Related papers: Interface-sensitive microwave loss in superconducting tantalum films sputtered on c-plane sapphire

Interface-sensitive microwave loss in superconducting tantalum films sputtered on c-plane sapphire

URL: http://arxiv.org/abs/2412.16730v1
Date: Sat, 21 Dec 2024 18:40:49 GMT
Title: Interface-sensitive microwave loss in superconducting tantalum films sputtered on c-plane sapphire
Authors: Anthony P. McFadden, Jinsu Oh, Lin Zhou, Trevyn F. Q. Larson, Stephen Gill, Akash V. Dixit, Raymond Simmonds, Florent Lecocq,
Abstract summary: tantalum (Ta) emerged as a promising material to address microscopic sources of loss found on niobium (Nb) or aluminum (Al) surfaces. Here we present a systematic study of the structural and electrical properties of Ta and Nb films sputtered on c-plane sapphire at varying growth temperatures.
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Abstract: Quantum coherence in superconducting circuits has increased steadily over the last decades as a result of a growing understanding of the various loss mechanisms. Recently, tantalum (Ta) emerged as a promising material to address microscopic sources of loss found on niobium (Nb) or aluminum (Al) surfaces. However, the effects of film and interface microstructure on low-temperature microwave loss are still not well understood. Here we present a systematic study of the structural and electrical properties of Ta and Nb films sputtered on c-plane sapphire at varying growth temperatures. As growth temperature is increased, our results show that the onset of epitaxial growth of $\alpha$-phase Ta correlates with lower Ta surface roughness, higher critical temperature, and higher residual resistivity ratio, but surprisingly also correlates with a significant increase in loss at microwave frequency. We determine that the source of loss is located at the Ta/sapphire interface and show that it can be fully mitigated by either growing a thin, epitaxial Nb inter-layer between the Ta film and the substrate or by intentionally treating the sapphire surface with \textit{in-situ} argon plasma before Ta growth. In addition to elucidating this interfacial microwave loss, this work provides adequate process details that should allow for the reproducible growth of low-loss Ta film across fabrication facilities.

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