Structure, composition and evolution of dispersive optical constants of sputtered TiO₂ thin films: effects of nitrogen doping

Films of nitrogen-doped TiO₂ have been successfully deposited on a Si substrate by radio frequency reactive sputtering in a mixture of argon, oxygen and nitrogen. The nitrogen gas ratio varies in the range 0.2–0.4 during the deposition, resulting in TiO_xN_y films with 3% ≤ y ≤ 6.55% as determined by x-ray photoelectron spectroscopy (XPS). Chemical bond state analysis by XPS indicates that nitrogen is effectively incorporated and produces an oxynitride centre as oxygen is replaced by nitrogen. Characterization by atomic force microscopy demonstrates that the incorporation of nitrogen has a significant effect on the morphology of the targeted TiO₂ thin films. Spectroscopic ellipsometry with a photon energy of 0.75–6.5 eV at room temperature has been carried out to derive the refractive index n and the extinction coefficient k on the basis of a new amorphous dispersive model. The optical constants such as absorption coefficient, complex dielectric functions and the optical band gap have been determined. The trend of a decrease in the optical band gap with an increase in nitrogen concentration is consistent with the observation determined by UV–visible spectroscopy. The reduced band gap is associated with the N 2p orbital in the TiO_xN_y films.