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Scientific Reports

Blurred interface induced control of electrical transport properties in Josephson junctions

Junling Qiu, Huihui Sun, Chuanbing Han, Xiaodong Ding, Bo Zhao, Shuya Wang, Lixin Wang, Zheng Shan

Published Jul 27, 2024• 1 citation
Publisher

Abstract

The interfacial microstructures of Josephson junctions are vital for understanding the microscopic mechanism to improve the performance of superconducting qubits further. However, there remain significant concerns about well understanding the correlation between atomic structures and electrical behaviors. Here, we propose a new method to define the interface of the barrier in Josephson junctions, and investigate the factors that affect the electrical properties of junctions using material analysis techniques and first principles. We find that the aluminium-oxygen ratio of the interface contributes greatly to the electrical properties of junctions, which is consistent with the conclusions obtained by utilizing the generative adversarial network for data augmentation. When the aluminium-oxygen ratio of the interface is 0.67-1.1, the model exhibits a lower barrier height and better electrical properties of the junction. Moreover, when the thickness of the barrier is fixed, the impact of the aluminium-oxygen ratio becomes prominent. A detailed analysis of Josephson junctions using a microscopic model has led to identifying of process defects that can enhance the yield rate of chips. It has a great boost for determining the relationship between microstructures and macroscopic performances.

DOI
10.1038/s41598-024-68285-y

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The interfacial microstructures of Josephson junctions are vital for understanding the microscopic mechanism to improve the performance of superconducting qubits further. However, there remain significant concerns about well understanding the correlation between atomic structures and electrical behaviors. Here, we propose a new method to define the interface of the barrier in Josephson junctions, and investigate the factors that affect the electrical properties of junctions using material analysis techniques and first principles. We find that the aluminium-oxygen ratio of the interface contributes greatly to the electrical properties of junctions, which is consistent with the conclusions obtained by utilizing the generative adversarial network for data augmentation. When the aluminium-oxygen ratio of the interface is 0.67-1.1, the model exhibits a lower barrier height and better electrical properties of the junction. Moreover, when the thickness of the barrier is fixed, the impact of the aluminium-oxygen ratio becomes prominent. A detailed analysis of Josephson junctions using a microscopic model has led to identifying of process defects that can enhance the yield rate of chips. It has a great boost for determining the relationship between microstructures and macroscopic performances.
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Al/AlOx/Al Josephson junctions are the building blocks of a wide range of superconducting quantum devices that are key elements for quantum computers, extremely sensitive magnetometers and radiation detectors. The properties of the junctions and the superconducting quantum devices are determined by the atomic structure of the tunnel barrier. The nanoscale dimension and disordered nature of the barrier oxide have been challenges for the direct experimental investigation of the atomic structure of the tunnel barrier. Here we show that the miniaturized dimension of the barrier and the interfacial interaction between crystalline Al and amorphous AlOx give rise to oxygen deficiency at the metal/oxide interfaces. In the interior of the barrier, the oxide resembles the atomic structure of bulk aluminium oxide. Atomic defects such as oxygen vacancies at the interfaces can be the origin of the two-level systems and contribute to decoherence and noise 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 Alumina Josephson junction has demonstrated a tremendous potential to realize superconducting qubits. Further progress towards scalable superconducting qubits urgently needs to be guided by novel analysis mechanisms or methods to reduce the thickness sensitivity of the junction critical current to the tunnel barrier. Here, it is first revealed that the termination mode of AlOx interface plays a crucial role in the uniformity of critical current, and we demonstrate that the O-terminated interface has the lowest resistance sensitivity to thickness. More impressively, we developed atomically structured three-dimensional models and calculated their transport properties using a combination of quantum ballistic transport theory with first-principles DFT and NEGF to examine the effects of the Al2O3 termination mode and thickness variations. This work clarifies that O-terminated interface can effectively improve the resistance uniformity of Josephson junction, offering useful guidance for increasing the yield of fixed-frequency multi-qubit quantum chips which require tight control on qubit frequency.
Superconducting quantum circuits are promising platforms for scalable quantum computing, where qubit coherence is critically determined by microscopic defects in the oxide tunneling barrier of Josephson junctions. Amorphous Al2_22​O3_33​ is widely used as a barrier material, but under irradiation, oxygen vacancy (VO_OO​) defects are readily generated, introducing noise sources that accelerate qubit decoherence. We systematically investigate the structural characteristics and electronic impact of VO_OO​ defects in amorphous Al2_22​O3_33​ using first-principles calculations and \textit{ab initio} molecular dynamics. Our results show that both the coordination environment and concentration of VO_OO​s strongly influence electrical conductivity. In particular, two- and three-coordinated VO_OO​s, unique to the amorphous structure, enhance conductivity more than conventional four-coordinated vacancies. Increasing VO_OO​ concentration amplifies conductivity fluctuations, which we link to critical current noise in Josephson junctions. Using a noise model, we estimate that higher VO_OO​ densities lead to shorter qubit coherence times. These findings provide insights for radiation-hard design of superconducting quantum devices.
An ultra-thin aluminum oxide layer is a key component for Josephson junctions (JJ) in superconducting quantum bits (qubits). This layer serves as a barrier layer for Cooper pairs tunneling between the superconducting electrodes and significantly influences the overall performance of the junction. In this study, we investigate the impact of aluminum deposition rates on the microstructure and chemical variation of the aluminum oxide layer, as well as the device’s yields and qubits’ lifetimes. Surprisingly, although the oxide layer roughness and thickness variation improve noticeably with an increasing Al deposition rate from 0.5 Å/s to 5 Å/s, the qubit’s coherence time is almost unchanged. We attribute this to the fact that Cooper pairs tunnel only through the thinnest region of the barrier. Additionally, we revealed a stress-induced grain boundary sliding short-circuit failure mechanism in the JJ. Our discovery provides a vital understanding of oxide microstructure and JJ functionality, which are critical for improving the coherence time of superconducting qubits.