Transport and defect engineering in NbN/AlN(GaN) Josephson junctions

Niobium nitride–polar nitride heterostructures are promising for scalable Josephson devices. We calculate interface energies and Schottky barriers for β-Nb₂N/δ-NbN–AlN/GaN interfaces and find positive barrier potentials for all interfaces with band offsets differing by ~2 eV for the two polarities of AlN/GaN. This results in a built-in electric field in these junctions. We then construct a Wannier tight-binding model for metallic NbN and evaluate a generalized supply function suitable for multiband metals. We use this in a Tsu–Esaki tunneling model to calculate the transmission, junction resistance, and critical current for NbN/AlN and NbN/GaN junctions. We find that experimentally achievable nm-scale barriers can support large critical currents which can be controlled by the interface structure. In parallel, first-principles formation-enthalpy calculations show that oxygen segregates to interfaces and surfaces, enabling trap-assisted tunneling that degrades ideal transport. These results provide concrete design rules for NbN/AlN(GaN) junctions.