Investigations on the structural and optical properties of RF magnetron sputtered TiO2 thin films: effect of substrate temperature and Ar:O2 ratio

TiO2 thin films were deposited onto quartz substrates by RF magnetron sputtering at different substrate temperature and Ar:O-2 ratios using TiO2 ceramic target and then annealed at 873 K for 2 h. The films deposited at low substrate temperature (ST) were amorphous-like but possessed short range order as shown by the Raman shifts. In films prepared by sputtering technique, we have found that better crystallinity to anatase phase is possible by thermal annealing only if films are deposited at suitable substrate temperature. Though band gap was found to increase with both substrate temperature and Ar:O-2 ratios, better tunability of band gap from 3.61-3.78 eV was achieved by increasing the Ar:O-2 ratio. The increase in optical band gap from that of the bulk in both cases is attributed to quantum confinement effect. Refractive index decreased when substrate temperature was increased upto 473 K after which it showed an increase. Increase of oxygen content resulted in the decrease of refractive index from 2.56 to 2.38, which is attributed to the variation of the packing density. It was observed that substrate temperature and Ar:O-2 ratio strongly affected the PL spectra of the films.