for n-type contacts through to the n-type regions despite the use of highly resistive porous layers. This eliminates the need to form conductive material layers and the need to contact both p-type and n-type layers in the LED device from the same side of the device. This avoids the formation of contacts for the n-type contact on the side of the device opposite the p-type contact. This is particularly advantageous for longer wavelength InGaN-based light emitting devices, such as LEDs formed from nitride materials configured to emit light having a dominant peak wavelength corresponding to red light, in which case strain-relieved islands of undoped InGaN can be used in place of thicker conductive layers of InGaN, which are typically of lower quality. Furthermore, allowing conduction to make an n-type contact on the side of the device opposite the p-type contact means that the pixels associated with the LED devices in the array can be packed more closely together, thereby improving the resolution of display devices based on such arrays.

[0079] Although the processes described herein are presented in a particular order, those skilled in the art will appreciate that in further examples, alternative and / or additional step and process orders may be implemented while still enabling the functionality provided.

Claims

1. 1. A method for forming a strain relieved layer in an epitaxial crystalline structure, said method comprising: providing a crystalline template layer comprising a material having a first naturally relaxed in-plane lattice parameter; forming a first epitaxial crystalline layer on the crystalline template layer, the first epitaxial crystalline layer having an initial conductivity higher than a conductivity of the crystalline template layer; forming a second epitaxial crystalline layer on the first epitaxial crystalline layer, the second epitaxial crystalline layer having a lower electrical conductivity than the initial electrical conductivity of the first epitaxial crystalline layer and comprising a material having a second naturally relaxed in-plane lattice parameter different from the first naturally relaxed in-plane lattice parameter of the crystalline template layer; forming pores in the first epitaxial crystalline layer by electrochemical etching of the first epitaxial crystalline layer, allowing strain relaxation in the second epitaxial crystalline layer by plastic deformation of bonds in the first epitaxial crystalline layer and / or plastic deformation of bonds at an interface between the first epitaxial crystalline layer and the second epitaxial crystalline layer; forming one or more channels comprising a conductive material through at least the first epitaxial crystalline layer and the second epitaxial crystalline layer, thereby enabling electrical connection to the crystalline template layer through the first epitaxial crystalline layer and the second epitaxial crystalline layer; A method comprising:

2. 10. The method of claim 1, wherein the pores are formed to have a density of greater than 50%, preferably greater than 60%, and more preferably greater than 70% of the volume of the first epitaxial crystal layer between the crystalline template layer and the second epitaxial crystal layer.

3. 3. The method according to claim 1, wherein the second epitaxial crystal layer includes at least one V-pit, and preferably the depth of the at least one V-pit is substantially the thickness of the first epitaxial crystal layer and the second epitaxial crystal layer combined together.

4. and preferably, widening the at least one V-pit.

4. The method of claim 3, wherein enlarging one V-pit comprises etching material from at least one sidewall of the at least one V-pit, thereby exposing at least a portion of the crystalline template layer.

5. 5. The method of claim 1, comprising etching through at least the second epitaxial crystalline layer, thereby forming one or more islands in the second epitaxial crystalline layer, and preferably comprising patterning the second epitaxial crystalline layer, wherein the patterning comprises at least one of a lithographic technique and a self-assembled Ni hard mask.

6. The method of any one of claims 1 to 5, comprising forming the crystalline template layer on a patterned template layer.

7. 7. The method of claim 6, wherein the patterned template layer is patterned to provide trenches, and preferably, forming the crystalline template layer includes forming overgrowth material in the trenches to provide V-grooves, thereby enabling the formation of two-dimensional regions defined at least in part by valleys in the second epitaxial crystalline layer.

8. 7. The method of claim 6, wherein the crystalline template layer is patterned to provide one or more overgrowth structures, and preferably forming the patterned template layer comprises at least partially masking the crystalline template layer, providing one or more holes, and forming overgrowth material in the one or more holes, thereby providing the one or more overgrowth structures, and preferably forming the one or more channels comprising the conductive material through at least the first epitaxial crystalline layer and the second epitaxial crystalline layer comprises removing material from one or more sidewalls of the one or more overgrowth structures.

9. 9. The method of claim 1, further comprising planarizing at least a portion of the second epitaxial crystal layer, thereby forming a third epitaxial crystal layer pseudomorphic to the second epitaxial crystal layer, wherein the conductive material preferably forms part of the third epitaxial crystal layer.

10. 10. The method of claim 9, wherein the third epitaxial crystalline layer is a conductive layer, preferably in electrical communication with the crystalline template layer, and / or wherein the third epitaxial crystalline layer forms at least a portion of an active region configured to emit light in response to carrier recombination.

11. forming a third epitaxial crystalline layer; 11. The method of claim 1, wherein at least one of the crystalline template layer, the first epitaxial crystalline layer, the second epitaxial crystalline layer, and the third epitaxial crystalline layer comprises a semiconductor material, preferably the semiconductor material is a III-V based material, and more preferably the III-V based material is a nitride based material.

12. An epitaxial crystalline structure, a crystalline template layer comprising a material having a first naturally relaxed in-plane lattice parameter; a first epitaxial crystalline layer formed on the crystalline template layer; a second epitaxial crystalline layer formed on the first epitaxial crystalline layer, the second epitaxial crystalline layer being aligned with the first naturally relaxed surface of the crystalline template layer; a second epitaxial crystalline layer comprising a material having a second naturally relaxed in-plane lattice parameter different from the in-plane lattice parameter; one or more pores formed in the first epitaxial crystal layer, whereby any strain in the second epitaxial crystal layer is relieved by plastic deformation of bonds within the first epitaxial crystal layer and / or plastic deformation of bonds at an interface between the first epitaxial crystal layer and the second epitaxial crystal layer; and one or more channels comprising a conductive material formed through at least the first epitaxial crystalline layer and the second epitaxial crystalline layer, thereby enabling electrical connection to the crystalline template layer through the first epitaxial crystalline layer and the second epitaxial crystalline layer; and An epitaxial crystal structure comprising:

13. 13. The epitaxial crystalline structure of claim 12, comprising a third epitaxial crystalline layer pseudomorphically matched to the second epitaxial crystalline layer.

14. 14. The epitaxial crystal structure of claim 12 or 13, wherein the one or more pores are formed to have a density of more than 50%, preferably more than 60%, and more preferably more than 70% of the volume of the first epitaxial crystal layer between the crystal template layer and the second epitaxial crystal layer.

15. The epitaxial crystal structure according to any one of claims 12 to 14, wherein the second epitaxial crystal layer includes at least one V-pit, and preferably the depth of the at least one V-pit is substantially the thickness of the first epitaxial crystal layer and the second epitaxial crystal layer combined together.

16. 16. The epitaxial crystal structure of claim 15, wherein the at least one V-pit exposes at least a portion of the crystalline template layer.

17. The epitaxial crystalline structure of any one of claims 12 to 16, comprising one or more islands formed in the second epitaxial crystalline layer.

18. 20. The epitaxial crystal structure of claim 17, comprising one or more valleys at least partially surrounding the one or more islands in the second epitaxial crystal layer, thereby enabling the formation of one or more two-dimensional regions.

19. 14. The epitaxial crystalline structure of claim 13, wherein said conductive material forms part of said third epitaxial crystalline layer.

20. a third epitaxial crystalline layer; 20. The epitaxial crystal structure of any one of claims 12 to 19, wherein the third epitaxial crystal layer is an electrically conductive layer, preferably in electrical communication with the crystalline template layer and / or forming part of an active region configured to emit light in response to carrier recombination.

21. a third epitaxial crystalline layer; The epitaxial crystal layer according to any one of claims 12 to 20, wherein at least one of the crystalline template layer, the first epitaxial crystal layer, the second epitaxial crystal layer, and the third epitaxial crystal layer comprises a semiconductor material, preferably the semiconductor material is a III-V based material, and more preferably the III-V based material is a nitride based material. Taxial crystal structure.

22. A light emitting device comprising the epitaxial crystal structure of any one of claims 12 to 21.

23. 23. An array of light-emitting pixels comprising at least one light-emitting device according to claim 22.

24. A method for forming a light emitting device comprising the method of any one of claims 1 to 11.

25. 25. A method of forming an array of light-emitting pixels comprising the method of claim 24.

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