Geometry effects on the performance of superconducting tunnel junctions

Superconducting tunnel junctions are predicted to have high energy resolving power at X-ray energies (∼4 eV FWHM at 6 keV). Possible mechanisms for the degradation of the energy resolution in an experimental configuration are spatial non-uniformities in the response of the STJs, induced by the loss of quasiparticles into the connecting leads or at the edges of the detectors. In order to investigate the effects of the device geometry on its performance as an X-ray detector, STJs of different sizes (from 10 × 10 to 200 × 200 μm2) and with different widths of the connecting leads (1 and 3 μm) have been fabricated on the same chip. The STJs are niobium based devices with 110 nm thick epitaxial base electrodes and 180 nm thick polycrystalline top electrodes. The response of devices to 6 keV X-rays is discussed.