Nanocrystalline superconducting $\gamma$-Mo$_2$N ultra-thin films for single-photon detectors

TL;DR

This study investigates how surface passivation affects the superconducting properties of ultra-thin $ ext{Mo}_2 ext{N}$ films, revealing that protective layers enhance critical temperature and analyzing flux-flow instability for potential single-photon detector applications.

Contribution

It demonstrates that surface passivation with AlN layers improves superconducting critical temperature and provides detailed analysis of flux-flow instability in ultra-thin $ ext{Mo}_2 ext{N}$ films for detector use.

Findings

Surface passivation increases critical temperature in ultra-thin films.

Flux-flow instability characterized and modeled using Larkin-Ovchinnikov theory.

Fast quasiparticle relaxation time (~45 ps) suitable for single-photon detectors.

Abstract

We analyze the influence of the surface passivation produced by oxides on the superconducting properties of $\gamma$-Mo$_2$N ultra-thin films. The superconducting critical temperature of thin films grown directly on Si (100) with those using a buffer and a capping layer of AlN are compared. The results show that the cover layer avoids the presence of surface oxides, maximizing the superconducting critical temperature for films with thicknesses of a few nanometers. We characterize the flux-flow instability measuring current-voltage curves in a 6.4 nm thick Mo$_2$N film with a superconducting critical temperature of 6.4 K. The data is analyzed using the Larkin and Ovchinnikov model. Considering self-heating effects due to finite heat removal from the substrate, we determine a fast quasiparticle relaxation time $\approx$ 45 ps. This value is promising for its applications in single-photon detectors.