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

TL;DR

This study uses transport measurements in magnetic fields to characterize thin NbN superconducting films, determining quasiparticle lifetime, grain size distribution, and other parameters crucial for single-photon detector performance.

Contribution

It introduces a nondestructive transport measurement method to extract key material parameters of NbN films relevant for superconducting photon detectors.

Findings

Quasiparticle relaxation time is about 2 ns.

Grain size distribution matches microscopy images.

Transport measurements reveal vortex dynamics and material uniformity.

Abstract

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