Voltage-dependent scanning tunneling microscopy images of the Ge(111)-c(2x8) surface

We present scanning tunneling microscopy images of the Ge(111)-c(2X8) surface where both clean and impurity-containing portions exhibit significant voltage-dependent variations. While only adatoms are observed in empty state images, both adatoms and rest atoms simultaneously appear in filled state images but changing contrast with varying bias voltage. The filled state image taken with a voltage as low as 0.5 V confirms that a surface state exists between rest-atom bands and the Fermi level and it is not dangling-bond-like, i.e., its associated charge is found to be distributed over both adatoms and rest atoms. The voltage-dependent images of the impurity-containing surface show delocalized features around the impurity. These delocalized features are observed for the first time on the surface other than III-V semiconductor surfaces. It is attributed to the charged impurity relative to the clean part, causing the band bending near the impurity. (C) 1998 American Vacuum Society.