High critical current density NbN/AlN/NbN tunnel junctions fabricated on ambient temperature MgO substrates

NbN/AlN/NbN tunnel junctions are fabricated at ambient temperature on MgO substrates, and a critical current density of 8 kA/cm2 is obtained in junctions with 1.5–nm–thick AlN barriers. Even though the NbN/AlN/NbN trilayers are deposited without intentional heating, the junctions show a large gap voltage (Vg=5 mV), sharp quasiparticle current rise (ΔVg=0.16 mV), and small subgap leakage current (Vm=25 mV and Rsg/RN=9). This report shows that high-quality NbN/AlN/NbN tunnel junctions can be made at ambient substrate temperature.