Ab initio study of height contrast in scanning tunneling microscopy of Ge/Si surface layers grown on Si(111) in presence of Bi

It has been recently demonstrated that Bi surfactants over Ge layers grown on Si(111) allow us to distinguish Ge and Si covered regions through a scanning tunneling microscope. We revised this problem by considering geometric (or structural) and electronic effects to explain the measured apparent height difference of relaxed surface layers. The local density of states and related decay lengths into vacuum are calculated within density functional theory for each surface of interest. The results are compared with recent experimental data from a scanning tunneling spectroscopy study [J. Myslivecek, F. Dvorak, A. Strozecka, and B. Voigtlander, Phys. Rev. B 81, 245427 (2010)].