Comparative microscopic and spectroscopic analysis of temperature-dependent growth of WO₃ and W_0.95Ti_0.05O₃ thin films

We present a comparative microscopic and spectroscopic study of the morphology and composition of WO₃ and W_0.95Ti_0.05O₃ 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 W_0.95Ti_0.05O₃ 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 WO_3−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.