Tungsten oxide thin films were prepared either by reactive sputtering or by thermal evaporation on SiO2 or Al2O3 and stabilized by annealing in dry air. The morphology and the surface structure were investigated by reflection high energy electron diffraction and atomic force microscopy. The structure of the tungsten oxide was monoclinic and the top surface was preferentially (001) oxygen terminated plane with many O vacancies, the density of which depends on the partial O pressure. The resistivity of the WO3 thin films was investigated as a function of temperature in various atmospheres. The activation energy for conduction deduced from the Arrhenius equation was found to depend upon the O partial pressure. This behavior was interpreted in terms of a so-called defect-band model which supposed that surface O vacancies introduced donor levels in the gap and that free electrons were produced by thermal activation. When the surface O vacancy concentration increased, the donor orbitals overlapped and led to the formation of a band which lessened the gap; resulting in a decrease in the activation energy which could be inhibited at high vacancy concentrations.

The Role of Surface Oxygen Vacancies in WO3 Conductivity. M.Gillet, C.Lemire, E.Gillet, K.Aguir: Surface Science, 2003, 532-535, 519-25