LEEM Studies of Phase Separations and Surface Depletion of Coand Nion Si(111)

Low energy electron microscopy images of Si(111) surfaces during sub monolayer deposition of Co or Ni at temperatures near 700°C reveal a phase separation into a disordered "ring cluster" phase and the ordered 7 × 7 reconstruction. For Co/Si(111) we find that temperature-driven phase separations at a fixed dose are reversible and occur at a fixed surface coverage. The shape of the phase boundary reveals a repulsive interaction between Co-containing ring clusters in the disordered phase. For Ni/Si(111), dissolution into the bulk prevents reversible separations between surface phases. At coverages close to 1/7 ML an ordered close-packed$\sqrt{7}\times\sqrt{7}$ring cluster phase forms for Co/Si(111), but not for Ni/Si(111). The$\sqrt{7}\times\sqrt{7}$phase disappears irreversibly on heating, due to the nucleation and growth of stable silicide islands.