INTERSTITIAL PRECURSOR TO SILICIDE FORMATION ON SI(111)-(7X7)

We show that cobalt atoms deposited on Si(111)-(7 x 7) at room tempera ture occupy near-surface interstitial sites of the silicon lattice at very low coverages. These sites are visible in scanning tunneling micr oscopy images as slightly lowered groups of 2 or 3 adjacent Si adatoms in an otherwise intact Si(111)-(7 x 7) surface. At 150-degrees-C the interstitials are mobile and preferentially occupy sites directly unde r 3-coordinated silicon surface atoms (''rest atoms'') on the faulted side of the 7 x 7 unit cell. An atom-displacing silicide reaction occu rs only for higher coverages, when 7 x 7 half-unit cells become multip y occupied.