In situ core-level and valence-band photoelectron spectroscopy of reactively sputtered tungsten oxide films

In this paper, we study tungsten oxides deposited by reactive magnetron sputtering. The total working pressure during deposition was varied in order to obtain different morphology. The effects on the electronic properties and chemical composition were studied by XPS and UPS. It was observed that, for the same argon to oxygen ratio in the gas feed, the decrease in total working pressure implies a decrease of the oxygen content in the film. In the nearly stoichiometric WO3 films, W 4f5/2 and W 4f7/2 form a distinct doublet peak. In sub-stoichiometric films, the films do not exhibit a well-resolved doublet and suggest multiple oxidation states of tungsten.The valence-band spectra of the sub-stoichiometric samples present an additional feature below the Fermi edge (~0.5 eV). This peak is assigned to W 5d1 because of the presence of W5+. It is consistent with the changes on the core-level spectra. XPS results, UPS features, and visual aspect are in agreement and suggest that the total working pressure has a strong influence on the oxygen content and therefore on the oxidation state of tungsten in the films.