The Rectification Process at Metal-Silicon Surface Barriers

Abstract

Published explanations for the rectification at metal-semiconductor contacts have ranged from those which attribute the establishment of the diffusion potential (Vd) entirely to work function differences to those which presume a major role for adsorbed gases and surface states. By making direct (differential capacitance) measurements of the barrier height (Vd) and E-I characteristics during fabrication, and as a function of ambient and surface treatment, it is shown that it is possible to rank the variables for the type of etched silicon surfaces commonly used for radiation detector fabrication in the following order of importance; (1) surface treatment, (2) adsorbed gases, (3) work function. It is also shown that the high reverse currents observed in Au-Si structures before exposure to air are directly correlated with a small Vd and that the Vd associated with various surface treatments agrees with the results of Buck. Preliminary work with liquid metal contacts supports the assumption of a secondary role for the work function. A theoretical model based on surface states and interfacial layers of finite thickness is presented to explain the behavior of metal contacts on etched silicon surfaces, including the fact that many metals (e. g., Al) have been observed to form both rectifying and non-rectifying contacts on etched Si.