Many portions of energy generated in the U.S. are not used and take the form of wasted heat due to a poor heat transfer efficiency. This fact leads research communities to focus on thermoelectrics as a means for using waste heat through direct thermal to electrical energy conversion. One way to enhance thermoelectric efficiency is to reduce thermal conductivity through nanostructuring. In nanostructures, understanding energy transport across the interface of two materials is important because interfaces dominate the resistance to overall thermal transport of the system and can be described by thermal boundary conductance (TBC). Also of note, an understanding of thermal transport cannot be achieved without an understanding of transfer via atomic vibration, known as phonons. In this study, two different techniques of molecular dynamics (MD) simulation are introduced in order to improve the understanding of the phonon transport at the interface of dissimilar materials and the impact of different material properties on TBC. Non-equilibrium MD simulations are used to study relative and combined contributions of mass and bond energy difference on TBC and phonon wave-packet simulations are used to obtain a detailed description of phonon interactions at the interface. At the end of this study, a simple analytical model for the prediction of effective thermal conductivity, using knowledge of thermal boundary resistance, an inverse of TBC, and the interface geometry, is developed.

Irreversible thermodynamics is a widely-applicable toolset that extends thermodynamics to describe systems undergoing irreversible processes. It is particularly useful for describing macroscopic flow of system components, whether conserved (e.g., particle number) or non-conserved (e.g., spin). We give a general introduction to this toolset and calculate the entropy production due to bulk and interfacial flow. We compare the entropy production and heating rate of bulk and interfacial transport, as well as interfacial charge and spin transport. We then demonstrate the power and applicability of this toolset by applying it to three systems. We first consider metal oxide growth, and discuss inconsistency in previous theory by Mott. We show, however, that Mott's solution is the lowest order of a consistent asymptotic solution, with the ion and electron concentrations and fluxes going as power series in t^-k/2, where k = 1, 2, .... We find that this gives corrections to the "parabolic growth law" that has oxide thickness going as t^1/2; the lowest order correction is logarithmic in t. We then consider the effect on spin of electric currents crossing an interface between a ferromagnet (FM) and non-magnetic material (NM). Previous theories for electrical potential and spin accumulation neglect chemical or magnetic contributions to the energy. We apply irreversible thermodynamics to show that both contributions are pivotal in predicting the spin accumulation, particularly in the NM. We also show that charge screening, not considered in previous theories, causes spin accumulation in the FM, which may be important in ferromagnetic semiconductors. Finally, we apply irreversible thermodynamics to thermal equilibration in a thin-film FM on a substrate. Recent experiments suggest that applying a thermal gradient across the length of the system causes a spin current along the thickness; this spin current is present much farther from the heat sources than expected. We find that, although the interaction between the separate thermal equilibration processes increases the largest equilibration length, thermal equilibration does not predict a length as large as the experimentally measured length; it does predict, however, a thermal gradient along the thickness that has the shape of the measured spin current.

To keep downsizing transistors, new materials must be explored since traditional 3D materials begin to experience tunneling and other problematic physical phenomena at small sizes. 2D materials are appealing due to their thinness and bandgap. The relatively weak van der Waals forces between layers in 2D materials allow easy exfoliation and device fabrication but they also result in poor heat transfer to the substrate, which is the main path for heat removal. The impaired thermal coupling is exacerbated in few-layer devices where heat dissipated in the layers further from the substrate encounters additional interlayer thermal resistance before reaching the substrate, which results in self-heating and degradation of mobility. This study explores the electro-thermal properties of five materials (MoS2, MoSe2, WS2, WSe2, and 2D black phosphorous) which have been identified as possible replacements for Si in future sub-5-nm channel-length devices. We have developed a coupled electro-thermal model to calculate device mobility. The carrier wavefunctions and distribution are obtained from solving the coupled Schrodinger and Poisson equations in the cross-plane direction. The screening length is then calculated from the screening wavenumber. We calculate TBC for each layer in the stack into the substrate from a model based on first-principles phonon dispersion. We determine the local temperature in each layer from a ratio of its dissipated energy and its TBC. We simulate various devices with self-heating (Delta T does not equal 0, where Delta T is the temperature rise of the few-layer device) under several parameters and examined the effects on mobility and change in device temperature. The effects are compared to the isothermal case (Delta T = 0). We observe that self-heating has a significant effect on temperature rise, layer-wise drain current, and effective mobility. Black phosphorous performs the best electrothermally and WS2 performs the worst overall. This thesis will inform future thermally aware designs of nanoelectronic devices based on 2D materials.

Cette thèse s'inscrit dans le domaine de la nanothermique, c'est à dire, l'étude des transferts de chaleur à l'échelle du nanomètre, pour lesquels la loi de Fourier n'est plus valide. A cette échelle, l'interface domine le transport thermique par sa résistance au flux d'énergie thermique. Cetterésistance se manifeste par une discontinuité de la température aux interfaces. Il est important de pouvoir la prédire afin de contrôler et aménager les flux thermiques dans les application en microélectroniques où la taille des transistors modernes devient nanomètrique. Le but de cette thèse est d'une part, d'étudier la transmission angulaire des phonons et prédire la résistance de Kapitza aux interfaces entre solides, de quantifier le transfert d'énergie assuré par les phonons au travers d'un gap de vide nanométrique, et d'autre part d'étudier le transfert thermique dans un système nanoparticule cœur-coquille immergé dans l'eau. Nous avons développé une nouvelle méthode de dynamique de réseau couplée à l'utilisation de constantes de force issus des calculs ab-initio. Cette méthode permet d'obtenir la transmission des phonons entre deux solides, en fonction de leur fréquence et vecteur d'onde, en tenant comptede la dispersion des phonons en volume dans les deux milieux. Nous avons également appliqué la méthode pour décrire le transfert thermique à travers un gap de vide entre deux solides. Enfin, nous nous sommes intéressés au transfert thermique autour d'une nanoparticule (NP) immergée dans l'eau et chauffée par un faisceau laser. Nous avons comparé l'efficacité de chauffage des NPs homogènes d'or à celles de type cœur-coquille or-silice / This thesis is devoted to the study of interfacial thermal transport at the nanoscale where Fourier’s law is not valid. This is because, at this scale, phonon mean free path becomes smaller to the characteristic length of the system, thus the heat transfer is no longer diffusive but rather ballistic. As a consequence, the thermal boundary resistance (TBR) becomes a determinant factor in heat transfer. The goal of this thesis is, firstly, to study phonon transmission and predict the thermal boundary conductance at interface between two solids. To this end, we have developed a new approach, which combines lattice dynamics calculations and inputs from ab initio, and we have applied our LD model to two types of solid structures: the face-centered cubic (FCC) crystal solid and the diamond-like crystal solid.Secondly, we aim to quantify the phononic contribution in heat transfer across a nanometric vacuum gap that separates two solids. We have used this ab initio LD model to predict the contribution of phonons in the heat transfer across a vacuum gap in two systems: the Au/vacuum-gap/Au and the Si/vacuum-gap/Si. Our results indicate that phonons do contribute significantly to heat transfer across a nanometric/subnanometric vacuum gap. Finally, we have investigated heat transfer in a system made of a core-shell nanoparticle (NP) immersed in water and heated by a laser pulse. We have used the four temperatures model, we have solved numerically the heat transfer equations in the system, taking into account the thermal boundary resistance (TBR) and the interfacial electron-phonon coupling

<p>Beta-gallium oxide (<i>β</i>-Ga₂O₃) is an emerging wide bandgap semiconductor for next generation power devices which offers the potential to replace GaN and SiC. It has an ultra-wide bandgap (UWBG) of 4.8 eV and a corresponding <i>E</i>_brof 8 MV/cm. <i>β</i>-Ga₂O₃also possesses a decent intrinsic electron mobility limit of 250 cm²/V<i>·</i>s, yielding high Baliga’s figure of merit of 3444. In addition, the large bandgap of <i>β</i>-Ga₂O₃gives stability in harsh environment operation at high temperatures. </p> <p>Although low-cost large-size <i>β</i>-Ga₂O₃native bulk substrates can be realized by melt growth methods, the unique property that (100) surface of <i>β</i>-Ga₂O₃has a large lattice constant of 12.23 Å allows it to be cleaved easily into thin and long nano-membranes. Therefore, <i>β</i>-Ga₂O₃FETs on foreign substrates by transferring can be fabricated and investigated before <i>β</i>-Ga₂O₃epitaxy technology becomes mature and economical viable. Moreover, integrating <i>β</i>-Ga₂O₃on high thermal conductivity materials has an advantage in terms of suppressing self-heating effects. </p><p>In this dissertation, structural and material innovations to overcome and improve critical challenges are summarized as follows: 1) Top-gate nano-membrane <i>β</i>-Ga₂O₃FETs on a high thermal conductivity diamond substrate with record high maximum drain current densities are demonstrated. The reduced self-heating effect due to high thermal conductivity of the substrate was verified by thermoreflectance measurement. 2) Local electro-thermal effect by electrical bias was applied to enhance the electrical performance of devices and improvements of electrical properties were shown after the annealing. 3) Thin thermal bridge materials such as HfO₂and ZrO₂were inserted between <i>β</i>-Ga₂O₃and a sapphire substrate to reduce self heating effects without using a diamond substrate. The improved thermal performance of the device was analyzed by phonon density of states plots of <i>β</i>-Ga₂O₃and the thin film materials. 4) Nano-membrane tri-gate <i>β</i>-Ga₂O₃FETs on SiO₂/Si substrate fabricated via exfoliation have been demonstrated for the first time. 5) Using the robustness of <i>β</i>-Ga₂O₃in harsh environments, <i>β</i>-Ga₂O₃ferroelectric FETs operating as synaptic devices up to 400 °C were demonstrated. The result offers the potential to use the novel device for ultra-wide bandgap logic applications, specifically neuromorphic computing exposed to harsh environments.<br></p>