The asymmetric or symmetric nature of the dimer adatom arrangement on reconstructed Si and Ge (001) surfaces is still controversial. There has not been a direct quantitative determination of the asymmetry. Using x-ray standing waves we have directly measured the asymmetry of Ge dimers on the Si(001) surface as 0.55±0.02 Å. Our results unequivocally rule out the possibility of symmetric dimers. At elevated temperatures the dimer asymmetry decreases, raising the possibility of a high temperature asymmetric to symmetric phase transition on the (001) surfaces.

• Received 26 October 1992

DOI:https://doi.org/10.1103/PhysRevLett.70.2790