Light emitters on coalesced selective area grown nanocolumns

Abstract

The light-emitting structure has quantum wells derived from nanocolumns grown on a coalesced substrate. The crystalline structure has very few defects and good light-generation efficiency. By growing the nanocolumns at a lower temperature, the quantum well structure is better matched to the coalesced substrate, improving efficiency.

Description

[Technical Field]

[0001] cross reference This application claims priority to U.S. Provisional Patent Application No. 63 / 483,818, filed February 8, 2023, which is incorporated herein by reference. U.S. Patent No. 11,799,054 claims priority to U.S. Provisional Patent Application No. 63 / 483,818, granted October 24, 2023, also filed February 8, 2023.

[0002] The present technology relates to semiconductor light sources, and more particularly to highly efficient selective area grown nanocolumn monochromatic photon sources. [Background technology]

[0003] All semiconductor emitters commercially available today are fabricated using metalorganic chemical vapor deposition (MOCVD) processes for epitaxial growth. Depending on the desired emission wavelength, a base template containing GaN, GaAs, or GaP is selected to achieve the best device performance. Quantum wells (QWs) are constructed by adding indium (In) and aluminum (Al) to tune the wavelength.

[0004] Current methods used to fabricate emitters are based on thin-film technology, using different materials for different wavelengths. However, high levels of crystal structure defects due to lattice mismatch between the template and the grown structure contribute to low external quantum efficiency (EQE) in thin-film devices, especially at short wavelengths and small sizes. Bulk materials used as growth templates with low defect densities are commercially available, but are expensive.

[0005] The growth of nanocolumns for numerous applications has been reported in the scientific literature. The growth of nanocolumns emitting in the UVB (340 nm) range is described in a doctoral thesis titled "III-Nitride Nanowire Heterostructures: p-Type Conduction and High Efficiency Infrared and Ultraviolet Light Sources" by Huy Binh Le, Department of Electrical and Computer Engineering, McGill University, Montreal, Canada, October 2016. Nanocolumns grown for visible light emission can be found in a doctoral thesis titled "Selective Area Epitaxy of GaN-based Nanowire Heterostructures for Application in Photonic and Electronic Devices" by Renjie Wang, Department of Electrical and Computer Engineering, McGill University, Montreal, Canada, July 2018. However, no completed devices demonstrating practical efficiency have been demonstrated. Both spontaneous growth and selective area growth have been mentioned and, while useful, have not demonstrated efficient devices.

[0006] Selective area grown (SAG) GaN nanocolumns (NCs) are an emerging platform for nanophotonic devices. Grown using molecular beam epitaxy (MBE), these nanocolumns are significantly less prone to the crystal lattice defects common in MOCVD-grown thin-film devices. The absence of these defects promises to improve the efficiency of electrical power-to-light conversion.

[0007] However, to maintain the crystalline quality of the GaN NCs and achieve selectivity, the SAG process is performed at higher substrate temperatures (up to 900 °C) compared to the self-assembly process, where the growth temperature is approximately 750 °C. Growth at high temperatures provides nucleation and growth within the nanoholes while attempting to avoid deposition and growth on the mask. Summary of the Invention

[0008] Therefore, the object of the present technology is to remedy at least some of the disadvantages present in the prior art.

[0009] Although forming coalesced GaN nanocolumn substrates to create heterojunction light-emitting devices is known in the art (see PhD thesis entitled "III-Nitride Nanowire Heterostructures: p-Type Conduction and High Efficiency Infrared and Ultraviolet Light Sources" by Huy Binh Le, October 2016, Department of Electrical and Computer Engineering, McGill University, Montreal, Canada), fabricating multiple quantum wells on coalesced GaN nanocolumn substrates is not known in the art.

[0010] The light-emitting structure has quantum wells grown on a coalesced substrate derived from nanocolumns. The crystal structure has very few defects and good light-generation efficiency. By growing the nanocolumns at a lower temperature, the quantum well structure is better matched to the coalesced substrate, improving efficiency.

[0011] Here, each QW refers to one InGaN well (which may also include an AlGaN well in some embodiments) sandwiched between two GaN barriers. Growing InGaN / GaN multiple quantum wells on a coalesced GaN nanocolumn substrate at temperatures (approximately 650°C) lower than the typical growth temperatures for conventional GaN nanocolumns (approximately 900°C) introduces defects into the crystal structure of the active layer of the light-emitting device, reducing its light-emitting efficiency.

[0012] Reducing the temperature of the growth base from the conventional temperature of about 900°C to about 810°C may result in the formation of material on the growth base mask, thus preventing the formation of GaN nanocolumns.

[0013] Applicants have discovered that GaN nanocolumns exhibiting desirable physical properties can be formed at temperatures in the range of 770°C to 795°C, i.e., about 785°C, which provides GaN structures with fewer defects than InGaN quantum wells formed at temperatures of about 650°C.

[0014] Applicant has also found that at temperatures as low as approximately 785°C, controlling the nitrogen flux supplied to the mask relative to the amount of gallium supplied to the mask is necessary to control the deposition of Ga adatoms on the mask / openings, thereby preventing the formation of GaN nanostructures throughout the mask and promoting GaN nanocolumn growth within the openings. The combination of a controlled nitrogen flux and a controlled reduction in the crystal growth temperature compared to conventional techniques promotes the formation of nanocolumns that initiate at the mask openings and subsequently grow over the openings by capturing Ga adatoms and nitrogen atoms, thereby growing GaN nanocolumns. This control of temperature and nitrogen flux variables allows GaN nanocolumns to be grown at lower temperatures. When quantum wells are formed by doping GaN with In, reducing the process temperature to approximately 650°C to avoid the instability of InGaN associated with increased process temperatures results in InGaN / GaN quantum wells with fewer defects than those grown at temperatures above 810°C, i.e., in the range of 810°C to 900°C.

[0015] In the context of this specification, unless otherwise specified, the term "forming" refers to the initial stages of the growth process of structures such as GaN nanocolumns, coalescence substrates, quantum wells, positively doped (p-doped) GaN layers, reflective layers, etc. It is understood that, for example, there may be stages of forming the base of the nanocolumn within an opening in a mask and growing the base of the nanocolumn above the opening until it slightly protrudes beyond the mask. The term "growing" refers to the entire process of growing the nanocolumns, including the initial stages of nanocolumn formation and the later stages of nanocolumn growth, up to the point where the nanocolumns coalesce and the coalescence substrate of the GaN nanocolumns is further grown.

[0016] A reduction in defects in a crystal structure may be referred to herein as "defect-free," "minimal defects," or "minimal detectable defects." For example, unless otherwise specified, a reduction in defects in a crystal structure refers to a crystal structure that has an acceptable threshold of defects, including internal and / or surface defects, making it more compatible with subsequent layers formed thereon. When a crystal structure is more compatible with the layers formed thereon, such a structure is said to have improved compatibility with the layers and structures grown thereon. A tolerable threshold of defects in the crystal structure of a light-emitting device provides the device with an appropriate EQE and / or appropriate light-emitting characteristics. Specifically, when the surface of a GaN nanocolumn coalescence substrate has minimal or reduced defects, it is more compatible with InGaN / GaN quantum wells, which allows for the formation of multiple quantum wells on the coalescence substrate, with the multiple quantum wells also having reduced defects, resulting in the desired EQE and desired light-emitting characteristics.

[0017] According to a first broad aspect of the present technology, there is provided a method for fabricating a light-emitting structure including an array of negatively or positively doped GaN (n-GaN or p-GaN) nanocolumns with a coalescing substrate. The doped GaN nanocolumns are grown using molecular beam epitaxy. The coalescing substrate for the doped GaN nanocolumns has a crystalline structure that can be considered defect-free, improving compatibility with a multi-quantum well structure formed / grown thereon. A multi-quantum well (hereinafter, MQW) is formed on the coalescing substrate by molecular beam epitaxy by including at least indium (In). A layer of positively doped (p-doped) GaN is formed on the multi-quantum well, preferably using molecular beam epitaxy. The method can include the following steps: a. providing a growth substrate having a mask defining openings for the nanocolumns; b. heating the growth base to a first temperature of about 785°C suitable for preventing defects between the negatively or positively doped GaN coalesced substrate and the multiple quantum wells; c. growing doped GaN nanocolumns in the openings using molecular beam epitaxy at a first temperature while controlling the flux of nitrogen relative to gallium to prevent material from growing on the mask between the openings until a coalesced substrate is formed; d. heating the growth base and the coalesced substrate to a second temperature of about 650°C or less suitable for forming MQWs comprising InGaN without generating defects; e. forming alternating GaN barrier layers and at least one InGaN layer using controlled indium flux levels to provide said MQWs on a coalesced substrate using molecular beam epitaxy; f. heating the growth base, coalescing substrate, and MQWs to a third temperature of about 760°C to about 785°C; g. forming a layer of positively or negatively doped GaN on said MQWs;

[0018] Making electrical contact with positively doped GaN (p-GaN) is difficult, and as is known in the art, a metallization layer of Ni / Au (e.g., 3 nm each) can be used to make electrical contact with p-GaN. In contrast, making electrical contact with negatively doped GaN (n-GaN) is easy and does not require a metallization layer.

[0019] In one aspect of the present technology, controlling the flux of nitrogen relative to gallium results in the formation of a coalesced substrate, e.g., a coalesced substrate with minimal crystalline surface defects, which provides improved compatibility with quantum wells formed on the surface.

[0020] In another aspect of the present technique, the first temperature is in the range of 770° C. to 795° C. The first temperature is therefore defined by the fact that it is suitable for producing a crystalline structure that minimizes or prevents defects between the negatively doped GaN coalesced substrate and the multiple quantum wells.

[0021] In another aspect of the present technique, forming a layer of positively doped GaN includes producing Mg-doped GaN.

[0022] In another aspect of the present technique, the mask is a titanium nitride mask. It will be appreciated that any suitable material may be used for the mask, as known in the art.

[0023] In another aspect of the present technology, the mask is processed using one of lithography and nanoimprinting techniques to create openings. It is understood that any suitable process that creates openings of appropriate size and depth can be used, as known in the art.

[0024] In another aspect of the present technology, the growth base includes a sapphire substrate. It is understood that any suitable substrate is acceptable for receiving the initial n-doped GaN growth layer thereon. Other substrates are known in the art and have different properties that may be suitable for the present technology. The n-doped GaN growth layer may be formed on a substrate of different materials, and this formation may be at different temperatures, but other aspects of the present technology related to the growth of n-doped GaN nanocolumns, MQWs, p-doped GaN layers, etc., are as described herein, regardless of the temperature, substrate material, and process that provides the n-doped GaN growth layer on the substrate.

[0025] In another aspect of the present technology, the growth base includes a mask disposed on a negatively doped GaN growth layer. As described in the previous paragraph, the n-doped GaN growth layer is disposed on a substrate, such as a sapphire substrate. After the n-doped GaN layer is disposed on the substrate, a mask is deposited on the n-doped GaN growth layer using any suitable technique known in the art.

[0026] In another aspect of the present technology, the negatively doped GaN growth layer comprises Si-doped GaN. It is understood that the n-doped GaN growth layer can comprise other materials that provide various desired properties for the light emitting device.

[0027] In another aspect of the present technique, the step of growing negatively doped GaN nanocolumns includes lateral growth caused by attachment of Ga adatoms to the sidewalls of the negatively doped GaN nanocolumns under nitrogen-rich conditions.

[0028] In another aspect of the present technology, the method further includes growing at least five quantum wells. It is understood that any number of quantum wells can be grown using the present technology, and the number of five quantum wells is provided for illustrative purposes. A light-emitting device can have one quantum well, two quantum wells, three quantum wells, etc. When referring to "multiple quantum wells" herein, applicants refer to any number of quantum wells greater than one.

[0029] In another aspect of the present technology, the method further includes controlling the height of the GaN nanocolumns relative to the size of the openings to reduce defects in the coalescence substrate. The applicants discovered that creating small openings in a mask creates small-diameter bases for GaN nanocolumns. Such GaN nanocolumns growing from small bases may begin their formation with one or more crystalline defects near the base or near / on the surface of the n-doped GaN growth layer. If such defects exist near the base, they are minimized, bent, or terminated in the remainder of the GaN nanocolumn's crystalline structure as the GaN nanocolumn grows toward coalescence with other GaN nanocolumns. It should be noted, in particular, that the nanocolumn height must be sufficient to minimize defects propagating from the base. The smaller the opening, the lower the nanocolumn height required to minimize such defects. Consequently, the larger the opening (i.e., the larger the base), the higher the nanocolumn must be grown before coalescing with other nanocolumns to minimize such defects near the base.

[0030] In another aspect of the present technology, smaller aperture sizes allow for smaller heights of GaN nanocolumns with the desired minimum of defects.

[0031] In another aspect of the present technique, the method further comprises coating the combined substrate, the multiple quantum wells, and the layer of positively doped GaN with an insulating layer.

[0032] In another aspect of the present technology, the insulating layer is a layer of either SiNx or SiO2. It is understood that the insulating layer material can be other than SiNx and SiO2, as long as it exhibits the desired insulating properties. In some embodiments, the insulating layer can be absent, and the surrounding air or environment can act as an insulator, provided that the physical properties of the air or environment are suitable for the operation of the light emitting device.

[0033] In another aspect of the present technology, the method further includes a dry etching process step to expose the positively doped GaN layer, a Ni metal stack deposition step, and a subsequent annealing process to achieve ohmic contact. If the insulator is air or the environment, the dry etching step to expose the p-doped GaN layer may be omitted, and in this case, the metal stack may be deposited on the p-doped GaN without the dry etching step. It is also understood that if the p-doped GaN layer is covered before connecting to the ohmic contact, the dry etching process may be replaced by any other suitable process that exposes this layer. It is also understood that the Ni metal stack may be replaced by any other suitable material that achieves ohmic contact with the p-doped GaN layer.

[0034] Another aspect of the present technology further includes controlling the concentration of indium (In) in the multiple quantum wells to control the wavelength of light emitted by the light emitting structure. For example, the In concentration may vary from quantum well to quantum well, resulting in the light emitting device emitting light at different wavelengths. Each of the quantum wells may also contain materials other than indium (In) that affect the ability of the light emitting device to emit light at different wavelengths. These wavelengths may be in the visible spectrum of the human eye, the ultraviolet spectrum, the infrared spectrum, etc.

[0035] In another aspect of the present technology, the MQW can include active layers of different bandgap materials to cause light to be emitted from the MQW at several wavelengths together.

[0036] In another aspect of the present technique, the method further comprises growing an AlxGa1-xN barrier layer having an Al content between each GaN quantum well.

[0037] In another aspect of the present technology, the method further comprises polishing the back surface of the sapphire facet and adding a microlens to the back surface, It is understood that while the light emitting device can emit light without polishing and adding a microlens, in some applications at least polishing or adding at least a microlens may be desirable.

[0038] According to a second broad aspect of the present technology, there is provided a method for fabricating a light-emitting structure including an array of negatively doped GaN nanocolumns having a base and a coalesced substrate. The coalesced substrate has multiple quantum wells formed thereon. The multiple quantum wells include indium. A layer of positively doped GaN is formed on the multiple quantum wells. The method can include any of the following steps: a) Providing a growth base having a sapphire surface covered with a negatively or positively doped GaN growth layer, the GaN growth layer being covered with a mask defining an opening therein designed to form the base of a doped GaN nanocolumn. b. heating the growth base to a first temperature in the range of 770-795°C; c. providing a nitrogen source and a gallium source to the growth base and controlling the nitrogen flux relative to the gallium supplied to the growth base so that the concentration of gallium relative to the gallium nitride in the mask does not initiate deposition of Ga adatoms on the mask, but rather promotes deposition of Ga adatoms into the openings by molecular beam epitaxy and growth of GaN nanocolumns therefrom; d. forming a coalescence substrate of negatively or positively doped GaN nanocolumns from the grown negatively or positively doped GaN nanocolumns such that the coalescence substrate is composed of several negatively or positively doped GaN nanocolumns. e. heating the growth base and the combined substrate to a second temperature of less than about 650°C and providing an indium flux; f. controlling the second temperature and the indium flux supplied to the coalesced substrate to grow multiple quantum wells with indium on the coalesced substrate using molecular beam epitaxy while minimizing defects in the multiple quantum wells during growth; g. heating the growth base, coalesced substrate, and multiple quantum wells to a temperature of about 770°C to about 795°C and growing a layer of positively doped or negatively doped GaN on the multiple quantum wells using molecular beam epitaxy.

[0039] In another aspect of the present technology, defect-free coalesced substrates are formed by controlling the nitrogen flux relative to the gallium supplied to the growth base. It is understood that achieving a completely defect-free crystalline structure is highly unlikely, and therefore, coalesced substrates having some defects that are acceptable for the purposes of fabricating light-emitting devices are included herein.

[0040] In the context of this specification, unless otherwise specified, the terms "nanocolumn" and "nanocolumn structure" are used interchangeably.

[0041] In the context of this specification, unless otherwise specified, the term "nanostructure" includes any structure on the nanoscale, including nanocolumns, nanocolumns with a coalesced substrate, coalesced substrates with MQWs, coalesced substrates with MQWs and p-doped GaN layers, nanostructures with isolation and / or reflective or other layers, etc.

[0042] In the context of this specification, unless otherwise specified, the terms "negatively-doped" and "n-doped" are used interchangeably.

[0043] In the context of this specification, unless otherwise specified, the terms "positively-doped" and "p-doped" are used interchangeably.

[0044] In the context of this specification, unless otherwise specified, the terms "first temperature," "second temperature," and "third temperature" are used to indicate that these temperatures are temperatures used at different times in the methods disclosed herein. These temperatures do not imply any hierarchical or numerical relationship between the temperatures, but are used merely to distinguish them from one another for ease of understanding. It is understood that the first, second, and third temperatures can have the same measurement in °C (the same number of °C) obtained at different times during the methods disclosed herein.

[0045] Each embodiment of the present technology will have at least one, but not necessarily all, of the above-described objects and / or aspects. It will be understood that some aspects of the present technology that arise from an attempt to achieve the above-described object may not meet that object and / or may meet other objects not specifically recited herein.

[0046] Additional and / or alternative features, aspects, and advantages of embodiments of the present technology will become apparent from the following description, the accompanying drawings, and the appended claims. [Brief explanation of the drawings]

[0047] For a better understanding of the present technology and other aspects and further features, reference is made to the following description taken in conjunction with the accompanying drawings.

[0048] [Figure 1]FIG. 1 is a perspective view of a growth base including a substrate, a GaN growth layer, and a mask with an array of openings defining selective area growth, according to an embodiment of the present technique.

[0049] [Figure 2] FIG. 1 is a perspective view of a growth base with nanocolumns and a coalesced substrate thereon, in accordance with an embodiment of the present technology.

[0050] [Figure 3] 1 is a partial cross-sectional schematic diagram of a growth base including a substrate, a GaN growth layer, and a mask with an array of openings, in accordance with an embodiment of the present technique. FIG.

[0051] [Figure 4] FIG. 1 is a schematic illustration of the process of Ga adatoms reaching and diffusing onto the mask, according to an embodiment of the present technique.

[0052] [Figure 5] FIG. 10 is a schematic illustration of the process in which Ga adatoms reach the openings in the mask and begin to form the base of the nanocolumns, according to one embodiment of the present technology.

[0053] [Figure 6] Schematic diagram showing that some of the Ga adatoms migrate to the sidewalls of the nanocolumns for lateral growth, while other Ga adatoms migrate to the top of the nanocolumns for vertical growth.

[0054] [Figure 7] 1 is a schematic diagram of two nanostructures selectively grown on a growth base, the nanostructures including n-doped GaN, an MQW active region, and p-doped GaN, according to one embodiment of the present technique.

[0055] [Figure 8] FIG. 1 is a schematic diagram of two nanostructures selectively grown on a growth base, according to one embodiment of the present technique, the nanostructures including an n-doped GaN, an MQW active region, a p-doped GaN, an insulating layer, and an n-contact connected to the n-doped GaN.

[0056] [Figure 9] 1 is a schematic diagram of two nanostructures selectively grown on a growth base, according to one embodiment of the present technique, the nanostructures including n-doped GaN, an MQW active region, p-doped GaN, an insulating layer, an n-contact connected to the n-doped GaN, and a p-contact connected to the p-doped GaN.

[0057] [Figure 10] FIG. 1 is a schematic diagram of two nanostructures selectively grown on a growth base, according to one embodiment of the present technique, the nanostructures including an n-doped GaN, an MQW active region, a p-doped GaN, an insulating layer, an n-contact connected to the n-doped GaN, a p-contact connected to the p-doped GaN, and a conformally deposited layer of aluminum reflector.

[0058] [Figure 11] FIG. 1 is a schematic diagram of two nanostructures selectively grown on a growth base, according to one embodiment of the present technique, the nanostructures including an n-doped GaN, an MQW active region, a p-doped GaN, an insulating layer, an n-contact connected to the n-doped GaN, a p-contact connected to the p-doped GaN, a conformally deposited layer of aluminum reflector, and a thick metal stack Ti / Au deposited as contact pads for bonding purposes.

[0059] [Figure 12] 1 is a schematic diagram of two nanostructures selectively grown on a growth base according to one embodiment of the present technique, the nanostructures including an n-doped GaN, an MQW active region, a p-doped GaN, an insulating layer, an n-contact connected to the n-doped GaN, a p-contact connected to the p-doped GaN, a layer of conformally deposited aluminum reflector, a thick metal stack Ti / Au deposited as contact pads for bonding purposes, and a growth base with an attached microlens;

[0060] [Figure 13]1 is a schematic diagram of a conventional thin film fabricated light emitting device in accordance with an embodiment of the present technology;

[0061] [Figure 14] FIG. 1 is a schematic diagram of a light-emitting device fabricated with coalesced nanocolumns, in accordance with an embodiment of the present technology.

[0062] [Figure 15] 1A-1D are schematic diagrams of method steps for fabricating a light-emitting device using a coalesced nanocolumn process, in accordance with one embodiment of the present technology. DETAILED DESCRIPTION OF THE INVENTION

[0063] Reference will now be made in detail to embodiments of the present invention, including certain modes of carrying out the invention contemplated by the inventors as being suitable for understanding the present technology, and to specific examples of embodiments of the present invention. Examples of specific embodiments are illustrated in the accompanying drawings. While the present technology has been described in connection with these specific embodiments, it will be understood that it is not intended to limit the invention to the described embodiments. On the contrary, it is intended to cover alternatives, modifications, and equivalents that may be included within the scope of the present invention as defined by the appended claims.

[0064] In the following description, specific details are set forth in order to provide a thorough understanding of the present technology. Certain exemplary embodiments of the present technology may be practiced without some or all of these specific details. In other instances, well-known process operations have not been described in detail in order to not unnecessarily obscure the present technology.

[0065] Various techniques and mechanisms of the present technology may be described in the singular for clarity, however, it should be noted that some embodiments include multiple iterations of a technique or multiple instantiations of a mechanism unless otherwise specified.

[0066] Furthermore, various techniques and mechanisms according to the present technology may describe a connection between two entities. Note that a connection between two entities does not necessarily imply a direct and unobstructed connection because various other entities may exist between the two entities. Therefore, unless otherwise specified, a connection does not necessarily imply a direct and unobstructed connection.

[0067] 1 and 2 show a schematic representation of the selective area growth process of nanocolumns according to the present technique.

[0068] In particular, Figure 1 shows an embodiment in which the growth base 100 comprises a sapphire substrate 101. The sapphire substrate 101 is covered by an n-doped GaN growth layer 106, which in turn is covered by a mask 102. The mask 102 has an opening 103 therein, exposing the growth layer 106 through the mask 102. In the illustration, the base and nanocolumns are n-GaN with p-GaN over MQWs, although it will be understood that p-GaN can be used for the base and nanocolumns with n-GaN over MQWs.

[0069] In this exemplary embodiment, the sapphire substrate 101 has a surface 107 for forming the n-doped GaN growth layer 106 polished to a flatness of 1-2 nanometers (hereinafter, nm) RMS, and the opposite surface 108 of the substrate 101 may be coated with 2 microns of molybdenum or another highly thermally conductive material. In embodiments where a material other than sapphire is used for the substrate 101 of the growth base 100, such substrate 101 is typically polished or otherwise processed / treated to form a sufficiently smooth surface on one surface 107 for forming the n-doped GaN growth layer 106, and the surface 108 may be coated with a thermally conductive material. Molybdenum or another highly thermally conductive material can be deposited on the surface 108 of the sapphire substrate 101 by any suitable technological process, such as sputtering or evaporation. Alternative materials to sapphire include Si and GaN.

[0070] While n-doped GaN growth layer 106 may be a thin layer of approximately 2 microns, those skilled in the art will appreciate that n-doped GaN growth layer 106 may have a thickness of approximately 2-5 microns. Typically, the thickness of growth layer 106 is determined by the specific characteristics of the manufacturing equipment used to produce the light emitting device and the desired characteristics of the manufacturer. n-doped GaN growth layer 106 may be grown on sapphire substrate 101 by any suitable process known in the art, such as molecular beam epitaxy, MOCVD, or other epitaxy process.

[0071] The surface 107 of the sapphire substrate 101 coated with the n-doped GaN growth layer 106 may be further coated with about 10 nm of titanium and then processed lithographically to create an opening 103, which allows the creation of the mask 102.

[0072] 2, the openings 103 are defined to create selected areas for forming the bases 109 of the nanocolumns 104 of the combined substrate 105. The array of nanoscale openings 103 can be performed by a top-down fabrication process on the Ti mask 102.

[0073] FIG. 2 shows a nanostructure 111 grown on a growth base 100. The nanostructure 111 is an example of a type of nanostructure that can be grown using molecular beam epitaxy by selective area growth techniques. Nanocolumns 104 are shown extending from their openings 103 onto the growth base 100. The nanocolumns 104 coalesce at a certain height to form a coalesced substrate 105. The coalesced substrate 105 is shown having other layers grown thereon, which may be one or more quantum well layers 112 and a layer of p-doped GaN 113.

[0074] It is understood that nanostructure 111 may have fewer or more nanocolumns 104, nanocolumns 104 may be of any suitable height, coalescing substrate 105 may be of any suitable height, quantum well layer 112 may be thicker if it includes multiple quantum wells, and p-doped GaN layer 113 may be of any suitable thickness.

[0075] 2 also shows that the top surface 114 of structure 111 is composed of ordered semipolar planes. The shape of surface 114 is determined by the crystalline shape of the coalesced substrate 105 of n-doped GaN NC 104 because coalesced substrate 105 has a surface composed of the same ordered semipolar planes (not shown in FIG. 1B).

[0076] 1 and 2 show that the selected growth regions are spaced apart at a fixed distance from each other, which is small enough so that as the n-doped GaN nanocolumns 104 grow both vertically and laterally, they coalesce at a certain height to form a common canopy, the coalescing substrate 105.

[0077] 3 shows a portion of growth base 100, specifically a portion of a cross section of growth base 100. The portion of the cross section of growth base 100 includes sapphire substrate 101, n-doped GaN growth layer 106, and mask 102 with an array of openings 103. It can be seen that openings 103 extend through mask 102, exposing n-doped GaN growth layer 106. N-doped GaN growth layer 106 covers the entire surface of sapphire substage 101, the side opposite bottom surface 108.

[0078] The opening 103 is a nanoscale opening. If the mask 102 is a titanium mask, the opening 103 can be created using a standard electron beam lithography process or a nanoimprinting technique followed by a dry etching process. The key is to create an opening that exposes the n-doped GaN growth layer 106 from under the titanium mask 102 while avoiding damaging the n-doped GaN growth layer 106 and exposing the sapphire substrate 101.

[0079] The n-doped GaN growth layer 106 is required to form the base 109 of the nanocolumns 104 .

[0080] To initiate the molecular beam epitaxy formation of the base 109 of the n-doped GaN nanocolumns 104 and the molecular beam epitaxy growth of the n-doped GaN nanocolumns 104, the growth base 100 must be heated to a first temperature in the range of 760°C to 795°C and a nitrogen source and a gallium source must be provided.

[0081] In the prior art, the Sekiguchi and Kishino groups pioneered the growth of GaN NCs using the SAG technique on a Ti mask. In their studies, the growth temperature (Tg) was always above 900 °C, and SAG did not occur below 900 °C [1-10]. At a low Tg of 880 °C, Ga desorption and diffusion were sufficiently suppressed, resulting in nucleation in the spaces between the nanoholes. The lowest Tg reported by this group to date is 880 °C, but this Tg was measured using an imprecise pyrometer [5].

[0082] Also in the prior art, Kristine Bertness, Norman Sanford, and John Schlager at the National Institute of Standards and Technology (NIST) reported SAG GaN NCs grown on Si templates at Tg values ​​between 835°C and 844°C. However, the NCs had poor crystalline quality and a high number of pores and defects in the structure

[11] . Furthermore, in the prior art, E. Calleja and his colleagues first grew SAG GaN NCs at very high Tg values ​​between 900°C and 960°C [12, 13]. Subsequently, by improving the growth technique, the lowest Tg achieved by this group was 840°C to 880°C [14-22]. Furthermore, in the prior art, Zetian Mi's group also demonstrated low-defect GaN NCs using SAG technology, with Tg values ​​ranging from 850°C to 1030°C [23-27]. Also, in the prior art, Yong Ho Ra et al. of the Korea Institute of Ceramic Engineering & Technology reported the success of growing SAG GaN NCs with high Tg of 920-1010 °C. Finally, in the prior art, Songgui Zhao of McGill University was able to grow SAG GaN NCs with reasonable morphology and crystalline quality at 865 °C

[28] .

[0083] Applicants have discovered that using a first temperature in a range substantially different from that described above is necessary to produce nanostructures 111 with minimal crystalline defects.

[0084] After the growth base 100 is heated to a first temperature in the range of 760-795°C, the MBE process is initiated by supplying a nitrogen flux controlled relative to the gallium supplied to the growth base 100. As shown in Figure 4, Ga adatoms 400 reach the mask 102 and diffuse onto it. By controlling the first temperature and the nitrogen flux relative to gallium, the Ga adatoms 400 mostly remain within the openings 103, react with N to form GaN, and bond to the GaN substrate, thereby beginning to form the bases 109 of the n-doped GaN nanocolumns. Thus, no GaN is deposited on the mask 102. If the N flux is too high, the Ga adatoms may begin to react during their short time on the mask surface, inhibiting selective area growth (SAG).

[0085] The first temperature is then maintained in the required range of 760°C to 795°C, and while continuing the MBE process to grow nanostructures 111, Ga adatoms 400, including impinging and diffusing atoms, react with N (and some Si) to form bases 109 that extend above the top surface 107 of mask 102, as shown in Figure 5. If control of the first temperature and nitrogen flux conditions is maintained, defects in the bases 109 of n-doped GaN nanocolumns 104 are minimized. If the N flux is high enough, Ga adatoms, still present on the sides of the nanocolumns, begin to react to form GaN while migrating to the top surface, resulting in nanocolumn expansion.

[0086] 5 shows that as the MBE growth process progresses, Ga adatoms 400, including impinging and diffusing atoms, migrate onto the sidewalls 402 of the n-doped GaN nanocolumns 104, thus promoting the lateral growth of the n-doped GaN nanocolumns 104, while other Ga adatoms 400, including impinging and diffusing atoms, contribute to the vertical growth of the n-doped GaN nanocolumns 104. Maintaining control of the first temperature and nitrogen flux conditions prevents the Ga adatoms 400 from reacting with N while on the top surface 107 of the mask 102, while growing nanocolumns 104 with minimal crystal defects.

[0087] 6 shows that during the MBE process, the n-doped GaN NCs 104 eventually coalesce with controlled size, diameter, and orientation to form the coalesced substrate 105 of the nanostructure 111. Control of the first temperature and nitrogen flux conditions must be maintained at least until the NCs 104 coalesce. The resulting coalesced substrate eliminates defects at the boundaries of the coalesced substrate 105.

[0088] In an exemplary embodiment of the MBE process according to the present technique, lateral growth of n-doped GaN NCs 104 was induced under nitrogen-rich conditions by heating the growth base 100 to 785°C, where Ga adatoms reacted with N and began to attach to the sidewalls of the nanocolumns.

[0089] In another exemplary embodiment, high quality SAG n-doped GaN NCs 104 are grown at a first temperature of 780° C. to 790° C. The growth conditions for the SAG n-doped GaN NCs 104 are 17 nm / min (approximately 1.1×10 15 atoms / cm 2 / sec), Ga flux of 5 nm / min (approximately 3.7 × 10 14 atoms / cm 2 / sec) and a first temperature of 790°C.

[0090] (S)TEM studies show that GaN NCs grown by MBE exhibit a nearly defect-free structure: structural defects, including strain due to lattice mismatch, threading dislocations, and boundary defects, are bent and terminated towards the sidewalls 402 of the n-doped GaN nanocolumns 104.

[0091] In the MBE process for growing n-GaN nanocolumns, if the N flux relative to the Ga flux is too low, GaN formation fails and nanocolumns do not form. In the MBE process for growing nanocolumns, if the N flux relative to the Ga flux is too high, GaN forms too quickly and also on the mask, resulting in loss of selective growth in the openings. Because the GaN formation rate is slower at 790°C than at 900°C, the relative flux of N to Ga must be increased at lower temperatures for growing NCs.

[0092] 4, 5, and 6, by controlling the epitaxial growth rates of the n-doped GaN NCs 104 in both directions, all of the n-doped GaN NCs 104 grown simultaneously as a group on the growth base 100 can meet or coalesce on top of the n-doped GaN NCs 104 to form a continuous horizontal film of extremely high purity n-doped GaN, i.e., coalesced substrate 105. The n-doped GaN NCs 104 coalesced substrate 105 is isolated from the lattice mismatch issues encountered with all other epitaxial growth techniques known in the art, resulting in a defect-free or nearly defect-free base for forming MQWs.

[0093] In one embodiment, each of the grown n-doped GaN nanocolumns 104 has a maximum diameter or width of about 200 nm. These n-doped GaN nanocolumns 104 are grown from openings 103 spaced apart by a distance of about 50 nm. Those skilled in the art will appreciate that other dimensions are within the scope of the present technology. For example, the distance between openings 103 can be in the range of 50 nm to 100 nm, the maximum diameter of each n-doped GaN nanocolumn can be in the range of 200 nm to 400 nm, and the height of the n-doped GaN nanocolumn can be in the range of 300 nm to 600 nm.

[0094] In another embodiment, each of the grown n-doped GaN nanocolumns 104 was grown to a height of 500 nm and had a diameter of less than 200 nm. Those skilled in the art will appreciate that the dimensions of the epitaxially grown n-doped GaN NCs 104 depend on the exact growth conditions.

[0095] Growth conditions that have proven effective to applicants in providing desired results are set forth below in Table 1. Table 1 shows exemplary epitaxial growth conditions for UVA n-doped GaN nanocolumns 104 with high EQE UVA emission characteristics.

[0096] [Table 1]

[0097] Table 1 also lists variable additional materials that can be included in nanostructures grown by MBE, depending on the desired properties of the light-emitting devices fabricated using the techniques taught herein. For example, the nanostructures may be composed of Si-doped GaN segments and Mg-doped GaN segments, which can function as n- and p-contact layers, respectively.

[0098] FIG. 7 shows a nanostructure 700 consisting of a growth base 100 on which n-doped GaN nanocolumns 104 are grown. The growth base 100 has a GaN-on-sapphire substrate 101 with an n-doped GaN growth layer 106 exposed through a mask 102. The bases 109 of the n-doped GaN nanocolumns 104 are formed on the n-doped GaN growth layer 106 of the GaN-on-sapphire substrate 101. The n-doped GaN nanocolumns 104 coalesce to form a coalesced substrate 105. A multiple quantum well 701 is grown on the coalesced substrate 105 of the n-doped GaN nanocolumns 104. The MQW 701 is sandwiched between the coalesced substrate 105 of the n-doped GaN nanocolumns 104 and a p-doped GaN layer 113. The p-doped GaN layer 113 has a top surface 114. The top surface 114 is adapted for the formation of other layers thereon, as described in more detail below.

[0099] 7 also shows barrier layers 702 that may be sandwiched between each quantum well 701. For example, the barrier layers 702 may comprise GaN (undoped), AlxGa1-xN layers with an Al content xAl of about 9-15%, or other forms of GaN.

[0100] According to the present technique, the MQWs 701, the barrier layers 702, and the p-doped GaN layer 113 are grown by molecular beam epitaxy.

[0101] At least one of the MQWs 701 is GaN doped with In. The level of In affects the wavelength of the emitted light. Al can also be added, particularly to emit shorter wavelengths. Increasing the Al level allows the AlGaN to form a barrier rather than an active layer, as mentioned above.

[0102] The MBE growth process of the active and barrier layers can be repeated multiple times, preferably 3 to 8 times, to form the MQW region on the combined substrate 105. The p-doped GaN layer can have a thickness of 50 nm and can be grown by doping with magnesium as per Table 1 above. AlGaN QWs emit UV wavelengths, with the exact wavelength depending on the concentration of Al in the AlGaN, which can be controlled by the density of each component flux. For visible light, the QW material can be InGaN, with the wavelength depending on the ratio of In to Ga in each material flux.

[0103] Those skilled in the art will appreciate that the thickness of each QW 701, barrier layer 702, and p-doped GaN layer 113 may vary according to the desired design characteristics of the fabrication process for nanostructure 700.

[0104] It is understood that varying the number of MQWs 701 grown in nanostructure 700 may yield other suitable results and is within the scope of the present technology. For example, nanostructure 700 may have one quantum well 701, two quantum wells 701, three quantum wells 701, etc., up to about 10 quantum wells.

[0105] It will be understood that growth bases 100 with different substrate 101 and mask 102 templates may yield other suitable results and are within the scope of the present technology.

[0106] The fabrication process defined herein is capable of producing light emitters of different diameters, defined by groups of nanocolumns 104 that coalesce into one coalesced substrate 105 or grow together into multiple coalesced substrates 105, forming a grouped common canopy with multiple quantum wells 701 layered on each or most of the coalesced substrates 105.

[0107] After the MQW is formed, an additional barrier of AlGaN can be added by MBE to prevent electron leakage.

[0108] p-GaN is grown on the MQW. As mentioned above, the temperature for growing p-GaN 113 can be high as long as the InGaN is not destroyed by heat. MBE is effective for growing p-GaN 113, but MOCVD can also be used.

[0109] FIG. 8 illustrates that nanostructure 700 can be coated with an isolation or insulating layer 800. Insulating layer 800 can comprise SiNx or SiO2 or any other suitable material and can be deposited on n-doped GaN nanocolumns 104, MQW portion 701, p-doped GaN layer 112, and mask 102 of nanostructure 700. In some embodiments, the deposited insulating layer 800 can be planarized to form a flat surface. Insulating layer 800 covers all groups of n-doped nanocolumns 104 that form coalesced substrate 105, as well as the mask 102 between each such group. It is understood that the process for depositing insulating layer 800 can use any process available in the art.

[0110] The n-contact 802 may be defined by a photolithography step. The n-contact 802 may be defined near the mask 102 on the n-doped GaN growth layer 106. Arrow 801 indicates where the p-contact may be defined on the nanostructure 700.

[0111] As shown in Figure 9, a p-contact 900 can be fabricated on each group of n-doped GaN nanocolumns 104 grown on the growth base 100 of a light-emitting device nanostructure 700. To add the p-contact 900 to the p-doped GaN layer 113, the insulator 800 must be removed from the top surface 114 of the p-doped GaN layer 113. This may be done by etching or any other suitable technique known in the art. After exposing the top surface 114 of the p-doped GaN layer 113, the p-contact can be formed by depositing about 3 nm of nickel and about 3 nm of gold, followed by annealing the nanostructure 700 at about 450°C for about 10 minutes.

[0112] The p-contact 900 described herein can have high transparency to UVA (e.g., >92%). The p-contact can also be an ohmic contact. Note that under certain conditions, a thickness of p-contact 900 significantly greater than 3 nm can promote nonlinearly high absorption of UVA. Furthermore, a p-contact 900 significantly greater than 3 nm can impair the efficiency and / or presence of the ohmic contact required for efficient conversion of electrical energy to light in the light-emitting nanostructure 700.

[0113] As shown in FIG. 7, a reflective layer 1000 is deposited on the insulating layer 800 and the p-contact layer 900. The reflective layer may be composed of a reflective material that is approximately 50 nm thick for 365 nm. The reflective material may be Al or any suitable reflective material known in the art. It is understood that various thicknesses of the reflective material layer 1000 are considered within the scope of the present technology, including thicknesses much greater than or less than approximately 50 nm.

[0114] The reflective layer 1000 reflects light generated within the nanostructure 700 of the light-emitting device through the p-contact 900. Any other light that is reflected by the reflective layer 1000 may be guided through the nanocolumns 104 and through the substrate 101 of the growth base 100, e.g., the sapphire substrate 101, to the bottom of the light-emitting device.

[0115] In some embodiments, nanostructure 700, comprised of a growth base 100 (GaN substrate 101 and mask 101) and a plurality of n-doped GaN nanocolumns 104 layered with MQWs 701 to create one or more canopies of a coalesced substrate 105 covered with a p-doped GaN layer 113, a p-contact layer 900, and a reflective layer 1000, may be topped by a reflective layer and, optionally, another isolation layer (not shown). This final isolation layer (not shown) may serve two purposes: to protect all conductive surfaces from electrical shorting with other structures in final packaging, and to provide a surface for placement of n-pad 1102 (FIG. 11) and p-pad 1101 (FIG. 11), which must be at a higher level than nanostructure 700 for packaging purposes in commercial light-emitting devices.

[0116] Light in the nanostructure 700 is emitted in all directions by the MQWs 701. The emitted light is reflected downward from the reflective layer 1000 towards the bottom surface 108 of the emitter. Light towards the sidewalls 1001 may be reflected off the reflective layer 1000, reflected back through the bottom surface 108 through multiple reflections within the nanostructure 700, and / or absorbed within the nanostructure 700. As a result, all emitted light may be emitted through the bottom surface 108 of the substrate 101 of the growth base 100.

[0117] In embodiments having an additional insulating layer, for example a SiO insulating layer (not shown) over the reflective layer 1000, the additional insulating layer can be etched to expose the underlying p-contact 900. A metal layer 1100 can then be deposited that forms a trace for connecting the p-contact 900 to an associated p-pad 1101. Figure 11 shows the metal layer 1100 connected to the p-pad 1101 and the p-contact 900.

[0118] The n-contact 800 is connected to the bottom n-doped GaN growth layer 106 and is connected to the n-pad 1102, for example, by running a metal trace from a common layer between the n-contact 800 and the n-pad 1102, as shown in FIG.

[0119] When each such p-pad 1101 is driven while the n-pad 1102 is connected negative, a potential differential exceeding the forward voltage drop of the light-emitting device having the nanostructure 700 can induce light emission in the light-emitting device.

[0120] 12 illustrates that one or more microlenses 1200 can be added to the bottom surface 108 of the substrate 101 of the growth base 100. The microlenses 1200 can improve the light-emitting properties of the nanostructures 700 of the light-emitting device. The presence of the microlenses 1200 can further enhance light emission and guide the light for certain applications, such as imaging on photosensitive materials, such as lithography. The microlenses 1200 may be etched by plasma on the output side of the sapphire substrate 101. It is understood that the microlenses 1200 may be added to the bottom surface 108 by any other technique known in the art.

[0121] Figures 13 and 14 illustrate the types of nanostructures that can be grown using known techniques (Figure 13) and the process outlined herein (Figure 14).

[0122] 13 shows a nanostructure 1300 with QWs 1305 formed on a surface 1306 of the nanostructure 1304. The nanostructure 1304 has a diameter d equal to the diameter of the opening 1307 in the mask 1302. The nanostructure 1304 is grown on a substrate 1301.

[0123] 14 shows a nanostructure 1400 with QWs 1407 formed on the surface 1406 of a coalesced substrate 1405. The coalesced substrate 1405 has a diameter d, but is grown on nanocolumns 1404 extending from a base 1404 with openings 1408, each with a much smaller diameter.

[0124] When structures 1300 and 1400 have the same diameter d, surface 1406 is larger than surface 1306. This provides QM 1407, which generates more light than QM 1305. Because coalescing substrate 1405 is constructed from the tops of nanocolumns 1403, the crystalline structure of each nanocolumn 1403 is able to correct many defects that may exist at the junction of substrate 1401 and base 1403 of each nanocolumn 1404 during vertical growth. Nanostructures 1304 often lack the ability to correct many similar defects.

[0125] The structures obtained from the above method are known to exhibit higher EQE, i.e., higher luminous intensity per area, at comparable or lower costs (due to reduced manufacturing steps) compared to current thin-film methods, thanks to the following key results of the method:

[0126] The GaN nanocolumns on which the 1 QW is constructed have very low lattice discontinuities, resulting in a higher EQE, which is achieved by growing the GaN columns on a thin GaN layer on a sapphire substrate.

[0127] 2 Nanocolumn emitters do not exhibit a decrease in EQE as the emitter size decreases because there is no plasma ion etching in thin-film emitters that causes the EQE to decrease as the emitter size decreases.

[0128] The top of the nanocolumns is not flat (see Figure 10), but has a semi-polar surface. This makes the top of the combined structure non-planar. This increases the area of ​​the emitter compared to a thin film emitter. The increase in area can be calculated as follows:

[0129]

number

[0130] Here, θ is the tilt angle of the semipolar plane in the wurtzite GaN lattice as shown in FIG.

[0131] Depending on the growth conditions, θ varies from 30° to 58°, with a corresponding increase in the active area of ​​25% to 100%.

[0132] Therefore, the total emission per area of ​​the emitter is higher than in thin film devices.

[0133] FIG. 15 illustrates steps in a method 1500 for manufacturing a light emitting device according to the present technique.

[0134] Although numerous specific details are set forth in the above description, embodiments of the present invention may be practiced without these specific details. Well-known circuits, structures, and techniques have not been shown in detail to avoid obscuring an understanding of this description. "Embodiments," "various embodiments," and the like indicate that the described embodiments may include a particular feature, structure, or characteristic, but not all embodiments necessarily include the particular feature, structure, or characteristic. Some embodiments may not have some, all, or any of the features described for other embodiments. "Connected" can indicate that elements are in direct physical or electrical contact with each other, and "coupled" can indicate that elements cooperate or interact with each other, but they may or may not be in direct physical or electrical contact. Also, although similar or identical numbers may be used to designate the same or similar parts in different figures, doing so does not imply that all figures containing similar or identical numbers constitute a single or the same embodiment.

[0135] Some of these steps are well known in the art and, therefore, have been omitted in certain portions of this specification for the sake of brevity.

[0136] Modifications and improvements to the above-described embodiments of the technology may become apparent to those skilled in the art. The foregoing description is intended to be illustrative, not limiting. Accordingly, the scope of the technology is intended to be limited only by the appended claims.

Claims

1. 1. A method for fabricating a light emitting structure comprising an array of negatively or positively doped GaN nanocolumns with a coalescence substrate, wherein the nanocolumns are grown using molecular beam epitaxy at low temperatures to improve the compatibility of the coalescence substrate with multiple quantum well structures; multiple quantum wells are formed on the coalescence substrate by molecular beam epitaxy through the inclusion of indium (In); and a layer of positively or negatively doped GaN is formed on the multiple quantum wells using molecular beam epitaxy; providing a growth substrate having a mask defining openings for the nanocolumns; heating the growth substrate to a first temperature of about 785°C suitable for reducing defects in the multiple quantum wells formed on the combined substrate; growing the negatively or positively doped GaN nanocolumns in the openings using molecular beam epitaxy at the first temperature while controlling the flux of nitrogen relative to gallium to prevent growth of material on the mask between the openings until the coalesced substrate is formed; heating the growth substrate and the combined substrate to a second temperature of about 650° C. or less suitable for forming the multiple quantum wells having at least indium with improved compatibility with the combined substrate; controlling the indium flux to form GaN barrier layers and at least one InGaN active layer, and providing the multiple quantum wells on the combined substrate using molecular beam epitaxy; heating the growth substrate and the combined substrate to a third temperature of about 760 to about 785°C; forming an upper layer of said positively or negatively doped GaN on said multiple quantum well (MQW).

2. The method of claim 1 , wherein the layer of positively doped GaN is formed using molecular beam epitaxy.

3. 3. The method of claim 1 or 2, wherein the first temperature is in the range of 770°C to 795°C.

4. 4. The method of claim 1, 2 or 3, wherein the positively doped GaN is Mg-doped GaN.

5. 5. The method of claim 1, wherein the mask is a titanium nitride mask.

6. The method of claim 1 , wherein the mask is processed using one of lithography and nanoimprint techniques to create the openings.

7. The method of claim 1 , wherein the growth substrate comprises a sapphire substrate.

8. The method of claim 1 , wherein the growth substrate comprises the mask on a negatively doped GaN growth layer.

9. The method of claim 8 , wherein the negatively doped GaN growth layer comprises Si-doped GaN.

10. 10. The method of claim 1, further comprising growing at least five quantum wells.

11. 11. The method of claim 1, further comprising coating the combined substrate, the multiple quantum wells, and the top layer with an isolation layer.

12. The separation layer is made of SiNx and SiO ２ The method of claim 11 , wherein the layer is one of:

13. 13. The method according to any one of claims 1 to 12, comprising the steps of a dry etching process to expose the top layer, followed by depositing Ni and Au, followed by an annealing process to form ohmic contacts.

14. 14. The method of claim 1, further comprising controlling the concentration of In in the multiple quantum wells to determine the wavelength of light emitted by the light emitting structure.

15. 15. The method of any one of claims 1 to 14, wherein the MQW emits light at different wavelengths due to layers having different bandgaps.

16. 16. The method of claim 15, wherein the different wavelengths combine to provide white light.

17. 17. The method of claim 1, further comprising growing an AlxGa1-xN barrier layer having an Al content between each GaN quantum well.

18. 18. The method of claim 1, further comprising polishing a backside of the sapphire side of the growth substrate and adding a microlens to the backside.

19. 19. The method of any one of claims 1 to 18, wherein the coalesced substrate is composed of ordered semi-polar planes.

Images