Influence of phase composition on optical properties of TiO2: Dependence of refractive index and band gap on formation of TiO2-II phase in thin films

Atomic layer deposition was applied to grow TiO2 thin films containing anatase, rutile and mixture of TiO2-II and rutile phases with the aim to investigate the dependence of optical parameters on the formation of metastable TiO2-II phase. The films were grown at a substrate temperature of 450 degrees C using TiCl4 and O-3 as the precursors. The crystal structure obtained depended on the substrates used. The films deposited on amorphous SiO2 substrates contained only the anatase phase when the film thicknesses did not exceed 88 nm while in thicker films, anatase with inclusions of rutile was formed. The films deposited on alpha-Al2O3 substrates contained epitaxial rutile without traces of other crystalline phases on alpha-Al2O3(0 1 (1) over bar 2) (r-sapphire) and epitaxial TiO2-II with inclusions of rutile phase on alpha-Al2O3(0 0 0 1) (c-sapphire) substrates. The films that contained TiO2-II had refractive indices of 2.8 at a wavelength of 500 nm and 2.61-2.67 at 633 nm. These values were higher than those of anatase films (2.6 and 2.48-2.49, respectively) and rutile films (2.74 and 2.57-2.62, respectively). Optical band gaps of 3.28-3.36 eV obtained for the films containing TiO2-II were by 0.20 +/- 0.08 eV wider than the band gaps of rutile films and by 0.06 +/- 0.05 eV narrower than the band gaps of anatase films. A band gap value of 3.00 +/- 0.08 eV was measured for a non-epitaxial film that was deposited on amorphous SiO2 and contained anatase with inclusions of rutile.