Characterization of thin-film NbN superconductor for single-photon detection by transport measurements

摘要

The fabrication of high-quality thin superconducting films is essential forsingle-photon detectors. Their device performance is crucially affected bytheir material parameters, thus requiring reliable and nondestructivecharacterization methods after the fabrication and patterning processes.Important material parameters to know are the resistivity, superconductingtransition temperature, relaxation time of quasiparticles, and uniformity ofpatterned wires. In this work, we characterize micro-patterned thin NbN filmsby using transport measurements in magnetic fields. We show that from theinstability of vortex motion at high currents in the flux-flow state of the$IV$ characteristic, the inelastic life time of quasiparticles can bedetermined to be about 2 ns. Additionally, from the depinning transition ofvortices at low currents, as a function of magnetic field, the sizedistribution of grains can be extracted. This size distribution is found to bein agreement with the film morphology obtained from scanning electronmicroscopy and high-resolution transmission electron microscopy images.