High critical magnetic field superconducting contacts to Ge/Si core/shell nanowires

Contacts between high critical field superconductors and semiconductor nanowires are important in the context of topological quantum circuits in which superconductivity must be sustained to high magnetic fields. Here we demonstrate gate-tunable supercurrent in NbTiN-Ge/Si core/shell nanowire-NbTiN junctions. We observe supercurrents up to magnetic fields of 800 mT. The induced soft superconducting gap measured by co-tunneling through a quantum dot is 220 {\mu}eV. To improve contact transparency, we deposit an aluminum interlayer prior to NbTiN and observe a systematic change in the device pinch-off voltages with aluminum thickness. We inform future multi-step device fabrication by observing that aluminum anneals with the Ge/Si nanowire at 180{\deg} celsius, the baking temperature of common electron beam lithography resists.