ASYMMETRY OF ADATOMS ON GE(111) OBSERVED BY SCANNING-TUNNELING-MICROSCOPY - THE CLEAN C(2X8) AND SN-INDUCED (7X7) STRUCTURES

Scanning tunneling microscopy (STM) is used to study two types of stru ctures formed on Ge(111): clean c(2 x 8) and the Sn-induced (7 x 7). F or the clean annealed Ge(111) surface, we have found for the first tim e co-existent symmetric and asymmetric c(2 x 8) structures in separate domains with typical sizes of 400 angstrom. Exposure to submonolayer amounts of Sn at high temperatures is found to form both (7 X 7) and ( 5 x 5) structures with domain sizes of 300 angstrom. Empty-state image s of the (7 x 7) reveal strong asymmetry among the adatoms in the dime r-adatom-stacking fault (DAS) model: the inner adatoms tend to form tw elve-member rings around the corner holes, suggesting preferential Sn adsorption in these rings. The relationship of these observations to p revious structural and spectroscopic studies of these surfaces is disc ussed.