FABRICATION OF NB-NBOX-PB JOSEPHSON TUNNEL-JUNCTIONS USING RF GLOW-DISCHARGE OXIDATION

An rf glow-discharge oxidation technique has been applied to the fabrication of Nb-NbOx-Pb Josephson tunnel junctions which exhibit quasiparticle current densities up to 104 A/cm2 at the sum of the gaps (i.e., junction resistances as low as 10-7 Ω cm2). The dependence of the impedance level and the shape of the junction V-I curve on the oxidation parameters (e.g., oxidation time, rf bias voltage, the composition, and the pressure of the argon-oxygen mixture) have been investigated. Good junctions were obtained by using very low rf peak-to-peak voltage (≃60 V) and an oxidation period of 10 min or less. The junction resistance can be controlled by varying the partial pressure of oxygen (less than 3×10-4 Torr) while using a partial pressure of argon high enough (≃10-2 Torr) to sustain a steady rf glow discharge. The results also indicate that sputter removal of niobium oxide is virtually absent in this rf glow-discharge oxidation process. The dependence of the junction resistance on the glow-discharge parameters is qualitatively explained by considering the effects of the parameters on the concentration of the oxidizing species in the vicinity of the substrate.