Abstract
Extremely low subgap leakage current NbN/Pb tunnel junctions, in which Vms exceed 100 mV, were fabricated using a low energy rf plasma oxidation technique. This was done under conditions where cathode self bias voltages VCSB 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 VCSBs above 200V. Ar ion bombardment at VCSBs 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 VCSB conditions for cleaning and oxidation.