Low-temperature tunneling spectroscopy of Ge(111)c(2×8) surfaces

Scanning tunneling spectroscopy is used to study p-type Ge(111)c(2×8) surfaces over the temperature range 7 to 61 K. Surface states arising from adatoms and rest atoms are observed. With consideration of tip-induced band bending, a surface band gap of 0.5±0.1 eV separating the bulk valence band from the surface adatom band is deduced. Peak positions of adatom states are located at energies of 0.09±0.02 eV and 0.24±0.03 eV above this gap. A spectral feature arising from the inversion of the adatom state occupation is also identified. A solution of Poisson’s equation for the tip-semiconductor system yields a value for the interband current in agreement with the observations, for an assumed tip radius of 100 nm. The rest-atom spectral peak, observed at ≈1.0 eV below the valence band maximum, is observed to shift as a function of tunnel current. It is argued that nonequilibrium occupation of disorder-induced surface states produces this shift.