Research progress on the thin film materials of superconducting strip single photon detectors

The single photon detection of a superconducting strip single photon detector (SSPD) is based on the photon-induced local superconducting-to-normal transition within the nanowire. Generally, the single-photon detection performance of SSPD relies heavily on the physical properties of the involved superconducting thin film materials and the nano-fabrication process for ultra-narrow and long nanowires. With respect to the nano-fabrication process, after twenty years’ development, the electron beam lithography and reactive ion etching-based fabrication technology can pattern the superconducting thin films into uniform nanowires with a wire width down to 30 nm, which significantly lowers the influence of nanowire constrictions on the single photon detection performance of SSPDs. The superconducting thin film materials and their modulations of physical properties, therefore, become the key to realizing high-performance SSPDs.This review briefly describes the development and progress of the superconducting thin film materials for SSPD fabrications. Based on the phenomenological hotspot model, we analyzed the key physical properties of superconducting materials that influence the photon-response performance of superconducting nanowires. We note that the single photon detection performance of superconducting nanowires relies on the photon energy conversion efficiency, which is related to the sophisticated quasiparticle energy relaxation process. For superconducting materials with strong electron-electron interaction but weaker electron-phonon interaction, namely τe−ph/τee≫1, the energy of the incident photons will more efficiently transfer into the electron subsystem, which in turn depairs more Cooper pairs and generates more quasiparticles. As a result, the superconductivity is more severely suppressed, and a larger hotspot can be generated within the nanowire. With a larger hotspot, the bias current within the nanowire can be completely expelled into the readout circuits at a relatively low threshold bias current, resulting in more saturated bias current dependence of the intrinsic detection performance.Moreover, based on the dynamic quasiparticle diffusion process, we figure out that the hotspot diameter is related to the diffusion coefficient of the normal state quasiparticles, the quasiparticle thermalization time, and the hotspot relaxation time. We compared the two most important types of superconducting materials that have been successfully applied for SSPD fabrications, namely the polycrystalline NbN or NbTiN and amorphous WSi or MoSi. For the polycrystalline NbN or NbTiN nanowires, due to their strong electron-phonon interactions, the hotspot in these nanowires is relatively small, and researchers commonly need to enhance the disorder to achieve a relatively good detection performance. While the amorphous superconducting films natively hold a rather strong disorder level and a weak electron-phonon interaction, ensuring the extraordinary photon sensitivity. Finally, we also discussed the single-photon detection performance of unconventional superconducting nanowires with high critical temperature. We further discussed the future materials for fabricating all-rounder SSPDs, SSPDs with a large sensitive area, and extended to middle-infrared photons, SSPDs with high operation temperature beyond the liquid helium temperature.