Energy gap revealed by low-temperature scanning-tunnelling spectroscopy of Si(111)-7x7 surface in illuminated slightly-doped crystals

Physical properties of Si(111)-7x7 surface of low-doped n- and p-type Si samples is studied in the liquid helium temperature region by the scanning-tunnelling microscopy and spectroscopy. Conduction required for the study is provided by illumination of the surface. Application of illumination removes completely the band bending near the surface and restores initial population of the surface states. Our results indicate the existence of the energy gap 2{\Delta} = 40 \pm 10 meV in intrinsically-populated Si(111)-7x7 surface.