  The scattering potential of the defects on Ge(001) surfaces is investigated
by first-principles methods. The standing wave in the spatial map of the local
density of states obtained by wave function matching is compared to the image
of the differential conductance measured by scanning tunneling spectroscopy.
The period of the standing wave and its phase shift agree with those in the
experiment. It is found that the scattering potential becomes a barrier when
the electronegativity of the upper atom of the dimer is larger than that of the
lower atom, while it acts as a well in the opposite case.
