Analysis of Low-Energy Localized Phonon Mode in SiGe Alloys and its impact on Lattice Thermal Conductivity: A Molecular Dynamics Study
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The origin of the low-energy localized phonon mode in SiGe alloys was investigated using classical molecular dynamics (MD) simulations. Inelastic X-ray scattering spectra have shown that the peak intensity of this mode is comparable to that of the Ge–Ge optical phonon mode. However, previous MD studies employing SiGe alloy models with randomly arranged atoms failed to reproduce the relative intensity of the localized phonon mode. In this work, we performed MD simulations using SiGe alloy models with controlled Ge cluster sizes. We found that the intensity of the localized phonon mode increases as the cluster size decreases, and the model containing isolated Ge atoms dispersed in the Si matrix exhibited the maximum intensity. Models containing Ge dimers or trimers simultaneously reproduce sharp localized phonon mode and Ge-Ge optical phonon mode. Considering that both the Ge-Ge optical phonon mode and the localized phonon mode are observed simultaneously, we conclude that Ge dimer or trimer clusters are the most plausible origin of the localized phonon mode. Also, we evaluated the lattice thermal conductivity of SiGe structures containing isolated Ge atoms dispersed in the Si matrix that exhibit the highest localized phonon mode intensity using the Green–Kubo method. Calculations were performed with Ge concentrations of 10%, 20%, and 30%. The thermal conductivity decreased with increasing Ge concentration, showing the lowest value in the model of 30%. The peak intensity of the localized phonon mode increased with Ge concentration and the 30% model exhibited highest value. These results suggest that the localized phonon mode contributes to the reduction in thermal conductivity.
