Materials losses in superconducting circuits based on tantalum thin films

Superconducting quantum circuits have very promising applications in
the fields of quantum computing and detection. In general, their performance
is limited by a variety of dissipation and noise sources. For
mitigating these, the focus has been on improving microwave design
and better fabrication processes. Another approach, recently in the
spotlight, is to explore new materials entirely. One promising candidate
superconducting material is Tantalum (Ta), which recently enabled
record-breaking Transmon qubit lifetimes. However, there is a
lack of conclusive evidence of the dominant loss mechanisms related to
Ta. Here, we present a study of losses in epitaxial Ta films deposited
using magnetron sputtering, with the aim of relating basic material
properties and loss mechanisms. A variation in the deposition parameters
(mainly substrate temperature) leads to structurally different
films. We characterized these using high-resolution X-ray diffraction,
scanning electron microscopy and measurements of the superconducting
transition temperature. In addition, we fabricated lumped element
resonators from the films using e-beam lithography and measured their
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quality factors as a function of photon number and temperature.