NBN-NB-AL SUPERCONDUCTING TUNNEL-JUNCTIONS AS PHOTON-COUNTING DETECTORS

Asymmetric NbN-Nb-Al-AlOx-Al-Nb superconducting tunnel junctions have been investigated as photon counting detectors at x-ray and ultraviole t (UV)-visible wavelengths. The inclusion of a thin sup passivation la yer on the top electrode of the devices in place of the natural niobiu m oxides has reduced the quasiparticle loss rates, thereby enhancing t he probability of multiple tunnel processes. As a consequence, the det ector responsivity has increased from 900e(-)/eV, up to values in exce ss of 2000e(-)/eV in the temperature range 0.3-0.8 K. Such a responsiv ity level has allowed single photon counting performance at wavelength s as long as 700 nm and at operating temperatures as high as 830 mK. T he devices show a linear response in the UV-visible range, while at 6 keV the expected nonlinearities in the energy response and moderate en ergy resolution similar to that found in Nb-Al junctions are observed. (C) 1998 American Institute of Physics.