The role of surface oxygen vacancies upon WO3 conductivity

Tungsten oxide thin films have been 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 have been investigated by reflection high energy electron diffraction and atomic force microscopy. The structure of the tungsten oxide is monoclinic and the top surface is preferentially (0 0 1) oxygen terminated plane with many oxygen vacancies, the density of which depends on the partial oxygen pressure. The resistivity of the WO3 thin films has been investigated as a function of temperature in various atmospheres. The activation energy for conduction deduced from the Arrhenius equation is found to depend on oxygen partial pressure. We interpret this behaviour with a “defect band” model which supposes that surface oxygen vacancies introduce donor levels in the gap and free electrons are produced by thermal activation. When the surface oxygen vacancy concentration increases the donor orbitals overlap and lead to the formation of a band which lessens the gap resulting in a decrease of the activation energy which can be inhibited for high vacancy concentration.