Photoconducting state and its perturbation by electrostatic fields in oxide-based two-dimensional electron gas

The two-carrier transport model as proposed for the two-dimensional electron gas formed at the interfaces of oxide heterostructures is investigated by means of a combined perturbation by near-ultraviolet radiation and an electrostatic field, applied both separately and simultaneously. Comparison of the photoresponse of prototype systems such as the band insulator LaAlO3 and Mott insulator LaTiO3 films on TiO2-terminated SrTiO3 shows remarkably similarities. Two types of nonequilibrium carrier are generated in each system, each having the signature of a particular type of perturbation characterized by distinctly different relaxation processes. While the photoconducting state diminishes in a stretched exponential manner, with a temperature-dependent activation energy varying from a few tens of meV to approximate to 1 to 2 meV on lowering the temperature and a relaxation time of several hours, the recovery from electrostatic gating occurs on the millisecond time scale. An attempt is also made to explain the experimental observations using ab initio density functional calculations. The calculations show that the electronic transitions associated with near-ultraviolet radiation emerge from bands located at similar or equal to 2 eV above and below the Fermi energy, which are the Ti 3d states of the SrTiO3 substrate and of the AlO2 (TiO2) layers of the LaAlO3 (LaTiO3) films, respectively. The slow decay of the photocurrent to the unperturbed state is explained in terms of the closely spaced Ti 3d(xy) states in the lower conduction band, which are manifested as flatbands (or localized states) in the band structure. Such localization leads to increased carrier lifetimes, through the energy-time relationship of the uncertainty principle.