Grain boundary states in germanium

We have investigated the electrical properties of large-angle grain boundaries in n- and p-type germanium bicrystals. Measurements of the two-dimensional conductivity in the boundary plane and of the thermally activated conductance across the boundary were performed on the same specimen for the first time. A sequence of measurements was chosen which allowed us to exclude contamination or other experimental differences as the reason for a fundamental discrepancy between earlier results of different authors. From a new analysis we are able to show that all experimental results are in fact consistent with the same model of a two-dimensional quasi-metallic hole band for the grain boundary states. In the neutral state, the Fermi level of this band is very close to the edge of the valence band with a few milli-electron-volts as an upper limit.