Scanning tunneling microscopy contrast in lateral Ge-Si nanostructures on Si(111)-√3x√3-Bi

We investigate the origin of scanning tunneling microscope (STM) contrast in lateral Ge-Si nanostructures prepared on the Si(111)-root 3x root 3-Bi surface [M. Kawamura, N. Paul, V. Cherepanov, and B. Voigtlander, Phys. Rev. Lett. 91, 096102 (2003)]. At low sample bias, the voltage-dependent apparent height difference between Si-and Ge-terminated areas in STM images corresponds exceptionally well to the difference in voltage-integrated scanning tunneling spectroscopy (STS) curves measured in Si-and Ge-terminated areas. The STS curves and the STM contrast reflect both differences in local density of states and in tip-induced effects in Si- and Ge-terminated areas. At higher bias voltage, the tunneling into unoccupied states on Ge-terminated areas is strongly influenced by lowering of the local height of the tunneling barrier with respect to Si. The lowering of the local tunneling barrier height vanishes for the occupied states and can be traced back to different tip-induced band bending on Si-and Ge-terminated areas.