Effects of nitrogen doping on optical properties of TiO2 thin films

The potential for extending the optical absorption range of TiO2 by doping with nonmetallic elements was examined in nitrogen-containing TiO2 thin films. Thin films of TiO2-xNx were synthesized on glass and silicon substrates by ion-beam-assisted deposition to obtain a wide range of nitrogen concentrations. The compositions of the films were determined by Rutherford backscattering spectrometry and X-ray photoelectron spectroscopy. The structures of the films were analyzed by X-ray diffraction, transmission electron microscopy, and atomic force microscopy. The optical properties of the films were measured by UV-Vis spectroscopy and ellipsometry. A characteristic decreasing trend in band-gap values of the films was observed within a certain range of increasing dopant concentrations. As the nitrogen concentration increased, the structure of the films evolved from a well-defined anatase to deformed anatase. The reduced band gaps are associated with the N 2p orbital in the TiO2-xNx films.