Ammonothermal Synthesis of III-Nitride Crystals

Ammonothermal synthesis of nitrides is reviewed, with an emphasis on gallium and aluminum nitrides due to their important applications as direct wide band gap semiconductors. Since the crystallization process of nitrides involves the formation of some intermediate compounds during ammonothermal synthesis where a mineralizer is used, some ternary amides and ammoniates of aluminum and gallium with alkali metals or halides are also reviewed briefly. The ammonothermal crystallization of GaN and AlN bulk crystals, which is analogous to the hydrothermal growth of oxides, is introduced. Retrograde solubility, mineralizers, pressure−temperature−volume−concentration (PTVC) relations, phase relations, and transport growth of GaN in alkaline solutions are discussed in detail. Recent progress of GaN single-crystal growth by the ammonothermal technique is reported. We have grown GaN bulk single crystals up to 10 mm 2 by 1-mm thick. Issues such as ammonia breakdown, impurity incorporation, and scale-up of the ammonothermal growth of III-nitrides and perspectives on the method are also discussed.