Light-induced metal-insulator transition inSrTiO3by photoresistance spectroscopy

Abstract

Photoresistivity and its spectral response has been systematically studied in oxygen deficient SrTiO3 single crystals for a wide range of resistivities, ρ, and carrier densities, n. At roomtemperature we have found a persistent photoresistance that gradually decreases as ρ 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 shows 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 resisitivity under illumination that experiences a metal-insulator transition below T ∼ 85 K. Lowtemperature photoresistance spectrum reveals as a suitable technique to understand the origin of this transition, pointing to an enhanced efficiency of the ∼ 3.25 eV gap to promote electrons to the bottom of the conduction band.

Surface effects on the photoconducting properties of SrTiO3 thin films

“…When illumination is applied, a temperature dependent metallic behavior is attained. This metal-insulator transition (MIT) induced by light has already been reported in bulk STO crystals 16 and it is related to the appearance of an optical phonon below T ' 77 K, 13,37 which enhances the efficiency of the indirect gap transition. We have also studied the photoconductivity spectrum at low-temperatures (T ¼ 37 K), and the results are 3.…”

“…It is believed that these shallow levels are related to oxygen vacancies, V O , which are generated during the fabrication and the post-annealing of the film. 12,16,[19][20][21][22] Hence, using the relation 35 NðE F Þ ¼ 18=k B T 0 a 3 H with an effective Bohr radius of a H ' 140 Å for STO, 36 it gives N(E F ) ' 4.4 Â 10 19 eV À1 cm À3 , which is a rather high value for an impurity band considering that for a metallic STO crystal 23 N(E F ) ' 5 Â 10 20 eV À1 cm À3 . When illumination is applied, a temperature dependent metallic behavior is attained.…”

“…12 Since STO has an UV indirect gap of $3.2 eV (whose transition involves the creation or destruction of a phonon in order to conserve the momentum) 13 and a direct one of $3.7 eV, 14 previous studies have explored the photoconducting properties of bulk crystals resulting in a persistent photoconductivity effect (PPC), 15,16 which is very dependent on the presence of deeplevels inside the gap generated by oxygen vacancies. 15,16 In thin films, it is expected that the surface effects becomes more pronounced and sensitive to external perturbations. In this sense, we have fabricated STO thin films in oxygen deficient conditions with reduced strain 17 and we have been able to tune their photoconducting properties by means of external parameters like excitation voltage, ambient conditions, and temperature.…”

“…Homoepitaxial STO thin films were grown by pulsed laser deposition (PLD) on metallic n-doped (001) SrTiO 3 substrates 12,16,18 at a temperature of 685 C and a base pressure of 7 Â 10 À5 Torr (see supplementary material for details). After the fabrication, the film was post-annealed at a temperature of 500 C and a pressure of 7 Â 10 À5 Torr during 30 min that is believed that generates a higher concentration of oxygen vacancies at the surface.…”

Review on fabrication methods of SrTiO3-based two dimensional conductive interfaces

“…Moreover, the annealing can effectively tune the electronic transport properties of STO and the resistance under irradiation is changed by the twelve orders of magnitude at 20 K. In addition, the oxygen-deficient oxides still exhibited many peculiar physical properties as same as the 2DEG at LAO/STO interfaces, such as persistent photoconductivity, [89,90] magnetism [91] or their coexistence. [92] Semi-logarithmic plots of the room temperature current-voltage characteristics measured in various reduced STO samples both in the dark (dashed lines) and under UV light illumination.…”