Ammonothermal crystal growth processes appear as the most feasible method for producing GaN bulk crystals of sufficient size and quality for commercial applications. Various kinds of mineralizers have been applied for the ammonothermal processes for solubility enhancement. For process design purpose, accurate knowledge of the GaN solubility in supercritical ammonia is essential. While few experimental solubility data have been reported for the basic ammonothermal systems, the data for the acidic ammonothermal systems have never been reported. In this study, we present our experimental study for the GaN solubility in acidic supercritical ammonia. We show firslty our experimental apparatus and procedures in details. The experimental method was based on a weight loss method. The mineralizer used was maninly ammonium chloride and the GaN sample used is the GaN crystals grown by H-VPE (Halide Vapor Phase Epitaxy). Experimental conditions were in the temperature range from 450 K to 770 K and at pressure about 100 MPa. The weight ratios of the ammonium chloride to ammonia were 5 to 20 wt%. It was found that the solubility increases with increasing temperature in the present experimental conditions. This type of temperature dependence of the GaN solubility has never been observed in the basic ammonothermal systems. This result suggests that we can assume same solubilization phenomena in the present experimental conditions. Also more importantly this result indicates that the acidic ammonothermal process can be designed wwith the similar principles developed by the hydrothermal crystal growth processes for quartz and zinc oxide.

By combining with the present ssolubility data and simulation results for ammonothermal autoclaves, spatial distributions of the GaN supersaturation degree in the autoclave can be determined. The ideal crystal growth rate in the whole region in the autoclave can be estimated. From these results, we will discuss configuration of the autoclave to provide good productivity of the GaN bulk crystals.

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