Room temperature growth of niobium titanium alloy thin films on sapphire substrates for low loss superconducting quantum devices

Superconducting microwave resonators with high quality factors (high-Q) have become a key technology in a wide range of applications. The quest for materials that have low loss property at low temperatures is an area of great interest for quantum computation and photon detection. Niobium-titanium (NbTi) is a disordered superconducting alloy with a relatively high transition temperature (T_c) and has been widely used in industrial applications as practical superconducting wires. Here, we present the growth of high quality NbTi thin films on c-plane sapphire (Al₂O₃) substrates using room temperature magnetron sputtering techniques. We investigate the strain, crystallinity, surface morphology, and superconducting properties of NbTi films with various thicknesses. A nanoscale surface morphology exhibits a sixfold symmetric grain structure following the hexagonal Al₂O₃ substrate. Detailed XRD analyses revealed that the bcc NbTi with higher T_c are grown epitaxially on Al₂O₃ substrates with bcc (110) and (100) aligned along Al₂O₃ (0001) and (10-10), respectively. We found that the film quality and the concomitant superconducting property can be widely controlled by changing the sputtering conditions and relevant parameters.