Mechanism of the irreversible back gate doping at the LaAlO₃/SrTiO₃ Interface

The two-dimensional electron gas (2DEG) formed at the LaAlO₃/SrTiO₃ (LAO/STO) interface exhibits rich physical properties such as superconductivity and magnetism. Additionally, due to the large dielectric constant of STO, the 2DEG can be controlled by a back gate voltage (V_G). When applying V_G to the system, a commonly observed phenomenon is that the sheet resistance follows an irreversible route when V_G is swept first forward and then backward. The explanation as given by Biscaras et al. [1] is that the Fermi energy is close to the top of the quantum well (QW) and high mobility electrons escape the QW and get trapped in STO when the carrier density goes beyond a critical value.
In this work, we study the mechanism of the irreversible field-effect doping by means of a back gate. By combining magnetotransport data and self-consistent Schroedinger-Poisson calculations, we come to a different picture. There is no critical carrier density for electron trapping but rather applying any positive V_G triggers it. Moreover, the analysis shows that it is not the mobile charges which get trapped but that the additionally injected electrons immediately find defects. This appears to be a universal phenomenon in STO-based heterostructures.

[1] Biscaras et al. Sci. Rep. 4, 6788, (2014).