Review of Germanium Surface Phenomena

Germanium surface behavior has become of great interest recently, chiefly for its importance in the understanding of diode and transistor technology. In general, the surface may be treated as an assemblage of allowed electron states occurring in the normally forbidden energy range. A review of the measurements of the electrical properties suggests that there are two distinct types of state. The ``fast'' state has a hole or electron capture time not greater than a microsecond and is chiefly involved in the recombination process. The ``slow'' state has capture times from a millisecond to several minutes and determines the density and type of carrier at the surface. ``Fast'' states are believed to occur at the interface between the germanium and the oxide layer, and their density of about 1011 cm−2 is determined by the initial surface treatment. ``Slow'' states are associated with the structure of the oxide layer and the gaseous ambient, and have a density greater than 1013 cm−2. Since these states determine the conductivity type at the surface, they contribute to surface ``leakage'' in diodes and transistors and, because of their long equilibrium times, to low-frequency noise. The adsorption of gases such as water vapor, not only controls the density and energy of the ``slow'' states but also leads to possible electrolytic conduction along the surface, in addition to the normal electron flow in the bulk semiconductor.