STRUCTURES OF CO/AG METALLIC SUPERLATTICES GROWN BY MOLECULAR-BEAM EPITAXY

Co/Ag metallic superlattices, were epitaxially grown onto Ag buffer layers pre-deposited on (0001)sapphire or (III)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.