Electrical and optical spectroscopic studies of TiO2 anatase thin films deposited by sputtering show that the metastable phase anatase differs in electronic properties from the well-known, stable phase rutile. Resistivity and Hall-effect measurements reveal an insulator–metal transition in a donor band in anatase thin films with high donor concentrations. Such a transition is not observed in rutile thin films with similar donor concentrations. This indicates a larger effective Bohr radius of donor electrons in anatase than in rutile, which in turn suggests a smaller electron effective mass in anatase. The smaller effective mass in anatase is consistent with the high mobility, bandlike conduction observed in anatase crystals. It is also responsible for the very shallow donor energies in anatase. Luminescence of self-trapped excitons is observed in anatase thin films, which implies a strong lattice relaxation and a small exciton bandwidth in anatase. Optical absorption and photoconductivity spectra show that anatase thin films have a wider optical absorption gap than rutile thin films.

Photoemission spectromicroscopy was used to investigate the electronic structure of TiO2 anatase single crystals and polycrystalline thin films. The stoichiometry and the degree of oxidation of as-grown crystals, as-deposited films, as well as of thermally annealed samples in different atmospheres, were analyzed, based on the Ti 2p and O 1s core levels, with an energy resolution of 0.4 eV. The experimental density of states (DOS) was found to be in agreement with the theoretical DOS reported in the literature for anatase crystals, and shows some characteristics similar to the experimental DOS reported for rutile crystals. In reduced samples, the experimental DOS is characterized by intense emission in the region of O 2p bonding orbitals, and does not exhibit an appreciable density of states in the band gap. As-grown crystals exhibit small band gap emission (a few percent of the valence band VB signal) at about 0.8 eV, which is attributed to Ti3+ (3d) defect states. Annealing the crystals at high temperatures in O2 or subsequent thermal reduction in an Ar–H2 mixture (95%–5%) produces nearly stoichiometric surfaces with smaller or undetectable density of Ti3+ states. In addition, some redistribution of the spectral weight is observed in the VB spectra.

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