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 (Ag) 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 Ag when the contact area is
small (Ag < 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 Ag is small, the Ga2O3/EGaIn electrode contacts the bottom-electrode
principally over this rough apex and forms irreproducible areas of electrical contact. When Ag 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 selfregulation
of pressure prevents damage to the SAM and makes the ratio of electrical contact area to geometric footprint
approximately constant.