Structure and Transport Properties of Epitaxial Oxide Thin Films: From Synthesis to Characterization

Epitaxial thin films and heterostructures based on perovskite oxides have attracted significant attention in physics since perovskites exhibit an enormous range of electrical, magnetic, and optical properties, making them exciting systems for studies of the fundamental physical mechanisms of interactions between electron, lattice, and spin degrees of freedom. This dissertation has been focused on ferroelectricity in lowdimensional ferroelectric materials using ultra-thin ferroelectric epitaxial films (BaTiO₃) with a metallic electrode (SrRuO₃) by studying polarized ordering of the crystal structure and electronic transport through the films. High quality and highly oxidized epitaxial films are a prerequisite for the clear observation of physical properties such as ferroelectricity which depends on a sensitive balance of lattice structure, dynamics, and charge distribution. Measurements in low dimensional, ultra-thin films require a controlled surface status through in-situ characterization. As is demonstrated here, fundamental physical phenomena on surfaces and in ultra-thin films are easily modified due to reactivity in ambient air, even for oxide materials generally considered inert. This study is centered on in-situ low energy electron diffraction and scanning tunneling spectroscopy of BaTiO₃ films grown on SrRuO₃ electrodes on a SrTiO₃ substrate. Results show out-of-plane polarized structure and polarization switching, which provide evidence of ferroelectricity in films down to 4 ML. Surface reconstruction in 1-2 ML thick BaTiO₃ films is seriously affected by the interface between BaTiO₃ films and SrRuO₃ bottom electrode. Our observation in epitaxial BaTiO₃ films indicates the existence of ferroelectricity with a lower limit (4 ML) for the minimum thickness than theoretical expectation (6 ML), which results from the difference of film stress, termination on films, and depolarizing screening.