The phonon dispersion relation in 〈100〉 Si nanowire (SiNW) is calculated by employing a realistic atomistic model surrounded by thin SiO2 layer. We performed molecular dynamics simulations to calculate the dynamical structure factor by the space-time Fourier transform of atomic trajectories, and extracted the phonon dispersion relations. In the SiNWs, low energy phonon branches spread into broad spectra due to the presence of the SiO2 film, which is considered as the origin of the thermal conductivity degradation. A softening of the transverse optical mode also appears due to the lattice strain induced by the outer oxide film. This work suggests that the presence of amorphous oxide layer is crucial factor to characterize phonon vibration properties in practical SiNWs.
We perform a series of molecular dynamics (MD) simulations to investigate the heat transport in Si nano-structures, while explicitly including oxide cover layers in the simulation system for the first time. The dependences of thermal diffusion velocity on the thicknesses of the SiO2 film and Si lattice are investigated. The results show that thermal diffusion velocity decreases with Si lattice thickness and does not depend on SiO2 film thickness.
A series of molecular dynamics (MD) simulations have been conducted to investigate the heat transport in terms of the phonon dynamics in nanoscale silicon (Si). This work is motivated by a concern over the stagnation of heat at the drain region of nanoscopic transistors, owing to this, a large amount of optical phonons with a low group velocity are emitted from hot electrons, which are ballistically transferred through channel region. The point of this work is the explicit inclusion of the SiO 2 film in the MD simulation of the Si lattice. The calculation results show that longitudinal optical (LO) phonons decay faster as Si lattice thickness decreases and turn into acoustic phonons. In contrast, thermal diffusion rate decreases with Si lattice thickness. Both the decay rate of LO phonons and thermal diffusion rate are not governed by oxide thickness. These results imply that the phonon scattering at the SiO 2 /Si interface is enhanced by thinning the Si layer. In nanoscopic devices, a thin Si layer is effective in diminishing the optical phonons with a low group velocity, but it hinders the subsequent heat transport.
The phonon dispersion relation in <100> Si nanowire (SiNW) is calculated by employing a realistic atomistic model surrounded by thin SiO 2 layers. We performed molecular dynamics simulation to calculate the dynamical structure factor by the space-time Fourier transform of atomic trajectories, and extracted the phonon dispersion relations. Although the bulk dispersion relations are maintained in the SiNWs on the whole, acoustic phonon branches are diffused beyond recognition, which is considered as the origin of the thermal conductivity degradation in SiNWs. A red shift of the transverse optical mode also appears probably due to the lattice strain induced by the outer oxide film. These results provide a foothold to estimate the electron-phonon scattering rates and the heat transport processes in realistic SiNWs.
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