Tantalum Superconducting Tunnel Junctions for photon counting detectors.

The properties of Superconducting Tunnel Junctions (STJ) make them very suitable for low light level astronomical observations from the visible to the near-infrared. We have developed a modified version of the "SNEP" fabrication process for making Ta/Al-AlOxAl/Ta/Nb photon counting STJ devices. These junctions show a very low sub-gap leakage current at 0.2 K. They use a double thin aluminum trapping layer and a Ta absorber grown epitaxially on a R-plane sapphire substrate. The parameters of the junctions have been investigated in order to optimize future devices, i.e. pixel area, Josephson current density, subgap leakage current, Ta mean free path. Photon counting ability in the near infrared at 0.78 micron has been demonstrated with such Ta-based junctions. [ABSTRACT FROM AUTHOR]