Nanoscale Electronic And Thermal Transport Properties In Iii V Re V Nanostructures

The incorporation of rare earth-V (RE-V) semimetallic nanoparticles embedded in III-V compound semiconductors is of great interest for applications in solid-state devices including multijunction tandem solar cells, thermoelectric devices, and fast photoconductors for terahertz radiation sources and receivers. With regard to those nanoparticle roles in device applications and material itself, electrical and thermal properties of embedded RE-V nanoparticles, including nanoscale morphology, electronic structure, and electrical and thermal conductivity of such nanoparticles are essential to be understood to engineer their properties to optimize their influence on device performance. To understand embedded RE-V semimetallic nanostructures in III-V compound semiconductors, nanoscale characterization tools are essential for analysis their properties incorporated in compound semiconductors. In this dissertation, we used atomic force microscopy (AFM) with other secondary detection tools to investigate nanoscale material properties of semimetallic RE-V and GaAs heterostructures, grown by molecular beam epitaxy. We used scanning capacitance microscopy and conductive AFM techniques to understand electronic and electrical properties of ErAs/GaAs heterostructures. For the electrical properties, this thesis investigates details of statistical analysis of scanning capacitance and local conductivity images contrast to provide insights into (i) nanoparticle structure at length scales smaller than the nominal spatial resolution of the scanned probe measurement, and (ii) both lateral and vertical nanoparticle morphology at nanometer to atomic length scales, and their influence on electrical conductivity. To understand thermal properties of ErAs nanoparticles, in-plane and cross-sectional plane of ErAs/GaAs superlattice structure were investigated with a scanning probe microscopy technique implemented with 3[omega] method for thermal measurement. By performing detailed numerical modeling of thermal transport between thermal probe tip and employed samples, and estimation of additional phonon scattering induced by ErAs nanoparticles, we could understand influences of ErAs nanoparticles on the host GaAs thermal conductivity. Investigation of ErAs semimetallic nanostructure embedded in GaAs matrix with scanned probe microscopy provided detailed understanding of their electronic, electrical and thermal properties. In addition, this dissertation also demonstrates that an atomic force microscope with secondary detection techniques is promising apparatus to understand and investigate intrinsic properties of nanostructure materials, nanoscale charge transports, when the system is combined with detailed modeling and simulations.

For the efficient utilization of energy resources and the minimization of environmental damage, thermoelectric materials can play an important role by converting waste heat into electricity directly. Nanostructured thermoelectric materials have received much attention recently due to the potential for enhanced properties associated with size effects and quantum confinement. Nanoscale Thermoelectrics describes the theory underlying these phenomena, as well as various thermoelectric materials and nanostructures such as carbon nanotubes, SiGe nanowires, and graphene nanoribbons. Chapters written by leading scientists throughout the world are intended to create a fundamental bridge between thermoelectrics and nanotechnology, and to stimulate readers' interest in developing new types of thermoelectric materials and devices for power generation and other applications. Nanoscale Thermoelectrics is both a comprehensive introduction to the field and a guide to further research, and can be recommended for Physics, Electrical Engineering, and Materials Science departments.

The book introduces modern atomistic techniques for predicting heat transfer in nanostructures, and discusses the applications of these techniques on three modern topics. The study of heat transport in screw-dislocated nanowires with low thermal conductivity in their bulk form represents the knowledge base needed for engineering thermal transport in advanced thermoelectric and electronic materials, and suggests a new route to lower thermal conductivity that could promote thermoelectricity. The study of high-temperature coating composite materials facilitates the understanding of the role played by composition and structural characterization, which is difficult to approach via experiments. And the understanding of the impact of deformations, such as bending and collapsing on thermal transport along carbon nanotubes, is important as carbon nanotubes, due to their exceptional thermal and mechanical properties, are excellent material candidates in a variety of applications, including thermal interface materials, thermal switches and composite materials.

Although silicon-based nanofabrication technology has satisfied computational demands for decades, the aggressive scaling of complementary metal oxide semiconductor (CMOS) technology to sub-5 nm geometries poses challenges that must be addressed at the materials level. One example is tuning electro-thermal transport in metal nanostructures to enhance the transfer of information and the dissipation of heat in integrated circuits. The manipulation of these pathways can be further optimized by integrating low-temperature passivation materials with varying thermal conductivities. Furthermore, the emergence of photonic interconnects presents an opportunity for the integration of electro-optic components that rely heavily on the movement, transfer, and recombination of charge carriers within photosensitive materials. All the above are governed by the fundamental limits of physical transfer mechanisms in semiconductors, bringing electron and phonon engineering --the control of heat and charge carriers in materials-- to the forefront of CMOS hardware design. This work explores the fundamental mechanisms and limits of electron-phonon transport in four individual material systems which can comprise different parts of a broader, electro-thermally optimized electronic system using primarily time-domain thermoreflectance (TDTR) and scanning ultrafast electron microscopy (SUEM) as probes. First, we discuss the electro-thermal characterization of iridium (Ir) as an emerging metal for high aspect ratio nanostructures on account of its favorable resistivity scaling with thickness. The exceptionally defect-free metal films offer minimal confounding microstructural effects and allow the probing of thermal anisotropy and cross-plane quasi-ballistic thermal transport in epitaxial Ir(001) interposed between Al and MgO(001). Such effects reveal a transition between three dominant cross-plane thermal transport mechanisms which include electron dominant, phonon dominant, and electron-phonon energy conversion dominant regimes at different thicknesses. Finally, we develop a phenomenological model that correctly describes the dominant transport regimes, providing insight into the thickness-dependent interplay between carriers in metals as well as enabling quick evaluation and potential scalability to broader material systems. Next, we describe defect-modulated thermal transport in sputtered aluminum nitride (AlN) thin films for enabling wide-bandgap (WBG), high-temperature, and high-power electronic devices deposited at back-end of the line (BEOL) compatible temperatures (

Heat in most semiconductor materials, including the traditional group IV elements (Si, Ge, diamond), III–V compounds (GaAs, wide-bandgap GaN), and carbon allotropes (graphene, CNTs), as well as emerging new materials like transition metal dichalcogenides (TMDCs), is stored and transported by lattice vibrations (phonons). Phonon generation through interactions with electrons (in nanoelectronics, power, and nonequilibrium devices) and light (optoelectronics) is the central mechanism of heat dissipation in nanoelectronics. This book focuses on the area of thermal effects in nanostructures, including the generation, transport, and conversion of heat at the nanoscale level. Phonon transport, including thermal conductivity in nanostructured materials, as well as numerical simulation methods, such as phonon Monte Carlo, Green’s functions, and first principles methods, feature prominently in the book, which comprises four main themes: (i) phonon generation/heat dissipation, (i) nanoscale phonon transport, (iii) applications/devices (including thermoelectrics), and (iv) emerging materials (graphene/2D). The book also covers recent advances in nanophononics—the study of phonons at the nanoscale. Applications of nanophononics focus on thermoelectric (TE) and tandem TE/photovoltaic energy conversion. The applications are augmented by a chapter on heat dissipation and self-heating in nanoelectronic devices. The book concludes with a chapter on thermal transport in nanoscale graphene ribbons, covering recent advances in phonon transport in 2D materials. The book will be an excellent reference for researchers and graduate students of nanoelectronics, device engineering, nanoscale heat transfer, and thermoelectric energy conversion. The book could also be a basis for a graduate special topics course in the field of nanoscale heat and energy.

The book is devoted to nanostructures and nanostructured materials containing both amorphous and crystalline phases with a particular focus on their thermal properties. It is the first time that theoreticians and experimentalists from different domains gathered to treat this subject. It contains two distinct parts; the first combines theory and simulations methods with specific examples, while the second part discusses methods to fabricate nanomaterials with crystalline and amorphous phases and experimental techniques to measure the thermal conductivity of such materials. Physical insights are given in the first part of the book, related with the existing theoretical models and the state of art simulations methods (molecular dynamics, ab-initio simulations, kinetic theory of gases). In the second part, engineering advances in the nanofabrication of crystalline/amorphous heterostructures (heavy ion irradiation, electrochemical etching, aging/recrystallization, ball milling, PVD, laser crystallization and magnetron sputtering) and adequate experimental measurement methods are analyzed (Scanning Thermal Microscopy, Raman, thermal wave methods and x-rays neutrons spectroscopy).

With the advancement of nanofabrication techniques, the sizes of semiconductor electronic and optoelectronic devices keep decreasing while the operating speeds keep increasing. High-speed operation leads to more heat generation and puts more thermal stress on the devices. Since the heat conduction in semiconductors is dominated by the lattice (i.e., phonons), understanding phonon transport in nanostructures is essential to addressing and alleviating the thermal-stress problem in these modern devices. In addition to the increased thermal stress, the advanced techniques that have allowed for the shrinking of the devices routinely rely on heterostructuring, doping, alloying, and the growth of intentionally strained layers to achieve the desired electronic and optical properties. These introduce impediments to phonon transport such as boundaries, interfaces, point defects (alloy atoms or dopants), and strain. Phonon transport is strongly affected by this nanoscale disorder. This dissertation examines how different types of disorder interact with phonons and degrade phonon transport. First, we study thermal transport in graphene nanoribbons (GNRs). GNRs are quasi-one-dimensional (quasi-1D) systems where the edges (boundaries) play an important role in reducing thermal conductivity. Additionally, the thermal transport in GNRs is anisotropic and depend on the GNR's chirality (GNR orientation and edge termination). We use phonon Monte Carlo (PMC) with full phonon dispersions to describe two highly-symmetric types of GNRs: the armchair GNR (AGNR) and the zigzag GNR (ZGNR). PMC tracks phonon in real space and we can explicitly include non-trivial edge structures. Moreover, the relatively low computational burden of PMC allows us to simulate samples up to 100 $\mu$m in length and predict an upper limit for thermal conductivity in graphene. We then investigate the thermal conductivity in III-V superlattices (SLs). SLs consist of alternating thin layers of different materials and III-V SLs are widely used in nanoscale thermoelectric and optoelectronic devices. The key feature in SLs is that it contains many interfaces, which dictates thermal transport. As III-V SLs are often fabricated using well-controlled techniques and have high-quality interfaces, we develop a model with only one free parameter---the effective rms roughness of the interfaces---to describe its twofold influence: reducing the in-plane layer thermal conductivity and introducing thermal boundary resistance (TBR) in the cross-plane direction. Both the calculated in-plane and cross-plane thermal conductivity of SLs agree with a number of different experiments. Finally, we study thermal conductivity of ternary III-V alloys. In modern optoelectronic devices, ternary III-V alloys are used more often than binary compounds because one can use composition engineering to achieve different effective masses, electron/hole barrier heights, and strain levels. Ternary alloys are usually treated under the virtual crystal approximation (VCA) where cation atoms are assumed to be randomly distributed and possess an averaged mass. This assumption is challenged by a discrepancy between different experiments, as well as the discrepancy between experiments and calculations. We use molecular dynamics (MD) to study the ternary alloy system as both atom masses and atom locations are explicitly tracked in MD. We discover that the thermal conductivity is determined by a competition between mass-difference scattering and the short-range ordering of the cations.

A first approach to engineering problems in nanosized systems requires a thorough understanding of how physical properties change as size decreases from the macroscale. One important class of properties that can be severely affected by such a downward size shift are transport properties - classical mass, momentum and energy transport. Using atomistic simulation techniques, primarily molecular dynamics, and statistical expressions for diffusion, viscosity, and thermal conductivity formulated in terms of atomistic properties, three case studies of transport in important, nanosized systems are investigated, including confined water systems, silicon-germanium nanos- tructures, and carbon nanostructures. In the first study of confined water systems, diffusion and viscosity are of primary interest, as recent experimental studies have shown notably increased rates of diffusion through nano-confined carbon nanotube structures. In this work, a full treatment of the transport properties is provided in both water clusters and water thin films, both having characteristic size scales under 11 nm. The diffusion, viscosity, and thermal conductivity in the nanosized systems are all shown to be significantly different from bulk water systems, with diffusion and thermal transport increasing and viscosity decreasing. For silicon-germanium nanostructures, the thermal transport properties are exclusively considered, with the problem of interest concerning the control of thermal transport through a strict control on the nanostructure. Quantum dot superlattices are shown to be effective structures for controlling the thermal transport properties, the available range of thermal conductivity using these structures being 0.1-160 W/mK. The final study concerns graphene nanostructures, which in terms of thermal transport have some of the highest thermal conductivities of any available materials. Control of thermal transport properties is again of primary importance, with various physical aspects - defects, shape, and size - being probed in graphene, graphene nano ribbons, carbon nanotubes, and fullerenes to determine their influence on transport; overall, these structures yield a large range of thermal transport, 10-2500 W/mK. The electronic version of this dissertation is accessible from http://hdl.handle.net/1969.1/149231

Nanotechnology is a vibrant research area and a growing industry. The properties of nanoparticles and nanofluids are different from those of macroparticles and macrofluids because the physical and chemical properties are very dissimilar when dimensions are at the nanometer range. The first successes in using nanofluids for cooling were achieved and commercialized for automobiles; hence, this subarea is rather profitable. Other nanotechnology research and developmental areas are cutting edge. The core scientific principles of all nanotechnology applications are based in physics, chemistry, and engineering. Nanotechnology is not taught in most programs of engineering yet, and this book on nanotechnology and energy includes a discussion of introducing nanotechnology to the curricula of engineering students. The book also introduces significant current research topics in nanoscience and nanotechnology. It is a textbook for advanced undergraduate- and graduate-level students of nanotechnology, as well as a useful reference book for researchers and professional engineers working in the fields of macromolecular science, nanotechnology, and chemistry, especially those with an interest in energy and the environment, and the automotive industry.

This work presents the development and application of analytical, numerical, and experimental methods for the study of thermal and electrical transport in nanoscale systems, with special emphasis on those materials and phenomena which can be important in thermoelectric and semiconductor device applications. Analytical solutions to the Boltzmann transport equation (BTE) using the relaxation time approximation (RTA) are presented and used to study the thermal and electrical transport properties of indium antimonide (InSb), indium arsenide (InAs), bismuth telluride (Bi2Te3), and chromium disilicide (CrSi2) nanowires. Experimental results for the thermal conductivity of single layer graphene supported by SiO2 were analyzed using an RTA-based model and compared to a full quantum mechanical numerical BTE solution which does not rely on the RTA. The ability of these models to explain the measurement results as well as differences between the two approaches are discussed. Alternatively, numerical solutions to the BTE may be obtained statistically through Monte Carlo simulation for complex geometries which may prove intractable for analytical methods. Following this approach, phonon transport in silicon (Si) sawtooth nanowires was studied, revealing that thermal conductivity suppression below the diffuse surface limit is possible. The experimental investigation of energy transport in nanostructures typically involved the use of microfabricated devices or non-contact optical methods. In this work, two such approaches were analyzed to ascertain their thermal behavior and overall accuracy as well as areas for possible improvement. A Raman spectroscopy-based measurement design for investigating the thermal properties of suspended and supported graphene was examined analytically. The resulting analysis provided a means of determining from measurement results the thermal interface conductance, thermal contact resistance, and thermal conductivity of the suspended and supported graphene regions. Previously, microfabricated devices of several different designs have been used to experimentally measure the thermal transport characteristics of nanostructures such as carbon nanotubes, nanowires, and thin films. To ascertain the accuracy and limitations of various microdevice designs and their associated conduction analyses, finite element models were constructed using ANSYS and measurements of samples of known thermal conductance were simulated. It was found that designs with the sample suspended were generally more accurate than those for which the sample is supported on a bridge whose conductance is measured separately. The effects of radiation loss to the environment of certain device designs were also studied, demonstrating the need for radiation shielding to be at temperatures close to that of the device substrate in order to accurately calibrate the resistance thermometers. Using a suspended microdevice like those analyzed using finite element analysis, the thermal conductivities of individual bismuth (Bi) nanowires were measured. The results were correlated with the crystal structure and growth direction obtained by transmission electron microscopy on the same nanowires. Compared to bulk Bi in the same crystal direction, the thermal conductivity of a single-crystal Bi nanowires of 232 nm diameter was found to be 3 - 6 times smaller than bulk between 100 K and 300 K. For polycrystalline Bi nanowires of 74 nm to 255 nm diameter the thermal conductivity was reduced by a factor of 18 - 78 over the same temperature range. Comparable thermal conductivity values were measured for polycrystalline nanowires of varying diameters, suggesting a grain boundary scattering mean free path for all heat carriers in the range of 15 - 40 nm which is smaller than the nanowire diameters. An RTA-based transport model for both charge carriers and phonons was developed which explains the thermal conductivity suppression in the single-crystal nanowire by considering diffuse phonon-surface scattering, partially diffuse surface scattering of electrons and holes, and scattering of phonons and charge carriers by ionized impurities such as oxygen and carbon of a concentration on the order of 1019 cm−3. Using a similar experimental setup, the thermoelectric properties (Seebeck coefficient, electrical conductivity, and thermal conductivity) of higher manganese silicide (HMS) nanostructures were investigated. Bulk HMS is a passable high temperature thermoelectric material which possesses a complex crystal structure that could lead to very interesting and useful nanoscale transport properties. The thermal conductivities of HMS nanowires and nanoribbons were found to be reduced by 50 - 60 % compared to bulk values in the same crystal direction for both nanoribbons and nanowires. The measured Seebeck coefficient data was comparable or below that of bulk, suggesting unintentional doping of the samples either during growth or sample preparation. Difficulty in determining the amorphous oxide layer thickness for nanoribbons samples necessitated using the total, oxide-included cross section in the thermal and electrical conductivity calculation. This in turn led to the determined electrical conductivity values representing the lower bound on the actual electrical conductivity of the HMS core. From this approach, the measured electrical conductivity values were comparable or slightly below the lower end of bulk electrical conductivity values. This oxide thickness issue affects the determination of the HMS nanostructure thermoelectric figure of merit ZT as well, though the lower bound values obtained here were found to still be comparable to or slightly smaller than the expected bulk values in the same crystal direction. Analytical modeling also indicates higher doping than in bulk. Overall, HMS nanostructures appear to have the potential to demonstrate measurable size-induced ZT enhancement, especially if optimal doping and control over the crystallographic growth direction can be achieved. However, experimental methods to achieve reliable electrical contact to quality four-probe samples needs to be improved in order to fully investigate the thermoelectric potential of HMS nanostructures.