Strain engineering of polar optical phonon scattering mechanism – an effective way to optimize the power-factor and lattice thermal conductivity of ScN

The tug-of-war between the thermoelectric power factor and the figure-of-merit complicates thermoelectric material selection, particularly for mid-to-high temperature thermoelectric materials. Approaches to reduce lattice thermal conductivity while maintaining a high-power factor are crucial in thermoelectric applications. Using strain engineering, we comprehensively investigated the microscopic mechanisms influencing the lattice thermal conductivity in this study. Scandium nitride (ScN) was chosen for this purpose since it has recently been discovered to be a potential mid-to-high temperature thermoelectric material. Our precise DFT+U calculations showed the exact electronic direct and indirect band gaps in ScN, which was subsequently subjected to compressive and tensile volume strain (up to 2%) within the crystal structure. Relevant thermoelectric properties such as Seebeck coefficient and electrical conductivity were obtained from both strained and unstrained ScN, whilst incorporating three key scattering sources, namely, ionized impurity (IMP), acoustic deformation potential (ADP), and polar optical phonon (POP). Based on the calculated scattering rates, we found that a POP scattering source is the dominant scattering mechanism that has a significant impact on transport properties at high temperatures. Our study revealed that modifying this POP scattering mechanism through strain in ScN has a considerable impact on the variation of lattice thermal conductivity without much reduction in the thermoelectric power factor values. A detailed description was provided with a focus on understanding the effects of strain on the scattering rates and thermoelectric properties of ScN.