Enhanced optical properties of Al-doped TiO2 thin films in oxygen or nitrogen atmosphere

Al-doped TiO2 (TiO2:Al) films were deposited by simultaneous RF magnetron sputtering of TiO2 and DC magnetron sputtering of Al. The advantage of this method is that the Al content could be independently controlled. By depositing in a mixed Ar–O2 or a mixed Ar–N2 atmosphere, the TiO2:Al film became more stoichiometric and the nanocrystallinity was enhanced. The nonlinear refractive index of TiO2:Al film deposited in a pure Ar, a mixed Ar–O2 or a mixed Ar–N2 atmosphere was measured by Moiré deflectometry, and was of the order of 10−8 cm2 W−1. For the TiO2:Al film deposited in a pure Ar atmosphere, the porosity was higher corresponding to the lower transmission. However, the porosity of TiO2:Al film decreased as the oxygen or nitrogen pressure increased. Especially, as the ratio of O2 to Ar pressure increased to 0.22, TiO2:Al film exhibited lower porosity, higher visible transmission, higher linear refractive index, lower stress and lower stress-optical coefficient.