Abstract:
Techniques for processing materials in supercritical fluids include processing in a capsule disposed within a high-pressure apparatus enclosure. The invention is useful for growing crystals of: GaN; AN; InN; and their alloys, namely: InGaN; AlGaN; and AlInGaN; for manufacture of bulk or patterned substrates, which in turn can be used to make optoelectronic devices, lasers, light emitting diodes, solar cells, photoelectrochemical water splitting and hydrogen generation, photodetectors, integrated circuits, and transistors.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to U.S. Provisional Application No. 61/356,489, filed Jun. 18, 2010; and U.S. Provisional Application No. 61/386,879, filed Sep. 27, 2010, each of which is incorporated herein by reference for all purposes. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    This invention relates to techniques for processing materials in supercritical fluids. Embodiments of the invention include techniques for material processing in a capsule disposed within a high-pressure apparatus enclosure. The invention can be applied to growing crystals of: GaN; AN; InN; and their alloys, namely: InGaN; AlGaN; and AlInGaN; and others for manufacture of bulk or patterned substrates. Such bulk or patterned substrates can be used for a variety of applications including optoelectronic devices, lasers, light emitting diodes, solar cells, photoelectrochemical water splitting and hydrogen generation, photodetectors, integrated circuits, and transistors, among other devices. 
         [0003]    Large area, high quality crystals and substrates, for example, nitride crystals and substrates, are needed for a variety of applications, including light emitting diodes, laser diodes, transistors, and photodetectors. In general, there is an economy of scale with device processing, so that the cost per device is reduced as the diameter of the substrate is increased. In addition, large area seed crystals are needed for bulk nitride crystal growth. 
         [0004]    There are known methods for fabrication of large area gallium nitride (GaN) crystals with a (0 0 0 1) c-plane orientation. In many cases, hydride vapor phase epitaxy (HVPE) is used to deposit thick layers of gallium nitride on a non-gallium-nitride substrate such as sapphire, followed by the removal of the substrate. These methods have demonstrated capability for producing free-standing c-plane GaN wafers 50-75 millimeters in diameter, and 100 millimeter diameters are expected. The typical average dislocation density, however, in these crystals, about 10 6 -10 8  cm −2 , is undesirably high for many applications. Techniques have been developed to gather the dislocations into bundles or low-angle grain boundaries, but it is still very difficult to produce dislocation densities below 10 4  cm −2  in a large area single grain by these methods, and the relatively high concentration of high-dislocation-density bundles or grain boundaries creates difficulties, performance degradation, and/or yield losses for the device manufacturer. 
         [0005]    The non-polar planes of gallium nitride, such as {1 0-1 0} and {1 1-2 0}, and the semi-polar planes of gallium nitride, such as {1 0-1±1}, {1 0-1±2}, {1 0-1±3}, and {1 1-2±2}, {2 0-2 1} are attractive for a number of applications. Unfortunately, no large area, high quality non-polar or semi-polar GaN wafers are generally available for large scale commercial applications. Other conventional methods for growing very high quality GaN crystals, for example, with a dislocation density less than 10 4  cm −2  have been proposed. These crystals, however, are typically small, less than 1-5 centimeters in diameter, and are not commercially available. 
         [0006]    Dwilinski, et al. [U.S. Patent Application No. 2008/0156254] suggested a method for merging elementary GaN seed crystals into a larger compound crystal by a tiling method. The method uses elementary GaN seed crystals grown by hydride vapor phase epitaxy (HVPE) and polishing the edges of the elementary crystals at oblique angles to cause merger in fast-growing directions. Dwilinski, et al., however, has limitations. Dwilinski, et al. did not specify the accuracy of the crystallographic orientation between the merged elementary seed crystals nor provide a method capable of providing highly accurate crystallographic registry between the elementary seed crystals, and observed defects resulting from the merging of the elementary seed crystals. 
         [0007]    Conventional techniques are inadequate for failing to meaningfully increase the available size of high-quality nitride crystals while maintaining extremely accurate crystallographic orientation across the crystals. 
       BRIEF SUMMARY OF THE INVENTION 
       [0008]    This invention provides a method for growth of a large-area, gallium-containing nitride crystal. The method includes providing at least two nitride crystals having a dislocation density below about 10 7  cm −2  together with a handle substrate. The nitride crystals are bonded to the handle substrate. Then the nitride crystals are grown to coalescence into a merged nitride crystal. The polar misorientation angle γ between the first nitride crystal and the second nitride crystal is less than 0.5 degree and azimuthal misorientation angles α and β are less than 1 degree. A semiconductor structure can be formed on the nitride crystals as desired. 
         [0009]    In another embodiment, the invention includes the steps above, but also includes providing a release layer and a high quality epitaxial layer on each of the two nitride crystals. The epitaxial layers are grown to cause coalescence into a merged nitride crystal. The polar misorientation angle γ between the first nitride crystal and the second nitride crystal is less than 0.5 degree and azimuthal misorientation angles α and β are less than 1 degree. 
         [0010]    The invention can provide a crystal that includes at least two single crystal domains having a nitride composition and a dislocation density within the domain less than 10 7  cm −2 . The two single crystal domains are separated by a line of dislocations with a linear density less than 50 cm −1  and preferably less than 5×10 5  cm −1 . The polar misorientation angle γ between the first domain and the second domain is less than 0.5 degree and the azimuthal misorientation angles α and β are less than 1 degree. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIGS. 1   a ,  1   b ,  1   c ,  1   d ,  1   e ,  1   f ,  1   g ,  1   h ,  1   i ,  1   j , and  1   k  are diagrams illustrating a method for wafer bonding of crystals; 
           [0012]      FIG. 2  is a diagram illustrating the crystallographic misorientation between two adjacent wafer-bonded crystals; 
           [0013]      FIG. 3  is a diagram illustrating arrangements of tiled crystals; 
           [0014]      FIGS. 4   a ,  4   b ,  5 , and  6  are diagrams illustrating a method for coalescence of wafer-bonded crystals; 
           [0015]      FIG. 7  is a diagram illustrating a merged crystal; and 
           [0016]      FIGS. 8 ,  9 , and  10  are diagrams illustrating lateral growth from a seed crystal. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0017]    Referring to  FIG. 1   a , a crystal  101  having a first surface  105  is provided. We will often refer to crystal  101  as a “nitride crystal”, as nitride crystals with a wurtzite crystal structure comprise a preferred embodiment. The method disclosed, however, has broader generality, and the term “nitride crystal” should be understood to include non-nitride crystals as well as nitride crystals. Examples of non-nitride crystals for which this invention may be applicable include diamond, cubic boron nitride, boron carbide, silicon, germanium, silicon germanium, indium phosphide, gallium phosphide, zinc oxide, zinc selenide, gallium arsenide, cadmium telluride, and cadmium zinc telluride. In preferred embodiments, nitride crystal  101  comprises GaN or Al x In y Ga( 1−x−y )N, where 0≦x, y≦1 and has a very high crystallographic quality. In another embodiment, crystal  101  has a wurtzite crystal structure and is ZnO, ZnS, AgI, CdS, CdSe, 2H-SiC, 4H-SiC, and 6H-SiC. Nitride crystal  101  preferably has a surface dislocation density less than about 10 7  cm −2 , 10 6  cm −2 , 10 5  cm −2 , 10 4  cm −2 , 10 3  cm −2 , or even less than about 10 2  cm −2 . Nitride crystal  101  also preferably has a stacking-fault concentration below 10 3  cm −1 , 10 2  cm −1   , 10 cm   −1  or even below 1 cm −1 . Nitride crystal  101  also has a symmetric x-ray rocking curve full width at half maximum (FWHM) less than about 300 arc sec, 200 arc sec, 100 arc sec, 50 arc sec, 35 arc sec, 25 arc sec, or even less than about 15 arc sec. Nitride crystal  101  has a crystallographic radius of curvature greater than 0.1 meter, 1 meter, 10 meters, 100 meters, or even greater than 1000 meters, in up to three independent or orthogonal directions. 
         [0018]    Nitride crystal  101  has regions having a relatively high concentration of threading dislocations separated by regions having a relatively low concentration of threading dislocations. The concentration of threading dislocations in the relatively high concentration regions may be greater than about 10 6  cm −2 , 10 7  cm −2 , or even greater than about 10 8  cm −2 . The concentration of threading dislocations in the relatively low concentration regions may be less than about 10 6  cm −2 , 10 5  cm −2 , or even less than about 10 4  cm −2 . The thickness of nitride crystal  101  is between about 100 microns and about 100 millimeters, or even between about 1 millimeter and about 10 millimeters. The diameter of the crystal  101  is at least about 0.5 millimeter, 1 millimeter, 2 millimeters, 5 millimeters, 10 millimeters, 15 millimeters, 20 millimeters, 25 millimeters, 35 millimeters, 50 millimeters, 75 millimeters, 100 millimeters, 150 millimeters, and can be at least about 200 millimeters. Surface  105  has a crystallographic orientation within 5 degrees, 2 degrees, 1 degree, 0.5 degree, 0.2 degree, 0.1 degree, 0.05 degree, 0.02 degree, or even within 0.01 degree of (0 0 0 1) Ga-polar, (0 0 0 −1) N-polar, {1 0-1 0} non-polar, or {1 1-2 0} non-polar a-plane. Surface  105  may have a (h k i l) semi-polar orientation, where i=−(h+k) and l and at least one of h and k are nonzero. 
         [0019]    In a specific embodiment, the crystallographic orientation of surface  105  is within 5 degrees, 2 degrees, 1 degree, 0.5 degree, 0.2 degree, 0.1 degree, 0.05 degree, 0.02 degree, or even 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}, {2 1-3±1}, or {3 0-3±4}. Nitride crystal  101  has a minimum lateral dimension of at least two millimeters, but it can be four millimeters, one centimeter, two centimeters, three centimeters, four centimeters, five centimeters, six centimeters, eight centimeters, or even at least ten centimeters. In another set of embodiments, crystal  101  has a cubic crystal structure. In some embodiments, crystal  101  has a cubic diamond structure and is selected from among diamond, silicon, germanium, or silicon germanium. In other embodiments, crystal  101  has a cubic zincblende structure and is selected from among cubic BN, BP, BAs, AlP, AlAs, AlSb, β-SiC, GaP, GaAs, GaSb, InP, InAs, ZnS, ZnSe, CdS, CdSe, CdTe, CdZeTe, and HgCdTe. In a specific embodiment, the crystallographic orientation of surface  105  is within 5 degrees, 2 degrees, 1 degree, 0.5 degree, 0.2 degree, 0.1 degree, 0.05 degree, 0.02 degree, or even within 0.01 degree of {1 1 1}, {1 1 0}, {1 0 0}, {3 1 1}, and {2 1 1}. 
         [0020]    In some embodiments, nitride crystal  101  is grown by hydride vapor phase epitaxy (HVPE) according to known methods. In other embodiments, nitride crystal  101  is grown by molecular beam epitaxy (MBE) or metalorganic chemical vapor deposition (MOCVD). Nitride crystal  101  may be grown on a heteroepitaxial substrate such as sapphire or gallium arsenide. In some embodiments, nitride crystal  101  is grown by a flux or high temperature solution method. In some embodiments, nitride crystal  101  is grown ammonothermally. 
         [0021]    One of the steps in the preparation of nitride crystal  101  can be lateral growth from a seed crystal, as described in U.S. patent application Ser. No. 12/556,562, filed Sep. 9, 2009, and U.S. patent application Ser. No. 61/250,476, filed Oct. 9, 2009. Referring to  FIG. 8 , in one set of embodiments a bar-shaped seed crystal  330  having two a-plane-oriented edges is provided. Ammonothermal growth may be performed, using conditions that favor rapid growth in the a-direction, producing laterally-grown wings  340  and  350 . The laterally-grown wings may be separated from the seed crystal, producing crystals with a shape approximating a half-rhombus, as shown in  FIG. 9 . Referring to  FIG. 10 , in another set of embodiments a bar-shaped seed crystal  780  having +c and −c-plane-oriented edges is provided. Ammonothermal growth may be performed, using conditions that favor rapid growth in the +c- and/or −c-directions, producing laterally-grown crystal  790 . If desired, the laterally-grown wings may be separated. 
         [0022]    Referring again to  FIG. 1   a , in some embodiments, the conditions for the final growth step for crystal  101  are chosen so that the crystal grows to the nominal orientation and is highly flat. For example, the growth condition may be chosen so that the growth rates in directions parallel to surface  105  are larger, by at least a factor of 5, a factor of 10, a factor of 20, or a factor of 50, than the growth rate perpendicular to surface  105 . Establishment of an on-axis orientation by direct growth may be particularly advantageous when surface  105  has an orientation selected from (0 0 0 1) Ga-polar, (0 0 0 −1) N-polar, {1 0-1 0} non-polar, or { 10 - 1 ± 1 } semi-polar. Additional steps in the preparation of nitride crystal  101  and of surface  105  may include sawing, lapping, polishing, dry etching, and chemical mechanical polishing. Surface  105  may be optically flat, with a deviation from flatness less than 1 micron, 0.5 micron, 0.2 micron, 0.1 micron, or even less than 0.05 micron. Surface  105  may be very smooth, with a root-mean-square roughness less than 5 nanometers, 2 nanometers, 1 nanometer, 0.5 nanometer, 0.2 nanometer, 0.1 nanometer, or even less than 0.05 nanometer, measured over an area of at least 10 microns×10 microns. 
         [0023]    In some embodiments, at least one edge, at least two edges, or at least three edges of nitride crystal  101  are as-grown. In some embodiments, at least one edge, at least two edges, or at least three edges of nitride crystal  101  are cleaved. In some embodiments, at least one edge, at least two edges, or at least three edges of nitride crystal  101  are sawed, ground, lapped, polished, and/or etched, for example, by reactive ion etching (RIE) or inductively-coupled plasma (ICP). In one specific embodiment, one or more edges of the surface of crystal  101  are defined by etching one or more trenches in a larger crystal. In some embodiments, at least one edge, at least two edges, or at least three edges of nitride crystal  101  have a {1 0-1 0} m-plane orientation. In one specific embodiment, nitride crystal  101  has a substantially hexagonal shape. In another specific embodiment, nitride crystal  101  has a substantially rhombus or half-rhombus shape. In still other embodiments, nitride crystal  101  is substantially rectangular. In one specific embodiment, nitride crystal  101  has a (0 0 0 1) +c-plane edge and a (0 0 0 −1) −c-plane edge. In another specific embodiment, nitride crystal  101  has two {1 1-2 0} edges. In yet another specific embodiment, nitride crystal  101  has two {1 0-1 0} edges. In still another specific embodiment, crystal  101  has a cubic crystal structure and at least one edge, at least two edges, or at least three edges have a { 111} orientation. In yet another, specific embodiment, crystal  101  has a cubic zincblende crystal structure and at least one edge, at least two edges, or at least three edges have a {110} orientation. 
         [0024]    Referring again to  FIG. 1   a , in one set of embodiments, surface  105  of nitride crystal  101  is implanted with ions, forming an implanted/damaged region  103  according to methods that are known in the art. The ion implantation may be performed with at least one of H + , H 2   + , He + , Ne + , Ar + , Kr + , Xe + , N + , or N 2   + . The implantation energy be between about 10 keV and about 1 MeV, or preferably between about 20 keV and about 200 keV. The ion fluence or dose may be between about 10 16  cm −2  and about 10 19  cm −2 , between about 10 17  cm −2  and about 10 18  cm −2 , or between about 2×10 17  cm −2  and about 4×10 17  cm −2 . In some embodiments, the back side of crystal  101  is also implanted with ions, forming a second implanted/damaged region (not shown), with a similar ion composition, energy, and fluence, so as to minimize bow in crystal  101 , as described by O. Moutanabbir and U. Gösele, J. Electronic Mater. 39, 482 (2010), which is hereby incorporated by reference in its entirety. 
         [0025]    Referring to  FIG. 1   b , in some embodiments a release layer  107  is provided. In some embodiments, release layer  107  has an optical absorption coefficient greater than 1000 cm −1  at at least one wavelength where nitride crystal  101  is substantially transparent, with an optical absorption coefficient less than 50 cm −1 , is then deposited on nitride crystal  101 . In some embodiments, the release layer has an optical absorption coefficient greater than 5000 cm −1  at at least one wavelength where nitride crystal  101  is substantially transparent. In some embodiments, release layer  107  can be selectively wet etched, electrochemically etched, or photoelectrochemically etched preferentially with respect to crystal  101  or with respect to high quality epitaxial layer  109 . In some embodiments, the release layer comprises Al x In y Ga 1−x−y N, where 0≦x, y, x+y≦1. In some embodiments the release layer further comprises at least one impurity, to render the release layer strongly absorbing at some wavelengths. A number of dopant impurities, including H, O, C, Mn, Fe, and Co, may render an Al x In y Ga 1−x−y N or GaN crystal colored. Heavy doping with cobalt, in particular, can render GaN black, that is, with a high optical absorption coefficient across the visible spectrum. In particular, the optical absorption coefficient may be greater than 5000 cm −1  across the entire visible spectrum, including the range between about 465 nm and about 700 nm. The optical absorption coefficient may also be greater than 5000 cm −1  between about 700 nm and about 3077 nm and at wavelengths between about 3333 nm and about 6667 nm. Incorporation of In can decrease the bandgap of GaN, leading to strong absorption at wavelengths where GaN or AlGaN are substantially transparent. However, the InGaN has inferior temperature stability and a larger lattice mismatch with respect to GaN or AlGaN than does heavily-doped GaN or AlGaN. Release layer  107  may be deposited epitaxially on nitride crystal  101  by metalorganic chemical vapor deposition (MOCVD), hydride vapor phase epitaxy (HVPE), molecular beam epitaxy (MBE), ammonothermal growth, or flux growth, as described further in U.S. patent application Ser. No. 12/546458, which is hereby incorporated by reference in its entirety. 
         [0026]    In another set of embodiments, the release layer  107  comprises nitrogen and at least one element selected from Si, Sc, Ti, V, Cr, Y, Zr, Nb, Mo, a rare earth element, Hf, Ta, and W. A metal layer may be deposited on the base crystal, to a thickness between about  1  nm and about 1 micron by sputtering, thermal evaporation, e-beam evaporation, or the like. The metal layer may then be nitrided by heating in a nitrogen-containing atmosphere such as ammonia to a temperature between about 600 degrees Celsius and about 1200 degrees Celsius. During the nitridation process the metal partially de-wets from the base crystal, creating nano-to-micro openings through which high quality epitaxy can take place. The nitridation step may be performed in an MOCVD reactor, in an HVPE reactor, or in an ammonothermal reactor immediately prior to deposition of a high quality epitaxial layer. 
         [0027]    In still another set of embodiments, the release layer  107  comprises Al x In y Ga 1−x−y N, where 0≦x, y, x+y≦1, but may not have an optical absorption coefficient larger than that of nitride crystal  101 . In a preferred embodiment, nitride crystal  101  comprises GaN and release layer  107  comprises Al 1−x In x N, where x is approximately equal to 0.17 so that the release layer is lattice-matched to nitride crystal  101 , also known as the nitride base crystal. Referring again to  FIG. 1   b , a high quality epitaxial layer  109  may be provided. In some embodiments, the high quality epitaxial layer is grown in a separate step, by MOCVD, by MBE, or by HVPE, after deposition of the release layer. In another embodiment, the high quality epitaxial layer is grown ammonothermally. The high quality epitaxial layer may have a thickness between about 0.05 micron and about 500 microns. In some embodiments the thickness of the high quality epitaxial layer is between about one micron and about 50 microns. 
         [0028]    The high quality epitaxial layer  109  has the same crystallographic orientation as nitride crystal  101 , to within about 0.5 degree, within 0.2 degree, within 0.1 degree, within 0.05 degree, within 0.02 degree, or within 0.01 degree, and very similar crystallographic properties. High quality epitaxial layer  109  may be between 0.1 micron and 50 microns thick, comprises nitrogen and may have a surface dislocation density below 10 7  cm −2 . In preferred embodiments, high quality epitaxial layer  109  comprises GaN or Al x In y Ga( 1−x−y )N, where 0≦x, y≦1 and has a very high crystallographic quality. High quality epitaxial layer  109  may have a surface dislocation density less than about 10 7  cm −2 , less than about 10 6  cm −2 , less than about 10 5  cm −2 , less than about 10 4  cm −2 , less than about 10 3  cm −2 , or less than about 10 2  cm −2 . High quality epitaxial layer  109  may have a stacking-fault concentration below 10 3  cm −1 , below 10 2  cm −1 , below 10 cm −1  or below 1 cm −1 . High quality epitaxial layer  109  may have a symmetric x-ray rocking curve full width at half maximum (FWHM) less than about 300 arc sec, less than about 200 arc sec, less than about 100 arc sec, less than about 50 arc sec, less than about 35 arc sec, less than about 25 arc sec, or less than about 15 arc sec. In some embodiments, the high quality epitaxial layer is substantially transparent, with an optical absorption coefficient below 100 cm −1 , below 50 cm −1 , below 5 cm −1 , or below 1 cm −1  at wavelengths between about 700 nm and about 3077 nm and at wavelengths between about 3333 nm and about 6667 nm. In some embodiments, the high quality epitaxial layer is substantially free of low angle grain boundaries, or tilt boundaries. In other embodiments, the high quality epitaxial layer comprises at least two tilt boundaries, with the separation between adjacent tilt boundaries not less than 3 mm. The high quality epitaxial layer may have impurity concentrations of O, H, C, Na, and K below 1×10 17  cm −3 , 2×10 17  cm −3 , 1×10 17  cm −3 , 1×10 16  cm −3  , and 1×10 16  cm −3 , respectively, as quantified by calibrated secondary ion mass spectrometry (SIMS), glow discharge mass spectrometry (GDMS), interstitial gas analysis (IGA), or the like. 
         [0029]    Referring again to  FIG. 1   b , the process of depositing a release layer and a high quality epitaxial layer may be repeated at least one, at least two, at least four, at least eight, or at least sixteen times. In one set of embodiments the high quality epitaxial layers comprise GaN and the release layers comprise lattice-matched Al 0.83 In 0.17 N. In another set of embodiments the roles are reversed, and the release layers comprise GaN and the high quality epitaxial layers comprise lattice-matched Al 0.83 In 0.17 N. The outermost surface  111  of the one or more high quality epitaxial layers has the same crystallographic orientation as surface  105 . 
         [0030]    Referring to  FIG. 1   c , in some embodiments a series of channels are provided through a high quality epitaxial layer. A pattern, for example, a series of stripes, may be defined by conventional photolithography. Channels may be etched by reactive ion etching (RIE), inductively-coupled plasma (ICP) etching, or the like. In some embodiments the channels are etched through only a single high quality epitaxial layer. The channel may or may not cut through the outermost release layer, but the release layer is exposed in each channel. In other embodiments the channels are cut through two or more high quality epitaxial layers. The spacing between adjacent channels may be between about 10 microns and about 10 millimeters, or between about 0.1 millimeter and 1 millimeter. 
         [0031]    Referring to  FIG. 1   d , in some embodiments nitride crystal  101  is affixed to block  112 . Block  112  may comprise stainless steel, steel, an iron-based alloy, a nickel-based alloy, a cobalt-based alloy, a copper-based alloy, or the like. Block  112  may have edges that are machined or ground very accurately. For example, at least two parallel faces on block  112  may be parallel to 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. At least two perpendicular faces on block  112  may be perpendicular to 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. Nitride crystal  101  may be affixed to block  112  by means of a cement, an epoxy, an adhesive, a Au-Sn eutectic, a solder bond, a braze joint, a polymer-based cement, or the like. One or more edges of nitride crystal  101  may also be ground very accurately. At least one edge of nitride crystal  101  may be co-planar with an edge of block  112 . In some embodiments, at least two edges of crystal  101  are co-planar with edges of block  112 . 
         [0032]    Referring to  FIGS. 1   e  and  1   f , a handle substrate  117  having a surface  115  is provided. Handle substrate  117  may comprise a single crystal, polycrystalline or amorphous material. Handle substrate  117  may comprise sapphire, aluminum oxide, mullite, silicon, silicon nitride, germanium, gallium arsenide, silicon carbide, MgAl 2 O 4  spinel, zinc oxide, indium phosphide, gallium nitride, indium nitride, gallium aluminum indium nitride, or aluminum nitride. Handle substrate  117  may comprise substantially the same composition as crystal  101 . In one specific embodiment, handle substrate  117  comprises crystals that have been merged or tiled together using another method. For example, handle substrate  117  may be formed using at least one of the tiling methods disclosed by Dwilinski et al. [U.S. Patent Application No. 2008/0156254] or the method disclosed in U.S. patent application Ser. No. 12/635645, which is hereby incorporated by reference in its entirety. In a preferred embodiment, handle substrate  117  comprises substantially the same composition as crystal  101  and has a crystallographic orientation within about 10 degrees, within about 5 degrees, within about 2 degrees, or within about 1 degree of that of crystal  101 . Handle substrate  117  may comprise a glass. Handle substrate  117  may comprise an oxide of at least one of Si, Ge, Sn, Pb, B, Al, Ga, In, Tl, P, As, Sb, Pb, Li, Na, K, Rb, Cs, Mg, Ca, Sr, Ba, Y, Ti, Zr, Hf, Mn, Zn, or Cd. In one specific embodiment, handle substrate  117  comprises borophosphosilicate glass. Handle substrate  117  may have a thermal expansion coefficient parallel to surface  115  between room temperature and about 700 degrees Celsius that is between about 2.5×10 −6  K −1  and about 7×10 −6  K −1 . Handle substrate  117  may have a thermal expansion coefficient parallel to surface  115  between room temperature and about 700 degrees Celsius that is between about 5.5×10 −6  K −1  and about 6.5×10 −6  K −1 . Handle substrate  117  may have a softening point, that is, where its viscosity has a value of about 10 8  Poise, at a temperature between about 500 degrees Celsius and about 1400 degrees Celsius. Handle substrate  117  may have a glass transition temperature between about 600 degrees Celsius and about 1200 degrees Celsius. Handle substrate  117  may have a softening point, that is, where its viscosity has a value of about 10 8  Poise, at a temperature between about 600 degrees Celsius and about 900 degrees Celsius. Surface  115  may be optically flat, with a deviation from flatness less than 1 micron, less than 0.5 micron, less than 0.2 micron, less than 0.1 micron, or less than 0.05 micron. Surface  115  may be very smooth, with a root-mean-square roughness less than 5 nanometers, less than 2 nanometers, less than 1 nanometer, less than 0.5 nanometer, less than 0.2 nanometer, less than 0.1 nanometer, or less than 0.05 nanometer, measured over an area of at least 10 microns×10 microns. 
         [0033]    An adhesion layer  113  may be deposited on surface  115  of handle substrate  117 . Adhesion layer  113  may comprise at least one of SiO 2 , GeO 2 , SiN x , AlN x , or B, Al, Si, P, Zn, Ga, Si, Ge, Au, Ag, Ni, Ti, Cr, Zn, Cd, In, Sn, Sb, Tl, or Pb, or an oxide, nitride, or oxynitride thereof. Adhesion layer  113  may further comprise hydrogen. The adhesion layer  113  may be deposited by thermal evaporation, electron-beam evaporation, sputtering, chemical vapor deposition, plasma-enhanced chemical vapor deposition, or the like, or by thermal oxidation of a deposited metallic film. The thickness of adhesion layer  113  may between about 1 nanometer and about 10 microns, or between about 10 nanometers and about 1 micron. In some embodiments, an adhesion layer is deposited on surface  105  of nitride crystal  101  or on surface  111  of high quality epitaxial layer  109  (not shown). The adhesion layer(s) may be annealed, for example, to a temperature between about 300 degrees Celsius and about 1000 degrees Celsius. In some embodiments, an adhesion layer is deposited on surface  105  of crystal  101  and annealed prior to forming an implanted/damaged layer by ion implantation. In some embodiments, at least one adhesion layer is chemical-mechanically polished. In a preferred embodiment, the root-mean-square surface roughness of at least one adhesion layer is below about 0.5 nanometer, or below about 0.3 nanometer over a 20×20 μm 2  area. 
         [0034]    Referring again to  FIGS. 1   e  and  1   f , surface  105  of nitride crystal  101 , surface  111  of high quality epitaxial layer  109 , or an adhesion layer placed thereupon, is placed in contact with adhesion layer  113  and/or with the surface  115  of the handle substrate  117  and wafer-bonded. In a preferred embodiment, the wafer bonding operation is performed in a clean room, with less than 10,000, less than 1,000, less than 100, or less than 10 particles per cubic centimeter in the air. Particles may be removed from at least one of the surfaces immediately prior to wafer bonding by spraying, brushing, or rinsing with ionized nitrogen, a CO 2  jet, CO 2  snow, high-resistivity water, an organic solvent, such as methanol, ethanol, isopropanol, acetone, or the like. In some embodiments, surface  105  or surface  109 , or the surface of an adhesion layer placed thereupon, and surface  113  or surface  115  are brought into contact while immersed in a liquid. Optionally, at least one of the surfaces is exposed to a plasma to enhance wafer bonding. 
         [0035]    The positional and orientational accuracy of the placement of nitride crystal  101  with respect to handle substrate  117  is precisely controlled. In one specific embodiment nitride crystal is placed on handle substrate  117  by a pick and place machine, or robot, or a die attach tool. Nitride crystal  101  may be picked up by a vacuum chuck, translated to the desired position above handle substrate  117  by a stepper-motor-driven x-y stage, re-oriented, if necessary, by a digital-camera-driven rotational drive, and lowered onto the handle substrate. The positional accuracy of placement may be better than 50 microns, better than 30 microns, better than 20 microns, better than 10 microns, or better than 5 microns. The orientational accuracy of placement may be better than 5 degrees, better than 2 degrees, better than 1 degree, better than 0.5 degree, better than 0.2 degree, better than 0.1 degree, better than 0.05 degree, better than 0.02 degree, or better than 0.01 degree. In another specific embodiment, block  112 , attached to nitride crystal  101 , is placed in a kinematic mount. The kinematic mount establishes orientational accuracy with respect to handle substrate  117  that is better than 1 degree, better than 0.5 degree, better than 0.2 degree, better than 0.1 degree, better than 0.05 degree, better than 0.02 degree, or better than 0.01 degree. Nitride crystal  101 , block  112 , and the kinematic mount may then be positioned with respect to handle substrate  117  with submicron accuracy using an x-y stage similar to that in a stepper photolithography tool, using stepper motors in conjunction with voice coils. In some embodiments, the azimuthal crystallographic orientations of crystal  101  and handle substrate  117  are equivalent to within about 10 degrees, within about 5 degrees, within about 2 degrees, or within about 1 degree. 
         [0036]    Nitride crystal  101  may be pressed against handle substrate  117  with a pressure between about 0.1 megapascals and about 100 megapascals. In some embodiments, van der Waals forces are sufficient to obtain a good wafer bond and no additional applied force is necessary. Nitride crystal  101  and handle substrate  117  may be heated to a temperature between about 30 degrees Celsius and about 950 degrees Celsius, between about 30 degrees Celsius and about 400 degrees Celsius, between about 30 degrees Celsius and about 200 degrees Celsius to strengthen the wafer bond. In some embodiments, heating of nitride crystal  101  and handle substrate  113  is performed while they are mechanically loaded against one another. 
         [0037]    In some embodiments, at least the surface region of bonded nitride crystal  101  having implanted/damaged region  103  and handle substrate  117  are heated to a temperature between about 200 degrees Celsius and about 800 degrees Celsius or between about 500 degrees Celsius and about 700 degrees Celsius to cause micro-bubbles, micro-cracks, micro-blisters, or other mechanical flaws within region  103 . In one specific embodiment, surface region  105  or  109  is heated by means of optical or infrared radiation through handle substrate  117 , and the distal portion of crystal  101 , which may be in contact with block  112 , may remain less than about 300 degrees Celsius, less than about 200 degrees Celsius, or less than about 100 degrees Celsius. In some embodiments, mechanical energy may be provided instead of or in addition to thermal energy. In some embodiments, an energy source such as a pressurized fluid is directed to a selected region, such as an edge, of bonded nitride crystal  101  to initiate a controlled cleaving action within region  103 . After the application of energy, the distal portion of nitride crystal  101  is removed, leaving a proximate portion of nitride crystal  101  bonded to handle substrate  117 . In some embodiments, distal portion of nitride crystal  101  remains bonded to block  112 . In some embodiments, the newly exposed surface of distal portion of nitride crystal  101  is polished, dry-etched, or chemical-mechanically polished. Care is taken to maintain the surface crystallographic orientation of the newly exposed surface of distal portion of nitride crystal  101  the same as the original orientation of surface  105 . In some embodiments, an adhesion layer is deposited on the newly exposed surface of distal portion of crystal  101 . In some embodiments, the adhesion layer is chemical-mechanically polished. 
         [0038]    Referring to  FIG. 1   g , in some embodiments, nitride crystal  101  is separated from high quality epitaxial layer  109  and handle substrate  117  by laser irradiation. The release layer  107  may be illuminated through nitride crystal  101  by laser radiation  125  having a wavelength at which the release layer has an optical absorption coefficient greater than 1000 cm −1  and the nitride crystal is substantially transparent, with an optical absorption coefficient less than 50 cm −1 . In another set of embodiments, the release layer is illuminated through handle substrate  117  by laser radiation  127  having a wavelength at which the release layer has an optical absorption coefficient greater than 1000 cm −1  and the handle substrate is substantially transparent, with an optical absorption coefficient less than 50 cm −1 . Absorption of the laser energy by the release layer  109  occurs on a very short length scale, causing considerable local heating. Without wishing to be bound by theory, we believe that the local heating causes partial or complete decomposition of the release layer and/or a thin portion of the nitride crystal in direct contact with the release layer, forming metal and N 2 , which may occur as a thin layer or as micro- or nano-bubbles. The thin layer or micro- or nano-bubbles of N 2  mechanically weakens the interface between the nitride crystal and the high quality epitaxial layer, enabling facile separation of the nitride crystal from the high quality epitaxial layer, which is in turn bonded to the handle substrate. The optimal degree of weakening of the interface, without causing undesired damage to the high quality epitaxial layer or the handle substrate, is achieved by adjusting the die temperature, the laser power, the laser spot size, the laser pulse duration, and/or the number of laser pulses. The laser fluence to effect separation may be between 300 and 900 millijoules per square centimeter or between about 400 mJ/cm 2  and about 750 mJ/cm 2 . The uniformity of the laser beam may be improved by inclusion of a beam homogenizer in the beam path, and the beam size may be about 4 mm by 4 mm. In some embodiments, the laser beam is scanned or rastered across the release layer rather than being held stationary. Separation may be performed at a temperature above the melting point of the metal produced by decomposition, e.g., above about 30 degrees Celsius in the case of gallium metal. 
         [0039]    In some embodiments, multiple release layers and high quality epitaxial layers are present in the wafer-bonded stack. In this case laser illumination is preferably applied through the handle substrate, and the fluence controlled so that substantial decomposition takes place only within the release layer closest to the handle substrate and the remaining release layers and high quality epitaxial layers remain bonded to the nitride crystal after liftoff. 
         [0040]    After separation of the high quality epitaxial layer from the nitride crystal, any residual gallium, indium, or other metal or nitride on the newly exposed back surface of the high quality epitaxial layer, on nitride crystal  101 , or on another newly-exposed high quality epitaxial layer still bonded to nitride crystal  101  may be removed by treatment with at least one of hydrogen peroxide, an alkali hydroxide, tetramethylammonium hydroxide, an ammonium salt of a rare-earth nitrate, perchloric acid, sulfuric acid, nitric acid, acetic acid, hydrochloric acid, and hydrofluoric acid. The surfaces may be further cleaned or damage removed by dry-etching in at least one of Ar, Cl 2 , and BCl 3 , by techniques such as chemically-assisted ion beam etching (CAIBE), inductively coupled plasma (ICP) etching, or reactive ion etching (RIE). The surfaces may be further treated by chemical mechanical polishing. 
         [0041]    In some embodiments, traces of the release layer may remain after laser liftoff or etching from the edges of the release layer. Residual release layer material may be removed by photoelectrochemical etching, illuminating the back side of the high quality epitaxial layer or the front side of nitride crystal  101  or of the front side of the outermost high quality epitaxial layer still bonded to nitride crystal  101  with radiation at a wavelength at which the release layer has an optical absorption coefficient greater than 1000 cm −1  and the high quality epitaxial layer is substantially transparent, with an optical absorption coefficient less than 50 cm −1 . 
         [0042]    Referring to  FIG. 1   h , in another set of embodiments, the high quality epitaxial layer bonded to the handle substrate is separated from the nitride crystal by means of chemical etching of the release layer. In one embodiment, one or more edges of the release layer is treated with at least one of 1,2-diaminoethane, hydrogen peroxide, an alkali hydroxide, tetramethylammonium hydroxide, an ammonium salt of a rare-earth nitrate, perchloric acid, sulfuric acid, nitric acid, acetic acid, hydrochloric acid, and hydrofluoric acid. In one specific embodiment, the edge of the release layer is etched by treatment in a mixture of 200 milliliters of deionized water, 50 grams of diammonium cerium nitrate, Ce(NH 4 ) 2 (NO 3 ) 6 , and 13 milliliters of perchloric acid, HClO 4 , at approximately 70 degrees Celsius. At least one edge of the release layer is etched away, mechanically weakening the interface between the nitride base crystal and the high quality epitaxial layer and enabling facile separation of the nitride base crystal from the high quality epitaxial layer, which is in turn bonded to at least one semiconductor device layer. The right degree of weakening of the interface, without causing undesired damage to the high quality epitaxial layer or the semiconductor structure, is achieved by adjusting the temperature and time of the chemical treatment. The time required for lateral etching of the release layer may be reduced by incorporating a pre-formed set of channels in the release layer. In the case that multiple, alternating release layers and high quality epitaxial layers are bonded to nitride crystal  101 , transfer may be restricted to the outermost high quality epitaxial layer by utilizing etch channels that penetrate only the outermost high quality epitaxial layer. 
         [0043]    In still another set of embodiments, the high quality epitaxial layer bonded to the handle substrate is separated from the nitride crystal by means of photoelectrochemical (PEC) etching of the release layer. For example, an InGaN layer or InGaN/InGaN superlattice may be deposited as the release layer. An electrical contact may be placed on the nitride crystal and the release layer illuminated with above-bandgap radiation, for example, by means of a Xe lamp and a filter to remove light with energy greater than the bandgap of the high quality epitaxial layer and/or the nitride crystal. In one set of embodiments, illustrated schematically in  FIG. 1   i , the illumination  127  is provided through the handle substrate and the intensity adjusted so that essentially all the light is absorbed by the release layer in closest proximity to the handle substrate. At least one edge of the release layer is exposed to an electrolyte, for example, a stirred, 0.004M HCl solution. The time required for lateral etching of the release layer may be reduced by incorporating a pre-formed set of channels in the release layer. In the case that multiple, alternating release layers and high quality epitaxial layers are bonded to nitride crystal  101 , transfer may be restricted to the outermost high quality epitaxial layer even when the etch channels penetrate multiple high quality epitaxial layers by ensuring that the light is fully absorbed by only the outermost release layer. Further details of the PEC etching process are described by Sharma et al., Applied Physics Letters 87, 051107 (2005) and references therein. In one set of embodiments, GaN is deposited as the release layer and lattice-matched AlInN comprises the high quality epitaxial layer, and the wavelength range of the illumination is chosen so that electron-hole pairs are generated in the GaN but not in the AlInN. 
         [0044]    In yet another set of embodiments, the high quality epitaxial layer bonded to the handle substrate is separated from the nitride crystal by means of selective oxidation followed by chemical etching of the release layer. For example, at least one release layer comprising Al x In y Ga 1−x−y N, where 0&lt;x, x+y≦1, 0≦y≦1, or Al 0.83 In 0.17 N, lattice matched to GaN, may be selectively oxidized. The selective oxidation may be performed by exposing at least one edge of the Al-containing release layer to a solution comprising nitriloacetic acid (NTA) and potassium hydroxide at a pH of approximately 8 to 11 and an anodic current of approximately 20 μA/cm 2 , to about 0.1 kA/cm 2 , as described by Dorsaz et al., Applied Physics Letters 87, 072102 (2005) and by Altoukhov et al., Applied Physics Letters 95, 191102 (2009) and references cited therein. The oxide layer may then be removed by treatment in a nitric acid solution at approximately 100 degrees Celsius. The time required for lateral etching of the release layer may be reduced by incorporating a pre-formed set of channels in the release layer. In the case that multiple, alternating release layers and high quality epitaxial layers are bonded to nitride crystal  101 , transfer may be restricted to the outermost high quality epitaxial layer by utilizing etch channels that penetrate only the outermost high quality epitaxial layer. 
         [0045]    Referring to  FIG. 1   j  and  1   k , the wafer bonding process is repeated. A second nitride crystal, or the distal portion of the first nitride crystal, is wafer bonded in close proximity to the first nitride crystal or to the proximate portion of first nitride crystal. The second nitride crystal may have an ion-implanted, damaged region or at least one release layer and at least one high quality epitaxial layer, similar to the first nitride crystal. The second nitride crystal or the outer most high quality epitaxial layer on the second nitride crystal has a surface  135  whose crystallographic orientation is essentially identical to that of surface  105  of the nitride crystals  101  or to that of surface  111  of the first high quality epitaxial layer. In some embodiments, accurate equality between the surface orientation of the first and second nitride crystals is achieved by growing each crystal to an accurately flat on-axis orientation, for example, (0 0 0 1) Ga-polar, (0 0 0 −1) N-polar, {1 0-1 0} non-polar, or {1 0-1±1} semi-polar. If the first and/or second nitride crystals are polished, dry-etched, or chemical-mechanically polished, care is taken so as not to significantly alter the surface orientation of either. In some embodiments, accurate equality between the surface orientation of the first and second nitride crystals is achieved by removing a uniform, thin proximate portion of the first nitride crystal to form the second nitride crystal. If the distal portion of the first nitride crystal, used also as the second nitride crystal, is polished, dry-etched, or chemical-mechanically polished, care is taken so as not to significantly alter the surface orientation. In other embodiments, accurate equality between the surface orientation of the first and second nitride crystals is achieved by removing a uniform, thin high quality epitaxial layer from the first nitride crystal to form the second nitride crystal. If the distal portion of the first nitride crystal, used also as the second nitride crystal, is polished, dry-etched, or chemical-mechanically polished, care is taken so as not to significantly alter the surface orientation. For example, the crystallographic orientations of first surfaces  105  or  111  and  135 , respectively, of the outermost surface or high quality epitaxial layers on the first and second nitride crystals may be identical to less than 0.5 degree, less than 0.2 degree, less than 0.1 degree, less than 0.05 degree, less than 0.02 degree, or less than 0.01 degree. In still other embodiments, accurate equality between the surface orientation of the first and second nitride crystals is achieved by very careful crystallographic orientation and grinding and/or polishing, for example, using a high-precision goniometer. After wafer bonding, a distal portion of the second nitride crystal may be removed. Gap  145  between the edges of two or more adjacent nitride crystals or proximate portions thereof may be less than 1 millimeter, less than 100 microns, less than 50 microns, less than 20 microns, less than 10 microns, less than 5 microns, less than 2 microns, or less than 1 micron. The wafer bonding process may be repeated more than two, more than 4, more than 8, more than 16, more than 32, or more than 64 times. 
         [0046]    The placement of the second nitride crystal is performed in such as way that the crystallographic orientations between the from the first nitride crystal and the second nitride crystal, or the high quality epitaxial layers thereupon, are very nearly identical. Referring to  FIG. 2 , coordinate system  221  (x 1  y 1  z 1 ) represents the crystallographic orientation of the high quality epitaxial layer from the first nitride crystal  201 , where z 1  is the negative surface normal of the nominal orientation of surface  111  (cf.  FIG. 1 ) and x 1  and y 1  are vectors that are orthogonal to z 1 . For example, if surface  111  has a (0 0 0 1) orientation, then z 1  is a unit vector along [0 0 0 −1], and x 1  and y 1  may be chosen to be along [1 0-1 0] and [1-2 1 0], respectively. If surface  111  has a (1 0-1 0) orientation, then z 1  is a unit vector along [−1 0 1 0] and x 1  and y 1  may be chosen to be along [1-2 1 0] and [0 0 0 1], respectively. Similarly, coordinate system  222  (x 2  y 2  z 2 ) represents the crystallographic orientation of the high quality epitaxial layer from the second nitride crystal  202 , where z 2  is the negative surface normal of the nominal orientation of surface  105  (cf.  FIG. 1 ) and x 2  and y 2  are vectors that are orthogonal to z 2 , where the same convention is used for the crystallographic directions corresponding to (x 2  y 2  z 2 ) as for (x 1  y 1  z 1 ). The crystallographic misorientation between the first nitride crystal and the second nitride crystal may be specified by the three angles α, β, and γ, where αis the angle between x 1  and x 2 , β is the angle between y 1  and y 2 , and γ is the angle between z 1  and z 2 . Because the surface orientations of the first and second nitride crystals are nearly identical, the polar misorientation angle γ is very small, for example, less than 0.5 degree, less than 0.2 degree, less than 0.1 degree, less than 0.05 degree, less than 0.02 degree, or less than 0.01 degree. Because of the precise control in the orientation of the nitride crystal during placement, the misorientation angles α and β are also very small, for example, less than 1 degree, less than 0.5 degree, less than 0.2 degree, less than 0.1 degree, less than 0.05 degree, less than 0.02 degree, or less than 0.01 degree. Typically, γ will be less than or equal to α and β. The crystallographic misorientation between additional, adjacent nitride crystals is similarly very small. 
         [0047]    Referring to  FIG. 3 , after placing and wafer bonding a number of similarly-sized and similarly-shaped high quality epitaxial layers from one or more nitride crystals, a tiled arrangement of high quality epitaxial layers may be formed, with each adjacent pair on the handle substrate being accurately aligned crystallographically with its neighbor(s). The tiling pattern may be (a) square, (b) rectangular, (c) hexagonal, or (d) rhombal. Other arrangements are also possible. The gaps between the edges of two or more adjacent high quality epitaxial layers may be less than 1 millimeter, less than 100 microns, less than 50 microns, less than 20 microns, less than 10 microns, less than 5 microns, less than 2 microns, or less than 1 micron. 
         [0048]    In some embodiments, a similar set of nitride crystals or high quality epitaxial layers is wafer-bonded to the back surface of the handle substrate by an analogous procedure to that used to form the tile pattern of nitride crystals or high quality epitaxial layers on the front surface of the handle substrate. In a preferred embodiment, the tile pattern on the back surface of the handle substrate is a mirror image of the tile pattern on the front surface of the handle substrate, with the front and back tile patterns in registry. 
         [0049]    In one set of embodiments, the at least two nitride crystals or high quality epitaxial layers on the handle substrate are used as substrate for fabrication of one or more devices. The two or more tiled high quality epitaxial layers or crystals bonded to the handle substrate may be prepared for lateral growth for epitaxial growth and/or for fusion of the tiled crystals into a single larger crystal. The lateral crystal growth may be achieved by techniques such as metalorganic chemical vapor deposition (MOCVD), hydride vapor phase epitaxy (HVPE), ammonothermal crystal growth, or crystal growth from a flux. 
         [0050]    In some embodiments, the handle substrate is suitable for exposure to the epitaxial growth environment without further treatment. In some embodiments, growth may proceed more smoothly, with fewer stresses, if the gaps between adjacent nitride crystals are undercut. Referring to  FIG. 4   a , a photoresist  447  may be spun onto the wafer bonded, tiled substrate comprising handle substrate  417 , first nitride crystal  401 , and second nitride crystal  402 . Photoresist  447  may be exposed through a mask, etched, and the exposed channel etched by dry etching, and the photoresist removed to form patterned nitride/handle substrate  450 . Referring to  FIG. 4   b , patterned nitride/handle substrate  450  may be used as a substrate for epitaxial nitride growth by MOCVD, HVPE, ammonothermal growth, or flux growth. Growth is performed as known in the art, and the at least two nitride crystals  401  and  402  grow both laterally and vertically to form a merged nitride crystal  455 . Because of the very low crystallographic misorientation between nitride crystals  401  and  402 , the coalescence front  457  may have a modest concentration of dislocations but a classical low angle grain boundary or tilt boundary may be difficult to detect. 
         [0051]    In some embodiments, the handle substrate and/or the adhesion layer may not be suitable for exposure to the epitaxial growth environment without further treatment. Exposed portions of the handle substrate may be coated with a suitable inert material. Referring to  FIG. 5 , nitride crystals  501  and  502  may be masked, for example, by a shadow mask or by photolithography with a photoresist, and the regions between the masked areas on the handle substrate  517  and/or adhesion layer  509  coated with inert coating  561 . Inert coating  561  may comprise at least one of Ag, Au, Pt, Pd, Rh, Ru, Ir, SiO 2 , SiN x , or AN. Inert coating  561  may further comprise an adhesion layer (not shown) in contact with the surface of handle substrate  517  and/or adhesion layer  509  comprising, for example, at least one of Ti, V, Cr, Al, or Ni. Inert coating  561  may be deposited by sputtering, thermal evaporation, electron beam evaporation, chemical vapor deposition, plasma enhanced chemical vapor deposition, or the like. Masked nitride/handle substrate  550  may be used as a substrate for epitaxial nitride growth by MOCVD, HVPE, ammonothermal growth, or flux growth. Flux growth may be performed, for example, using liquid Ga under a nitrogen pressure of 1-3 GPa, using an alloy comprising Ga and at least one alkali metal under a pressure of a nitrogen-containing gas at a pressure of 10-200 MPa, or using one or more halide, nitride, or amide salts under a pressure of a nitrogen-containing gas at a pressure of 0.1-200 MPa. Growth is performed as known in the art, and the at least two nitride crystals  501  and  502  grow both laterally and vertically to form a merged nitride crystal  555 . Because of the very low crystallographic misorientation between nitride crystals  501  and  502 , the coalescence front  557  may have a modest concentration of dislocations but a classical low angle grain boundary or tilt boundary may be difficult to detect. 
         [0052]    The etching/patterning and masking steps may be combined. Referring to  FIG. 6 , nitride crystals  601  and  602  with an etched gap between them may be masked, for example, by a shadow mask or by photolithography with a photoresist, and the regions between the masked areas on handle substrate  617  and/or adhesion layer  609  coated with inert coating  661 . Masked/patterned/etched nitride/handle substrate  670  may be used as a substrate for epitaxial nitride growth by MOCVD, HVPE, ammonothermal growth, or flux growth. Growth is performed as known in the art, and the at least two nitride crystals  601  and  602  grow both laterally and vertically to form a merged nitride crystal  655 . Because of the very low crystallographic misorientation between nitride crystals  601  and  602 , the coalescence front  657  may have a modest concentration of dislocations but a classical low angle grain boundary or tilt boundary may be difficult to detect. 
         [0053]    The merged nitride crystal may be grown to a thickness greater than 5 microns, greater than 50 microns, greater than 0.5 millimeters, or greater than 5 millimeters. After cooling and removal from the reactor, the merged nitride crystal may be separated from the handle substrate. The inert coating, if present, may be removed from at least a portion of the edge of the handle substrate by scribing, abrasion, or the like. The handle substrate may be dissolved or etched away, for example, by placing in contact with an acid, a base, or a molten flux, preferably in a way that produces negligible etching or other damage to the merged nitride crystal. For example, a glass, silicon, or germanium substrate may be etched away without damaging the merged nitride crystal by treatment in a solution comprising HF and/or H 2 SiF 6 . Alternatively, a a glass or zinc oxide substrate may be etched away without damaging the merged nitride crystal by treatment in a solution comprising NaOH, KOH, or NH 4 OH. A gallium arsenide or zinc oxide substrate may be etched away without damaging the merged nitride crystal by treatment in a solution comprising aqua regia or one or more of HCl, HNO 3 , HF, H 2 SO 4 , and H 3 PO 4 . A sapphire or alumina substrate may be etched away without damaging the merged nitride crystal by treatment in molten KBF 4 . After removal of the handle substrate, one or more surface of the merged nitride crystal may be lapped, polished, and/or chemical-mechanically polished. The merged nitride crystal may be sliced (sawed, polished, and/or chemical-mechanically polished) into one or more wafers. 
         [0054]    Referring to  FIG. 7 , the merged nitride crystal comprises two or more domains separated by one or more lines of dislocations. Depending on the geometry of the original nitride crystals, the pattern of domains may be (a) square, (b) rectangular, (c) hexagonal, or (d) rhombal. Other patterns are also possible. The polar misorientation angle y between adjacent domains may be less than 0.5 degree, less than 0.2 degree, less than 0.1 degree, less than 0.05 degree, less than 0.02 degree, or less than 0.01 degree. The misorientation angles α and β between adjacent domains may be less than 1 degree, less than 0.5 degree, less than 0.2 degree, less than 0.1 degree, less than 0.05 degree, less than 0.02 degree, or less than 0.01 degree. Typically, γ will be less than or equal to α and β. The density of dislocations along the lines between adjacent domains may be less than 5×10 5  cm −1 , less than 2×10 5  cm −1 , less than 1×10 5  cm −1 , less than 5×10 4  cm −1 , less than 2×10 4  cm −1 , less than 1×10 3  cm −1 , less than 5×10 3  cm −1 , less than 2×10 3  cm −1 , or less than 1×10 3  cm −1 . The density of dislocations along the lines between adjacent domains may be greater than 50 cm −1 , greater than 100 cm −1 , greater than 200 cm −1 , greater than 500 cm −1 , greater than 1000 cm −1 , greater than 2000 cm −1 , or greater than than 5000 cm −1 . 
         [0055]    Within individual domains, the merged nitride crystal may have a surface dislocation density less than about 10 7  cm −2 , less than about 10 6  cm −2 , less than about 10 5  cm −2 , less than about 10 4  cm −2 , less than about 10 3  cm −2 , or less than about 10 2  cm −2 . The domains may have a stacking-fault concentration below 10 3  cm −1 , below 10 2  cm −1 , below 10 cm −1  or below 1 cm −1 . The merged nitride crystal may have a symmetric x-ray rocking curve full width at half maximum (FWHM) less than about 300 arc sec, less than about 200 arc sec, less than about 100 arc sec, less than about 50 arc sec, less than about 35 arc sec, less than about 25 arc sec, or less than about 15 arc sec. The merged nitride crystal may have a thickness between about 100 microns and about 100 millimeters, or between about 1 millimeter and about 10 millimeters. The merged nitride crystal may have a diameter of at least about 5 millimeters, at least about 10 millimeters, at least about 15 millimeters, at least about 20 millimeters, at least about 25 millimeters, at least about 35 millimeters, at least about 50 millimeters, at least about 75 millimeters, at least about 100 millimeters, at least about 150 millimeters, at least about 200 millimeters, or at least about 400 millimeters. The surface of the merged nitride crystal may have a crystallographic orientation 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) Ga-polar, (0 0 0 −1) N-polar, {1 0-1 0} non-polar, or {1 1-2 0} non-polar a-plane. The surface of the merged nitride crystal may have a (h k i l) semi-polar orientation, where i=−(h+k) and l and at least one of h and k are nonzero. In a specific embodiment, the crystallographic orientation of the merged nitride crystal 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}, {2 1-3±1}, or {3 0-3±4}. The merged nitride crystal has a minimum lateral dimension of at least four millimeters. In some embodiments, the merged nitride crystal has a minimum lateral dimension of at least one centimeter, at least two centimeters, at least three centimeters, at least four centimeters, at least five centimeters, at least six centimeters, at least eight centimeters, at least ten centimeters, or at least twenty centimeters. 
         [0056]    In some embodiments, the merged nitride crystal is used as a substrate for epitaxy. The merged nitride crystal may be sawed, lapped, polished, dry etched, and/or chemical-mechanically polished by methods that are known in the art. One or more edges of the merged nitride crystal may be ground. The merged nitride crystal, or a wafer formed therefrom, may be placed in a suitable reactor and an epitaxial layer grown by MOCVD, MBE, HVPE, or the like. In a preferred embodiment, the epitaxial layer comprises GaN or Al x In y Ga( 1−x−y )N, where 0≦x, y≦1. The morphology of the epitaxial layer is uniform from one domain to another over the surface because the surface orientation is almost identical. 
         [0057]    In some embodiments, the merged nitride crystal is used as a substrate for further tiling. For example, referring to  FIGS. 11-1k , the nitride crystal  101  may be chosen to be a merged nitride crystal. The tiling, coalescence, and re-tiling operation may be iterated more than twice, more than 4 times, more than 8 times, or more than 16 times. In this way, by successive tiling operations, a merged nitride crystal with excellent crystalline quality and very large diameter may be fabricated. 
         [0058]    The merged nitride crystal crystal, or a wafer sliced and polished from the merged nitride crystal crystal, may be used as a substrate for fabrication into optoelectronic and 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. In some embodiments, the positions of the devices with respect to the domain structure in the merged nitride crystal are chosen so that the active regions of individual devices lie within a single domain of the merged nitride crystal. 
         [0059]    In other embodiments, the merged nitride crystal crystal, or a wafer sliced and polished from the merged nitride crystal crystal, is used as a seed crystal for bulk crystal growth. In one specific embodiment, the tiled crystal, or a wafer sliced and polished from the merged nitride crystal crystal, is used as a seed crystal for ammonothermal crystal growth. In another embodiment, the tiled crystal, or a wafer sliced and polished from the merged nitride crystal crystal, is used as a seed crystal for HVPE crystal growth. 
         [0060]    In still other embodiments, the at least two nitride crystals or high quality epitaxial layers on the handle substrate, non-merged, are used as a substrate for fabrication into optoelectronic and 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. The at least one device may flip-chip mounted onto a carrier and the handle substrate removed. 
         [0061]    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 invention which is defined by the appended claims.