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arXiv:2301.10306 · Applied Physics Letters

Two-level systems in nucleated and non-nucleated epitaxial alpha-tantalum films

Loren D. Alegria, Daniel Tennant, Kevin Chaves, Jonathan R. I. Lee, Sean R. O’Kelley, Yaniv Jacob Rosen, Jonathan L. DuBois

Published Aug 7, 2023• 16 citations
arXivHTMLPDF

Abstract

Abstract Building usefully coherent superconducting quantum processors depends on reducing losses in their constituent materials.1 Tantalum, like niobium, has proven utility as the primary superconducting layer within highly coherent qubits.2,3 But, unlike Nb, high temperatures are typically used to stabilize the desirable body-centered-cubic phase, α-Ta, during thin film deposition. It has long been known that a thin Nb layer permits the room-temperature nucleation of α-Ta,4-6 although neither an epitaxial process nor few-photon microwave loss measurements have been reported for Nb-nucleated Ta films prior to this study. We compare resonators patterned from Ta films grown at high temperature (500 °C) and films nucleated at room temperature, in order to understand the impact of crystalline order on quantum coherence. In both cases, films grew with Al2O3 (001) || Ta (110) indicating that the epitaxial orientation is independent of temperature and is preserved across the Nb/Ta interface. We use conventional low-power spectroscopy to measure two level system (TLS) loss, as well as an electric-field bias technique to measure the effective dipole moments of TLS in the surfaces of resonators. In our measurements, Nb-nucleated Ta resonators had greater loss tangent (1.5 ± 0.1 × 10-5) than nonnucleated (5 ± 1 × 10-6) in approximate proportion to defect densities as characterized by X-ray diffraction (0.27 ° vs 0.18 ° [110] reflection width) and electron microscopy (30 nm vs 70 nm domain size). The dependence of the loss tangent on domain size indicates that the development of more ordered Ta films is likely to lead to improvements in qubit coherence times.1,7 Moreover, lowtemperature α-Ta epitaxy may enable the growth of new, microstate-free heterostructures which would not withstand high temperature processing.8

DOI
10.1063/5.0157654

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•Apr 21, 2025•0 citations
Abstract Building usefully coherent superconducting quantum processors depends on reducing losses in their constituent materials.1 Tantalum, like niobium, has proven utility as the primary superconducting layer within highly coherent qubits.2,3 But, unlike Nb, high temperatures are typically used to stabilize the desirable body-centered-cubic phase, α-Ta, during thin film deposition. It has long been known that a thin Nb layer permits the room-temperature nucleation of α-Ta,4-6 although neither an epitaxial process nor few-photon microwave loss measurements have been reported for Nb-nucleated Ta films prior to this study. We compare resonators patterned from Ta films grown at high temperature (500 °C) and films nucleated at room temperature, in order to understand the impact of crystalline order on quantum coherence. In both cases, films grew with Al2O3 (001) || Ta (110) indicating that the epitaxial orientation is independent of temperature and is preserved across the Nb/Ta interface. We use conventional low-power spectroscopy to measure two level system (TLS) loss, as well as an electric-field bias technique to measure the effective dipole moments of TLS in the surfaces of resonators. In our measurements, Nb-nucleated Ta resonators had greater loss tangent (1.5 ± 0.1 × 10-5) than nonnucleated (5 ± 1 × 10-6) in approximate proportion to defect densities as characterized by X-ray diffraction (0.27 ° vs 0.18 ° [110] reflection width) and electron microscopy (30 nm vs 70 nm domain size). The dependence of the loss tangent on domain size indicates that the development of more ordered Ta films is likely to lead to improvements in qubit coherence times.1,7 Moreover, lowtemperature α-Ta epitaxy may enable the growth of new, microstate-free heterostructures which would not withstand high temperature processing.8
Tantalum films incorporated into superconducting circuits have exhibited low surface losses, resulting in long-lived qubit states. The remaining loss pathways originate in microscopic defects that manifest as two level systems (TLSs) at low temperatures. These defects limit performance, so careful attention to tantalum film structures is critical for optimal use in quantum devices. In this work, we investigate the growth of tantalum using magnetron sputtering on sapphire, Si, and photoresist substrates. In the case of sapphire, we present procedures for the growth of fully-oriented films with α-Ta [1 1 1]//Al2O3 [0 0 0 1] and α-Ta [1 −1 0]//Al2O3 [1 0 −1 0] orientational relationships and having residual resistivity ratio (RRR) ∼ 60 for 220 nm thick films. On Si, we find a complex grain texturing with Ta [1 1 0] normal to the substrate and RRR ∼ 30. We further demonstrate airbridge fabrication using Nb to nucleate α-Ta on photoresist surfaces. For the films on sapphire, resonators show TLS-limited quality factors of 1.3 ± 0.3 × 106 at 10 mK (for a waveguide gap and conductor width of 3 and 6 μm, respectively). Structural characterization using scanning electron microscopy, x-ray diffraction, low temperature transport, secondary ion mass spectrometry, and transmission electron microscopy reveal the dependence of residual impurities and screw dislocation density on processing conditions. The results provide practical insights into the fabrication of advanced superconducting devices including qubit arrays and guide future works on crystallographically deterministic qubit fabrication.
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. Notably, this high level of loss is not observed in Nb films prepared in the same way and having very similar structure. By experimentally controlling the surface on which the Ta film is nucleated, we determine that the source of loss is only present in samples having an epitaxial Ta/sapphire interface and show that it is apparently mitigated by either growing a thin, epitaxial Nb inter-layer between the Ta film and the substrate or by intentionally treating, and effectively damaging, the sapphire surface with an in-situ argon plasma before Ta growth. In addition to elucidating this interfacial microwave loss, this work provides adequate process details to aid reproducible growth of low-loss Ta films across fabrication facilities.
Abstract Tantalum (Ta) films with a body-centered cubic α-phase play a crucial role in high-performance Ta-based superconducting quantum processors. The growth of pure α-Ta at low temperatures has been a long-term goal. Existing experience with Ta growth on silicon substrates has led to the belief that α-Ta can only be stabilized at high temperatures. In this study, we systematically analyzed the growth mechanism of Ta films on sapphire substrates using first-principles calculations, revealing a narrow growth energy window for pure α-Ta at room temperature (RT). This finding indicates the feasibility of obtaining pure α-Ta at significantly lower temperatures. Experimentally, we successfully achieved pure α-Ta at RT via magnetron sputtering by meticulously adjusting the growth parameters. The resulting superconducting resonators based on the developed RT α-Ta exhibit an internal quality factor ( Q i ) of approximately 2.1 × 10 6 at single-photon levels, comparable to those fabricated at high temperatures. Additionally, fixed-frequency transmon qubits fabricated using the RT α-Ta film demonstrate a coherence time exceeding 280 µs, underscoring the film’s high quality. The RT α-Ta without a seed layer simplifies the fabrication process, offering a more efficient and flexible approach for producing Ta-based superconducting quantum processors.
Tantalum thin films sputtered on unheated silicon substrates are characterized with microwaves at around 10 GHz in a 10 mK environment. We show that the phase of tantalum with a body-centered cubic lattice (α-Ta) can be grown selectively by depositing a niobium buffer layer prior to a tantalum film. The physical properties of the films, such as superconducting transition temperature and crystallinity, change markedly with the addition of the buffer layer. Coplanar waveguide resonators based on the composite film exhibit significantly enhanced internal quality factors compared with a film without the buffer layer. The internal quality factor approaches 2 × 107 at a large-photon-number limit. While the quality factor decreases at the single-photon level owing to two-level system (TLS) loss, we have deduced that one of the causes of TLS loss is the amorphous silicon layer at the film–substrate interface, which originates from the substrate cleaning before the film deposition rather than the film itself. The temperature dependence of the internal quality factors shows a marked rise below 200 mK, suggesting the presence of TLS–TLS interactions. The present low-loss tantalum films can be deposited without substrate heating and thus have various potential applications in superconducting quantum electronics.
Abstract Tantalum (Ta) is a promising material for superconducting qubits due to its stable chemical properties, formation of a single oxide, and resistance to reaction with other substances. However, achieving the desired α -Ta typically requires high-temperature substrate heating, leading to a time-consuming process and stringent requirements for the film deposition system. Here we demonstrate the deposition of high-quality α -Ta at room temperature using a niobium (Nb) film as a buffer layer. The buffered films are characterized using x-ray diffraction, transmission electron microscopy, and scanning electron microscopy. Transmons with capacitors, transmission lines, and readout resonators fabricated with such Nb-buffered Ta exhibit excellent performance. We observed an “inflection point” in the dependence of the Q i of the resonators on the input microwave photons, which can be fitted well with an expression for the superposition of the buffered resonators. These experimental results show that the buffered films deposited at room temperature have promising applications in superconducting devices.
Abstract Tantalum (Ta) is a promising material for superconducting qubits due to its stable chemical properties, formation of a single oxide, and resistance to reaction with other substances. However, achieving the desired α-Ta typically requires high-temperature substrate heating, leading to the time-consuming process and stringent requirements of the film deposition system. Here we demonstrate the deposition of high-quality α-Ta at room temperature using the niobium (Nb) film as a buffer layer. The multilayer films are characterized using X-ray diffraction, transmission electron microscopy, and scanning electron microscopy. Transmons with capacitors, transmission lines, and readout resonators fabricated with such Nb buffered Ta exhibit excellent performance. We observe an “inflection point” in the dependence of the Qi of the resonators on the input microwave photons, which can be fitted well with an expression for the superposition of the multilayer resonators. These experimental results show that the multilayer films deposited at room temperature have promising applications in superconducting devices.