Dependence of the scatter of the electrical properties on localnon-uniformities of the tunnel barrier in Nb/Al-AlOx/Nbjunctions

In this paper, we study the effect of the tunnel barrier thickness non-uniformity inNb/Al-AlOx/Nb tunneljunctions using the measurement results of the junctioncapacitance(C) and the normal resistance (Rn). Thelocal thickness distribution of the AlOx tunnel barrier inNb/Al-AlOx/Nb trilayer (RnA ∼ 30 Ωμm2) was studied by high resolution transmission electronmicroscopy. The specific resistance (RnA)values of the measured junctions range from 8.8 to 68 Ω μm2. Weobserved scatterin both the junction specific resistance and capacitance data, which isconsiderably higher than the measurement uncertainty. We also observed noticeablescatter in theRnC product, which does not stem from junction area estimationuncertainties. We discuss the possible reasons that contribute to this scatter. We suggest that thelocal thickness non-uniformity of the tunnel barrier significantly contributes to thescatter in theRnC product. We confirm this conclusion through anillustrative model based on the barrier imaging data, which results in the variation ofthe RnC data consistent with the measurements in thispaper.