Monte Carlo based statistical approach to solve Boltzmann Transport Equation (BTE) has become a norm to investigate heat transport in semiconductors at sub-micron regime, owing mainly to its ability to characterize realistically sized device geometries qualitatively. One of the primary issues with this technique is that the approach predominantly uses empirically fitted phonon dispersion relations as input to determine the properties of phonons so as to predict the thermal conductivity of specified material geometry. The empirically fitted dispersion relations assume harmonic approximation thereby failing to account for thermal expansion, interaction of lattice waves, effect of strain on spring stiffness, and accurate phonon-phonon interaction. To circumvent this problem, in this work, a coupled molecular mechanics-Monte Carlo (MM-MC) platform has been developed and used to solve the phonon Boltzmann Transport Equation (BTE) for the calculation of thermal conductivity of several novel and emerging nanostructures. The use of the quasi-anharmonic MM approach (as implemented in the open source NEMO 3-D software toolkit) not only allows one to capture the true atomicity of the underlying lattice but also enables the simulation of realistically-sized structures containing millions of atoms. As compared to the approach using an empirically fitted phonon dispersion relation, here, a 17% increase in the thermal conductivity for a silicon nanowire due to the incorporation of atomistic corrections in the LA (longitudinal acoustic) branch alone has been reported. The atomistically derived thermal conductivity as calculated from the MM-MC framework is then used in the modular design and analysis of (i) a silicon nanowire based thermoelectric cooler (TEC) unit, and (ii) a GaN/InN based nanostructured light emitting device (LED). It is demonstrated that the use of empirically fitted phonon bandstructure parameters overestimates the temperature difference between the hot and the cold sides and the overall cooling efficiency of the system, thereby, demanding the use of the BTE derived thermal conductivity in the calculation of thermal conductivity. In case of the light-emitting device, the microscopically derived material parameters, as compared to their bulk and fitted counterparts, yielded [approximately] 3% correction (increase) in optical efficiency. A non-deterministic approach adopted in this work, therefore, provides satisfactory results in what concerns phonons transport in both ballistic and diffusive regimes to understand and/predict the heat transport phenomena in nanostructures.

The main objective of this thesis is to study phonon thermal transport in two-dimensional (2D) materials using first-principles density functional theory (DFT) calculations and the full solution of the Boltzmann transport equation (BTE). A wide range of 2D materials including graphene, 2D structures of group-VA, and recently emerged NX (X=P, As, Sb) compound monolayers are considered. Special attention is given to a mode-by-mode study of the thermal tunability via strain and functionalization. First, this thesis investigated the sensitivity of the DFT-calculated intrinsic thermal conductivity and phonon properties of 2D materials to the choice of exchange-correlation (XC) and pseudopotential (PP). It was found that the choice of the XC-PP combination results in significant discrepancies among predicted thermal conductivities of graphene at room temperature, in the range of 5442-8677 Wm^(-1)K^(-1). The LDA-NC and PBE-PAW combinations predicted the thermal conductivities in best agreement with available experimental data. This sensitivity analysis was an essential first step towards using DFT to engineer the phonon thermal transport in 2D systems. Next, DFT was used to systematically investigate the strain-dependent lattice thermal conductivity of -arsenene and -phosphorene, 2D monolayers of group-VA. The results showed that the thermal conductivity in both monolayers exhibits an up-and-down behavior when biaxial tensile strain is applied in the range from 0% to 9%. An interplay between phonon group velocities, heat capacities, and relaxation times, is found to be responsible for this behaviour. Finally, this project investigated the thermal conductivity of nitrogen functionalized - NX (X=P, As, Sb) monolayers. The results showed that the room-temperature thermal conductivities of -NP, -NAs, and -NSb are about 1.1, 5.5, and 34.0 times higher than those of their single-element -P, -As, and -Sb monolayers, respectively. The phonon transport analysis revealed that higher phonon group velocities, as well as higher phonon lifetimes were responsible for such an enhancement in the thermal conductivities of - NX compounds compared to single-element group-VA monolayers. Also, it was found that -NP has the minimum thermal conductivity among -NX monolayers, while it has the minimum average atomic mass. This thesis provides valuable insight into phonon physics and thermal transport in novel 2D materials using advanced DFT calculations.

The book covers all aspects from the expansion of the Boltzmann transport equation with harmonic functions to application to devices, where transport in the bulk and in inversion layers is considered. The important aspects of stabilization and band structure mapping are discussed in detail. This is done not only for the full band structure of the 3D k-space, but also for the warped band structure of the quasi 2D hole gas. Efficient methods for building the Schrödinger equation for arbitrary surface or strain directions, gridding of the 2D k-space and solving it together with the other two equations are presented.

Over the last decade, substantial attention has been paid to novel nanostructures based on two-dimensional (2D) materials. Among the hundreds of 2D materials that have been successfully synthesized in recent years, graphene, boron nitride, and molybdenum disulfide are the ones that have been intensively studied. It has been demonstrated that these materials exhibit thermal conductivities significantly higher than those of bulk samples of the same material. However, little is known about the physics of phonons in these materials, especially when tensile strain is applied. Properties of these materials are still not well understood, and modelling approaches are still needed to support engineering design of these novel nanostructures. In this thesis, I use state-of-the-art atomistic simulation techniques in combination with statistical thermodynamics formulations to obtain the phonon properties (lifetime, group velocity, and heat capacity) and thermal conductivities of unstrained and strained samples of graphene, boron nitride, molybdenum disulfide, and also superlattices of graphene and boron nitride. Special emphasis is given to the role of the acoustic phonon modes and the thermal response of these materials to the application of tensile strain. I apply spectral analysis to a set of molecular dynamics trajectories to estimate phonon lifetimes, harmonic lattice dynamics to estimate phonon group velocities, and Bose-Einstein statistics to estimate phonon heat capacities. These phonon properties are used to predict the thermal conductivity by means of a mode-dependent equation from kinetic theory. In the superlattices, I study the variation of the frequency dependence of the phonon properties with the periodicity and interface configuration (zigzag and armchair) for superlattices with period lengths within the coherent regime. The results showed that the thermal conductivity decreases significantly from the shortest period length to the second period length, 13% across the interfaces and 16% along the interfaces. For greater periods, the conductivity across the interfaces continues decreasing at a smaller rate of 11 W/mK per period length increase, driven by changes in the phonon group velocities (coherent effects). In contrast, the conductivity along the interfaces slightly recovers at a rate of 2 W/mK per period, driven by changes in the phonon relaxation times (diffusive effects).

Deterministic simulation of the particle transport in semiconductor devices is an interesting alternative to the common Monte Carlo approach. In this book, a state-of-the-art technique called the multigroup approach is presented and applied to a variety of transport problems in bulk semiconductors and semiconductor devices. High-field effects as well as hot-phonon phenomena in polar semiconductors are studied in detail. The mathematical properties of the presented numerical method are studied, and the method is applied to simulating the transport of a two-dimensional electron gas formed at a semiconductor heterostructure. Concerning semiconductor device simulation, several diodes and transistors fabricated of silicon and gallium arsenide are investigated. For all of these simulations, the numerical techniques employed are discussed in detail. This unique study of the application of direct methods for semiconductor device simulation provides the interested reader with an indispensable reference on this growing research area.

This is a graduate level textbook in nanoscale heat transfer and energy conversion that can also be used as a reference for researchers in the developing field of nanoengineering. It provides a comprehensive overview of microscale heat transfer, focusing on thermal energy storage and transport. Chen broadens the readership by incorporating results from related disciplines, from the point of view of thermal energy storage and transport, and presents related topics on the transport of electrons, phonons, photons, and molecules. This book is part of the MIT-Pappalardo Series in Mechanical Engineering.

The book details sources of thermal energy, methods of capture, and applications. It describes the basics of thermal energy, including measuring thermal energy, laws of thermodynamics that govern its use and transformation, modes of thermal energy, conventional processes, devices and materials, and the methods by which it is transferred. It covers 8 sources of thermal energy: combustion, fusion (solar) fission (nuclear), geothermal, microwave, plasma, waste heat, and thermal energy storage. In each case, the methods of production and capture and its uses are described in detail. It also discusses novel processes and devices used to improve transfer and transformation processes.

Nonequilibrium hot charge carriers play a crucial role in the physics and technology of semiconductor nanostructure devices. This book, one of the first on the topic, discusses fundamental aspects of hot carriers in quasi-two-dimensional systems and the impact of these carriers on semiconductor devices. The work will provide scientists and device engineers with an authoritative review of the most exciting recent developments in this rapidly moving field. It should be read by all those who wish to learn the fundamentals of contemporary ultra-small, ultra-fast semiconductor devices. - Topics covered include - Reduced dimensionality and quantum wells - Carrier-phonon interactions and hot phonons - Femtosecond optical studies of hot carrier - Ballistic transport - Submicron and resonant tunneling devices

The monograph is devoted to the investigation of physical processes that govern the phonon transport in bulk and nanoscale single-crystal samples of cubic symmetry. Special emphasis is given to the study of phonon focusing in cubic crystals and its influence on the boundary scattering and lattice thermal conductivity of bulk materials and nanostructures.