Visualizing Local Superconductivity of NbTiN Nanowires to Probe Inhomogeneity in Single-Photon Detectors

NbTiN has a high critical temperature (T_c) of up to 17 K, making it a great candidate for superconducting nanowire single-photon detectors (SNSPDs) and other applications requiring a bias current close to the depairing current. However, superconducting inhomogeneities are often observed in superconducting thin films, and superconducting inhomogeneities can influence the vortex nucleation barrier and furthermore affect the critical current I_c of a superconducting wire. Superconducting inhomogeneities can also result in stochastic variations in the critical current between identical devices, and therefore, it is crucial to have a detailed understanding of inhomogeneities in SNSPDs in order to improve device efficiency. In this study, we utilized scanning tunneling microscopy/spectroscopy (STM/STS) to investigate the inhomogeneity of superconducting properties in meandered NbTiN nanowires, which are commonly used in SNSPDs. Our findings show that variations in the superconducting gap are strongly correlated with the film thickness. By using time-dependent Ginzburg-Landau simulations and statistical modeling, we explored the implications of the reduction in the critical current and its sample-to-sample variations. Our study suggests that the thickness of NbTiN plays a critical role in achieving homogeneity in superconducting properties.