In this work the structure of electron gun thermally evaporated films of Er/Er2O3 on Bridgman-grown GaSb substrates has been studied by means of atomic force microscopy and scanning electron microscopy. The microcrystalline structure of the uppermost Er film consists in hexagonal microcrystals, being around 1 mum in size with a surface roughness of about 200 nm for the evaporation conditions used here. In order to characterise the overall composition of this bilayer structure, Rutherford backscattering spectroscopy and secondary ion mass spectrometry analysis have also been carried out. The composition of the different layers could be determined and the analysis has also revealed diffusion processes between the layers and the substrate. The presence of Sb diffused from the substrate has been observed both in the oxide and metal layer and the formation of an Sb-rich region between the substrate and the oxide layer has also been proved. It has also been identified that the presence of a small amount of Er into the substrate and the SIMS spectra have established the metal-oxide-semiconductor nature of the structure. (C) 2004 Elsevier B.V. All rights reserved.