Optical properties of radio-frequency magnetron sputtered α-(Cr1-xAlx)2O3 thin films grown on α-Al2O3 substrates at different temperatures

We report the optical properties of the (Cr1-xAlx)(2O3) thin films (0.08 >= x >= 0.16) with a thickness of similar to 300 nm grown on c-plane (0001) alpha-Al2O3 substrates at different temperatures (i.e. room temperature, 320 degrees C and 400 degrees C) by non-reactive radio-frequency magnetron sputtering. (0001) oriented single-phase corundum structured crystalline thin films were grown at temperatures of 320 degrees C and 400 degrees C, while the films deposited at room temperature are amorphous. The stoichiometric composition of the films was analyzed and verified by electron probe micro-analyses. The phase composition and lattice parameter c were characterized by X-ray diffraction in Bragg-Brentano geometry. The crystal quality increases with increasing the deposition temperature independent of the film composition, and the lattice constant c increases with decreasing the Al concentration which is independent of the deposition temperature. Raman spectra supported the interpretation of phase composition analysis and showed a significant shift of phonon frequency with A1 concentration and the substrate temperature. Infrared reflection absorption spectroscopy of the films was carried out to determine the absorption band shifts in relation to the lattice parameter c which the smaller lattice constant c the larger frequency of the E-u mode. Further, the band gap of the thin films was calculated from visible light absorption spectra.