QUASI-PARTICLE DIFFUSION AND LOSS PROCESSES IN SUPERCONDUCTING TUNNEL-JUNCTIONS

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