Growth and crystallographic characterization of molecular beam epitaxial WO3 and MoO3/WO3 thin films on sapphire substrates

•Single-crystalline WO3 and MoO3/WO3 films were grown on sapphire substrates by MBE. Molecular beam epitaxy of tungsten trioxide (WO3) on (011¯2)-oriented (r-plane) sapphire substrates and molybdenum trioxide (MoO3) on the WO3 was studied by focusing on their crystallogrhaphic properties. Although polycrystalline monoclinic (γ-phase) WO3 films were grown at 500°C and they became single-crystalline (001)-oriented γ-phase at 700°C, the latter films were oxygen-deficient from stoichiometry and contained dense and deep thermal etchpits. By using a two-step growth method where only the initial 15nm was grown at 700°C and the rest part was grown at 500°C, (001)-oriented γ-phase single-crystalline WO3 films with stoichiometric composition and smooth surface were obtained. On top of the 15-nm-thick WO3 initiation layer, (110)-oriented orthorhombic (α-phase) MoO3 films with smooth surface were obtained.