The specific capacitance of Nb/Al-AlOx/Nb SIS junctions with extremely low RnA product

Paper in proceeding, 2017

This paper provides new insight regarding the specific capacitance of Nb/Al-AlOx/Nb SIS junctions with low RnA product. Employing the direct junction capacitance measurement method, the specific capacitance (Cs) and RnA of several junctions with various RnA values ranging from 8.8 to 68 Ω.µm2 was studied. We noticed non-negligible scatter in the measured RnC (normal resistance times junction capacitance) product for the junctions with the same RnA value. We demonstrated that the local variations in the thickness distribution of the tunnel barrier could have resulted in the scatter of the RnC data. We also show that, even at such low microwave frequencies as in our direct measurement method, the previously neglected nonlinear susceptance should be accounted, especially for junctions with low RnA values. We present the measured Cs vs RnA data for Nb/Al-AlOx/Nb junctions.