  Scalable architectures characterized by quantum bits (qubits) with low error
rates are essential to the development of a practical quantum computer. In the
superconducting quantum computing implementation, understanding and minimizing
materials losses is crucial to the improvement of qubit performance. A new
material that has recently received particular attention is indium, a
low-temperature superconductor that can be used to bond pairs of chips
containing standard aluminum-based qubit circuitry. In this work, we
characterize microwave loss in indium and aluminum/indium thin films on silicon
substrates by measuring superconducting coplanar waveguide resonators and
estimating the main loss parameters at powers down to the sub-photon regime and
at temperatures between 10 and 450 mK. We compare films deposited by thermal
evaporation, sputtering, and molecular beam epitaxy. We study the effects of
heating in vacuum and ambient atmospheric pressure as well as the effects of
pre-deposition wafer cleaning using hydrofluoric acid. The microwave
measurements are supported by thin film metrology including secondary-ion mass
spectrometry. For thermally evaporated and sputtered films, we find that
two-level states (TLSs) are the dominating loss mechanism at low photon number
and temperature. Thermally evaporated indium is determined to have a TLS loss
tangent due to indium oxide of ~5x1e-05. The molecular beam epitaxial films
show evidence of formation of a substantial indium-silicon eutectic layer,
which leads to a drastic degradation in resonator performance.
