Interface effects for metal oxide thin films deposited on another metal oxide .2. SnO2 deposited on SiO2

SnO2 overlayers are deposited on SiO2 by evaporation from a SnO2 source and treatment with a plasma of oxygen. According to an analysis by XPS/ISS, the SnO2 overlayers spread on the surface of SiO2. XPS and EELS confirm that the oxidation state of tin under different experimental conditions is Sn4+ and that the stoichiometry of the films is SnO2. A systematic comparison between the results obtained for SnO2 and SnO shows that these two materials can be easily differentiated by XPS by looking not only at photoelectron peaks, but also at the values of the Auger parameters of O and Sn in each case. Moreover, at low coverages, XPS also shows a shift of about 1 eV in the BE of the Sn 3d(5/2) peak and another of about 1.7 eV in the Auger parameter (alpha') in respect to the values found for a high coverage of SnO2 on SiO2 (i.e. similar to the bulk material). These shifts are discussed in terms of the interaction of the SnO2 overlayer with the SiO2 support. In this respect, a correlation is found between the alpha' values at different coverages and the position of the valence band edge. Simulation of this interaction by means of molecular orbital calculations with cluster models confirms this effect of the SiO2 support and justify the correlation found between alpha' and the band gap of the deposited oxide.