Control of the stoichiometry in the deposition of cobalt oxides on SiO2

Both CoO and Co3O4 overlayers have been deposited on SiO2 by evaporation from metallic Co and subsequent oxidation with oxygen and a plasma of oxygen. The combined use of ion scattering spectroscopy and XPS shows that both oxides grow in the form of small particles on the surface of SiO2. Ion scattering spectroscopy also shows that the surface of cobalt oxide exposed to a plasma of oxygen is enriched in oxygen ions with respect to the surface of the cobalt oxide formed by exposure to oxygen. The Co 2p spectra corresponding to the deposits obtained by oxidation with O2 are characteristic of CoO, while those corresponding to the deposits obtained after oxidation with a plasma are typical of Co3O4. Moreover, the OCo/Co ratios determined by XPS and factor analysis indicate the formation of CoO stoichiometry in the former case and Co3O4 stoichiometry in the latter. It has also been observed that no shift in either binding energy or modified Auger parameter α′ appears as a function of coverage. This absence of shifts is interpreted as a consequence of the type of screening mechanism that dominates the relaxation of the photoholes in these oxides. © 1998 John Wiley & Sons, Ltd.