Comparison of electrical conductivity and optical properties of substoichiometrically and electrochemically coloured WO x films of different crystallinity

Abstract

Thin films of tungsten oxide were prepared by sputtering on 330°C hot as well as on unheated substrates in order to get different degrees of crystallinity. The crystallinity was characterized by X-ray-Diffraction Spectroscopy (XRD), Infra-Red-spectroscopy (IR) and Scanning Force Microscopy (SFM). Two kinds of coloration were compared, the electrochemical and the coloration by oxygen deficiency, i.e. reduction of the oxygen pressure during the sputtering. Electrical conductivity as well as transmittance and reflectance were measured at room temperature and for the substoichiometric films from 300 K down to 100 K. It turned out that the optical behaviour is the same for the two kinds of colorations, but not the electrical conductivity. The different crystallinities lead to a two-phase-model, according to which the sputtered films are constituted by crystallites with the size in the range of the film thickness surrounded by amorphous material, the substrate temperature increasing the number of the crystallites rather than the size. Even for films sputtered on unheated substrates it is necessary to consider a mixture of two phases. The stability and homogeneity of the substoichiometric samples allowed an evaluation of the optical constants n and k for different degrees of coloration and crystallinity. The conductivity measurements disprove an isolator–metal transition at increasing coloration at least in substoichiometrically sputtered films, but also in electrochemically coloured samples such a transition has to be doubted. An alternative model, assuming an impurity band with the fermi-energy located in a pseudo-gap, is suggested.