Microstructural analysis and electrical conductivity of hexagonal WO3 thin films during annealing

WO3 thin films have been deposited by thermal evaporation on (0001) surfaces of alpha-Al2O3 single crystal and annealed between 423 K and 623 K in air with a mean humidity of 50%. The structural and morphological properties of the films have been characterized by transmission electron microscopy and compatible selected area electron diffraction. During annealing, important changes in grain size and structure occur. As the annealing temperature increases slightly, the WO3 center dot 1/3H(2)O structure successively transforms to hexagonal WO3 and Magneli WO3-x phases. During these structural transformations, the grains are constructed of twinned microdomains in different directions with largest lattice parameters of WO3. The conductivity of the WO3 thin films has been investigated as a function of annealing temperature for each stable crystallographic phase. The activation energy for conduction deduced from the Arrhenius equation is found to be dependent on surface structure during various WO3 phases. (C) 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim