Characterization of superconducting NbTiN films using a Dispersive Fourier Transform Spectrometer

TL;DR

This paper introduces a Terahertz Dispersive Fourier Transform Spectrometer to analyze the frequency-dependent complex conductivity of superconducting NbTiN films, providing detailed characterization relevant for THz detector applications.

Contribution

It presents a novel spectrometer method for characterizing superconducting films at terahertz frequencies, with experimental validation on Nb and NbTiN films showing good agreement with theoretical models.

Findings

Determined the strong-coupling coefficient for Nb and NbTiN films.

Validated the spectrometer method against extended Mattis-Bardeen theory.

Characterized film quality at terahertz frequencies.

Abstract

We have built a Terahertz Dispersive Fourier Transform Spectrometer \cite{Birch1987} to study frequency properties of superconducting films used for fabrication of THz detectors. The signal reflected from the tested film is measured in time domain, which allows to separate it from the other reflections. The complex conductivity of the film depends on frequency and determines the reflection coefficient. By comparing the film reflection in the superconducting state (temperature is below $T_c$) with the reflection of the normal state, we characterise the film quality at terahertz frequencies. The method was applied to 70 and 200nm thick Nb films on a silicon wafer and to 360nm thick NbTiN films on silicon and quartz wafers. The strong-coupling coefficient, $\alpha$, was found to be 3.52 for Nb, and 3.71-4.02 for the NbTiN films. The experimental results were fitted using extended Mattis-Bardeen theory \cite{Noguchi2012} and show a good agreement.