Deep-level optical spectroscopy investigation of N-doped TiO₂ films

N-doped TiO₂ films were deposited on n⁺-GaN/Al₂O₃ substrates by reactive magnetron sputtering and subsequently crystallized by annealing at 550 °C in flowing N₂ gas. The N-doping concentration was ∼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 O 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.