Optical and near-infrared photon counting detector using superconducting tunnel junctions

The properties of Superconducting Tunnel Junctions (STJ) make them very suitable for low light level astronomical observations. The applications are of great importance for the future in the field of high-angular resolution astronomy where STJ surpass all other detectors. STJ detectors will be used to improve the sensitivity of such a system to be implemented on ground-based interferometer like the GI2T in France or IOTA in USA. All this instrumentation is developed in the framework of the Very Large Telescopes Interferometer built by the European Southern Observatory in Chile. We have developed a modified version of the "SNEP" fabrication process for making Nb/Al-AlOx-Al/Nb and Ta/Al-AlOx-Al/Ta/Nb STJs, showing very low sub-gap leakage current at 0.2 K by using double thin aluminium trapping layers and the higher energy gap of the Ta or Nb absorber grown epitaxially on the R-plane sapphire substrate. The key points of the junction fabrication process have been studied: experimental parameters suitable to grow high quality, (2 0 0) oriented epitaxial niobium films with rho(300 K)/rho(10 K) > 20 by DC-magnetron sputtering on 3 in.-diameter R-plane sapphire substrates heated at 600 degrees C and AlOx barrier oxidation conditions. The influence of such processing parameters and trapping layer thickness have been analysed on the [I-V] curves measured down to 0.2 K. (C) 2000 Elsevier Science B.V. All rights reserved.