The role of hydrogen in the persistent photoconductivity of SrTiO₃

Strontium titanate (STO) is a perovskite oxide with an indirect bandgap of 3.25 eV at room temperature. It has been often used as substrate to grow other materials, and it is the basis of widely reported perovskite heterostructures that show high-density two-dimensional electron gas. Recently, an extremely long persistent photoconductivity (PPC) at room temperature has been observed in annealed strontium titanate single crystals upon illumination with sub-gap light [M. C. Tarun, F. A. Selim, and M. D. McCluskey,Phys. Rev. Letters, 111, 187403 (2013)]. Annealed studies indicate the importance of hydrogen impurities as well as oxygen vacancies to induce PPC. We use electronic structure calculations based on density functional theory (DFT) to investigate possible mechanisms behind the PPC effect. We present results of defect formation energies, defect-related absorption, and defect migration barriers, in an attempt to identify the defect associated with the observed PPC in STO. The results are combined to explain the experimental observations, and we further generalize the model to predict similar effects in other oxide like TiO₂ and BaTiO₃.