Optical properties of TiO2 thin films with crystal structure

The optical properties of TiO2 thin films with polymorphs were investigated by spectroscopic ellipsometry (SE) and transmittance measurements. Films were prepared by RF sputtering and e-beam evaporation and post-annealing, and their microstructures were examined by SEM and XRD. The results reveal that the crystal structure of the TiO2 film is strongly affected by the post-annealing temperature. The optical constant spectra of the films showed a gradual change above 3.5 eV. As the crystal structure of the film changes from amorphous phase to rutile phase, the extinction coefficient spectra are enhanced and shift toward lower energy. The direct band gap energies obtained by SE measurements were well matched with those obtained by transmittance measurements. The SE analysis was supported by comparison of the measured and calculated transmittances of the films, and both spectra showed good agreement over the entire spectral range.