Phase selection of microcrystalline GaN synthesized in supercritical ammonia

Microcrystalline GaN was synthesized in supercritical ammonia with acidic, neutral, and basic mineralizers. For both acidic and neutral mineralizers, a mixture of wurtzite GaN and zinc-blende GaN was obtained, whereas wurtzite GaN was preferentially synthesized with basic mineralizers. Gibbs free energy for wurtzite GaN and zinc-blende GaN was calculated to investigate phase stability of GaN. The free energy of formation for wurtzite GaN was smaller than that for zinc-blende GaN by 720 J/mol (8 meV) at 600 K and 1310 J/mol (14 meV) at 800 K. Given this small energy difference, the selection of phase in ammonothermal synthesis turned to depend more on kinetics than on thermodynamics. Water-dissolving powder which was occasionally synthesized in basic conditions suggested that the reaction path in basic conditions is different from that in acidic and neutral conditions. The difference in the reaction path is thought to affect the phase selection.