No abstract

We present scanning tunneling microscopy observations of the reaction of cobalt with Si(111)-(7×7). For deposition at 320 °C (reactive epitaxy), flat-topped monolayer islands of triangular shape with vertices along 〈1̄1̄2〉Si nucleate on the faulted side of the 7×7 structure then grow in size attaining edge lengths that are quantized to integer multiples of the 7×7 unit cell. A 2×2 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 7×7 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 Å suggesting that they are (111)CoSi2. For postdeposition annealing at 600 °C (solid phase epitaxy), large, multilayer islands of CoSi2 form, in coexistence with a √7 submonolayer phase. General aspects of the nucleation, growth, and coarsening in this reactive, epitaxial system are discussed.

No abstract

We characterize a ‘‘ring-cluster’’ (RC) structure produced by high temperature annealing of transition metals on Si(111), studied by scanning tunneling microscope and low-energy electron microscopy. This structure consists of a single metal atom on a substitutional site in the top layer of Si(111) plus an overlying ring of six Si adatoms with an unusual bridge bonded topology. We find that the same structure occurs for all near noble metals specifically Fe, Co, Ni, Pd, Pt and Ir. It does not occur for refractory metals (Ti, W) or for noble metals (Cu, Ag). Clear phase separation of clean Si(111)-7×7 and large triangular domains containing a lattice gas of RCs occurs only for quenching from above the 7×7 to 1×1 phase transition at T=840 °C. The density of RCs within a domain increases with average metal coverage. The domain orientation is defined by its boundary with the faulted half of 7×7 unit cells. The large scale morphology for RC induced structures and its dependence on quench rate suggests a precipitation reaction. Dissolution rate of nickel RCs into bulk Si was found to be t=20 s at 800 °C assuming exponential decay.

No abstract