Thermoelectric properties and electronic structure of Cr(Mo,V)Nx thin films studied by synchrotron and lab-based x-ray …

Chromium-based nitrides are used in hard, resilient coatings and show promise for thermoelectric applications due to their combination of structural, thermal, and electronic properties. Here, we investigate the electronic structures and chemical bonding correlated to the thermoelectric properties of epitaxially grown chromium-based multicomponent nitride $\mathrm{Cr}(\mathrm{Mo},\phantom{\rule{0.16em}{0ex}}\mathrm{V}){\mathrm{N}}_{x}$ thin films. The small amount of N vacancies causes Cr $3d$ and N $2p$ states to appear at the Fermi level and reduces the band gap in ${\mathrm{Cr}}_{0.51}{\mathrm{N}}_{0.49}$. Incorporating holes by alloying of V in N-deficient CrN results in an enhanced thermoelectric power factor with marginal change in the charge transfer of Cr to N compared with ${\mathrm{Cr}}_{0.51}{\mathrm{N}}_{0.49}$. Further alloying of Mo, isoelectronic to Cr, increases the density of states at the Fermi level due to hybridization of the (Cr, V) $3d$ and Mo $4d$-N $2p$ states in $\mathrm{Cr}(\mathrm{Mo},\phantom{\rule{0.16em}{0ex}}\mathrm{V}){\mathrm{N}}_{x}$. This hybridization and N off-stoichiometry result in more metal-like electrical resistivity and reduction in Seebeck coefficient. The N deficiency in $\mathrm{Cr}(\mathrm{Mo},\phantom{\rule{0.16em}{0ex}}\mathrm{V}){\mathrm{N}}_{x}$ also depicts a critical role in reduction of the charge transfer from metal to N site compared with ${\mathrm{Cr}}_{0.51}{\mathrm{N}}_{0.49}$ and ${\mathrm{Cr}}_{0.50}{\mathrm{V}}_{0.03}{\mathrm{N}}_{0.47}$. In this paper, we envisage ways for enhancing thermoelectric properties through electronic band engineering by alloying and competing effects of N vacancies.