The microstructures of GaN buffer layers which had been grown onto (111)Al2MgO4 substrates by means of metal-organic vapor-phase epitaxy was studied by using transmission electron microscopy. It was observed that initial deposition of the GaN buffer layers at a relatively low temperature produced a continuous island sub-layer (5nm thick) with an hexagonal crystallographic structure. Subsequent GaN buffer deposition led to crystal columns which were composed of nano-crystalline slices with mixed, cubic and hexagonal, phases. Following high-temperature annealing, the crystallinity of the nano-crystalline slices and the island sub-layer was found to have improved. Formation of threading dislocations in the GaN film was attributed not only to the lattice mismatch of GaN/Al2MgO4 interfaces, but also to stacking mismatch at the crystal-column boundaries.

Microstructure Evolution of GaN Buffer Layer on MgAl2O4 Substrate. H.F.Yang, P.D.Han, L.S.Cheng, Z.Zhang, S.K.Duan, X.G.Teng: Journal of Crystal Growth, 1998, 193[4], 478-83