Physics and Astronomy, Department of

 

Date of this Version

Summer 8-28-2011

Comments

A DISSERTATION Presented to the Faculty of The Graduate College at University of Nebraska In Partial Fulfillment of Requirements For the Degree of Doctor of Philosophy, Major: Physics and Astronomy, Under the Supervision of Professor Evgeny Y. Tsymbal. Lincoln, Nebraska: August, 2011

Copyright 2011 Yong Wang

Abstract

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 effects of polarization 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. 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. It is found that compressive epitaxial strain introduces a polarization in SrTiO3 pointing away from the interface. This polarization strongly affects the 2DEG carrier density through a polarization charge formed at the interface. The critical thickness to form a 2DEG at the interface of the heterostructure increases with the compressive strain, while the saturated carrier density decreases.

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 hope that this prediction will stimulate experimental investigations to achieve the spin-polarized 2DEG.

Adviser: Evgeny Y. Tsymbal