Heteroepitaxial growth of tungsten oxide films on sapphire for chemical gas sensors

Abstract

The performance of chemiresistive gas sensors made from semiconducting metal oxide films is influenced by film stoichiometry, crystallographic structure, surface morphology and defect structure. To obtain well-defined microstructures, heteroepitaxial WO 3 films were grown on r-cut and c-cut single crystal sapphire substrates using rf magnetron Ar/O 2 reactive sputtering of a W target. On r-cut sapphire, an epitaxial tetragonal WO 3 phase is produced at a 450°C deposition temperature whereas 650°C growth stabilizes an epitaxial monoclinic WO 3 phase. On c-cut sapphire, a metastable hexagonal WO 3 phase is formed. RHEED and X-ray diffraction indicate that the films have a ‘polycrystalline epitaxial structure’ in which several grains are present, each having the same crystallographic orientation. STM analysis of the film surfaces reveals morphological features that appear to be derived from the substrate symmetries. The monoclinic phase has a step/terrace growth structure, has the smallest mosaic spread in XRD rocking curves and exhibits the highest degree of reproducibility suggesting that it is the best suited for sensor applications. Measurements of film conductivity versus temperature indicate that the charge transport mechanisms are also dependent on the crystallographic phase and microstructure of the WO 3 films.