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.
The improvement of thermoelectric figure of merit of silicon nanowire (SiNW) can be achieved by lowering its thermal conductivity. In this work, non-equilibrium molecular dynamics method was used to demonstrate that the thermal conductivity of bulk silicon crystal is drastically reduced when it is crafted as SiNW and that it can be reduced remarkably by including vacancy defects. It has been found that 'centre vacancy defect' contributes much more in reducing the thermal conductance than 'surface vacancy defect'. The lowest thermal conductivity that occurs is about 52.1% of that of pristine SiNW, when 2% vacancy defect is introduced in the nanowire. The vibrational density of states analysis was performed to understand the nature of this reduction and it has been found that the various boundary scatterings of phonon significantly reduce the thermal conductivity. Also, larger mass difference due to voids induces smaller thermal conductivity values. These results indicate that the inclusion of vacancy defects can enhance the thermoelectric performance of SiNWs.
The frequency domain perfectly matched layer (FDPML) approach is used to study phonon transport in a series of large 2D domains with randomly embedded nanoparticles over a wide range of nanoparticle loadings and wavelengths. The effect of nanoparticle packing density on the mean free path and localization length is characterized. We observe that, in the Mie scattering regime, the independent scattering approximation is valid up to volume fractions exceeding 10% and often higher depending on scattering parameter, indicating that the mean free path can usually be calculated much less expensively using the number density and the scattering cross section of a single scatterer. We also study localization lengths and their dependence on particle loading. For heavy nanoparticles embedded in a lighter material, using the FDPML approach, we only observe localization at volume fractions [Formula: see text] and only for short wavelength modes where vibrational frequencies exceed those available in the embedded nanoparticles. Using modal analysis, we show that localization in nanoparticle laden materials is primarily due to energetic confinement rather than Anderson localization. We then show that, by using light particles in a heavy matrix, the fraction of confined modes can be substantially increased.
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