Two-Level Systems in Nucleated and Non-Nucleated Epitaxial alpha-Tantalum films

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. But, unlike Nb, high temperatures are typically used to stabilize the desirable body-centered-cubic phase, alpha-Ta, during thin film deposition. It has long been known that a thin Nb layer permits the room-temperature nucleation of alpha-Ta, 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 {\deg}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 x 10^-5) than non-nucleated (5 +/- 1 x 10^-6) in approximate proportion to defect densities as characterized by X-ray diffraction (0.27 {\deg} vs 0.18 {\deg} [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. Moreover, low-temperature alpha-Ta epitaxy may enable the growth of new, microstate-free heterostructures which would not withstand high temperature processing.