Structure determination of theCoSi2(111) surface using medium-energy ion scattering

The surface structure of epitaxially grown ${\mathrm{CoSi}}_2$ crystals on Si(111) has been investigated with use of medium-energy ion scattering. A Co- or a Si-rich surface composition is obtained, depending on the preparation conditions. The structure of the Co-rich surface is shown to be bulklike, i.e., the crystal is terminated by a Si-Co-Si triple layer. The Si-rich surface is found to have, on top of the last Si-Co-Si triple layer, a Si double layer of the same orientation as the ${\mathrm{CoSi}}_2$ bulk lattice. This accounts for the difficulty to grow a 180°-rotated Si film on top of ${\mathrm{CoSi}}_2$(111) by normal molecular-beam-epitaxy techniques. The topmost Co atoms of the Si-rich surface are eightfold coordinated.