  Over the past decades, superconducting qubits have emerged as one of the
leading hardware platforms for realizing a quantum processor. Consequently,
researchers have made significant effort to understand the loss channels that
limit the coherence times of superconducting qubits. A major source of loss has
been attributed to two level systems that are present at the material
interfaces. We recently showed that replacing the metal in the capacitor of a
transmon with tantalum yields record relaxation and coherence times for
superconducting qubits, motivating a detailed study of the tantalum surface. In
this work, we study the chemical profile of the surface of tantalum films grown
on c-plane sapphire using variable energy X-ray photoelectron spectroscopy
(VEXPS). We identify the different oxidation states of tantalum that are
present in the native oxide resulting from exposure to air, and we measure
their distribution through the depth of the film. Furthermore, we show how the
volume and depth distribution of these tantalum oxidation states can be altered
by various chemical treatments. By correlating these measurements with detailed
measurements of quantum devices, we can improve our understanding of the
microscopic device losses.

Materials Science;Quantum Physics