STM/STS measurements of adatom vacancies on Si(111)-7x7 reconstruction surface

Local electronic structure around adatom vacancies on Si (111)-7 x 7 surface has been investigated using STM (scanning tunneling microscopy) and STS (scanning tunneling spectroscopy). The nearest neighbor adatom to a vacancy appears bright in the STM observations, while the brightness of the other neighbor adatoms are almost unchanged. The nearest neighbor adatom slightly approaches to the vacancy, while the other neighbor adatoms do not change their location. These results suggest that the electronic states between the vacancy and its nearest neighbor adatom are changed. In the STS spectra for each type of adatom vacancies, a new state appears at about 0.5 eV below the Fermi level. The new state also appears in the STS spectra for the nearest neighbor adatom of the vacancies. Thus, it is clear that the state about 0.5 eV below Fermi level is associated with the localized vacancy level.