Post-deposition annealing effect on RF-sputtered TiO2 thin-film properties for photonic applications

Abstract

Titanium dioxide (TiO2) thin films were grown on glass substrates at room temperature using RF magnetron sputtering technique. Effect of the post-annealing for 1 h at 400–600 °C on the structural, morphological, optical and waveguide properties was investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), UV–visible spectrophotometry and m-lines spectroscopy (MLS). XRD studies show that as-grown and post-annealed TiO2 films exhibit (101) XRD peak corresponding to the anatase phase of TiO2. Higher annealing temperatures result in a significant increase in crystallinity. The grain size values were calculated and found to be about 15–37 nm. From the analyses made on the SEM micrographs and AFM images, it was revealed that the morphology and surface roughness of the thin films were influenced by the heat treatment temperature. The UV–visible spectroscopy analyses show that as-grown TiO2 films were transparent in the visible region with an average transmittance of more than 75 % and the transmittance decreases slightly with an increase in annealing temperature. Annealed TiO2 films also exhibit an increase in the values of direct optical band gap. MLS measurements at 633-nm wavelength put into evidence that TiO2 planar waveguides demonstrate a well-guided fundamental mode for both transverse electric and transverse magnetic polarized light. Moreover, the refractive index was found to increase with temperature and to approach to the anatase TiO2 single-crystal value for the TiO2 film annealed at 600 °C.