Growth, Characterization, and Thermal Stability of CeO2 Thin Films

Sammanfattning: Rare earth oxides are potentially useful materials in various optical electronic, and catalytic devices. The characteristics of such devices are strongly dependent on the interface structure of heterojunctions, hence engineering of the performance can be achieved through tailoring of the micro- and/ or surface structure. In this thesis thin films of cerium dioxide(Ce02) are investigated, with the aim of realising Ce02 films with a welldefined microstructure and surface crystallography.Ce02 thin films have been sputter deposited in a reactive oxygen/ argon atmosphere at a substrate temperature of 800 °C on α - Al203 (102), LaAl03 (001), and MgO (111) substrates. The microstructure, orientation, surface morphology and chemical state of the Ce02 films have been investigated by a combination of transmission electron microscopy (TEM), x-ray diffraction (XRD), atomic force microscopy(AFM), x-ray photoelectron spectroscopy (XPS) and Rutherford backscattering spectroscopy (RBS).The Ce02 films are shown to grow with an epitaxial relation to the α-Al203 substrate, with Ce02 (001) ║ Al203 (102). TEM reveals a microstructure with highly oriented columns parallel to the growth direction. Upon close examination a faceted top surface terminated by {111} planes can be observed. In order to examine possible changes in the surface morphology or the crystalline quality of the CeO2 layers, selected samples were post-deposition annealed at various temperature and gas ambient. Annealing at 850 °C in Ar/ 02 render a truncation of the extremely sharp microfacets on the CeO2 surface as verified by high resolution TEM (HREM). AFM analysis establish the formation of a flat surface morphology through annealing at higher temperatures in Ar, and XPS show an accompanying reduction in the oxidation state of Ce4+. Identical annealing procedures in 02 prevent, to a high degree, such reduction reactions, however, at high temperatures, the film layer is no longer continuous, but rather a network of material on the substrate surface.When a YBa2Cu3O7-x (YBCO) is sputtered on CeO2 - buffered α-Al2O3 HREM reveals a remarkably smooth interface between the YBCO and CeO2 layers. Also, the microwave surface resistance is comparable to that of YBCO films grown directly on single-crystalline substrates, being approximately 185 mW at 6,1 Ghz.The total sputtering pressure,P significantly influence the orientation and surface morphology of CeO2 thin films grown on α-Al2O3(102) substrates, using both off-axis and on-axis substrate/magnetron configurations. XRD analysis show that growth at 12.9 Pa favor the (001)CeO2 orientation giving a rough surface morphology, while decreasing the pressure to 0.4 Pa results in a (111) CeO2 preferred orientation with a relatively smooth surface morphology. Additionally, RBS examination reveal a stoichiometric film composition with abrupt interfaces. Identical studies on CeO2 films grown on LaA1O3 substrates show no observable change in film orientation, however the surface roughness varies with the total pressure in a similar way to that of films on Al2O3.The CeO2 films exhibit a strong epitaxial preference on MgO (111), with Ce02 (111)│MgO(111) in the case of thicker films. The top surfaces of these films are characterized by fairly flat areas, exposing the (111)-surface of the CeO2 crystal, interrupted by a few, taller (001)-oriented grains. However, thin ( 20 nm) CeO2 films consist of islands that exhibit a preferred (001) orientation.