Persistent photoconductivity in barium titanate

Annealed bulk crystals of barium titanate (BaTiO3) exhibit persistent photoconductivity (PPC) at room temperature. Samples were annealed in a flowing gas of humid argon and hydrogen, with a higher flow rate corresponding to larger PPC. When exposed to sub-bandgap light, a broad infrared (IR) absorption peak appears at 5000 cm−1 (2 μm), attributed to polaronic or free-carrier absorption from electrons in the conduction band. Along with the increased IR absorption, electrical resistance is reduced by a factor of approximately two. The threshold photon energy for PPC is 2.9 eV, similar to the case of SrTiO3. This similarity suggests that the mechanisms are similar: an electron in substitutional hydrogen (HO) is photoexcited into the conduction band, causing the proton to leave the oxygen vacancy and attach to a host oxygen atom. The barrier to recover to the ground state is large such that PPC persists at room temperature.