L'objectif de ce travail porte sur la généralisation de la modélisation de la gravure du silicium dans les plasmas de fluor ou de chlore à celle de la gravure des éléments de la colonne IV et des composés III-V de structure cristalline de type diamant ou zinc-blende dans les plasmas d'halogènes, i.e. fluor, chlore, brome et iode. Dans ce contexte, les effets stériques et de diffusion en volume et/ou en surface en constituent les problématiques principales. Cette généralisation s'appuie sur le modèle de gravure de Petit et Pelletier qui, par rapport aux modèles antérieurs, prend en compte un certain nombre d'hypothèses distinctes ou additionnelles telles que les interactions répulsives entre adatomes d'halogènes proches voisins, les mécanismes de Langmuir-Hinshelwood pour la formation des produits de réaction, la nature mono-couche ou multi-couches de l'adsorption, et la diffusion des adatomes en surface. Les effets stériques relatifs à la diffusion des atomes d'halogènes à travers les surfaces (100) des structures cristallines des éléments de la colonne IV et des composés III-V définissent une première loi de similitude entre la maille du réseau cristallin et le rayon ionique de Shannon des atomes d'halogènes concernant leurs conditions de diffusion en volume. Cette loi se traduit par un diagramme prévisionnel, commun aux éléments de la colonne IV et aux composés III-V, délimitant les systèmes de gravure de types mono-couche et multi-couches. Les effets stériques relatifs aux mécanismes réactionnels de gravure sur les surfaces (100) aboutissent à des secondes lois de similitude entre la maille du réseau et le rayon covalent des adatomes d'halogènes caractérisant la nature de la gravure : gravure isotrope, gravure anisotrope, ou absence de gravure. Ces lois de similitude, distinctes pour les éléments de la colonne IV et les composés III-V (stœchiométrie différente des produits de réaction), se traduisent par deux diagrammes prévisionnels délimitant les différents domaines de gravure. Les diagrammes prévisionnels pour les éléments de la colonne IV ont pu être validés, d'une part, à partir des résultats expérimentaux antérieurs, et, d'autre part, en l'absence de données, à partir d'études expérimentales complémentaires : gravure de Si et Ge en plasma de brome et d'iode, gravure de Sn en plasma d'iode.

Ce travail porte sur la conception, l’épitaxie, et la caractérisation structural et optique de deux types de nanostructures, à savoir des boîtes quantiques AlGaN/AIN et des nanodisques AlGaN/AIN sur nanofils GaN. Ces nanostructures ont été synthetisées par épitaxie par jets moléculaires assistée par plasma (PA-MBE) et ont été conçues pour être le matériau actif d’une lampe ultraviolette à pompage électronique (EPUV) pour la purification de l'eau. En modifiant la composition Al et la géométrie des boîtes quantiques AlGaN/AlN, leur longueur d'onde d'émission peut être réglée dans la gamme 320-235 nm tout en gardant une grande efficacité quantique interne (> 35%). Le confinement quantique a été confirmé par la stabilité de l'intensité et du temps de déclin de la photoluminescence avec la température. La quantité optimale d’AlGaN dans les boîtes pour obtenir une luminescence maximale à la température ambiante est un compromis entre densité de boîtes quantiques et rendement quantique interne. L'effet de la variation du rapport hauteur/diamètre de base sur les transitions interbande et intrabande dans les boîtes a été explorée par ajustement des données expérimentales à des calculs tridimensionnels du diagramme de bande et des niveaux quantiques. En ce qui concerne les nanodisques d’AlGaN sur nanofils GaN, l'interdiffusion Al-Ga aux interfaces et l'hétérogénéité de l'alliage ternaire sont attribuées aux processus de relaxation des contraintes. Cette interprétation a été prouvée par la corrélation des données expérimentales avec des calculs de distribution déformation en trois dimensions effectuées sur des structures qui imitent la séquence de croissance réelle. Malgré le défi du manque d'homogénéité, la longueur d'onde d'émission des nanodisques AlGaN/AIN peut être réglée dans la gamme ultraviolette en préservant une haute efficacité quantique interne. Un prototype de lampe EPUV a été fabriqué en utilisant une région active à base de boîtes quantiques AlGaN/AIN avec les valeurs optimals d'épaisseur de la région active, d'épaisseur de la barrière AlN, et de quantité d’AlGaN dans chaque couche de boîtes. Pour ce premier dispositif, le SiC a été utilisé comme substrat pour éviter les problèmes associés à l’évacuation de charge ou de chaleur. Un essai de purification de l'eau par une telle lampe a été réalisé. Tous les échantillons ont été purifiés avec succès à la dose prévue. / This work reports on the design, epitaxial growth, and the structural, and optical characterization of two types of nanostructures, namely AlGaN/AlN Stranski-Krastanov quantum dots (SK-QD) and AlGaN/AlN nanodisks (NDs) on GaN nanowires (NWs). These nanostructures were grown using plasma-assisted molecular beam epitaxy (PA-MBE) and were conceived to be the active media of electron-pumped ultraviolet (EPUV) emitters for water purification, operating in mid-ultraviolet range. The peak emission wavelength of three-dimensional SK-QD can be tuned in mid-ultraviolet range while keeping high internal quantum efficiency (IQE > 35%) by modifying the Al composition and the QD geometry. The efficient carrier confinement was confirmed by the stability of the photoluminescence intensity and decay time with temperature. The optimal deposited amount of AlGaN in AlGaN/AlN QDs which grants maximum luminescence at room temperature was determined by finding a compromise between the designs providing maximum IQE and maximum QD density. The effect of the variation of the QD height/base-diameter ratio on the interband and intraband optical properties was explored by fitting the experimental data with three-dimensional calculations of the band diagram and quantum levels. Regarding AlGaN/AlN NDs on GaN NWs, the Al-Ga intermixing at Al(Ga)N/GaN interfaces and the alloy inhomogeneity in AlGaN/AlN NDs are attributed to the strain relaxation process. This interpretation was proved by correlation of experimental data with three-dimensional strain distribution calculations performed on structures that imitate the real growth sequence. Despite the challenge of inhomogeneity, the emission wavelength of AlGaN/AlN NDs can be tuned in mid-ultraviolet range while preserving high IQE by adjusting the ND thickness and Al content. A prototype of EPUV emitter was fabricated using the AlGaN/AlN SK-QDs active region with proposed optimal design of active region thickness, AlN barrier thickness, and amount of AlGaN in each QD layer. For this first device, SiC was used as a substrate to prevent problems associated to charge or heat evacuation. A water purification test by such prototype EPUV emitter was carried out by irradiating E-coli bacteria, showing that all the specimens were successfully purified at the predicted ultraviolet dose.

abstract: This dissertation addresses challenges pertaining to multi-junction (MJ) solar cells from material development to device design and characterization. Firstly, among the various methods to improve the energy conversion efficiency of MJ solar cells using, a novel approach proposed recently is to use II-VI (MgZnCd)(SeTe) and III-V (AlGaIn)(AsSb) semiconductors lattice-matched on GaSb or InAs substrates for current-matched subcells with minimal defect densities. CdSe/CdTe superlattices are proposed as a potential candidate for a subcell in the MJ solar cell designs using this material system, and therefore the material properties of the superlattices are studied. The high structural qualities of the superlattices are obtained from high resolution X-ray diffraction measurements and cross-sectional transmission electron microscopy images. The effective bandgap energies of the superlattices obtained from the photoluminescence (PL) measurements vary with the layer thicknesses, and are smaller than the bandgap energies of either the constituent material. Furthermore, The PL peak position measured at the steady state exhibits a blue shift that increases with the excess carrier concentration. These results confirm a strong type-II band edge alignment between CdSe and CdTe. The valence band offset between unstrained CdSe and CdTe is determined as 0.63 eV±0.06 eV by fitting the measured PL peak positions using the Kronig-Penney model. The blue shift in PL peak position is found to be primarily caused by the band bending effect based on self-consistent solutions of the Schrödinger and Poisson equations. Secondly, the design of the contact grid layout is studied to maximize the power output and energy conversion efficiency for concentrator solar cells. Because the conventional minimum power loss method used for the contact design is not accurate in determining the series resistance loss, a method of using a distributed series resistance model to maximize the power output is proposed for the contact design. It is found that the junction recombination loss in addition to the series resistance loss and shadowing loss can significantly affect the contact layout. The optimal finger spacing and maximum efficiency calculated by the two methods are close, and the differences are dependent on the series resistance and saturation currents of solar cells. Lastly, the accurate measurements of external quantum efficiency (EQE) are important for the design and development of MJ solar cells. However, the electrical and optical couplings between the subcells have caused EQE measurement artifacts. In order to interpret the measurement artifacts, DC and small signal models are built for the bias condition and the scan of chopped monochromatic light in the EQE measurements. Characterization methods are developed for the device parameters used in the models. The EQE measurement artifacts are found to be caused by the shunt and luminescence coupling effects, and can be minimized using proper voltage and light biases. Novel measurement methods using a pulse voltage bias or a pulse light bias are invented to eliminate the EQE measurement artifacts. These measurement methods are nondestructive and easy to implement. The pulse voltage bias or pulse light bias is superimposed on the conventional DC voltage and light biases, in order to control the operating points of the subcells and counterbalance the effects of shunt and luminescence coupling. The methods are demonstrated for the first time to effectively eliminate the measurement artifacts. / Dissertation/Thesis / Ph.D. Electrical Engineering 2012

Cette thèse se situe dans le cadre de l'électronique moléculaire qui vise à réaliser une unité de calcul constituée d'une molécule connectée à des électrodes mésoscopiques. La première étape est de choisir une surface qui soit isolante, afin de découpler les états électroniques de la molécule de ceux du substrat et sur laquelle il soit possible de faire croître des îlots métalliques " 2D ", permettant la connexion de la molécule à un réservoir d'électrons, tout en ayant la possibilité de l'imager en NC-AFM. Notre choix s'est porté sur le nitrure d'aluminium (AlN), en raison de sa grande énergie de bande interdite (6,2 eV) et de sa similarité avec le nitrure de gallium (GaN, 3,4 eV) sur lequel il est possible de faire croitre des îlots 2D de magnésium. Le travail de cette thèse porte sur la croissance par épitaxie par jets moléculaire de films minces d'AlN sur substrats de silicium (Si(111)) et de carbure de silicium (SiC(0001)) et leur étude in-situ par NC-AFM et KPFM sous ultra vide. Les études NC-AFM ont permis d'adapter les protocoles de croissance afin de diminuer significativement les défauts de surface des films d'AlN. Des calculs théoriques DFT ont aidé à adapter ces protocoles de croissance afin d'obtenir de façon reproductible la reconstruction de surface (2x2) de l'AlN pour laquelle la surface est terminée par des adatomes d'azote. A l'issu de cette thèse, les films d'AlN obtenus présentent des surfaces adaptées au dépôt de molécules et d'îlots métalliques. / This thesis is part of molecular electronics, which aims to realize a calculation unit based on a single molecule connected to mesoscopic electrodes. The first step is to find a suitable surface, i.e. an insulating or large gap semi-conductor surface to decouple the electronic states of the molecule from the electronic states of the substrate. It must also be compatible with the growth of flat metallic nano-pads allowing the connection of the molecule to an electron tank, while having the possibility of imaging it in NC-AFM. Our choice was focused on the large gap semi-conductor Aluminum Nitride (AlN, 6.2 eV). Indeed it has been shown that the growth of magnesium on a similar substrate (GaN, 3.4 eV) yields one mono-layer high islands. The present work is focused on the growth by molecular beam epitaxy of AlN thin layers on silicon (Si(111)) and silicon carbide (SiC(0001)) substrates and in-situ study by NC-AFM and KPFM under ultrahigh vacuum. The NC-AFM studies helped to adapt the growth protocols in order to significantly reduce the surface defects of the AlN films. Theoretical calculations (DFT) helped to adapt these growth protocols which allows to reproducibly obtain the (2x2) surface reconstruction for which the surface is terminated by a layer of N atoms. At the end of this thesis, the AlN films obtained present suitable surfaces for depositing metallic electrodes and molecules.

<div>With the transistor dimensions scaling down to a few atoms, quantum phenomena - like quantum tunneling and entanglement - will dictate the operation and performance of the next generation of electronic devices, post-CMOS era. While quantum tunneling limits the scaling of the conventional transistor, Tunneling Field Effect Transistor (TFET) employs band-to-band tunneling for the device operation. This mechanism can reduce the sub-threshold swing (S.S.) beyond the Boltzmann's limit, which is fundamentally limited to 60 mV/dec in a conventional Si-based metal-oxide-semiconductor field-effect transistor (MOSFET). A smaller S.S. ensures TFET operation at a lower supply voltage and, therefore, at lesser power compared to the conventional Si-based MOSFET.</div><div><br></div><div>However, the low transmission probability of the band-to-band tunneling mechanism limits the ON-current of a TFET. This can be improved by reducing the body thickness of the devices i.e., using 2-Dimensional (2D) materials or by utilizing heterojunction designs. In this thesis, two promising methods are proposed to increase the ON-current; one for the 2D material TFETs, and another for the III-V heterojunction TFETs.</div><div><br></div><div>Maximizing the ON-current in a 2D material TFET by determining an optimum channel thickness, using compact models, is presented. A compact model is derived from rigorous atomistic quantum transport simulations. A new doping profile is proposed for the III-V triple heterojunction TFET to achieve a high ON-current. The optimized ON-current is 325 uA/um at a supply voltage of 0.3 V. The device design is optimized by atomistic quantum transport simulations for a body thickness of 12 nm, which is experimentally feasible.</div><div> </div><div>However, increasing the device's body thickness increases the atomistic quantum transport simulation time. The simulation of a device with a body thickness of over 12 nm is computationally intensive. Therefore, approximate methods like the mode-space approach are employed to reduce the simulation time. In this thesis, the development of the mode-space approximation in modeling the triple heterojunction TFET is also documented.</div><div><br></div><div>In addition to the TFETs, quantum computing is an emerging field that utilizes quantum phenomena to facilitate information processing. An extra chapter is devoted to the electronic structure calculations of the Si:P delta-doped layer, using the empirical tight-binding method. The calculations agree with angle-resolved photoemission spectroscopy (ARPES) measurements. The Si:P delta-doped layer is extensively used as contacts in the Phosphorus donor-based quantum computing systems. Understanding its electronic structure paves the way towards the scaling of Phosphorus donor-based quantum computing devices in the future.</div>

In der Arbeit wurden sowohl optische, als auch Schwingungs- und Struktur-Eigenschaften poröser Strukturen auf der Basis von n-GaP, n-GaAs und n-InP mit Hilfe der Raman- und FTIR- Spektroskopie untersucht. Speziell den in polaren Materialien auftretenden Fröhlich-Moden wurde dabei besondere Aufmerksamkeit gewidmet. Es wurden experimentelle Ergebnisse theoretischen Modellen auf der Basis der Effektiven Medien Theorie gegenübergestellt. Die Ergebnisse sollen helfen, Grundkenntnisse über die Eigenschaften poröser Halbleiterstrukturen zu gewinnen, um zum einen Anwendungsmöglichkeiten zu erschließen und zum anderen das theoretische Modell zur Beschreibung heterogener Medien zu vervollkommnen.

The vertical-cavity surface-emitting laser or VCSEL is a special type of diode laser, which has established itself in optoelectronic applications asa low-cost, high-quality miniaturized light source. The development of VCSELs can be largely promoted with support from computer simulations. In this study, we have used such simulations, on one hand to understand and improve the VCSEL performance, and on the other hand to prepare for analyzing new device concepts such as transistor-VCSELs. This thesis starts with a background introduction to the principle idea of VCSELs and then states the significance of this simulation work.Then it briefly introduces the previously used simulation workbench Sentaurus and explains the mathematical approach and the computation methods of the finally chosen simulator PICS3D. The case study of a fabricated and characterized epitaxially regrown VCSEL is the major component of this work. First the device configuration is demonstrated with detailed discussion on several design features. Second the physical models of electrical, optical and thermal phenomena along with their key parameters are presented and so are the advanced models for the active region. The main results of simulation, including steady-state characteristics and small-signal modulation, show good agreement with the experimental results and reveal some imperfections of the device design and processing, such as the overestimated stability of the regrown junction and the variation of cavity length caused by over-etch. This work is also treated as an evaluation of the simulator PICS3D, and two problems are identified: one is the troublesome way to construct a 3D device by coupling several 2D layer structures together, requiring the mesh for each layer structure to be compatible; the other would be the tricky boundary setting for the adopted method, Effective Index Method (EIM), for the transverse field calculation when only a weak index guiding effect exits in the cavity. Finally, we summarize this work and suggest some tasks for further simulations.

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