Quasiparticle diffusion and loss processes in superconducting tunnel junctions

Superconducting Tunnel Junctions (STJ) are promising as X-ray detectors. However, they still do not reach their theoretical energy resolution due to various loss processes such as the diffusion of quasiparticles out of the electrode volume and trapping in regions of reduced energy gap. Low Temperature Scanning Electron Microscopy (LTSEM) allows the investigation of inhomogeneities in the response of superconducting Nb/AlOx/Nb tunnel junctions with high spatial resolution. In this way diffusion of quasiparticles and local trapping sites can be identified directly. The impact of these processes on the homogeneity of the signal height and the energy resolution can be visualized. Furthermore, the diffusion length and the lifetime of the quasiparticles is derived. Numerical simulations show that it is necessary to further reduce signal inhomogeneities due to diffusion processes and local traps to ensure a mayor improvement in the energy resolution. (orig.)