Comprendre et maîtriser le passage de type I à type II de puits quantiques d'In(x)Ga(1-x)As(y)Sb(1-y) sur substrat de GaSb

Les antimoniures sont des semi-conducteurs III-V prometteurs pour le développement de dispositifs optoélectroniques puisqu'ils ont une grande mobilité d'électrons, une large gamme spectrale d'émission ou de détection et offrent la possibilité de former des hétérostructures confinées dont la recombinaison est de type I, II ou III. Bien qu'il existe plusieurs publications sur la fabrication de dispositifs utilisant un alliage d'In(x)Ga(1-x)As(y)Sb(1-y) qui émet ou détecte à une certaine longueur d'onde, les détails, à savoir comment sont déterminés les compositions et surtout les alignements de bande, sont rarement explicites. Très peu d'études fondamentales sur l'incorporation d'indium et d'arsenic sous forme de tétramères lors de l'épitaxie par jets moléculaires existent, et les méthodes afin de déterminer l'alignement des bandes des binaires qui composent ces alliages donnent des résultats variables. Un modèle a été construit et a permis de prédire l'alignement des bandes énergétiques des alliages d'In(x)Ga(1-x)As(y)Sb(1-y) avec celles du GaSb pour l'ensemble des compositions possibles. Ce modèle tient compte des effets thermiques, des contraintes élastiques et peut aussi inclure le confinement pour des puits quantiques. De cette manière, il est possible de prédire la transition de type de recombinaison en fonction de la composition. Il est aussi montré que l'indium ségrègue en surface lors de la croissance par épitaxie par jets moléculaires d'In(x)Ga(1-x)Sb sur GaSb, ce qui avait déjà été observé pour ce type de matériau. Il est possible d'éliminer le gradient de composition à cette interface en mouillant la surface d'indium avant la croissance de l'alliage. L'épaisseur d'indium en surface dépend de la température et peut être évaluée par un modèle simple simulant la ségrégation. Dans le cas d'un puits quantique, il y aura une seconde interface GaSb sur In(x)Ga(1-x)Sb où l'indium de surface ira s'incorporer. La croissance de quelques monocouches de GaSb à basse température immédiatement après la croissance de l'alliage permet d'incorporer rapidement ces atomes d'indium et de garder la seconde interface abrupte. Lorsque la composition d'indium ne change plus dans la couche, cette composition correspond au rapport de flux d'atomes d'indium sur celui des éléments III. L'arsenic, dont la source fournit principalement des tétramères, ne s'incorpore pas de la même manière. Les tétramères occupent deux sites en surface et doivent interagir par paire afin de créer des dimères d'arsenic. Ces derniers pourront alors être incorporés dans l'alliage. Un modèle de cinétique de surface a été élaboré afin de rendre compte de la diminution d'incorporation d'arsenic en augmentant le rapport V/III pour une composition nominale d'arsenic fixe dans l'In(x)Ga(1-x)As(y)Sb(1-y). Ce résultat s'explique par le fait que les réactions de deuxième ordre dans la décomposition des tétramères d'arsenic ralentissent considérablement la réaction d'incorporation et permettent à l'antimoine d'occuper majoritairement la surface. Cette observation montre qu'il est préférable d'utiliser une source de dimères d'arsenic, plutôt que de tétramères, afin de mieux contrôler la composition d'arsenic dans la couche. Des puits quantiques d'In(x)Ga(1-x)As(y)Sb(1-y) sur GaSb ont été fabriqués et caractérisés optiquement afin d'observer le passage de recombinaison de type I à type II. Cependant, celui-ci n'a pas pu être observé puisque les spectres étaient dominés par un niveau énergétique dans le GaSb dont la source n'a pu être identifiée. Un problème dans la source de gallium pourrait être à l'origine de ce défaut et la résolution de ce problème est essentielle à la continuité de ces travaux.

Comprendre et maîtriser le passage de type I à type II de puits quantiques d'In(x)Ga(1-x)As(y)Sb(1-y) sur substrat de GaSb

Les antimoniures sont des semi-conducteurs III-V prometteurs pour le développement de dispositifs optoélectroniques puisqu'ils ont une grande mobilité d'électrons, une large gamme spectrale d'émission ou de détection et offrent la possibilité de former des hétérostructures confinées dont la recombinaison est de type I, II ou III. Bien qu'il existe plusieurs publications sur la fabrication de dispositifs utilisant un alliage d'In(x)Ga(1-x)As(y)Sb(1-y) qui émet ou détecte à une certaine longueur d'onde, les détails, à savoir comment sont déterminés les compositions et surtout les alignements de bande, sont rarement explicites. Très peu d'études fondamentales sur l'incorporation d'indium et d'arsenic sous forme de tétramères lors de l'épitaxie par jets moléculaires existent, et les méthodes afin de déterminer l'alignement des bandes des binaires qui composent ces alliages donnent des résultats variables. Un modèle a été construit et a permis de prédire l'alignement des bandes énergétiques des alliages d'In(x)Ga(1-x)As(y)Sb(1-y) avec celles du GaSb pour l'ensemble des compositions possibles. Ce modèle tient compte des effets thermiques, des contraintes élastiques et peut aussi inclure le confinement pour des puits quantiques. De cette manière, il est possible de prédire la transition de type de recombinaison en fonction de la composition. Il est aussi montré que l'indium ségrègue en surface lors de la croissance par épitaxie par jets moléculaires d'In(x)Ga(1-x)Sb sur GaSb, ce qui avait déjà été observé pour ce type de matériau. Il est possible d'éliminer le gradient de composition à cette interface en mouillant la surface d'indium avant la croissance de l'alliage. L'épaisseur d'indium en surface dépend de la température et peut être évaluée par un modèle simple simulant la ségrégation. Dans le cas d'un puits quantique, il y aura une seconde interface GaSb sur In(x)Ga(1-x)Sb où l'indium de surface ira s'incorporer. La croissance de quelques monocouches de GaSb à basse température immédiatement après la croissance de l'alliage permet d'incorporer rapidement ces atomes d'indium et de garder la seconde interface abrupte. Lorsque la composition d'indium ne change plus dans la couche, cette composition correspond au rapport de flux d'atomes d'indium sur celui des éléments III. L'arsenic, dont la source fournit principalement des tétramères, ne s'incorpore pas de la même manière. Les tétramères occupent deux sites en surface et doivent interagir par paire afin de créer des dimères d'arsenic. Ces derniers pourront alors être incorporés dans l'alliage. Un modèle de cinétique de surface a été élaboré afin de rendre compte de la diminution d'incorporation d'arsenic en augmentant le rapport V/III pour une composition nominale d'arsenic fixe dans l'In(x)Ga(1-x)As(y)Sb(1-y). Ce résultat s'explique par le fait que les réactions de deuxième ordre dans la décomposition des tétramères d'arsenic ralentissent considérablement la réaction d'incorporation et permettent à l'antimoine d'occuper majoritairement la surface. Cette observation montre qu'il est préférable d'utiliser une source de dimères d'arsenic, plutôt que de tétramères, afin de mieux contrôler la composition d'arsenic dans la couche. Des puits quantiques d'In(x)Ga(1-x)As(y)Sb(1-y) sur GaSb ont été fabriqués et caractérisés optiquement afin d'observer le passage de recombinaison de type I à type II. Cependant, celui-ci n'a pas pu être observé puisque les spectres étaient dominés par un niveau énergétique dans le GaSb dont la source n'a pu être identifiée. Un problème dans la source de gallium pourrait être à l'origine de ce défaut et la résolution de ce problème est essentielle à la continuité de ces travaux.

Growth and application of planar III-V semiconductor nanowires grown with MOCVD

The semiconductor nanowire has been widely studied over the past decade and identified as a promising nanotechnology building block with application in photonics and electronics. The flexible bottom-up approach to nanowire growth allows for straightforward fabrication of complex 1D nanostructures with interesting optical, electrical, and mechanical properties. III-V nanowires in particular are useful because of their direct bandgap, high carrier mobility, and ability to form heterojunctions and have been used to make devices such as light-emitting diodes, lasers, and field-effect transistors. However, crystal defects are widely reported for III-V nanowires when grown in the common out-of-plane <111>B direction. Furthermore, commercialization of nanowires has been limited by the difficulty of assembling nanowires with predetermined position and alignment on a wafer-scale. In this thesis, planar III-V nanowires are introduced as a low-defect and integratable nanotechnology building block grown with metalorganic chemical vapor deposition. Planar GaAs nanowires grown with gold seed particles self-align along the <110> direction on the (001) GaAs substrate. Transmission electron microscopy reveals that planar GaAs nanowires are nearly free of crystal defects and grow laterally and epitaxially on the substrate surface. The nanowire morphology is shown to be primarily controlled through growth temperature and an ideal growth window of 470 +\- 10 °C is identified for planar GaAs nanowires. Extension of the planar growth mode to other materials is demonstrated through growth of planar InAs nanowires. Using a sacrificial layer, the transfer of planar GaAs nanowires onto silicon substrates with control over the alignment and position is presented. A metal-semiconductor field-effect transistor fabricated with a planar GaAs nanowire shows bulk-like low-field electron transport characteristics with high mobility. The aligned planar geometry and excellent material quality of planar III-V nanowires may lead to highly integrated III-V nanophotonics and nanoelectronics.

High-order fractional microwave-induced resistance oscillations in two-dimensional systems

We report on the observation of microwave-induced resistance oscillations associated with the fractional ratio n/m of the microwave irradiation frequency to the cyclotron frequency for m up to 8 in a two-dimensional electron system with high electron density. The features are quenched at high microwave frequencies independent of the fractional order m. We analyze temperature, power, and frequency dependencies of the magnetoresistance oscillations and discuss them in connection with existing theories.

Investigation of resins suitable for the preparation of biological sample for 3-D electron microscopy.

In the last two decades, the third-dimension has become a focus of attention in electron microscopy to better understand the interactions within subcellular compartments. Initially, transmission electron tomography (TEM tomography) was introduced to image the cell volume in semi-thin sections (∼500nm). With the introduction of the focused ion beam scanning electron microscope, a new tool, FIB-SEM tomography, became available to image much larger volumes. During TEM tomography and FIB-SEM tomography, the resin section is exposed to a high electron/ion dose such that the stability of the resin embedded biological sample becomes an important issue. The shrinkage of a resin section in each dimension, especially in depth, is a well-known phenomenon. To ensure the dimensional integrity of the final volume of the cell, it is important to assess the properties of the different resins and determine the formulation which has the best stability in the electron/ion beam. Here, eight different resin formulations were examined. The effects of radiation damage were evaluated after different times of TEM irradiation. To get additional information on mass-loss and the physical properties of the resins (stiffness and adhesion), the topography of the irradiated areas was analysed with atomic force microscopy (AFM). Further, the behaviour of the resins was analysed after ion milling of the surface of the sample with different ion currents. In conclusion, two resin formulations, Hard Plus and the mixture of Durcupan/Epon, emerged that were considerably less affected and reasonably stable in the electron/ion beam and thus suitable for the 3-D investigation of biological samples.

Cryo-negative staining: advantages and applications for three-dimensional electron microscopy of biological macromolecules

Les échantillons biologiques ne s?arrangent pas toujours en objets ordonnés (cristaux 2D ou hélices) nécessaires pour la microscopie électronique ni en cristaux 3D parfaitement ordonnés pour la cristallographie rayons X alors que de nombreux spécimens sont tout simplement trop << gros D pour la spectroscopie NMR. C?est pour ces raisons que l?analyse de particules isolées par la cryo-microscopie électronique est devenue une technique de plus en plus importante pour déterminer la structure de macromolécules. Néanmoins, le faible rapport signal-sur-bruit ainsi que la forte sensibilité des échantillons biologiques natifs face au faisceau électronique restent deux parmi les facteurs limitant la résolution. La cryo-coloration négative est une technique récemment développée permettant l?observation des échantillons biologiques avec le microscope électronique. Ils sont observés à l?état vitrifié et à basse température, en présence d?un colorant (molybdate d?ammonium). Les avantages de la cryo-coloration négative sont étudiés dans ce travail. Les résultats obtenus révèlent que les problèmes majeurs peuvent êtres évités par l?utilisation de cette nouvelle technique. Les échantillons sont représentés fidèlement avec un SNR 10 fois plus important que dans le cas des échantillons dans l?eau. De plus, la comparaison de données obtenues après de multiples expositions montre que les dégâts liés au faisceau électronique sont réduits considérablement. D?autre part, les résultats exposés mettent en évidence que la technique est idéale pour l?analyse à haute résolution de macromolécules biologiques. La solution vitrifiée de molybdate d?ammonium entourant l?échantillon n?empêche pas l?accès à la structure interne de la protéine. Finalement, plusieurs exemples d?application démontrent les avantages de cette technique nouvellement développée.<br/><br/>Many biological specimens do not arrange themselves in ordered assemblies (tubular or flat 2D crystals) suitable for electron crystallography, nor in perfectly ordered 3D crystals for X-ray diffraction; many other are simply too large to be approached by NMR spectroscopy. Therefore, single-particles analysis has become a progressively more important technique for structural determination of large isolated macromolecules by cryo-electron microscopy. Nevertheless, the low signal-to-noise ratio and the high electron-beam sensitivity of biological samples remain two main resolution-limiting factors, when the specimens are observed in their native state. Cryo-negative staining is a recently developed technique that allows the study of biological samples with the electron microscope. The samples are observed at low temperature, in the vitrified state, but in presence of a stain (ammonium molybdate). In the present work, the advantages of this novel technique are investigated: it is shown that cryo-negative staining can generally overcome most of the problems encountered with cryo-electron microscopy of vitrified native suspension of biological particles. The specimens are faithfully represented with a 10-times higher SNR than in the case of unstained samples. Beam-damage is found to be considerably reduced by comparison of multiple-exposure series of both stained and unstained samples. The present report also demonstrates that cryo-negative staining is capable of high- resolution analysis of biological macromolecules. The vitrified stain solution surrounding the sample does not forbid the access to the interna1 features (ie. the secondary structure) of a protein. This finding is of direct interest for the structural biologist trying to combine electron microscopy and X-ray data. developed electron microscopy technique. Finally, several application examples demonstrate the advantages of this newly

Thermoelectric properties of TiS2 mechanically alloyed compounds

Bulk polycrystalline samples in the series Ti1−xNbxS2 (0 ≤ x ≤ 0.075) were prepared using mechanical alloying synthesis and spark plasma sintering. X-ray diffraction analysis coupled with high resolution transmission electron microscopy indicates the formation of trigonal TiS2 by high energy ball-milling. The as-synthesized particles consist of pseudo-ordered TiS2 domains of around 20–50 nm, joined by bent atomic planes. This bottom-up approach leads, after spark plasma sintering, to homogeneous solid solutions, with a niobium solubility limit of x = 0.075. Microstructural observations evidence the formation of small crystallites in the bulk compounds with a high density of stacking faults. The large grain boundary concentration coupled with the presence of planar defects, leads to a substantial decrease in the thermal conductivity to 1.8 W/mK at 700 K. This enables the figure of merit to reach ZT = 0.3 at 700 K for x = 0.05, despite the lower electron mobility in mechanically alloyed samples due to small crystallite/grain size and structural defects.

Structural Characterization of Nanocrystalline Sb-Doped SnO2 Xerogels by Multiedge X-ray Absorption Spectroscopy

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

A nonzero gap two-dimensional carbon allotrope from porous graphene

Graphene has been one of the hottest topics in materials science in the last years. Because of its special electronic properties graphene is considered one of the most promising materials for future electronics. However, in its pristine form graphene is a gapless semiconductor, which poses some limitations to its use in some transistor electronics. Many approaches have been tried to create, in a controlled way, a gap in graphene. These approaches have obtained limited successes. Recently, hydrogenated graphene-like structures, the so-called porous graphene, have been synthesized. In this work we show, based on ab initio quantum molecular dynamics calculations, that porous graphene dehydrogenation can lead to a spontaneous formation of a nonzero gap two-dimensional carbon allotrope, called biphenylene carbon (BC). Besides exhibiting an intrinsic nonzero gap value, BC also presents well delocalized frontier orbitals, suggestive of a structure with high electronic mobility. Possible synthetic routes to obtain BC from porous graphene are addressed. © 2012 Materials Research Society.

Matrizes semicondutoras GaAs e Sn'X IND. 2' dopado com terras-raras Ce ou Eu: investigação do transporte elétrico

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Structure, organization and dynamics of functional supramolecular materials studied by solid-state NMR

Functional materials have great importance due to their many important applications. The characterization of supramolecular architectures which are held together by non-covalent interactions is of most importance to understand their properties. Solid-state NMR methods have recently been proven to be able to unravel such structure-property relations with the help of fast magic-angle spinning and advanced pulse sequences. The aim of the current work is to understand the structure and dynamics of functional supramolecular materials which are potentially important for fuel-cell (proton conducting membrane materials) and solar-cell or plastic-electronic applications (photo-reactive aromatic materials). In particular, hydrogen-bonding networks, local proton mobility, molecular packing arrangements, and local dynamics will be studied by the use of advanced solid-state NMR methods. The first class of materials studied in this work is proton conducting polymers which also form hydrogen-bonding network. Different materials, which are prepared for high 1H conduction by different approaches are studied: PAA-P4VP, PVPA-ABPBI, Tz5Si, and Triazole-functional systems. The materials are examples of the following major groups; - Homopolymers with specific functional groups (Triazole functional polysiloxanes). - Acid-base polymer blends approach (PAA-P4VP, PVPA-ABPBI). - Acid-base copolymer approach (Triazole-PVPA). - Acid doped polymers (Triazole functional polymer doped with H3PO4). Perylenebisimide (PBI) derivatives, a second type of important functional supramolecular materials with potent applications in plastic electronics, were also investigated by means of solid-state NMR. The preparation of conducting nanoscopic fibers based on the self-assembling functional units is an appealing aim as they may be incorporated in molecular electronic devices. In this category, perylene derivatives have attracted great attention due to their high charge carrier mobility. A detailed knowledge about their supramolecular structure and molecular dynamics is crucial for the understanding of their electronic properties. The aim is to understand the structure, dynamics and packing arrangements which lead to high electron conductivity in PBI derivatives.

Novel organic semiconductors and their applications in electronics

The main goals of this thesis were the design, synthesis, and characterization of novel organic semiconductors, together with their applications in electronics, such as OFETs, OPVs, and OLEDs. The results can be summarized as follows:rn1. In chapter II, two novel angular n-type molecules were presented. Their different alkyl chains play a pivotal role in the molecular orientation relative to surface. One molecule with longer branched chains is tilted with respect to the substrate, thereby resulting in poor device performance, while the other adopt an edge-on orientation with an OFET electron mobility of 0.01 cm2 V-1 s-1.rn2. In chapter III, fused bis-benzothiadiazoles with different molecular geometries, namely linear benzoquinone-fused bis(benzothiadiazole) and V-shaped sulfone-fused bis(benzothiadiazole), were shown. This work not only contributes to the diversity of electron acceptors based on bis-benzothiadiazole moieties, but also highlights the important role of molecular shape for the solid-state packing of organic conjugated materials. In chapter IV, we demonstrated the synthesis of layered acceptors via dimerization of thiadiazole end-capped acenes. Interestingly, they feature huge differences in their photophysical properties. One compound showed a new strong emission in the near-infrared region introduced by the aggregation effect. The planosymmetric compound featured intramolecular excimer (IEE) fluorescence in solution. rn3. In chapter V and VI, we have demonstrated the synthesis of novel spiro-bifluorene based asymmetric and symmetric cruciform electron acceptors with dicyanovinylene substitutions. The solar cells based on PTB7:asymmetric acceptor yields the highest PCE of 0.80%. Such results demonstrate for the first time that dicyanovinylene substituted acceptor could be an alternative to fullerene-based acceptors. rn4. In chapter VII, two novel blue-emitting compounds were shown, which consist of dihydroindenofluorenyl units and ladder-type poly-p-phenylene groups, respectively. The two novel cruciform rigid compounds present not only excellent thermal and electrochemical stability but also high PLQYs. Through analysis of their triplet energy levels, both molecules can be served as hosts for other normal fluorescent or phosphorescent materials.rn

Electron transport in E x B devices

A Hall thruster, an E × B device used for in-space propulsion, utilizes an axial electric field to electrostatically accelerate plasma propellant from the spacecraft. The axial electric field is created by positively biasing the anode so that the positivelycharged ions may be accelerated (repelled) from the thruster, which produces thrust. However, plasma electrons are much smaller than ions and may be accelerated much more quickly toward the anode; if electrons were not impeded, a "short circuit" due to the electron flow would eliminate the thrust mechanism. Therefore, a magnetic field serves to "magnetize" plasma electrons internal to the thruster and confines them in gyro-orbits within the discharge channel. Without outside factors electrons would be confined indefinitely; however, electron-neutral collisions provide a mechanism to free electrons from their orbits allowing electrons to cross the magnetic field toward the anode, where this process is described by classical transport theory. To make matters worse, cross-field electron transport has been observed to be 100-1000 times that predicted by classical collisional theory, providing an efficiency loss mechanism and an obstacle for modeling and simulations in Hall thrusters. The main difficulty in studying electron transport in Hall thrusters is the coupling that exists between the plasma and the fields, where the plasma creates and yet is influenced by the electric field. A device has been constructed at MTU’s Isp Lab, the Hall Electron Mobility Gage, which was designed specifically to study electron transport in E × B devices, where the coupling between the plasma and electric field was virtually eliminated. In this device the two most cited contributors to electron transport in Hall thrusters, fluctuation-induced transport, and wall effects, were absent. Removing the dielectric walls and plasma fluctuations, while maintaining the field environment in vacuum, has allowed the study of electron dynamics in Hall thruster fields where the electrons behave as test particles in prescribed fields, greatly simplifying the environment. Therefore, it was possible to observe any effects on transport not linked to the cited mechanisms, and it was possible to observe trends of the enhanced mobility with control parameters of electric and magnetic fields and neutral density– parameters that are not independently variable in a Hall thruster. The result of the investigation was the observation of electron transport that was ~ 20-100 times the classical prediction. The cross-field electron transport in the Mobility Gage was generally lower than that found in a Hall thruster so these findings do not negate the possibility of fluctuations and/or wall collisions contributing to transport in a Hall thruster. However, this research led to the observation of enhanced cross-field transport that had not been previously isolated in Hall thruster fields, which is not reliant on momentum-transfer collisions, wall collisions or fluctuations.

Mobility and bandwidth prediction in virtualized LTE systems: Architecture and challenges

Long Term Evolution (LTE) represents the fourth generation (4G) technology which is capable of providing high data rates as well as support of high speed mobility. The EU FP7 Mobile Cloud Networking (MCN) project integrates the use of cloud computing concepts in LTE mobile networks in order to increase LTE's performance. In this way a shared distributed virtualized LTE mobile network is built that can optimize the utilization of virtualized computing, storage and network resources and minimize communication delays. Two important features that can be used in such a virtualized system to improve its performance are the user mobility and bandwidth prediction. This paper introduces the architecture and challenges that are associated with user mobility and bandwidth prediction approaches in virtualized LTE systems.

Fabrication of luminescent nanostructures by electron-beam direct writing of PMMA resist

We report on the conversion of non-luminescent conventional poly(methylmethacrylate) (PMMA)-based electron-beam resists into luminescent materials when used as negative-tone resists, that is, when exposed to high electron irradiation doses. Raman spectroscopy reveals the chemical transformation induced by electron irradiation which is responsible for the observed luminescence in the visible (blue) region. The emission intensity from exposed PMMA-based patterns can be controlled by the electron irradiation dose employed to create them.