Relation between Polarization Phenomenon and Space-Charge-Limited Currents in Rutile Single Crystals (TiO2)

A considerable depolarization current was observed in rutile single crystals. It was ascribed to electrons trapped at the positive electrode in levels which probably exist only in superficial layers of the crystal. The charge piled up corresponds to, at minimum, ${10}^{14}/{\mathrm{cm}}^2$ elementary charges e. The dependence of the depolarization current on the depolarization time, $\text{I\hspace{0.17em}∼\hspace{0.17em}1/t}$ , could be explained by assuming that the time dependence is governed by the probability of electron transitions from traps to the conduction band and these under the influence of the strong electric field of the space charge. Assuming that there are also ${10}^{14}/{\mathrm{cm}}^2$ ionized donors at the negative electrode, the observed current-voltage characteristics in thin flakes can be explained by taking into account that this gives rise to a field of the order of ${10}^6\hspace{0.17em}\mathrm{V/cm}$ at the negative electrode. Electrons then can enter the crystal by field emission from the negative electrode, but the current will be space-charge-limited in the bulk of the crystal. This explains the observed ${\mathrm{I\hspace{0.17em}\sim \hspace{0.17em}V}}^2$ dependence.