Origin of subgap current in an SIS junction

Abstract

According to the BCS theory, the subgap current in an SIS junction is reduced exponentially as the temperature decreases. However, experimentally, the reduction is saturated below a certain temperature. This phenomenon was a bottleneck in the development of sensitive SIS photon detectors with NEP of 10-19 W√HZ at 650 GHz, because the NEP is limited by the shot noise in the subgap current. To investigate the origin of the residual subgap current under various temperatures we have measured I - V curves of a Nb/Al-AIN/Nb junction. Obtained I - V curves have been reproduced by a model: singleand two-particle tunneling under reduced quasiparticle lifetime. Our result suggests that increasing the lifetime is essential to reduce the subgap current. One of possible methods to increase the lifetime is to form an epitaxial Nb film. © 2009 American Institute of Physics.