Band-gap narrowing of TiO2 films induced by N-doping

N-doped TiO2 films were deposited on n(+)-GaN/Al2O3 substrates by reactive magnetron sputtering and subsequently crystallized by annealing at 550 degrees C in flowing N-2 gas. The N-doping concentration was similar to 8.8%, as determined from X-ray photoelectron spectroscopy measurements. Deep-level optical spectroscopy measurements revealed two characteristic deep levels located at 1.18 and 2.48 eV below the conduction band. The 1.18 eV level is probably attributable to the 0 vacancy state and can be active as an efficient generation-recombination center. Additionally, the 2.48 eV band is newly introduced by the N-doping and contributes to band-gap narrowing by mixing with the O 2p valence band. (c) 2006 Elsevier B.V. All rights reserved.