Correction of systematic errors in scanning tunneling spectra on semiconductor surfaces: the energy gap of Si(111)-7x7 at 0.3 K

The investigation of the electronic properties of semiconductor surfaces using scanning tunneling spectroscopySTS is often hindered by nonequilibrium transport of the injected charge carriers. We propose acorrection method for the resulting systematic errors in STS data, which is demonstrated for the well-knownSi111-77 surface. The surface has an odd number of electrons per surface unit cell and is metallic above20 K. We observe an energy gap in the ground state of this surface by STS at 0.3 K. After the correction, themeasured width of the gap is 70+-15 meV, which is compatible with previous less precise estimates. No sharppeak of the density of states at the Fermi level is observed, in contrast to proposed models for theSi(111)-7x7 surface.