Fabrication of High Quality NbN/Pb Josephson Tunnel Junctions with Plasma Oxidized Barriers

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Abstract
Extremely low subgap leakage current NbN/Pb tunnel junctions, in which V m s exceed 100 mV, were fabricated using a low energy rf plasma oxidation technique. This was done under conditions where cathode self bias voltages V CSB were below 200 V. XPS study shows that NbN surface oxide layer thickness increases from 1.5 nm to 4.5 nm after Ar ions bombarded the NbN surface at V CSB s above 200V. Ar ion bombardment at V CSB s above 200V produces a damaged layer on the NbN surface. Artifical initial oxide layer formation subsequent to NbN film preparation is effective in reducing subgap current under lower V CSB conditions for cleaning and oxidation.

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