Thermal annealing effects on SCAATTM substrate grown toward the c- and m-directions

The effect of the thermal annealing on an acidic ammonothermal gallium nitride (GaN) crystal grown toward the c- and m-directions was investigated. The specific annihilation of hydrogenated Ga vacancies was confirmed for the GaN crystal grown toward the m-direction but not for that grown toward the c-direction. A transparent and yellow luminescence-free ammonothermal GaN substrate was realized. The annihilation of the hydrogenated Ga vacancies was explained by the difference in the diffusion of F incorporated as a mineralizer between the c- and m-directions.