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

Abstract

We show that cobalt atoms deposited on Si(111)-(7\ifmmode\times\else\texttimes\fi{}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\ifmmode\times\else\texttimes\fi{}7) surface. At 150 \ifmmode^\circ\else\textdegree\fi{}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\ifmmode\times\else\texttimes\fi{}7 unit cell. An atom-displacing silicide reaction occurs only for higher coverages, when 7\ifmmode\times\else\texttimes\fi{}7 half-unit cells become multiply occupied.