Electronic structure of thermally oxidized tungsten

The electronic structure of a pure tungsten surface oxidized at an oxygen pressure of 1 Torr and a temperature of 1000 K has been in situ studied by photoelectron spectroscopy in ultrahigh vacuum. Photoemission spectra from the valence band and O 1$s$, O 2$s$, and W 4$f$ core levels at synchrotron excitation in the photon energy range of 80–600 eV have been analyzed. A semiconductor tungsten oxide film is found to form, which contains different oxides of tungsten with the oxidation state from 6+ to 4+. Oxides of tungsten with the oxidation state of 6+ are mainly formed on the surface; their fraction gradually decreases while moving away from the surface, while the amount of oxides of tungsten with the oxidation state of 4+ increases.