Atomistic Analysis Reveals Hidden Structural Variants in NbN Superconducting Trilayers Limiting Circuit Performance
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Microscopic inconsistencies within superconducting films represent a significant challenge to advancing circuit performance and scalability, and researchers are actively seeking materials that overcome these limitations. Prachi Garg, Danqing Wang, Hong X. Tang, and colleagues investigate these inconsistencies in all-nitride Josephson Junctions, a promising material for enhanced coherence and higher temperature operation. Their work diagnoses atomic-scale limitations preventing niobium nitride (NbN) superconducting trilayers from reaching their full potential, revealing that structural and chemical variations, rather than barrier thickness, are the primary source of performance limitations. Through advanced microscopy, the team identifies nanoscale inclusions of a specific NbN polymorph, epsilon-Nb2N2, coexisting within the dominant delta-NbN electrodes, and importantly, links these defects to the initial film nucleation process and their detrimental effect on electrical performance, establishing a crucial material-to-device correlation for future phase engineering strategies. ALD NbN Films, Defect Origins, Qubit Performance This research investigates how nanoscale defects within niobium nitride (NbN) films limit the performance of superconducting qubits. Scientists identified inclusions of a different NbN phase, epsilon-Nb₂N₂, within the primary delta-NbN film, and detected oxygen at interfaces within the material. These defects act as points of energy loss, reducing qubit coherence and hindering superconducting transport. The study demonstrates a clear correlation between these structural imperfections and diminished superconducting properties, providing valuable insights into improving qubit design and fabrication. Optimizing ALD parameters and developing post-deposition annealing processes could minimize defect formation and enhance qubit performance. Atomic Mapping of Defects in Superconducting Junctions This research pioneers a materials-to-device correlation in superconducting Josephson Junctions (JJs), identifying atomic-scale defects that limit performance and scalability. Scientists employed a sophisticated analytical approach, integrating advanced microscopy with electrical measurements to diagnose limitations within NbN/AlN/NbN JJs, focusing on characterizing the distribution of different niobium nitride phases. To achieve this, researchers utilized Atom Probe Tomography (APT), a technique capable of resolving materials at the atomic scale, and developed a novel detection method for precise phase mapping. Quantitative analysis revealed a systematic increase in the δ-NbN fraction from the substrate towards the surface, and a maximum ε-Nb₂N₂ fraction of approximately 3. 8% in the bottom electrode, hypothesized to arise from thermodynamic instability and interfacial strain during growth. The study suggests vacancy formation and impurity stabilization also play a role, establishing a crucial feedback loop between chemistry, structure, and superconducting properties, paving the way for reproducible, high-coherence, and scalable devices.
Consistent Barrier Limits Supercurrent in Junctions This work diagnoses atomic-scale limitations hindering the performance of niobium nitride/aluminum nitride/niobium nitride Josephson junctions, revealing the origins of suppressed critical current and soft quasiparticle tunneling behavior. Electrical measurements of a junction demonstrate a low critical current density, indicating susceptibility to environmental noise, while complementary analysis confirms an effective insulating barrier with consistent thickness across the wafer. Despite this structural integrity, the absence of supercurrent and a soft onset of quasiparticle current isolates the performance limitation to inhomogeneities within the electrodes and barrier. Detailed atomic probe tomography (APT) analysis reveals localized oxygen concentration within the aluminum nitride barrier region, alongside sharp barrier interfaces. Further investigation using transmission electron microscopy and clustering analysis identifies three reproducible clusters based on lattice periodicities within the superconducting electrodes, with two exhibiting median d-spacings differing from the dominant delta-niobium nitride phase, indicating the coexistence of epsilon-niobium nitride inclusions. These findings establish a material-to-device correlation, linking atomic-scale defects to detrimental electrical signatures and providing a targeted strategy for phase engineering towards reproducible, high coherence, and scalable devices.
Atomic Defects Limit Superconducting Junction Performance This research establishes a clear link between atomic-scale defects and the performance of superconducting devices, specifically all-nitride Josephson Junctions. Through a combined analysis of electrical measurements and advanced microscopy, scientists identified nanoscale inclusions of a hexagonal niobium nitride polymorph within the dominant superconducting material. These inclusions, originating from the initial stages of film growth, contribute to variations in critical current and a softened transition to the normal state, ultimately limiting device performance. The significance of this work lies in demonstrating how subtle structural and chemical variations at the atomic level directly impact the macroscopic behavior of superconducting circuits. By tracing the origin of these defects to the initial film nucleation process, the team provides a targeted strategy for material engineering, aiming to improve the coherence and reproducibility of these devices. The statistical methodology developed, including bond length mapping and data clustering techniques, offers a generalizable approach for analyzing polymorphic coexistence in ultrathin superconducting electrodes. 👉 More information 🗞 Hidden Structural Variants in ALD NbN Superconducting Trilayers Revealed by Atomistic Analysis 🧠 ArXiv: https://arxiv.org/abs/2512.07095 Tags: