Superconducting Nb-Ta-Al-AlOx-Al tunnel junctions for x-ray detection

We report progress on the microlithographic fabrication of Nb-Ta-Al-AlOx-Al structures designed for x-ray detection. These structures use bandgap engineering both for quasiparticle trapping to increase the collection efficiency and to prevent quasiparticle diffusion out through the leads. Non-standard tunnel junction geometries are used to reduce the magnetic field needed to suppress the Josephson current for stable biasing. The performance of these devices as alpha particle detectors is presented.