Identify SIS/NIS behavior of the NbN/NbOx/In interface in non-passivated NbN flip-chip devices

Determine the nonlinear current–voltage characteristics of the NbN/NbO_x/In interface in the non-passivated niobium nitride flip-chip device across temperature, in order to establish whether the interface exhibits superconductor–insulator–superconductor (SIS) behavior below the superconducting transition temperature of indium and normal-metal–insulator–superconductor (NIS) behavior above, thereby supporting or refuting the interpretation proposed from resistance measurements.

Background

In the non-passivated NbN flip-chip device, the authors observed an additional sharp increase in resistance around 3.5 K (the Tc of indium) and attributed it to a thin native oxide layer on NbN. They interpret the NbN/NbOx/In interface as forming an SIS junction below the Tc of indium with an estimated critical current from the Ambegaonkar–Baratoff relation, and as an NIS junction above the Tc. Although the temperature dependence of voltage resembles NIS behavior, definitive identification requires direct nonlinear I–V characterization across temperatures.

The authors explicitly state that additional measurements are needed to confirm this interpretation and defer this task to future work.