High-temperature growth of ultrathin NbTiN films for waveguide integrated SNSPDs on LNOI

A key challenge towards monolithic integrated quantum photonics experiments on lithium niobate-on-insulator (LNOI) is the integration of single photon detectors. Niobium titanium nitride (NbTiN) superconducting nanowire single photon detectors (SNSPDs) are a strong candidate for this application. In this work, we use DC reactive magnetron sputtering to grow high quality 4 nm thick NbTiN films using a custom made ultra-high vacuum deposition system with a base pressure of 1E-10 mbar. Enabled by the low concentration of background impurities in this system, we investigate the relatively unexplored impact of substrate temperature during NbTiN growth. At 825 K we achieve superconducting films with a critical temperature (Tc) of 12.3 K and residual-resistance ratio above 1. This is among the highest reported Tc to date for sub-10 nm NbTiN films. AFM measurements indicate that our films evolve from a porous pillar structure when grown at low temperatures to densely packed fibrous grains at higher temperatures. Furthermore, we successfully integrate these films with the LNOI platform and investigate the obtained interfaces with STEM measurements. Lastly, we characterize the NbTiN films and their native oxide using XRD and XPS measurements and link these results to detector performance.