Spectroscopic analysis of temperature dependent growth of WO3 and W0.95Ti0.05O3 thin films

We present a comparative spectroscopic study of the morphology and composition of tungsten oxide WO$_{3}$ and W$_{0.95}$Ti$_{0.05}$O$_{3}$ thin films, grown by radio frequency magnetron reactive sputtering at substrate temperatures varied from room temperature (RT) to 500~$^{\circ}$C, using Raman and X-ray photoelectron spectroscopy (XPS). The Raman results demonstrate the occurrence of a phase transformation from a monoclinic WO$_{3}$ structure to an orthorhombic or tetragonal configuration in the W$_{0.95}$Ti$_{0.05}$O$_{3}$ thin films. This remark is based on the observed shifting, with Ti doping, to lower frequencies of the Raman peaks corresponding to W-O-W stretching modes of WO$_{3}$ at 806 and 711 cm$^{-1}$, to 793 and 690 cm$^{-1}$, respectively. Also, higher growth temperatures are required to obtain crystalline microstructure for Ti-doped WO$_{3}$ films than for WO$_{3}$ films. XPS data indicate that the doped material has a reduced WO$_{3-x}$ stoichiometry at the surface, with the presence of W$^{+6}$ and W$^{+5}$ tungsten oxidation states; this observation could also be related to the existence of a different structural phase of this material, corroborating with the Raman measurements.