Heteroepitaxy of tungsten oxide films on sapphire and silicon for chemiresistive sensor applications

This work focuses on deposition of heteroepitaxial WO₃ films on sapphire and silicon substrates for use as chemiresistive sensing elements. The approach of using controlled heteroepitaxial growth leads to well defined nanostructured films that yield stable sensor response characteristics. Tetragonal, monoclinic, or hexagonal phase WO₃ films can be produced in pure form, and they can be modified with monolayer amounts of metal (Au, Ag) or metal oxide (CuO, SnO₂, TiO₂, SiO₂) surface catalysts to achieve selectivity towards different target gases. Structural control is achieved by using rf magnetron sputtering and electron cyclotron resonance (ECR) oxygen plasma-assisted evaporation methods coupled with in situ RHEED monitoring of film structure. Phase, crystallography, and stoichiometry are strongly dependent on deposition temperature, growth rate, and plasma gas composition, as well as post-deposition annealing treatments. Highly polished and insulating r-cut and c-cut sapphire substrates provide a reproducible template consisting of well-defined step-terrace structures that promote heteroepitaxial oxide growth under appropriate growth conditions. Alternatively, heteroepitaxial WO₃ growth on Si [100] substrates can be realized using ultrathin [111] oriented insulating BaF₂ buffer layers.