GaN thin films were deposited on GaAs (0 0 1) substrates using chemical beam epitaxy (CBE) and plasma-assisted molecular beam epitaxy (MBE) methods. The effect of in situ substrate cleaning and pre-treatment on the interfacial structure was investigated. Time-of-flight mass spectroscopy of recoiled ions was utilized to measure in situ the surface composition. The microstructure of GaN films was studied with selected area electron diffraction, and both conventional and high-resolution transmission electron microscopy. It was found that simultaneous electron cyclotron resonance (ECR) plasma treatment using an Ar/N-2 mixture increases the yield of a cubic phase in GaN films during CBE growth. Resulting films are less defective than mixed phase GaN films grown after pure N-2 plasma nitridation. In the case of plasma-assisted MBE growth, a low-temperature annealing followed by N-2 ECR nitridation process yields cubic GaN thin films while N-2 ECR nitridation without a sample annealing to 600degreesC yields hexagonal phase GaN films. (C) 2002 Elsevier Science B.V. All rights reserved.