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

Journal Article · Mon Aug 07 00:00:00 UTC 2023 · Applied Physics Letters

DOI:https://doi.org/10.1063/5.0157654· OSTI ID:2203348

Alegria, Loren D. ^[1]; Tennant, Daniel M. ^[2]; Chaves, Kevin R. ^[1]; Lee, Jonathan I. ^[1]; O'Kelley, Sean R. ^[1]; Rosen, Yaniv J. ^[1]; DuBois, Jonathan L. ^[1]

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1. Lawrence Livermore National Laboratory (LLNL), Livermore, CA (United States)
2. Lawrence Livermore National Laboratory (LLNL), Livermore, CA (United States); Rigetti Computing, Berkeley, CA (United States)

Building usefully coherent superconducting quantum processors depends on reducing losses in their constituent materials. Tantalum, like niobium, has proven utility as the primary superconducting layer within highly coherent qubits. However, 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, but here we observe the epitaxial process and present few-photon microwave loss measurements in Nb-nucleated Ta films. 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 the 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 non-nucleated (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 vs 70 nm domain size). Furthermore, 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. Moreover, low-temperature α-Ta epitaxy may enable the growth of microstate-free heterostructures, which would not withstand high temperature processing.

• Research Organization: Lawrence Livermore National Laboratory (LLNL), Livermore, CA (United States); University of Wisconsin-Madison, WI (United States)
• Sponsoring Organization: USDOE Laboratory Directed Research and Development (LDRD) Program; USDOE National Nuclear Security Administration (NNSA); USDOE Office of Science (SC), Basic Energy Sciences (BES)
• Grant/Contract Number: AC52-07NA27344; SC0020313
• OSTI ID: 2203348
• Alternate ID(s): OSTI ID: 2916883
• Report Number(s): LLNL--JRNL-844082; 1067128
• Journal Information: Applied Physics Letters, Journal Name: Applied Physics Letters Journal Issue: 6 Vol. 123; ISSN 0003-6951
• Publisher: American Institute of Physics (AIP)
• Country of Publication: United States
• Language: English

Related Subjects

75 CONDENSED MATTER PHYSICS
SUPERCONDUCTIVITY AND SUPERFLUIDITY
77 NANOSCIENCE AND NANOTECHNOLOGY
Crystal structure
Dielectric properties
Electron microscopy
Heterostructures
Quantum coherence
Quantum computing
Resonator device
Thin film deposition
Transition metals
X-ray diffraction