Comparison of SAM-Based Junctions with Ga₂O₃/EGaIn Top Electrodes to Other Large-Area Tunneling Junctions

This paper compares the J(V) characteristics obtained for self-assembled monolayer (SAM)-based tunneling junctions with top electrodes of the liquid eutectic of gallium and indium (EGaIn) fabricated using two different procedures: (i) stabilizing the EGaIn electrode in PDMS microchannels and (ii) suspending the EGaIn electrode from the tip of a syringe. These two geometries of the EGaIn electrode (with, at least when in contact with air, its solid Ga2O3 surface film) produce indistinguishable data. The junctions incorporated SAMs of SCn–1CH3 (with n = 12, 14, 16, or 18) supported on ultraflat, template-stripped silver electrodes. Both methods generated high yields of junctions (70–85%) that were stable enough to conduct measurements of J(V) with statistically large numbers of data (N = 400–1000). The devices with the top electrode stabilized in microchannels also made it possible to conduct measurements of J(V) as a function of temperature, almost down to liquid nitrogen temperatures (T = 110–293 K). The J(V) characteristics were independent of T, and linear in the low-bias regime (−0.10 to 0.10V); the current density decreased exponentially with increasing thickness of the SAM. These observations indicate that tunneling is the main mechanism of charge transport across these junctions. Both methods gave values of the tunneling decay coefficient, β, of ∼1.0 nC–1 (∼0.80 Å–1), and the pre-exponential factor, J0 (which is a constant that includes contact resistance), of ∼3.0 × 102 A/cm2. Comparison of the electrical characteristics of the junctions generated using EGaIn by both methods against the results of other systems for measuring charge transport indicated that the value of β generated using EGaIn electrodes is compatible with the consensus of values reported in the literature. Although there is no consensus for the value of J0, the value of J0 estimated using the Ga2O3/EGaIn electrode is compatible with other values reported in the literature. The agreement of experimental values of β across a number of experimental platforms provides strong evidence that the structures of the SAMs—including their molecular and supramolecular structure, and their interfaces with the electrodes—dominate charge transport in both types of EGaIn junctions. These results establish that studies of J(V) characteristics of AgTS-SAM//Ga2O3/EGaIn junctions are dominated by the structure of the organic component of the SAM, and not by artifacts due to the electrodes, the resistance of the Ga2O3 surface film, or to the work functions of the metals.