Interstitial precursor to silicide formation on Si(111)-(7×7)

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