Submonolayer phases of Pb on Si(111).

We present conclusive results on the structures and coverages of the submonolayer phases of Pb on the Si(111) 7\ifmmode\times\else\texttimes\fi{}7 surface based on scanning-tunneling-microscopy, low-energy-electron-diffraction, and Rutherford backscattering measurements. For room-temperature deposition at low coverage on the 7\ifmmode\times\else\texttimes\fi{}7 surface, we find that Pb atoms occupy sites above and between the Si adatoms (with a preference for the faulted half of the unit cell) leaving the 7\ifmmode\times\else\texttimes\fi{}7 structure intact. After annealing to 450 \ifmmode^\circ\else\textdegree\fi{}C, some Pb atoms occupy Si(111) 7\ifmmode\times\else\texttimes\fi{}7 adatom sites, while others have nucleated to form small areas of an unusual \ensuremath{\surd}3 \ifmmode\times\else\texttimes\fi{} \ensuremath{\surd}3 phase which has a 1:1 ratio of Si:Pb.