The responsivity of Ta-based STJ devices as a function of temperature and bias

The potential of superconducting tunneling junctions (STJ) as energy resolving photon counters was proved in the range of 1 eV to 6 keV. In our laboratory we are producing and studying Ta/Al1/AlOx/Al2/Ta STJs. The responsivity of a STJ depends on the energy of the photon, the geometry of the STJ, the temperature and the bias point. We measured the dependence of the responsivity on the bias and the temperature. We model our devices with four quasiparticle populations coupled by the usual Rothwarf-Taylor balance equations. This simple approach appears to explain some of the observed features and doesn't require the introduction of local traps in the quasiparticle dynamics.