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

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 E_F into the conduction band while retaining dispersive Cr 3d bands. These features are conducive for both high electrical conductivity σ and high Seebeck coefficient α and, hence, a high thermoelectric power factor α²σ. To investigate this possibility, epitaxial CrW_xN_z films were grown on c-sapphire by dc-magnetron sputtering. However, even films with the lowest W content (x = 0.03) in our study contained metallic h-Cr₂N, which is not conducive for a high α. Nevertheless, the films exhibit a sizeable power factor of α²σ ∼ 4.7 × 10⁻⁴ W m⁻¹ K⁻² due to high σ ∼ 700 S cm⁻¹, and a moderate α ∼ − 25 μV/K. Increasing h-Cr₂N fractions in the 0.03 < x ≤ 0.19 range monotonically increases σ, but severely diminishes α leading to two orders of magnitude decrease in α²σ. This trend continues with x > 0.19 due to W precipitation. These findings indicate that dilute W additions below its solubility limit in CrN are important for realizing a high thermoelectric power factor in CrW_xN_z films. [ABSTRACT FROM AUTHOR]