380 research outputs found

    Spintronics: Fundamentals and applications

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    Spintronics, or spin electronics, involves the study of active control and manipulation of spin degrees of freedom in solid-state systems. This article reviews the current status of this subject, including both recent advances and well-established results. The primary focus is on the basic physical principles underlying the generation of carrier spin polarization, spin dynamics, and spin-polarized transport in semiconductors and metals. Spin transport differs from charge transport in that spin is a nonconserved quantity in solids due to spin-orbit and hyperfine coupling. The authors discuss in detail spin decoherence mechanisms in metals and semiconductors. Various theories of spin injection and spin-polarized transport are applied to hybrid structures relevant to spin-based devices and fundamental studies of materials properties. Experimental work is reviewed with the emphasis on projected applications, in which external electric and magnetic fields and illumination by light will be used to control spin and charge dynamics to create new functionalities not feasible or ineffective with conventional electronics.Comment: invited review, 36 figures, 900+ references; minor stylistic changes from the published versio

    Ultrafast carrier dynamics investigated by a novel pump-and-probe-terahertz technique

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    In dieser Arbeit wird eine neuartige Methode, die so genannte "Pump-and-Probe-THz-" Technik zur Untersuchung der Ladungsträgerdynamik in dem Halbleiter GaAs und dem Hochtemperatursupraleiter YBa2Cu3O7-x (YBCO) vorgeschlagen. Hierbei wird THz-Strahlung von der zu untersuchenden Probe emittiert. Diese wird zusätzlich durch einen Pumplaserpuls beeinflusst und zeitlich aufgelöst gemessen. Es wird gezeigt, dass sich mit dieser Methode Ladungsträger-Relaxationszeiten im GaAs in ihrer Abhängigkeit vom äußeren elektrischen Feld, der Ladungsträgerdichte und dem Ladungsträgereinfang in lokalisierten Zuständen im Inneren der Energielücke, bestimmen lassen. Im Fall des YBCO kann die Relaxationsrate der Quasiteilchen und die Rekombinationszeit der Cooperpaare im supraleitenden Zustand bestimmt werden und somit der Einfluss verschiedener Streumechanismen in Abhängigkeit von der Temperatur unterhalb TC

    Diffusive Transport in Quasi-2D and Quasi-1D Electron Systems

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    Quantum-confined semiconductor structures are the cornerstone of modern-day electronics. Spatial confinement in these structures leads to formation of discrete low-dimensional subbands. At room temperature, carriers transfer among different states due to efficient scattering with phonons, charged impurities, surface roughness and other electrons, so transport is scattering-limited (diffusive) and well described by the Boltzmann transport equation. In this review, we present the theoretical framework used for the description and simulation of diffusive electron transport in quasi-two-dimensional and quasi-one-dimensional semiconductor structures. Transport in silicon MOSFETs and nanowires is presented in detail.Comment: Review article, to appear in Journal of Computational and Theoretical Nanoscienc

    Nanoscale heat conduction with applications in nanoelectronics and thermoelectrics

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    Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2006.Includes bibliographical references.When the device or structure characteristic length scales are comparable to the mean free path and wavelength of energy carriers (electrons, photons, phonons, and molecules) or the time of interest is on the same order as the carrier relaxation time, conventional heat transfer theory is no longer valid. Tremendous progress has been made in the past two decades to understand and characterize heat transfer in nanostructures. However most work in the last decade has focused on heat transfer in simple nanostructures, such as thin films, superlattices and nanowires. In reality, there is a demand to study transport process in complex nanostructures for engineering applications, such as heat transfer in nanoelectronic devices and the thermal conductivity in nanocomposites which consists of nanowires or nanoparticles embedded in a matrix material. Another class of problems which are rich in physics and might be explored for better design of both nanoelectronic devices and energy conversion materials and devices are coupled electron and phonon transport. Experimentally, most past work has been focused on thermal conductivity characterization of various nanostructures and very little has been done on the fundamental transport properties of energy carriers.(cont.) This thesis work contributes to the following aspects of heat transfer, nanoelectronics, and thermoelectrics. 1) Simulation tools are developed for transient phonon transport in multidimensional nanostructures and used to predict the size effect on the temperature rise surrounding a nanoscale heat source, which mimics the heating issue in nano-MOSFETs. 2) Semiconductor nanocomposites are proposed for highly efficient thermoelectric materials development where low thermal conductivity is a blessing for efficiency enhancement. Both the deterministic solution and Monte Carlo simulation of the phonon Boltzmann equation are established to study the size effect on the thermal conductivity of nanocomposites where nanoparticles and nanowires are embedded in a host material. 3) Explored the possibility of creating nonequilibrium conditions between electrons and phonons in thermoelectric materials using high energy flux coupling to electrons through surface plasmons, and thus to develop highly efficient thermoelectric devices.(cont.) 4) Established a sub-pico second optical pump-probe measurement system where a femtosecond laser is employed and explored the possibility of extracting phonon reflectivity at interfaces and the phonon relaxation time in a material, which are the two most fundamental phonon properties for nanoscale energy transport from the pump-probe measurements.by Ronggui Yang.Ph.D

    Phonon Spectroscopy and Low-Dimensional Electron Systems: The Effect of Acoustic Anisotropy and Carrier Confinement

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    The generation and propagation of pulses of nonequilibrium acoustic phonons and their interaction with semiconductor nanostructures are investigated. Such studies can give unique information about the properties of low-dimensional electron systems, but in order to interpret the experiments and to understand the underlying physics, a comparison with theoretical models is absolutely necessary. A central point of this work is therefore a universal theoretical approach allowing the simulation and the analysis of phonon spectroscopy measurements on low-dimensional semiconductor structures. The model takes into account the characteristic properties of the considered systems. These properties are the elastic anisotropy of the substrate material leading to focusing effects and highly anisotropic phonon propagation, the anisotropic nature of the different electron-phonon coupling mechanisms, which depend manifestly on phonon wavevector direction and polarization vector, and the sensitivity to the confinement parameters of the low-dimensional electron systems. We show that screening of the electron-phonon interaction can have a much stronger influence on the results of angle-resolved phonon spectroscopy than expected from transport measurements. Since we compare theoretical simulations with real experiments, the geometrical arrangement and the spatial extension of phonon source and detector are also included in the approach enabling a quantitative analysis of the data this way. To illustrate the influence of acoustic anisotropy and carrier confinement on the results of phonon spectroscopy in detail we analyse two different applications. In the first case the low-dimensional electron system acts as the phonon detector and the phonon induced drag current is measured. Our theoretical model enables us to calculate the electric current induced in low-dimensional electron systems by pulses of (ballistic) nonequilibrium phonons. The theoretical drag patterns reproduce the main features of the experimental images very well. The sensitivity of the results to variations of the confining potential of quasi-2D and quasi-1D electrons is demonstrated. This provides the opportunity to use phonon-drag imaging as unique experimental tool for determining the confinement lengths of low-dimensional electron systems. By comparing the experimental and theoretical images it is also possible to estimate the relative strength of the different electron-phonon coupling mechanisms.In the second application the low-dimensional electron system acts as the phonon pulse source and the angle and mode dependence of the acoustic phonon emission by hot 2D electrons is investigated. The results exhibit strong variations in the phonon signal as a function of the detector position and depend markedly on the coupling mechanism, the phonon polarization and the electron confinement width. We demonstrate that the ratio of the strengths of the emitted longitudinal (LA) and transverse (TA) acoustic phonon modes is predicted correctly only by a theoretical model that properly includes the effects of acoustic anisotropy on the electron-phonon matrix elements, the screening, and the form of the confining potential. A simple adoption of widely used theoretical assumptions, like the isotropic approximation for the phonons in the electron-phonon matrix elements or the use of simple variational envelope wavefunctions for the carrier confinement, can corrupt or even falsify theoretical predictions.We explain the `mystery of the missing longitudinal mode' in heat-pulse experiments with hot 2D electrons in GaAs/AlGaAs heterojunctions. We demonstrate that screening prevents a strong peak in the phonon emission of deformation potential coupled LA phonons in a direction nearly normal to the 2D electron system and that deformation potential coupled TA phonons give a significant contribution to the phonon signal in certain emission directions.Die vorliegende Arbeit beschäftigt sich mit der Ausbreitung von akustischen Nichtgleichgewichtsphononen und deren Wechselwirkung mit Halbleiter-Nanostrukturen. Güte und Effizienz moderner Halbleiter-Bauelemente hängen wesentlich vom Verständnis der Wechselwirkung akustischer Phononen mit niederdimensionalen Elektronensystemen ab. Traditionelle Untersuchungsmethoden, wie die Messung der elektrischen Leitfähigkeit oder der Thermospannung, erlauben nur eingeschränkte Aussagen. Sie mitteln über die beteiligten Phononenmoden und eine Trennung der einzelnen Wechselwirkungsmechanismen ist nur näherungsweise möglich ist. Demgegenüber erlaubt die in der Arbeit diskutierte Methode der winkel- und zeitaufgelösten Phononen-Spektroskopie ein direktes Studium des Beitrags einzelner Phononenmoden, d.h. in Abhängigkeit von Wellenzahlvektor und Polarisation der Phononen. Im Mittelpunkt der Arbeit steht die Fragestellung, wie akustische Anisotropie und Ladungsträger-Confinement die Ergebnisse der winkel- und zeitaufgelösten Phononen-Spektroskopie beeinflussen und prägen. Dazu wird ein umfassendes theoretisches Modell zur Simulation von Phononen-Spektroskopie-Experimenten an niederdimensionalen Halbleitersystemen vorgestellt. Dieses erlaubt sowohl ein qualitatives Verständnis der ablaufenden physikalischen Prozesse als auch eine quantitative Analyse der Messergebnisse. Die Vorteile gegenüber anderen Modellen und Rechnungen liegen dabei in dem konsequenten Einbeziehen der akustischen Anisotropie, nicht nur für die Ausbreitung der Phononen, sondern auch für die Matrixelemente der Wechselwirkung, sowie eine saubere Behandlung des Confinements der Elektronen in den niederdimensionalen Systemen. Dabei werden die Grenzen weit verbreiteter Näherungsansätze für die Elektron-Phonon-Matrixelemente und das Elektronen-Confinement deutlich aufgezeigt. Für den quantitativen Vergleich mit realen Experimenten werden aber auch solche Größen, wie die endliche räumliche Ausdehnung von Phononenquelle und Detektor, die Streuung der Phononen an Verunreinigungen oder die Abschirmung der Elektron-Phonon-Kopplung durch die Elektron-Elektron-Wechselwirkung berücksichtigt.Im zweiten Teil der Arbeit wird der theoretische Apparat auf typische experimentelle Fragestellungen angewandt. Im Falle der Phonon-Drag-Experimente an GaAs/AlGaAs Heterostrukturen wird der durch akustische Nichtgleichgewichtsphononen in zwei- und eindimensionalen Elektronensystemen induzierte elektrische Strom (Phonon-Drag-Strom) als Funktion des Ortes der Phononenquelle bestimmt. Das in der Arbeit hergeleitete theoretische Modell kann die experimentellen Resultate für die Winkelabhängigkeit des Drag-Stromes sowohl für Messungen mit und ohne Magnetfeld qualitativ gut beschreiben. Außerdem wird der Einfluss unterschiedlicher Confinementmodelle und unterschiedlicher Wechselwirkungsmechanismen studiert. Dadurch ist es möglich, aus Phonon-Drag-Messungen Rückschlüsse auf die elektronischen und strukturellen Eigenschaften der niederdimensionalen Elektronensysteme zu ziehen (Fermivektor, effektive Masse, Elektron-Phonon-Kopplungskonstanten, Form des Confinementpotentials). Als weiteres Anwendungsbeispiel wird das Problem der Energierelaxation (aufgeheizter)zweidimensionaler Elektronensysteme in GaAs Heterostrukturen und Quantentrögen untersucht. Für Elektronentemperaturen unterhalb 50 K werden die Gesamtemissionsrate als Funktion der Temperatur und die winkelaufgelöste Emissionsrate (als Funktion der Detektorposition) berechnet. Für beide Größen wird erstmals eine gute Übereinstimmung zwischen Theorie und Experiment gefunden. Es zeigt sich, dass akustische Anisotropie und Abschirmungseffekte zu überraschenden neuen Ergebnissen führen können. Ein Beispiel dafür ist der unerwartet große Beitrag der mittels Deformationspotential-Wechselwirkung emittierten transversalen akustischen Phononen, der bei einer Emission der Phononen näherungsweise senkrecht zum zweidimensionalen System beobachtet werden kann

    Quantifying nanoscale carrier diffusion with ultrafast optical and photocurrent microscopy

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    Aplicat embargament des de la data de defensa fins el 30 de setembre de 2019Heat transport in solids is one of the oldest problems in physics, dating back to the earliest formulations of thermodynamics. The classical laws of heat conduction are valid as long as the observed time and length scales are larger than the relaxation time and mean free path of the underlying microscopic heat carriers, such as electrons and phonons. With the advent of ultrafast lasers and nanoscale systems these regimes can now be surpassed and new refined models of heat transport are needed. In particular, the interaction of ultrashort light pulses with matter can excite electrons to high temperatures, leading to a local non-equilibrium of electrons and phonons. Under these conditions, also the transport properties of the carriers are altered. So far, these effects have typically been studied in the time domain. The cooling of photo-excited hot electrons has been studied both in metals as well as novel 2D materials, such as graphene. However, due to a lack of spatio-temporal resolution, it has not been possible to distinguish the effects of hot-electron diffusion from other cooling mechanisms, such as electron-phonon coupling. In this thesis, I directly track such ultrafast heat and carrier diffusion in space and time with ultrafast microscopy. By using the recently developed technique of probe-beam-scanning transient-absorption microscopy on thin gold films I directly resolve, for the first time, a transition from hot-electron diffusion to phonon-limited diffusion on the picosecond timescale. I support the understanding of these complex dynamics by theoretical modeling of the thermo-optical response based on a two-temperature model. I apply the same technique to study hot carrier diffusion in atomically thin monolayer graphene. By comparing differently prepared samples, I study the strong influence of external parameters, such as production type, substrate, and environment on carrier diffusion. Finally, I study hot carrier diffusion in exfoliated and encapsulated graphene devices with a novel technique of ultrafast spatio-temporal photocurrent microscopy based on the photothermoelectric effect. I extract diffusion dynamics for electrically characterized samples with the help of theoretical spatio-temporal modeling, thereby testing the fundamental relationship between electrical and thermal carrier transport. The precise quantification of ultrafast and nanoscale carrier transport with these state-of-the-art techniques leads to a broader understanding of non-equilibrium dynamics and could ultimately help the design, optimization, and heat management of the next generation of ultra-compact (opto-) electronic devices, such as solar cells, photodetectors, or integrated circuits.El transporte de calor en sólidos es uno de los problemas más antiguos de la física, que se remonta a las primeras formulaciones de la termodinámica. Las leyes clásicas de la conducción de calor son válidas cuando las escalas de tiempo y longitud observadas sean mayores que el tiempo de relajación y la trayectoria libre media de los portadores de calor microscópicos subyacentes, como los electrones y los fonones. Con la llegada de los láseres ultrarrápidos y los sistemas a nanoescala, estos regímenes ahora pueden superarse por lo cual se necesitan nuevos modelos refinados de transporte de calor. En particular, la interacción de pulsos de luz ultracortos con la materia puede excitar electrones a altas temperaturas, lo que lleva a un desequilibrio local de electrones y fonones. En estas condiciones, también se modifican las propiedades de transporte de los portadores de calor. Hasta ahora, estos efectos han sido típicamente estudiados en el dominio del tiempo. El enfriamiento de electrones calientes fotoexcitados se ha estudiado tanto en metales como en nuevos materiales bidimensionales, como el grafeno. Sin embargo, debido a la falta de resolución espacio-temporal, no ha sido posible distinguir los efectos de la difusión de electrones calientes de otros mecanismos de enfriamiento, como el acoplamiento de electrones y fonones. En esta tesis, hago un seguimiento directo de la difusión del calor y sus portadores en el espacio y el tiempo con microscopía ultrarrápida. Al utilizar la técnica recientemente desarrollada de microscopía de absorción transitoria con escaneo de sonda en películas de oro delgadas, resuelvo directamente, por primera vez, una transición de la difusión de electrones calientes a la difusión limitada por fonones en la escala de tiempo de picosegundos. Apoyo la comprensión de estas dinámicas complejas mediante el modelado teórico de la respuesta termo-óptica basada en un modelo de dos temperaturas. Aplico la misma técnica para estudiar la difusión de portadores calientes en una capa de grafeno atómicamente delgado. Al comparar muestras preparadas de manera diferente, estudio la fuerte influencia de los parámetros externos, como el tipo de producción, el sustrato y el entorno sobre la difusión del portador. Finalmente, estudio la difusión de portadores en dispositivos de grafeno exfoliados y encapsulados con una técnica novedosa de microscopía de fotocorriente espacio-temporal ultrarrápida basada en el efecto fototermoeléctrico. Extraigo dinámicas de difusión para muestras caracterizadas eléctricamente con la ayuda del modelado espacio-temporal teórico, probando así la relación fundamental entre el transporte eléctrico y térmico. La cuantificación precisa del transporte de los portadores ultrarrápido y a nanoescala con estas técnicas de vanguardia lleva a una comprensión más amplia de la dinámica del no equilibrio y podría, en última instancia, ayudar al diseño, la optimización y la gestión del calor de la próxima generación de dispositivos (opto-)electrónicos ultracompactos, como células solares, fotodetectores o circuitos integrados.Postprint (published version

    Semiconductor Spintronics

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    Spintronics refers commonly to phenomena in which the spin of electrons in a solid state environment plays the determining role. In a more narrow sense spintronics is an emerging research field of electronics: spintronics devices are based on a spin control of electronics, or on an electrical and optical control of spin or magnetism. This review presents selected themes of semiconductor spintronics, introducing important concepts in spin transport, spin injection, Silsbee-Johnson spin-charge coupling, and spindependent tunneling, as well as spin relaxation and spin dynamics. The most fundamental spin-dependent nteraction in nonmagnetic semiconductors is spin-orbit coupling. Depending on the crystal symmetries of the material, as well as on the structural properties of semiconductor based heterostructures, the spin-orbit coupling takes on different functional forms, giving a nice playground of effective spin-orbit Hamiltonians. The effective Hamiltonians for the most relevant classes of materials and heterostructures are derived here from realistic electronic band structure descriptions. Most semiconductor device systems are still theoretical concepts, waiting for experimental demonstrations. A review of selected proposed, and a few demonstrated devices is presented, with detailed description of two important classes: magnetic resonant tunnel structures and bipolar magnetic diodes and transistors. In most cases the presentation is of tutorial style, introducing the essential theoretical formalism at an accessible level, with case-study-like illustrations of actual experimental results, as well as with brief reviews of relevant recent achievements in the field.Comment: tutorial review; 342 pages, 132 figure

    MULTISCALE SIMULATIONS OF THERMAL TRANSPORT IN GRAPHENE-BASED MATERIALS AND ACROSS METAL-SEMICONDUCTOR INTERFACES

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    The rapid advance in modern electronics and photonics is pushing device design to the micro- and nano-scale, and the resulting high power density imposes immense challenges to thermal management. When device size shrinks to the same order of or even below the wavelength or mean-free-path of heat carriers, the transport of heat carriers and the interaction between them will differ from those in the macroscopic regime. This imposes challenges on designing micro/nano-devices with required thermal performance, while, at the same time, also opens the door for designing novel materials and structures with promising thermal properties
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