Bias voltage dependence of the signal polarity of superconducting tunnel junction detectors

Using Low Temperature Scanning Electron Microscopy (LTSEM) we have measured the bias voltage dependence of the charge output of superconducting tunnel junction detectors, which have different energy gaps Δb and Δc for the base and the counter electrode, respectively. The time dependence of the detector response was measured for various bias voltages Vb after perturbing the counter electrode with short electron beam pulses of 10 ns duration and about 50 keV total energy. For Nb/Al/AlOx/Nb tunnel junctions with Δc - Δb approximately equals 0.2 meV we observed a negative and positive quasiparticle tunneling current for Vb less than or equal 0.2 mV and Vb > 0.2 mV, respectively. The observed change of the signal polarity can be explained using standard tunnelling theory taking into account the different bias voltage dependence of electron and hole like quasiparticle current.