Spectroscopic study of nanocrystalline TiO_2 thin films grown by atomic layer deposition

摘要

Photoluminescence characteristics of nanocrystalline TiO_2 films grown by atomic layer deposition were studied. In dependence of growth conditions the films consisted of anatase, rutile and TiO_2-II phases. A broad band and two sharp peaks were observed in the photoluminescence spectra measured under continuous-wave Ar~+ laser excitation at temperatures 5-165 K. At 5 K the maximum of the broad band was at 2.24 eV in the anatase films, and at 2.37-2.40 eV in the rutile and TiO_2-II films. The intensity of sharp lines peaking at 3.31 and 3.37 eV depended on the crystal structure of the films and increased significantly after X-ray irradiation. The temperature dependence and decay times of different emission bands were also investigated. The data obtained allowed a defect-trapped-exciton interpretation of the sharp peaks although the free-exciton origin of the 3.31 eV peak could still be argued. The broad-band emission at 2.24-2.40 eV was obviously due to self-trapped exciton recombination.