SCANNING-TUNNELING-MICROSCOPY STUDIES OF NUCLEATION AND GROWTH IN A REACTIVE, EPITAXIAL SYSTEM - CO/SI(111)

We present scanning tunneling microscopy observations of the reaction of cobalt with Si (111)-(7 x 7). For deposition at 320-degrees-C (reactive epitaxy), flat-topped monolayer islands of triangular shape with vertices along [112BAR]Si nucleate on the faulted side of the 7 X 7 structure then grow in size attaining edge lengths that are quantized to integer multiples of the 7 x 7 unit cell. A 2 X 2 reconstruction with large corrugation occurs on some of the islands, and is believed to be an ordered array of silicon adatoms. The shape and orientation of the islands appears to be determined by energetics, not growth kinetics. They coexist with the 7 X 7 structure, implying that metal atoms readily diffuse through the silicon matrix before attaching to an existing island. From the areal, density of islands we estimate an activation energy for ''surface diffusion'' of 0.8 eV, and argue that the process involves metal atom transport through near-surface silicon interstitials. At lower temperatures, no ordered silicide forms, while at higher temperatures, multilayer islands form with a predominant layer spacing of 3.1 angstrom suggesting that they are (111)CoSi2. For postdeposition annealing at 600-degrees-C (solid phase epitaxy), large, multilayer islands of CoSi2 form, in coexistence with a square-root 7 submonolayer phase. General aspects of the nucleation, growth, and coarsening in this reactive, epitaxial system are discussed.