Molecular Beam Epitaxial Growth and Structures of Al/Ag Superlattices

We studied epitaxial growth and structures of Al/Ag superlattices prepared using molecular beam epitaxy. Samples were grown at temperatures of 20, 50, 70 and 100°C, at a growth rate of 0.1 monolayers per second, on 50-nm-thick (100) Ag buffer layers preformed on (100) MgO substrates. In situ reflection high-energy electron diffraction (RHEED) during growth and ex situ X-ray diffraction were performed to analyze the growth behavior and the structures. In all samples, (100) epitaxial growth was confirmed at early stages. Moreover, RHEED intensity oscillations were observed when Al layers were grown on Ag buffer layers. However, (111)-grown domains subsequently appeared and gradually became dominant upon prolonged growth. After sixty Al/Ag bilayer depositions, all surface areas were occupied by (111)-grown domains. Persistence of (100) epitaxial growth was strongly temperature dependent; (100) growth persisted during a few tens of bilayer depositions at 50°C, while at 20 and 70°C the growth orientation changed to (111) much faster. Other features, including the structures, are also described.