Abstract:
A method of forming at least one single crystal of a Group III metal nitride. The method includes the steps of: providing a flux material and a source material comprising at least one Group III metal selected from the group consisting of aluminum, indium, and gallium, to a reaction vessel; sealing the reaction vessel; heating the reaction vessel to a predetermined temperature and applying a predetermined pressure to the vessel. The pressure is sufficient to suppress decomposition of the Group III metal nitride at the temperature. Group III metal nitrides, as well as electronic devices having a Group III metal nitride substrate formed by the method are also disclosed.

Description:
BACKGROUND OF INVENTION 
     The invention relates to the growth of crystals of Group III metal nitrides. More particularly, the invention relates to the growth of Group III metal nitride crystals by recrystallization from a flux. Even more particularly, the invention relates to the growth of Group III metal nitride crystals by recrystallization from either a solvent or a melt under high pressure, high temperature conditions. 
     During the past decade there has been considerable interest in the nitrides of the Group III metals (also referred to hereinafter as “Group III metal nitrides”); namely aluminum nitride (also referred to hereinafter as “AlN”), gallium nitride (also referred to hereinafter as “GaN”), and indium nitride (also referred to hereinafter as “InN”) based optoelectronic devices, including, for example, light emitting diodes (LEDs) and laser diodes (LDs). The active layers in such devices typically comprise solid solutions of GaN, AlN, and InN and typically include n-doped layers, p-doped layers, heterostructures, and the like. The performance of these devices, including light emission efficiency, lifetime, and reverse bias current, is often degraded by the presence of threading dislocations, vacancies, and impurities in the active layers and in underlying and overlaying epitaxial layers. Such devices are typically grown on lattice-mismatched substrates such as sapphire or SiC, resulting in a high concentration of threading dislocations that propagate into the active layer. In the case of GaN-based devices, for example, the use of a high quality GaN substrate would greatly reduce the concentration of threading dislocations and other defects in the homoepitaxial active layers and improve device performance. 
     Gallium nitride single crystals that are of suitable quality for electronic applications have been obtained by reacting nitrogen (N 2 ) gas with gallium metal at pressures and temperatures in the range of 10–20 kilobar and 1200° C. to 1500° C., respectively. Other methods that have been used to grow crystalline GaN include chemical vapor deposition (CVD), hydride vapor phase epitaxy, crystallization in gallium/sodium alloys, and recrystallization from supercritical ammonia. The GaN crystals grown under these conditions exhibit varying degrees of quality and are limited in size. In addition, the growth rate of GaN crystals obtained by these processes is generally low (about 0.1 mm/hr). 
     The methods that are currently used to grow gallium nitride crystals are unable to produce large crystals of Group III metal nitrides at acceptable growth rates and that are of high quality. Therefore, what is needed is a method of growing Group III metal nitride crystals that are sufficiently large to serve as commercially viable substrates for electronic devices. What is also needed is a method of growing Group III metal nitride crystals that are of high quality and have low concentrations of impurities and dislocations. What is further needed is a method of growing Group III metal nitride crystals at a high growth rate. 
     SUMMARY OF INVENTION 
     The present invention meets these and other needs by providing a method of forming at least one single crystal of a Group III metal nitride by mixing poorly crystallized, amorphous, or crystalline Group III metal nitride powder or polycrystalline Group III metal nitride material with a suitable flux material and processing the mixture at high pressures and high temperatures. As described in the present invention, flux is understood to be a molten inorganic salt that is solid at near room temperature (about 30° C.). 
     Accordingly, one aspect of the invention is to provide a method of forming at least one single crystal of a Group III metal nitride. The method comprises the steps of: providing a source material to a reaction vessel, wherein the source material comprises at least one Group III metal selected from the group consisting of aluminum, indium, and gallium, and wherein the reaction vessel has an inner chamber and is sealable with respect to nitrogen and chemically inert; providing a flux material to the reaction vessel, wherein the flux material is a solid at about 30° C.; sealing the reaction vessel; heating the reaction vessel to a predetermined temperature and applying a predetermined pressure to the vessel, wherein the predetermined pressure is sufficient to suppress decomposition of the Group III metal nitride at the predetermined temperature; and forming at least one single crystal of the Group III metal nitride. 
     A second aspect of the invention is to provide a method of dissolving a source material for a Group III metal nitride in a flux material. The method comprises the steps of: providing the source material to a reaction vessel, wherein the source material comprises at least one Group III metal selected from the group consisting of aluminum, indium, and gallium, and wherein the reaction vessel has an inner chamber and is sealable with respect to nitrogen and is chemically inert; providing a flux material to the reaction vessel, wherein the flux material is a solid at about 30° C.; sealing the reaction vessel; heating the reaction vessel to a predetermined temperature and applying a predetermined pressure to the vessel, wherein the predetermined pressure is sufficient to suppress decomposition of the Group III metal nitride at the predetermined temperature; melting the flux material; applying a predetermined pressure to the vessel, wherein the predetermined pressure is sufficient to suppress decomposition of the Group III metal nitride at the predetermined temperature; and dissolving the source material in the flux material. 
     A third aspect of the invention is to provide a method of forming at least one single crystal of a Group III metal nitride. The method comprises the steps of: providing a source material to a reaction vessel, wherein the source material comprises at least one Group III metal selected from the group consisting of aluminum, indium, and gallium, and wherein the reaction vessel has an inner chamber and is sealable with respect to nitrogen and chemically inert; providing a flux material to the reaction vessel, wherein the flux material is a solid at about 30° C.; sealing the reaction vessel; heating a first end of the reaction vessel containing the source material to a first temperature and a second end of the reaction vessel opposite the first end to a second temperature, wherein the first temperature is different from the second temperature; applying a predetermined pressure to the vessel, wherein the predetermined pressure is sufficient to suppress decomposition of the Group III metal nitride at the predetermined temperature; melting the flux material; dissolving the source material in the flux material; and forming at least one single crystal of the Group III metal nitride in the second end of the reaction vessel. 
     A fourth aspect of the invention is to provide a Group III metal nitride single crystal. The Group III metal nitride single crystal is formed by a process comprising the steps of: providing a source material to a reaction vessel, wherein the source material comprises at least one Group III metal selected from the group consisting of aluminum, indium, and gallium, and wherein the reaction vessel is sealable with respect to nitrogen and chemically inert; providing a flux material to the reaction vessel, wherein the flux material is a solid at about 30° C.; sealing the reaction vessel; heating the reaction vessel to a predetermined temperature and applying a predetermined pressure to the vessel, wherein the predetermined pressure is sufficient to suppress decomposition of the Group III metal nitride at the predetermined temperature; and forming at least one single crystal of the Group III metal. 
     A fifth aspect of the invention is to provide at least one single crystal of a Group III metal nitride. The Group III metal nitride single crystal is formed by a process comprising the steps of: providing a source material to a reaction vessel, wherein the source material comprises at least one Group III metal selected from the group consisting of aluminum, indium, and gallium, and wherein the reaction vessel is sealable with respect to nitrogen and chemically inert; providing a flux material to the reaction vessel, wherein the flux material is a solid at about 30° C.; sealing the reaction vessel; heating a first end of the reaction vessel containing the source material to a first temperature and a second end of the reaction vessel opposite the first end to a second temperature, wherein the first temperature is different from the second temperature; melting the flux material; dissolving the source material in the flux material; and forming at least one single crystal of the Group III metal nitride in the second end of the reaction vessel. 
     These and other aspects, advantages, and salient features of the present invention will become apparent from the following detailed description, the accompanying drawings, and the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic representation of a cross-section of a reaction vessel of the present invention disposed in a pressure cell; 
         FIG. 2  is a schematic representation of a cross-section of a reaction vessel of the present invention disposed in an autoclave; and 
         FIG. 3  is a photograph of gallium nitride (GaN) crystals grown by the method of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     In the following description, like reference characters designate like or corresponding parts throughout the several views shown in the figures. It is also understood that terms such as “top,” “bottom,” “outward,” “inward,” and the like are words of convenience and are not to be construed as limiting terms. 
     Referring to the drawings in general and to  FIG. 1  in particular, it will be understood that the illustrations are for the purpose of describing a preferred embodiment of the invention and are not intended to limit the invention thereto. While the detailed description may at some point describe the growth of gallium nitride (GaN) in particular, it should be understood that the invention also encompasses the growth of nitrides of the other Group III metals, namely, aluminum and indium, as well as mixed nitrides of these Group III metals. 
     The nitrides of the Group III metals aluminum, gallium, and indium (also referred to hereinafter as “Group III metal nitrides”) are generally formed from a source material  102  comprising the particular Group III metal (or metals) in elemental or compound form. Gallium nitride single crystals, for example, are formed from a solid source material comprising gallium. The source material  102  may in the case of GaN comprise GaN powder, polycrystalline GaN, elemental gallium, or a gallium compound other than GaN. The GaN powder may be one of a polycrystalline, monocrystalline, amorphous GaN, or sintered GaN powder. The source material  102  may also comprise compounds containing at least one Group III metal. Such compounds include, but are not limited to Group III metal halides (e.g., gallium halides: i.e., gallium fluorides, chlorides, bromides and iodides). The source material may be provided in densified form. Densification may be achieved by cold-pressing the source material  102  into a pill or by sintering, as described by D&#39;Evelyn et al. in U.S. patent application Ser. No. 10/001,575, filed on Nov. 2, 2001, which is incorporated herein by reference in its entirety. Additionally the source material  102  may be heated or ‘baked’ at a predetermined temperature prior to reaction. In one embodiment, the source material  102  is baked to a temperature of between about 500° C. and about 1600° C. in a nitrogen-containing atmosphere, such as ammonia or a mixture of inert gas, such as He, Ne, Ar, or Kr, and at least one of ammonia and nitrogen. 
     The solid source material  102  is brought into contact with a flux material  106 . The flux material  106  is a solid at about 30° C. and comprises at least one of nitrides, amides, metal halides, urea and derivatives thereof, azides, ammonium salts, alkaline earth fluoronitrides, and combinations thereof. In one embodiment, the flux material  106  comprises at least one metal halide, wherein the metal halide is one of an alkali metal halide and an alkaline earth halide. Nitrides that may be used as the flux material  106  include, but are not limited to, lithium nitride (Li 3 N), magnesium nitride (Mg 3 N 2 ), calcium nitride (Ca 3 N 2 ) and copper nitride (CuN x ). Amides that may be used as flux material include, but are not limited to, lithium amide (LiNH 2 ), sodium amide (NaNH 2 ), and potassium amide (KNH 2 ). Among the azides that are suitable for use as flux material  106  is sodium azide (NaN 3 ). Ammonium salts that may be used as the flux material  106  include, but are not limited to, ammonium fluoride (NH 4 F), ammonium chloride (NH 4 Cl), ammonium bromide (NH 4 Br), and ammonium iodide (NH 4 l). Alkaline earth fluoronitrides that may be used as the flux material  106  include, but are not limited to, Mg2NF, Mg3NF3, and Ca2NF. Alternatively, the flux material  106  may comprise compounds formed by reaction of the aforementioned flux materials with reagents containing at least one Group III metal (e.g., GaCl 3 ). In one embodiment, the flux material  106  is baked at a temperature of between about 80° C. and about 1200° C. in an oxygen-free atmosphere, such as vacuum, at least one inert gas, such as He, Ne, Ar, or Kr, and at least one of ammonia and nitrogen. In order to more efficiently use the limited space within reaction vessel  100 , flux material  106  may additionally be densified and consolidated by cold pressing or hot pressing. 
     In one embodiment, the source material  102  may be brought into contact with the flux material  106  by first mixing both materials together and pressing the mixture into a pill. One or more well-defined crystals of a Group III metal nitride may also be added to the mixture to serve as seed crystals for the crystal growth process. The source material/flux material pill is placed in a reaction vessel  100  and processed under high pressure and high temperature (also referred to hereinafter as “HPHT”) conditions. HPHT conditions include processing pressures and temperatures ranging from about 1 atm to about 80 kbar, and from about 500 to about 3000° C., respectively. Under HPHT conditions, some or all of the source material  102  dissolves in the flux material  106 . Ostwald ripening occurs, as large and well-crystallized Group III metal nitride crystals grow while smaller and less-well-crystallized crystals of the Group III metal nitride shrink. Additional single crystals of the Group III metal nitride may precipitate from the flux material  106  upon cooling of the reaction vessel  100 . 
     In another embodiment, shown in  FIG. 1 , source material  102  is placed in one end of reaction vessel  100 , at least one crystalline Group III metal nitride seed  104  is placed in the opposite end of the reaction vessel  100 , and flux material  106  is placed between source material  102  and crystalline Group III metal nitride seed  104 . In yet another embodiment, a non-Group III metal nitride seed (i.e., a seed material other than a Group III metal nitride) is placed in reaction vessel  100  in the end opposite source material  102 . The non-Group III metal nitride seed should have a lattice constant within about 20% of that of the Group III metal nitride crystal to be grown. More preferably, the non-Group III metal nitride seed has a lattice constant within about 5% of that of the Group III metal nitride crystal to be grown. In the case of GaN or AlN, for example, silicon carbide or sapphire may be employed as a seed crystal. The seed is not limited to Group III metal nitride and could be single silicon carbide and sapphire. A portion of flux  106  may optionally be mixed with source material  102 . 
     In one embodiment, a baffle  110  separates source material  102  from the main body of flux  106 . Fluid communication between source material  102  and the main body of flux  106  is provided by through holes (not shown) included in baffle  110 . Baffle  110  with through holes has a fractional open area in the range of between about 1% and about 40%. In order to prevent dissolution of the crystalline Group III metal nitride seed  104  prior to the onset of growth, a diffusion barrier  108 , such as, but not limited to, a thin foil of a suitable material, such as platinum, tantalum, and the like, may be provided to protect the crystalline Group III metal nitride seed  104 . The crystalline Group III metal nitride seed  104  may either be wrapped within diffusion barrier  108 , or diffusion barrier  108  may be positioned to separate the seed and a small quantity of flux material  106  from the main body of flux material  106 . 
     Positioning of source material  102 , flux material  106 , and crystalline Group III metal nitride seed  104  within reaction vessel  100  depends upon the relative densities of these materials. For example, gallium nitride has a density of about 6.1 g/cc. If, under GaN growth conditions, the density of flux material  106  is greater than that of GaN, any spontaneously nucleated GaN crystals will float upward. In this case, source material  102  would be optimally arranged in the top of reaction vessel  100  and crystalline Group III metal nitride (GaN) seed crystal  104  with the seed crystal would be optimally arranged in the bottom of reaction vessel  100 , as shown in  FIG. 1 . If, conversely, flux material  106 , under growth conditions, has a density less than that of GaN, any spontaneously-nucleated GaN crystals will sink to the bottom of reaction vessel  100 . Here the arrangement of source material  102  and crystalline Group III metal nitride (GaN) seed  104  is inverted from that shown in  FIG. 1 ; i.e., with crystalline Group III metal nitride (GaN) seed  104  located at the top of reaction vessel  100 . 
     The source material  102 , flux  106 , at least one crystalline Group III metal nitride seed  104 , and, if included, baffle  110  and diffusion barrier  108  are enclosed within a reaction vessel  100 , which is sealable and impermeable with respect to nitrogen. Reaction vessel  100  is also chemically inert with respect to both source material  102  and flux  106  under crystal growth conditions. Once filled and sealed, reaction vessel  100  may undergo a passivation reaction with other cell components such as, for example, source material  102 , and/or flux  106 . Such passivation reactions are permissible as long as reaction vessel  100 , following passivation, is sealable, impermeable to nitrogen, and inert to further chemical reaction. 
     Reaction vessel  100  comprises at least one layer and may additionally include at least one liner  112  and at least one coating  114 . Each of reaction vessel  100 , baffle  110 , liner  114 , and at the least one coating  112  comprises at least one of: copper; silver; gold; platinum; palladium; iridium; rhodium; ruthenium; osmium; rhenium; iron; nickel; phosphorus; MC x N yO z, wherein M is at least one metal selected from magnesium, calcium, strontium, barium, aluminum, boron, silicon, titanium, vanadium, chromium, yttrium, zirconium, lanthanum, a rare earth metal, hafnium, tantalum, tungsten, molybdenum, niobium, and wherein 0≦x, y, and z≦3; and combinations thereof; pyrophyllite; talc; olivine; calcium carbonate; merylinite clay; bentonite clay; and sodium silicate. Liner  114  and the at least one coating  112  typically comprise a material that is different from that (or those) used to form reaction vessel  100 . In one embodiment, reaction vessel  100  has a melting point of greater than 1600° C. Reaction vessel  100  may be gas tight upon initial filling and sealing, or may become gas tight during processing at high pressure and high temperature. 
     Once filled and sealed, reaction vessel  100  is then placed into a reaction cell  120 , as shown in  FIG. 1 . In one embodiment, reaction cell  120  includes a heating element  124  and a pressure transmission medium  122 . Heating element  124  comprises at least one of graphite, nichrome, niobium, titanium, tantalum, stainless steel, nickel, chromium, zirconium, molybdenum, tungsten, rhenium, hafnium, platinum, silicon carbide, and combinations thereof. Heating element  124  may take the form of a resistively heated tube, foil, ribbon, bar, wire, or combinations thereof. Pressure transmission medium  122  is thermally stable at least up to the temperature at which crystal growth of the Group III metal nitride takes place. During HPHT processing, pressure transmission medium  122  preferably remains a solid with a relatively low shear strength and internal friction. Pressure transmission medium  122 , for example, has an internal friction below about 0.2. In one embodiment, pressure transmission medium  122  comprises at least one alkali halide, such as NaCl, NaBr, or NaF. Alternatively, transmission medium  122  may comprise at least one of talc, pyrophyllite, molybdenum disulfide, graphite, hexagonal boron nitride, silver chloride, calcium fluoride, strontium fluoride, calcium carbonate, magnesium oxide, zirconium oxide, merylinite clay, bentonite clay, and sodium silicate. 
     Reaction cell  120  containing reaction vessel  100  is then placed in a high pressure apparatus (not shown). In one embodiment, the high pressure apparatus comprises a belt-type apparatus, with a reinforced die and at least two punches or anvils. Alternatively, the high pressure apparatus may comprise one of a piston press, a multi-anvil press with at least four anvils, a toroid-type apparatus with two recessed anvils, and a split-sphere apparatus. 
     In yet another embodiment, shown in  FIG. 2 , the pressure apparatus comprises an autoclave  200  such as, but not limited to, a Morey autoclave, a Tuttle/Roy cold-cone seal autoclave, a modified Bridgman autoclave, a full Bridgman autoclave, a delta ring autoclave, and a Walker-Buehler type autoclave. The at least one Group III metal nitride seed  104 , flux  106 , and source material  102  are placed in autoclave  200 . In one embodiment, a baffle  110  separates source material  102  and the region containing the at least one Group III metal nitride seed  104 . In one embodiment, the at least one Group III metal nitride seed  104 , flux  106 , source material  102 , and baffle  110  are placed in reaction vessel  100  prior to insertion into autoclave  200 . Reaction vessel  100  and baffle  110  each comprise at least one of: copper; silver; gold; platinum; palladium; iridium; rhodium; ruthenium; osmium; rhenium; iron; nickel; phosphorus; MC x N yO z, where M is at least one metal selected from magnesium, calcium, strontium, barium, aluminum, boron, silicon, titanium, vanadium, chromium, yttrium, zirconium, lanthanum, a rare earth metal, hafnium, tantalum, tungsten, molybdenum, niobium, and combinations thereof, and where 0≦x, y, and z≦3; pyrophyllite; talc; olivine; calcium carbonate; merylinite clay; bentonite clay; and sodium silicate. In one embodiment, reaction vessel  100  has a melting point of greater than 1600° C. Reaction vessel  100  may be gas tight upon initial filling and sealing, or may become gas tight during processing at high pressure and high temperature. Optionally, an outer liner  206  may be inserted into autoclave  200  to improve chemical inertness. 
     Once sealed, reaction vessel  100  is processed under HPHT conditions. Processing pressures and temperatures ranges from about 1 atm to about 80 kbar, and from about 500° C. to about 3000° C., respectively. The solubility of the Group III metal nitrides in most fluxes generally increases as a function of temperature. In this case, the end of reaction vessel  100  containing source material  102  is maintained at a higher temperature (T 2  in  FIGS. 1 and 2 ) during processing at HPHT than the end of reaction vessel  100  containing the at least one Group III metal nitride seed  104 . If the solubility of the Group III metal nitride in flux  106  decreases as a function of temperature, the end of reaction vessel  100  containing source material  102  is maintained at a lower temperature (T 2  in  FIGS. 1 and 2 ) during processing at HPHT than the end of reaction vessel  100  containing the at least one Group III metal nitride seed  104 . The difference in temperature between source  102  (T 2 ) and the at least one Group III metal nitride seed  104  (T 1  in  FIGS. 1 and 2 ) is between about 5° C. and about 300° C. Under HPHT conditions, the source material dissolves in the flux material and is transported through the flux to the crystalline GaN seed. Because the Group III metal nitride solubility at the end of reaction vessel  100  containing source material  102  is greater than at the end of reaction vessel  100  containing the at least one Group III metal nitride seed  104 , the concentration of dissolved Group III metal nitride decreases in the direction from source  102  to the at least one Group III metal nitride seed  104 . As a result of this difference in solubility, the Group III metal nitride diffuses from source material  102  through flux  106  to the at least one Group III metal nitride seed  104 , where the Group III metal nitride precipitates onto the at least one Group III metal nitride seed  104  to form a large single crystal, or boule, of the Group III metal nitride. 
     The temperature gradient may be achieved in the apparatus of  FIG. 1  by locating one end of reaction vessel  100  asymmetrically within the heating zone of the HPHT reaction cell  120 . Alternatively—or in addition to locating reaction vessel  100  asymmetrically in reaction cell  120 —the temperature gradient may be produced by providing a heating element  124  having a non-uniform resistivity along its length. Non-uniform resistivity may be achieved by providing a heating element  124  having at least one of a non-uniform thickness, perforations at selected points, and a laminate structure of at least two materials of differing resistivity at selected points along its length. In one embodiment, at least two independent temperature sensors (not shown) are provided to measure and control the temperature gradient between the opposite ends of reaction vessel  100 . The temperature difference may also be achieved by either providing an auxiliary heater (not shown) proximate to one end of reaction vessel  100 , or by differentially cooling one end of reaction vessel  100 , for example, by providing a coolant at different temperatures to the two ends of the apparatus. In addition, the temperature difference may also be achieved by altering the cooling conditions at the top of apparatus and the bottom of apparatus. The temperature gradient may be adjusted during the single crystal growth to optimize quality and growth rate. 
     The reaction vessel shown in  FIG. 2  may be heated by means of at least one heating element  224  or a furnace external to the outer wall of autoclave  200 . The desired temperature gradient may be achieved by means of two or more hot zones within the furnace. 
     In another embodiment, source material  102 , comprising at least one of amorphous or polycrystalline Group III metal nitride powder, such as, for example, GaN, is pressed into a pill. A flux material  106 , such as, for example, but not limited to, NaN 3 , is pressed separately into a sleeve and placed around the pill such that the sleeve makes contact with the pill. The pill and surrounding sleeve are then placed inside reaction vessel  100 . The presence of the nitrogen-containing flux material  106  maintains the chemical potential of nitrogen within the reaction vessel at a sufficiently high level in order to obtain stoichiometric Group III metal nitride, such as AlN, InN, and GaN. For growth of InN and GaN crystals, the reaction vessel is then pressurized to between about 55 and about 80 kbar and heated to a temperature of between about 1200° C. and about 3000° C. The temperature is sufficient to melt the source material  102  at one end of the reaction vessel (T 2  in  FIGS. 1 and 2 ), while the pressure is sufficient to inhibit decomposition. The approximate melting point of AlN is about 3200° C., and the nitrogen (N 2 ) pressure needed to inhibit decomposition is about 0.2 kbar. For GaN, the approximate melting point is about 2500° C., and the nitrogen (N 2 ) pressure needed to inhibit decomposition is about 45 kbar. The approximate melting point of InN is about 1900° C., and the nitrogen (N 2 ) pressure needed to inhibit decomposition is about 60 kbar. During processing at HPHT, one end of reaction vessel  100  is maintained at a higher temperature (T 2  in  FIGS. 1 and 2 ) than the opposite end of reaction vessel  100  (T 1  in  FIGS. 1 and 2 ), with the difference in temperature being between about 5° C. and about 300° C. After being held for a predetermined time at HPHT, reaction vessel  100  is cooled at a predetermined cooling rate of between about 0.02° C./hr to about 100° C./hr so that a single GaN crystal nucleates at the “cold”—or low temperature (T 1  in FIGS.  1  and  2 )—end of reaction vessel  100 . The remaining molten Group III metal nitride then crystallizes onto the single nucleated crystal as the entire reaction vessel  100  cools below the melting point of the Group III metal nitride. Group III metal nitride seed  104  may optionally be located at the low temperature end of reaction vessel to provide a nucleation site. A similar procedure may be used to obtain single crystals of other Group III metal (i.e., Al, In, and combinations of Ga, Al, and In) nitrides. 
     After processing at HPHT conditions for a predetermined time, reaction vessel  100  is cooled and the pressure on reaction vessel  100  is released. Reaction vessel  100  is disassembled and the Group III metal nitride single crystals are removed, typically by washing the interior of reaction vessel  100  with mineral acids such as HCl and HNO 3 . 
     The following examples serve to illustrate the features and advantages offered by the present invention, and are not intended to limit the invention thereto. 
     EXAMPLE 1 
     Commercial grade GaN powder, having a nominal purity of 99.9%, was mixed with lithium nitride (Li 3 N) powder in a 6:1 ratio by weight. The mixed powders were pressed into a pill, wrapped in tantalum foil, and placed inside a magnesium oxide outer capsule reaction vessel. The encapsulated powders were then placed in a cell and pressed at about 50 kbar and about 1500° C. in a belt-type press apparatus for about 15 minutes. The reaction vessel was then cooled and opened. The GaN crystals were separated from the lithium nitride flux material by washing with water and HNO 3 . The GaN crystals were approximately 20 microns in size. X-ray diffraction patterns obtained for the GaN starting material and the GaN crystals that were grown under HPHT conditions are compared in Table 1. The x-ray diffraction pattern obtained for the GaN crystals grown under HPHT conditions included diffraction peaks not observed in the starting material and significantly greater peak heights than the x-ray diffraction pattern obtained for the GaN starting material, indicating that the GaN crystals grown under HPHT conditions had a significantly higher degree of crystallinity than the GaN starting material. 
     EXAMPLE 2 
     Commercially available gallium nitride powder, having the same nominal purity and relatively poor crystallinity as described in Example 1, was compacted into a pill weighing about 1.4 g. Ammonium iodide (NH 4 I) powder was compacted into a second pill, weighing 2.6 g. The two pills were placed into a reaction vessel comprising two opposing cups fabricated from hot-pressed boron nitride. The reaction vessel was then placed within a cell and treated at high pressure and high temperature in a temperature gradient cell in a belt-type press apparatus. The pressure was approximately 30 kbar. The temperature of the top of the cell was about 1435° C., and the temperature at the bottom of the cell was about 1360° C. After a treatment time of about 20 hours, the cell was cooled and removed from the press. Residual NH 4 I was washed out of the cell with water, leaving residual GaN powder and well-crystallized GaN crystals, having an average diameter of about 0.5 mm, which are shown in  FIG. 3 . X-ray diffraction studies confirmed that the crystals are pure gallium nitride. 
     EXAMPLE 3 
     Gallium nitride powder was compacted into a pill weighing about 1.25 g. A second pill was compacted from a mixture comprising about 2.6 g of ammonium iodide (NH 4 I.) powder and about 0.1 g GaN powder. The two pills were placed into a reaction vessel comprising two opposing cups fabricated from hot-pressed boron nitride. The reaction vessel was then placed within a cell and treated at high pressure and high temperature in a temperature gradient cell in a belt-type press apparatus. The pressure was approximately 40 kbar. The temperature of the top of the cell was about 1450° C., and the temperature at the bottom of the cell was about 1375° C. After a treatment time of 24 hr, the cell was cooled and removed from the press. Residual NH 4 I was washed out of the cell with water, leaving residual GaN powder and well-crystallized GaN crystals. 
     EXAMPLE 4 
     Gallium nitride powder was compacted into a pill weighing about 1.25 g. A second pill was compacted from a mixture comprising about 2.4 g of ammonium bromide (NH 4 Br) powder and about 0.1 g. GaN powder. The two pills were placed into a reaction vessel comprising two opposing cups fabricated from hot-pressed boron nitride. The reaction vessel was then placed within a cell and treated at high pressure and high temperature in a temperature gradient cell in a belt-type press apparatus. The pressure was approximately 40 kbar. The temperature of the top of the cell was about 1330° C., and the temperature at the bottom of the cell was about 1255° C. After a treatment time of about 16 hr, the cell was cooled and removed from the press. Residual NH 4 Br was washed out of the cell with water, leaving residual GaN powder and GaN crystals. 
     EXAMPLE 5 
     Gallium nitride powder was compacted into a pill weighing about 1.25 g. A second pill was compacted from a mixture comprising about 2.6 g of ammonium iodide (NH 4 I) powder and about 0.1 g GaN powder. A single crystal of SiC, 1 mm×1 mm×0.2 mm, served as a seed. The two pills were separated by a tungsten baffle and then placed into a reaction vessel comprising two opposing cups fabricated from hot-pressed boron nitride. The reaction vessel was then placed within a cell and treated at high pressure and high temperature within a temperature gradient cell, which was placed in a belt-type press apparatus. The pressure was approximately 40 kbar. The temperature of the top of the cell was about 1450° C., and the temperature at the bottom of the cell was about 1375° C. After a treatment time of 30 hr, the cell was cooled and removed from the press. Residual NH 4 I was washed out of the cell with water, leaving residual GaN powder and 200 micrometer well-crystallized GaN crystals. 
     While typical embodiments have been set forth for the purpose of illustration, the foregoing description should not be deemed to be a limitation on the scope of the invention. Accordingly, various modifications, adaptations, and alternatives may occur to one skilled in the art without departing from the spirit and scope of the present invention. 
     Table 1. X-Ray diffraction patterns obtained for GaN starting material and GaN crystals were grown under HPHT conditions. 
     
       
         
               
               
             
               
               
               
               
             
               
               
               
               
             
           
               
                   
               
               
                 GaN 
                 GaN 
               
               
                 Starting Material 
                 Grown under HPHT 
               
             
          
           
               
                 d spacing 
                 Peak Height 
                 d spacing 
                 Peak Height 
               
               
                 (angstroms) 
                 Arbitrary units 
                 (angstroms) 
                 Arbitrary units 
               
               
                   
               
             
          
           
               
                   
                   
                 2.7551 
                 1614 
               
               
                 2.7571 
                 491 
                 2.5882 
                 1310 
               
               
                   
                   
                 2.4339 
                 1505 
               
               
                 2.4385 
                 560 
                 1.8872 
                 427 
               
               
                   
                   
                 1.5925 
                 913 
               
               
                   
                   
                 1.4633 
                 609 
               
               
                   
                   
                 1.3784 
                 132 
               
               
                   
                   
                 1.3566 
                 376 
               
               
                   
                   
                 1.3332 
                 482 
               
               
                   
                   
                 1.2953 
                 117 
               
               
                   
                   
                 1.2181 
                 148 
               
               
                   
                   
                 1.1719 
                 69 
               
               
                   
                   
                 1.0779 
                 156 
               
               
                   
                   
                 1.0432 
                 121 
               
               
                   
                   
                 1.0226 
                 250