Effect of substrate structures on epitaxial growth and electrical properties of WO3 thin films deposited on (1¯012) and (0001) α-Al2O3 surfaces

The effect of surface structures of annealed (1¯012) and (0001) α-Al2O3 substrates on epitaxial growth and electrical properties of electron beam deposited WO3 thin films has been investigated. (0001) and (1¯012) α-Al2O3 surfaces were used in (1 × 1) stoichiometric and reconstructed forms. The structure and the morphology of WO3 films were determined by transmission electron microscopy (TEM), selected area electron diffraction (SAED) and reflection high energy electron diffraction (RHEED). Generally the films consist of micro-grains of monoclinic WO3 and the (010) planes are parallel to the substrate surfaces. Certain epitaxial relationships between WO3 films and the substrate surfaces were found. These phenomena are interpreted by nucleation and growth theories in relation to a variation of the density of surface oxygen vacancies of the α-Al2O3 substrates. The electrical conductivity of the WO3 films was measured as a function of annealed temperatures of the substrates. The activation energy for conduction deduced from the Arrhenius equation is found to be dependent on the grain size and the morphology of WO3 films.