Controlled growth of WO3 films

We have used reactive rf magnetron sputtering of a tungsten target in Ar/O-2 mixtures and direct electron-beam evaporation of WO3 pellets with and without the presence of an electron cyclotron resonance (ECR) oxygen plasma to grow WO3 films on alpha-Al2O3(10<(12)over bar>) single-crystal substrates (r-cut sapphire). The WO3 films exhibit a range of microstructures depending on deposition conditions. Using any of the deposition methods, the films are amorphous when grown at room temperature. Postdeposition annealing in O-2 induces the formation of a random polycrystalline microstructure and an increased surface roughness. Growth of crystalline WO3 films can be achieved at deposition temperatures above 200 degrees C. During electron-beam evaporation of WO3 at 600 degrees C, reflection high-energy electron diffraction observations indicate that a tetragonal phase of WO3 grows epitaxially on r-cut sapphire with the (100) tetragonal plane coincident with the rectangular mesh of the r-cut sapphire substrate. Deposition of WO3 using plasma species generated by either a rf magnetron or ECR source promotes the formation of some nuclei of an orthorhombic and/or monoclinic phase coexisting with the tetragonal phase; the nearly identical lattice parameters make the orthorhombic and monoclinic phases difficult to distinguish Compared to the polycrystalline films, the epitaxial films exhibit both a higher base-line conductivity and a larger conductivity change upon exposure to H2S gas. (C) 1997 American Vacuum Society.