Multifunctional Irradiation-Induced Defects for Enhancing Thermoelectric Properties of Scandium Nitride Thin Films

Scandium nitride (ScN) is an emerging rocksalt indirect bandgap semiconductor with the potential to overcome some of the limitations of traditional wurtzite III (A)-nitride semiconductors in next-generation optoelectronic and thermoelectric applications. Epitaxial ScN thin films contain point defects such as oxygen impurities that result in a high carrier concentration and help achieve a high thermoelectric power factor. However, due to its high thermal conductivity, the thermoelectric figure-of-merit (zT) of ScN is relatively low. Recent theoretical calculations have suggested that scandium and nitrogen vacancies in ScN introduce asymmetric states close to the Fermi energy, increasing its Seebeck coefficient. Increased phonon scattering by these native defects should also reduce thermal conductivity to help achieve higher zT. However, incorporating such native defects in as-deposited ScN is challenging due to their high formation energies. In this work, we introduce native defects in molecular beam epitaxy (MBE)-deposited ScN thin films by lithium-ion irradiation and study the impact of such native defects on ScN's thermoelectric properties. Consistent with theoretical calculations, we find that the Seebeck coefficient in irradiated ScN thin films increases significantly. Thermal conductivity decreases by more than half to 7 ± 1 W/(m·K) at room temperature due to phonon scattering by the irradiation-induced defects. Despite a reduced electrical conductivity due to scattering from defects, irradiated ScN thin films exhibit a high power factor ∼(1–2) × 10–3 W/(m·K2) in the 300–950 K range and show a modest increase in the overall zT. This work highlights the multifunctionality of irradiation-induced defects in engineering thermoelectric properties of transition metal nitride semiconductors.