Abstract:
A method for growth of indium-containing nitride films is described, particularly a method for fabricating a gallium, indium, and nitrogen containing material. On a substrate having a surface region a material having a first indium-rich concentration is formed, followed by a second thickness of material having a first indium-poor concentration. Then a third thickness of material having a second indium-rich concentration is added to form a sandwiched structure which is thermally processed to cause formation of well-crystallized, relaxed material within a vicinity of a surface region of the sandwich structure.

Description:
FIELD 
     The present disclosure relates generally to techniques for growing indium-containing nitride films. More specifically, the disclosure includes a process for forming materials such as InGaN and/or AlInGaN or AlInN that are nucleated heteroepitaxially directly on a substrate without first forming crystalline GaN. In various embodiments, deposition is performed at relatively low temperature, via molecular beam epitaxy, hydride vapor phase epitaxy, metalorganic chemical vapor deposition, or atomic layer epitaxy. There are other embodiments as well. 
     BACKGROUND 
     Indium-containing nitride films are important in a number of applications, including epitaxial InGaN layers in light-emitting diodes and laser diodes. Typically, these films are grown on a GaN layer, which in turn is either deposited heteroepitaxially on a non-GaN substrate such as sapphire or silicon carbide, or homoepitaxially on a bulk or quasi-bulk GaN substrate. Unfortunately, these conventional techniques are often inadequate. Thus, improved method and system for forming indium-containing nitride films are desirable. 
     BRIEF SUMMARY 
     The present disclosure relates generally to techniques for growing indium-containing nitride films. More specifically, the disclosure includes a process for forming materials such as InGaN and/or AlInGaN or AlInN that are nucleated heteroepitaxially directly on a substrate without first forming crystalline GaN. In various embodiments, deposition is performed at relatively low temperature, via molecular beam epitaxy, hydride vapor phase epitaxy, metalorganic chemical vapor deposition, or atomic layer epitaxy. There are other embodiments as well. 
     According to one embodiment, the disclosure provides a method for fabricating a gallium, indium, and nitrogen containing material. The method includes providing a substrate having a surface region. The method also includes forming a first thickness of material having a first indium-rich concentration. The method includes forming a second thickness of material having a first indium-poor concentration overlying the first thickness of material. The method additionally includes forming a third thickness of material having a second indium-rich concentration to form a sandwiched structure including at least the first thickness of material, the second thickness of material, and third thickness of material. Moreover, the method includes processing the sandwiched structure using at least a thermal process to cause formation of well-crystallized, relaxed material within a vicinity of a surface region of the sandwiched structure. The sandwiched structure has an overall thickness of 100 nm and greater. 
     According to another embodiment, the present disclosure provides an indium, gallium and nitrogen containing substrate structure. The structure includes a substrate comprising a surface region. The structure also includes a thickness of a gallium, indium, and nitrogen containing crystalline material overlying the surface region and configured in a stain free manner from an alternating sequence of at least two materials including an indium and nitrogen containing material and a gallium and nitrogen containing material. The thickness is at least 100 nm and is substantially strain-free and well-crystallized. 
     Among the benefits of the disclosed techniques are InGaN and/or AlInGaN or AlInN are nucleated heteroepitaxially directly on a substrate without first forming crystalline GaN. Deposition is performed at relatively low temperature, via molecular beam epitaxy, hydride vapor phase epitaxy, metalorganic chemical vapor deposition, or atomic layer epitaxy, in order to avoid undesired segregation and facet formation. The crystalline InGaN or AlInGaN or AlInN film formed thereupon has superior homogeneity at thicknesses greater than about 500 nm than the prior art. In a specific embodiment, the techniques according to the present disclosure can be performed in a relatively simple and cost effective manner. Depending upon the embodiment, the methods and systems according to the present disclosure can be performed using conventional materials and/or equipments according to one of ordinary skill in the art. There are other benefits as well. 
     A further understanding of the nature and advantages of the techniques of the disclosure may be realized by reference to the following specification and attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is simplified diagram illustrating a substrate for processing according to an embodiment of the present disclosure. 
         FIG. 2  is a simplified diagram illustrating using a reactor  200  for processing one or more substrates according to an embodiment of the present disclosure. 
         FIG. 3  shows a cross-sectional view of a reactor near the susceptor according to an embodiment of the present disclosure. 
         FIG. 4  is a simplified diagram illustrating using a reactor  400  for processing one or more substrates according to an embodiment of the present disclosure. 
         FIG. 5  illustrates a suitable sequence of gas flows to enable deposition of alternating indium-rich and indium-poor layers according to an embodiment of the present disclosure. 
         FIG. 6  illustrates a showerhead system for processing a substrate according to an embodiment of the present disclosure. 
         FIG. 7  is a simplified diagram illustrating a gas flow system for processing a substrate according to an embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     The present disclosure relates generally to techniques for growing indium-containing nitride films. More specifically, the disclosure includes a process for forming materials such as InGaN and/or AlInGaN or AlInN that are nucleated heteroepitaxially directly on a substrate without first forming crystalline GaN. In various embodiments, deposition is performed at relatively low temperature, via molecular beam epitaxy, hydride vapor phase epitaxy, metalorganic chemical vapor deposition, or atomic layer epitaxy. There are other embodiments as well. 
     As explained above, conventional techniques for forming crystalline GaN and indium-containing nitride films are often inadequate and have various limitations. It is well known that as the fraction of InN in the InGaN layer increases beyond a certain point, the internal quantum efficiency for light emission decreases severely—a phenomenon known widely as the “green gap.” The lattice constant of InN is significantly larger than that of GaN and, for higher In fractions in an InGaN layer, the crystallinity of the layer tends to be degraded for layers thick enough to be useful for many device applications. In some cases an InN or indium-containing nitride layer is deposited directly onto a dissimilar substrate, but the crystallinity and homogeneity are typically poor. 
     Thus it is to be appreciated that embodiments of the present disclosure provide an improved method for growing high quality indium-containing nitride layers and films with high crystallinity, homogeneity, and large area. 
     To provide a point of reference, it is useful to review the typical practice used to grow epitaxial indium-containing nitride layers such as In y Ga 1-y N and Al x In y Ga 1-x-y N, where 0≦x≦1 and 0&lt;y, x+y≦1. 
     In the large majority of cases, a heteroepitaxial GaN layer is first deposited onto a substrate such as sapphire, silicon carbide, gallium arsenide, MgAl 2 O 4  spinel, lithium aluminate, or the like. Heteroepitaxial growth is desirable from the standpoint of achieving large area, for example, because large-area sapphire and silicon carbide substrates are readily available. Nakamura developed a technique whereby a sapphire wafer is heated above 1000° C. in H 2  and/or NH 3 , then cooled to 450-600° C. and a GaN nucleation layer a few hundred Angstroms thick is grown by metalorganic chemical vapor deposition (MOCVD). The substrate is then heated to about 1000-1100° C. and a relatively high crystalline quality GaN layer is grown by MOCVD. In some cases a relatively high crystalline quality In y Ga 1-y N layer is deposited by MOCVD above the GaN layer, typically using trimethylindium, In(CH 3 ) 3 , as the indium source and a substrate temperature between about 700° C. and about 850° C. and a film thickness less than about 500 nanometers. If significantly thicker In y Ga 1-y N films are grown the morphology typically is degraded, especially for very high InN content InGaN. A straightforward generalization of this process to direct nucleation of InGaN films, unfortunately, does not work. 
     The mechanism by which the nucleation layer enables a relatively high quality GaN layer to be grown is reasonably well understood, as described by Koleske et al. As the nucleation layer is heated it undergoes partial decomposition and, through a combination of surface diffusion and evaporation of Ga atoms followed by reaction with incoming NH 3  molecules, begins to form nuclei of well-crystallized GaN which grow and eventually coalesce into a continuous film. Without wishing to be bound by theory, we believe that this process does not work with directly-nucleated InGaN layers for several reasons. InN and GaN decompose at different temperatures, have different surface diffusion characteristics, and different vapor pressures for Ga and In. If isolated nuclei of InGaN form, they will likely have facets of more than one crystallographic orientation. InN and GaN will tend to become incorporated into the different facets at different ratios, leading to a film with gross local stoichiometry variations once the nuclei have coalesced to form a continuous film. In addition, decomposition of In(CH 3 ) 3  becomes difficult at temperatures low enough for no significant decomposition or structural rearrangement of the nucleation layer to occur. 
     Referring to the diagram  100  of  FIG. 1 , a substantially indium-free substrate  101  is provided. This diagram is merely an example, which should not unduly limit the scope of the claims. One of ordinary skill in the art would recognize many variations, alternatives, and modifications. The substrate  101  may comprise one of sapphire, silicon carbide, gallium arsenide, silicon, germanium, a silicon-germanium alloy, MgAl 2 O 4  spinel, ZnO, BP, ScAlMgO 4 , YFeZnO 4 , MgO, Fe 2 NiO 4 , LiGa 5 O 8 , Na 2 MoO 4 , Na 2 WO 4 , In 2 CdO 4 , LiAlO 2 , LiGaO 2 , Ca 8 La 2 (PO 4 ) 6 O 2 , lithium aluminate, gallium nitride, or aluminum nitride. The substrate may have a wurtzite crystal structure and the surface orientation may be within 5 degrees, within 2 degrees, within 1 degree, within 0.5 degree, within 0.2 degree, within 0.1 degree, within 0.05 degree, within 0.02 degree, or within 0.01 degree of (0 0 0 1)+c plane, (0 0 0 −1)−c plane, {1 0−1 0} m-plane, {1 1−2 0} a-plane, or a (h k i l) semi-polar plane, where l and at least one of h and k are nonzero and i=−(h+k). In a specific embodiment, the crystallographic orientation of the substrate is within 5 degrees, within 2 degrees, within 1 degree, within 0.5 degree, within 0.2 degree, within 0.1 degree, within 0.05 degree, within 0.02 degree, or within 0.01 degree of {1 0−1±1}, {1 0−1±2}, {1 0−1±3}, {1 1−2±2}, {2 0−2±1}, {3 0−3±1}, {2 1−3±1}, or {3 0−3±4}. The substrate may have a cubic crystal structure and the surface orientation may be within 5 degrees, within 2 degrees, within 1 degree, within 0.5 degree, within 0.2 degree, within 0.1 degree, within 0.05 degree, within 0.02 degree, or within 0.01 degree of (1 1 1), (−1 −1 −1), {0 0 1}, or {1 1 0}. The substrate may have a diameter greater than about 5 millimeters, greater than about 10 millimeters, greater than about 15 millimeters, greater than about 25 millimeters, greater than about 40 millimeters, greater than about 70 millimeters, greater than about 90 millimeters, greater than about 140 millimeters, or greater than about 190 millimeters. 
     In some embodiments, a mask  103  is deposited on the substrate and a one- or two-dimensional array of openings provided in the mask. The mask may comprise at least one of SiO 2 , SiN x , or a metal oxynitride. In some embodiments the substrate is patterned. And, in some embodiments, the substrate  101  further comprises a release layer, as described in U.S. patent application Ser. No. 12/546,458, which is hereby incorporated by reference in its entirety. 
     The substrate  101  may be placed in a reactor. The surface to be grown upon may be positioned facing up, facing down, facing horizontally, or facing in an oblique direction. 
     In one set of embodiments, differential incorporation of In into facets of non-coalesced films is avoided by deposition of alternating layers of high-indium layers  107  and low-indium layers  105 , as shown in  FIG. 1 . At least one low-indium layer overlies a high-indium layer and at least one high-indium layer overlies a low-indium layer to form a sandwich structure. The high-indium layers may comprise InN and GaN, AlN, or AlGaN. For an indium-containing composition comprising In y Ga 1-y N or Al x In y Ga 1-x-y N, where 0≦x≦1 and 0&lt;y, x+y≦1, the high-indium or indium-rich layer may have a value y high  that is greater than 0.05, greater than 0.1, greater than 0.2, or greater than 0.4. For an indium-containing composition comprising In y Ga 1-y N or Al x In y Ga 1-x-y N, where 0≦x≦1 and 0&lt;y, x+y≦1, the low-indium or indium-poor layer may have a value y low  that is less than y high , less than 0.05, less than 0.02, or less than 0.01. The low-indium layers may comprise GaN, AlN, AlGaN, and/or up to about 5% InN. A plurality of the high-indium layers and of the low-indium layers may have a thickness between about 0.01 monolayer and about 100 monolayers, or between about 0.1 monolayer and about 20 monolayers. The term monolayer refers to a single atomic layer of group III metal atoms (In, Ga, and/or Al) plus a single atomic layer of nitrogen atoms. The number of pairs of alternating layers may be greater than 1, greater than 10, greater than 100, greater than 1000, or greater than 10,000. The alternating layers may be deposited as nucleation layers, that is, may be poorly crystallized. The alternating layers may be deposited as well-crystallized layers. The alternating layers may be deposited directly on substrate  101 , on release layer  103 , or on a faceted nucleation layer formed on substrate  101  or on release layer  103 . The thickness of the alternating layers may be greater than 100 nm, greater than 500 nm, greater than 1 micron, greater than 2 microns, greater than 5 microns, or greater than 10 microns. 
     In one set of embodiments, alternating high-indium and low-indium layers are deposited by hydride vapor phase epitaxy (HVPE). Referring to  FIG. 2 , one or more substrates  201  may be placed on a susceptor  210  within reactor  200 . Susceptor  210  may be provided with one or more recessed portions (not shown) so that the outward-facing surfaces of one or more substrates  201  may be approximately flush with respect to the outward-facing surface of susceptor  210 . At least two metal halide sources  211  and  221  are provided. High-indium source  211  may comprise a crucible  213  which contains an indium-rich alloy  215 . Low-indium source  221  may comprise a crucible  223  which contains an indium-poor alloy  225 . Indium-rich alloy  215  may comprise indium. Indium-poor alloy  225  may comprise gallium. Halogen-containing gases  217  and  227  are passed over indium-rich alloy  215  and indium-poor alloy  225 , respectively. Halogen-containing gases  217  and  227  may comprise X 2  or HX, where X is F, Cl, Br, or I. In a preferred embodiment, halogen-containing gases  217  and  227  comprise Cl 2 . At least one of halogen-containing gases  217  and  227  may further comprise a carrier gas, such as hydrogen, helium, neon, argon, krypton, xenon, or nitrogen. Nitrogen-containing gases  219  and  229  are mixed with the effluent gases from high-indium and low-indium sources  211  and  221 , respectively. In a preferred embodiment, nitrogen-containing gases  219  and  229  comprise ammonia. At least one of nitrogen-containing gases  219  and  229  may further comprise a carrier gas, such as hydrogen, helium, neon, argon, krypton, xenon, or nitrogen. The gases in the high-indium zone and in the low-indium zone may be inhibited from mixing prior to impinging up one or more substrates  201  by one or more baffles  231 . Baffle  231  may have a relatively small clearance with respect to substrates  201  and susceptor  210 , for example, less than 1 millimeter, less than 0.5 millimeter, less than 0.2 millimeter, or less than 0.1 millimeter. Reactor  200  may further be provided with temperature-controlled heating source for susceptor  210 , high-indium source  211 , and low-indium source  221 . Susceptor  210  may rotate, so that one or more substrates  201  are alternately exposed to indium-rich and indium-poor growth environments. The absolute and relative rates of deposition of indium-rich and indium-poor layers may be adjusted by setting the temperatures of the indium-rich and of the indium-poor sources, the flow rate and mole fraction of halogen-containing gases over the indium-rich and the indium-poor alloys, the flow of nitrogen-containing gas, the reactor pressure, and the substrate temperature to pre-determined values. The number of alternating layers may be adjusted by setting the deposition time and the rotation rate of the susceptor to pre-determined values. The pressure at which the HVPE is performed may be below about 1 Torr, between about 1 Torr and about 10 Torr, between about 10 Torr and about 100 Torr, between about 100 Torr and about 760 Torr, between about 1 atmosphere and about 2 atmospheres, or above about two atmospheres. 
     In order to enable deposition of well-crystallized indium-rich and indium-poor layers at approximately the same substrate temperature, the reactor may be further equipped with one or more wires or heating elements  280  positioned proximate to the substrate(s), with a spacing of between about 0.1 millimeter and about 10 centimeters to the surface. The wires or heating elements may be characterized by a temperature ranging from about 500 to about 2000 Degrees Celsius. The wires may be configured in any of the reactors described herein or illustrated schematically in  FIGS. 2 ,  3 ,  4 , and  6 . In some embodiments, the one or more wires are positioned between about 1 millimeter and about 15 millimeters from the surface. The one or more wires may be substantially parallel to one another. The one or more wires may be fixtured so as to be under a controlled tension, so that they remain substantially straight when heated. The one or more wires may lie substantially in a common plane, with a spacing between one another between about 0.1 millimeter and about 10 centimeters. In one specific embodiment, the wires comprise a wire cloth having a two-dimensional weave. The one or more wires may comprise at least one of Hf, Ta, W, Re, Mo, Ni, Cr, Fe, Al, Ir, Pt, Pd, Rh, or Ru. The wires may comprise a compound, for example, MoSi 2 . The one or more wires may further comprise a coating, such as ThO 2 . In some embodiments, one or more ribbons or foil portions are used in addition to or in place of one or more wires. 
       FIG. 3  shows a cross-sectional view of a reactor near the susceptor. This diagram is merely an example, which should not unduly limit the scope of the claims. One of ordinary skill in the art would recognize many variations, alternatives, and modifications. One or more substrates  301  are placed in or on susceptor  310 , which is capable of rotating with respect to reactor  300 . One or more baffles  331  separate reactor  300  into at least two zones. The at least two zones may include an indium-rich zone  320  and an indium-poor zone  330 . At least one purge zone  340  may also be included. An inert or purge gas, such as at least one of hydrogen, helium, argon, or nitrogen, may flow over the substrate while in a purge zone. The purge zone may further inhibit inter-mixing of the gases from the indium-rich and indium-poor growth zones. The susceptor may rotate so that the time each substrate  301  is within a zone is between about 0.01 second and about 100 seconds, between about 0.1 second and about 10 seconds, or between about 0.3 second and about 3 seconds. 
     In another set of embodiments, the HVPE reactor is configured to be vertical instead of horizontal, as shown in  FIGS. 2 and 3 . In a specific embodiment, the one or more substrates face downward, with the indium-rich and indium-poor sources directed upwards. In another specific embodiment the one or substrates face upward, with the indium-rich and indium-poor sources directed downwards. 
     In a set of embodiments, the at least one substrate is exposed to alternating indium-rich and indium-poor growth conditions by means of a time-varying gas flow rather than by means of physical motion of the substrate. Referring to the reactor  400  of  FIG. 4 , at least one substrate  401  is placed proximate to susceptor  410 . At least two metal halide sources  411  and  421  are provided. High-indium source  411  may comprise a crucible  413  which contains an indium-rich alloy  415 . Low-indium source  421  may comprise a crucible  423  which contains an indium-poor alloy  425 . Indium-rich alloy  415  may comprise indium. Indium-poor alloy  425  may comprise gallium. Halogen-containing gases  417  and  427  may be passed over indium-rich alloy  415  and indium-poor alloy  425 , respectively. Halogen-containing gases  417  and  427  may comprise X 2  or HX, where X is F, Cl, Br, or I. In a preferred embodiment, halogen-containing gases  417  and  427  comprise Cl 2 . Halogen-containing gases  417  and  427  may further comprise a carrier gas, such as hydrogen, helium, neon, argon, krypton, xenon, or nitrogen. Nitrogen-containing gas  419  is mixed with the effluent gases from high-indium and low-indium sources  411  and  421 , respectively. In a preferred embodiment, nitrogen-containing gas  419  comprises ammonia. Nitrogen-containing gas  419  may further comprise a carrier gas, such as hydrogen, helium, neon, argon, krypton, xenon, or nitrogen. Reactor  400  may further be provided with temperature-controlled heating source for susceptor  410 , high-indium source  411 , and low-indium source  421 . Substrate  401  is exposed to alternating indium-rich and indium-poor gas compositions by time variation of gases  417  and  427  over indium-rich alloy  415  and indium-poor alloy  425 , respectively. The reactor may be further equipped with one or more wires  480  positioned proximate to the substrate(s). 
       FIG. 5  shows, schematically, a suitable sequence of gas flows to enable deposition of alternating indium-rich and indium-poor layers. This diagram is merely an example, which should not unduly limit the scope of the claims. One of ordinary skill in the art would recognize many variations, alternatives, and modifications. Time-traces  501  and  503  may be taken to represent the flow of halogen-containing gas and of carrier gas, respectively, in the gas stream over the indium-rich alloy. The carrier gas flow is on when the halogen-containing gas flow is off, and vice-versa, so that the overall gas flow rate above the indium-rich alloy is approximately constant. Time-traces  505  and  507  may be taken to represent the flow of halogen-containing gas and of carrier gas, respectively, in the gas stream over the indium-poor alloy. The carrier gas flow is on when the halogen-containing gas flow is off, and vice-versa, so that the overall gas flow rate above the indium-poor alloy is approximately constant. The flow of halogen-containing gas above the indium-rich alloy is off while the flow of halogen-containing gas above the indium-poor alloy is on, and vice-versa. The on-times of the halogen-containing gas may be shorter than the off-times, allowing for separation in time of the indium-rich and indium-poor metal halide gas streams impinging on the substrate. The flow of nitrogen-containing gas, for example, ammonia, may be approximately constant during this sequence. The “on” times of the halogen-containing gas flows may be between about 0.01 second and about 100 seconds, between about 0.1 second and about 10 seconds, or between about 0.3 second and about 3 seconds. 
     Referring again to  FIG. 4 , in order to allow for a relatively clean transition between halogen-containing gas and carrier gas within each gas stream  417  and  427 , the gas flow rate, gas pressure, and the diameter of sources  411  and  421  may be chosen so that the Peclet number for the gas flows is greater than 1, greater than 3, greater than 10, greater than 30, or greater than 100. The Peclet number Pe=uL/D, where u is the gas flow velocity, L is a characteristic length, and D is the diffusion coefficient, provides a quantitative measure of convective versus diffusive gas transport. The shapes of crucibles  413  and  423  may be chosen to minimize turbulence, eddies, and the like in the flow of gases  417  and  427 , respectively. 
     The absolute and relative rates of deposition of indium-rich and indium-poor layers may be adjusted by setting the temperatures of the indium-rich and of the indium-poor sources, the flow rates, mole fractions, and on-times of the halogen-containing gases over the indium-rich and the indium-poor alloys, the flow rate of nitrogen-containing gas, the reactor pressure, and the substrate temperature to pre-determined values. The number of alternating layers may be adjusted by setting the deposition time and the cycle time for the alternating gas flows to pre-determined values. The pressure at which the HVPE is performed may be below about 1 Torr, between about 1 Torr and about 10 Torr, between about 10 Torr and about 100 Torr, between about 100 Torr and about 760 Torr, between about 1 atmosphere and about 2 atmospheres, or above about two atmospheres. 
     In another set of embodiments, alternating high-indium and low-indium layers are deposited by metalorganic chemical vapor deposition (MOCVD). In one set of embodiments, the deposition is performed in a reactor in which the substrate position is varied to alternate between indium-rich and indium-poor growth conditions. In one specific embodiment, the reactor is a tubular reactor. An example of a suitable apparatus is described in MA Tishchler and SM Bedair, Appl. Phys. Lett. 48, 1681 (1986), which is hereby incorporated by reference in its entirety. Note, however, that the inventive procedure is very different than that described by Tischler and Bedair. The instant process involves alternative exposure to indium-rich and indium-poor (gallium rich) metalorganic gas streams with continuous exposure to a nitrogen-containing gas, for example, ammonia. Tischler and Bedair teach alternating exposures to a metalorganic gas stream and an ammonia gas stream to achieve atomic layer epitaxy. Referring again to  FIG. 3 , a susceptor  310  holding one or more substrates  301  may be rotated proximate to at least two deposition zones. The gas flux in indium-rich deposition zone  320  may comprise at least one of trimethylindium, triethylindium, and cyclopentadienylindium and ammonia. The gas flux in indium-poor deposition zone  330  may comprise at least one of trimethylgallium, triethylgallium, and ammonia. At least one of the indium-rich and indium-poor metalorganic gas flows and the nitrogen-containing gas flow may further comprise a carrier gas, such as hydrogen, helium, neon, argon, krypton, xenon, or nitrogen. At least one purge zone  340  may also be included. 
     The absolute and relative rates of deposition of indium-rich and indium-poor layers may be adjusted by setting the flow rate and mole fraction of the indium-rich and the indium-poor metalorganic sources, the flow of nitrogen-containing gas, the reactor pressure, and the substrate temperature to pre-determined values. The number of alternating layers may be adjusted by setting the deposition time and the rotation rate of the susceptor to pre-determined values. The pressure at which the MOCVD is performed may be below about 1 Torr, between about 1 Torr and about 10 Torr, between about 10 Torr and about 100 Torr, between about 100 Ton and about 760 Torr, between about 1 atmosphere and about 2 atmospheres, or above about two atmospheres. 
     In another set of embodiments, the deposition of alternating indium-rich and indium-poor layers is performed by MOCVD, but using a time-varying gas flow rather than by means of physical motion of the substrate. In one specific embodiment, the reactor in which the alternating flows of metalorganic gases is performed is a tubular reactor. Examples of a suitable reactor are described by SP DenBaars and PD Dapkus, Appl. Phys. Lett. 51, 1530 (1987) and by MA Khan et al., Appl. Phys. Lett. 60, 1366 (1992), which are hereby incorporated by reference in their entirety. Note, however, that the inventive procedure is very different than that described by the aforementioned authors. The instant process involves alternative exposure to indium-rich and indium-poor (e.g., gallium rich) metalorganic gas streams with continuous exposure to a nitrogen-containing gas, for example, ammonia. The aforementioned authors teach alternating exposures to a metalorganic gas stream and an ammonia gas stream to achieve atomic layer epitaxy. A suitable sequence of gas flows is shown schematically in  FIG. 5 . Time-traces  501  and  503  may be taken to represent the flow of metalorganic gas and of carrier gas, respectively, in the indium-rich gas stream. The carrier gas flow is on when the metalorganic gas flow is off, and vice-versa, so that the overall gas flow rate is approximately constant. Time-traces  505  and  507  may be taken to represent the flow of metalorganic gas and of carrier gas, respectively, in the indium-poor gas stream. The carrier gas flow is on when the metalorganic gas flow is off, and vice-versa, so that the overall indium-poor gas flow rate is approximately constant. The flow of indium-rich metalorganic gas is off while the flow of indium-poor metalorganic gas is on, and vice-versa. The on-times of the metalorganic gases may be shorter than the off-times, allowing for separation in time of the indium-rich and indium-poor metalorganic gas streams impinging on the substrate. The flow of nitrogen-containing gas, for example, ammonia, may be approximately constant during this sequence. 
     In still another set of embodiments, the deposition of alternating indium-rich and indium-poor layers is performed by MOCVD using a time-varying gas flow and a showerhead with at least two chambers and interpenetrating gas nozzles. One version of a showerhead is known as a Close Coupled Showerhead, a registered trademark of Aixtron AG. Another suitable showerhead is shown schematically in  FIG. 6 . Substrate  601  is placed proximate to susceptor  610  and to showerhead  600 . Showerhead  600  comprises a first chamber  647  with array of outlet nozzles  645 , second chamber  651  with array of outlet nozzles  649 , and may further comprise third chamber  655  with array of outlet nozzles  653 . In some embodiments showerhead  600  further comprises a fourth chamber with an array of outlet nozzles, a fifth chamber with an array of outlet nozzles, and the like. In a preferred embodiment, the time constant of each chamber, that is, the volume of the chamber divided by the product of the number of nozzles and the conductance of each nozzle, is less than about 3 seconds, less than about 1 second, less than about 0.3 second, or less than about 0.1 second. The showerhead design may facilitate scaleup of the reactor to large substrate diameters and/or to large number of substrates while retaining relatively high Peclet numbers, which is desirable for efficient switching of gas flow composition quickly. The showerhead may also minimize pre-reaction between the group III precursors and the nitrogen-containing gas. The reactor may be further equipped with one or more wires  680  positioned proximate to the substrate(s). 
     Referring again to  FIG. 5 , time-traces  501  and  503  may be taken to represent the flow of metalorganic gas and of carrier gas, respectively, in an indium-rich gas stream flowing through a first chamber in the showerhead. The carrier gas flow is on when the metalorganic gas flow is off, and vice-versa, so that the overall gas flow rate is approximately constant. Time-traces  505  and  507  may be taken to represent the flow of metalorganic gas and of carrier gas, respectively, in an indium-poor gas stream flowing through a second chamber in the showerhead. The carrier gas flow is on when the metalorganic gas flow is off, and vice-versa, so that the overall indium-poor gas flow rate is approximately constant. The flow of indium-rich metalorganic gas is off while the flow of indium-poor metalorganic gas is on, and vice-versa. The on-times of the metalorganic gases may be shorter than the off-times, allowing for separation in time of the indium-rich and indium-poor metalorganic gas streams impinging on the substrate. A flow of nitrogen-containing gas, for example, ammonia, may be approximately constant during this sequence. The flow of nitrogen-containing gas may mixed into the gas flow of the indium-rich and/or the indium-poor gas flow streams or may be admitted separately through a separate (e.g., third) chamber of the showerhead. 
     The absolute and relative rates of deposition of indium-rich and indium-poor layers may be adjusted by setting the flow rate and mole fraction of the indium-rich and the indium-poor metalorganic sources, the flow of nitrogen-containing gas, the reactor pressure, and the substrate temperature to pre-determined values. The number of alternating layers may be adjusted by setting the deposition time and the cycle time for the alternating gas flows to pre-determined values. The pressure at which the MOCVD is performed may be below about 1 Torr, between about 1 Torr and about 10 Torr, between about 10 Torr and about 100 Torr, between about 100 Torr and about 760 Torr, between about 1 atmosphere and about 2 atmospheres, or above about two atmospheres. 
     In another set of embodiments, an In-containing film is nucleated and deposited by atomic layer epitaxy (ALE). ALE has been used to deposit a number of binary compounds and semiconductor materials [T. Suntola, Materials Science Reports 4, 265 (1989); M. A. Khan, R. A. Skogman, J. M. Van Hove, D. T. Olson, and J. N. Kuznia, Appl. Phys. Lett. 60, 1366 (1992)] but, to the best of our knowledge, has not been applied to InGaN. The deposition of In-containing layers and Ga-containing layers may be performed either simultaneously or sequentially. Without wishing to be limited by theory, we believe that the alternating deposition of group III and nitride monolayers may avoid differential indium incorporation into facets of differing crystallographic orientation, as occurs during typical continuous growth. In preferred embodiments, the Peclet number of the gas flow is greater than 1, greater than 2, greater than 5, greater than 10, or greater than 100. The pressure at which the ALE is performed may be below about 1 Torr, between about 1 Torr and about 10 Torr, between about 10 Torr and about 100 Ton, between about 100 Torr and about 760 Torr, between about 1 atmosphere and about 2 atmospheres, or above about two atmospheres. 
     In one set of embodiments, the ALE is performed using metalorganic precursors in a reactor equipped with a showerhead gas inlet with at least two interpenetrating gas pathways, as illustrated in  FIG. 6 . Referring again to  FIG. 5 , time-traces  501  and  503  may be taken to represent the flow of mixed metalorganic gas (that is, with a common indium mole fraction) and of carrier gas, respectively, flowing through a first chamber in the showerhead. The carrier gas flow is on when the metalorganic gas flow is off, and vice-versa, so that the overall gas flow rate is approximately constant. Time-traces  505  and  507  may be taken to represent the flow of nitrogen-containing and of carrier gas, respectively, flowing through a second chamber in the showerhead. The carrier gas flow is on when the nitrogen-containing gas flow is off, and vice-versa, so that the overall gas flow rate is approximately constant. The flow of metalorganic gas is off while the flow of nitrogen-containing gas is on, and vice-versa. The on-times of the metalorganic and nitrogen-containing gases may be shorter than the off-times, allowing for separation in time of the metalorganic and nitrogen-containing gas streams impinging on the substrate. In a preferred embodiment, the metalorganic mole fraction and flow rate, the mole fraction and flow rate of nitrogen-containing gas, the respective on-times, the reactor pressure, and the substrate temperature are chosen so that approximately one monolayer of indium-containing nitride is deposited per cycle. In this specific embodiment the mole fraction of indium in the indium-containing nitride layer is controlled by the mole fractions of indium-containing and indium-free (for example, gallium-containing) metalorganic precursors, along with the other reaction parameters. The rate of deposition is controlled by the cycle time (one monolayer per cycle), and the thickness of the layer by the deposition time (number of cycles). The pressure at which the ALE is performed may be below about 1 Torr, between about 1 Torr and about 10 Torr, between about 10 Torr and about 100 Torr, between about 100 Torr and about 760 Torr, between about 1 atmosphere and about 2 atmospheres, or above about two atmospheres. 
     In another embodiment, ALE is performed with metalorganic precursors, but deposition of indium-rich and indium-poor layers (preferably, monolayers) is performed separately. Referring to  FIG. 7  and making reference again to  FIG. 6 , time-traces  701  and  703  may be taken to represent the flow of indium-poor metalorganic gas (for example, gallium-rich) and of carrier gas, respectively, flowing through a first chamber in the showerhead. The carrier gas flow is on when the indium-poor metalorganic gas flow is off, and vice-versa, so that the overall gas flow rate is approximately constant. Time-traces  705  and  707  may be taken to represent the flow of indium-rich metalorganic gas and of carrier gas, respectively, flowing through a second chamber in the showerhead. The carrier gas flow is on when the indium-rich metalorganic gas flow is off, and vice-versa, so that the overall gas flow rate is approximately constant. Time-traces  709  and  711  may be taken to represent the flow of nitrogen-containing gas and of carrier gas, respectively, flowing through a third chamber in the showerhead. The carrier gas flow is on when the nitrogen-containing gas flow is off, and vice-versa, so that the overall gas flow rate is approximately constant. The flow of metalorganic gases is off while the flow of nitrogen-containing gas is on, and vice-versa. The on-times of the metalorganic and nitrogen-containing gases may be shorter than the off-times, allowing for separation in time of the metalorganic and nitrogen-containing gas streams impinging on the substrate. In a preferred embodiment, the metalorganic mole fraction and flow rate, the mole fraction and flow rate of nitrogen-containing gas, the respective on-times, the reactor pressure, and the substrate temperature are chosen so that approximately one monolayer of indium-poor or indium-rich nitride is deposited per cycle of nitrogen-containing gas. In this specific embodiment the mole fraction of indium in the indium-containing nitride layer is controlled digitally, by the ratio of the number of indium-rich and indium-poor (for example, gallium-containing) metalorganic precursor pulses, along with the other reaction parameters. The rate of deposition is controlled by the cycle time (one monolayer per cycle), and the thickness of the layer by the deposition time (number of cycles). The pressure at which the ALE is performed may be below about 1 Torr, between about 1 Torr and about 10 Torr, between about 10 Torr and about 100 Torr, between about 100 Torr and about 760 Torr, between about 1 atmosphere and about 2 atmospheres, or above about two atmospheres. 
     In still another embodiment, ALE is performed by HVPE, but deposition of indium-rich and indium-poor layers (preferably, monolayers) is performed separately. Referring to  FIG. 7  and making reference again to  FIG. 4 , time-traces  701  and  703  may be taken to represent the flow  427  of halogen-containing gas and of carrier gas, respectively, flowing through the indium-poor source  421 . The carrier gas flow is on when the halogen-containing gas flow is off, and vice-versa, so that the overall gas flow rate is approximately constant. Time-traces  705  and  707  may be taken to represent the flow  417  of halogen-containing gas and of carrier gas, respectively, flowing through the indium-rich source  411 . The carrier gas flow is on when the halogen-containing gas flow is off, and vice-versa, so that the overall gas flow rate is approximately constant. Time-traces  709  and  711  may be taken to represent the flow  419  of nitrogen-containing gas and of carrier gas, respectively, flowing through the body of the reactor. The carrier gas flow is on when the nitrogen-containing gas flow is off, and vice-versa, so that the overall gas flow rate is approximately constant. The flow of halogen-containing gases is off while the flow of nitrogen-containing gas is on, and vice-versa. The on-times of the halogen-containing and nitrogen-containing gases may be shorter than the off-times, allowing for separation in time of the halogen-containing and nitrogen-containing gas streams impinging on the substrate. In a preferred embodiment, the indium-rich and indium-poor source temperatures, flow rates of halogen-containing gases, the mole fraction and flow rate of nitrogen-containing gas, the respective on-times, the reactor pressure, and the substrate temperature are chosen so that approximately one monolayer of indium-poor or indium-rich nitride is deposited per cycle of nitrogen-containing gas. In this specific embodiment the mole fraction of indium in the indium-containing nitride layer is controlled digitally, by the ratio of the number of indium-rich and indium-poor (for example, gallium-containing) halogen-containing pulses, along with the other reaction parameters. The rate of deposition is controlled by the cycle time (one monolayer per cycle), and the thickness of the layer by the deposition time (number of cycles). The pressure at which the ALE is performed may be below about 1 Torr, between about 1 Torr and about 10 Ton, between about 10 Torr and about 100 Torr, between about 100 Torr and about 760 Torr, between about 1 atmosphere and about 2 atmospheres, or above about two atmospheres. 
     The substrate may undergo a pre-treatment prior to depositing an In-containing nitride layer. The pre-treatment may include heating to a temperature between about 500° C. and about 1200° C. in vacuum or in an atmosphere comprising at least one of H 2  and NH 3 . 
     An indium-containing nitride nucleation layer may be deposited. The surface may be exposed to a flux comprising an indium-containing precursor and a nitrogen-containing precursor. The indium-containing precursor may comprise In(R 1 R 2 R 3 ), where each of R 1 , R 2 , and R 3  are alkyl and/or aryl groups. In one specific embodiment, the indium-containing precursor comprises triethylindium, In(C 2 H 5 ) 3 . In another specific embodiment, the indium-containing precursor comprises trimethylindium, In(CH 3 ) 3 . In still another specific embodiment, the indium-containing precursor comprises cyclopentadienylindium, C 5 H 5 In. In yet another specific embodiment, the indium-containing precursor comprises pentamethylcyclopentadienylindium, (CH 3 ) 5 C 5 In. In another set of embodiments, the indium-containing precursor comprises an indium halide, InX a , where X represents F, Cl, Br, and/or I, and a=1, 2, or 3. In a specific embodiment, the indium-containing precursor comprises InCl or InCl 3 . The nitrogen-containing precursor may comprise at least one of ammonia, NH 3 , nitrogen, N 2 , hydrazine, N 2 H 4 , and hydrazoic acid, HN 3 . The surface may additionally be exposed to a flux comprising a gallium-containing precursor and/or an aluminum-containing precursor. The gallium-containing precursor may comprise Ga(R 1 R 2 R 3 ), where each of R 1 , R 2 , and R 3  are alkyl and/or aryl groups. In one specific embodiment, the gallium-containing precursor comprises triethylgallium, Ga(C 2 H 5 ) 3 . In another specific embodiment, the gallium-containing precursor comprises trimethylgallium, Ga(CH 3 ) 3 . In another set of embodiments, the gallium-containing precursor comprises a gallium halide, GaX a , where X represents F, Cl, Br, and/or I, and a=1, 2, or 3. In a specific embodiment, the gallium-containing precursor comprises GaCl or GaCl 3 . The aluminum-containing precursor may comprise Al(R 1 R 2 R 3 ), where each of R 1 , R 2 , and R 3  are alkyl and/or aryl groups. In one specific embodiment, the aluminum-containing precursor comprises triethylaluminum, Ga(C 2 H 5 ) 3 . In another specific embodiment, the aluminum-containing precursor comprises trimethylaluminum, Ga(CH 3 ) 3 . In still another specific embodiment, the aluminum-containing precursor comprises trimethylaminealane, (CH 3 ) 3 NAlH 3 . In another set of embodiments, the aluminum-containing precursor comprises an aluminum halide, AlX a , where X represents F, Cl, Br, and/or I, and a=1, 2, or 3. In a specific embodiment, the aluminum-containing precursor comprises AlCl or AlCl 3 . 
     During the exposure of the surface to a flux comprising an indium-rich precursor, an indium-poor precursor, and/or a nitrogen-containing precursor, the substrate may be held at a first temperature between about 300° C. and about 500° C. The pressure in the reactor may be maintained at a value between about 10 −5  Torr and about 10 atmospheres. In some embodiments, the pressure is maintained at a value between about 10 −5  Torr and about 1 Torr. In other embodiments, the pressure is maintained at a value between about 1 Torr and about 600 Torr. In still other embodiments, the pressure is maintained at a value between about 0.9 atmospheres and about 1.2 atmospheres. In still other embodiments, the pressure is maintained at a value between about 1.3 atmospheres and about 10 atmospheres. The one or more wires positioned proximate to the surface may be held at a temperature between about 500° C. and about 2500° C., causing partial or complete decomposition of at least one of an indium-containing precursor, a gallium-containing precursor, an aluminum-containing precursor, and a nitrogen-containing precursor. A DC electrical bias between about 1 volt and about 1000 volts may be placed between the one or more wires positioned proximate to the surface and the surface. In one set of embodiments, the surface is biased positively with respect to the one or more wires, causing the substrate to be bombarded by electrons generated thermionically from the one or more wires. In another set of embodiments, the surface is biased negatively with respect to the one or more wires, causing bombardment of the substrates by ions. In some embodiments, a plasma is generated in the region between the one or more wires and the substrate. In some embodiments, an AC bias is placed between the one or more wires and the substrate. In some embodiments, the AC bias is modulated at radio frequencies (rf). In some embodiments, a DC or AC bias is placed between the one or more wires and the substrate only during certain portions of a cycle of time-varying gas flows. For example, referring to  FIGS. 5 and 7 , a DC or AC bias may be placed between the one or more wires and the substrate during exposure of the substrate to one or more of an indium-rich gas flow, an indium-poor (but metal containing) gas flow, a nitrogen-containing gas flow, or a gas flow comprising carrier gas only. Without wishing to be bound by theory, we believe that the addition of energy by means of the at least one hot wire, electron bombardment or irradiation, ion bombardment, a plasma, or the like, facilitates partial or complete decomposition of at least one of a an indium-containing precursor and a nitrogen-containing precursor and enables deposition of an indium-containing nitride film at a lower substrate temperature than would be feasible in the absence of the energy. In another embodiment, a microwave plasma is provided proximate to the surface region of the substrate. The thickness of the nucleation layer may be between about 10 Ångstroms and about 1000 Ångstroms. 
     The temperature of the surface may be ramped to a second temperature higher than the first temperature. The second temperature may be between about 500° C. and about 900° C. The heating rate may be between about 0.1° C. per minute and about 100° C. per minute. The surface may be exposed to a flux of at least one of an indium-rich precursor, an indium-poor but metal-containing precursor, and a nitrogen-containing precursor during the temperature ramp. The fluxes of one or more precursors during the ramp may be lower, equal, or higher than the corresponding flux during deposition of the nucleation layer. 
     After the substrate reaches the second temperature, a well-crystallized layer of indium-containing nitride is deposited. The surface may be exposed to a flux comprising at least one indium-containing precursor and at least one nitrogen-containing precursor. The surface may additionally be exposed to a flux comprising at least one gallium- and/or aluminum-containing precursor. Additional energy may be provided during the deposition via hot wires, electron bombardment, ion bombardment, plasma excitation, or the like. The thickness of the well-crystallized layer may be between about 10 nanometers and about 100 millimeters, or between about 100 nanometers and about 10 microns. 
     In another set of embodiments, an In-containing film is nucleated and deposited using molecular beam epitaxy (MBE). An advantage of this technique is the ability to deposit or grow high quality InGaN films at significantly lower temperatures (e.g. lower than about 700° C. and as low as about 300° C.) relative to vapor deposition techniques such as MOCVD, HVPE and other growth techniques which typically require temperatures greater than about 700° C. In particular, an advantage of MBE is that the source materials for growth do not require elevated substrate temperatures in order to be available for growth at the surface of a substrate. In a preferred embodiment, conventional Knudsen cells (“K-cells”) are used to contain high purity elemental group III sources such as Ga, In, and Al, among others. Such cells are commonly used in MBE growth and are extremely well-known in the art. Such cells provide group III source elements at a growth surface in an MBE growth chamber through evaporation in a molecular beam under appropriate temperature and pressure conditions and with the cells appropriately positioned relative to the growth surface. These conditions are independent of the substrate temperature or “growth temperature” making possible element and/or layer deposition and/or crystal growth at very low temperatures. 
     Similarly, active group V (nitrogen) species for growth can be provided at the substrate surface in an MBE growth chamber in a manner which does not rely on or require elevated substrate temperatures. Specifically, in a preferred embodiment the source of active nitrogen for growth is a radio frequency plasma source. In this embodiment a nitrogen plasma is provided in the growth chamber, and is directed at the growth surface or substrate, which contains one or more active nitrogen species for growth. The nitrogen source could alternatively comprise an ammonia gas injector, or a combination of ammonia gas and nitrogen plasma in order to provide active nitrogen species for growth. 
     Additionally, it is well-known in the art that for MBE growth, elemental cells or sources (gas and/or solid sources) are each equipped with a shutter in the vicinity of the outlet of the cell or source. Shutters are opened or closed in order to provide or prohibit, respectively, fluxes of source elements and/or gases at the growth surface or substrate. In one embodiment, the MBE deposition is performed with both group III (e.g. Ga and In) and group V (e.g. N) shutters open to expose fluxes of constituent elements to the growth surface or substrate. In a preferred embodiment, the total group III/group V flux ratio during growth is maintained to be greater than 1. Group III fluxes in the range of about 10 −7  to 10 −6  Torr beam equivalent pressure (BEP) can be utilized to achieve this condition for nitrogen-limited growth rates of about 0.1-0.5 microns/hour. This condition is known as “metal-rich” growth and provides for a build-up of the excess group III element on the surface known as the “wetting layer”. Such a wetting layer has been shown to be advantageous for increasing surface adatom mobilities and hence promoting higher quality 2-D growth modes such as step flow growth or layer-by-layer growth rather than 3-D Volmer-Weber (island) or Stranski-Krastanov (layer-plus-island) growth modes. The presence or absence of a metallic wetting layer and the exact thickness of the wetting layer (in monolayers) can be actively monitored and controlled during MBE growth by two techniques: Reflection high energy electron diffraction (RHEED) and quantitative mass spectroscopy (QMS) [refs]. 
     In another embodiment, growth can be initiated with a total group III/group V flux ratio of less than 1 and then the conditions changed in a second or later step of the growth to achieve a total group III/group V flux of greater than 1. In this case roughened surfaces may be obtained during the initial growth due to dramatically reduced adatom surface mobility, but the subsequent presence of a metallic wetting layer in later steps of the growth has been shown to provide for surface recovery to smooth surfaces and high quality reduced-dislocation heteroepitaxial GaN films. Further, if the total group III/group V flux is much greater than 1, metallic droplets of the excess group III element can form on the surface which can be undesirable. In this case a pulsed growth method can be employed during which all shutters are closed periodically to allow in-situ desorption of excess group III. In a preferred embodiment a short-period pulsed growth is utilized for InGaN heteroepitaxy over large area substrates. This embodiment is advantageous for obtaining high quality InGaN films over large areas and provides a more highly manufacturable method due to reduced sensitivity to temperature variations which may occur across a large area substrate during growth. In this embodiment, a large-area substrate (for example, 2″, 4″, 6″ substrate) is first exposed to a high flux of Indium and/or group III and group V elements with a III/V ratio&gt;&gt;1, such that cooler areas may accumulate metallic group III droplets and warmer areas maintain a wetting layer of 1-2 monolayers but do not accumulate droplets due to competing desorption of species in hotter areas. Periodically, the excess group III elements are allowed to desorb in-situ returning the growth surface to the pre-growth state in terms of adatom coverage and then the process is repeated. 
     In some cases, growth initiation under metal-rich or high group III/group V ratios may not be desirable due to unwanted reactions between constituent elements of the substrate material and the group III sources. For example, (Al, In, Ga)N growth on Si or SiC during which Si can dissolve into Ga or Al and will incorporate into films, or (Al,In,Ga) N growth on GaAs during which As can incorporate into the growing film and alter the crystal structure from wurtzite to cubic, for example. In such cases, a multiple step growth which is initiated under group V-rich conditions and later changed to group III-rich conditions would be desirable. 
     In another embodiment, shutters are alternated between group III and group V at specified intervals. In yet other embodiments, growth interrupts or pauses are incorporated during which time one or more shutters are closed for specified lengths of time in order to enhance adatom surface mobility and provide for higher quality growth. This type of technique is known as migration enhanced epitaxy (MEE). 
     In some embodiments, the shutters are opened and closed in a predetermined way so as to deposit alternating layers of an indium-rich nitride and an indium-poor nitride. In one specific embodiment, the shutters are opened and closed in a predetermined way so as to deposit alternating layers of an InN and GaN. 
     Thus MBE offers several advantages for direct deposition of heteroepitaxial InGaN. Further, since growth temperatures are lower, high quality InGaN crystals can be grown across the whole compositional range. 
     In the above embodiments using MBE growth, one side of the substrate may first be coated with a metal such as Ti or Pt, a refractory metal such as W, Mo, Ta, a silicide or nitride of one of these metals, among others, or any combination of these, in order to facilitate heat transfer to the substrate during MBE growth. The thickness of the metal containing layers may range in thickness from 0.5 to 2 microns, but is not limited to this range. The metal containing layers may be deposited by e-beam evaporation, sputtering, chemical vapor deposition, plasma-enhanced chemical vapor deposition, or a combination of these or other deposition techniques. The substrate may then be de-greased in solvents such as, but not limited to, acetone, methanol and isopropanol, with or without use of an ultrasonic bath, and spin-dried or nitrogen blow-dried. 
     The prepared substrate may then be mounted on a substrate holder with the metalized surface facing the direction of the heater during growth, and loaded into a chamber of the MBE system. The type of sample mounting and the configuration of the substrate holder may vary depending on the type and geometry of the MBE system being used. The substrate holder could include, but is not limited to, ceramic diffuser plates, backing wafers such as silicon, quartz, or sapphire, molybdenum, ceramic, or tungsten retaining rings, and molybdenum or tantalum faceplates, for example. In some cases the substrate could be indium-bonded to a carrier wafer such as silicon prior to placement in the substrate holder. The substrate and holder may then be baked at high temperature, for example in the range of 100-800° C., and more typically in the range of 400° C.-600° C., prior to growth. The baking may be performed in the growth chamber or preferably in a separate chamber which is attached to the growth chamber, for ˜1 hour or a sufficient amount of time to outgas water and other impurities from the surface. The outgassing can be monitored by the chamber pressure and potentially using a residual gas analyzer. In one embodiment, the growth chamber of the MBE system is equipped with a radio frequency plasma source, an ammonia gas injector, a combination of these, or another suitable source of active nitrogen for growth. The inlet nitrogen source gas is preferably further purified using a getter-filter prior to introduction to the growth chamber. Conventional Knudsen cells are available for group III sources such as Ga and In. Pumping of any or all of the MBE system chambers to vacuum pressures can be achieved utilizing cryogenic pumps, turbo pumps, ion pumps, among others, or a combination of these. Base pressures of the growth chamber prior to introducing source gases is typically in the range of 10 −8 -10 −10  Torr, and preferably on the order of 10 −10  Torr or less. 
     InGaN, InN, and/or GaN may be deposited in the temperature range of about 300° C. to about 700° C., and preferably in the range of about 400° C.-600° C. In one embodiment, the growth pressure is 1 the range of about 1×10 −5  to 1×10 −6  Torr. It is possible that dopants could be intentionally incorporated in the epitaxial layers. A typical n-type dopant is Si, a typical p-type dopant is Mg, and typical compensating dopants are Be or C, for example. Under certain conditions Mg doping can result in polarity inversion in GaN. If Mg doping is utilized the appropriate conditions should be utilized to control polarity. 
     After MBE growth, the sample is removed from the chamber. The metal containing layer on backside of the substrate can be removed by wet etching in hydrofluoric acid, hydrochloric acid, nitric acid, sulfuric acid, among others, or a combination of these. The metal containing layer could also be removed by dry etching techniques such as RIE. 
     The indium-containing film may be annealed, that is, subjected to a thermal treatment, to improve crystallinity and/or to promote relaxation. The thermal or annealing treatment may comprise heating to a temperature above 600 degrees Celsius, above 700 degrees Celsius, above 800 degrees Celsius, above 900 degrees Celsius, or above 1000 degrees Celsius for a period between about 1 second and about 4 hours. To inhibit decomposition, a capping layer may be deposited on the indium-containing film prior to the thermal or annealing treatment. The capping layer may comprise at least one of GaN, AN, AlGaN, SiO 2 , and SiN x . At least an outermost portion of the indium-containing film has a wurtzite structure, is substantially relaxed and unstrained, and is well-crystallized. The x-ray diffraction full-width-at-half-maximum (FWHM) of the lowest-order on-axis (symmetric) reflection may be less than about 500 arc-sec, less than about 300 arc-sec, less than about 200 arc-sec, less than about 150 arc-sec, or less than about 100 arc-sec. The x-ray diffraction full-width-at-half-maximum of the lowest-order off-axis (asymmetric) reflection may be less than about 1000 arc-sec, less than about 800 arc-sec, less than about 500 arc-sec, less than about 300 arc-sec, less than about 200 arc-sec, less than about 150 arc-sec, or less than about 100 arc-sec. In a specific embodiment, the indium-containing film has a c-plane orientation, an x-ray rocking-curve (002) reflection FWHM less than about 300 arc-sec and a (102) reflection FWHM less than about 1000 arc-sec. In another specific embodiment, the indium-containing film has a c-plane orientation, an x-ray rocking-curve (002) reflection FWHM less than about 150 arc-sec and a (102) reflection FWHM less than about 300 arc-sec. 
     In some embodiments, an additional indium-containing nitride layer is deposited. The second deposition may be performed in a second reactor, after cooldown from the second temperature and removal from the first reactor. In some embodiments, the second deposition is performed using MOCVD. In other embodiments, the second deposition is performed using hydride vapor phase epitaxy (HVPE). The additional indium-containing nitride layer may have a thickness between about 1 micron and about 100 millimeters, or between about 20 microns and about 10 millimeters, or between about 100 microns and about 5 millimeters. The additional deposition thickness can assist in reducing threading dislocation density, and thus improve crystallinity of the indium-containing nitride layer. 
     In some embodiments, the indium-containing nitride layer is removed from the substrate. The substrate may be removed by spontaneous delamination, by laser liftoff, by selective chemical etching, or the like, according to methods that are known in the art, to produce a free-standing indium-containing nitride boule or wafer. It is to be appreciated that the indium-containing nitride layer provided by the methods described in this invention is substantially near its native, strain-free state. For example, the in-plane lattice constant for a (0001) indium-gallium-nitride of the present invention will be higher than that of GaN (3.19 Å). In particular, the in-plane lattice constant of the (0001) indium-gallium-nitride layer may be greater than 3.20 Å, greater than 3.22 Å, greater than 3.24 Å, greater than 3.26 Å, or greater than 3.28 Å. The relationship between materials parameters and compositions for semiconductor alloys are described, for example, in I. Vurgaftman, J. R. Meyer, and L. R. Ram-Mohan, J. Appl. Phys. 89, 11, pp. 5815-5875 (2001), which is hereby incorporated by reference in its entirety. Therefore, the teachings in this invention provide a large-area, homogeneous, near-strain-free indium-containing nitride layer which overcomes the limitations of the prior art and can be employed for improved optoelectronic devices. 
     One or more active layers may be deposited on the well-crystallized indium-containing nitride layer. The active layer may be incorporated into an optoelectronic or electronic devices such as at least one of a light emitting diode, a laser diode, a photodetector, an avalanche photodiode, a transistor, a rectifier, and a thyristor; one of a transistor, a rectifier, a Schottky rectifier, a thyristor, a p-i-n diode, a metal-semiconductor-metal diode, high-electron mobility transistor, a metal semiconductor field effect transistor, a metal oxide field effect transistor, a power metal oxide semiconductor field effect transistor, a power metal insulator semiconductor field effect transistor, a bipolar junction transistor, a metal insulator field effect transistor, a heterojunction bipolar transistor, a power insulated gate bipolar transistor, a power vertical junction field effect transistor, a cascode switch, an inner sub-band emitter, a quantum well infrared photodetector, a quantum dot infrared photodetector, a solar cell, and a diode for photoelectrochemical water splitting and hydrogen generation. 
     While the above is a full description of the specific embodiments, various modifications, alternative constructions and equivalents may be used. Therefore, the above description and illustrations should not be taken as limiting the scope of the present disclosure which is defined by the appended claims.