Ramp-edge josephson junctions with nd_1.85ce_0.15cuo_4-y barriers

We have fabricated YBa₂Cu₃O_7-δ ramp-edge Josephson junctions using Nd_1.85Ce_0.15CuO_4-y as the barrier material. In the fabrication process we introduced an in situ cleaning process for the ramp surface in the base electrode prior to the deposition of the barrier layer and the top electrode. In the cleaning process a low energy oxygen and/or argon ion beam is used. Varying the barrier thickness d, the critical current density J_c and the normal resistance times area ρ_n of the junctions were found to be proportional to exp(-d/ξ_n) and exp(d/ξ_r) with characteristic decay lengths ξ_n = 3.8 nm and ξ_r = 7.2 nm, respectively. For junctions with d > 10 nm, J_c ρ_n products above 1 mV at 77 K were obtained. The spatial distribution of the critical current density was imaged directly by low-temperature scanning electron microscopy (LTSEM) and showed considerable inhomogeneities on a μm scale.