Epitaxial cerium oxide films deposited on r-plane sapphire substrates: A comprehensive study of growth mechanisms

• CeO 2 thin films are epitaxially grown on r-plane sapphire substrates with high quality. • Growth competition mechanisms and orientation control in CeO 2 films are explored. • Dislocations and misorientation in CeO 2 thin films are directly imaged and analyzed. Epitaxial growth of CeO2 thin films onto sapphire single crystals is of great interest for high-quality heterostructures. Here, the growth mechanisms and crystallographic characteristics of CeO2 films, from 5 to 40 nm-thick deposited by rf-sputtering on r-plane sapphire substrates, were explored by X-ray diffraction, atomic-force microscopy and high-resolution transmission electron microscopy. A growth competition between (1 1 1)/(0 0 2) orientations was identified during the early stages of growth. While both orientations coexist within the first nanometers, the (0 0 2)-orientation progressively dominates due to its higher growth rate, while (1 1 1)-oriented domains remain contained close to the substrate surface. Post-annealing was achieved to enhance the crystalline quality of the films, removing (1 1 1)-oriented domains and providing (0 0 2)-single-oriented CeO2 epitaxial layers. Misorientation was linked to strain relaxation induced by periodic dislocations, as revealed by TEM analysis. Additionally, in-plane dislocations are present within the entire film thickness. Both contribute significantly to the local misorientation. This study provides detailed pictures of defects and quantifies their effect on the film crystallographic quality. It offers insights into both fundamental growth mechanisms and practical strategies to facilitate the epitaxial growth of complex oxides onto the surface of CeO2 thin films, thereby paving the way for innovative applications in catalysis, electronics and/or energy applications.