Role of oxygen vacancies in anodic TiO₂ thin films

Defects play an important role in the electronic and optical properties of amorphous solids in general. Here we present both experimental and theoretical investigations on the nature and origin of defect states in anodic rutile TiO₂ thin films (thickness 5-20 nm). There is experimental evidence that the observed gap state at 0.7 eV below the edge of the conduction band is due to an oxygen vacancy. For this reason oxygen vacancies are used in our model. A comparison of the calculated bulk-photoconductivity to photospectroscopy experiment reveals that the films have bulk-like transport properties. On the other hand, a fit of the surface density of states to the scanning tunneling microscopy (STM) on the (001) surfaces has suggested a surface defect density of 5% of oxygen vacancies. To resolve this discrepancy, we calculated the DC-conductivity where localization effects are included. Our results show an impurity band formation at about p_c=9% of oxygen vacancies. We concluded that the gap states seen in STM are localized and the oxygen vacancies are playing the role of trapping centers (deep levels) in the studied films.