Imaging of Thermal Domains in ultrathin NbN films for Hot Electron Bolometers

TL;DR

This study uses low-temperature scanning electron microscopy to visualize thermal domains in ultrathin NbN superconducting microbridges, revealing inhomogeneities and validating hotspot models relevant for hot electron bolometers.

Contribution

It introduces LTSEM as a tool to image thermal structures in NbN microbridges, providing insights into inhomogeneities affecting bolometer performance.

Findings

Detection of micron-scale inhomogeneities in NbN films

Confirmation of hotspot model features through experimental comparison

Identification of spatial variations impacting device behavior

Abstract

We present low-temperature scanning electron microscopy (LTSEM) investigations of superconducting microbridges made from ultrathin NbN films as used for hot electron bolometers. LTSEM probes the thermal structure within the microbridges under various dc current bias conditions, either via electron-beam-induced generation of an unstable hotspot, or via the beam-induced growth of a stable hotspot. Such measurements reveal inhomogeneities on a micron scale, which may be due to spatial variations in the NbN film or film-interface properties. Comparison with model calculations for the stable hotspot regime confirm the basic features of common hot spot models.