Microstructure, defects, and superconductivity of high-power impulse magnetron sputtered NbTiN thin films

Superconducting NbTiN thin films, one of the promising candidates for quantum and superconducting radio-frequency (SRF) cavity applications, were deposited using high-power impulse magnetron sputtering (HiPIMS), a high-quality thin film deposition technique. The effects of deposition pressure, duty cycle, substrate temperature, and substrate bias on microstructure, vacancy-type defects, and superconducting properties were systematically studied. Using positron annihilation lifetime spectroscopy (PALS) from the radiation source ELBE combined with Doppler broadening spectroscopy (DBS), together with XRD, SEM, and AFM, a direct correlation between defect landscape and the superconducting transition temperature ( T c ) was experimentally demonstrated for the first time. The results indicate that higher T c values are typically associated with films exhibiting lower defect concentration, denser microstructures, and a preferred (200) orientation of NbTiN. We also found cases where T c improved even in the presence of higher defect concentrations, as long as the films exhibited dense microstructure and well-connected grains. This highlights the complex interplay between defects, microstructure, and superconducting properties. Optimal deposition conditions yielded the highest T c of ∼ 15.49 K in this work. These findings suggest that minimising open volumes in the material, combined with microstructural refinement through coordinated parameter optimisation, is key to optimising the superconducting properties of NbTiN films.