High resolution microstructural characterization of oxide thin films and interfaces

Doctoral thesis, 2013

Oxide interfaces can introduce properties that are different compared to the corresponding crystal bulk properties. The properties can be tuned by changing the structure. It has become clear that these oxide interface systems are potentially suitable for future electronic devices with unique and tailored properties. The fine scale structure of the interfaces affects the properties and enables the control of them provided that a knowledge about the correlation between structure and properties has been established. In this thesis work, the microstructure of oxide interfaces has been correlated to the electrical transport properties mainly by imaging and spectroscopy using transmission electron microscopy. SrTiO3 (STO) and LaAlO3 (LAO) are well-studied dielectric materials with wide band gaps of about 3.2 and 5.6 eV respectively. A highly conducting layer forms at the interface between thin films of these insulators under certain circumstances. The induced electrical conductivity is proposed to be due to oxygen vacancies, intermixing and/or a polar discontinuity at the LAO/STO interface. In order to further clarify the mechanisms that occur at the interfaces, details about the atomic structure at interfaces between LAO and (001) STO have been studied by atomic number contrast in HAADF STEM. In addition to the perovskite oxide systems, also the combined fluorite/perovskite heterostructures have attracted interest. There is a need to increase understanding for the electrical transport properties in heterostructures between large misfit structures. These nanostructured systems have already showed potential to be used in solid oxide fuel cells (SOFC) as well as gas sensors. Bulk Yttrium stabilized Zirconium (YSZ) is a widely used ionic conductor material at elevated temperatures in solid electrolytes in solid oxide fuel cells. In this work, the microstructure of thin films of YSZ on top of STO and LAO substrates have been investigated. We show important changes in the film orientation and morphology as a function of substrate and deposition temperature. The techniques used in this thesis work include high angle annular dark field scanning transmission electron microscopy (HAADF STEM), high resolution transmission electron microscopy (HRTEM), electron diffraction, energy dispersive X-ray spectroscopy (EDS), x-ray diffraction (XRD), medium energy ion spectroscopy iv (MEIS) and atomic force microscopy (AFM). The oxide interfaces were introduced in thin films deposited using the pulsed laser deposition technique.