Chlorine termination of mixed $\mathrm{G}\mathrm{e}/\mathrm{S}\mathrm{i}(100)$ surfaces substantially enhances the contrast between Ge and Si sites in scanning tunneling microscopy observations. This finding enables a detailed investigation of the spatial distribution of Ge atoms deposited on Si(100) by atomic layer epitaxy. The results are corroborated by photoemission measurements aided by an unusually large chemical shift between Cl adsorbed on Si and Ge. Adsorbate-substrate atomic exchange during growth is shown to be important. The resulting interface is thus graded, but characterized by a very short length scale of about one monolayer.

• Received 7 October 2002

DOI:https://doi.org/10.1103/PhysRevLett.90.046102