A high-quality GaN layer has been achieved using an AlN buffer layer directly grown on a sapphire Substrate at high temperature. The microstructure was investigated by high-resolution transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM) and energy-dispersive X-ray spectroscopy (EDX). A high density of V-shaped pits, with sizes of 20-40 rim, was observed at the interface between AlN and GaN layers. Unlike the previously reported V-shaped defects in nitride-based ternary alloys, these V-shaped pits were formed on top of the AlN layer due to H-2 etching of the surface during a high-temperature growth interruption between AlN and GaN layers. The pits were preferentially formed around the surface terminations of threading dislocations (TDs) in the AlN, similar to chemical etching-induced surface pits. The TD density of the AlN layer is generally very high, leading to a high density of V-shaped pits. Subsequent growth of GaN resulted in the bending over of the pre-existing TDs, significantly reducing the threading dislocation density in the overlaying GaN. The high crystal quality is further proved by highly improved photoluminescence intensity, compared to the case of conventional GaN. (c) 2005 Elsevier B.V. All rights reserved.