Effects of W alloying on the electronic structure, phase stability and thermoelectric power factor in epitaxial CrN thin films

TL;DR

This study investigates how dilute tungsten alloying affects the electronic structure, phase stability, and thermoelectric properties of epitaxial CrN thin films, revealing potential for energy harvesting applications.

Contribution

It combines ab initio calculations with experimental epitaxial film growth to analyze the impact of W alloying on CrN's thermoelectric performance, highlighting the importance of solubility limits.

Findings

W alloying induces electronic band modifications favorable for thermoelectricity.

High electrical conductivity observed despite alloying.

Increased W content leads to decreased Seebeck coefficient and power factor.

Abstract

CrN-based alloy thin films are of interest as thermoelectric materials for energy harvesting. Ab initio calculations show that dilute alloying of CrN with 3 at.% W substituting Cr, induce flat electronic bands and push the Fermi level EF into the conduction band, while retaining dispersive Cr 3d bands. These features are conducive for both high electrical conductivity \sigma and high Seebeck coefficient \alpha, and hence the thermoelectric power factor {\alpha}^2\sigma. To investigate this possibility, epitaxial CrWxNz films were grown on c-plane sapphire by dc-magnetron sputtering. However, even films with the lowest W concentration (x = 0.03) in our study contained metallic h-Cr2N, which is not conducive for a high \alpha. Nevertheless, the films exhibit a sizeable power factor of {\alpha}^2\sigma ~ 4.7 x 10-4 Wm-1K-2 due to high \sigma ~ 700 Scm-1, and a moderate \alpha ~ -25 ~{^^^^00b5}V/K. Increasing h-Cr2N fractions in the 0.03 < x \le 0.19 range monotonically increases \sigma, but severely diminishes \alpha leading to two orders of magnitude decrease in {\alpha}^2\sigma. This trend continues with x > 0.19 due to W precipitation. These findings indicate that dilute W additions below its solubility limit in CrN is important for realizing high thermoelectric power factor in CrWxNz alloy films.