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 sigma and high Seebeck coefficient alpha and, hence, a high thermoelectric power factor alpha(2)sigma. To investigate this possibility, epitaxial CrWxNz 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 alpha. Nevertheless, the films exhibit a sizeable power factor of alpha(2)sigma similar to 4.7 x 10(-4) W m(-1) K-2 due to high sigma similar to 700 S cm(-1), and a moderate alpha similar to - 25 mu V/K. Increasing h-Cr2N fractions in the 0.03 < x <= 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 are important for realizing a high thermoelectric power factor in CrWxNz films.