Time dependence of the growth morphology of GaN single crystals prepared in a Na-Ga melt

The yields of GaN prepared in a Na-Ga melt at 700-800°C and 1-5 MPa of N₂ for 200 h were measured. The morphology of the GaN crystals changed from pyramidal (yields 6-13%) to prismatic (yields 19-100%), and finally to thin platelets (yields 61-100%) with increasing temperature and N₂ pressure. A time dependence of the morphology was observed for the sample prepared at 750°C and 5 MPa of N₂. The morphology changed from pyramidal, prismatic to thick platelets with heating times up to 50 h. The yield of GaN increased linearly during this period. The formation rate of GaN increased after 50 h, and the crystal growth perpendicular to the c axis was enhanced. The crystal growth was completed within 200 h, and thin platelet single crystals with a size of 1-2 mm were formed. Microphotoluminescence spectra were measured at the cross section of a thin platelet GaN crystal. A large broad luminescence peak at 3.26 eV, probably associated with Mg or Si acceptors, was observed in the spectra obtained from the regions near the (0001) Ga polar plane.