ELECTRICAL SURFACE VERSUS BULK PROPERTIES OF FE-DOPED TIO2 SINGLE-CRYSTALS

Thermopower measurements, impedance spectroscopy and work-function measurements of Fe-doped TiO2 single crystals were carried out under conditions (T, P-O2) which correspond to the near-stoichiometric composition domain. Fe3+ ions form acceptor centers: Fe'(Ti) which leads to a significant shift of the bulk n-p-type transition to lower oxygen partial pressures as compared with undoped materials. It was shown that Fe-doped TiO2 is a mixed conductor within the n-p-type transition domain. The surface of iron-doped TiO2 exhibits a shift of the transition region to lower P-O2 compared with undoped samples. In the transition region the surface properties are fully controlled by the acceptor type dopant. The increase of the work-function is enhanced by the strong iron segregation in oxidized condition, which was confirmed by SIMS depth profile analysis.