Observation of a Band of Silicon Surface States Containing One Electron Per Surface Atom

We have studied the energy distribution of electrons photoemitted from surface states on the cleaved (111) face of a 0. 001--cm n-type Si crystal. We find a 1. 8-eV-wide surface band with a peak at 1. 1 eV and a shoulder at 0. 5 eV below the Fermi energy containing about 810^14 electrons/cm^2, i. e. , approximately one electron per surface atom. The strong peak lies below the valence-band maximum at the surface. The surface nature of this structure is confirmed by its disappearance when exposed to vacuum contamination at 10^-10 Torr or to oxygen.