High-Bias Instability of Atomic and Molecular Junctions

Abstract

Atom-sized contacts are appealing interconnects in atomicand molecular-scale devices. Because these contacts are smaller than the mean free path of electrons, conduction electrons are rarely scattered when they pass through the contact. This absence of scattering events not only ensures high electron mobility but also achieves small energy dissipation at the contact. As a result, we can obtain in atom-sized contacts a huge current density that is unattainable for present-day microfabricated interconnects. A single-atom contact of Au, for example, is capable of sustaining a current density as high as 8× 1010A/cm (Yanson et al., 1998), which is orders of magnitude higher than the current density realized in microelectronics devices. However, atom-sized contacts would eventually become unstable and break down when they are subject to sufficiently high biases or high currents. This stability limit of atom-sized contacts is of much value both in application and in academia. In electronics applications, the magnitude of allowable bias or current is a critical parameter because it directly determines the maximum rating of atom-sized contacts when they are incorporated into real devices. For atom-sized contacts, even a low voltage such as 3 V, commonly used in CMOS devices, can be regarded as a high bias which, in the case of an Au single-atom contact, would generate a current density that well exceeds the maximum value mentioned above. On the other hand, typical molecular FETs would operate under biases much higher than 3 V and hence achieve current densities far exceeding the maximum value. It is therefore practically quite important to know the stability limit of the atom-sized contacts under high-bias/current conditions. The same stability problem also provides us a fertile ground of physical investigations. The high-bias/current instability of atom-sized contacts involves various microscopic processes which have not yet been fully worked out. When one applies a high bias to an atom-sized contact, hot electrons are injected from one electrode into the contact. Though the contact is smaller in size than the electronic mean free path, there still remains a non-zero chance for the hot electrons to interact with the lattice and partly transfer their kinetic energy, and/or momentum, to the contact atoms, often causing their vibrational heating and electromigration. At the downstream electrode, electrons dissipate their energy which diffuses out to the bulk through the lattice heat conduction. All these processes are individually well studied in macroscopic materials but little understood for ultrasmall conductors such as the atom-sized contacts. To obtain some insight on this topic of practical and academic interest, some High-Bias Instability of Atomic and Molecular Junctions