ATOMIC-STRUCTURE OF COBALT SILICIDE ISLANDS FORMED BY REACTIVE EPITAXY

We report STM observations of the reaction of cobalt deposited on Si(1 11) at 320-degrees-C. At low coverage, coherent monolayer silicide isl ands form in a matrix of 7 x 7 reconstructed silicon. The islands take one of three forms: flat and recessed 0.3 angstrom, 2 x 2 corrugated and raised 0.8 angstrom or flat and raised 1.0 angstrom, where heights are given with respect to Si(111)-7 x 7. These structures are assigne d to CoSi2(111) with 7-fold (111) interface bonding, CoSi2 with a 2 x 2 array of Si adatoms, and CoSi(111), respectively. Atomic models are given for the lateral silicide-silicon interface along the island edge s. The role of these metastable, precursor structures in the kinetic p ath of the silicide reaction is described.