Influence of growth rate on the epitaxial orientation and crystalline quality of CeO2 thin films grown on Al2O3(0001)

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 (3D) islands and two-dimensional layers at growth rates of 1-7 A/min and ≥9 A/min, respectively. The formation of epitaxial CeO2(100) and CeO2(111) thin films occurs at growth rates of 1 A/min and ≥9 A/min, respectively. Glancing-incidence x-ray diffraction measurements have shown that the films grown at intermediate growth rates (2-7 A/min) consist of polycrystalline CeO2 along with CeO2(100). The thin film grown at 1 A/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 to 7 A/min, and three out of six in-plane domains gradually decrease and eventually disappear, as confirmed by XRD pole figures. At growth rates ≥9 A/min, CeO2(111) film with single in-plane domain was identified. The formation of CeO2(100) 3D islands at growth rates of 1-7 A/min is a kinetically driven process unlike at growth rates ≥9 A/min which result in an energetically and thermodynamically more stable CeO2(111) surface.