The defect structure and morphology of films of the hexagonal phase, grown by means of metal-organic chemical vapor deposition onto (00•1) sapphire substrates, were studied by using focussed ion-beam milling, field emission scanning electron microscopy and transmission electron microscopy. Two different types of grain boundary defect, having an anisotropic distribution, were observed. In the first type, the grain boundaries surrounded pyramid-shaped grains. These defects originated from the mismatch of the GaN film and the sapphire substrate. The grain boundary defects penetrated the GaN epilayer, and the average grain size increased with increasing layer thickness up to about 500nm from the interface to the upper surface of the GaN film. In the second type, the grain boundaries surrounded voids formed by impurities in the film. Both types of grain boundary had a distribution with an hexagonal symmetry. This was suggested to be the cause of the anisotropic transport properties which had been observed in GaN films.

Anisotropic Defect Structure of GaN Film Grown by MOCVD D.P.Feng, Y.Zhao, C.C.Sorrell, G.Y.Zhang: Solid State Communications, 2000, 114[4], 237-40