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

Highlights: •Crystalline α-(Cr_1-xAl_x)₂O₃ (0.08 ≤ x ≤ 0.16) were grown on α-Al₂O₃ at 320 °C and 400 °C. •The films deposited at room temperature are amorphous. •The Raman and infrared vibration are dependent on the lattice constant c. •The band gap of the thin films is between 2.72 eV and 2.85 eV. -- Abstract: We report the optical properties of the (Cr_1-xAl_x)₂O₃ thin films (0.08 ≤ x ≤ 0.16) with a thickness of ~300 nm grown on c-plane (0001) α-Al₂O₃ substrates at different temperatures (i.e. room temperature, 320 °C and 400 °C) by non-reactive radio-frequency magnetron sputtering. (0001) oriented single-phase corundum structured crystalline thin films were grown at temperatures of 320 °C and 400 °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 Al 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.