The effect of nitrogen annealing at 1123 K for 30 min on the structural characteristics of thin (15 nm) thermal Ta2O5 layers on Si was examined by X-ray photoelectron spectroscopy (XPS). The results indicate that the stoichiometric Ta2O5 detected at the surface of as-deposited films is reduced to suboxides at the interface with Si. Si-O bonds in the form of SiO2 exist in a small quantity through the whole thickness of the films. The existence of excess Si was established in the interfacial transition region. The annealing improves the stoichiometry and microstructure of both the bulk oxide and the interfacial region, which manifests as a reduced amount of suboxides. The interfacial region is a composite oxide of suboxides of Ta and Si before and after N-2 treatment but the anneal process reduces the excess Si and decreases the width of the interface. Thus, a trend to more abrupt interface is observed. (C) 2003 Elsevier B.V. All rights reserved.