Microscopic loss sources in alpha-tantalum superconducting resonators grown on silicon*

The performance of state-of-the-art superconducting quantum devices is currently limited by microwave dielectric losses at different surfaces and interfaces. The use  of  alpha-tantalum in these devices leads to a reduction in their dielectric loss and to an improvement of device performance due to its thin low-loss oxide. However, this tantalum phase was so far realized only on sapphire substrates, which is incompatible with advanced processing in industry-scale fabrication facilities. Here, we demonstrate the fabrication of alpha-tantalum resonators directly on a silicon wafer over a variety of metal deposition conditions, achieving internal quality factors up to five million at low photon number. Complementarily, we combine a comprehensive device material characterization study including, transmission electron microscopy, X-ray photoelectron spectroscopy, X-ray diffraction, atomic force microscopy and resistivity, with resonator loss modelling and simulation. By comparing experimental and simulated resonator loss, we demonstrate that two-level-system loss is dominated by surface oxides contributions. Our study paves the way to large scale manufacturing of low-loss superconducting circuits and to materials-driven advancements in superconducting circuit performance.