Impact of Ge clustering on the thermal conductivity of SiGe nanowires: atomistic simulation study.

Using non-equilibrium molecular dynamics simulations, we demonstrate that the thermal conductivity of SiGe alloy nanowires is remarkably sensitive to inhomogeneous composition distributions. Specifically, the effects of Ge clustering on the thermal conductivity of SiGe nanowires are studied. The results showed that clustering Ge atoms can improve the thermal conductivity of SiGe alloy nanowires due to the reduction of random alloy scattering centers. When the number of Ge atoms in the nanocluster increases, the thermal conductivity of such nanowires grows monotonically compared with that of random alloy nanowires. To reveal the role of inhomogeneous Ge distributions on the thermal conductivity, we performed vibrational eigenmode analyses and found the remarkable delocalization of phonon modes after Ge clustering. Through such analyses, we found that the increase in thermal conductivity was correlated with the phonon delocalization in the SiGe nanowires, where stronger delocalization indicates a better thermal performance of the nanowires. Our results are helpful not only in understanding the clustering effects on heat transport but also in modulating the thermal conductivity of SiGe nanowires.

Enhancing the Seebeck effect in Ge/Si through the combination of interfacial design features.

Due to their inherent physical properties, thin-film Si/SiGe heterostructures have specific thermal management applications in advanced integrated circuits and this in turn is essential not only to prevent a high local temperature and overheat inside the circuit, but also generate electricity through the Seebeck effect. Here, we were able to enhance the Seebeck effect in the germanium composite quantum dots (CQDs) embedded in silicon by increasing the number of thin silicon layers inside the dot (multi-fold CQD material). The Seebeck effect in the CQD structures and multi-layer boron atomic layer-doped SiGe epitaxial films was studied experimentally at temperatures in the range from 50 to 300 K and detailed calculations for the Seebeck coefficient employing different scattering mechanisms were made. Our results show that the Seebeck coefficient is enhanced up to ≈40% in a 3-fold CQD material with respect to 2-fold Ge/Si CQDs. This enhancement was precisely modeled by taking into account the scattering of phonons by inner boundaries and the carrier filtering by the CQD inclusions. Our model is also able to reproduce the observed temperature dependence of the Seebeck coefficient in the B atomic layer-doped SiGe fairly well. We expect that the phonon scattering techniques developed here could significantly improve the thermoelectric performance of Ge/Si materials through further optimization of the layer stacks inside the quantum dot and of the dopant concentrations.

A model for predicting the thermal conductivity of SiO2-Ge nanoparticle composites.

We present a simple theoretical model that predicts the thermal conductivity of SiO2 layers with embedded Ge quantum dots (QDs). Overall, the resulting nanoscale architecture comprising the structural relaxation in the SiO2 matrix, deviation in mass density of the QDs compared to the surrounding matrix and local strains associated with the dots are all likely to enhance phonon scattering and thus reduce the thermal conductivity in these systems. We have found that the conductivity reduction can be predicted by the dot-induced local elastic perturbations in SiO2. Our model is able to explain not only this large reduction but also the magnitude and temperature variation of the thermal conductivity with size and density of the dots. Within the error range, the theoretical calculations of the temperature-dependent thermal conductivity in different samples are in close agreement with the experimental measurements. Including the details of the strain fields in oxidized Si nanostructured layers is therefore essential for a better prediction of the heat pathways in on-chip thermoelectric devices and circuits.