Spectroscopic study of nanocrystalline TiO2 thin films grown by atomic layer deposition

Photoluminescence characteristics of nanocrystalline TiO2 films grown by atomic layer deposition were studied. In dependence of growth conditions the films consisted of anatase, rutile and TiO2-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 TiO2-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. (C) 1998 Elsevier Science S.A. All rights reserved.