Structural and optical properties of rutile and anatase TiO2 thin films: Effects of Co doping

Pure and Co-doped TiO2 thin films were grown on Al2O3(0001) substrates by reactive radio-frequency magnetron sputtering and sol-gel method. Rutile TiO2 films oriented to the a-axis of the tetragonal structure were obtained by the sputtering while polycrystalline anatase TiO2 films were obtained by the sol-gel method. Post-annealing of the deposited films in the air at 800 deg. 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 TiO2 film. Optical properties of the rutile and anatase TiO2 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 TiO2 films are determined to be 3.25 and 3.75 eV, respectively. The band-gap energy of the Co-doped rutile TiO2 films is found to increase with increasing Co composition