First principles calculation of LaAlO₃/SrTiO₃interfacial transport properties

Abstract

Since 2004 many interesting phenomena have been discovered at the high-mobility two dimensional electron gas (2DEG) between two band insulators LaAlO₃ (LAO) and SrTiO₃ (STO). The model of polar catastrophe and electronic reconstruction successfully explains formation of the LAO/STO 2DEG. However, the research of LAO/STO is now turning from intrinsic LAO/STO system into other directions such as surface modulation by polar liquid that may lead to novel properties and applications. The objective of my thesis is to provide theoretical understanding on the mechanism behind these new phenomena. Project density of states (DOS) and carrier density calculation results reveal that, the conduction properties at LAO/STO interface can be greatly influenced by water molecules adsorbed on the LAO surface. Due to the strong polar field produced by H₂O molecules, the surface electronic structure can be affected by just a few H₂O layers. The carrier density of the 2DEG interface can be greatly increased by a few layers of H₂O molecules. Work function of LAO/STO heterostructure will also change significantly, and this may explain the rectified I-V curve for the conduction through the polar molecule covered part and the uncovered part of LAO/STO. Recently, there are also some studies on different oriented LAO/STO interface. (110) and (111) LAO/STO transport properties and the results may challenge the polar catastrophe model in the formation of 2DEG. Based on the calculations reported in this thesis, it is unlikely to obtain polar catastrophe at both (110) and (111) LAO/STO interfaces. At the (110) LAO/STO, the conduction electrons should be localized along [1 -1 0], but transport easily along [0 0 1]. While at the (111) LAO/STO interface, electrons may be localized in all plane directions. Doping effects are also studied. The results of n-type doping (110) and (111) interfaces reveal that the electrons, unlike those at (001) interface, are not confined strongly at interfaces, and the results of p-type doping show that holes spread over LAO and STO layers.