Transport limitations in tunneling spectroscopy of Ge(111)c(2×8) surfaces

Scanning tunneling spectroscopy is used to study p-type Ge(111)c(2×8) surfaces at temperatures between 7 and 61 K and over a wide range of tunnel currents. The spectral feature arising from Ge rest atoms is found to shift in voltage with increasing tunnel current. A comparison of the current dependence of the results with electrostatic computations of tip-induced band bending yields poor agreement. A model is discussed in which the observed shift in the rest-atom state arises from an accumulation of nonequilibrium carriers at the surface.