Ultraviolet-visible absorption spectra of N-doped TiO2 film deposited on sapphire

Abstract

The optical-response properties of nitrogen(N)-doped titanium dioxide (TiO2) films are investigated by means of a combination of ultraviolet-visible absorption spectroscopy and first-principles density-functional calculations. The TiO2 films were epitaxially grown on the sapphire substrate by the pulsed laser deposition method. The doping of N atoms was achieved by 70 keV of N+ ion implantation, followed by postirradiation heat treatment at 550 degrees C for 2 h in air. We find that when 5x10(16) (1x10(17)) N ions/cm(2) were implanted into the epitaxially grown TiO2 film, the absorption edge is reproducibly shifted to lower energy by about 0.06 (0.12) eV together with a significant optical absorption extending into the visible-light region. These experimental data can be explained by our calculated band structure of N-doped TiO2, where the bands originating from N 2p states locate above the valence band edge, while the band gap narrowing due to the mixing of N with O 2p states is 0.04 eV.