Tunability of Artificial Interface Phases in LaAlO₃/SrTiO₃ Heterostructures

Abstract

Format

This is a copy of the slides presented at the meeting but not formally written up for the volume.

Abstract

Research into new concepts for oxide-electronic devices has been enriched by the emerging field of functional interfaces. A high control of the materials down to the atomic level enables the improvement of existing oxide devices, like magnetic tunnel junctions, but also the formation of new artificial interface phases. Previous work revealed the existence of a metallic electron gas at the interface between the two band-insulators, LaAlO₃ and SrTiO₃, for a certain atomic arrangement [1,2]. Several studies on single epitaxial connections between LaAlO₃ and SrTiO₃ have revealed them to be either high-mobility electron conductors or insulating, depending on the atomic stacking sequences. An important point to take into account is the formation of oxygen vacancies for low deposition pressures (<10^-5 mbar). For this growth regime the transport properties are fully dominated by the presence of the oxygen vacancies. In this talk we will show a detailed investigation of the controllable electronic properties of coupled interfaces in SrTiO₃-LaAlO₃-SrTiO₃ heterostructures. Recently we reported a critical separation distance of 6 perovskite unit cell layers (~23 Å) for the electronic coupling of closely-spaced complementary interfaces in SrTiO₃/LaAlO₃ multilayer structures [3]. We showed that a decrease of the interface conductivity and carrier density occurs when the LaO:TiO₂ and AlO₂:SrO interfaces are brought closer together. Interestingly, the high carrier mobilities characterizing the separate conducting interfaces were found to be maintained in such coupled structures down to sub-nanometer interface spacing. Here, we will explain in more detail the electronic properties of the closely spaced LaO:TiO₂ and AlO₂:SrO interfaces below the critical separation distance. The carrier density at room temperature for d_LAO5 is similar to a single LaO:TiO₂ interface and has a value of ~1.5X10¹⁴ cm^-2, corresponding to ~0.23 electrons per unit cell area on the LaO:TiO₂ interface. In this, the contribution by the AlO₂:SrO interface to the sheet carrier density is neglected, due to its much lower conductivity. When both interfaces are brought closer to each other the electronic coupling between them is increased and the charge density at room temperature is reduced to 0.15, 0.11 and 0.07 electrons per unit cell area for a spacing of respectively 3, 2 and 1 unit cells. However, for lower temperatures the sheet carrier density decreases and becomes constant for all coupled heterostructures at temperatures below 10 K. This constant low temperature carrier density has a value of 2.0×10¹³ cm^-2 and corresponds to 0.003 electrons per unit cell area. These results show the ability to control the electronic properties in SrTiO₃ heterostructures and to vary the carrier density homogeneously over the interface.