Electronic structure of the 7×7 Si(111): Differences due to preparation

The 7×{}7 Si(111) surface has been investigated with regard to the character of the surface-state band and the existence of a band gap. From the differences between energy-loss spectra taken at surfaces prepared by cleavage in ultrahigh vacuum and those prepared by polishing and heating, it has been proven experimentally that these differences are due to carbon contamination of the polished surfaces. Ther\'e is strong evidence that this contamination is localized below the surface but within the space-charge region and cannot therefore be detected by Auger electron spectroscopy, resulting in the conclusion that the surface Fermi level of the intrinsic 7×{}7 Si(111) lies within a gap and is therefore not metallic. This conclusion has to be considered when the 7×{}7 Si(111) surface is described theoretically.