LEEM studies of phase separations and surface depletion of Co and Ni on Si(111)

Low energy electron microscopy images of Si(111) surfaces during submonolay er deposition of Co or Ni at temperatures near 700 degrees C reveal a phase separation into a disordered "ring cluster" phase and the ordered 7 x 7 re construction. For Co/Si(111) we find that temperature-driven phase separati ons at a fixed dose are reversible and occur at a fixed surface coverage. T he shape of the phase boundary reveals a repulsive interaction between Go-c ontaining ring clusters in the disordered phase. For Ni/Si(111), dissolutio n into the bulk prevents reversible separations between surface phases. At coverages close to 1/7 ML an ordered close-packed root 7 x root 7 ring clus ter phase forms for Co/Si(111), but not for Ni/Si(111). The root 7 x root 7 phase disappears irreversibly on heating, due to the nucleation and growth of stable silicide islands.