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GaN-based HFETs demonstrate ubiquitous high power and high frequency performance and attract tremendous research efforts. Even though significant advances have been achieved, there still exist some critical issues needed to be investigated and solved. In particular, high defect densities due to inhomogeneous growth and operation under high power conditions bring many unique problems which are not so critical in the conventional Si and GaAs materials systems. In order to reduce the defect density and heat dissipation of GaN-based HFETs, research work on the realization of GaN-based HFETs on bulk GaN substrate has been carried out and the key problems have been identified and solved. Hot phonon scattering is the bottleneck which limits the enhancement of electron velocity in the GaN 2DEG channel. It is found that the plasmon-phonon coupling is the mechanism for converting of hot phonons into high group velocity acoustic phonons. In order to push more electrons into the GaN 2DEG channel in the plasmon-phonon coupling regime and to further reduce the hot phonon lifetime, a novel AlGaN/GaN dual channel HFET structure has been proposed. The growth, fabrication and characterization of such a AlGaN/GaN dual channel HFET structure has been carried out. Conventionally GaN-based light emitting diodes and laser diodes are grown and fabricated using the c-plane III-nitride expitaxy layers. In c-plane III-nitride epi-layers, the polarization-induced electric field introduces spatial separation of electron and hole wave functions in quantum wells (QW)s used LEDs and laser diodes LDs and degrades quantum efficiency. As well, blueshift in the emission wavelength becomes inevitable with increasing injection current unless very thin QWs are employed. The use of nonpolar orientations, namely, m-plane or a-plane GaN, would solve this problem. So far, m-plane GaN has been obtained on LiAlO2 (100), m-plane SiC substrates, and m-plane bulk GaN, which all have limited availability and/or high cost. Silicon substrates are very attractive for the growth of GaN due to their high quality, good thermal conductivity, low cost, availability in large size, and ease with which they can be selectively removed before packaging for better light extraction and heat transfer when needed To realize the low cost and improve the internal quantum efficiency of GaN based light emitting diodes, the process for m-plane GaN growth on Si (112) substrates has been studied and optimized. The continuous m-plane GaN film is successfully grown on Si (112) substrates.

Sustainable and renewable energy is an incredibly important area in today’s society and investigation into avenues to improve this wide ranging technology are underway in many different fields. Thermoelectric devices possess the ability for the direct solid-state interconversion of heat and electrical power, which not only allows for sustainable refrigeration but also waste heat recovery. One current restriction on the efficiency of thermoelectric devices is the disparity in thermoelectric performance of p-type and n-type materials. Furthermore, a key physical attribute shared by the majority of high performing thermoelectric materials is low thermal conductivity. Thus in this thesis three separate p-type material systems exhibiting low thermal conductivity will be discussed. The Cu-excessed quaternary chalcogenides, CuM2InTe4 (M = Zn, Cd), and ternary chalcogenide, CuSbS2, were investigated due to their intrinsically low thermal conductivity. Whereas, skutterudites typically have good electrical properties but do not exhibit an intrinsically low thermal conductivity. Nevertheless low thermal conductivity can be achieved by taking advantage of their unique crystal structure by filling large voids with loosely bound atoms that act as phonon scattering centers. Therefore double-filled Fe substituted skutterudites with nominal compositions Yb0.4In0.02Co3FeSb12 and Yb0.8In0.02Co2.5Fe1.5Sb12 were also investigated. The CuM2InTe4 (M = Zn, Cd) and skutterudite specimens were synthesized by direct reactions, whereas the CuSbS2 specimens were synthesized by mechanical alloying. Structural and stoichiometric compositions were analyzed by a combination of X-ray diffraction, Rietveld refinement and energy dispersive spectroscopy. High-temperature transport properties were measured for all specimens and will be discussed in detail. The Cu-excessed quaternary chalcogenides display intrinsically low thermal conductivity that appears to be unaffected by the change in electrical properties that is a result of differing stoichiometries. This may provide a possible route to furthering the enhancement of the thermoelectric properties of these materials. Similarly the CuSbS2 ternary chalcogenides display a very low thermal conductivity due to stereochemically active lone-pair electrons and would potentially allow an optimization of the power factor without a significant increase of the very low thermal conductivity, thus improving the figure of merit. For the case of p-type skutterudites, (Yb, In) double-filled skutterudites have a maximum ZT of 0.6, which is promising in the hunt for improved p-type materials. This fundamental investigation provides insight that can lead to a deeper understanding of all three material systems outlined in this thesis and provides a platform for new research in the quest for materials suitable for thermoelectric applications.

The ability of uniquely functional thermoelectric materials to convert waste heat directly into electricity is critical considering the global energy economy. Profitable, energy-efficient thermoelectrics possess thermoelectric figures of merit ZT ≥ 1. We examined the effect of metal nanoparticle – oxide film interfaces on the thermal conductivity κ and Seebeck coefficient α in bilayer and multilayer thin film oxide thermoelectrics in an effort to improve the dimensionless figure of merit ZT. Since a thermoelectric's figure of merit ZT is inversely proportional to κ and directly proportional to α, reducing κ and increasing α are key strategies to optimize ZT. We aim to reduce κ by phonon scattering due to the inclusion of metal nanoparticles in the bulk of thermoelectric thin films deposited by Pulsed Laser Deposition. XRD, AFM, XPS, and TEM analyses were carried out for structural and compositional characterization. The electrical conductivities of the samples were measured by a four-point probe apparatus. The Seebeck coefficients were measured in-plane, varying the temperature from 100K to 310K. The thermal conductivities were measured at room temperature using Time Domain Thermoreflectance. / Master of Science

The technological interest in a material depends very much on its electrical, magnetic, optical and/or mechanical properties. In carbon nanotubes the atoms form a cylindrical structure with a diameter of the order 1 nm, but the nanotubes can be up to several hundred micrometers in length. This makes carbon nanotubes a remarkable model for one-dimensional systems. A lot of efforts have been dedicated to manufacturing carbon nanotubes, which is expected to be the material for the next generation of devices. Despite all the attention that carbon nanotubes have received from the scientific community, only rather limited progress has been made in the theoretical understanding of their physical properties. In this work, we attempt to provide an understanding of the electron-phonon and electron-defect interactions in semiconducting zigzag carbon nanotubes using a tight-binding approach. The electronic energy dispersion relations are calculated by applying the zone-folding technique to the dispersion relations of graphene. A fourth-nearest-neighbour force constant model has been applied to study the vibrational modes in the carbon nanotubes. Both the electron-phonon interaction and the electron-defect interaction are formulated within the tight-binding approximation, and analyzed in terms of their quantum mechanical scattering rates. Apart from the scattering rates, their components in terms of phonon absorption, phonon emission, backscattering and forward scattering have been determined and analyzed. The scattering rates for (5,0), (7,0), (10,0), (13,0) and (25,0) carbon nanotubes at room temperature and at 10K are presented and discussed. The phonon scattering rate is dependent on the lattice temperature in the interval 0-0.17 eV. We find that backscattering and phonon emission are dominant over forward scattering and phonon absorption in most of the energy interval. However, forward scattering and phonon absorption can be comparable to backscattering and phonon emission in limited energy intervals. The phonon modes associated with each peak in the electron-phonon scattering rates have been identified, and the similarities in the phonon scattering rates between different nanotubes are discussed. The dependence of the defect scattering rate on the tube diameter is similar to that of the phonon scattering rate. Both the phonon and the defect scattering rates show strong dependence on the tube diameter (i.e., the scattering rate decreases as a function of the index of the nanotube). It is observed that the backscattering and forward scattering for electrons interacting with defects occur with same frequency at all energies, in sharp contrast to the situation for phonon scattering. It is demonstrated that the differences in the scattering rate between different tubes are mainly due to the differences in their band structures.

Thesis advisor: Zhifeng Ren / Solid-state cooling and power generation based on thermoelectric effects are attractive for a wide range of applications in power generation, waste heat recovery, air-conditioning, and refrigeration. There have been persistent efforts on improving the figure of merit (ZT) since the 1950's; only incremental gains were achieved in increasing ZT, with the (Bi1-xSbx)2(Se1-yTey)3 alloy family remaining the best commercial material with ZT ~ 1. To improve ZT to a higher value, we have been pursuing an approach based on random nanostructures and the idea that the thermal conductivity reduction that is responsible for ZT enhancement in superlattices structures can be realized in such nanostructures. The synthesis and characterization of various nanopowders prepared by wet chemical as well as high energy ball milling methods will be discussed in this dissertation. The solid dense samples from nanopowders were prepared by direct current induced hot press (DC hot press) technique. The thermoelectric properties of the hot pressed samples have been studied in detail. By ball milling ingots of bulk alloy crystals and hot pressing the nanopowders, we had demonstrated a high figure-of-merit in nanostructured bulk bismuth antimony telluride. In this dissertation, we use the same ball milling and hot press technique, but start with elemental chunks of bismuth, antimony, and tellurium to avoid the ingot formation step. We show that a peak ZT of about 1.3 can be achieved. Our material also exhibits a ZT of 0.7 at 250 °C, close to the value reached when ingot was used. This process is more economical and environmentally friendly than starting from bulk alloy crystals. The ZT improvement is caused mostly by the low thermal conductivity, similar to the case using ingot. Transmission electron microscopy observations of the microstructures suggest that the lower thermal conductivity is mainly due to the increased phonon scattering from the high density grain boundaries and defects. The performance of thermoelectric materials is determined by its dimensionless figure-of-merit (ZT) which needs to be optimized within a specific temperature range for a desired device performance. Hence, we show that by varying the Bi/Sb ratio, the peak ZT can be shifted to a higher or lower temperature for power generation applications or a cooling mode operation. A peak ZT of about 1.3 is achieved from a Bi0.4Sb1.6Te3 composition which is highest among the different compositions. These nanostructured bulk samples have a significantly low lattice thermal conductivity compared to the bulk samples due to the increased phonon scattering in the grain boundaries and defects. This study shows that Bi0.5Sb1.5Te3 may potentially perform better for cooling devices, while Bi0.3Sb1.7Te3 should be able to show better power generation efficiency. Several issues related to accurate measurement of thermoelectric properties were identified and many of them were solved during my studies and these are discussed in this thesis. With the data we obtained, it is clear that nanopowder-based thermoelectric materials hold significant promise. Therefore, a review of synthesis of nanostructured materials by solution-based methods, including a hydrothermal process for the Bi2Te3, Bi2Se3, and Bi2Te2.25Se0.75 nanoparticles, a solvothermal route for Sb2Te3 nanostructures, and a polyol process for the preparation of Bi nanostructures is presented in this dissertation. These new nanostructures may find applications in enhancing the thermoelectric performance. Although small sized and well dispersed nanopowders of various thermoelectric materials could be prepared by a solution method in large scale, contamination and partial oxidation are always big challenges in a chemical approach. Hence, a high energy ball milling technique to prepare thermoelectric nanopowders in large scale and without major contamination is still found to be more efficient and preferred. / Thesis (PhD) — Boston College, 2009. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Physics.

Since the advent of nanoscale material based electronic devices, there has been a considerable interest in exploring carbon nanotubes from fundamental science and technological perspectives. In carbon nanotubes, the atoms form a cylindrical structure with a diameter of the order 1nm. The length of the nanotubes can extend up to several hundred micrometers. Carbon nanotubes exhibit a variety of intriguing electronic properties such as semiconducting and metallic behaviour, due to the quantum confinement of the electrons in the circumferential direction. Much of the study dedicated to describe the behaviour of carbon nanotube-based devices assumes for simplicity the nanotube to be a ballistic material. However, in reality the phonon scattering mechanism exists also in nanotubes, of course, and can generally not be neglected, except in very short nanotubes. In this work, we focus attention on exploring the steady-state electron transport properties of semiconducting single-walled carbon nanotubes, including both phonon scattering and defect (vacancy) scattering, using the semi-classical bulk single electron Monte Carlo method.   The electron energy dispersion relations are obtained by applying the zone folding technique to the dispersion relations of graphene, which are calculated using the tight-binding description. The vibrational modes in the carbon nanotubes are studied using a fourth nearest-neighbour force constant model. Both the electron-phonon and the electron-defect interactions are formulated within the tight-binding framework, and their corresponding scattering rates are computed and analyzed. In particular, the dependence of the phonon scattering rate and the defect scattering rate on the diameter of the nanotube, on temperature and on electron energy is studied. It is shown that the differences observed in the scattering rate between different nanotubes mainly stem from the differences in their band structure.   A bulk single electron Monte Carlo simulator was developed to study the electron transport in semiconducting zigzag carbon nanotubes. As a first step, we included only electron-phonon scattering, neglecting all other possible scattering mechanisms. With this scattering mechanism, the steady-state drift velocity and the mobility for the nanotubes (8,0), (10,0), (11,0), (13,0) and (25,0) were calculated as functions of the electric-field strength and lattice temperature, and the results are presented and analysed here. The dependence of the mobility on the lattice temperature can be clearly seen at low electric-field strengths. At such electric-field strengths, the scattering is almost entirely due to acoustic phonons, whereas at high electric-field strengths optical phonon emission processes dominate. It is shown that the saturation of the steady-state drift velocity at high electric-field strengths is due to the emission of high-energy optical phonons. The results indicate the presence of Negative differential resistance for some of the nanotubes considered in this work. The discrepancy found in the literature concerning the physical reason for the appearance of negative differential resistance is clarified, and a new explanation is proposed. It is also observed that the backward scattering is dominant over the forward scattering at high electric-field strengths.                                                                                   We then included also defect scattering, actually electron-vacancy scattering, for the nanotubes (10,0) and (13,0). The steady-state drift velocities for these nanotubes are calculated as functions of the density of vacancies, electric-field strength and the lattice temperature, using three different vacancy concentrations. The results indicate the presence of Negative differential resistance at very low concentration of defects, and how this feature may depend on the concentration of defects. The dependence of the steady-state drift velocity on the concentration of defect and the lattice temperature is discussed. The electron distribution functions for different temperatures and electric field strengths are also calculated and investigated for all the semiconducting nanotubes considered here. In particular, a steep barrier found in the electron distribution function is attributed to the emission of high energy optical phonons.

Le formalisme des fonctions de Green hors-équilibre (NEGF pour « Non-equilibrium Green’s function) a suscité au cours des dernières décennies un engouement fort pour étudier les propriétés du transport quantique des nanostructures et des nano-dispositifs dans lesquels les interactions inélastiques, comme la diffusion des électrons-phonons, jouent un rôle significatif. L'incorporation d'interactions inélastiques dans le cadre du NEGF s’effectue généralement dans l'approximation auto-cohérente de Born (SCBA pour « Self-consistent Born approximation) qui représente une approche itérative plus exigeante en ressources numériques. Nous proposons dans ce travail de thèse une méthode efficace alternative dite LOA pour (« Lowest Order Approximation. Son principal avantage est de réduire considérablement le temps de calcul et de décrire physiquement la diffusion électron-phonon. Cette approche devrait considérablement étendre l'accessibilité de l'utilisation de codes atomistiques de transport quantique pour étudier des systèmes 3D réalistes sans faire à des ressources numériques importantes. / Non-equilibrium Green’s function (NEGF) formalism during recent decades has attracted numerous interests for studying quantum transport properties of nanostructures and nano-devices in which inelastic interactions like electron-phonon scattering have a significant impact. Incorporation of inelastic interactions in NEGF framework is usually performed within the self-consistent Born approximation (SCBA) which induces a numerically demanding iterative scheme. As an alternative technique, we propose an efficient method, the so-called Lowest Order Approximation (LOA) coupled with the Pade approximants. Its main advantage is to significantly reduce the computational time, and to describe the electron-phonon scattering physically. This approach should then considerably extend the accessibility of using atomistic quantum transport codes to study three-dimensional (3D) realistic systems without requiring numerous numerical resources.

Narrow silicon nanowires host a rich set of physical phenomena. Understanding these phenomena will open new opportunities for applications of silicon nanowires in optoelectronic devices and adds more functionality to silicon especially in those realms that bulk silicon may not operate remarkably. Compatibility of silicon nanowires with the mainstream fabrication technology is also advantageous. The main theme of this thesis is finding the possibility of using silicon nanowires in light sources; laser and light emitting diodes. Using Tight Binding (TB) and ab-initio Density Functional Theory (DFT) methods it was shown that axial strain can induce significant changes in the effective mass, density of states and bandgap of silicon nanowires. Generality of the observed effects was proven by investigating nanowires of different crystallography, diameter and material (e.g. germanium nanowires). The observed direct to indirect bandgap conversion suggests that strain is able to modulate the light emission properties of silicon nanowires. To investigate this possibility, spontaneous emission time was formulated using perturbation theory including Longitudinal Optical (LO) and Acoustic (LA) phonons. It was observed that corresponding to bandgap conversion, the spontaneous emission time can be modulated by more than one order of magnitude. This emanates from bandgap conversion and symmetry change of wave function in response to strain. A mechanism for population inversion was proposed in the thesis which is based on the Ensemble Monte Carlo (EMC) study of carrier statistics in direct and indirect conduction sub bands. By calculating all possible electron-phonon scattering mechanisms which may deplete the already populated indirect subband, it was shown that at different temperatures and under different electric fields there is a factor of 10 difference between the population of indirect and direct sub bands. This suggests that population inversion can be achieved by biasing an already strained nanowire in its indirect bandgap state. The light emission is possible then by releasing or inverting the strain direction. A few ideas of implementing this experiment were proposed as a patent application. Furthermore the photo absorption of silicon nanowires was calculated using TB method and the role of diameter, optical anisotropy and strain were investigated on band-edge absorption.