The Auger parameter and the study of chemical and electronic interactions at the Sb2Ox/SnO2 and Sb2Ox/Al2O3 interfaces

This paper reports a study by X-ray photoelectron spectroscopy (XPS) and reflection electron energy loss spectroscopy (REELS) of very thin films of antimony oxide deposited on sputter-cleaned Al2O3 and SnO2 substrates. The work aims the determination of the stability of the different oxidation states of antimony and the characterisation of the electronic interactions generated at the interface between substrate and deposited phase. Oxidation of the deposited moieties of the antimony oxide with a plasma of oxygen leads to the formation of a mixed valence compound consisting of a mixture of Sb3+ and Sb5+ oxidation states. A quantitative evaluation of this partially oxidised phase by XPS was not possible because the highest oxidised state of antimony was unstable in vacuum under X-ray radiation. Despite this effect, it was possible to show that the remaining Sb5+ species depicted characteristic features in XPS and REELS spectra that can be clearly differentiated from Sb3+ related contributions. Stoichiometric Sb2O3 deposits were obtained by direct evaporation of this oxide. The characterization of the Sb2O3/MOy systems revealed that the electronic parameters determined through photoemission (i.e., binding energy (BE) of the photoemission peaks and Auger parameter (alpha')) are dependent on the amount of deposited oxide and on the type of substrate on which it is deposited. The changes, much larger on Al2O3 (DeltaBE = 0.7 eV, Deltaalpha' = -1.3 eV) than on SnO2 (DeltaBE = 0.2 eV, Deltaalpha' = -0.2 eV), have been discussed in the frame of the Wagner plots and the chemical state vector concept. It is also deduced that the electronic and bonding interactions developed at the interface between substrate and deposited phase determine the observed variations of these parameters as a function of surface coverage. The results with the Sb2O3 model system show that the BE or a' values cannot be taken as unequivocal parameter to assign oxidation states of cations in deposited oxides. (C) 2003 Elsevier Science B.V. All rights reserved.