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

Abstract

Building usefully coherent superconducting quantum processors depends on reducing losses in their constituent materials [I. Siddiqi, Nat. Rev. Mater. 6, 875–891 (2021)]. Tantalum, like niobium, has proven utility as the primary superconducting layer within highly coherent qubits [Place et al., Nat. Commun. 12(1), 1–6 (2021) and Wang et al., npj Quantum Inf. 8(1), 1–6 (2022)]. 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 [Westwood et al., Tantalum Thin Films (Academic Press, 1975); D. W. Face and D. E. Prober, J. Vac. Sci. Technol. A 5, 3408–3408 (1987); and Colin et al., Acta Mater. 126, 481–493 (2017)], 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) ǁ 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). 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 [I. Siddiqi, Nat. Rev. Mater. 6, 875–891 (2021) and Premkumar et al., Commun. Mater. 2(1), 1–9 (2021)]. Moreover, low-temperature α-Ta epitaxy may enable the growth of microstate-free heterostructures, which would not withstand high temperature processing [McSkimming et al., J. Vac. Sci. Technol. A 35, 021401 (2017)].

Low-loss α-tantalum coplanar waveguide resonators on silicon wafers: fabrication, characterization and surface modification

“…On average, the Q i,low values after the HF treatment obtained in this work are around 2•10 6 . These values are in the range of α-Ta CPW resonators of similar size reported on both Si [46] and sapphire [44,48]. Transmon qubits made of α-Ta with record Q i of 7.1•10 6 and 11.8•10 6 have also been reported by Place et al [32] and Wang et al [33], respectively, while our record Q i,low is 4.5•10 6 .…”

“…While the deposition of α-Ta directly on thermally oxidized Si [42] substrates has been known since the 90s, it is widely recognized that the silicon oxide at the silicon-tantalum interface has a detrimental effect on the performance of superconducting qubits and resonators. To circumvent the issue, α-Ta coplanar-waveguide resonators, which are short loop proxies for superconducting qubits, have been fabricated with the help of seed layers such as Nb [43,44], tantalum nitride (TaN) [45], and titanium nitride (TiN) [46]. Nonetheless, this approach adds to the processing complexity during both the deposition and etching stages of the metallic layers.…”