Tantalum Josephson Junctions with Ta O Barriers Fabricated to Enhance Coherence Times in Quantum Circuits

Tantalum is rapidly becoming a crucial material in the pursuit of more powerful and reliable superconducting circuits, offering the potential for significantly extended coherence times. Raahul Potluri, Rohin Tangirala, and colleagues from the University of Washington, alongside Sage Bauers from the National Renewable Energy Lab, Alejandro Barrios and Praveen Kumar from the Colorado School of Mines, and Peter V. Sushko from the Pacific Northwest National Laboratory, systematically investigate methods for creating high-quality tantalum oxide layers, essential components in Josephson junctions. Their work addresses a key challenge, namely the widespread use of aluminium oxide despite tantalum’s promise of fewer defects that cause energy loss. The team successfully demonstrates that plasma oxidation consistently produces the smoothest and highest-quality tantalum oxide, and importantly, achieves epitaxial growth of tantalum layers on top of this oxide, establishing a pathway towards fabricating advanced trilayer tantalum/tantalum oxide/tantalum Josephson junctions with exceptionally clean and low-loss interfaces.

Tantalum Trilayers for Josephson Junction Fabrication

This work details the fabrication and characterisation of tantalum-based Josephson junctions incorporating thin tantalum pentoxide barriers. The objective is to create reproducible and high-quality tunnel barriers for superconducting devices, specifically focusing on trilayer structures consisting of tantalum, tantalum pentoxide, and tantalum. The fabrication process involves depositing tantalum and tantalum pentoxide films using magnetron sputtering, followed by patterning using electron beam lithography and reactive ion etching. Structural analysis employs transmission electron microscopy and energy-dispersive X-ray spectroscopy to determine the thickness, uniformity, and composition of the fabricated trilayers.

The results demonstrate the successful creation of tantalum/tantalum pentoxide/tantalum trilayers with controlled thicknesses ranging from 0. 5 nanometres to 2. 0 nanometres for the tantalum pentoxide barrier. Furthermore, the analysis confirms the amorphous nature of the tantalum pentoxide films and reveals a sharp interface between the tantalum and tantalum pentoxide layers, crucial for achieving high-performance Josephson junctions. This approach provides a pathway towards fabricating highly reproducible and reliable superconducting devices with improved performance characteristics.

Tantalum Films Demonstrate Superconducting Quality and Structure

Researchers confirmed that both tantalum films, deposited on different substrates, exhibited superconducting transitions and good film quality, validating their suitability for Josephson junction fabrication. X-ray diffraction showed a preferred crystalline orientation, confirming these results. Scanning electron microscopy revealed the formation of a distinct tantalum oxide layer during oxidation, with measured thicknesses corresponding well with other measurement techniques, validating the accuracy of thickness control crucial for junction characteristics. Energy-dispersive X-ray spectroscopy revealed a surprising finding: significant oxygen content not only in the tantalum oxide barrier layer but also within both tantalum electrodes.

The oxygen peak intensity in the tantalum electrodes was comparable to that of the tantalum oxide barrier, suggesting oxygen diffusion into the tantalum during or after oxidation. This is problematic because oxygen contamination can degrade superconducting properties and reduce junction performance. Computational modeling using Density Functional Theory showed that interstitial oxygen diffusion in amorphous tantalum oxide is possible. The calculated oxygen diffusion barrier suggests relatively facile diffusion, especially under high oxygen chemical potential. This provides a theoretical explanation for the experimental observation of oxygen contamination in the tantalum electrodes and confirms that the barrier isn’t a perfect oxygen diffusion barrier.

Tantalum Oxide Growth for Josephson Junctions

Researchers successfully grew tantalum oxide on tantalum thin films using thermal oxidation, rapid thermal annealing, and plasma oxidation. Plasma oxidation proved particularly effective at creating smooth oxide surfaces suitable for building layered structures, or trilayers, of tantalum/tantalum oxide/tantalum. The team elucidated the mechanisms behind tantalum oxidation, offering a means to control the composition and thickness of the oxide layer by adjusting plasma parameters during the growth process. Notably, the researchers demonstrated the epitaxial growth of tantalum on both naturally formed and plasma-oxidized tantalum films, establishing a pathway for fabricating trilayer Josephson junctions using tantalum and tantalum oxide.

These structures hold promise for reducing the number of two-level system defects, a major source of decoherence, within the barrier and at the interfaces of superconducting circuits. The team suggests that in-situ growth of these trilayer structures, without breaking vacuum, is achievable. Tantalum has recently emerged as a promising low-loss material, enabling record coherence times in superconducting qubits, largely attributed to its stable native oxide which hosts fewer two-level system defects. The authors acknowledge that oxygen contamination was observed within the tantalum layers during analysis, likely due to exposure to air during sample preparation for microscopy. Future work will focus on optimizing these growth techniques and incorporating the resulting junctions into superconducting qubits, with the goal of achieving improved coherence times and extending the performance of these quantum devices beyond current limitations of aluminum-based junctions.