Fermi level pinning at the Ge(001) surface - A case for non-standard explanation

TL;DR

This paper investigates the unexpected Fermi level pinning at Ge(001) surfaces, revealing that traditional electronic explanations are insufficient and proposing a more complex ionic mechanism.

Contribution

It introduces a non-standard explanation for Fermi level pinning at germanium surfaces, challenging the conventional electronic band bending model.

Findings

Fermi level remains pinned despite surface passivation.

Surface Fermi level location varies unexpectedly with doping.

Ionic effects may play a role in Fermi level pinning.

Abstract

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.