Fabrication of Nb-NbOx-Pb Josephson tunnel junctions using rf glow-discharge oxidation

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Abstract
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.