The reliability of the asperity contact model in determining charge injection across interfaces

Current profiles across mechanically contacted materials usually differ from the ones obtained by corresponding evaporated contacts. The asperity contact model has been brought up to cover such discrepancies. However, there is a lack of experimental evidence concerning its applicability on electronic injection across the interfaces. The work presented in this paper uses I-V curves of a well documented device, the metal-semiconductor contact, as a tool to examine the validity of the asperity contact model and the implications of the interfacial layer, the axial contact force, the interfacial field and the relative permittivity of the surrounding space, on the injection process. Namely, the influence of interfacial layers has been studied in Ultra High Vacuum (U.H.V.) environment (10^-10 mbar) using cleaved silicon samples, contacted by hemispherical metal electrodes (Au, Cu, In, Al) covered in situ by fresh overlayers. The applied axial forces were controlled by electromagnets which displaced stainless steel electrodes to contact chemically prepared and cleaved [110] Si samples in a U.H.V. environment. The importance of the interfacial fields has been examined by using Si and GaAs samples having specific surface profiles i.e. mesas with 10 μm diameter and 1 μm height, fabricated by plasma etching or wet chemistry process. Finally, the effect of the relative permittivity of the surrounding space has been investigated by applying sinusoidal 50 Hz high current densities on metal-metal contacts in laboratory and high vacuum (10^-8 mbar) environments. The obtained results are in good agreement with the expected implications of the above examined factors in the framework of the theory of the asperity contact model.