  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.
