The structure and electrical conductivity of vacuum-annealed WO 3 thin films

Abstract

We have investigated the surface structure of monoclinic WO 3 thin films on α-Al 2O 3(0001) substrates during thermal treatments in ultra high vacuum and oxygen exposure. For an annealing temperature of 450 °C, reflection high-energy electron diffraction shows the formation of a second WO 3 phase with a pseudo cubic structure and the partial reduction to metallic tungsten clusters on the WO 3 surface. Electron energy loss spectrometry confirms these changes of the surface structure. The reverse process, i.e. the oxidation of W, occurs if the samples are heated in air at 450 °C. Under oxygen exposure films in the monoclinic WO 3 phase follow a semiconductor like behaviour with an activation energy of −0.34 eV. Reduced WO 3 films exhibit a metal-like conductivity with an activation energy of approximately zero. The temperature-dependent conductivities as well as the conductivity effects that we found upon CO exposures to the WO 3 thin films depend much on the thermal pre-treatments and the associated surface structures. CO exposure leads to an increase of the conductivity; this acceptor-type interaction of CO with samples that have been annealed at 450 °C is explained by a dissociation into carbon and oxygen, the latter interacting with oxygen vacancies.