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

Area of Science:

• Materials Science
• Quantum Computing
• Solid State Physics

Background:

• Epitaxial all-nitride Josephson junctions are crucial for advancing superconducting qubits.
• Nanoscale defects significantly hinder the performance of these quantum devices.
• Understanding the interplay between material structure, defect chemistry, and device behavior is essential for improvement.

Purpose of the Study:

• To investigate the atomic-scale structural and chemical defects in different epitaxial nitride Josephson junction heterostructures.
• To elucidate the relationship between niobium nitride polymorph selection, impurity distribution, and device performance.
• To provide mechanistic insights into defect chemistry's role in epitaxial nitride Josephson junctions.

Main Methods:

• Epitaxial growth of 𝛿-NbN/AlN/𝛿-NbN, 𝛾-Nb₄N₃/AlN/𝛾-Nb₄N₃, and β-Nb₂N/AlN/β-Nb₂N heterostructures using molecular beam epitaxy.
• Integration of advanced microscopy techniques with density functional theory (DFT) calculations.
• Analysis of impurity distribution and segregation behavior across different polymorphs.

Main Results:

• Observed varying impurity (oxygen) distribution across different niobium nitride polymorphs.
• δ-NbN electrodes showed significant oxygen content, while oxygen segregated to the AlN barrier in β-Nb₂N/AlN/β-Nb₂N heterostructures.
• DFT calculations confirmed phase-dependent oxygen energetics and diffusion kinetics, explaining observed impurity trapping and mobility differences.

Conclusions:

• Structural and chemical differences in epitaxial nitride Josephson junctions are linked to polymorph-dependent defect chemistry.
• Impurity distribution significantly influences transport behavior and may contribute to decoherence in superconducting quantum circuits.
• This work provides critical insights for mitigating defects and enhancing the performance of superconducting qubits.