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

Scanning tunneling spectroscopy is used to study $p$-type $\mathrm{Ge}(111)c(2\ifmmode\times\else\texttimes\fi{}8)$ surfaces over the temperature range $7\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}61\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. Surface states arising from adatoms and rest atoms are observed. With consideration of tip-induced band bending, a surface band gap of $0.5\ifmmode\pm\else\textpm\fi{}0.1\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ separating the bulk valence band from the surface adatom band is deduced. Peak positions of adatom states are located at energies of $0.09\ifmmode\pm\else\textpm\fi{}0.02\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ and $0.24\ifmmode\pm\else\textpm\fi{}0.03\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ 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 $100\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$. The rest-atom spectral peak, observed at $\ensuremath{\approx}1.0\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ 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.