Hexagonal GaN (H-GaN) films have been grown on GaAs(0 0 1) substrates with AlAs buffer layers in a molecular beam epitaxy system. Transmission electron microscopy observations show the tilt of the sub-grains of H-GaN film with respect to the substrate. Such mis-orientation is derived from the steps on AlAs surface due to nitridation. Atomic force microscopy observations show a large density of pinholes on the surface of GaN film grown this way. But when an AlN buffer layer is applied, on AlAs/GaAs(0 0 1) substrate, the quality of the GaN film can be much improved with the elimination of mis-orientation and pinholes. (C) 2003 Elsevier Science B.V. All rights reserved.