  Josephson junctions based on InAs semiconducting nanowires and Nb
superconducting electrodes are fabricated in-situ by a special shadow
evaporation scheme for the superconductor electrode. Compared to other metallic
superconductors such as Al, Nb has the advantage of a larger superconducting
gap which allows operation at higher temperatures and magnetic fields. Our
junctions are fabricated by shadow evaporation of Nb on pairs of InAs nanowires
grown selectively on two adjacent tilted Si (111) facets and crossing each
other at a small distance. The upper wire relative to the deposition source
acts as a shadow mask determining the gap of the superconducting electrodes on
the lower nanowire. Electron microscopy measurements show that the fully
in-situ fabrication method gives a clean InAs/Nb interface. A clear Josephson
supercurrent is observed in the current-voltage characteristics, which can be
controlled by a bottom gate. The excess current of 0.68 indicates a high
junction transparency. Under microwave radiation, pronounced integer Shapiro
steps are observed suggesting a sinusoidal current-phase relation. Owing to the
large critical field of Nb, the Josephson supercurrent can be maintained to
magnetic fields exceeding 1 T. Our results show that in-situ prepared Nb/InAs
nanowire contacts are very interesting candidates for superconducting quantum
circuits requiring large magnetic fields.
