Influence of a NbN overlayer on Nb/Al–AlOx/Nb high quality Josephson tunnel junctions for x-ray detection

Nb/Al–AlOx/Nb–NbN Josephson junctions for use as x‐ray detectors have been fabricated. The NbN overlayer has been used to realize the trapping of quasiparticles in the Nb top layer with an increase of the charge collection efficiency. The temperature dependence of quasiparticle and Josephson critical current have been investigated and compared with that without a NbN overlayer, showing low leakage currents, about 200 pA (V=0.5 mV, T=0.67 K, A=20×20 μm2), and high dynamical resistances in the subgap region. Preliminary measurements under 6 keV x‐ray irradiation have shown an increased value of the maximum collected charge.