Linköping Studies in Science and Technology
Dissertation No.554

Growth, Characterization, and Thermal Stability of CeO₂ Thin Films

Sissel N. Jacobsen

Akademisk Avhandling

som för avläggande av teknologie doktorsexamen vid Linköpings tekniska högskola kommer att försvaras offentlig i hörsal Planck, Fysikhuset, Linköpings Universitet, onsdagen den 16 december 1998, kl. 10¹⁵. Opponent är Dr. David K. Fork, Xerox Palo Alto Research Center, Palo Alto, CA, USA.

Abstract

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 (CeO₂) are investigated, with the aim of realising CeO₂ films with a well-defined microstructure and surface crystallography.

CeO₂ thin films have been sputter deposited in a reactive oxygen/argon atmosphere at a substrate temperature of 800 °C on , LaAlO₃ (001), and MgO (111) substrates. The microstructure, orientation, surface morphology and chemical state of the CeO₂ 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 CeO₂films are shown to grow with an epitaxial relation to the alfa-Al₂O₃ substrate, with . 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 CeO₂layers, selected samples were post-deposition annealed at various temperature and gas ambient. Annealing at 850 °C in Ar/O₂ 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 O₂ 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 YBa₂Cu₃O_7-x (YBCO) is sputtered on CeO₂ - buffered alfa-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_t, significantly influence the orientation and surface morphology of CeO₂ thin films grown on 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 LaAlO₃ 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 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.

Thin Film Physics Division
Department of Physics and Measurement Technique
Linköping University, S-581 83 Linköping, Sweden
Linköping 1998

ISBN: 91-7219-362-X ISSN: 0345-7524