High resolution microstructural characterization of oxide thin films and interfaces
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
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
(MEIS) and atomic force microscopy (AFM). The oxide interfaces were introduced in
thin films deposited using the pulsed laser deposition technique.