Cobalt on rhenium(0001) - an example of thermally activated layer intermixing and surface alloying

The growth and morphology of cobalt thin films deposited onto a Re(0001) su rface at 300, 400 and 550 K were followed in the coverage range 0 ML<Theta< 6 ML by combined low-energy electron diffraction (LEED) and scanning tunnel ing microscopy (STM). The interaction phenomena are complex and depend stro ngly on temperature. At 300 K, cobalt nucleates homogeneously on terraces a nd heterogeneously at steps forming dendritic islands. Larger cobalt covera ges lead to incomplete layer growth. Interdiffusion and alloying play a min or role only at 300 K, but become dominant for T>400 K in that different (2 x 2) phases form within the first Re-Co bilayer, one within the rhenium su bstrate surface, the others within the cobalt islands. The (2 x 2) phases c an be associated with Re/Co surface alloys of different stoichiometry, depe nding on cobalt coverage. As the cobalt coverages exceed two monolayers (ML ), genuine but incomplete cobalt layers grow. Within the third and fourth c obalt layer, periodic triangular features with a lattice constant of simila r to 28 Angstrom appear in STM, followed by a Moire pattern for Theta>4 ML. Both structures produce an incomplete (10 x 10) LEED pattern. After growth of the fifth or sixth layer the lattice misfit is overcome, and cobalt ess entially grows layer-by-layer in a pseudo Frank-van der Merwe mechanism, th e details being strongly temperature-dependent. (C) 1999 Elsevier Science B .V. All rights reserved.