To explore the origin of the Fermi level pinning in germanium we investigate
the Ge(001) and Ge(001):H surfaces. The absence of relevant surface states in
the case of Ge(001):H should unpin the surface Fermi level. This is not
observed. For samples with donors as majority dopants the surface Fermi level
appears close to the top of the valence band regardless of the surface
structure. Surprisingly, for the passivated surface it is located below the top
of the valence band allowing scanning tunneling microscopy imaging within the
band gap. We argue that the well known electronic mechanism behind band bending
does not apply and a more complicated scenario involving ionic degrees of
freedom is therefore necessary. Experimental techniques involve four point
probe electric current measurements, scanning tunneling microscopy and
spectroscopy.Comment: 5 pages, 4 figure
