It was shown that Co atoms, when deposited onto (111) 7 x 7 surfaces at room temperature, occupied near-surface interstitial sites of the Si lattice at very low coverages. These sites could be seen, in scanning tunnelling microscopic images, as slightly lowered groups of 2 or 3 adjacent Si adatoms in an otherwise intact (111) 7 x 7 surface. At 150C, the interstitials were mobile and preferentially occupied sites which were directly under 3-coordinated Si surface atoms on the faulted side of the 7 x 7 unit cell. An atom-displacing silicide reaction occurred only at higher coverages; when the 7 x 7 half-unit cells became multiply occupied.

P.A.Bennett, D.G.Cahill, M.Copel: Physical Review Letters, 1994, 73[3], 452-5