TiO2 thin films prepared by sol–gel method for waveguiding applications: Correlation between the structural and optical properties

Thin films of transparent titanium oxide (TiO2) are prepared by the sol–gel dip-coating technique. Structural and optical properties of TiO2 thin films are investigated for different annealing temperatures and different number of coatings. X-ray diffraction (XRD) and Raman spectroscopy analysis show that the anatase crystalline phase appears beyond 350 °C for the four layers TiO2 film. At higher temperatures and for thicker films, we observe in addition to anatase the formation of brookite and rutile phases. The grain size calculated from XRD patterns increases as the temperature of annealing and number of dipping increase, from 11.9 to 17.1 nm for anatase and decreases as the number of dipping increases, from 24.2 to 10.2 nm for brookite. Film thickness, refractive index, and porosity are found to vary with treatment temperature and the number of coating. The obtained films are transparent in the visible range and opaque in the UV region. Waveguiding properties are studied using m-lines spectroscopy. The best results indicate that our films are monomodes TE0 at 632.8 nm with optical losses of 2 dB cm−1.