Electron Trapping and Detrapping in an Oxide Two-Dimensional Electron Gas: The Role of Ferroelastic Twin Walls

Abstract

The choice of electrostatic gating over the conventional chemical doping for phase engineering of quantum materials 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 SrTiO 3 (STO)-based two-dimensional electron gases. Such peculiar behavior is associated with electron trapping under an external electric field. However, the microscopic nature of the trapping centers remains an open question. In this paper, we investigate electric-field-induced chargetrapping and charge-detrapping phenomena at the conducting interface between the band insulators γ -Al 2 O 3 and STO. Our transport measurements reveal that the charge trapping under a positive back-gate voltage (V g ) above the tetragonal-to-cubic structural transition temperature (T c ) of STO has a contribution from electric-field-assisted thermal escape of electrons from the quantum well, and from clustering of oxygen vacancies as well. We observe an additional source of trapping below T c , which arises from the trapping of free carriers at ferroelastic twin walls in the STO. Application of a negative V g results in charge detrapping, which vanishes above T c . This feature demonstrates the crucial role of structural domain walls in the electrical transport properties of STO-based heterostructures. The number of charges trapped (detrapped) at (from) a twin wall is controlled by the net polarity of the wall and is completely reversible with a sweep of V g .

Non-volatile electric control of spin-orbit torques in an oxide two-dimensional electron gas

“…As seen in Fig. 2b , the R s contrast decreases upon increasing the temperature until full extinction of the Rs contrast at about 105 K, corresponding to the antiferrodistortive transition from cubic to the tetragonal phase of SrTiO 3 31 , in agreement with others charge trapping studies in SrTiO3-based 2DEG 30 . The dynamic switching between the high and low-resistivity states was studied using a sequence of pulsed gate-electric field.…”

“…When applying a positive gate-electric field, the 2DEG is enriched in electrons, until reaching a saturated low 2DEG resistivity state. Some electrons gets trapped within the SrTiO 3 , very probably on oxygen vacancies located at the vicinity of the 2DEG 29 , 30 . Because of these trapped electrons, the 2DEG is easily depleted when the gate-electric field is swept from positive to negative.…”

Non-collinear and asymmetric polar moments at back-gated SrTiO3 interfaces

“…An important role is also played by the twodimensional electron gas (2DEG) whose electrons can get trapped at the oxygen vacancy clusters and thereby reduce their effective charge and the resulting local electric fields. Such a trapping/detrapping of the 2DEG electrons at the oxygen vacancies, and in particular, at the AFD domain boundaries has been observed in recent transport experiments and calculations [41][42][43][44][45][46] . The local electric field in the vicinity of the interface is therefore determined by the interplay between the electromigration and clustering of the positive oxygen vacancies at the AFD domain boundaries on the one hand and the trapping/detrapping of some of the 2DEG electrons on the other hand.…”