Electric-field-induced point defect redistribution in single-crystal TiO2-x and effects on electrical transport

The spatial redistribution of non-stoichiometric point defects in rutile TiO2 is studied as a function of voltage and time. Single crystals are equilibrated initially to a well-defined stoichiometry with n-type conductivity and a carrier concentration on the order of 1018 cm−3. The crystals are subsequently electroded with Pt contacts that exhibit Schottky behavior. When subjected to an applied voltage of 15 V, a time-dependent increase and saturation in the leakage current is observed, which is associated with an accumulation of point defects and an attendant decrease in stoichiometry at the cathode electrode. This local change in stoichiometry degrades the Schottky barrier, leading to asymmetric electrodes and thus macroscopic rectifying behavior. Cathodoluminescence spectroscopy shows that Ti interstitials dominate the point defect redistribution process. Under larger applied voltages, of around 30 V, qualitatively different behavior is observed in which the resistivity increases as a function of time. This behavior is associated with condensation of point defects into a region of extended defects and Magnéli phases near the cathode, sufficient to increase the bulk stoichiometry and resistivity. These experiments demonstrate that a one-dimensional drift-diffusion process, as opposed to filamentary growth, dominates in these experimental conditions and that the Pt-TiO2-Pt system remains closed, with no significant oxygen transport across the Pt-TiO2 interfaces. We believe this is the first observation of a second higher-voltage regime in which the bulk stoichiometry and thus resistivity is increased as large concentrations of defects condense into metallic Magnéli phases in the near-electrode regions.