Superconducting tunnel junctions on Nb- and Ta-single crystal substrates for X-ray spectrometry

We present the first high quality Nb/Al-based tunnel junctions, fabricated with a superconducting interface onto single crystal substrates of Nb and Ta. Current-voltage measurements down to 1.4 K show a subgap current which is dominated by thermal excitation of quasiparticles, no ohmic or superconducting leakages could be observed. The fabrication process and development steps, such as preparation of ultra-smooth crystal surfaces are described.