EPITAXY OF FCC AND BCC CO, NI, AND CU STUDIED BY X-RAY PHOTOELECTRON DIFFRACTION

The structures of epitaxial transition-metal films with thicknesses of only a few monolayers were determined by X-ray photoemission diffraction (XPD) techniques. Ni ultrathin films were studied on two Cu single-crystal surfaces, and Cu/Ni multilayers (or ''sandwich'' structures) were also grown. On Cu(100), the initial Ni epitaxy is found to result in expansion of the Ni lattice constant to achieve lateral coherence with the substrate, along with a reduction in the Ni interplanar spacing perpendicular to the substrate, approximately maintaining a fixed Ni-Ni nearest-neighbor distance. This structure is a body-centered tetragonal distortion of the normal fcc(100) lattice. The behavior on Cu(111) is different, in that the epitaxial Ni films retain the c/a axis relationship of bulk Ni(111). Films of Co, Ni, and Cu, which are normally fcc or hcp crystals at room temperature, were grown on single-crystal Fe(100) substrates to induce bcc ordering. Both Co and Ni grow as bcc(100) films on iron, but the Cu films are disordered. This is in contrast to epitaxy of Cu on strained layer Fe(100) films, which do support the growth of bcc Cu(100). Modelling of ultrathin film XPD using multiple-scattering calculations was used for bond-length determinations with high accuracy, and for a determination of growth modes.