Comparative microscopic and spectroscopic analysis of temperature-dependent growth of WO3 and W0.95Ti0.05O3 thin films

We present a comparative microscopic and spectroscopic study of the morphology and composition of WO3 and W0.95Ti0.05O3 thin films, grown by radio-frequency magnetron reactive sputtering at substrate temperatures varied from room temperature to 500 °C, using atomic force microscopy (AFM), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). With increasing growth temperature, the AFM results show increase in the average crystallite size and in the surface roughness for both undoped and doped samples. The AFM data, along with the Raman results, clearly indicate that for the given set of experimental conditions, higher growth temperatures are required to obtain crystalline Ti-doped WO3 films than for WO3 films. Also, the Raman results suggest a potential phase transformation from a monoclinic WO3 structure to an orthorhombic, but more probably a tetragonal, configuration in the W0.95Ti0.05O3 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 WO3 at 806 and 711 cm−1 to 793 and 690 cm−1, respectively. XPS data indicate that the doped material has a reduced WO3−x stoichiometry at the surface, with the presence of W6+ and W5+ oxidation states; this observation could also be related to the existence of a different structural phase of this material, corroborating with the Raman measurements.