Bias voltage induced-electrical conductivity at electrode interfaces of a rutile TiO[formula omitted] single crystal: Two different mechanisms depending on temperature

Abstract

The electric field-induced conductivity of TiO2 is important in various emerging practical applications, including the recently proposed selective trace hydrogen sensing. In the present study, the bias voltage-induced electrical characteristics were investigated between 295 K (ambient temperature) and 600 K for a semi-insulated rutile TiO2 single crystal. The appearance of conductivity by elevating the temperature was shown to be due to the electromigration of mobile dopants including defects. It was found that there are two different mechanisms causing the conductivity depending on the temperature. At low temperature, close to room temperature, hydrogen migrating primarily along the open [001] channel triggers the conductivity. On the other hand at high temperature above 450 K, the migration of oxygen vacancy (VO) perpendicular to [001] induces the conductivity. It is proposed that VO is injected at the anode interface, which then migrates to the cathode interface and aggregates there. It was observed that the VO injection is not accompanied by the formation of the Magnèli phase. The proposed mechanism was examined by spectroscopic techniques such as time-of-flight secondary ion mass spectrometry and hard X-ray photoemission spectroscopy.