Room-temperature persistent photoconductivity in strontium titanate

Strontium titanate (SrTiO$_{3})$ is an oxide material with unique properties and is often used as a substrate for oxide thin films such as high-temperature superconductors. Persistent photoconductivity (PPC) is investigated in SrTiO$_{3}$ single crystals at room temperature. Defects and impurities can have a strong effect on the electrical properties of SrTiO$_{3}$. Our prior work showed that hydrogen impurities form a defect complex in SrTiO$_{3}$ which we tentatively ascribed to a strontium vacancy passivated by two hydrogen atoms. The defect, alternatively, could be a partially passivated titanium vacancy. When a thermally treated sample is exposed to sub-bandgap light at room temperature, the free-electron concentration increases by over two orders of magnitude. After the light is terminated, the enhanced conductivity persists for several days, with negligible decay. We tentatively attribute the PPC to the excitation of an electron from a titanium vacancy defect into the conduction band, with a high barrier for recapture. The presence of titanium vacancies was investigated through positron lifetime measurements. Wavelength-dependence measurements showed an optical excitation threshold for photoconductivity of 2.9 eV.