Substrate-vacuum interface modification in tantalum superconducting resonators

The performance of cutting-edge superconducting quantum devices is primarily limited by microwave dielectric losses at various surface and interface regions. Potential enhancements in device performance may be achieved through the strategic modification of these interfaces, specifically through the recess of the Si substrate to shift the lossy substrate-vacuum interfaces away from areas with high electric field intensities. This will facilitate the understanding of the substrate-vacuum participation to the overall device loss. In this study, we investigate the performance of alpha-tantalum resonators in which Si has been anisotropically recessed by a wet etch treatment. We conduct an extensive characterization study of the device material using transmission electron microscopy, X-ray photoemission spectroscopy, and critical temperature measurements. At shallow recess depths, the internal quality factors as a function of power exhibit typical TLS-dominated loss behaviour, with quality factors at the single-photon level being comparable to a non-recessed reference. However, as the recess depth increases, the quality factors at the single-photon level rise, reaching up to 8x10⁶. Despite this improvement, a shift away from TLS behaviour is observed. The quality factors remain stable up to around ~ n =10⁴, after which they begin to decline, likely due to the incorporation of hydrogen into the Ta film.