An Investigation of the Heat-Transfer Related Properties of Silicon and Germanium at Nanoscale via Molecular Dynamics Simulations
Date Issued
2009
Date
2009
Author(s)
Weng, Chien-Chou
Abstract
This thesis employs the equilibrium molecular dynamics (EMD) simulation to calculate the phonon dispersion relations and density of states of Si/Ge bulk materials, thin films, nanowires as well as superlattice thin films. It also applies the non-equilibrium molecular dynamics (NEMD) simulation to a calculation of the in-plane thermal conductivity of silicon thin films and the axial thermal conductivity of silicon nanowires. In the simulations, the Stillinger-Weber (SW) potential, including both the two-body and the three-body interactions, is adopted.As far as the dispersion relations are concerned, remarkable differences between bulk materials and thin films/nanowires are observed. In particular, the latter possesses many discrete vibration modes. These modes are also observed in the in-plane spectrum of the superlattice thin films; mini-Brillouin zones are generated in the cross-plane spectrum on the other hand. As the film/wire thickness decreases, the phonon DOS tends to shift to the lower frequencies. The lower dimension, the more the shift is. The superlattice period shows little effect on the DOS of superlattice thin films and the DOS of each layer is similar to its bulk counterpart, in spite of numerous additional peaks corresponding to the rhombus structures appearing in the cross-plane spectrum.n obtaining the thermal conductivity, both the quantum correction based on the measured DOS of the associated nanostructure and the elimination of the finite-size error by an extrapolation technique are performed. It is found as the temperature increases, the thermal conductivities of thin films and nanowires increase first because of the increased excited phonon modes and then decrease due to the enhanced Umklapp scattering. A peak value is thus observed at some temperature. Finally, an attempt is made to control the surface roughness of thin films by adjusting the model parameters associated with the surface potential. However, the computed thermal conductivity does not as expected decrease with the increasing surface roughness, which is attributed to the non-negligible statistic error.
Subjects
Molecular Dynamics
Nanostructure
Phonon Dispersion Relation
Quantum Correction
Lattice Thermal Conductivity
Surface roughness
Type
thesis
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