Functional two-dimensional electronic gases at interfaces of oxide heterostructures

A quasi-two dimensional electron gas (2DEG) in oxide heterostructures such as LaAlO3/SrTiO3 has unique properties that are promising for applications in all-oxide electronic devices. In this dissertation, we focus on understanding and predicting novel properties of the 2DEG by performing first-principles electronic calculations within the frame work of density-functional theory (DFT). The investigation is made upon adding new functionalities in oxide heterostructures, such as ferroelectric polarization, epitaxial strain, and spin polarization that can be employed to control 2DEG properties. Based on first-principles calculations the effects of different polarization magnitudes and alignments in all-oxide heterostructures incorporating different ferroelectric constituents, such as KNbO3/ATiO3 (A = Sr, Ba, Pb), are investigated. It is found that screening charge at the interface that counteracts the depolarizing electric field in the ferroelectric material significantly changes the free electron density of 2DEG at the interface. Using this mechanism, nonvolatile metal-insulating transition can be achieved at the interface by switching the ferroelectric spontaneous polarization. Growing on different substrates, LaAlO3/SrTiO3 heterostructures experience different epitaxial strains. Our first-principles calculations reveal that compressive epitaxial strain introduces a polarization in SrTiO3 pointing away from the interface, which is consistent with the experimental observations. This polarization strongly affects the 2DEG carrier density through a polarization charge formed at the interface. Our theoretical investigation finds that the critical thickness to form a 2DEG at the interface of the heterostructure increases with the compressive strain, while the saturated carrier density decreases which is consistent with the experimental results. Adding a spin degree of freedom to 2DEG may be interesting for the application of 2DEGs in a spintronic device. We explore a LaAlO3/EuO interface as a potential candidate to create a spin-polarized 2DEG. The exchange splitting of unoccupied Eu-5d conduction band in bulk EuO makes it possible to realize spin-polarized 2DEG. We predict that LaAlO3/EuO interface forms a spin-polarized 2DEG with spin polarization of about 50%. We hope that this prediction will stimulate experimental investigations to achieve the spin-polarized 2DEG