Inversion layer on the Ge(001) surface from the four-probe conductance measurements

We report four-probe conductance measurements with sub-micron resolution on atomically clean Ge(001) surfaces. A qualitative difference between n-type and p-type crystals is observed. The scaling behavior of the resistance on n-type samples indicates two-dimensional current flow, while for the p-type crystal a three-dimensional description is appropriate. We interpret this in terms of the formation of an inversion layer at the surface. This result points to the surface states, i.e., dangling bonds, as the driving force behind band bending in germanium. It also explains the intrinsic character of band bending in germanium.