METHOD FOR THE SYNTHESIS OF GROUP III NITRIDE CRYSTALS

Abstract: A novel method for the synthesis of Group III nitride crystals is disclosed. The method contemplates the reaction of a liquid form of the Group III nitride metal with active nitrogen. The active nitrogen may be generated by a plasma or the thermal decomposition of ammonia and may be neutral, ionic, excited or a mixture thereof. Using the method of the present invention, the crystalline Group III nitride can be grown at pressures of one atmosphere and below. Polycrystalline films grown by this method are of sufficient quality to show luminescence at room temperature.

Description: METHOD FOR THE SYNTHESIS OF GROUP III NITRIDE CRYSTALS Background of the Invention This application bases its priority on provisional application Serial No. 60/030,131 filed on November 4, 1996. 1. Field of the Invention This invention relates to a method for the synthesis of Group III nitride crystals. More particularly, the invention relates to a method for the synthesis of Group III nitride crystals at atmospheric pressure and below thereby negating the need for the extremely high pressures required in all currently-known processes. 2. Description of the Prior Art Gallium nitride is a III-IV semiconductor from the same family as gallium arsenide. The band gap in GaN (3.4 eV) is direct and rather wide. Accordingly, GaN has very favorable light emission properties in the ultraviolet and blue. Potential applications, then, for GaN and other Group III nitrides such as cubic boron nitride (c-BN), aluminum nitride (A1N), and indium nitride (InN) include the following: lasers and light emitting diodes; sensors; imagers; millimeter wave devices; cold cathodes for vacuum electronics; and, luminescent displays. Apart from the foregoing applications, cubic boron nitride (c-BN) is very hard and can also be used in grinding, cutting and machining applications. It is also useful for ferrous alloys, which cannot be machined with diamond. Crystals of c-BN may also be used as a closely lattice-matched substrate for growing single crystals of diamond by chemical vapor deposition. The potential market for these applications is truly enormous. As a consequence, GaN and other Group III nitrides have been the subject of intense worldwide research and development effort. The synthesis of bulk gallium nitride (GaN) crystals and other Group III nitride crystals has in the past conventionally been done at high pressures, e.g.. 1000 bar and higher, because of the very high equilibrium pressure of molecular nitrogen (N 2 ) over GaN at synthesis temperatures. At 1200°C, for example, the equilibrium N 2 pressure is approximately 1 ,000 bar. At 1500°C, the pressure is approximately 10,000 bar. This prior art process is disclosed in, for example, J. Karpinski and S. Porowski, J. Crystal Growth 66, 1 1 (1984); S. Porowski, J. Jun, P. Perlin, I. Grzegory, H. Teisseyre and T. Suski, Inst. Phys. Conf. Series, No. 137, Chapter 4, (5th Conf. on SiC and Related Materials, Washington, DC) p. 369, 1993.; P. Perlin, I. Gorszyca, N.E. Christensen, I. Grzegory, H. Teisseyre, and T. Suski, EMIS Data Reviews Series, No. 11, J.S. Edgar, Editor, INSPEC, Institution of Electrical Engineers, London, UK, 1994. At high pressures, for example, from 1000 bar to 10,000 bar, the synthesis reaction for gallium nitride may be written as follows: Ga M. -> 2(g) GaN, ( *> (1) Use of this synthesis reaction ( 1 ), which requires extremely high pressure, is disadvantageous due to complexity, poor control over crystal quality and impurities, and cost. At these high pressures, the process is difficult to implement and scale up to large sizes. Thus, the need exists for a new method of growing bulk Group III nitride crystals at low pressures, which can be easily implemented to provide both polycrystalline and single crystal Group III nitrides of a high quality and purity. Brief Summary of the Invention The present invention provides a new method for the synthesis of both polycrystalline and single crystal gallium nitride (GaN) and other Group III nitrides at low pressures. Further, the method can be used to grow these crystals with incorporated doping agents. This new method avoids the difficulties found in those methods disclosed in the past. For example, the method of the present invention contemplates the preparation of bulk Group III nitrides using a pressure of one atmosphere or less. Further, bulk crystals can be grown without the use of a substrate. Polycrystalline films grown by this new method are of sufficient quality to show luminescence at room temperature. Thus, these polycrystalline films can be used in device and display applications. Also, these polycrystalline Group III nitrides may be used as source materials for growing single crystals of these materials by other means such as sublimation. According to a first aspect of the invention, a method for producing Group III nitride crystals is disclosed. The method comprises the steps of: (a) optionally pretreating a liquid comprising a Group III metal to clean the surface thereof; (b) raising the temperature of the liquid from about the melting point of the Group III metal to a temperature below which net decomposition of the nitride crystals occurs; (c) saturating the liquid Group III metal using active nitrogen to form a melt comprising the Group III metal and nitrogen; (d) crystallizing the Group III nitride crystals; and (e) optionally etching the Group III nitride crystals to remove any unreacted Group III metal or solvent metal. The Group III metal is one selected from the group consisting of gallium, boron, aluminum and indium. Steps (a) and (b) of the method can be interchanged. The liquid may also comprise one or more metals different than the first Group III metal. Suitable examples of these metals would include another Group III metal, tin and bismuth. According to a second aspect of the invention, a method for producing Group III nitride crystals is disclosed. The method comprises the steps of directing a plasma comprising active nitrogen onto a surface of a liquid comprising a Group III metal; and crystallizing the Group III nitride. The method is carried out at atmospheric pressure or below. According to a third aspect of the invention, a method for producing gallium nitride crystals is disclosed. The method comprises the steps of saturating a liquid comprising gallium metal using active nitrogen and crystallizing the gallium nitride crystals. The active nitrogen may be generated by a plasma and can be neutral, ionic, electronically excited or a mixture thereof, fhc active nitrogen may also be generated by the decomposition of ammonia (Ni l,). According to a fourth aspect of the invention, polycrystalline Group III nitride crystals are disclosed, which are characterized by an ability to luminesce at room temperature. The crystals are prepared by a process comprising the steps of saturating a liquid comprising the Group III metal using active nitrogen, followed by crystallizing the Group III nitride crystals. One advantage of the present invention is that Group III nitride crystals can be synthesized in the absence of extremely high pressures. Another advantage of the present invention is that Group III nitrides can be synthesized at pressures of one atmosphere and lower. Still another advantage of the present invention is that Group III nitrides can be simply and economically synthesized. Still another advantage of the present invention is that Group III nitride crystals can be grown in high yields. Still another advantage of the present invention is that Group III nitrides can be synthesized in bulk without the use of a lattice matched substrate. Still another advantage of the present invention is that Group III nitride crystals grown using the new method described herein can be used as substrates for further growth of Group III nitride films using conventional growth methods such as molecular beam epitaxy and metal-organic vapor deposition. Still another advantage of the present invention is that c-BN crystals grown using the new method can be used as a substrate for the growth of diamond single crystals by chemical vapor deposition. Still another advantage of the present invention is the use of c-BN crystals for machining, cutting, and grinding, especially of ferrous metals and alloys. Still another advantage of the present invention is that polycrystalline Group III nitride films grown at temperatures of one atmosphere and lower are of sufficient quality to show luminescence at room temperature and can therefore be used in device applications. Still other benefits and advantages of the invention will become apparent to those skilled in the art upon a reading and understanding of the following detailed specification. Brief Description of the Figures Fig. 1 is a schematic drawing of the system used for growing gallium nitride and other Group III nitrides at low pressures; Fig. 2 is a schematic drawing of a portion of the phase diagram of the Ga/N system; Fig. 3 is a schematic drawing of a method used for horizontal freezing of GaN from Ga/N melts; Fig. 4 is a schematic drawing of a side view of a crucible used for vertical freezing and of the imposed temperature gradient; Fig. 5 is a polycrystalline GaN "dome" completely covering a liquid gallium drop; Fig. 6 is a scanning electron micrograph from a portion of the concave surface of the polycrystalline GaN "dome" of Fig. 5; Fig. 7 is a scanning electron micrograph of a portion of the convex surface of the polycrystalline GaN "dome" of Fig. 5; Fig. 8 is a transmission electron micrograph of a portion of a polycrystalline GaN sample of the present invention; Fig. 9 is a transmission electron micrograph of a portion of a polycrystalline GaN sample of the present invention; Fig. 10 is a steady state luminescence spectra taken at a lattice temperature of (a) 10K and (b) 300K; Fig. 1 1 is a room temperature Raman spectra of two different 1 micron spots on the same polycrystalline GaN sample; and Figure 12 is a scanning electron micrograph of GaN crystals grown from a liquid melt of Ga, In, and N. Description of the Preferred and Alternate Embodiments Referring now to the figures, wherein the showings are for purposes of illustrating the preferred and alternate embodiments of the invention only and not for purposes of limiting the same, Fig. 1 illustrates a schematic drawing of the growth system of the method of the present invention. In the method of the prescnt invention, active nitrogen is used to saturate a pool of liquid gallium with active nitrogen followed by the subsequent crystallization of solid gallium nitride. The solid gallium nitride may be formed in bulk as distinguished from thin films and microcrystalline materials. As used in the present disclosure "bulk" distinguishes crystals and clusters of crystals of macroscopic size, i.e., ones that can be picked up and handled, from thin films and microcrystalline materials. This method was specifically developed for the growth of gallium nitride crystals. But this method is equally applicable to the growth of other Group III nitrides, including aluminum nitride (A1N), cubic boron nitride (c-BN), and indium nitride (InN). Fig. 2 shows a schematic of a portion of the phase diagram of the gallium/nitrogen system (not drawn to scale) and the processes believed to take place during the growth of the gallium nitride. Path a to b shows the dissolution of nitrogen into the liquid gallium from the active nitrogen in the gas phase. At point b, solid gallium nitride starts to precipitate. Along path b to c more solid gallium nitride is formed. Path c to d shows the melting of the solid gallium nitride with increasing temperature. At point d, all of the solid gallium nitride is melted so that only a liquid phase is present. The path d to c can be retraced to recrystallize solid gallium nitride, e.g., as a single crystal. Other temperature/time profiles to induce crystallization can be used with this invention. For example, the melt can be saturated with nitrogen until the solubility limit of GaN is just reached (curved line on Figure 2). At that point the melt is cooled, and solid GaN is precipitated. This crystallization path is indicated by the line d to c. Still other process paths can be used and will be evident to a skilled person. The present invention contemplates saturation of the gallium or Group III metal using active nitrogen as opposed to molecular nitrogen. The terminology "active nitrogen" is meant to include neutral atomic nitrogen (N), ionized atomic nitrogen (N + ), excited states of molecular nitrogen (N 2 * ) or mixtures thereof and is conveniently generated by a plasma, e.g., a microwave plasma, a DC plasma or a radio frequency plasma. The active nitrogen can also be generated by thermal decomposition of ammonia (NH 3 ), for example, by simply heating the NH 3 to temperatures of approximately 900°C using a furnace. Other nitrogen containing gases can also be used. Use of this method to generate the active nitrogen is disadvantageous, since ammonia is a toxic material. In addition, the ammonia introduces large amounts of hydrogen into the growth process, which can be undesirable. Furthermore, ammonia, as opposed to nitrogen gas, is more difficult to obtain in a pure form. Whether generated by a plasma or the decomposition of ammonia, the flow rate of the active nitrogen must be sufficient to replenish the nitrogen in the melt as the solid Group III nitride forms. Molecular nitrogen, without activation, has no utility in the practice of the present invention. At reasonable growth temperatures, the equilibrium pressure for molecular nitrogen is very high, e.g., for GaN it is approximately 1500 bar at 1500K and approximately 25000 bar at 2000K. Because of the results of the equilibrium thermodynamic analysis, efforts at bulk synthesis of GaN using molecular nitrogen have in the past been directed at high pressures. Thc plasma process of the present invention circumvents these extremely high equilibrium pressures by use of a highly non-equilibrium (super-equilibrium) concentration oi ' active nitrogen species derived by dissociating N 2 in a plasma. Thermodynamic calculations show that the formation of solid GaN is highly favored at atomic nitrogen partial pressures that are easily obtained in conventional microwave, RF, or DC plasmas. Moreover, these calculations have been verified experimentally in the method of the present invention by the formation of GaN from a pool of liquid Ga from an atomic nitrogen beam. The results of the thermodynamic calculations are summarized in Table I, which sets out the estimated equilibrium pressures of N 2 and N over solid GaN in equilibrium with liquid Ga/N solutions. The thermodynamic data for GaN (s) , Ga (]) , N 2 and N were taken from the compilation of Knacke, et al., "Thermochemical Properties of Inorganic Substances," Second Edition, Springer Verlag, Berlin, 1991. The calculations were performed using the equilibrium N 2 pressures given by Karpinski, et al., J. Crystal Growth 66, 11 (1984). The partial pressures of N were calculated from the equilibrium of the reaction N 2 = 2N using tabulated thermodynamic data for N 2 and N. The data in Table I may be interpreted as follows: at atomic N pressures above the equilibrium pressure, P N , the formation of solid GaN from liquid Ga/N solutions is favored thermodynamically. The equilibrium P N pressures may easily be obtained in microwave ECR, in microwave ball plasmas, in RF plasmas, in DC plasmas, or using hollow cathodes, all of which can produce partial pressures of activated species in the millitorr range, i.e., » 10 "6 bar. FABLE 1 Equilibrium Pressures of N, and N Over GaN It is very surprising that atomic nitrogen can be used to form gallium nitride from liquid gallium, because the recombination of atomic nitrogen, N, to form molecular nitrogen, N 2 , is a highly favored reaction. IN (,gas) → N, 2,(gas ,) (2) Reaction (2) is very favored thermodynamically and is aided by the presence of surfaces. It was previously believed, therefore, that the atomic nitrogen would recombine to form N 2 and not enough nitrogen would remain to form the GaN. By the method of the present invention, this difficulty can be circumvented. We have determined that recombination of N is sufficiently slow at our reaction conditions to permit the parallel formation of GaN. In the high pressure process, which has been previously used, the pressure of N 2 is increased to the point where the following reaction is thermodynamically favored: Ga ι (1 r ). + — -> M Α,,g .) Ga is) (3) 2 In the low pressure plasma-based process of the present invention, the plasma is used to produce a sufficient partial pressure of N and other active nitrogen species. We have discovered that a saturated melt of nitrogen and liquid gallium can be formed using active nitrogen. This melt can be subsequently frozen to form gallium nitride at atmospheric pressure and below. It should be understood, though, that the process of the present invention is not limited to an operating pressure of one atmosphere and below. The process of the present invention could also be performed at pressures exceeding atmospheric pressure. For example, in the event that the thermal decomposition of ammonia is used as the source of active nitrogen, pressures of several atmospheres could be used. The overall process of the invention is set out below in equation (4). N to + G - GaN (s) (4) At BOCK (1027°C) the equilibrium N pressure is only 5.5 x 10 '9 millitorr (7 x 10 " ' 5 bar) for this reaction; at 1500°K (1227°C), the equilibrium N pressure increases to 4.5 x 10 " '° millitorr (6 x 10 " 12 bar). In contrast, the equilibrium N 2 pressure at 1200 ° C is approximately 1000 bar. Better quality crystals are grown if the concentration of nitrogen in the Ga/N melt is not extremely low. At 1200°C the nitrogen solubility is only 0.01 atomic percent; at 1600°C the solubility is approximately 1 %. In the preferred embodiment the solubility is as high as possible, since it is easier to grow crystals at higher solubility. In this invention, the increased solubilities are achieved ( 1 ) by addition of a second metal to the melt that does not form a stable solid nitride and (2) by increasing the melt temperature. The second metal can be another Group III metal or a metal from another group, e.g., tin. Adding a second metal to the melt permits the lowering of growth temperatures and/or increases the solubility of nitrogen in the melt. The second metal should be soluble in the Group III metal, should not form a stable solid nitride at the growth conditions, and should have a low vapor pressure. Suitable examples of the second metal include other Group III metals, tin and bismuth. In some situations it may be advantageous to use one or more metals for the second metal, for example, grow A1N from Al/Ga/In N melts. Growth temperatures as high as feasibly possible are used to maximize the concentration of nitrogen in the Ga/N melt. The maximum melt temperature that can be used is limited by several factors. It can be limited by the vapor pressure Ga over the liquid Ga/N solution. If the vapor pressure of the Ga is too high, it can interfere with the operation of the plasma source. For the ECR source, the Ga vapor pressure should be kept below 1 millitorr. Therefore, the melt temperature is limited to approximately 1200K. For the ball plasma microwave source, higher Ga pressures can be tolerated and the melt temperature can be as high as approximately 1400K. Other sources of activc nitrogen, for example, the inductively coupled plasma, can tolerate even higher Ga pressures and consequently higher melt temperatures. The Ga vapor pressures, P Ga , over pure liquid Ga are summarized in Table II below. This table can be used to estimate allowable melt temperatures depending on the Ga pressure that can be tolerated by the source of active nitrogen. It should be noted that the actual Ga vapor pressures over the Ga/N solution will be somewhat less than the values set out in Table II; however, how much less is not known. TABLE II Vapor Pressure of Gallium The method of the present invention was conducted at a maximum melt temperature of approximately 900°C (1173K) when GaN was synthesized. However, the melt temperature range over which the Group III nitrides can be synthesized by this method depends on the Group III nitride being synthesized. For example, the feasible melt temperature ranges from a few degrees above the melting point of the Group II I metal, to an upper temperature limited either by the decomposition rate of the Group I II nitride or by the vapor pressure of Ga. The temperature at which the Group III nitride undergoes net decomposition is the temperature at which the decomposition rate is equal to the formation rate. The formation rate of the nitride depends on the flux of active nitrogen, so the decomposition temperature depends on the source of active nitrogen being used. Using the ECR source, temperatures ranging from approximately 400K to 1300K can be used in the method of the present invention for synthesis of GaN. However, InN can only be synthesized with the ECR source at temperatures ranging from approximately 500K to 1000K because of the higher melting point of In and because of the greater decomposition rate of InN. Similar considerations hold for the other Group III nitrides and will be apparent to anyone skilled in the art. The average rate of growth of GaN crystals depends on the rate at which nitrogen can be supplied to the gallium surface. The present method using the ECR source yields average linear growth rates of about 8 μm per hour. The growth rate is significantly greater at the beginning of the synthesis before the liquid gallium is totally covered with the solid GaN crust. It is believed that this rate is limited by the solid GaN crust formed on the liquid gallium. If the melt temperature is increased, a solid crust of GaN does not form and the growth rate of the GaN is not limited by the formation of a crust. For example, at 900 C, 0.95 gram of solid GaN was grown in 3 hours. The linear growth rate for this run was approximately 600 microns per hour. The size of the crystals grown using the method of the present invention is on the order of a millimeter. However, it is contemplated that crystal size up to several inches across is feasible using the present method. The ability to synthesize GaN depends critically on having a low rate of recombination of N to N 2 , which is strongly favored thermodynamically. A small rate of recombination compared to the rates of liquid phase diffusion in reaction to form GaN enhances the amount of saturated Ga/N solution that can be formed. In a quiescent pool of gallium, the characteristic thickness, L, of the nitrogen containing layer is to order of magnitude given by where D is the diffusion coefficient of N in liquid Ga and k is the (linearized) rate constant for the recombination of N to N 2 in liquid Ga. The effective value of D is increased by stirring the pool of liquid gallium. The rate constant, k, is not known. It is not known why the recombination of N to form N 2 does not take place more rapidly than it does. However, one factor reducing the recombination rate may be the fact that the nitrogen in the Ga melt is coordinated by four or more Ga atoms. These loosely bound Ga atoms, which form a shell surrounding the N atoms, can shield the nitrogen atoms from one another, thereby reducing their ability to interact to form N 2 . Since in the preferred form of the invention, the reactants are pure Ga and N, possible contamination from the use of NH 3 or metal organics is avoided. Crucibles made from pyrolytic boron nitride can be used for the synthesis of the nitrides. No evidence of contamination of the GaN by boron is found using this type of crucible. A related issue is contamination of the gallium surface, which might hinder dissolution of N and catalyze recombination of N to N 2 on the surface. Gallium is very reactive and easily oxidized by residual water vapor and oxygen. Non-metallic impurities in the gallium, such as arsenic and sulphur, are known to segregate at the surface. Surface concentrations can be large, even for elements present only in trace amounts in the bulk gallium. In the present invention, the gallium surface is pretreated with a beam of atomic argon followed by a beam of atomic hydrogen. This pretreatment step reduces any Ga 2 0 3 and also removes arsenic and sulfur by forming volatile arsines and hydrogen sulfide. The use of argon ion sputtering prior to the formation of the gallium nitride is also helpful in reducing contamination, particularly when an ECR source is used to generate nitrogen plasma. The argon ion sputtering is done prior to the application of the hydrogen beam. At 298°K, Ga and GaN have almost the same density (6.1 g/cm 3 ). The linear expansion coefficient of Ga is 18.1 x 10 "6 °K " ' and for GaN is 5.6 x 10 " 6 °K ' '. At elevated temperatures, therefore, GaN should sink in liquid Ga. The GaN floats on Ga at 1000°C because of surface tension. The crystallization of the solid gallium nitride can be accomplished in many various ways, including, with and without a substrate or lattice matched seed crystal. Possible methods include, but are not limited to, directional freezing, remelting and freezing a crust, thermal gradient transport from a crust, Czochralski growth on a seed, inverted submerged Czochralski growth, and growth through a thin liquid gallium film onto a substrate. The preferred method depends on the type of product that is desired and there is some overlap between the methods. However, for growth of large single crystals, directional freezing is preferred; thermal gradient transport from a crust is simpler to implement but will produce smaller crystals. For producing large polycrystalline masses of crystals, remelting and refreezing a crust is preferred. For producing oriented or single crystal films, growth through a thin liquid gallium film is preferred. In each of these cases the advantage of operating below atmospheric pressure is obtained. To synthesize large single crystals the molten Ga/N alloy is slowly cooled in a directional manner. This can be done in two general ways: (1) freezing horizontally, or (2) freezing vertically. In the horizontal method, a temperature gradient is imposed across the liquid surface and the average temperature slowly decreased while the atomic nitrogen beam is maintained. A freezing front progresses across the Ga/N liquid melt, yielding a large crystal of GaN. To reduce the probability of multiple nucleation centers, the interior of the crucible is structured in the shape of a point as shown in Fig. 3. In this method, the solid GaN grows on a liquid surface so that stress and stress- induced dislocations are minimized. ln the second method shown in Fig. 4, a vertical temperature gradient is imposed across the liquid Ga/N alloy, with the highest temperature at the surface. The level of the Ga/N melt is just above a submerged substrate. The average temperature is reduced until nucleation of GaN occurs on the substrate. It is advantageous to maintain the distance between the liquid surface and the substrate constant as the solid GaN grows. This may be done in several different ways. The preferred way is to raise the liquid level by the slow insertion of a boron nitride piece. Alternatively, the substrate can be slowly lowered or gallium added to the melt. Another mode of operation is to transport nitrogen within the melt from regions of high temperature, where the solid nitrides are more soluble, to regions of low temperature, where the solid nitrides are less soluble. One way of doing this is to grow a polycrystalline crust of the solid nitride on top of the melt. One side of the crust is then maintained at a higher temperature than the other side. The solid nitride crust dissolves at the high temperature side; nitrogen is transported through the melt to the low temperature side, where the solid nitride recrystallizes. This mode of operation can be used to convert small crystals of solid nitride into larger crystals. Another possible mode is to grow a polycrystalline crust and then impose a vertical temperature gradient, in which the entire crust is kept at a higher temperature than the bottom of the melt. The crust dissolves and the nitrogen diffuses down the cooled substrate or seed crystal where the solid nitride crystallizes. Subsequent to crystallization, the formed GaN can be etched with a mixture of hydrochloric acid and nitric acid to remove excess gallium. The conccntratcd acids are used at room temperature in approximately a 1 to 1 ratio by volume. While seed crystals are not necessary in this invention, in some circumstances the use of a seed crystal to initiate nucleation of the nitride may be useful. The seed can be a crystal of the same material, e.g., GaN seed for growing GaN, or it can be a different material, e.g., sapphire seed for growing GaN or diamond seed for growing cubic boron nitride. The present invention is easily adapted for growth of doped nitride crystals. Doping can be used to enhance the conductivity or change the optical properties of the nitride crystals. The desired doping agent, e.g., magnesium, is added to the original metal or metals that make up the melt. The rest of the cleaning and growth procedure remains the same. The doping agent will be incorporated into the solid nitride crystal as it grows. The amount of doping agent incorporated into the solid nitride crystal will depend on several factors, including the concentration of doping agent in the melt and the distribution coefficient of the doping agent between the melt and the nitride crystal. The distribution coefficient can not be predicated and is best determined by experiment. Applying a negative bias to the melt during the growth phase has a beneficial effect on growth. In order to bias the melt effectively with a DC power supply, it is helpful if the crucible is electrically conducting. For example, the crucible can be made of graphite or thin layers of conducting nitride grown on a metal such as molybdenum or titanium. iasing may suppress recombination of N to N 2 on the surface of the melt. The bias voltage that is applied, typically several hundred volts, is sufficiently high that the nitrogen ions will be implanted a few atomic distances below the surface of the melt. Individual nitrogen atoms in the melt may have a weaker tendency to recombine to form molecular nitrogen, N 2 , because they are coordinated to metal atoms. These metal atoms, e.g., gallium, form a protective coordination shell around the nitrogen atoms that keep the nitrogen atoms from approaching each other closely. This would suppress the tendency to form N 2 . If the bias is not applied, the nitrogen atoms will not be implanted into the melt, but would be more likely to remain on the melt surface where they may have a greater tendency to recombine. Although this mechanism is speculative, it is in agreement with the experimental observations. It is found that the nitrogen containing melts strongly wet other nitrides. For example, gallium/nitrogen melts at temperatures above 900°C form a concave meniscus with the boron nitride crucible. Indium/nitrogen melts actually creep out of nitride crucibles upon heating. This effect can be used in another growth mode that is based on the principles of this invention. A thin film of liquid Group III metal, e.g., indium or gallium, flows out on a substrate that it wets well, for example a nitrided substrate. The thin film of Group III metal is then reacted with the active nitrogen from the plasma. The thin layer of Group III metal is converted into the nitride. If the substrate is latticed- matched with the nitride being formed, oriented crystals can be formed by this method. Single crystal layers can also be formed. Partially latticed-matched substrates that can be used are sapphire and silicon carbide. Other possible substrates are derived from the Group Ill/Group V compounds, e.g., gallium arsenide and indium phosphide. One advantage of this approach is that there is a very small distance that the nitrogen in the melt must be transported before it crystallizes to form the solid nitride. Also, large single crystal layers may be formed for device applications. The invention will now be described in detail in the following detailed examples. EXAMPLE I Liquid gallium was held in a pyrolytic boron nitride crucible (6 mm diameter and 4 mm high), which could be independently heated to temperatures up to 1 100°C. The electron cyclotron resonance microwave (ECR) source (Wavemat Model MPDR 610) was mounted directly above the crucible. The ECR plasma discharge was maintained with 200 W of microwave power. An ion flux density of approximately 10 16 cm ~ V was obtained. The beam was estimated to be approximately 10%N, 10%N + , with the remainder N 2 * and N 2 . The growth procedure was initiated by evacuating the reaction chamber to 10 "9 torr by a combination of a turbo-pump, sublimation-pump, cryo-pump and diffusion-pump. The liquid gallium was next heated at 600°C and 10 "7 torr for 45 minutes to promote the desorption of trapped gas. An argon beam plasma was used for 10 minutes followed by a hydrogen plasma for 30 minutes to remove gallium oxide and other impurities from the surface of the gallium. The hydrogen flow to the ECR source was then replaced by nitrogen and the temperature raised slowly to approximately 700 " C. During this step, the chamber was evacuated with the turbo-pump and the pressure controlled at 0.5 mtorr by controlling the nitrogen How rate, fhc nitrogen plasma used 10 seem of high purity nitrogen as source gas. After approximately 15 minutes, at a temperature of approximately 700 ° C, a change in the reflectivity of the gallium liquid was observed. The surface became rougher and lost the typical specular appearance of metallic gallium, indicating the formation of a crust of polycrystalline gallium nitride. The nitrogen plasma beam was maintained for 12 hours at the final temperature of 700 ° C. The pressure in the chamber was maintained at 0.5 mtorr throughout. A supersaturation of nitrogen was obtained and spontaneous crystallization occurred without cooling. After removal from the chamber, the sample was etched with a 1 to 1 mixture of concentrated hydrochloric acid and nitric acid to remove the excess gallium. Fig. 5 shows the polycrystalline GaN "dome" formed in Example I, which completely covered the liquid gallium at the end of a run. The "dome" shown in Fig. 5 was approximately 0.10 mm thick, weighed 40 mg, and had a surface area of 70 mm 2 . The average linear growth was approximately 8 micrometers per hour. X-ray and electron diffraction were used to analyze the crystalline GaN found on the liquid surface. The solid sample was confirmed by X-ray and electron diffraction to be wurtzitic GaN. Energy dispersive X-ray analysis performed with both TEM and SEM showed only the presence of gallium and nitrogen in equal amounts within the instrumental error of ±5%. A scanning electron micrograph of a portion of the concave surface of the polycrystalline GaN dome of Example I is shown in Fig. 6. A dendritic morphology, typical of uncontrolled freezing from a supersaturated solution was seen. In Fig. 7, a scanning electron micrograph of a portion of the convex surface of the dome is seen. Thin platelets of GaN aligned normal to the surface are evident. X-ray diffraction from this region of the sample indicated a strong < 1 1 - 20 > texturing. A transmission electron micrograph of a portion of a polycrystalline GaN sample of Example I is shown in Fig. 8. A small hexagonal platelet was visible in the center of Fig. 8. The insert was the electron diffraction pattern of the crystal tilted to the [11-20] zone axis. Streaks were noted along the [0001] directions, indicating the presence of planar defects. Diffraction spots from the [1-100], [0001] and [10-11] planes were also obtained and matched well with the w-GaN. Dark bands in the transmission electron micrographs at Fig. 9 are believed to arise from stacking faults and microtwins. Dislocations were not imaged. Typical steady state photoluminescence spectra, taken at room temperature and at approximately 10K, of the convex surface of the GaN dome are shown in Fig. 10. The excitation source was the 325 nm line of a HeCd laser, with 0.5 mW power focused to a 50 μm diameter spot. The detection system was a photon counting photomultiplier tube with a 0.8 meter double monochrometer, with the spectral resolution set to 0.4 nm (approximately 4 meV) at a photon energy of 3 eV. The sample was mounted with colloidal graphite on the copper block tail of a liquid-helium-cooled cold finger dewar. Care was taken to avoid artifacts in the spectra arising from the copper block or the graphite. Spectra at both room temperature and 10K. show the characteristic broad "yellow band" luminescence, peaking near 2.2 eV and multipeak near-band-edge luminescence. The scale of the abscissa is expanded in the inset to Fig. 10, which shows the near-band-edge peaks more clearly for the 1 OK spectrum. The shoulder in the luminescence spectrum at approximately 3.47 eV may be the A exciton as shown in Monemar, Phys. Rev. B 10, 676 (1974) or the exciton bound to a neutral donor as shown in T.W. Weeks, M.D. Bremser, K.S. Ailey, E. Carlson, W.G. Perry, and R.F. Davis, Appl. Phys. Lett. 67, 401 (1995). Peaks at 3.38 and 3.28 eV are in the energy range for impurity bound excitons and donor- acceptor pair recombination as well as for the first and second LO phonon replicas of the free or neutral donor bound exciton as shown in C.FI. Hong, D. Pavlidis, S.W. Brown, and S.C. Rand, J. Appl. Phys. 77, 1705 (1995); B.K. Meyer, D. Volm, A. Graber, H.C. Alt, T. Detchprohm, A. Amano, and I. Akasaki, Solid State Commun. 95, 597 (1995); and S. Strite and H. Morcoc, J. Vac. Sci. Tech. B 10, 1237 (1992), and references therein. Between 10K and 300K, the near-band-edge luminescence decreased by a factor of 50, and the yellow band by a factor of 2. The spectrally integrated luminescence intensity decreased by a factor of 10. This decrease was due to thermal activation of some non-radiative recombination mechanism. Depolarized Raman spectra of two different spots are shown in Fig. 1 1. The spectra were taken in backscattering geometry, with the spot size of approximately 1 micron. It was assumed that the crystallites are randomly oriented. The arrow in the main figure indicated the location of the 144 cm " ' E2 line for wurtzitic GaN, which was clearly observed. The arrows in the inset showed the locations of the Al (TO) line at 531 cm " ', the El (TO) line at 560 cm " '. and the E2 line at 568 cm '1 as shown in P. Perlin, C. Jauberthie-Carillon, J.P. Itie, A. San Miguel, I. Grzegory, and A. Polian, Phys. Rev. B 45, 83 ( 1992). Also shown is the location of the TO phonon for cubic GaN at 554 cm " 1 as shown in M. Giehler, M. Ramsteiner, O. Brandt, H. Yang, and K.H. Ploog, Appl. Phys. Lett. 67, 733 (1995). Peaks corresponding to all of these lines occurred at varying degrees in the two spectra, consistent with a mix of crystallographic orientations, consisting primarily of the wurtzite phase, over the sampled volume. EXAMPLE 2 Example 1 was repeated with the exception that an alloy containing 60 weight percent Ga and 40 weight percent In was used as the starting metal and the melt temperature was 670°C. The total growth period was 1 1 hours and 30 minutes. Upon crystallization, only polycrystalline GaN was obtained; no InN was formed despite the presence of In in the melt. Crystals from this example are shown in Figure 12. The crystals were characterized by photoluminescence spectroscopy. The band-edge luminescence was not shifted from the wavelength for pure GaN, which indicates that the crystals of GaN contained no dissolved InN. The crystallite size was larger, up to 0.1 mm, than in Example 1 where pure Ga was used as the starting metal. This is due to the beneficial effect of the enhanced solubility brought about by the addition of the second metal. EXAMPLE 3 Example 1 was repeated in an identical manner with the exception that In was used as the starting metal and the crystallization temperature was 650 ° C. The total growth period was 1 1 hours. During the growth period bubbles were observed coming from the melt. Upon crystallization, only polycrystalline InN was obtained. The InN was characterized by electron diffraction and by elemental analysis. EXAMPLE 4 Example 1 was repeated in an identical manner with the exception that an alloy containing 6.69 weight percent aluminum and 93.31 weight percent gallium was used as the starting metal. Upon crystallization, only polycrystalline AIN was obtained; no GaN was formed despite the presence of Ga in the melt. EXAMPLE 5 Example 1 was repeated in an identical manner with the exception that during growth a microwave ball plasma source was substituted for the microwave ECR source. The ball plasma source operated at 14 torr pressure. The Astex microwave source operated at 2.45 GHz and a power level of 1 kw. The initial charge of pure Ga was placed in a graphite plate that was machined to have a shallow, concave upper surface. The melt temperature was 950 C. During the run, the melt was negatively biased at 200V and 0.02 amperes using a DC power supply. After a run time of 2 hours and 32 minutes, a mass of polycrystalline GaN was obtained that weighed 0.9449 grams. The sample was confirmed to be GaN by photoluminescence spectroscopy and Raman spectroscopy. EXAMPLE 6 Example 5 was repeated in an identical manner except that pure In was used as the starting metal, which was held in a pyrolytic boron nitride plate that was machined to have a shallow, concave upper surface. The temperature of the susceptor holding the crucible was held at 200°C. The growth period was 1 hour 30 minutes. At the end of a the run the In was partially converted into InN. EXAMPLE 7 Example 6 was repeated except that a crucible was made from a piece of molybdenum foil. The foil was nitrided by subjecting it to active nitrogen from a microwave plasma before the growth run. During the growth run, approximately 30 minutes after the nitrogen plasma was started, the melt was visually observed to climb out of the bottom of the crucible and to flow out over the top of the crucible lip. After the growth run was completed, many well-faceted hexagonal plates of indium nitride were observed by optical and scanning electron microscopy. The platelets were in clusters; within each cluster the platelets were well oriented with respect to each other. The platelets were identified as indium nitride by electron diffraction and elemental analysis. EXAMPLE 8 Example 1 was repeated in an essentially identical manner except that a crystal of single crystal sapphire was added to the crucible. The sapphire floated on the surface of the melt. Upon crystallization, gallium nitride was found to have preferentially crystallized on the sapphire seed crystal. Scanning electron microscopy showed that in some places on the sapphire, the gallium nitride crystals had a preferential orientation with respect to the sapphire. EXAMPLE 9 Example 6 was repeated except that the nitrogen plasma was turned off and on in an intermittent manner. The melt surface observed visually while the plasma cycled from on to off. When the plasma was on, a polycrystalline crust of InN was observed. When the plasma was off, this crust disappeared almost instantaneously because of decomposition of the InN. This experiment shows that the synthesis takes place under conditions where the InN is unstable and will decompose unless there is a counterbalancing flux of active nitrogen. The invention has been described with reference to preferred and alternate embodiments. Obviously, modifications and alterations will occur to others upon the reading and understanding of this specification. It is intended to include all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.

Claims: CLAIMS: ~ 0~ Having thus described the invention, it is claimed: 1. A method for producing Group III nitride crystals comprising the steps of: (a) optionally pretreating a liquid comprising a first Group III metal to clean the surface thereof; (b) raising the temperature of the liquid from about the melting point of the first Group III metal to a temperature below which net decomposition of the nitride crystals occurs; (c) saturating the liquid using active nitrogen to form a melt comprising the first Group III metal and nitrogen.; (d) crystallizing the Group III nitride crystals; and (e) optionally etching the Group III nitride crystals to remove any unreacted Group III metal. 2. The method of claim 1 further comprising the step of transporting the nitrogen in the melt from a hotter region to a cooler region of the melt. 3. The method of claim 1 wherein the liquid further comprises a second Group III metal, the first Group III metal being different than the second Group III metal. 4. The method of claim 1 wherein the liquid further comprises one or more metals different than the first Group III metal. 5. The method of claim 4 wherein the one or more metals arc selected from the group consisting of a second Group III metal different than the first Group 111 metal, tin, bismuth and mixtures thereof. 6. The method of claim 1 wherein the pretreating of the liquid is carried out by directing a plasma comprising atomic argon at the surface of the liquid, followed by directing a plasma comprising atomic hydrogen at the surface of the liquid. 7. The method of claim 1 wherein the liquid is held in a container formed of boron nitride. 8. The method of claim 1 wherein the active nitrogen is neutral, ionic, excited or a mixture thereof. 9. The method of claim 1 wherein the active nitrogen is generated by a plasma comprising N, N + , N 2 * and the remainder N 2 . 10. The method of claim 1 wherein the active nitrogen is generated by the decomposition of ammonia (NH 3 ). 1 1. The method of claim 1 wherein the Group III metal is one selected from the group consisting of gallium, boron, aluminum and indium. 12. The method of claim 1 wherein the melt is comprised of about 0.01 atomic percent nitrogen with a remainder being a metal. 13. The method of claim 1 wherein the crystallizing of the Group III nitride crystals is carried out by freezing horizontally or freezing vertically. 14. The method of claim 13 wherein the Group III nitride crystals are single crystals. 15. The method of claim 1 wherein the Group III nitride crystals are polycrystalline. 16. The method of claim 15 wherein the polycrystalline Group III nitride crystals are characterized by an ability to luminesce at room temperature. 17. The method of claim 1 wherein the etching is carried out with a solution comprising hydrochloric acid and nitric acid. 18. The method of claim 1 wherein the saturating of the liquid is carried out at about atmospheric pressure or below. 19. The method of claim 1 wherein the crystallizing of the Group III nitride crystals is carried out by cooling the melt. 20. The method of claim 1 wherein the crystallizing of the Group III nitride crystals is carried out by supersaturating the liquid with active nitrogen. 21. The method of claim 1 wherein the crystallizing of the Group III nitride crystals is carried out at about atmospheric pressure or below. 22. A method for producing Group III nitride crystals comprising the steps of: directing active nitrogen onto a surface of a liquid comprising a first Group III metal; and, crystallizing the Group III nitride crystals. 23. The method of claim 22 wherein the Group III metal is one selected from the group consisting of gallium, boron, aluminum and indium. 24. The method of claim 22 wherein the active nitrogen is neutral, ionic, excited or a mixture thereof. 25. The method of claim 22 wherein the active nitrogen is generated by a plasma or by the thermal decomposition of ammonia. 26. The method of claim 22 wherein the step of crystallizing the Group III nitride crystals is carried out by a process selected from the group consisting of directional freezing, controlled gradient freezing, Czochralski growth on a seed, inverted submerged Czochralski growth and growth onto a substrate. 27. The method of claim 22 further comprising the step of heating the liquid from about the melting point of the first Group III metal to a temperature below which net decomposition of the Group III nitride occurs prior to the step of directing the active nitrogen onto the surface of the liquid. 28. The method of claim 27 further comprising the step of pretreating the liquid with a plasma comprising atomic argon prior to the step of directing the active nitrogen onto the surface of the liquid. 29. The method of claim 26 further comprising the step of pretreating the liquid with a plasma comprising atomic hydrogen prior to the step of directing the active nitrogen onto the surface of the liquid. 30. The method of claim 22 wherein the Group III nitride crystals have a length of about 1 mm and greater. 31. The method of claim 25 wherein the plasma is generated using an electron cyclotron resonance microwave source, a ball plasma microwave source, a DC source or a radio frequency source. 32. The method of claim 22 wherein the average growth rate of the Group III nitride crystals is about 8 μmh ' 1 . 33. The method of claim 22 wherein the step of directing the active nitrogen onto the surface of the liquid and the step of crystallizing the Group III nitride crystals are carried out at about atmospheric pressure or below. 34. The method of claim 22 wherein the liquid further comprises a second metal selected from the group consisting of a Group III metal different from the first Group III metal, bismuth, tin and mixtures thereof. 35. A method for producing gallium nitride crystals comprising the steps of: saturating a liquid comprising gallium metal with active nitrogen; and, crystallizing the gallium nitride crystals. 36. The method of claim 35 wherein the step of saturating the liquid gallium metal is carried out by impinging a plasma comprising active nitrogen onto the surface of the liquid. 37. The method of claim 35 wherein the active nitrogen is neutral, ionic, excited or a mixture thereof. 38. The method of claim 36 wherein the plasma is comprised of N, N ' , N 2 * and the remainder N 2 . 39. The method of claim 35 wherein the step of crystallizing the gallium nitride crystals is carried out by freezing horizontally or freezing vertically. 40. The method of claim 39 wherein the gallium nitride crystals are single crystals. 41. The method of claim 35 wherein the liquid is held in a container formed of boron nitride or a metal with a nitride coating. 42. The method of claim 35 further comprising the step of heating the liquid to a temperature below which net decomposition of the solid gallium nitride occurs prior to crystallizing the gallium nitride crystals. 43. The method of claim 35 wherein the step of saturating the liquid is carried out at atmospheric pressure or below. 44. The method of claim 35 wherein the step of crystallizing the gallium nitride is carried out at about atmospheric pressure or below. 45. The method of claim 35 wherein the liquid further comprises one or more metals different than the gallium metal. 46. The method of claim 35 wherein the gallium nitride crystals are polycrystalline and are characterized by an ability to luminesce at room temperature. 47. The method of claim 35 wherein the liquid further comprises one or more metals selected from the group consisting of a second Group III metal different than gallium, bismuth and tin. 48. Group III nitride crystals formed by a process comprising the steps of: saturating a liquid comprising the Group III metal with active nitrogen followed by crystallizing the Group III nitride crystals, each of said steps carried out at atmospheric pressure or below, said crystals being polycrystalline and characterized by an ability to luminesce at room temperature. 49. The Group III nitride crystals of claim 48 having a length of about 1 mm and greater. 50. The Group III nitride crystals of claim 48 wherein the Group III metal is one selected from the group consisting of gallium, boron, aluminum and indium. 51 . A method for producing Group I II nitride crystals comprising the steps of: applying a film comprising a first Group III metal onto a substrate; saturating the film with nitrogen using active nitrogen; and, crystallizing the Group III nitride crystals. 52. The method of claim 51 wherein the substrate is a lattice- matched substrate selected from the group consisting of sapphire, silicon carbide, Group Ill/Group V compounds and derivatives of Group Ill/Group V compounds. 53. The method of claim 51 wherein the active nitrogen is generated by a plasma comprising N, N + , N 2 * and the remainder N 2 . 54. The method of claim 51 wherein the Group III metal is one selected from the group consisting of gallium, boron, aluminum and indium. 55. The method of claim 51 wherein the film further comprises one or more metals different than the first Group III metal. 56. The method of claim 55 wherein the one or more metals are selected from the group consisting of a second Group III metal different than the first Group III metal, tin, bismuth and mixtures thereof.

Inventors: ANGUS JOHN C, ARGOITIA ALBERTO, HAYMAN CLIFF C, ANGUS, JOHN, C., ARGOITIA, ALBERTO, HAYMAN, CLIFF, C.
Applicants: ANGUS JOHN C, ARGOITIA ALBERTO, HAYMAN CLIFF C, UNIV CASE WESTERN RESERVE, ANGUS, JOHN, C., ARGOITIA, ALBERTO, CASE WESTERN RESERVE UNIVERSITY, HAYMAN, CLIFF, C.
Application year: 1997
Main classification: C01B21
IPC codes: C01B21, C30B11, C30B15, H01L21