Stranski-Krastanov Growth of Al on Ag Layers during Molecular Beam Epitaxy of (111) Al/Ag Superlattices

We studied the growth behavior of (111) Al/Ag superlattices during molecular beam epitaxy, using in situ reflection high-energy electron and ex situ X-ray diffractions. Epitaxial growth proceeded with an orientational relationship of Al(111)[110] ∥Ag(111)[110]. Although the first Al layer with 3-monolayer (ML) thickness grown on a Ag buffer layer did not show island growth, the second and succeeding (111) Al layers with 3-ML thickness were found to grow on Ag underlayers in the Stranski-Krastanov mode, in contrast to the layer growth of (001) Al/Ag superlattices reported previously. The critical Stranski-Krastanov thickness was seemingly 2 ML, and three-dimensional islands emerged upon further growth. Thus, superlattices with 3-ML-thick Al layers showed rough Al/Ag interfaces due to the presence of Al islands, while superlattices with 2-ML-thick Al layers exhibited rather flat interfaces. The strain relief of Al upon the formation of islands was detected. We discussed the possible cause of the difference in growth behavior between the (111) and (001) superlattices.