Energetics and bias-dependent scanning tunneling microscopy images of Si ad-dimers on Ge(001)

We report an ab initio study of the energetics and scanning tunneling micro scopy (STM) images of Si ad-dimers on Ge(001) and energetics of Ge ad-dimer s on Si(001). As in the case of Si dimers on Si(001), we find for both syst ems that the D dimer configuration, lying between the substrate dimer rows and parallel to them, is highest in energy. Conversely, recent STM experime nts for Si ad-dimers on Ge(001) deduce the D configuration to be most stabl e. Our theoretical STM images for this system find that both the D and C co nfigurations (the latter also between the rows) have similar STM images for the experimental voltages. We propose an experimental test (low-bias STM i maging) which would unambiguously distinguish between the D and C configura tions. [S0163-1829(99)04631-7].