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 (TS) in the 100–400 °C range. The Ti composition and carrier concentration (n) of the films are found to increase with increasing TS, reaching 5.9 at.% and 1.2×1020 cm−3, respectively, for TS=400 °C. Spectroscopic ellipsometry measurements on the films show that the band-gap energy increases for TS=100 and 200 °C from that of pure ZnO while it decreases for TS=300 and 400 °C. Such band-gap variation along with a large increase in n between TS=200 and 300 °C 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.