S. V. Khare, R. V. Kulkarni, D. Stroud, and J. W. Wilkins, Department of Physics, Ohio State University, Columbus, OH 43210

We report an ab initio study of the energetics and STM images of Si ad-dimers on Ge(001) and energetics of Ge ad-dimers on Si(001). As in the case of Si dimers on Si(001), we find for both systems that the D dimer configuration, lying between the substrate dimer rows and parallel to them, is highest in energy. Conversely, recent STM experiments for Si ad-dimers on Ge(001) deduce the D configuration to be most stable. Our theoretical STM images for this system find that both the D and C configurations (the latter also between the rows) have similar STM images for the experimental voltages. We propose an experimental test (low-bias STM imaging) which would unambiguously distinguish between the D and C configurations.