DIRECT MEASUREMENT OF THE ASYMMETRIC DIMER BUCKLING OF GE ON SI(001)

The asymmetric or symmetric nature of the dimer adatom arrangement on reconstructed Si and Ge(001) surfaces is still controversial. There ha s not been a direct quantitative determination of the asymmetry. Using x-ray standing waves we have directly measured the asymmetry of Ge di mers on the Si(001) surface as 0.55+/-0.02 angstrom. Our results unequ ivocally rule out the possibility of symmetric dimers. At elevated tem peratures the dimer asymmetry decreases, raising the possibility of a high temperature asymmetric to symmetric phase transition on the (001) surfaces.