Structural and optical properties of rutile and anatase TiO 2 thin films: Effects of Co doping

Abstract

Pure and Co-doped TiO 2 thin films were grown on Al 2O 3(0001) substrates by reactive radio-frequency magnetron sputtering and sol–gel method. Rutile TiO 2 films oriented to the a-axis of the tetragonal structure were obtained by the sputtering while polycrystalline anatase TiO 2 films were obtained by the sol–gel method. Post-annealing of the deposited films in the air at 800 °C resulted in an improvement of the crystalline quality for both the rutile and anatase films. For Co doping up to 15 at.% the sputter-grown films maintained the a-axis-oriented rutile structure with the same lattice constant as in the pure TiO 2 film. Optical properties of the rutile and anatase TiO 2 films were measured by spectroscopic ellipsometry at room temperature in the 1.5–5 eV photon energy region. Band-gap energies of the rutile and anatase TiO 2 films are determined to be 3.25 and 3.75 eV, respectively. The band-gap energy of the Co-doped rutile TiO 2 films is found to increase with increasing Co composition.