Atomic structure of cobalt silicide islands formed by reactive epitaxy

We report STM observations of the reaction of cobalt deposited on Si(111) at 320°C. At low coverage, coherent monolayer silicide islands form in a matrix of 7 × 7 reconstructed silicon. The islands take one of three forms: flat and recessed 0.3 Å, 2 × 2 corrugated and raised 0.8 Å or flat and raised 1.0 Å, where heights are given with respect to Si(111)-7 × 7. These structures are assigned to CoSi₂(111) with 7-fold (111) interface bonding, CoSi₂ with a 2 × 2 array of Si adatoms, and CoSi(111), respectively. Atomic models are given for the lateral silicide-silicon interface along the island edges. The role of these metastable, precursor structures in the kinetic path of the silicide reaction is described.