  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 ($\alpha$-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 $2\times 10^7$ 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 identified
the primary cause of TLS loss to be 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.
