Improvement and stabilization of the performance of tantalum superconducting resonators via organic molecule-passivation

Tantalum shows great potential for superconducting quantum circuits. However, two-level systems (TLS), which are often hosted by native oxides at various surfaces and interfaces, give rise to dielectric losses, which limit the performance. To mitigate TLS losses, the growth of these native oxides has to be prevented post-fabrication, which can be achieved via suitable surface passivation of the freshly oxide-etched, exposed surfaces. We explore the impact of passivating Ta co-planar waveguide resonators on Si substrates with alkene molecules. The highly ordered growth of organic, nanometer-thick films from these molecules on both, the superconducting Ta and the open (exposed) Si surface in-between, is indicated by Contact Angle measurements, Ellipsometry, AFM and FTIR spectroscopy. The coating method was then transferred to passivate Ta resonators, and the effect of passivation on the temporal stability of the resonators exposed to ambient air was investigated. Power-dependent (at T∼100mK) and temperature-dependent (T∼100-1000mK) RF measurements were conducted in the few GHz regime to determine the internal Q-factors. Passivated resonators demonstrated Q-factors of about ∼0.8 million, showing an approximately 60% enhancement over un-passivated reference resonators, in the low power regime at 100mK. Additionally, passivated resonators maintained excellent stability over time, which can be attributed to restricted surface oxide formation, in contrast to un-passivated resonators.