Quasiparticle Freeze-Out in Superconducting Tunnel Junction X-ray Detectors with Killed Base Electrode

The current–voltage characteristics of superconducting tunnel junction (STJ) X-ray detectors were measured in the temperature range from 4.2 to 0.1 K. The freeze-out of the thermal tunneling current was compared between an STJ detector with a traditional Nb/Al/Al 2 O 3 /Al/Nb layer structure and a Ti/Nb/Al/Al 2 O 3 /Al/Nb/NbN detector whose low-gap Ti film kills the X-ray response of the base electrode. The current decrease and the linear low-temperature I(V) characteristics for the detector with the killed electrode can be qualitatively explained by tunneling current contributions from the subgap states of the Ti film. The data are analyzed on the basis of the proximity theory in the dirty limit.