Features of phonon transport in stressed Si and Ge-based nanowires

The dissertation presents a study of thermal transport in silicon- and germanium-based nanowires of various morphologies and the peculiarities of the strain impact on the thermal conductivity of SiGe nanowires.
The dissertation consists of an introduction, four original chapters, general conclusions, a list of references, and an appendix.
The introduction substantiates the relevance of the chosen research topic and formulates the aim of the dissertation, which is to establish the mechanisms of thermal transport in strained semiconductor nanowires based on silicon and germanium of various morphologies. The object of the study is the processes of phonon transport in one-dimensional nanostructures of silicon and germanium with structural inhomogeneities and/or fields of mechanical stresses. The subject of the study is silicon and germanium nanowires of different component compositions and morphologies.
The first chapter presents an overview of the current state of research on thermal transport processes in solid-state structures of various morphologies and dimensions. The practical significance of studying the thermal properties of nanostructures is considered, particularly for enhancing energy conversion efficiency in thermoelectric modules or thermal management in modern electronics and optoelectronics. The mechanisms of thermal transport in dielectrics and conductors at different temperatures are analyzed. The main results of studies on the thermal properties of semiconductor nanowires are presented, based on their geometric parameters (length, diameter, crystallographic orientation), chemical composition, and morphology (solid nanowires, hollow nanowires, core-shell structures, etc.). A review of the current state of research related to the strain effects on the properties of nanostructures of various dimensions, particularly their thermophysical characteristics, is provided.
The second chapter describes the research methodology. In particular, it presents the types of nanowires studied and the features of creating the modeled structures. It outlines the basics of non-equilibrium and equilibrium molecular dynamics methods for calculating the thermal conductivity coefficient of nanowires, as well as methods for calculating other thermophysical parameters, including the density of vibrational states in the studied nanowires, phonon lifetime, and participation ratio. The chapter also describes the empirical interatomic interaction potentials used in the studies, along with the numerical values of the potential coefficients for silicon and germanium.
The third chapter presents the results of studies on the thermal properties of silicon and germanium nanowires containing structural inhomogeneities. In particular, the impact of an amorphous SiO2 shell on the thermal conductivity coefficient of silicon nanowires is investigated. It is shown that for nanowires with a fixed radius of the crystalline silicon core, the thermal conductivity coefficient monotonically decreases with an increase in the thickness of the SiO2 oxide layer. For nanowires with a constant thickness of the amorphous SiO2 layer, the thermal conductivity coefficient nonlinearly increases with an increase in the radius of the crystalline core. These results are explained based on the two-channel heat transfer model.
For the first time in this work, the influence of germanium clustering processes on the thermal properties of silicon-germanium solid solution nanowires has been modeled. It has been demonstrated that the formation of nanoclusters affects both the magnitude of the thermal conductivity coefficient of the nanowires and the overall shape of its temperature dependence. It has been established that with the increasing size of the germanium nanoclusters, the thermal conductivity coefficient of SiGe nanowires increases across the entire investigated temperature range. It has been proven that in nanowires with germanium clusters, the temperature dependence of the thermal conductivity coefficient is determined by two competing mechanisms of phonon scattering: scattering due to the difference in atomic masses of silicon and germanium in the solid solution and scattering on the surface of the formed nanoclusters.
Based on the analysis of the density of vibrational states and the phonon participation ratio, it has been shown that the formation of germanium clusters in SiGe nanowires leads to phonon mode delocalization. As a result, there is an increase in the thermal conductivity coefficient.
For the first time, molecular dynamics simulation has been conducted to study thermal transport processes in hollow silicon nanowires. It has been demonstrated that increasing the size of the cylindrical cavity results in a decrease in the thermal conductivity coefficient of the nanowire, accompanied by a transformation of its temperature dependence. In the study, it has been demonstrated that the appearance of cavities in silicon nanowires is accompanied by their structural transformation, leading to the formation of amorphized surface regions. The volume fraction of these regions increases with the radius of the cavity. This phenomenon results in the emergence of localized vibrational modes in the hollow nanowires, which corresponds to a change in their thermal conductivity coefficient.
In the fourth chapter, the results of research on the strain effects on the mechanical response and thermal properties of SiGe nanowires are described. Specifically, the process of uniaxial strain of SiGe nanowires with varying cavity size, component content, and temperature has been modeled. It has been shown that increasing the size of the cavities in nanowires leads to a transition from brittle to ductile fracture, accompanied by a decrease in the Young's modulus of the material. It has been established that the main factor driving this evolution of mechanical properties in nanowires is structural defects in the surface regions caused by the presence of cavities.
It has been found that changing the ratio of components in SiGe nanowires does not affect the mechanism of their brittle fracture but only alters the Young's modulus of the structure and the fracture stress. Meanwhile, it has been demonstrated that increasing the temperature has almost no effect on the Young's modulus but leads to a decrease in the fracture stress of the nanowires and the appearance of plastic strain areas at high temperatures.
It has been shown that uniaxial tensile strain causes a decrease in the thermal conductivity coefficient of silicon and silicon-germanium solid solution nanowires over a wide range of temperatures, whereas compression strain exhibits a reverse effect. It has been found that the relative change in the thermal conductivity coefficient induced by uniaxial strain is higher for silicon nanowires compared to silicon-germanium nanowires. It has been proven that the main reasons for the strain-induced change in the thermal conductivity coefficient of silicon-germanium nanowires are the alteration of the longitudinal acoustic phonon velocity, phonon lifetime, and stiffness of the interatomic bonds in the material.
The impact of internal strain fields on the thermal conductivity of «core-shell» nanowires has been analyzed. It has been demonstrated that accounting for internal stresses caused by lattice mismatch between the core and shell results in an increase in the thermal conductivity coefficient of the structure, regardless of its type: whether it is «Si-core - Ge-shell» or «Ge-core - Si-shell».

Семчук С. С. Особливості фононного транспорту в напружених нанонитках на основі Si та Ge : дис. д-ра філософії : 104 Фізика та астрономія / наук. кер. В. В. Курилюк. Київ, 2024. 136 с.