Structures of Co/Ag metallic superlattices grown by molecular beam epitaxy

Abstract

Co/Ag metallic superlattices were epitaxially grown onto Ag buffer layers pre-deposited on (0001)sapphire or (111)Si substrates by molecular beam epitaxy. In-situ time-resolved reflection high-energy electron diffraction observations during superlattice growth revealed the structures of Ag and Co layers. Ag layers had an FCC structure with growth direction 〈111〉 FCC. On the other hand, the structure of Co layers on Ag layers was initially close-packed with random stacking normal to the close-packed plane and after about 7 monolayer growth it was changed to FCC at temperatures below 323 K and to HCP at above 323 K. Interface roughness was deduced from the period of time when both patterns from Co and from Ag were being observed. Ex-situ X-ray diffraction (XRD) measurements and high-resolution transmission electron microscopy observations disclosed the atomic-level structures, and showed that the interfaces between Co layers and Ag layers were wavy at 273 K and the Co layers grown at 373 K discontinuous. A wave-model and an island-model were proposed to interpret the experimental XRD profiles. XRD with 4-circle diffractometer revealed that Ag layers on sapphire had a local lattice distortion, but that Ag on Si did not.