Phase-Dependent Oxygen Defect Energetics in Epitaxial NbN/AlN/NbN Films

Epitaxial all-nitride Josephson junctions are promising components for high coherence superconducting qubits, yet nanoscale defects often limit their performance. The key to mitigating these defects lies in understanding the atomic-scale relationship between polymorph selection, defect chemistry, and device performance. Here, we investigate structural and chemical defects in epitaxially grown α-NbN/AlN/α-NbN, γ-Nb₄N₃/AlN/α-Nb₄N₃, β-Nb₂N/AlN/β-Nb₂N heterostructures on c-plane sapphire using molecular beam epitaxy. Advanced microscopy integrated with density functional theory shows varying impurity distribution across different polymorphs. The chemical distribution reveals that δ-NbN electrodes contain significant oxygen, whereas in β-Nb₂N/AlN/β-Nb₂N heterostructures oxygen preferentially segregates to the AlN barrier. DFT calculations indicate that these differences arise from phase-dependent oxygen energetics and diffusion kinetics, with oxygen remaining kinetically trapped in δ-NbN while exhibiting greater mobility in β-Nb₂N. These structural and chemical differences are consistent with the distinct transport behavior observed in the two junction architectures and provide mechanistic insight into the role of defect chemistry in epitaxial nitride Josephson junctions. The observed impurity distribution may influence superconducting properties and contribute to the formation of two-level systems, a major source of loss and decoherence in superconducting quantum circuits.