Optical and electrical properties of Ti-doped ZnO films: observation of semiconductor-metal transition

Ti-doped ZnO films have been prepared on Al2O3 (0001) substrates by RF magnetron sputtering of a target with Ti fraction of 10 at.% for varying substrate temperature (T-s) in the 100-400degreesC range. The Ti composition and carrier concentration (n) of the films are found to increase with increasing T-S, reaching 5.9 at.% and 1.2 X 10(20) cm(-3), respectively, for T-s = 400degreesC . Spectroscopic ellipsometry measurements on the films show that the band-gap energy increases for T-s = 100 and 200degreesC T front that of pure ZnO while it decreases for T-s =. 300 and 400degreesC. Such band-gap variation along with a large increase in n between T-s = 200 and 300degreesC T can be explained in terms of a merging of the donor and the conduction bands, leading to a semiconductor- metal transition. The increase of the donor density is interpreted as due to the increase of the defect density caused by the increased Ti doping. (C) 2002 Elsevier Science Ltd. All rights reserved.