Probing Buried Interface Properties in Ta/Sapphire Superconducting Resonators

Dielectric loss significantly impacts the coherence time of superconducting qubits, suggesting that surfaces and interfaces are the primary limiting factors. Despite constituting a smaller fraction of the qubit’s electromagnetic mode, they have the potential to exert significant influence as sources of high-loss tangents. This study investigates the structure and composition of the interfacial layer in Ta/sapphire-based superconducting qubits. Ta (222) films, comprising a pure α-phase, were sputtered onto a C-plane sapphire substrate at a growth temperature of 750 °C. Characterization of the film, including thickness, roughness, and electron density, was performed using synchrotron-based X-ray reflectivity (XRR) and Hard X-ray photoemission spectroscopy techniques. Notably, our analysis revealed an unexplored layer at the metal-substrate interface that is not possible to detect with the lab-based XRR. To establish the composition and the spatial extent of this interface layer, high-angle annular dark field–scanning transmission electron microscopy (HAADF-STEM) coupled with core-level electron energy loss spectroscopy (EELS) was employed. HAADF- STEM technique provided evidence of an interfacial layer approximately 0.7 nm thick between the metal-substrate layer, aligning well with the measurements from synchrotron XRR fitting. Further examination of the elemental composition of these interface layers using HAADF-STEM with EELS revealed an intermixing layer containing Al, O, and Ta atoms. Ab initio calculations suggest the substrate-metal interface structure’s dependence on the sapphire termination before Ta deposition, offering the potential to modulate the Ta film structure through pre-treatment of the sapphire surfaces. These findings offer valuable insights into controlling the structure and composition of the substrate-metal interface that may be able to increase qubit coherence times.