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 trappi ng of quasiparticles in the Nb top layer with an increase of the charg e collection efficiency. The temperature dependence of quasiparticle a nd 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=20X20 mu m(2)), and high dynamical resistan ces in the subgap region. Preliminary measurements under 6 keV x-ray i rradiation have shown an increased value of the maximum collected char ge. (C) 1995 American Institute of Physics.