Temperature-Dependent Dielectric Function of Tantalum Nitride Formed by Atomic Layer Deposition for Tunnel Barriers in Josephson Junctions

TL;DR

This study characterizes the temperature-dependent dielectric properties of atomic layer deposited tantalum nitride films, demonstrating their suitability as insulating tunnel barriers in Josephson junctions for quantum computing.

Contribution

It provides the first detailed dielectric function analysis of ALD TaN films across a range of temperatures, establishing their potential for superconducting quantum circuits.

Findings

ALD TaN films are insulating with no free carrier absorption at all tested temperatures.

The dielectric function shows transparency in the mid-infrared, suitable for tunnel barriers.

ALD TaN's stability and properties enable thicker barriers and improved device performance.

Abstract

We report the dielectric functions of insulating tantalum nitride (TaN) films, deposited using atomic layer deposition (ALD) on 300 mm Si/SiO2 substrates, to demonstrate their suitability as tunnel barriers in tantalum-based Josephson junctions (JJ) for superconducting quantum circuits. The temperature-dependent ellipsometric angles were measured using ALD TaN films with nominal thicknesses of 13 nm and 25 nm at an incidence angle of 70 degrees, across photon energy ranges of 0.03 eV to 0.7 eV (80-300 K) and 0.5 eV to 6.5 eV (80-600 K). This data was used to develop a dispersion model for insulating ALD TaN films that incorporates a Tauc-Lorentz oscillator with a band gap of 1.5-1.8 eV to model the interband optical transitions. The extracted dielectric function of ALD TaN films shows an insulating behavior (mid-infrared transparency) at all temperatures and for both film thicknesses tested. ALD TaN does not exhibit infrared absorption due to free carriers, even at elevated temperatures, demonstrating its insulating nature, which is required for the tunnel barrier of the JJ in quantum applications. The results of transmission electron microscopy, including selected area electron diffraction, and X-ray diffraction are also discussed. Sputter depth-profile X-ray photoelectron spectroscopy (XPS) shows an N/Ta ratio of ~1.2 throughout the film. The lower band gap, low roughness, and thermal stability of ALD TaN compared to AlOx suggest the possibility of fabricating JJs with thicker barriers while achieving critical current densities required for qubits, better control of thickness and composition, reduced topography, and resistance to aging.