Aluminum superconducting tunnel junction as X-ray detector: Technological aspects and phonon decoupling from the substrate

We have investigated Al/Al2O3/Al Superconducting Tunnel Junctions (STJs) for their use as X-ray detector. The junctions have been fabricated on sapphire substrates both with and without a SiO buffer layer. It was found that the presence of the SiO significantly changes the Al2O3 tunnel barrier properties. Experiments under X-ray irradiation from Fe-55 X-ray source are reported. The spectra show that the SiO buffer layer succeeds in reducing the events coming from the phonons generated into the substrate. Besides, the charge yield is about 85% for STJs with the SiO buffer layer, quite high in comparison with the 5% of STJs without the buffer layer.