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 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\ifmmode\pm\else\textpm\fi{}0.02 \AA{}. 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.