Influence of the Contact Area on the Current Density across Molecular Tunneling Junctions Measured with EGaIn Top-Electrodes

This paper describes the relationship between the rates of charge transport (by tunneling) across self-assembled monolayers (SAMs) in a metal/SAM//Ga₂O₃/EGaIn junction and the geometric contact area (A_g) between the conical Ga₂O₃/EGaIn top-electrode and the bottom-electrode. Measurements of current density, J(V), across SAMs of decanethiolate on silver demonstrate that J(V) increases with A_g when the contact area is small (A_g < 1000 μm²), but reaches a plateau between 1000 and 4000 μm², where J(0.5 V) ≈ 10^-0.52±0.10 A/cm². The method used to fabricate Ga₂O₃/EGaIn electrodes generates a tip whose apex is thicker and rougher than its thin, smoother sides. When A_g is small, the Ga₂O₃/EGaIn electrode contacts the bottom-electrode principally over this rough apex and forms irreproducible areas of electrical contact. When A_g is large, the contact is through the smoother regions peripheral to the apex and is much more reproducible. Measurements of contact pressure between conical EGaIn electrodes and atomic force microscope cantilevers demonstrate that the nominal contact pressure (governed by the mechanical behavior of the oxide skin) decreases approximately inversely with the diameter of geometric contact. This self-regulation of pressure prevents damage to the SAM and makes the ratio of electrical contact area to geometric footprint approximately constant.