Low temperature scanning electron microscope measurements on a Nb/Ta junction

Until now X-ray detectors based on superconducting tunnel junctions do not meet the theoretical expectations with regard to their energy resolution. To investigate a possible dependence of the signal on the absorption position of the X-ray photon, measurements on a Nb/Ta-junction with a low temperature scanning electron microscope are performed. The results indicate a large lifetime of the quasiparticles as long as they are in the central part of the counter electrode. Near the edges of the counter electrode the superconducting gap is reduced. For intermediate and large bias voltages, this causes a strong reduction of the response at the edges. For small bias voltages the tunnel current of electron-like quasiparticles in the central part of the junction is canceled by the tunnel current of hole-like quasiparticles. For these small bias voltages the signal is mainly created by tunneling from areas with a reduced superconducting gap which results in a flat response over the junction area. For very large amounts of energy dumped in the junction self-recombination losses show up, especially at the corners and edges.