Electric field induced charge trapping/detrapping in SrTiO₃ based two dimensional electron gas

The choice of electrostatic gating over the conventional chemical doping is attributed to the fact that the former can reversibly tune the carrier density without affecting the system’s level of disorder. However, this proposition seems to break down in field-effect transistors involving SrTiO₃ (STO) based two-dimensional electron gases. Such peculiar behavior is associated with the electron trapping under the external electric field. However, the microscopic nature of trapping centers remains an open question. Our work on the conducting interface between defect spinel oxide γ- Al₂O₃ and cubic perovskite STO reveals that the charge trapping under +ve back gate voltage (V_g) above the tetragonal to cubic structural transition temperature (T_c) of STO is contributed by the electric field-assisted thermal escape of electrons from the quantum well, and the clustering of oxygen vacancies (OVs). Interestingly, an additional source of trapping emerges below T_c, which arises from the trapping of free carriers at the ferroelastic twin walls of STO. Application of -ve V_g results in a charge detrapping from the twin wall, which also vanishes above T_c. The amount of trapped/detrapped charges at the twin wall is controlled by the net polarity of the wall and scales almost linearly with V_g.