Random adatom heights in Ge/Si(111)-5x5 surfaces

Germanium-induced 5 x 5 reconstruction on Si(111) surfaces was investigated by scanning tunneling microscopy and spectroscopy. Although all adatoms in the Ge/Si(111)-5 x 5 structure were crystallographically equivalent, their topographic heights were not the same in the filled-state image. Height differences up to similar to 0.8 Angstrom between neighboring adatoms in the same unit were recognized. Site specific tunneling spectroscopy showed that according to the height depression the adatom dangling bond states shifted towards the empty-state across the surface Fermi level. The adatom undulation may originate from mixing of the substrate silicon atoms into the Ge/Si(111)-5 x 5 structure. Preferential sites for Ge/Si replacement are discussed in terms of the strain energy due to the mixing. The distributions of the height differences did not change for Ge thicknesses from 2-6 ML, indicating that 5 x 5 reconstruction may contain substrate Si atoms even for a 6 ML thick Ge layer.