Physical properties of the Si(111)-7×7 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 2Δ = 40 ± 10 meV in the intrinsically populated Si(111)-7×7 surface.

Electronic properties of the Si(111)-7×7 surface are studied using four-and two-probe conductivity measurements and tunneling spectroscopy. We demonstrate that the temperature dependence of the surface conductivity corresponds to the Efros-Shklovskii law at least in 10 − 100 K temperature range. The energy gap at the Fermi level observed in tunneling spectroscopy measurements at T ≥ 5 K vanishes by thermal fluctuations at T ≈ 30 K, without any sign of the metal-insulator transition. We show that the low-temperature energy gap observed by the tunneling spectroscopy technique is actually the consequence of the Coulomb blockade effect.

Electronic transport properties of the Si(111)-( √ 3 × √ 3)-Sn surface formed on low doped Si substrates are studied using two-probe conductivity measurements and tunnelling spectroscopy. We demonstrate that the ground state corresponds to Mott-Hubbard insulator with a band gap 2∆ = 70 meV, which vanishes quickly upon temperature increase. The temperature dependence of the surface conductivity above T > 50 K corresponds to the Efros-Shklovskii hopping conduction law. The energy gap at the Fermi level observed in tunnelling spectroscopy measurements at higher temperatures could be described in terms of dynamic Coulomb blockade approximation. The obtained localization length of electron is ξ = 7Å.