Experimental and theoretical investigations were made of the nature and origin of defect states in anodic rutile TiO2 thin films (thickness: 5 to 20nm). There was experimental evidence that the observed gap state at 0.7eV below the edge of the conduction band was due to an oxygen vacancy. For this reason oxygen vacancies were used in the present model. A comparison of the calculated bulk-photoconductivity to photospectroscopy experiment revealed that the films had bulk-like transport properties. On the other hand, a fit of the surface density of states to the scanning tunnelling microscopic data on the (001) surfaces suggested a surface defect density of 5% of oxygen vacancies. To resolve this discrepancy, the DC-conductivity where localization effects were included was calculated. The results revealed impurity band formation at about 9% of oxygen vacancies. It was concluded that the gap states seen in scanning tunnelling microscopy were localized and that the oxygen vacancies played the role of trapping centers (deep levels) in the studied films.

Role of Oxygen Vacancies in Anodic TiO2 Thin Films. Tit, N., Halley, J.W., Michalewicz, M.T., Shore, H.: Applied Surface Science, 1993, 65-66, 246-51