Electron Trapping Mechanism in LaAlO3/SrTiO3 Heterostructures

Abstract

In LaAlO 3 =SrTiO 3 heterostructures, a still poorly understood phenomenon is that of electron trapping in back-gating experiments. Here, by combining magnetotransport measurements and self-consistent Schrödinger-Poisson calculations, we obtain an empirical relation between the amount of trapped electrons and the gate voltage. The amount of trapped electrons decays exponentially away from the interface. However, contrary to earlier observations, we find that the Fermi level remains well within the quantum well. The enhanced trapping of electrons induced by the gate voltage can therefore not be explained by a thermal escape mechanism. Further gate sweeping experiments strengthen that conclusion. We propose a new mechanism which involves the electromigration and clustering of oxygen vacancies in SrTiO 3 and argue that such electron trapping is a universal phenomenon in SrTiO 3-based two-dimensional electron systems.

Irreversible Multi‐Band Effects and Lifshitz Transitions at the LaAlO3/SrTiO3 Interface Under Field Effect

“…This indicates that major temperature effects are saturated below 20 K. The data presented in Figure 2 substantially confirm experiments that were presented in previous papers. [ 20,22,28 ]…”

“…In ref. [ 22 ] , an alternative ionic mechanism was proposed, in terms of oxygen vacancy electromigration and clustering in the SrTiO 3 substrate under the driving force of the back gate voltage. Such oxygen vacancy clusters would form in‐gap states that trap the free charge of the quantum well during the first positive gate polarization and do not release it as long as the sample is kept at low temperature.…”

“…The hysteretic effects are related to irreversible changes in the resistance that are systematically observed in these systems as the back gate is swept toward positive voltages at low temperatures, for the first time after cooling. [ 19–23 ] Such effects have been traditionally ignored in the early stages of the investigation of an electric field effect in this system. [ 13 ] The first positive back gate voltage application was applied to the samples as a “forming step,” and the whole field‐effect analysis was typically performed on the irreversibly created perturbed state, characterized by a lower conductivity as compared to the pristine state.…”

“…[ 24–26 ] In the LaAlO 3 /SrTiO 3 system, the observed behavior has been interpreted either in terms of Fermi level lying close to the top of the quantum well and charge leaking away from the quantum well into trap states during a positive gate polarization, also modifying the confining potential landscape [ 19,23 ] or in terms of charges trapped at oxygen vacancy sites in the SrTiO 3 substrate that electromigrate and cluster under the driving force of the back gate voltage. [ 22 ]…”

“…We carried out isothermal magnetotransport measurements under field effect at different fixed temperatures, adopting the gate voltage sweep protocol of ref. [ 22 ] and we fitted experimental data within a two‐band framework, taking into account sample inhomogeneity in the depletion regime. We could thus independently monitor the populations of d xy and d yz , d zx carriers as a function of an electric field cycle and clarify how each type of band is affected by the reversible and irreversible mechanisms induced by field effect, in both the crystalline and amorphous sample, and for different operating temperature.…”