Molecular Beam Epitaxial Growth and Structures of Al/Ag Superlattices

Abstract

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.

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

“…17) In the previous study, 13) we found that (001) Al/Ag superlattices grow epitaxially in the F-M growth mode, although the stability of (001) growth depends on the conditions of MBE growth such as substrate temperature. In the present study, however, we observed the S-K growth mode of (111) Al on a (111) Ag layer during MBE growth of (111) Al/Ag superlattices.…”

“…growth behavior and structures of Al based, metallic superlattices grown by MBE, including (001) Al/Ag, 13) Al/Pd, 14) Al/Cr 15,16) and Al/Ag/Cr. 17) In the previous study, 13) we found that (001) Al/Ag superlattices grow epitaxially in the F-M growth mode, although the stability of (001) growth depends on the conditions of MBE growth such as substrate temperature.…”

Stranski-Krastanov growth of Al on Cr layers during molecular-beam epitaxial growth and its influence on the structure of epitaxial (001) Al/Cr superlattices

“…This expectation is supported by our recent study on the persistence of ͑001͒ growth of Al/Ag SLs. 13 Under this situation, a large part of the surface of the three-dimensional islands formed on the layer-grown ͑001͒ Al can be composed of ͕111͖ surfaces, leading to the minimum increase of the total surface energy. Therefore, its contribution to the increase of the surface free energy may be small.…”

“…This is consistent with the observations on growth of ͑001͒ Al/Ag SLs. 13 Figure 8͑c͒ shows the surface of the first Cr layer grown on this Al layer. Streaks are still observed indicating that the Cr layer has a smooth surface.…”

“…Especially, little research work has been done on epitaxial growth of Al layers in metallic SLs using molecular-beam epitaxy ͑MBE͒. 12,13 One reason for this is that Al is reactive and hence is not easy to handle.…”