Imaging of thermal domains in ultrathin NbN films for hot electron bolometers

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.

Hot Spots and Waves inBi2Sr2CaCu2O8Intrinsic Josephson Junction Stacks: A Study by Low Temperature Scanning Laser Microscopy

“…The feature seen here is very indicative of a hot spot. For conventional superconducting thin films, for in-plane current flow such hot spots have been imaged by LTSEM [28,29]. Here, when the beam touches the edge of the hot spot it increases its size, leading to a signal ∆V> 0.…”

Mapping of the quantum efficiency of a superconducting single electron detector

“…Here we present experiments in which we take advantage of the tightly focussed electron beam to test the local single-particle response of the detector. We note that our work differs from previous lowtemperature scanning-electron microscopy (LTSEM) studies of superconducting films [6], [7] that focussed on response in the stationary regime. Using single-electron detection we observe wire inhomogeneities with a spatial resolution considerably improved with respect to both previous LTSEM work and also with optical scanning methods [8].…”

Two-stage dissipation in a superconducting microbridge: experiment and modeling

“…Since ρ h > ρ b , equation (7) indicates that the temperature increase in the damaged zone is bigger than in the rest of the bridge. Since the model assumes that T * c (x, I ) is larger outside the weak zone at any given current, a threshold I th3 exists for which the dissipation stops everywhere except in the weak section, which now becomes a localized hot spot.…”

“…While the jumps in I -V curves associated with hot spots have an extraordinary importance for applications, a full understanding of the phenomenon demands the use of thermal visualization techniques. Low temperature scanning electron microscopy has been used to identify unstable hot spots generated by an electron beam in ultrathin NbN superconductors [7]. Hot spots in intrinsic stacks of Josephson junctions have been recently visualized in Bi 2 Sr 2 CaCu 2 O 8 (BSCCO) superconductors by means of low temperature scanning laser microscopy [8].…”