Temperature-dependent thermal and thermoelectric properties of n -type and p -type Sc1−xMgxN

Abstract

Scandium Nitride (ScN) is an emerging rocksalt semiconductor with octahedral coordination and an indirect bandgap. ScN has attracted significant attention in recent years for its potential thermoelectric applications, as a component material in epitaxial metal/semiconductor superlattices, and as a substrate for defect-free GaN growth. Sputter-deposited ScN thin films are highly degenerate $n$-type semiconductors and exhibit a large thermoelectric power factor of $\ensuremath{\sim}3.5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}\phantom{\rule{0.16em}{0ex}}\mathrm{W}/\mathrm{m}\text{\ensuremath{-}}{\mathrm{K}}^{2}$ at 600--800 K. Since practical thermoelectric devices require both n- and p-type materials with high thermoelectric figures-of-merit, development and demonstration of highly efficient p-type ScN is extremely important. Recently, the authors have demonstrated p-type $\mathrm{S}{\mathrm{c}}_{1\ensuremath{-}x}\mathrm{M}{\mathrm{g}}_{x}\mathrm{N}$ thin film alloys with low $\mathrm{M}{\mathrm{g}}_{\mathrm{x}}{\mathrm{N}}_{\mathrm{y}}$ mole-fractions within the ScN matrix. In this article, we demonstrate temperature dependent thermal and thermoelectric transport properties, including large thermoelectric power factors in both n- and p-type $\mathrm{S}{\mathrm{c}}_{1\ensuremath{-}x}\mathrm{M}{\mathrm{g}}_{x}\mathrm{N}$ thin film alloys at high temperatures (up to 850 K). Employing a combination of temperature-dependent Seebeck coefficient, electrical conductivity, and thermal conductivity measurements, as well as detailed Boltzmann transport-based modeling analyses of the transport properties, we demonstrate that p-type $\mathrm{S}{\mathrm{c}}_{1\ensuremath{-}x}\mathrm{M}{\mathrm{g}}_{x}\mathrm{N}$ thin film alloys exhibit a maximum thermoelectric power factor of $\ensuremath{\sim}0.8\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}\phantom{\rule{0.16em}{0ex}}\mathrm{W}/\mathrm{m}\text{\ensuremath{-}}{\mathrm{K}}^{2}$ at 850 K. The thermoelectric properties are tunable by adjusting the $\mathrm{M}{\mathrm{g}}_{\mathrm{x}}{\mathrm{N}}_{\mathrm{y}}$ mole-fraction inside the ScN matrix, thereby shifting the Fermi energy in the alloy films from inside the conduction band in case of undoped $n$-type ScN to inside the valence band in highly hole-doped $p$-type $\mathrm{S}{\mathrm{c}}_{1\ensuremath{-}x}\mathrm{M}{\mathrm{g}}_{x}\mathrm{N}$ thin film alloys. The thermal conductivities of both the n- and p-type films were found to be undesirably large for thermoelectric applications. Thus, future work should address strategies to reduce the thermal conductivity of $\mathrm{S}{\mathrm{c}}_{1\ensuremath{-}x}\mathrm{M}{\mathrm{g}}_{x}\mathrm{N}$ thin-film alloys, without affecting the power factor for improved thermoelectric performance.