Optical dispersion analysis of TiO2 thin films based on variable-angle spectroscopic ellipsometry measurements

Pure and doped TiO2 thin films, deposited by reactive r.f. sputtering onto glass substrates, have been investigated by spectroscopic ellipsometry in the wavelength range between 360 and 830 nm. Doping with Ce and Nb induces structural changes in TiO2 and modifies its optical constants. While undoped TiO2 films crystallize in a mixed rutile/anatase phase at a substrate temperature of 250°C, the doped films exhibit the anatase phase only. Using a polynomial and a single oscillator model dispersion function the spectral dependency of the refractive index and the extinction coefficient, as well as the optical band-gap have been determined. It turned out that even a moderate surface roughness in the range of 10 nm significantly influences the absolute value of the refractive index. Therefore, the surface roughness has been measured by atomic force microscopy and has been taken into account in the ellipsometric model.