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 sourc e and treatment with a plasma of oxygen. According to an analysis by X PS/ISS, the SnO2 overlayers spread on the surface of SiO2. XPS and EEL S 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 s hows that these two materials can be easily differentiated by XPS by l ooking 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 respec t to the values found for a high coverage of SnO2 on SiO2 (i.e. simila r to the bulk material). These shifts are discussed in terms of the in teraction of the SnO2 overlayer with the SiO2 support. In this respect , a correlation is found between the alpha' values at different covera ges and the position of the valence band edge. Simulation of this inte raction 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.