An Investigation of the Lattice Thermal Transport Phenomenon in Low-Dimensional Si/Ge via the Molecular Dynamics Simulation
Date Issued
2010
Date
2010
Author(s)
Chang, Tai-Ming
Abstract
The low-dimensional structures have been widely investigated because of their low thermal conductivities for improving the efficiency of thermoelectric materials. Past theoretical and experimental studies found that the lattice thermal conductivity of thin-film, nanowire, superlattice, quantum dot (QD) superlattice etc., can further reduce 1~ 2 order of magnitude to their bulk counter parts. Nonetheless, it’s much difficult to analyze or fabricate the complicated nanostructures through the theoretical model or the experiment. Hence, in this thesis we attempt to establish the NEMD simulation to calculate the thermal properties of low-dimensional materials. We first arrange and build a reasonable procedure of NEMD approach by employing the simulation of single crystal of the silicon thin film, and further simulate the QD heterostructures to observe the influence on thermal properties of QD. The Stillinger-Weber potential which contains two-body and three-body interactions is adopted for silicon and germanium in the simulation.
In the simulation of thin film, since the calculation of temperature in MD belongs to classical mechanics, it’s necessary to make the quantum correction of temperature when lower than Debye temperature. Most of investigations made the correction by using the Debye or bulk DOS. In this thesis, however, we adopt the thin film DOS via EMD simulation. Besides, the so-called finite-size-effect is caused by absence of phonons with long wave lengths in the finite size of in-plane thin film. To obtain the results of infinite domain, samples of various lengths are simulated and an extrapolation technique is employed. The investigation shows the thermal conductivities should be corrected when simulated temperature is close to room temperature. Moreover, the corrected results by the thin-film DOS presented in this thesis are reasonable and agree excellently with the theoretical predictions having a similar surface roughness based on the phonon Boltzmann equation.
As for simulation of Si/Ge heterostructures, two commonly used methods, control heat flux and control temperature, are compared first for producing the heat transfer in the NEMD simulation. It is found that the longer simulation time is required to attain the steady state and to take the time average because of the existence of interface. Besides, the use of control temperature method has the better convergence. Next, to clarify QD phenomenon, we employed three heterostructures, a Si/Ge QD/Ge heterostructure, a Si/Ge QD/Si heterostructure, and a QD superlattice in this thesis. From the simulation results of former two heterostructures, we found that acoustic mismatch can be alleviated by the QDs embedded inside. However, the existence of the wetting layer may create an additional interface and cause the destruction of phonon transport by wave interference. The large Ge concentration of QD may also lead to reduction the effective thermal conductivity because the low thermal conductivity of Ge material. Since many interfaces are existed in the QD superlattices, the effective thermal conductivity only decreases with the increasing QD density, and it qualitatively and quantitatively agree with the experimental measurements.
Subjects
Thin-film
Superlattice
Quantum dot
Lattice thermal conductivity
Density of states
Non-equilibrium molecular dynamics
Type
thesis
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