INVARIANCE OF THE MOBILITY EDGE IN ANODIC TITANIUM-OXIDES

We present a theoretical investigation to explain the electronic and optical properties of anodic rutile TiO2 thin films of different thicknesses (ranging from 5nm to 20nm). There is experimental evidence that the observed gap state at 0.7eV below the edge of conduction-band is due to an oxygen vacancy. For this reason, oxygen vacancies are used as defects in our model. A comparison of the calculated bulk-photoconductivity to photospectroscopy experiment reveals that the films have bulk-like transport properties with a bandgap E(g) = 3.0eV. On the other hand, a fit of the surface density of states to the scanning tunneling microscopy (STM) experiment 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 studied films have defect densities below the threshold of impurity band formation. As a consequence the gap states seen in STM are localized (i.e: the oxygen vacancies are playing the role of trapping centers, deep levels) and the mobility edge is invariant.