Optical and transport properties of NbN thin films revisited

TL;DR

This paper revisits the optical and electronic properties of disordered NbN thin films, proposing a modified model that incorporates quantum corrections to resolve previous discrepancies in parameter estimation.

Contribution

It introduces a modified Drude-Lorentz model including quantum corrections, providing consistent parameters aligned with experimental and ab initio data.

Findings

Quantum corrections are essential for accurate optical conductivity modeling.

Revised parameters include electron relaxation rate, Fermi velocity, and density of states.

The modified model resolves previous inconsistencies in NbN thin film analysis.

Abstract

Highly disordered NbN thin films exhibit promising superconducting and optical properties. Despite extensive study, discrepancies in its basic electronic properties persist. Analysis of the optical conductivity of disordered ultra-thin NbN films, obtained from spectroscopic ellipsometry by standard Drude-Lorentz model, provides inconsistent parameters. We argue that this discrepancy arise from neglecting the presence of quantum corrections to conductivity in the IR range. To resolve this matter, we propose a modification to the Drude-Lorentz model, incorporating quantum corrections. The parameters obtained from the modified model are consistent not only with transport and superconducting measurements but also with ab initio calculations. The revisited values describing conduction electrons, which differ significantly from commonly adopted ones, are the electron relaxation rate $\Gamma\approx1.8~\textrm{eV}/\hbar$, the Fermi velocity $v_F \approx 0.7 \times 10^{6}~\textrm{ms}^{-1}$ and the electron density of states $N(E_F)=2~$states of both spins/eV/$V_{\textrm{f.u.}}$.