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Advanced Science

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

Prachi Garg, Kedarsh Kaushik, Danqing Wang, Naomi A. Pieczulewski, Anand Ithepalli, John Wright, Debdeep Jena, David Anthony Muller, Hong X. Tang, Cyrus E. Dreyer, Baishakhi Mazumder

Published Aug 5, 2026• 0 citations
Publisher

Abstract

ABSTRACT 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 𝛿‐NbN/AlN/𝛿‐NbN, 𝛾‐Nb 4 N 3 /AlN/𝛾‐Nb 4 N 3 , β‐Nb 2 N/AlN/β‐Nb 2 N 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‐Nb 2 N/AlN/β‐Nb 2 N 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‐Nb 2 N. 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.

DOI
10.1002/advs.76944

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ABSTRACT 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 𝛿‐NbN/AlN/𝛿‐NbN, 𝛾‐Nb 4 N 3 /AlN/𝛾‐Nb 4 N 3 , β‐Nb 2 N/AlN/β‐Nb 2 N 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‐Nb 2 N/AlN/β‐Nb 2 N 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‐Nb 2 N. 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.
Microscopic inhomogeneity within superconducting films is a critical bottleneck hindering the performance and scalability of quantum circuits. All-nitride Josephson Junctions (JJs) have attracted substantial attention for their potential to provide enhanced coherence times and enable higher temperature operation. However, their performance is often limited by local variations caused by polymorphism, impurities, and interface quality. This work diagnoses atomic-scale limitations preventing superconducting NbN/AlN/NbN JJs from reaching their full potential. Electrical measurements reveal suppressed critical current density and soft onset of quasiparticle current. However, inverse proportionality between resistance and junction area confirms homogenous barrier thickness. This isolates structural and chemical variations in electrodes and barrier as the source of performance limitation. The observed characteristics are attributed to complex materials problems: NbN polymorphism, phase coexistence, and oxygen impurities. Using advanced microscopy and machine learning integrated approach, nanoscale inclusions of epsilon-Nb2N2 are found to coexist within dominant delta-NbN electrodes. DC performance of JJs may be affected by these defects, leading to unresolved supercurrent and soft transition to normal state. By identifying specific atomic scale defects, tracing its origin to initial film nucleation, and linking to its detrimental electrical signature, this work establishes a material-to-device correlation and provides targeted strategy for phase engineering towards reproducible, high coherence and scalable quantum devices.
Abstract Improving the coherence of superconducting qubits is a fundamental step towards the realization of fault-tolerant quantum computation. However, coherence times of quantum circuits made from conventional aluminum-based Josephson junctions are limited by the presence of microscopic two-level systems in the amorphous aluminum oxide tunnel barriers. Here, we have developed superconducting qubits based on NbN/AlN/NbN epitaxial Josephson junctions on silicon substrates which promise to overcome the drawbacks of qubits based on Al/AlOx/Al junctions. The all-nitride qubits have great advantages such as chemical stability against oxidation, resulting in fewer two-level fluctuators, feasibility for epitaxial tunnel barriers that reduce energy relaxation and dephasing, and a larger superconducting gap of ~5.2 meV for NbN, compared to ~0.3 meV for aluminum, which suppresses the excitation of quasiparticles. By replacing conventional MgO by a silicon substrate with a TiN buffer layer for epitaxial growth of nitride junctions, we demonstrate a qubit energy relaxation time $${T}_{1}=16.3\;{{\upmu }}{{{{{\rm{s}}}}}}$$ T 1 = 16.3 μ s and a spin-echo dephasing time $${T}_{2}=21.5\;{{\upmu }}{{{{{\rm{s}}}}}}$$ T 2 = 21.5 μ s . These significant improvements in quantum coherence are explained by the reduced dielectric loss compared to the previously reported $${T}_{1}\approx {T}_{2}\approx 0.5\;{{\upmu }}{{{{{\rm{s}}}}}}$$ T 1 ≈ T 2 ≈ 0.5 μ s of NbN-based qubits on MgO substrates. These results are an important step towards constructing a new platform for superconducting quantum hardware.
We report on atomic-scale analyses of oxygen distribution and segregation at grain boundaries (GBs) of Nb and Ta-encapsulated Nb (Ta/Nb) thin films for superconducting qubits using atom-probe tomography (APT) and transmission electron microscopy (TEM). We observe oxygen segregation at grain boundaries (GBs) relative to the oxygen concentration within the grains for both Nb and Ta-capped Nb thin films for superconducting qubits and find that higher oxygen concentration in the interior of Nb grains lead to greater oxygen segregation levels at GBs. This finding emphasizes that controlling oxygen impurities in Nb during film deposition and fabrication processing is important to reduce the level of oxygen segregation at GBs in Nb. The enrichment factor (Cgb/Cgrain) for oxygen segregation at GBs in Nb is 2.7 (error bar: 0.3) for Nb films, and Ta-capped Nb thin films exhibit slightly reduced Nb GB enrichment factors of 2.3 (error bar: 0.3) while GBs in the Ta capping layer itself possess higher enrichment factors of 3.0 (error bar: 0.3). We hypothesize that the Ta capping layer can trap oxygen and thereby affect oxygen in-diffusion and segregation at GBs in the underlying Nb thin films. Finally, we find that increases in the oxygen concentration in both Nb grains and GBs correlate with a suppression in the critical temperature for superconductivity (Tc). Together, our comparative chemical and charge transport property analyses provide atomic-scale insights into a potential mechanism contributing to decoherence in superconducting qubits.
The Josephson junction is a crucial element in superconducting devices, and niobium is a promising candidate for the superconducting material due to its large energy gap relative to aluminum. AlOx has long been regarded as the highest quality oxide tunnel barrier and is often used in niobium-based junctions. Here, we propose ZrOx as an alternative tunnel barrier material for Nb electrodes. We theoretically estimate that zirconium oxide has excellent oxygen retention properties and experimentally verify that there is no significant oxygen diffusion leading to NbOx formation in the adjacent Nb electrode. We develop a top–down, subtractive fabrication process for Nb/Zr–ZrOx/Nb Josephson junctions, which enables scalability and large-scale production of superconducting electronics. Using cross-sectional scanning transmission electron microscopy, we experimentally find that depending on the Zr thickness, ZrOx tunnel barriers can be fully crystalline with chemically abrupt interfaces with niobium. Further analysis using electron energy loss spectroscopy reveals that ZrOx corresponds to tetragonal ZrO2. Room temperature characterization of fabricated junctions using Simmons’ model shows that ZrO2 exhibits a low tunnel barrier height, which is promising in merged-element transmon applications. Low temperature transport measurements reveal sub-gap structure, while the low-voltage sub-gap resistance remains in the megaohm range.
Epitaxial NbN/AlN/NbN Josephson junctions are promising alternatives to conventional Al/AlOx_xx​/Al junctions for superconducting qubits due to their crystalline tunnel barriers and the high transition temperature of NbN. However, their integration into quantum circuits typically requires complex fabrication processes involving amorphous interlayer dielectrics, which can degrade qubit coherence times. To address these issues, we developed sidewall spacer passivated epitaxial NbN/AlN/NbN junctions using a fabrication process that reduces both the volume of amorphous dielectric material and fabrication complexity. The fabricated junctions exhibited a large gap voltage of approximately 5.6 mV and low subgap leakage currents. By varying the thickness of the AlN tunnel barrier, high-quality junctions were fabricated over a wide range of critical current densities Jc{\it J}_cJc​, from 0.3 to 104^44 A/cm2^22. In addition, the comparison of the current-voltage characteristics before and after buffered hydrofluoric acid treatment indicated that the wet etching process did not degrade the electrical characteristics of the junctions. These results establish a scalable fabrication process for epitaxial Josephson junctions and provide a promising platform for high-coherence nitride-based superconducting qubits.
Abstract Tunneling defects in disordered materials form spurious two-level systems which are a major source of decoherence for micro-fabricated quantum devices. For superconducting qubits, defects in tunnel barriers of submicrometer-sized Josephson junctions couple strongest to the qubit, which necessitates optimization of the junction fabrication to mitigate defect formation. Here, we investigate whether defects appear predominantly at the edges or deep within the amorphous tunnel barrier of a junction. For this, we compare defect densities in differently shaped Al/AlO x /Al Josephson junctions that are part of a Transmon qubit. We observe that the number of detectable junction-defects is proportional to the junction area, and does not significantly scale with the junction’s circumference, which proposes that defects are evenly distributed inside the tunnel barrier. Moreover, we find very similar defect densities in thermally grown tunnel barriers that were formed either directly after the base electrode was deposited, or in a separate deposition step after removal of native oxide by Argon ion milling.