Method and system for preparing polycrystalline group III metal nitride

#The present disclosure generally relates to processing of materials for growth of crystals. More particularly, the present disclosure provides a crystalline nitride material suitable for use as a raw material for crystal growth of a gallium-containing nitride crystal by an ammonobasic or ammonoacidic technique, but there can be others.

#Group III nitride containing crystalline materials serve as substrates for manufacture of optoelectronic devices such as violet or blue light emitting diodes and laser diodes and electronic devices such as diodes and switches. Such optoelectronic and electronic devices have been commonly manufactured on sapphire, silicon, or silicon carbide substrates that differ in composition from the deposited nitride layers. The use of group III nitride substrates can provide a number of benefits, including increased performance and lifetime.

#Most bulk GaN substrates today are manufactured by hydride vapor phase epitaxy (HVPE), a vapor phase technique that deposits a thick layer of GaN on a substrate, typically sapphire or gallium arsenide. To date, HVPE substrates have been relatively expensive, and the crystalline quality may be insufficient for certain applications, such as high performance laser diodes or vertical GaN-on-GaN power electronic devices.

#FIG. 1 is a schematic diagram illustrating a reactor according to an embodiment of the present disclosure;

#FIG. 2 is a schematic diagram of a composition of matter according to an embodiment;

#FIG. 3 is a schematic diagram illustrating a reactor according to an embodiment;

#The inventors have discovered various shortcomings of previous methods for synthesizing polycrystalline group III nitrides, or polycrystalline GaN in particular. GaN powder is typically unsuitable for use as a nutrient for ammonothermal crystal growth, for at least two reasons. First, powders are more susceptible to the presence of oxides, moisture, and other impurities by virtue of their relatively high surface area. Second, it is difficult to maintain a suitable fluid flow through a bed of GaN powder, where the fluid is able to freely pass between GaN particles and the GaN particles remain in place without becoming entrained in the fluid flow.

#The inventors have found that the morphology of polycrystalline GaN formed by conversion-in-place processes, in which a thick GaN plate is formed overlying a pool of liquid Ga, is very sensitive to the process conditions, including the ratio of a halogen-containing gas and ammonia. These gases have widely differing molecular weights and densities, so it is difficult to keep them well mixed at the low overall gas flow velocities that are optimum for material utilization efficiency. As a result, the process is difficult to scale up and to control. In addition, by its nature the conversion-in-place process generates polycrystalline GaN that is somewhat porous, comprising a plurality of through-holes or through-pores, as Ga must be transported through these pores in order for the process to proceed. This residual porosity may entrap contaminants, such as oxides, moisture, dust, particulates, or oils, which may compromise a subsequent crystal growth process utilizing this material as a nutrient. Furthermore, the through-holes or through-pores that Ga must be transported through during this process make full conversion of the Ga difficult. If full Ga to GaN conversion does not occur, contamination of the ammonothermal process with oxygen can be an issue since the unreacted Ga readily reacts to form a Ga-oxide when exposed to an oxygen containing environment.

#In the case of downstream synthesis processes, multiple chambers within a reactor are typically required, for example, a source chamber in which a halogen-containing gas flows above molten gallium to form a gaseous gallium halide, and a deposition chamber in which a nitrogen-containing gas (typically ammonia) is mixed with the gaseous gallium halide. These chambers are commonly synthesized from fused quartz, which is inexpensive, easy to fabricate, and leak-tight joints can be fabricated by welding, but can contribute both Si and O contamination to the resulting polycrystalline GaN. In addition, polycrystalline GaN deposits on quartz surfaces can be difficult to remove and, because of a large difference in the thermal expansion coefficients of the two materials, the quartz can crack. Other container materials, such as boron nitride, have been disclosed but fabrication with these materials is more difficult and the inventors are unaware of disclosure of suitable means for forming joints that do not compromise process performance. Downstream deposition has generally been performed on surfaces that are either flat, such as the top surface of a susceptor, or cylindrical, such as the perimeter of a susceptor or the inner diameter of a vessel wall. Materials of construction for the deposition surfaces have included quartz, graphite, silicon carbide, SiC- or C-coated graphite, boron nitride, and silicon nitride, all of which are rigid and from which removal of polycrystalline GaN deposits can be difficult.