The growth of ultra-thin epitaxial CeO2 films on r-plane sapphire

Ultra-thin CeO2 films with thicknesses in the range of 0.3-10 nm were deposited on r-plane sapphire substrates by sputtering. The growth of the films was investigated by X-ray diffraction measurements and by Rutherford backscattering spectroscopy. At a substrate temperature of 735 °C and a deposition rate of 0.16 nm/min the films grow epitaxially, (100) oriented revealing in their mosaic distribution a twofold appearance, namely a very narrow part with a breadth depending on the resolution of the X-ray system and a broad part. Both parts are observed from the beginning of the growth; the narrow distribution is ascribed to pseudomorphic growth showing partial coverage of the substrate, the broad part grows simultaneously but is relaxed by defect incorporation. The partial coverage starts with growth blocks with a minimum thickness of four unit cells up to a coverage of 36%; further growth proceeds on top of the first blocks and uncovered parts of the substrates until full coverage is achieved at thicknesses of around 8 nm. In the nominally thinnest films, i.e. at the shortest deposition times, the lattice constant shows a drastic increase of up to 3.7% in comparison to bulk material. This effect probably is due to oxygen loss in accordance with observations in CeO2 nanoparticles.