Light-induced metal-insulator transition in SrTiO3 by photoresistance spectroscopy

Photoresistivity and its spectral response has been systematically studied in oxygen-deficient SrTiO3 single crystals for a wide range of resistivities rho and carrier densities n. At room temperature we found a persistent photoresistance that gradually decreases as rho is diminished or n is increased, in addition to relaxation times of seconds to a few minutes, suggesting that trapping of carriers is playing a major role. An analysis of the photoresistance excitation spectra showed two distinctive features that are related to the indirect gap of SrTiO3 at (3.25 +/- 0.04) eV and to a direct transition at (3.40 +/- 0.03) eV. The photoresistive crystals present a temperature-dependent resistivity under illumination that experiences a metal-insulator transition below T similar to 85 K. The low-temperature photoresistance spectrum reveals a suitable technique to understand the origin of this transition, pointing to an enhanced efficiency of the similar to 3.25 eV gap to promote electrons to the bottom of the conduction band.