Ultrathin epitaxial iron films on a highly asymmetrical substrate: Fe/Cu(311)

The structure and morphology of ultrathin epitaxial Fe films on Cu(311) is examined in situ by spot-profile analyzing low-energy electron diffraction. Cu(311) is a highly asymmetrical substrate which is characterized by a troughlike surface structure composed of uniaxially arranged close-packed Cu atom rows. Deposition at 140 K initially leads to the formation of pseudomorphic Fe. The constraint to lower dominant stress along the close-packed atom rows forces the formation of pseudomorphic patches with a quasiperiodic separation of 35 Angstrom. With proceeding growth, these patches act as nucleation sites for three-dimensional islands that gradually relax via a Pitsch transformation towards a strained bcc structure. Coalescence occurs at similar to 5 ML and leads to a periodically faceted growth front of the overlayer. The resulting surface topography corresponds to a regular up-and-down staircase and remains unchanged for coverages >5 ML: the periodic facet separation measures 35 Angstrom; 7-8 layers contribute to the growth front. As a consequence of coalescence, the film locks into a lattice arrangement which is predominantly dictated by the substrate and is likely to correspond to an fee-like structure. The periodic facetting of the film surface, on the other hand, allows partial relaxation towards strained bcc structure to persist in the top layers of the film.