Tantalum thin films on a-plane sapphire for low-loss superconducting circuits

Superconducting quantum circuits are a promising hardware platform in the fields of quantum computing. Ways to reduce noise and dissipation are of major importance in order to improve device performance and particularly promising in this respect is the exploration of novel materials. α-Tantalum (Ta) thin films grown on c-plane sapphire have been shown to improve Transmon qubit lifetimes when used for their shunting capacitor. However, achieving the high film quality needed to fabricate low loss devices, requires high substrate temperature during sputter deposition. At intermediate sample temperatures (< 650°C), that can be realized in many deposition systems, the film growth is characterized by the competing growth of (110) Ta and (111) Ta . The resulting films, when patterned into superconducting resonators, exhibit loss tangents larger than observed for resonators patterned from films grown at higher temperature. This can be avoided by growing the films on the a-plane surface of sapphire, instead. There, only (110) Ta is formed at intermediate temperatures. We correlate the film properties with quality factors of lumped element resonators fabricated from films grown on the different sapphire surfaces.