Phonon transport in confined structures and at interfaces

Lin Sun, Purdue University

Abstract

The objective of this thesis is to develop a fundamental understanding of the role of the interface in thermal transport in nanostructures. This task is accomplished using molecular dynamics (MD) simulations and the atomistic Green’s function (AGF) method as the primary tools. A molecular dynamics simulation tool for large-scale MD simulation is developed and validated through a variety of tests. First, the thermal conductivity of bulk silicon is computed using the Green-Kubo method and its dependence on temperature and system size explored. The dispersion relation along the [100] direction of silicon is predicted and compared to experimental data. Finally, non-equilibrium MD is applied to predict the out-of-plane thermal conductivity of silicon thin films as a function of film thickness. A wave-packet MD technique is developed to evaluate interface transmissivities. It is firstly applied to study thermal transport across interfaces with controlled roughness between silicon and an artificially heavy silicon system. The transmissivity is found to be strongly dependent on phonon frequency, polarization and surface asperity. In the low-frequency limit, the roughness structure at interfaces is transparent to acoustic phonons and the transmission coefficients nearly equal the ideal-interface results. In the mid-frequency range, phonon transmission is significantly decreased when the roughness characteristic length is comparable to the phonon wave length. Complex phonon mode conversions are observed and wave interference at this range is conjectured to be the reason for decreased transmission. At frequencies close to the cutoff frequency, the transmissivity drops rapidly to zero and the roughness influence is not evident. The method is also applied to the study of an Si/Ge rough interface. Strong phonon wave interference effects are found to restore the transmissivity as the number of roughness layers goes larger. The AGF method is employed and verified by application to a 1-D atomic chain. The AGF is then implemented to simulate phonon transport between two semi-infinite semiconductors to obtain the phonon transmission function with respect to interface atomic configuration, roughness layer thickness and phonon frequency. It is found that the in-plane structure of the roughness strongly modify the transmission while the roughness thickness has only a small effect to the extent that it modifies the device DOS. Finally, the development of a massively-parallel molecular dynamics program that has been used through the thesis work is presented. The parallelization scheme and message passing treatment are described in detail. Code performance is tested on a wide range of computer platforms and its efficiency and scalability on different architectures are discussed.

Degree

Ph.D.

Advisors

Murthy, Purdue University.

Subject Area

Mechanical engineering

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