Superconducting via contacts to graphene

Highly transparent superconducting contacts is of great interest to research exploring the superconducting proximity effect in two-dimensional materials. In prior studies on NbSe₂/graphene heterostructures [Li et al, Phys. Rev. B 101, 195405 (2020)], Andreev reflections with efficiencies of 80% were demonstrated. This work suggests that a clean van der Waals interface facilitates the superconducting proximity coupling. Here we report on the fabrication and performance of superconducting NbN contacts to graphene, realized through transferred via contacts. The NbN via contacts are sputtered into etched pits of a h-BN sheet, and then transferred to graphene to form a van der Waals-like interface. Our NbN films have a superconducting transition temperature of T_c ~7 K, and an upper critical field of H_c2 > 9 T at 2K. Differential conductance measurements on NbN/graphene junctions show enhancement at small DC voltage biases, as a result of Andreev reflection occurring at the NbN/graphene interface. We characterize the performance of junctions fabricated using different processes. The large H_c2 of NbN will facilitate the studies of the superconducting proximity effect in the quantum Hall regime.