Thermoelectric properties of epitaxial ScN films deposited by reactive magnetron sputtering onto MgO(001) substrates

Epitaxial ScN(001) thin films were grown on MgO(001) substrates by dc reactive magnetron sputtering. The deposition was performed in an Ar/N-2 atmosphere at 2 x 10(-3) Torr at a substrate temperature of 850 degrees C in a high vacuum chamber with a base pressure of 10(-8) Torr. In spite of oxygen contamination of 1.6 +/- 1 at. %, the electrical resistivity, electron mobility, and carrier concentration obtained from a typical film grown under these conditions by room temperature Hall measurements are 0.22 m Omega cm, 106 cm(2) V-1 s(-1), and 2.5 x 10(20) cm(-3), respectively. These films exhibit remarkable thermoelectric power factors of 3.3-3.5 x 10(-3) W/mK(2) in the temperature range of 600 K to 840 K. The cross-plane thermal conductivity is 8.3 W/mK at 800 K yielding an estimated ZT of 0.3. Theoretical modeling of the thermoelectric properties of ScN calculated using a mean-free-path of 23 nm at 300 K is in very good agreement with the experiment. These results also demonstrate that further optimization of the power factor of ScN is possible. First-principles density functional theory combined with the site occupancy disorder technique was used to investigate the effect of oxygen contamination on the electronic structure and thermoelectric properties of ScN. The computational results suggest that oxygen atoms in ScN mix uniformly on the N site forming a homogeneous solid solution alloy. Behaving as an n-type donor, oxygen causes a shift of the Fermi level in ScN into the conduction band without altering the band structure and the density of states. (C) 2013 AIP Publishing LLC [http://dx.doi.org/10.1063/1.4801886]