Strong quasiparticle trapping In A 6×6 array of vanadium-aluminum superconducting tunnel junctions

A 6×6 array of symmetrical V/Al/AlOx/Al/V Superconducting Tunnel Junctions (STJs) was fabricated. The base electrode is a high quality epitaxial film with a residual resistance ratio (RRR) of ∼30. The top film is polycrystalline with an RRR of ∼10. The leakage currents of the 25×25 μm2 junctions are of the order of 0.5 pA/μm2 at a bias voltage of 100 μV, which corresponds to a dynamical resistance of ∼3105 Ω. When the array was illuminated by 6 keV X-ray photons from a 55Fe radioactive source the single photon charge output was found to be low and strongly dependent on the temperature of the devices. This temperature dependence at X-ray energies can be explained by the existence of a very large number of quasiparticle (QP) traps in the Vanadium. QPs are confined in these traps, having a lower energy gap than the surrounding material, and are therefore not available for tunneling. The number of traps can be derived from the energy dependence of the responsivity of the devices (charge output per electron vo...