The role of interfaces in oxide thin films

Doctoral thesis, 2007

The role of interfaces in oxide thin films has been investigated from a microstructural point of view. Thin films of manganites and titanates have been investigated by transmission electron microscopy (TEM). The analysis included techniques as high-resolution TEM, bright and dark filed imaging, electron diffraction as well as chemical analysis using electron energy loss spectroscopy (EELS) and energy dispersive x-ray spectroscopy (EDS). Ferromagnetic films of perovskite derivative structures as La0.67Sr0.33MnO3 and La0.67Ca0.33MnO3 have been characterized in terms of atomic and defect arrangement. The film microstructure has been linked to magnetotransport properties. The work on the Sr doped manganites concerned grain boundary tunnel junctions. Artificial grain boundaries of (001) bicrystal tilt substrates has been analysed in both cross sectional and plan view specimens. The calcium-doped manganite has been deposited on various substrates with a different fit to the film. The microstructural investigation confirmed the importance of the lattice fit for the formation of different structural phases and effects of thermal treatment. Ferroelectric films of Ba0.25Sr0.75TiO3 and BaTiO3 incorporated with a bottom electrode on silicon substrates have been analysed and linked to dielectric performance and frequency response. The growth of the ferroelectric films was in many ways different to the manganite films, since the films were not epitaxial. The influence of deposition technique, electrode configuration and deposition temperature has been determined. During the initial stage of the growth of the thin film oxides, the interface between the substrate and the film affected the film microstructure. Factors such as substrate morphology and chemical stability at the growth temperature were important, together with the film interaction. As the film grew, the lateral, angular and thermal expansion match between the substrate and film not only had an influence on the film quality but were also crucial for the properties of the film. Independent of oxide film, this work confirmed the need of microstructural information on an atomic scale in order to understand the properties of functional oxides. The TEM is a unique tool for correlating complex structures to magnetic and electric properties that cannot easily be done by combining surface analysis techniques with x-ray diffraction that very often is practiced in the field.