Influence of defects upon the forward bias admittance of metal-semiconductor interfaces

Static and dynamic electrical properties of several metal-semiconductor interfaces in which the metal thickness is a few hundred or thousand Å are analysed. First, admittance measurements have been made under forward bias in two types of Al/n-GaAs contacts. The Al and n-GaAs layers have been prepared by molecular beam epitaxy on an n⁺-GaAs wafer. In the first type, which shows ideal current-voltage characteristics, only an inductive effect is observed. Such a result cannot be fitted by any effect attributable to interface states but rather by minority carrier injection into the bulk semiconductor. In the second type, differing only in the n-GaAs surface which was prepared under inferior vacuum conditions, non-ideality of current-voltage characteristics and excess capacitances are both seen. The back contact being the same in both types of samples, the onset of excess capacitance must be related to the change in the defect density near the interface. Second, polycrystalline YSi_1.7/Si samples are studied. For all these imperfect metal/semiconductor contacts, the capacitance measured under forward bias shows behavior which can be explained by the existence of states lying within the band gap of the semiconductor and in equilibrium with the majority carrier band.