Epitaxial base layer Nb superconducting tunnel junctions with Ta absorbers

Superconducting tunnel junction detectors have shown theoretical promise in high resolution X-ray spectrometry. Here we present results for epitaxial Nb tunnel junctions with relatively thick Ta absorbers. In these devices the epitaxial Ta absorber also forms the base electrode for the junction. We report on several junction geometries where both the total size of the junction and the size of the absorber is varied. Current results show a maximum resolution of 65 eV at 5.89 keV, limited primarily by external noise in the junction signal. Significant differences in junction performance are observed for the variation in absorber size, although variation in total junction size from 15 × 15 μm to 40 × 40 μm shows no consistent effect on the resolution. The results indicate a significant loss of recombination phonons into the substrate.