Preparation and characteristics of Nb/Al-oxide-Nb tunnel junctions

Abstract

Preparation and characteristics of refractory Nb/Al-oxide-Nb Josephson tunnel junctions with critical current densities jc from ∼103 to ∼4×104 A/cm2 were studied with respect to junction quality, uniformity, reproducibility and jc control. The key steps leading to improved junction parameters are emphasized and compared with previously used procedures. The results include jmaxc/jminc of 1.2, on a 1-in. chip, Vm up to 56 mV in junctions with jc∼103 A/cm2, and Vm up to 24 mV with jc∼6×103 A/cm2. Good run-to-run reproducibility and control of jc are also demonstrated. In particular, jc is shown to increase by a factor of ∼40 when the oxygen pressure during the in-situ oxidation of the Al overlayer is reduced from 1 to 0.01 Torr.