Responsivity of Ta-based STJ's to visible light as a function of Al trapping layer thickness

Superconducting tunneling junction devices are being developed as energy resolving photon counting imagers. We have fabricated a series of Ta/Al/AlOx/Al/Ta devices with variable Al layer thickness, d(Al), and measured their response to a weak visible photon source. Single photon sensitivity is observed. We model our system with a set of modified Rothwarf-Taylor equations including quasiparticle losses, trapping, (back)tunneling and calculate the charge responsivity as a function of dAl with one fitting parameter. The quasiparticle loss time for Al and Ta is found to be tau(l)(Al) = 35 mus and tau(1)(Ta) = 0.14 mus in our devices, respectively. (C) 2003 Elsevier B.V. All rights reserved.