First-principles studies of a two-dimensional electron gas at the interface in ferroelectric oxide heterostructures

The discovery of a two-dimensional electron gas (2DEG) at the interface between two insulating oxides has recently stimulated intense research activity in this field. The 2DEG has unique properties that are promising for applications in all-oxide electronic devices. For such applications it is desirable to have the ability to control 2DEG properties by external stimulus. Here we employ first-principles calculations to investigate KNbO₃/ATiO₃(001) (A=Sr, Pb, and Ba) heterostructures where perovskites ferroelectrics, KNbO₃, PbTiO₃, and BaTiO₃, are used as oxide constituents to create the interface 2DEG. Our results suggest that the polar (NbO₂)⁺/(AO)⁰ interface in these heretostructures favors the formation of 2DEG similar to that at the (LaO)⁺/(TiO₂)⁰ interface in a LaAlO₃/SrTiO₃ heterostructure. We predict that the presence of spontaneous ferroelectric polarization which can be switched between two stable states allows modulations of the carrier density and consequently the conductivity of the 2DEG. The effect occurs due to the screening charge at the interface that counteracts the depolarizing electric field and depends on polarization orientation. The magnitude of the effect of polarization on the 2DEG properties strongly depends on contrast between polarizations of the two constituents of the heterostructure: the larger is the difference in the two polarizations, the bigger is the effect. For a sufficiently large polarization difference, we predict a metal-insulator transition at the interface driven by polarization reversal. This behavior is found for the KNbO₃/PbTiO₃ interface and for the KNbO₃/BaTiO₃ interface when polarizations of KNbO₃ and BaTiO₃ are antiparallel. The proposed concept of ferroelectrically controlled interface conductivity may be very interesting for memory and logic applications and we hope that our predictions will stimulate experimental studies in this field.