PNNL

Abstract

Growth rate-induced epitaxial orientations and crystalline quality of CeO2 thin films grown on Al2O3(0001) by oxygen plasma-assisted molecular beam epitaxy were studied using in-situ and ex-situ characterization techniques. CeO2 grows as three-dimensional (3-D) islands and two-dimensional (2-D) layers at growth rates of 1-7 Å/min, and ?9 Å/min, respectively. The formation of epitaxial CeO2(100) and CeO2(111) thin films occurs at growth rates of 1 Å/min and ? 9 Å/min, respectively. Glancing incidence x-ray diffraction (GIXRD) measurements have shown that the films grown at intermediate growth rates (2-7 Å/min) consist of polycrystalline CeO2 along with CeO2(100). The thin film grown at 1 Å/min exhibits six in-plane domains, characteristic of well-aligned CeO2(100) crystallites. The content of the poorly-aligned CeO2(100) crystallites increases with increasing growth rate from 2 Å/min to 7 Å/min, and three out of six in-plane domains gradually decrease and eventually disappear, as confirmed by XRD pole figures. At growth rates ?9 Å/min, CeO2(111) film with single in-plane domain was identified. The formation of CeO2(100) 3-D islands at growth rates of 1-7 Å/min is a kinetically driven process unlike at growth rates ?9 Å/min which result in an energetically and thermodynamically more stable CeO2(111) surface.