The role of self-assembled monolayers at substrate–metal interfaces in limiting coherence of superconducting quantum circuits

In planar superconducting quantum circuits, two-level system (TLS) defects within amorphous dielectrics at various circuit interfaces, including substrate–metal (SM) and air-interfaces, are a critical source of decoherence. We employed molecular self-assembled monolayer (SAM) at different interfaces to mitigate TLS losses in superconducting coplanar waveguide (CPW) resonators. Previous work has reported an increase in the resonator quality factor when a SAM is applied to air-interfaces, but a contrasting decrease is observed when the SAM is applied to the metal–substrate interfaces. To investigate the reason behind this decrease, we fabricated and characterized niobium (Nb)-based CPW resonator chips on silicon substrates, both with and without a SAM at the SM interface. Quality factor measurements performed at 100 mK across a wide range of driving powers confirmed that the presence of the SAM at the SM interface led to a reduction in the quality factor. This observed decrease is attributed to an increased TLS defect density, as suggested by the topography of niobium deposited on the SAM. Our findings demonstrate that the roughness of the deposited Nb at the SM interface, which is influenced by the SAM, significantly impacts the density of TLS defects at this interface. Furthermore, simulations of TLS losses in CPW resonators supported our experimental observations by evaluating resonator quality factors under various interface modifications, particularly showing a decrease in the quality factor when the SM interface exhibited high loss.