Fermi-level pinning and charge neutrality level in germanium

The Schottky barrier height in metal/Ge contacts shows weak dependence on the metal work function indicating strong Fermi-level pinning close to the Bardeen limit. The pinning factor $S$ is about 0.05 and the charge neutrality level (CNL) is only about $0.09\mathrm{eV}$ above the top of the valence band. Because of this, the Fermi level in Ge lies higher than CNL in most cases of interest so that unpassivated acceptorlike gap states at the interface are easily filled, building up a net negative fixed charge. This could prevent efficient inversion of a $p$ -type Ge surface in a metal-oxide-semiconductor structure.