Stoichiometry and microstructure effects on tungsten oxide chemiresistive films

Highly oriented tungsten trioxide thin films have been grown on (0 1 2) r-cut sapphire substrates using reactive rf magnetron sputtering of a tungsten metal target in various oxygen/argon mixtures. Parameters for growth of stoichiometric tungsten trioxide (WO3) films were determined by X-ray photoelectron spectroscopy (XPS). In situ four-point Van der Pauw conductivity measurements were performed on as-grown films and after post-deposition annealing to study changes in the oxygen vacancy population. Reflection high-energy electron diffraction (RHEED) and X-ray diffraction (XRD) indicate that stoichiometric WO3 films deposited at 450°C are dominated by the tetragonal phase with (0 0 1) orientation along the growth direction, and films deposited at 650°C possess coexisting (0 0 2), (0 2 0), and (2 0 0) in-plane orientations of the monoclinic phase. Tetragonal and monoclinic-phase WO3 films exhibit a change in conductivity of 0.1 Ω−1 cm−1 to 20 ppm H2S at 250°C, but display different response kinetics.