Medium-energy ion scattering investigation of the surface and subsurface structure of three-dimensional HoSi2x grown on Si(111)

The surface structure of the three-dimensional (3D) holmium silicide grown on Si(111) has been determined using medium-energy ion scattering. Using this technique, it has been possible for the first time to elucidate the structure of the surface bilayer, free from complications due to lower layer atoms. It has been established that this surface generally displays sixfold symmetry which has been attributed to a mixture of reversed and nonreversed buckled bilayers (B-type and A-type respectively). A threefold surface has also been observed which is due to the preferential growth of an A-type buckled bilayer. It is proposed that the underlying vacancy network is the cause of the mixed phase on the surface. Both phases show consistent lattice spacings with the surface Si bilayer in the A/B mixed phase sitting 1.97 +/- 0.01 A above the first Ho layer, with a vertical buckling separation of 0.79 +/- 0.04 A. Such terminations are present on top of the established layered structure of the 3D silicide, with a Ho-Ho vertical spacing of 4.01 +/- 0.02 A, which is compressed relative to the bulk value due to the relief of strain at the interface.