Influence of N-doping and air annealing on the structural and optical properties of TiO2 thin films deposited by reactive DC sputtering at room temperature

N-doped TiO2 thin films have been grown over glass substrates without intentional heating by DC sputtering of a metallic Ti target immersed in a reactive gas mixture of argon, oxygen and nitrogen. The oxygen partial pressure has been adjusted to achieve effective N-doping, intended to decrease the band gap energy with respect to analogous undoped layers. These N-doped TiO2 films are found amorphous by X-ray analysis, even after post-deposition heating in air up to 550 °C, whereas the analogous intrinsic TiO2 layers exhibit rutile crystalline structure before and after heating. The obtained results agree with the localized level model that has been developed for titania crystals and applies here for amorphous TiO2:N layers, which show an effective redshift of 0.88 eV with respect to the undoped samples. Nevertheless, the widest gap value of 3.22 eV is achieved for the several TiO2 and TiO2:N thin films after annealing in air at the highest temperature. This is attributed to the suppression of oxygen vacancies and Ti3+ oxidation states during such thermal treatment.