Experiments on Ge-Sn barrier Josephson junctions

Supercurrents have been observed through thick (∼500Å) barriers of co-evaporated Germanium - tin mixtures. The Critical current density is a strong function of the barrier composition and thickness. For 600Å thick barriers, a change in composition from 20% Sn to 50% Sn yields a factor of 10 6 change in current density. Typically the critical current of the junctions decreases by a factor of 10 for every 80Å increase in barrier thickness. In general, the low current-density junctions show I-V characteristics which are well described by tunneling. High current-density junctions usually have characteristics which, depending on junction preparation, indicate either tunneling, or metal-to-metal conduction. By using smooth base-electrodes it is possible to make junctions with barriers as thin as 400Å that are effectively pinhole free and whose I-V characteristics show true tunneling. The relatively thick barriers of these junctions allow a substantial decrease in the junction capacitance per unit area compared to oxide barrier junctions. This lower capacitance allows the fabrication of junctions with non-hysteretic I-V characteristics having critical current densities as small as 10 3 A/cm 2 . Using small-area, non-hysteretic junctions, we have made a high-sensitivity D. C. SQUID with an intrinsic energy resolution of 10 -31 Joules/Hz.