Ferroelectric Control of the Conduction at the LaAlO₃ /SrTiO₃ Heterointerface

Abstract

Complex oxide heterointerfaces have emerged as one of the most exciting subjects in condensed matter, owing to their unique physical properties and new possibilities for next-generation electronic devices. In the push for practical applications, it is desirable to have the ability to modulate the interface functionalities by an external stimulus. In this Communication, we propose a generic approach in which a functional layer is inserted into the heterostructure to acquire non-volatile control of the intriguing properties at oxide interfaces. The LaAlO₃ /SrTiO₃ (LAO/STO) interface serves as a model system in which a highly mobile quasi-two-dimensional electron gas (2DEG) forms between two band insulators, exhibiting 2D superconductivity and unusual magnetotransport properties. Although a modulation of the carrier density and mobility of the LAO/STO interface has been achieved using the electric fi eld effect, it is essential to extend the control concepts to gain non-volatile and reversible capabilities for practical applications. Recently, non-volatile modifi cation of the local conduction at the LAO/STO interface has been demonstrated by scanning probe techniques. Several possible mechanisms have been proposed to explain this interesting behavior based on the electrostatic effects attributed to either induced ferroelectricity or surface charge. In the study reported here, we added a ferroelectric Pb(Zrₒ.₂ Tiₒ.₈ )O₃ (PZT) layer near the LAO/STO interface. The ferroelectric polarization of the PZT layer serves as a control parameter to modulate the 2DEG conducting behavior. The as-grown polarization ( Pup state) leads to charge depletion and consequently low conduction. Switching the polarization direction ( Pdown state) results in charge accumulation and enhances the conduction at the LAO/STO interface. The origin of this modulation is attributed to a change in the electronic structure due to the ferroelectric polarization states, evidenced by X-ray photoelectron spectroscopy (XPS) and cross-sectional scanning tunneling microscopy/spectroscopy (XSTM/S). Control of the conduction at this oxide interface suggests that the concept can be generalized for other oxide systems to design functional interfaces.