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

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