Electronic, Magnetic, and Dielectric Properties of Titanate Films and Heterostructures

Abstract

Titanates are the earliest discovered perovskite oxides and have been studied extensively ever since. Owing to their versatile properties (ferroelectricity, superconductivity, ferromagnetism, just to name a few), perovskite titanates are not only scientifically interesting, but also technologically attractive in many aspects such as catalysis, memory, computing, energy storage, power harvesting, etc. Further excitement has been stimulated by the two-dimensional electron gas (2DEG) discovered at the SrTiO3/LaAlO3 interface, where both components are insulators in bulk.In this thesis work, we present a detailed study of electronic, magnetic, and dielectric properties of perovskite titanate thin films and heterostructures grown by the hybrid molecular beam epitaxy approach. We first start with a systematic study of the prototypical perovskite SrTiO3 (STO) with carrier densities ranging from 1017 to 1020 cm-3. Detailed transport measurements reveal that the electronic and structural instabilities of STO are intimately coupled, and, the superconducting dome of STO thin films is found to be dramatically different from that in bulk. Dielectric measurements of homoepitaxial STO thin films also manifest the strong influence of structural transitions. Then we turn to rare earth titanates, a model system for the study of strong electron-electron interactions. Two members with ferromagnetic ground states, YTiO3 and DyTiO3, are picked, and their growth, band structure, and magnetic properties are investigated. For YTiO3, the transport is found to be dominated by small hole polaron hopping and the Mott Hubbard gap is determined to be around 1.5 eV. A ferrimagnetic ground state has been identified in DyTiO3 films, and the magnetic properties turn out to be extremely sensitive to the cation stoichiometry. The final part of this thesis work is focused on heterostructures based on perovskite titanates. A hopping process is found to be responsible for the transport behavior of SrTiO3/Nd1-xTiO3/SrTiO3 heterostructures, and the detailed hopping mechanism varies according to the Nd vacancy concentration. We also demonstrate SrTiO3-based transistor devices and identify potential routes to improve the performance through dielectric analysis.