Crystal structure of TiO2 thin films grown on sapphire substrates by RF sputtering as a function of temperature

This study investigated the dependence of crystal structure on growth temperature in the TiO2 thin films deposited on c-, a-, and r-plane sapphire substrates by reactive RF magnetron sputtering. Deposition of the films was carried out at temperatures ranging from 400°C to 700°C. X-ray diffraction patterns revealed that TiO2 with a rutile structure was epitaxially grown on substrates independent of substrate orientations. TiO2 thin films were grown with a dominant peak of (200) on c-plane sapphire, and their crystallization and crystal quality were improved with growth temperature. For the films formed on a-plane and r-plane sapphires, the preferential orientation was [101]. However, the intensities of the (101) peak were very weak and were not dependent on growth temperature. The TiO2 thin films formed on the sapphire had a band gap of about 3.7 eV, which was larger than that of bulk (3.03 eV).