Дисертації з теми "ESD physics"

Title from PDF of title page.
Document formatted into pages; contains 43 pages.
Thesis (M.S.)--University of South Florida, 2003.
Includes bibliographical references.
Text (Electronic thesis) in PDF format.
ABSTRACT: Skutterudites have been of great interest for thermoelectric applications over the last ten years. Scientific interest has focused on the unique transport properties Skutterudites possess due to the unique crystal structure. Technical interest has grown since it was discovered that some compounds rival the current best thermoelectric materials. To further the understanding of this material system, and optimize its thermoelectric properties, the synthesis and characterization of polycrystalline n- and p-type CoGe&esc;b1&esc;s&&dotb;esc;b5&esc;sSe&esc;b1&esc;s&&dotb;esc;b5&esc;s was undertaken. Structural, morphological, chemical, electrical, thermal and magnetic properties were studied. These data are compared to those of the binary Skutterudite CoSb3. The results of this study show a very sensitive dependence of the physical properties on stoichiometry.
ABSTRACT: While the thermoelectric figure of merit is low in these materials, it is apparent that optimization via doping and "void filling" will lead to improved thermoelectric properties.
System requirements: World Wide Web browser and PDF reader.
Mode of access: World Wide Web.

Superconducting radio frequency (SRF) technology is widely adopted in particle accelerators. The shallow penetration (∼ 40 nm) of the RF into superconducting niobium lends great importance to SRF cavity interior surface chemistry and topography. These in turn are strongly influenced by the chemical etching "surface clean-up" that follows fabrication.;The principal surface smoothing methods are buffered chemical polish (BCP) and electropolish (EP). The resulting topography is characterized by atomic force microscopy (AFM). The power spectral density (PSD) of AFM data provides a more thorough description of the topography than a single-value roughness measurement. In this work, one dimensional average PSD functions derived from topography of BCP and EP with different controlled starting conditions and durations have been fitted with a combination of power law, K-correlation, and shifted Gaussian models to extract characteristic parameters at different spatial harmonic scales. While the simplest characterizations of these data are not new, the systematic tracking of scale-specific roughness as a function of processing is new and offers feedback for tighter process prescriptions more knowledgably targeted at beneficial niobium topography for SRF applications.;Process development suffers because the cavity interior surface cannot be viewed directly without cutting out pieces, rendering the cavities unavailable for further study. Here we explore replica techniques as an alternative, providing imprints of cavity internal surface that can be readily examined. A second matter is the topography measurement technique used. Atomic force microscopy (AFM) has proven successful, but too time intensive for routine use. We therefore introduce white light interferometry (WLI) approach as an alternative. We examined real surfaces and their replicas, using AFM and WLI. We find that the replica/WLI is promising to provide the large majority of desired information, so that use of the time-intensive AFM approach can be limited to where it is genuinely necessary.;The prevalent idea is that sharp features could lead to magnetic quench or enhance the thermal quench. In this report, a calculation on magnetic field is numerically given on fine structure by finite element and conformal mapping methods. Corresponding RF Ohmic loss will be simulated. A certain thermal tolerant will be calculated. A Q∼E curve will be predicted from this model.;A perturbation model is utilized to calculate rough surface additional RF loss based on PSD statistical analysis. This model will not consider that superconductor will become iormal at field higher than transition field. Therefore, it is only expected to explain midfield Q performance. One can calculate the RF power dissipation ratio between rough surface and ideal smooth surface within this field range. Additionally, the resistivity of Nb is temperature and magnetic field dependent from classic thermal feedback model theory. Combining with topographic PSD analysis and Rs temperature and field ependency, a middle field Q slope model could be modeled and the contribution from topography can be simulated.

Ce mémoire de thèse porte sur l'étude de la réactivité induite par les électrons de basse énergie (0-20 eV) dans les systèmes condensés. Ces électrons induisent dans ces systèmes la formation de fragments réactifs pouvant ensuite initier des réactions soit avec le substrat : c'est la fonctionnalisation de la surface, soit au sein même du film, ce qui conduit à la synthèse de nouvelles molécules. La première partie traite de la caractérisation et de la fonctionnalisation par des groupements organiques de la surface de diamant hydrogéné polycristallin. Il est en particulier montré que les électrons peuvent induire la chimisorption de groupements (CH2CN) après l'irradiation de deux monocouches d'acétonitrile CH3CN condensées sur le diamant, suivant une réaction, résonante à 2 eV pour laquelle un mécanisme est proposé. Cette étude ouvre ainsi la voie à l'utilisation des électrons de basse énergie comme vecteur de la fonctionnalisation contrôlée des surfaces. La deuxième partie concerne l'étude de la synthèse induite dans les glaces à basse température. Il est montré la formation de CO2 à partir de différents acides carboxyliques, ainsi que la synthèse de molécules plus complexes, comme l'acide aminé glycine à partir du film mixte CH3COOH:NH3 ou l'acide carbamique à partir du mélange CO2:NH3. Dans ces deux cas, un mécanisme de réaction est proposé. Cette étude montre que les électrons de basse énergie, présents en grand nombre en tant qu'électrons secondaires dans tout système condensé sous rayonnement ionisant (glaces interstellaires ou stratosphériques) peuvent jouer un rôle dans la chimie de ces milieux, en particulier dans la chimie prébiotique du milieu interstellaire.

The differential cross-section and polarization observables in the process γ + p --> K++ Λ are studied within an isobaric approach that includes resonances with total angular momentum J ≤ 5/2 over a center of mass energy range from W = 1.6 GeV to W=2.6 GeV. The model is used to fit recent experimental data as a function of the coupling products at the photon and strong vertices for the well established low energy resonances, as well as the total decay width for the high energy less well-established resonances. The model employed in this study is based on an effective hadronic lagrangian using a tree-level approximation. The model uses Feynman diagrammatic techniques to extract the interaction vertices at a first order level in perturbation theory. To extract the coupling strength products involved in the reaction, a X2- minimization technique is used to fit experimental data. The results suggests that both differential cross-section and double polarization observables need to be fit simultaneously to obtain an accurate description of the data. In addition, it was found that while resonances with angular momentum J= 5/2 do not couple strongly to the KΛ channel, higher energy states with J = 3/2 do couple strongly to the KΛ channel and are highly relevant for an accurate description of the data at energies beyond 1.9 GeV.

For three-dimensional compact lattice quantum electrodynamics, the meson-meson energy spectrum is obtained from a truncated time-correlation matrix of field operators. The energy levels indicate a slightly attractive residual interaction between the mesons. From the finite-volume spectrum, the scattering phase shifts can be calculated for the various irreducible representations of the lattice symmetry group. The s-wave phase shifts indicate short-range repulsion, while the d-wave data indicate intermediate-range attraction. This work serves as a model for describing the strong nuclear force from basic principles.

Experiments were conducted using the Time of Flight (TOF) method to identify the final product states of the dissociative recombination reaction of krypton and xenon. In the dissociative recombination (DR) reaction the molecular ion breaks up into product atoms whose velocities can be measured. These velocities can then be used to identify the final product states. The DR of krypton had been studied by Shiu and Biondi using spectrometric techniques. They observed the 5p states. Hardy et al. using TOF techniques had observed the 5s states. Mitchell et al. studied the DR of xenon. They observed the 6p and 5d states of xenon. In this laboratory using the TOF method I have recently identified the 5s, 6p and the 4d final states of the DR of krypton. Then I was able to identify the 5d, 7s, 6d, and 6p' final product states of vi the DR of xenon. The study of the DR of these heavy inert gases can shed light on the theory of the DR of heavy polyatomic gases, which is not well developed.

Metallic nanowires have useful applications in scanning tunneling microscopes and atomic force microscopes due to their unique sensitivity to force and electricity. These unique properties arise because of the large surface area to volume ratio. One of these properties is that introducing twinning planes the mechanical properties of metallic nanowires can be altered. The effects of twinning planes on metallic nanowires were studied using molecular dynamics simulations. Silver, copper, and nickel nanowires with and without twinning planes were simulated with engineering strain until the first yielding stress was obtained. The radial simulations showed that as the radius of twinned nanowires increased, the strength gained by introducing the twinning planes increased. The temperature simulations showed that nanowires with twinning planes were stronger than their un-twinned counterparts as temperature increased. The purpose of this investigation was to better understand the effect twinning planes had on metallic nanowires, so that future technological advances would benefit from the results.

Gallium nitride is an important III-V semiconductor which is used in many optoelectronic and high-frequency devices. The nature of the GaN surface and its electrical characteristics can impact the performance of such devices. In this study, several GaN surfaces are locally charged using an atomic force microscope, and then subsequently studied by measuring the surface potential with scanning Kelvin probe microscopy (SKPM). The charging and discharging behavior of the surface appears to be strongly influenced by surface preparation and the presence of a surface oxide layer. If a substantial oxide layer exists, then both positive and negative charging is possible on n-type and p-type samples. Surface treatments and photoemission spectroscopy (XPS) data confirm the presence and influence of the oxide layer on surface charging behavior. In the case of forward-bias charging, a small change in surface contact potential (0.1 – 0.3 eV) is observed that is primarily due to a small voltage drop across the surface oxide. Reverse-bias charging produces a substantially larger change in surface potential (~1 – 3 eV) that must be explained by a large increase in surface band bending. Temperature-dependent SKPM measurements also indicate that the decay behavior of deposited surface charge in dark involves a thermionic mechanism.

Immunoglobulin E (IgE)-conjugated gold nanoparticles were produced via in situ conjugation of gold nanoparticles with immunoglobulin E by laser ablation of Au in a liquid solution. The colloidal stability and the size distribution of the resulting bio-nanoconjugates were examined with UV-Visible spectroscopy (UV-Vis), dynamic light scattering (DLS), and transmission electron microscopy (TEM). These techniques showed that the Au nanoparticles in aqueous solutions were highly monodispersed spherical particles with a very narrow size distribution. The particles net diameter using TEM, was found to be D5 =3.8±0.9nm and D10 =4.7±1.3nm while the hydrodynamic diameter obtained with DLS was found to be h5 D =171±12nm , h10 D =164±18nm for 5min and 10min laser ablation time respectively. Enzyme-Linked immunosorbent Assay (ELISA) and flow cytometry measurements of the conjugates confirmed that the gold-bound protein remained biologically active, thus paving the way for the application of these nanoparticles in immuno-diagnostics, particularly in tumor-targeted drug delivery.

First principles calculations, using the density-functional theory and particularly the local density approximation (LDA), have achieved remarkable success in studying the properties of solid state systems. Although the basic results of these calculations are the electronic structures (eigenvalues, eigenfunctions, etc.) and the total energy of ground state, many other related physical properties can be deduced from them by investigating their response under external perturbations. Using the linear response method with linearized-augmented-plane-wave (LAPW) basis, we have calculated lattice dynamical properties of important semiconductors CuCl, SiC and ferroelectric KNbO{dollar}\sb3.{dollar} CuCl is known to exhibit large anharmonic effects and possibly a complicated multi-well Born Oppenheimer surface reminiscent of the instabilities in perovskite ferroelectrics and the high-temperature cuprate superconductors. However, we have determined its phonon dispersion from first-principles calculations and find it to be in good agreement with the low temperature experimental results. For zincblende SiC, the calculated phonon dispersions, Grunensen's parameters, dielectric constant, Born effective charge, elastic constants, and the equation of state agree very well with the available experimental data. Additionally, we find that its dielectric constant decreases with pressure and Born effective charge increases with pressure up to 80 GPa without any saturation, calling into question the recent interpretation of experimental results to the contrary. Our calculations for KNbO{dollar}\sb3{dollar} find unusually large Born effective charges on Nb and O that originate from the strong covalent interactions between Nb 4d and O 2p orbitals. The Born effective charges and dielectric tensors are found to change up to 20% from the cubic structure to the rhombohedral structure, demonstrating that the polarizability of the perovskite ferroelectrics is very sensitive to the small atomic displacements involved in the transitions. The phonon modes for the cubic and rhombohedral structures are also calculated and their implications are discussed.

Le ricerche e le scoperte fatte nell'ambito della fisica sono fortemente dipendenti dall'efficienza e dall'affidabilità degli esperimenti ad alta energia. L'obiettivo principale è lo studio delle particelle che costituiscono la materia in termini di cariche elementari, loro interazioni e prodotti secondari che ne possono derivare. L'LHC (Large Hadron Collider) lavora al CERN ogni giorno con l’obiettivo di scoprire nuovi dettagli su particelle cariche come neutroni e Bosoni di Higgs. Queste sono generate e accelerate all’interno dell’LHC e vengono rilevate da opportuni detector organizzati in una struttura a shell. In questo modo, è possibile avere una caratterizzazione in termini di momento, carica elettrica, energia, tempo di volo e distanza associati alla particella rilevata. La progettazione dei rilevatori è importante come anche quella dell’elettronica vicina. Un grande esperimento richiede un duro lavoro di scienziati e ingegneri. Negli ultimi anni, l’elettronica è sempre più efficiente e compatta grazie alla sostituzione dei componenti discreti con circuiti integrati CMOS. La progettazione deve essere però fatta considerando sia le reti di interfacciamento con i sensori sia l’ambiente radiattivo circostante. Le radiazioni, infatti, possono modificare parzialmente o totalmente le performance e la scelta della tecnologia scalata può però essere di grande aiuto. In questo scenario, sono stati progettati, integrati e misurati 3 circuiti di lettura per esperimenti di fisica delle alte energie. 2 prototipi sono stati realizzati in tecnologia 130nm per l'esperimento ATLAS in collaborazione dell’Istituto Max-Plank di Monaco. Questi prototipi sono pensati per rilevare cariche fino a 100fC e convertirle in un segnale di tensione di ampiezza variabile che sarà processato in digitale per avere informazioni sull’istante di arrivo della carica e sulla sua intensità. A tal fine, gli integrati hanno uno stadio di discriminazione ed un Wilkinson ADC in grado di convertire in un tempo il segnale in tensione ricevuto. Il secondo prototipo è molto simile al primo. Esso è stato migliorato principalmente per poter essere più immune ai disturbi provenienti da masse e alimentazioni. Il terzo circuito presentato in questa tesi è un sistema di lettura progettato per Pixel detectors in tecnologia CMOS 28nm. Il canale integrato include un preamplificatore di carica con un comparatore in cascata. L'utilizzo della tecnologia 28nm con la sua ridotta alimentazione comporta una serie di difficoltà nella progettazione ma anche una maggiore resistenza alle radiazioni, consumi ridotti e una minor area occupata. I circuiti sono stati progettati per due differenti scenari in termini di capacità parassita del rilevatore, cariche di ingresso rilevabili, alimentazioni, soglie, consumi di potenza e rumore. In tutti i casi, però, i sistemi sono in grado di fornire le informazioni sulla carica rilevata in tempi relativamente rapidi (entro 25ns). Questo aspetto è molto importante e permette di evitare errori. Collisioni successive potrebbe causare segnali spuri e si potrebbe rilevare come unico evento due eventi distinti e consecutivi. Questo lavoro è organizzato come segue. La Parte I include una breve introduzione sull'intera attività svolta nei tre anni di attività di ricerca. La Parte II è dedicata alla descrizione semplificata del campo di applicazione ed ai target previsti per i prossimi esperimenti di fisica. In particolare, sono forniti alcuni dettagli su come l'elettronica può essere influenzata dalla presenza delle radiazioni. Le parti III e IV rappresentano il core della tesi perché mirano all'analisi dettagliata dei circuiti progettati e descritti precedentemente in maniera generica. L'analisi prevede una caratterizzazione completa degli integrati con simulazioni e misure. Infine, prima di concludere, la Parte V è dedicata alla pubblicazioni correlate all'attività di ricerca.
Physic researches and discoveries depend heavily from efficient and reliability of the High-Energy Physics (HEP) experiments. The main goal is to study the fundamental constituents of the matter in terms of elementary charge particles, their interactions and their secondary products. The Large Hadron Collider (LHC) at the CERN works every day to discover details on new charged particles as neutrinos and Higgs Bosons. Charges are generated and accelerated from beam collisions inside the LHC. Different detectors are organized in shell structures and are designed to detect few particles topology. Typically, the parameters useful to identify a charged particle are momentum, electrical charge, energy, time of flight and distance. Detectors design is important but it is enhanced from proper electronic readout systems. In the last years, electronics parts are more and more efficient and compact. CMOS integrated solution are preferred to discrete one allowing major reliability, cost reduction and performance improvement. The design is not trivial but not impossible. Some characteristics depend on the electronic designer and his capability to manage the external parasitic effects, as the parasitic capacitance of the connected detector. Unfortunately, phenomena as radiation effects on electronics must be taken in account but they are not completely eliminated. CMOS technology influences strongly the integrated circuit performance and radiation hardness. In this scenario, 3 readout frontend circuits for HEP experiments have been designed, integrated and measured. 2 of them represent 2 different prototypes realized in IBM 130nm technology for ATLAS experiment at CERN laboratory with Max-Plank Institute for Physics collaboration. They include an analog chain in cascade with a digital one. Input charges (up to 100fC) are detected and converted into voltage signals. Their amplitude are proportional to the input and are sent to the following digital part. The digital part provides information about arrival time and amount of the input charge. When the discriminator switches, an event is detected and the Wilkinson ADC starts the voltage-to-time conversion. The full chips have a JTAG section to manage all programmable parameters (i.e. thresholds, hysteresis, deadtime, etc.) The second prototype is designed improving the previous version in terms of supply rejection noise, deadtime range and hysteresis management. The third circuit presented in this thesis is the first readout frontend for Pixel detectors in 28nm technology. The channel includes a charge sensitive preamplifier with an inverter switched based comparator. Reduced supply voltage and 28nm technology imply some difficult in the design with a major tolerance to the radiations, a lower area occupancy and a lower power consumption. The circuits are been designed for 2 different scenarios in terms of detector parasitic capacitance, detectable input charges, supply voltage, threshold voltage, power consumption and noise. In overall cases, the integrated systems provide information about amount of detected input charge and arrival time within 25ns. This aspect is very important and allows avoiding mistakes. Successive collisions lead to spurious signals presence and a single detection could have information about two different events. Maintaining the processing time within 25ns, consecutive collisions are detected as different events. This work is organized as follows. Part I includes a brief summary of the entire work in order to fix the goals of my activities. Then, the Part II is dedicated on a simplified description of the application field and the next target of the future experiments. In particular, some details on the effects induced by the radiation to integrated electronic component are provided. Part III and Part IV represent the core, including 3 readout frontend circuits design and measurements. Finally, there are correlated publications and conclusions.

A search for supersymmetry is presented based on proton-proton collision events containing identified hadronically decaying top quarks (i.e., events with no identified leptons), and an imbalance EmissT in transverse momentum. The data were collected with the Compact Muon Solenoid (CMS) detector at the CERN Large Hadron Collider (LHC) at a center-of-mass energy of 13 TeV, and correspond to an integrated luminosity of 35.9 fb-1. The 84 exclusive search regions are defined in terms of the multiplicity of bottom quark jet and top quark candidates, the missing energy, the scalar sum of jet transverse momenta HT, and the transverse mass variable sensitive to the pair production of heavy particles, each of which decays into an invisible particle MT2. A novel and robust top quark reconstruction algorithm which is based on multivariate approach and is capable of identifying top quarks in the wide spectrum of top quark transverse momentum is developed. Methods in the modeling of events arising from quantum chromodynamics and electroweak boson production, which are major backgrounds in searches for new physics at the LHC, are also presented. No statistically significant excess of events is observed relative to the expectation from the standard model. Discussion of the non-excluded regions of the model parameter space is given. Lower limits on the masses of supersymmetric particles are determined at 95% confidence level in the context of simplified models with top quark production. For a model with direct top squark pair production followed by the decay of each top squark to a top quark and a neutralino, top squark masses up to 1020 GeV and neutralino masses up to 430 GeV are excluded. For a model with pair production of gluinos followed by the decay of each gluino to a top quark-antiquark pair and a neutralino, gluino masses up to 2040 GeV and neutralino masses up to 1150 GeV are excluded. These limits extend previous results obtained with 8 TeV data and 2.3 fb-1 13 TeV data.

For the sub-32 nm technological nodes, a high dielectric constant oxide (high-k) and a metal gate electrode have to be implemented in MOSFET devices. Hf-based oxides are identified as the best option for the high-k. Titanium and tantalum nitrides are the mûst prûmising metals. This work focuses on the characterization of oxygen and nitrogen interdiffusions in gate stacks elaborated in the Gate First approach, containing TiN or TaN metal gates on HfOz. We show that metal gate deposition using ALD or A VD induces nitrogen diffusion to hafnia and the pedestal silicon oxide. Due to their high thermal budget, the poly-Si deposition and the spike anneal at 1050°C for dopant activation both give rise to oxygen diffusion and increase the amount of nitrogen in the pedestal 8iOz. The amount of atoms that diffuse at each step increase with the metal thickness, which expIrons the EOT and mobility degradations observed for devices with thicker metal electrodes. Finally, studying agate stack where Hf Oz has been plasma nitrided enables to demonstrate that defects initiaUy present in the high-k layer behave like traps for species diffusing during the gate electrode elaboration
La miniaturisation des dispositifs MOSFET au-delà du nœud technologique 32 nm requiert l'utilisation d'un oxyde à forte permittivité (high-k), associé à une électrode de grille métallique. Côté high-k, les oxydes à base d'hafnium constituent la meilleure option. Côté métal, les nitrures de titane et de tantale sont pressentis. Ce travail porte sur la 1 caractérisation des interdiffusions dans des empilements à grille TiN et TaN sur HfOz, élaborés selon le schéma i d'intégration Gate First. Nous montrons que le dépôt du métal nitruré provoque une diffusion d'azote dans le EfOz i et le SiOz piédestal. Les forts budgets thermiques associés au dépôt du poly-Si et au recuit d'activation des dopants induisent une diffusion d'oxygène et amplifient la quantité d'azote dans le SiOz. La quantité d'atomes diffusant est plus importante pour un métal épais. Ceci explique la dégradation de l'EOT et de la mobilité des porteurs pour les 1 dispositifs comportant des grilles plus épaisses. Enfin, la nitruration du Hf Oz par plasma permet de montrer que les défauts dans le high-k agissent comme des pièges à atomes diffusant lors de l'élaboration du module de grille

Il trend esponenziale delle tecnologie CMOS previsto dalla legge di Moore è stato ampiamento dimostrato nel corso degli ultimi tre decenni. Si è osservato uno scaling costante, caratterizzato da dispositivi sempre più piccoli, per soddisfare le esigenze delle applicazioni digitali in termini di velocità, complessità, densità circuitale e basso consumo di potenza. Ogni nodo tecnologico è rappresentato dalla minima lunghezza ottenibile, che corrisponde alla lunghezza del canale del più piccolo transistor che si può integrare. Con l'arrivo delle tecnologie al di sotto dei 100nm, le performance dei circuiti digitali sono ulteriormente aumentate, a scapito dei progettisti analogici che si ritrovano ad affrontare nuove problematiche. Infatti, da una parte lo scaling tecnologico comporta dei vantaggi per i circuiti digitali: aumento della velocità, basso consumo di potenza, alto livello di integrazione. I circuiti analogici invece risentono negativamente dello scaling, a causa di un peggioramento del comportamento del transistor, soprattutto per le tecnologie ultra-scalate. In queste ultime infatti, effetti del secondo ordine, fino a prima del tutto trascurabili, diventano importanti e iniziano ad essere dominanti, influenzando il funzionamento e le performance dei dispositivi. Per esempio, basso guadagno intrinseco del MOS, swing ridotto, problemi di punto operativo e elevata variabilità dei parametri, sono solo alcune delle difficoltà derivanti dallo scaling. I progettisti analogici devono far fronte a questi problemi in diverse fasi della progettazione, sia circuitale che di layout. Nonostante ciò, la progettazione di circuiti analogici in tecnologie così scalate in molti casi è determinante. Per esempio, in molti sistemi mixed-signal, dove coesistono circuiti analogici e digitali e sono necessarie alte performance ad alta frequenza, l’uso di queste tecnologie anche per la parte analgica diventa una scelta obbligata. Oppure ci sono gli esperimenti di fisica ad alta energia, dove la scelta di tecnologie scalate è strategica. Infatti in queste applicazioni, i circuiti elettronici sono esposti ad alti livelli di radiazione con conseguente peggioramento delle performance e fenomeni di malfunzionamento. Dato che il danno da radiazione è proporzionale allo spessore dell'ossido, è evidente che per i dispositivi più piccoli, il danneggiamento è inferiore. In questa tesi, i trend e le principali problematiche derivanti dall'uso di tecnologie molto scalate sono analizzati nel primo capitolo, seguiti poi dalla presentazione di circuiti integrati in tecnologia CMOS 28nm. Il primo circuito presentato nel secondo capitolo è un Fast-Tracker front-end (FTfe) per la rilevazione di cariche. In particolare il sistema di read-out è stato implementato a partire dalle principali specifiche e soluzioni circuitali già usate per la rilevazione di muoni nell'esperimento ATLAS. Il front-end proposto è in grado di rilevare un evento e subito dopo resettare il sistema in maniera tale da rendere il FTfe già pronto per il prossimo evento, evitando lunghi tempi morti. Il secondo circuito, presentato nel terzo capitolo ed anch'esso integrato in tecnologia CMOS 28nm, è un amplificatore per strumentazione di tipo Chopper. Gli amplificatori per strumentazione sono elementi chiave nelle applicazioni per sensori, dove vengono usati per amplificare segnali tipicamente piccoli (dell'ordine dei mV) e a bassa frequenza. Per questo motivo risulta importante ridurre o addirittura eliminare l'offset e il rumore flicker in ingresso, segnali che si sovrappongono al segnale utile da rilevare ed introdotto dallo stesso circuito elettronico. L'amplificatore per strumentazione proposto usa una tecnica di modulazione, chiamata chopper, per ridurre i contributi di rumore flicker ed offset. Inoltre l'intero amplificatore è stato progettato per lavorare in regione di sottosoglia, dati i problemi dovuti alla tecnologia fortemente scalata.
The exponential trend of the complementary metal-oxide-semiconductor (CMOS) technologies predicted by Moores law has been successfully demonstrated over the last three decades. A constant downscaling of CMOS technologies with smaller and smaller device size has been developed, in order to comply with requirements on speed, complexity, circuit density and power consumption of advanced high performance digital applications. The minimum reachable length, which corresponds to the half the length of the channel of the smallest transistor that can be manufactured, represents every following technological node. With the arrival of nanoscale (sub-100nm) CMOS technologies, digital performance improve further, but many new challenges have been introduced for analog designers. In fact, for the digital circuits CMOS scaling-down leads to several benefits: speed improvement, reduced power consumption, high integration and complexity level. The analog circuits, instead, strongly suffers from the ScalTech trend, because the MOS behavior dramatically changes through the different technological nodes and especially for the ultra-scaled ones, where second order effects, previously negligible, become very important and start to be dominant, affecting its performance. For instance, lower intrinsic DC-gain, reduced dynamic range, operating point issues and larger parameter variability are some of the problems due to scaling-down. Analog designers must face this problems at different phases of the design, circuital and layout. Despite that, the design of analog circuit in sub-nm technologies is mandatory in some cases or can be even helpful in others. For example, in mainly mixed-signal systems, the read-out electronic requires high frequency performance, so the choice of deep submicron technology is mandatory, also for the analog part. Other types of applications where using scaled technology is even strategical are the high-energy physics experiments, where read-out circuits are exposed to very high radiation levels with consequent performance degradation and breakdown events. Since radiation damage is proportional to gate oxide volume, smaller devices exhibit lower radiation detriment. It has been demonstrated in fact, that 28nm CMOS technology devices are capable to sustain 1Grad-TID exposure, not possible with previous technologies. In this thesis, the main key challenges in ultra-scaled technologies are analyzed in the first chapter, and then integrated circuits designed in 28nm CMOS technology are presented. The first circuit design, presented in the second chapter and integrated in 28nm CMOS technology, is a Fast-Tracker front-end (FTfe) for charge detection. The read-out system has been developed starting from the main specifications and circuital solutions already adopted for muon detection in ATLAS experiment. The proposed front-end is able to detect an event and soon after to reset the system in order to make the FTfe already available for the following event, avoiding long dead times. The architecture is analyzed in detail, followed by the layout choices and the performance results. The second circuit design presented in the third chapter and always integrated in 28nm CMOS technology, is a Chopper instrumentation amplifier. Instrumentation amplifiers are the key building blocks in sensor and monitoring applications, where they are used to sense and amplify usually very small (sub-mV) and low frequency signals. For this reason it is important to reduce or eliminate the input offset and flicker noise, which cover and disturb the main signal to be detected. The proposed amplifier use a modulation technique, called chopper, in order to meet the low offset and low flicker noise requirements. Moreover it has been modeled to operate in sub-threshold region, in order to address the scaling problems. After the architecture description, layout and results of the integrated prototype are shown.

Thesis (MSc)--Stellenbosch University, 2013.
This work is a thesis regarding the energy scaling of end-pumped Ho:YLF amplifiers. The work includes: a brief review of laser physics and models, the development of a suitable three dimensional time resolved numerical model, a parametric study of double pass ampli ers simulated using the model, comparison between the simulation and the experimental results of a double pass ampli er system, and simulation of a high energy single pass ampli er. A three dimensional time resolved numerical model of an end-pumped ampli er was developed. A rate equation model was used to simulate the absorption and emission of light, energy transfer upconversion, and spontaneous emission within the gain medium. In the traveling wave approximation the propagation of light through the gain medium was modelled with the use of a split step method that included di raction and gain. A parametric study was performed to nd the design parameters for an end-pumped two pass ampli- er. Limited optimisation of several ampli er parameters was performed. The study focused on the optimisation of the energy per pulse through changes to the following parameters: crystal length, laser beam size, pump beam sizes, and pump wavelength. The nal design speci cations for an experimental system were for a 100 mm long 0.5 % (atm.) doped Ho:YLF gain medium, pump and seed beams with spot sizes with e ective beam sizes of 1 mm and 0.95 mm respectively and a pump wavelength of 1892 nm. The simulation predicted pulse energies above 480 mJ when seeded by a 55 mJ pulse at repetition rates of 50 Hz. The experimentally realised system with similar design parameters produced the highest reported energy, 330 mJ, from an end-pumped Ho:YLF ampli er. Comparison between the simulation and the experimental results showed signi cant deviation. The deviation was explained by the e ect of parameters not included previously in the simulation. These parameters were the power of the continuous component of the seed beam, and the energy transfer upconversion rate. Limitations and delity of the numerical model with respect to the experimental system are discussed, notably the model of the highly divergent pump beam was simplistic. Preliminary simulation results of a high energy single pass ampli er predict that energy scaling in Ho:YLF follows linearly with respect to pump power and that in the ideal case, multi-Joule operation is possible at 50 Hz with optical to optical e ciencies of 19%.

GaN is a III-V semiconductor that is a promising material used in production of light emitting devices and high power/high frequency electronics. The electronic and optical properties of GaN are subdued by defects that occur during the growth processes of this material. The emitted photoluminescence (PL) from optically excited GaN gives insight into the origins and effects of point defects within the crystal lattice structure of GaN. In this study, PL spectroscopy is used to examine and analyze the point defects that occur in Zn-doped GaN. The blue luminescence band seen in undoped and Zn-doped GaN have identical fine structure and properties. This band is attributed to a ZnGa acceptor. In Zn-doped, the PL intensity quenches abruptly at certain temperatures, which increase with increasing excitation intensity. This behavior is different from the PL quenching in undoped GaN. The PL behavior was simulated with a phenomenological model based on rate equations. A program created with mathematical modeling software, in conjunction with the basic rate equations, was used to explain the unusual behavior of the abrupt thermal quenching observed in Zn-doped GaN.

Photoluminescence (PL) has been studied from different types of bulk GaN samples grown by hydride vapor phase epitaxy technique at Kyma Technologies. Point defects in bulk and at the surface affect the electrical and optical properties of GaN and could be analyzed by PL. The surface of the samples was polished with different techniques: one is chemical mechanical polish (CMP) and another is mechanical polish (MP). PL data from MP and CMP surfaces show that PL intensity from the CMP-treated surface is much higher than that from the MP-treated surface. This can be explained by defects formed during the process of MP polish. However, after the MP-treated surface is etched with RIE method, the optical quality of the MP-treated surface improves. In particular, as the depth of etching increases from 50 nm to 700 nm, the PL intensity increases by a factor of 1000. PL from the CMP surfaces of undoped bulk GaN samples contains a broad red luminescence (RL) band and a broad green luminescence (GL) band. However, PL from the CMP surfaces of Fe-doped GaN samples contained a blue luminescence band (labeled as BL2 in literature) and the yellow luminescence (YL) band. PL from MP-treated surfaces (both undoped and Fe-doped) was very weak and it contained relatively narrow red and green bands. These bands, labeled RL2 and GL2, respectively, are quenched at relatively low temperatures, in contrast to the RL and GL bands which are almost independent of temperature in the range from 15 to 300 K.

This investigation utilized collaborative strategies to look at how a more social approach to teaching physics curriculum would affect students' interest, knowledge and self-efficacy towards the science of physics. Students went on field trips to meet physicists and worked together in the regular classroom on physics concept questions through Interactive Engagement teaching methods called the 'Collaborative Group Concept Conflict Process' and 'Physics by Inquiry'. The Force Concept Inventory was used as a formative and summative assessment tool and student percentiles ranked at the top of existing data that utilizes Normalized Gain as a formula for summative assessment. It was found that students gained curricular knowledge, interest and self-efficacy towards the field of physics.

Beginning with a survey of functional variation methods in classical physics, we derive the Hartree-Fock theory from canonical quantization. Following a development of density functional theory, many-body perturbation theory, and other techniques of computational condensed matter physics, we perform a systematic study of metal-polyhydride impurities in T8 and T12 polyhedral oligomeric silsesquioxane (POSS) cage molecules. Second-quantized methods motivate the derivations throughout.

A study was done to examine the effect of surface orientation as well as heterogeneous epitaxy at an interface between two materials with a large lattice mismatch. Silver nanoparticles of different diameters were grown in an effort to study methods of preferentially orienting the geometry of metal nanoparticles. Arrays of calcium fluoride nanorods were grown on silicon substrates using oblique angle thermal vapor deposition. The chamber operated at an ultra high vacuum pressure of 10^-10 Torr during the deposition of the rods and an oblique angle of 75° was kept between the silicon substrate normal and the direction of incident flux. A method was then developed to grow silver nanoparticles exclusively on the (111) facet of the calcium fluoride tips. This was accomplished by once again using oblique angle deposition with an angle of 75° along with the larger size of the (111) calcium fluoride tip facet. Cross sectional scanning electron microscopy and transmission electron microscopy imaging was used to verify that the nanoparticles adhered exclusively to the desired facet of the tip. Using selected area diffraction, (SAED) and dark field in the TEM, it was shown that the nanoparticles did grow at a (111) orientation at the interface between them and the calcium fluoride rods. Different thicknesses and diameters of nanoparticles were then grown to determine what an ideal size was to achieve the most (111) orientation of the nanoparticles. Thicknesses of the particles varied between 5 nanometers and 15 nanometers. Through characterization it was shown that all three of the different thicknesses grown exhibited (111) orientation of the silver nanoparticles, both at the interface and in the overall nanoparticle as well with the 10 nanometer sample being the most ideal in terms of the desired result. Lattice straining of the silver nanoparticles was also observed by characterization through diffraction and SAED.

Negative ions play an important role in chemistry as building blocks of salts and oxidizing agents. Halogen atoms, due to their ability to attract electrons, readily form negative ions. Considerable interest exists in the design and synthesis of new negative ions called superhaogens whose electron affinities are much higher than those of halogen atoms. This thesis deals with the design of such species. Using density functional theory I have studied two classes of superhalogens. First one involves d1 transition metal (Sc, Y, La) atoms surrounded by Cl while the second one involves simple metals (Na, Mg, Al) surrounded by pseudohalogens such as CN. Geometry, electronic structure, and electron affinity of these species containing up to 5 ligands have been calculated. Studies reveal a fundamental difference between the interaction of transition and metal atoms with electronegative ligands. In addition, pseudohalogens can be used to synthesize a new class of superhalogens.

This thesis reports the main results obtained from the Ph.D. research activity of the candidate. The activity was focused on the study of defects and physical mechanisms that limit the ESD robustness of Light Emitting Diodes (LEDs). In particular, most of the research activity was mainly focused on the analysis of GaN-based LEDs, which are the basis for the realization of blue and UV emitters, and white LEDs based on phosphor conversion. After an initial overview on the most important theoretical concepts necessary for the understanding of physical results, four main sections can be identified in this thesis, concerning the presentation of research activity: - First, we report an extensive analysis of the defect-related localized emission processes occurring in InGaN/GaN-based light emitting diodes at low reverse and forward-bias conditions. The analysis is based on combined electrical characterization and spectrally and spatially resolved electroluminescence (EL) measurements. Results of this analysis show that: (i) under reverse bias, LEDs can emit a weak luminescence signal, which is directly proportional to the injected reverse current. Reverse-bias emission is localized in submicrometer-size spots; the intensity of the signal is strongly correlated to the threading dislocation (TD) density, thus suggesting that TDs are preferential paths for leakage current conduction. (ii) Under low forward-bias conditions, the intensity of the EL signal is not uniform over the device area. Spectrally resolved EL analysis of green LEDs identifies the presence of localized spots emitting in the yellow spectral region, whose origin is ascribed to localized tunneling occurring between the quantum wells and the barrier layers of the diodes, with subsequent defect-assisted radiative recombination. - Afterwards, we propose an extensive study of the electroluminescence characteristics of InGaN-based LEDs with color-coded structure, i.e. with a triple quantum well structure in which each quantum well has a different indium content, in order to analyze the carrier distribution inside the quantum wells of the active region. The analysis is based on combined electroluminescence measurements and two-dimensional simulations, carried out at different current and temperature levels. Results indicate that: (i) the efficiency of each of the quantum wells strongly depends on device operating conditions (current and temperature); (ii) at low current and temperature levels, only the quantum well closer to the p-side has a significant emission; (iii) emission from the other quantum wells is favored at high current levels. The role of carrier injection, hole mobility, carrier density and non-radiative recombination in determining the relative intensity of the quantum wells is also discussed. - At this point, we propose the results obtained from the analysis of physical mechanisms that limit the ESD robustness of GaN-based LEDs. The analysis was carried out on several LED families with different ESD robustness. Each of analyzed sample family is characterized by two different parameters: the failure rate measured after the application of a single ESD pulse, named First level failure F1, and the failure rate measured after the application of a second ESD pulse, named Second level failure F2. After an initial electro-optical characterization, we have analyzed the LEDs by means of slow capacitance transient, deep level optical spectroscopy (DLOS) and deep level transient spectroscopy (DLTS) measurements. The experimental results show that: (i) the overall junction capacitance is strongly correlated to the First level failure F1, thus suggesting also a correlation between the maximum junction electric field and the First level failure F1 of LEDs; (ii) the amplitude of capacitance transients, related to trapping phenomena, is strongly correlated to the Relative failure parameter, which is defined as the ratio Second level failure F2/First level failure F1. Thus, the presence of defects in the LED structures can influence the ESD robustness measured after the application of two consecutive ESD pulses; (iii) the correlation between trapping and Relative failure is confirmed by both DLOS and DLTS measurements. - To conclude the study of physical mechanisms that limit the ESD robustness of Light Emitting Diodes (LEDs), we present an ESD characterization carried out on commercially available LEDs. In particular, we present an extensive analysis of the failure mechanisms of RGB (multichip) LEDs submitted to ESD testing: the tests have been carried out on several commercially available LEDs of four different suppliers. In order to better understand the failure mechanisms, we have submitted LEDs to ESD tests under reverse and forward bias condition separately, by means of a Transmission Line Pulser (TLP). The experimental results indicates that: (i) red LEDs (based on AlInGaP) have an higher ESD robustness with respect to green and blue samples (based on InGaN), both under reverse and under forward bias test; (ii) TLP negative pulses with a current smaller than the failure threshold can induce a decrease of the leakage current in GaN-based LEDs, due to a partial annihilation of defective paths responsible for reverse conduction; (iii) typical failure mechanism of devices is represented by a catastrophic event, with short-circuiting of the junction. However, some of the analyzed red LEDs had shown “soft” failure, with gradual increase of the leakage current and corresponding decrease of the optical power, even without a catastrophic damage. Finally, also the temperature dependence of the ESD robustness of GaN-based devices has been studied. Useful information on the research activity can be also found in the papers co-authored by the candidate and listed in the next section.
La seguente tesi riporta i principali risultati ottenuti dall’attività di ricerca di Dottorato del candidato. L’attività è stata focalizzata sullo studio dei difetti e dei meccanismi fisici the limitano la robustezza alle scariche elettrostatiche (ESD) dei diodi emettitori di luce (LED). In particolare, la maggior parte dell’attività di ricerca è stata principalmente focalizzata sull’analisi dei LED basati su nitruro di gallio (GaN), che sono la base per la realizzazione di emettitori blu e UV e di LED bianchi basati sulla conversione dei fosfori. Dopo una panoramica iniziale dei concetti teorici più importanti necessari per la comprensione dei risultati fisici, in questa tesi possono essere identificate quattro sezioni principali che riguardano la presentazione dell’attività di ricerca: - In primo luogo riportiamo un’estesa analisi dei processi di emissione localizzati legati ai difetti, che si verificano nei diodi emettitori di luce basati sulla struttura InGaN/GaN. L’analisi è basata su una caratterizzazione elettrica combinata con misure di elettroluminescenza (EL) risolte spettralmente e spazialmente. I risultati di questa analisi mostrano che: (i) in condizioni di polarizzazione inversa i LED possono emettere un debole segnale di luminescenza, che è direttamente proporzionale alla corrente inversa iniettata. L’emissione in polarizzazione inversa è localizzata in spot di dimensione submicrometrica; l’intensità del segnale è fortemente correlata alla densità di threading dislocation (TD), suggerendo quindi che le threading dislocation sono percorsi preferenziali per la conduzione della corrente di leakage. (ii) In condizioni di bassa polarizzazione diretta, l’intensità del segnale EL non è uniforme sull’area del dispositivo. L’analisi EL risolta spettralmente dei LED verdi identifica la presenza di spot localizzati che emettono nella regione spettrale gialla, la cui origine è stata attribuita a tunneling localizzato che si verifica tra le buche quantiche e gli strati di barriera dei diodi, con successiva ricombinazione radiativa assistita da difetti. - Successivamente proponiamo uno studio esteso delle caratteristiche di elettroluminescenca dei LED basati su InGaN con struttura color-coded, cioè una struttura a tripla buca quantica nella quale ciascuna buca quantica ha un contenuto di indio differente, allo scopo di analizzare la distribuzione di portatori all’interno delle buche quantiche della regione attiva. L’analisi è basata su misure di elettroluminescenza combinate con simulazioni bidimensionali, eseguite a differenti livelli di corrente e temperatura. I risultati indicano che: (i) l’efficienza di ciascuna delle buche quantiche dipende fortemente dalle condizioni operative del dispositivo (corrente e temperatura); (ii) a bassi livelli di corrente e temperatura solo la buca quantica più vicina al lato p ha un’emissione significativa; (iii) l’emissione dalle altre buche quantiche è favorita ad elevati livelli di corrente. Sarà anche discusso il ruolo dell’iniezione dei portatori, della mobilità delle lacune, della densità di portatori e della ricombinazione non radiativa nel determianre l’intensità relativa delle buche quantiche. - A questo punto proponiamo i risultati ottenuti dall’analisi dei meccanismi fisici che limitano la robustezza alle scariche elettrostatiche (ESD) dei LED basati su GaN. L’analisi è stata eseguita su numerose famiglie di LED con differenti robustezze ESD. Ciascuna delle famiglie di campioni analizzate è caratterizzata da due differenti parametri: il tasso di failure misurato dopo l’applicazione di un singolo impulso ESD, denominato First level failure F1, e il tasso di failure misurato dopo l’applicazione di un secondo impulso ESD, denominato Secondo level failure F2. Dopo un’iniziale caratterizzazione elettro-ottica, abbiamo analizzato i LED per mezzo di misure di transienti capacitivi lenti, deep level optical spectroscopy (DLOS) e deep level transient spectroscopy (DLTS). I risultati sperimentali mostrano che: (i) la capacità di giunzione complessiva è fortemente correlata al First level failure F1, suggerendo quindi anche una correlazione tra il massimo campo elettrico di giunzione e il First level failure F1 dei LED; (ii) l’ampiezza dei transienti capacitivi, legata a fenomeni di intrappolamento, è fortemente correlata al parametro Relative failure, che è definito come il rapporto Second level failure F2/First level failure F1. Quindi la presenza di difetti nelle strutture LED può influenzare la robustezza ESD misurata dopo l’applicazione consecutiva di due impulsi ESD; (iii) la correlazione tra il trapping e il Relative failure è confermata sia dalle misure di DLOS, sia da quelle DLTS. - Per concludere lo studio dei meccanismi fisici che limitano la robustezza ESD dei diodi emettitori di luce (LED), presentiamo una caratterizzazione ESD eseguita su dei LED disponibili commercialmente. In particolare presentiamo un’estesa analisi dei meccanismi di failure dei LED RGB (multichip) sottoposti a test ESD: i test sono stati eseguiti su numerosi LED disponibili commercialmente di quattro differenti produttori. Allo scopo di comprendere meglio i meccanismi di failure, abbiamo sottoposto i LED a test ESD in condizioni di polarizzazione inversa e diretta separatamente, per mezzo di un Transmission Line Pulser (TLP). I risultati sperimentali indicano che: (i) i LED rossi (basati su AlInGaP) hanno una robustezza ESD più alta rispetto ai campioni verdi e blu (basati su InGaN), sia nei test in polarizzazione inversa, sia in quelli in polarizzazione diretta; (ii) impulsi TLP negativi con una corrente inferiore alla soglia di failure possono indurre una diminuzione della corrente di leakage nei LED basati su GaN, a causa di un parziale annientamento dei percorsi difettosi responsabili per la conduzione inversa; (iii) il tipico meccanismo di failure dei dispositivi è rappresentato da un evento catastrofico, con cortocircuitazione della giunzione. Tuttavia, alcuni dei LED rossi analizzati hanno mostrato “soft” failure, con graduale aumento della corrente di leakage ed una corrispondente diminuzione della potenza ottica, anche in assenza di un danno castastrofico. Infine, è stata studiata anche la dipendenza dalla temperatura della robustezza ESD dei dispositivi basati su GaN. Utili informazioni sull’attività di ricerca possono essere trovate negli articoli in cui ha collaborato il candidato ed elencati nella successiva sezione.

We investigate the dynamic environment of the double asteroid system (65803) Didymoswith its satellite Didymos B for stable trajectories of more than 30 days for a model cubesatcalled APEX, foreseen to visit Didymos with the HERA mission, to be launched in 2023.We find semi-stable orbits with lifetimes up to 15 days under the influence of solar radiationpressure. In comparison, in the absence of solar radiation pressure, orbit trajectories reachlifetimes of up to 90 days. We present the distribution of orbital stability for positionsof initial deployment and the related orbital lifetime for inclinations of i = 0, 6 and 30 to the ecliptic plane of the sun, to investigate expected seasonality of the Didymossystem. The influence on trajectories through solar radiation pressure on the arrangementof the solar panels of APEX is compared to trajectories without solar radiation pressure. Itbecomes clear that the influence of solar radiation pressure is the main variable for stabilityof trajectories and must be further investigated with great care. We also discuss briefly theoperation of APEX in relation to orbital corrections and the limitations for such corrections.

The instrumentation of radiation detectors for high energy physics calls for the development of very low-noise application-specific integrated-circuits and demanding system-level design strategies, with a particular focus on the minimisation of inter-ference noise from power anagement circuitry. On the other hand, the aggressive pixelisation of sensors and associated front-end electronics, and the high radiation exposure at the innermost tracking and vertex detectors, requires radiation-aware design and radiation-tolerant deep sub-micron CMOS technologies. This thesis explores circuit design techniques towards radiation tolerant power management integrated circuits, targeting applications on particle detectors and monitoring of accelerator-based experiments, aerospace and nuclear applications. It addresses advantages and caveats of commonly used radiation-hard layout techniques, which often employ Enclosed Layout or H-shaped transistors, in respect to the use of linear transistors. Radiation tolerant designs for bandgap circuits are discussed, and two different topologies were explored. A low quiescent current bandgap for sub-1 V CMOS circuits is proposed, where the use of diode-connected MOSFETs in weak-inversion is explored in order to increase its radiation tolerance. An any-load stable LDO architecture is proposed, and three versions of the design using different layout techniques were implemented and characterised. In addition, a switched DC-DC Buck converter is also studied. For reasons concerning testability and silicon area, the controller of the Buck converter is on-chip, while the inductance and the power transistors are left on-board. A prototype test chip with power management IP blocks was fabricated, using a TSMC 65 nm CMOS technology. The chip features Linear, ELT and H-shape LDO designs, bandgap circuits and a Buck DC-DC converter. We discuss the design, layout and test results of the prototype. The specifications in terms of voltage range and output current capability are based on the requirements set for the integrated on-detector electronics of the new CGEM-IT tracker for the BESIII detector. The thesis discusses the fundamental aspects of the proposed on-detector electronics and provides an in-depth depiction of the front-end design for the readout ASIC.

Ce travail porte sur l'élaboration et les caractérisations physico-chimiques de couches très minces de Ta2O5 et ZrO2 (<10 nm) pour différentes applications en microélectronique. Les films sont élaborés par atomisation électrostatique (ESD) et/ou dépôt de couches atomiques (ALD). Les paramètres expérimentaux sont optimisés pour l'obtention de films denses et minces (environ 10 nm), homogènes en épaisseur et conformes. En ESD, la solution précurseur choisie contient un acétylacétonate de tantale ou de zirconium (0,00625 M) dissous dans 20 % vol. Ethanol et 80 % vol. Butyl carbitol. Les paramètres tels que température du substrat, distance aiguille-substrat, débit de la solution et temps de dépôt sont fixés à 240°C, 20 mm, 0,49 ml/h et 5 minutes, quelque soit le matériau déposé. En ALD, les précurseurs utilisés pour déposer Ta2O5 sont l'éthoxyde de tantale chauffé à 100°C dans une nacelle interne et l'eau maintenue à 20°C dans un bulleur externe. Les dépôts ALD sont réalisés pendant 200 à 2000 cycles (0,2/2/2/2s) à 209°C et 100 Pa. L' étude structurale originale des films montre une cristallisation vers 800°C pour Ta2O5 et 600°C pour ZrO2. Ta2O5 présente une transformation de phases monoclinique à rhomboédrique réversible en température lors d'un refroidissement lent. ZrO2 cristallise majoritairement dans la symétrie quadratique. Les tests électriques de capacités MOS réalisés avec une sonde à mercure ont permis d'évaluer la constante diélectrique. Enfin, les performances de barrière de chacun des matériaux ont été étudiées et la défaillance a lieu au delà de 600°C.

This thesis uses the techniques of atomic force microscope (AFM) and conductive AFM (C-AFM) to study the conduction properties of n-type GaN films. A total of 16 samples were examined and grouped according to their surface morphologies and conduction behaviors. The most common type of surface morpliology was that of Ga-rich samples having undulating "hillocks" with interspersed holes. Although most of the samples had this common morphology, their local conduction behaviors were not all similar. Local I-V spectra of the tip-sample Schottky contact could be grouped according to three major types: low leakage, high leakage, and "p-type". The highest quality samples with low leakage were usually grown at moderate temperatures (~650°C). For such samples, localized leakage only occurred at screw dislocations located at small pits terminating surface hillocks. I-V spectra taken on and off such hillocks were fit in forward bias to determine whether field emission or Frenkel-Poole conduction were dominant. Although field emission is a good fit compared to Frenkel-Poole, yielding reasonable values for the barrier height, the results are not yet conclusive without variable temperature studies.

This dissertation is concerned with the understanding of physico-chemical properties of energetic materials (EMs). Recently, a substantial amount of work has been directed towards calculations of equations of state and structural changes upon compression of existing EMs, as well as elucidating the underlying chemistry of initiation in detonating EMs. This work contributes to this effort by 1) predicting equations of state and thermo-physical properties of EMs, 2) predicting new phases of novel EMs, and 3) examining the initial stages of chemistry that result in detonation in EMs. The motivation for the first thrust, is to provide thermodynamic properties as input parameters for mesoscale modeling. Such properties are urgently sought for a wide range of temperatures and pressures, and are often difficult or even impossible to obtain from experiment. However, thermo-physical properties are obtained by calculating structural properties and vibration spectra using density function theory and employing the quasi-harmonic approximation. The second thrust is directed towards the prediction and investigation of novel polymorphs of known azide compounds to identify precursor materials for synthesis of polymeric nitrogen EMs. Structural searches are used to identify new polymorphs, while theoretical Raman spectra for these polymorphs are calculated to aid experimentalists in identifying the appearance of these azide compounds under high pressure. The final thrust is concerned with elucidating the initial chemical events that lead to detonation through hypervelocity collision simulations using first-principles molecular dynamics. The chemical mechanisms of initiation are determined from the atomic trajectory data, while heats of reaction are calculated to quantify energy trends of chemical transformations.

In this research physics ranking tasks were introduced to see if they could increase students' conceptual knowledge in general and calculus based physics courses. Assessments were given both pre and post in order to calculate a class's percent gain. Although students did not seem to enjoy or appreciate these types of tasks at the beginning, analysis of the percent gain did show a remarkable increase in the conceptual concepts that were assessed due to the physics ranking tasks.

Thesis (PhD (Physics))--University of Stellenbosch, 2010.
ENGLISH ABSTRACT: The output power of end-pumped lasers is mainly limited by thermal effects in the bulk crystal gain material. The thermal effects either fracture the crystal or cause degradation in the laser beam quality and output power. This is especially pronounced in Nd:YVO4 and Nd:GdVO4 which exhibit strong thermal lensing. These two Nd3+ vanadate materials are of great value because of their high emission cross sections which makes them excellent gain materials for mode-locked, high repetition rate Q-switched and intra-cavity frequency doubled lasers. The two Nd3+ vanadates have very similar spectral properties but many publications claim that the more expensive Nd:GdVO4 is thermo-optically superior to Nd:YVO4. However, a debate ensued after theoretical calculations as well as measurements of the thermal conductivity and thermo-optical coefficients indicated that the opposite is true. To our knowledge there has never been a direct comparison of the thermal lensing of these two materials under identical pumping and lasing conditions. In order to contribute to the debate we did such measurements for different crystals of these two materials with equal low doping using three different measurement methods. We subsequently determined that Nd:YVO4 has slightly lower thermal lensing for the stronger gain -polarisation. One of the measurement methods we used is a novel more reproducible one that we developed for this purpose. It is more reproducible because it selectively measures only the focal length of the central, relatively unaberrated part of the thermal lens. Another measurement, utilising a probe beam through the laser crystal, found that there was almost no increase in the temperature when lasing is interrupted. This indicated that there is almost no upconversion present in the crystals which is probably due to their optimally chosen low doping. A further consequence of the vanadate debate is that there is still confusion about the value of the important thermo-optical coefficient for the higher gain -polarisation (dne/dT ) of Nd:YVO4. This parameter is of great importance in thermal calculations since the strength of the thermal lens is largely dependent on it. We therefore numerically modelled the thermal lensing in our crystals using different dne/dT values and found that the value given by Sato & Taira (2007) matches our experimental results the best. Our measurements also indicated that the thermal lens dioptric power increased nonlinearly with pump power. This appeared to contradict theory of thermal lensing since we knew that there was no upconversion in the crystals (which is the standard explanation for the nonlinear increase). We proceeded to use our numerical modelling to identify the main source of the nonlinear increase as the varying spectral output of the diode pump laser. The findings in this thesis therefore extend knowledge of the thermo-optical properties of the vanadates and increase understanding of the strongly aberrated thermal lenses formed inside them. Furthermore, the findings now enable the power-scaling of end-pumped vanadates lasers to higher levels.
AFRIKAANSE OPSOMMING: Die uitset drywing van longitudinale-gepompte vaste-toestand lasers word hoofsaaklik beperk deur termiese effekte in die laser kristal. Die kristal word of gekraak of die laser se bundel kwaliteit en uitset drywing verminder. Dit is veral ’n problem in Nd:YVO4 en Nd:GdVO4 kristalle waarin sterk termiese lense voorkom. Hierdie twee Nd3+ vanadaat kristalle is waardevol vanwee hulle ho¨e emissie deursnitte wat hulle uitstekend maak as versterkings materiale vir modus-gesinkroniseerde, ho¨e repitisie, Q-geskakelde en binne-resonator frekwensie-verdubbelde lasers. Die twee vanadate het baie soortgelyke spektrale eienskappe, maar verskeie publikasies beweer dat die duurder Nd:GdVO4 materiaal termo-opties beter is as Nd:YVO4. Onlangse teoretiese berekeninge asook metings van die termiese en termo-optiese kwaliteite van die twee kristalle toon egter die teenoorgestelde. Sover ons weet is daar nog geen direkte vergelyking van die termiese lens in hierdie twee materiale onder identiese kondisies gedoen nie. Ons het dus so ’n meting aangepak vir kristalle met identiese lae konsentrasie van die Nd3+ ioon deur drie verskillende meet metodes te gebruik. Een van die meet metodes is ’n nuwe, meer reproduseerbare metode wat ons ontwikkel het vir hierdie doel. Dit is meer reproduseerbaar omdat dit slegs die binneste deel van die termiese lens meet wat min sferiese aberrasie het. ’n Ander meeting, wat ’n toets-bundel deur die kristal stuur, het getoon dat daar byna geen verhoging in die die temperatuur van die kristal was toe ossilasie in die resonator onderbreek was nie. Dit is ’n aanduiding dat dat daar byna geen op-omskepping teenwoordig is in die kristalle nie wat te danke is aan hul optimale lae konsentrasie van die Nd3+ ioon. ’n Verdere gevolg van die debat is dat daar nog verwarring in die literatuur bestaan oor die waarde van Nd:YVO4 se termo-optiese dne/dT koeffisi¨ent. Hierdie parameter is van groot belang in berekinge van die termiese lens se fokale lengte vir die ho¨er wins -polarisasie. Deur numeriese modellering te gebruik het ons bevind dat die waarde wat verskaf word deur Sato & Taira (2007) ons eksperimentele data die beste pas. Ons metings het ook aangedui dat die dioptriese krag van die termiese lens nie linie¨er toeneem ten opsigte van die geabsorbeerde pomp krag nie. Dit was o¨enskynlik teenstryding met teorie oor termiese lense. Dit is omdat ons bevestig het dat daar geen op-omskepping in die kristalle teenwoordig was nie, wat die standaard verklaring vir die nie linie¨eriteit is. Ons het dus ons numeriese modellering gebruik om die hoofbron van die nie-lini¨ere toename te identifiseer as die veranderende spektrale uitset van die diode pomp laser. Die bevindings in hierdie tesis bou dus kennis op oor die termo-optiese eienskappe van die vanadate en versterk begrip van die sterk termiese lense binne hulle. Verder stel die bevindings ons nou in staat om die uitset drywing van longitudinale-gepompte vanadaat lasers na ho¨er vlakke te skaal.

This thesis describes the construction of an Optical Tweezer apparatus to be used in conjunction with a confocal Raman spectrometer. The tweezer utilizes an infrared (λ=1064 nm) laser directed into an inverted microscope with NA=1.4 oil immersion 100x objective lens that strongly focuses the laser light into a sample to function as a single-beam gradient force trap. The long term goal of this research program is to develop a single molecule Raman tweezers apparatus that allows one to control the position of a Raman nanoplasmonic amplifier. This thesis describes the construction of the Raman tweezer apparatus along with several Raman spectra obtained from optically trapped samples of polystyrene fluorescent orange, amine-modified latex beads. In addition, I explored the Raman spectra of bulk cytochrome c mixed with or injected onto Ag aggregates for SERs enhancement.

In this thesis, we first present a brief overview of various theoretical approaches used to examine the electronic structure of defects in GaN. Using the recently developed hybrid density functional theory (HSE06) along with the experimental measurements, we propose a new explanation of the nature of the yellow luminescence band in carbon-doped GaN. We conduct a systematic study of electronic and optical properties of defects (Carbon, Oxygen, Silicon related) that are candidates for the origin of yellow luminescence. We show that the CN-ON complex is significantly more likely to form compared to isolated carbon configurations. In contrast to the properties of the isolated carbon acceptor, calculated defect levels and optical transitions involving deep level of the CN-ON complex agree quite well with our thermal luminescence quenching data as well as with the experimentally measured C-doped GaN luminescence spectra. Hence, the CN-ON complex, rather than isolated C impurity, is more likely to resolve a long-standing problem of the yellow luminescence in GaN.

We examine the design of a high speed atomic force microscope using an optical pickup from a commercially available compact disc/digital versatile disc drive. An investigation of the commercial optical pickup is done with the goal of determining how it can be used for dimensional measurements on nanometer scale. An evaluation of noise sources, imaging capabilities, and functionality is performed.

The effect of ambient on photoluminescence (PL) from GaN was studied. We found that the PL intensity in vacuum was nearly four times higher than in air. The PL intensity also increased after etching the sample in Aqua Regia and BOE to remove the native oxide layer. After etching, the PL intensity was very stable in vacuum, but substantially degraded in air ambient. In HCl vapor (low pH), the PL intensity increased as compared to air ambient, while in NH3 vapor (high pH) it decreased. The quantum efficiency of the exciton and blue luminescence bands increased significantly with increasing excitation power density. This increase could not be explained by reduction of the depletion region width (field effect mechanism), but could be explained by changes in the nonradiative recombination rate at the surface (recombination mechanism). We therefore assume that in vacuum and acid vapor some surface species are desorbed or passivated, resulting in a decreased nonradiative recombination rate and increased PL intensity.

The processes leading to the widespread presence of crystalline silicates throughout the galaxy and the origin of silicon nanoparticles thought to be responsible for the observed extended red emission in diffuse galactic background are still far from being understood. One of the most abundant oxygen bearing species in molecular astronomical regions is SiO. It has been conjectured that silicate formation probably proceeds via the agglomeration of these molecular species; however there are no studies to reveal the microscopic mechanism. We have used a synergistic approach combining experiments in molecular beams and first principles theoretical calculation to demonstrate that the passage from SiO to Si02 proceeds via gradual oxygen enrichment of SinOm clusters and that the smallest cascade involves Si203, S304, Si405, Si506 as the intermediate products. We also demonstrate that as the SiO molecules cluster together, the chemistry drives the agglomerates towards configurations such that the central core are pure Sin clusters while the outer shell are SiO2 molecules. The gap between the highest occupied molecular orbital and the lowest unoccupied molecular orbital range from 0.84 to 3.84 eV and hence can contribute to the observed extended red emission and blue luminescence. The findings are of general interest in Astrophysics but are also critical to a fundamental understanding of the interstellar extinction.

Nanopores have been shown to be a useful analytical tool for single molecule detection. They have been used to study the composition of DNA and other molecules of interest. These pores are usually α-hemolysin which is a toxin from Staphylococcus aureus or more recently nanoscale synthetic solid state pores. Now we are beginning to look at other molecules or proteins by sending them into the nanopores and measuring a characteristic partial current blockade. In this thesis we look at polyethylene glycol (PEG) as it enters and blocks current through a single alpha hemolysin pore. We report the effects of ionic strength, PEG size, and applied voltage on the depth and duration of the current blockades. We also apply autocorrelation analysis on the arrival times of PEG molecules to the pore see if we can identify if the PEG is translocating through the pore or escaping from the same side it enters. This suggests a new approach to current blockade analysis.

The scalar wave equation is investigated for a scalar field propagating in a spacetime background ds²=e^{2a}(-dt²+dr²)+R(e^{-2ψ}dφ²+e^{2ψ}dz²). The metric is compactified in the radial direction. The spacetime slices of constant φ and z are foliated into outgoing null hypersurfaces by the null coordinate transformation u=t-r. The scalar field imitates the amplitude behavior of a light ray, or a gravitational wave, traveling along a null hypersurface when the area function R is a constant or is a function of u. These choices for R restrict the gravitational wave factor ψ to being an arbitrary function of u.

In this dissertation, I present two studies in the field of gravitational wave astrophysics applied to compact binary systems. In the first project, I investigate simulated encounters between a binary system comprised of two stellar mass black holes with a galactic supermassive black hole. It is found that binaries disrupted by the supermassive black hole form extreme mass ratio inspirals (EMRIs), which would begin with very high eccentricity, e ≈ 1 − O(10−2), but circularize dramatically by the emission of gravitational wave radiation. At the time when the stable orbit turns over to a plunge orbit, the EMRIs still have some small residual eccentricity, e ≈ 0.05 on average, which is slightly larger than previous estimates. The surviving binaries are classified based on their final relation with the supermassive black hole. When inspecting the merger lifetime of the surviving binaries, a mean new merger lifetime of ˜ T = 0.8T0 is found. Factoring in this new lifetime with other relevant data, I calculate the merger rate of these systems in the range of the advanced Laser Interferometer Gravitational Wave Observatory to be about 0.25 yr−1, which represents a small percentage of the current predicted CBC rates. In the second project I propose and explore a new method of estimating the radius of the accretion disc in cataclysmic variable binary systems though the use of coupled electromagnetic and gravitational wave observations. By identifying the angle of the hot spot formed by the impact of the accretion stream with the disc, φHS, the radius of the disc can be recovered. I test the proposed method against fully simulated lightcurve output, as well as the true observed AM CVn lightcurve. In both cases, I find our method capable of estimating the disc radius to high precision. I calculate a disc radius of ˆRD/a ≈ 0.476°”0.025 for the fully simulated data and ˆRD/a ≈ 0.481 °” 0.05 for the true lightcurve data. These estimates agree with the accepted value of RD = 0.478a within the uncertainties, and differ from the accepted value by 0.4% and 0.6%, respectively. Because this method does not rely on eclipses, it will be applicable to a much broader population of binaries.

This Ph.D work reports the studies of photovoltaic devices produced by solution processable methods. Two material systems are of interest: one is based on organic semiconductors, and another on organic/inorganic hybrid composites. Specifically, organic photovoltaic (OPV) devices are made using photoactive materials consisted of a -conjugated polymer [Poly(3-hexylthiophene), or P3HT] and fullerene derivative [phenyl-C60-butric acid methyl ester, or PCBM] in a bulk heterojunction (BHJ) structure of donor/acceptor network. On the other hand, hybrid photovoltaic (HPV) devices are made from blend of quantum dots and -conjugated polymers. The QD material presented here are of the lead sulfide (PbS), and lead selenide (PbSe), whereas the polymers are either P3HT or Poly(3-dodecyl thienylene vinylene) (PTV)with controlled regio-regularity. For OPV devices, two different device geometries are investigated, namely, the conventional or normal structure where indium tin oxide (ITO) is used an anode, and a metal cathode is fabricated by thermal vapor deposition (TVD). In this geometry, thin layer (about 30~35nm) of poly(3,4ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is deposited from aqueous solution onto ITO as hole transport layer (HTL). The second geometry, called the inverted structure, uses ITO as the cathode of the device. A thin layer of cesium carbonate (Cs2CO3) (about 1~2nm) is applied over the ITO and functions as electron transport layer (ETL), thereby decreasing the work function of ITO and allowing it to function as the cathode. In this case, PEDOT:PSS is mixed with 5vol.% of dimethylsulfoxide to increase conductivity for serving as anode. Two solution processable methods, spin-coating and spray processes were investigated, and a detailed study of nanomorphology influence under different annealing conditions, different solvents and thickness are reported. The main contribution of this Ph.D. work was the development and implementation of a layer-by-layer (LBL) all-spray solution-processable technique to fabricate large-scale OPV arrays, with more than 30% transmission in the visible to near IR range. Comparing with traditional laboratory OPV fabrication based on spin-coating and using metal as cathode contact, which greatly limits transparency of solar cells and posts difficulty for large scale manufacturing, this LBL spray process solves these two problems simultaneously. This technology eliminates the need for high-vacuum, high temperature, low rate and high-cost manufacturing associated with current silicon and in-organic thin film photovoltaic products. Furthermore, this technology could be used on any type of substrate including cloth and plastic. Single cell OPV with active area of 4mm2 was used as preliminary test device to obtain fabrication parameters for multi-cell OPV arrays. Three different sizes of OPV arrays were fabricated and tested under various illumination conditions. Starting from a 4" x 4" array with 50 cells in series connection 4" x 4" substrate consisting of 50 cells with total active area of 30cm2, a scaled up 1' x 1' array was fabricated as a proof of concept, and whose results are reported. Scaled down arrays, called micro arrays, are also presented in this work. OPV micro array has the potential application in DC power supplies for electrostatic Microelectromechanical systems (MEMS) devices. The first generation micro array consists of 20 small (1mm2) solar cells connected in series for a total device area of approximately 2.2cm2. The 2nd generation micro array with 60 cells shares the same size substrates and single cell active area as the first generation. However, the 2nd generation micro array cell has a new design with reduced series resistance and improved cell occupancy by 3 fold. Infrared quantum dots (QD) such as PbS and PbSe have potential in photovoltaic applications. These solution processable quantum dots with tunable electronic properties offer very attractive approach for expanding spectral sensitivity of -conjugated polymers to infrared region of solar spectrum. However, these QDs often have defects originated from either incomplete surface passivation or imperfections in the quantum Dots. The electronic levels of defects often are within the bandgap of the semiconductor. These in-gap states are of great importance since they affect the final destiny of excitons. Continuous wave photoinduced absorption spectroscopy has proven to be a convenient and successful technique to study long-lived photoexcitations of in-gap states. Part of this Ph.D work was the investigation of a peculiar gap state found in films of PbS QDs. This gap state bears confinement dependence, with a lifetime about 2μs. A detailed analysis of the Stokes shift, temperature dependence of PL, absorption and photoinduced absorption reveals the unconventional GS is a new state of a trapped exciton in a QD film. This gap state is directly relevant to exciton dissociation and carrier extraction in this class of semiconductor quantum dots. As synthesized PbSe and PbS quantum dots usually have bulky ligands such as oleic acids or TOPO (trioctylphosphine oxide). This capping layer is necessary to prevent nanocrystals from coalescence, however, the bulky ligands hinder charge extraction from and charge transport through the nanocrystals, as well as exciton dissociation at the nanocrystal/polymer interface. Common ways to manipulate ligands include ligand wash and ligand exchange in solution, and ligand removal on films. Through this Ph.D. work, a novel method using electric field to manipulate quantum dots ligands for interface of quantum dots and polymer, which possibly could facilitate charge extraction from the quantum dots and charge transfer between quantum dots and polymers, without the need of harmful chemicals. Over four orders improvement of photoconductivity at zero bias and more than six orders improvement at 5V reverse bias in a sandwich structure quantum dots photovoltaic device, and more than 5x improve in film smoothness. After thorough fundamental study on QD optoelectronic properties, hybrid photovoltaic (HPV) device was fabricated using a blend solution of PbS QDs and P3HT. Two different solution processes are used to form the QD/polymer active layer, one is the traditional spin coating method, and another is the spray technique developed in this Ph.D. Work. Different film morphology was observed with these two methods. Although the film is slightly rougher in the case with sprayed QD/polymer active layer, the phase segregation is more distinct and with smaller domain, which is beneficial for charge transport.

Improving science, technology, engineering, and mathematics (STEM) education has become a national priority and the call to modernize secondary science has been heard. A Physics First (PF) program with the curriculum sequence of physics, chemistry, and biology (PCB) driven by inquiry- and project-based learning offers a viable alternative to the traditional curricular sequence (BCP) and methods of teaching, but requires more empirical evidence. This study determined impact of a PF program (PF-PCB) on math achievement (SAT math scores) after the first two cohorts of students completed the PF-PCB program at Matteo Ricci High School (MRHS) and provided more quantitative data to inform the PF debate and advance secondary science education. Statistical analysis (ANCOVA) determined the influence of covariates and revealed that PF-PCB program had a significant (p < .05) impact on SAT math scores in the second cohort at MRHS. Statistically adjusted, the SAT math means for PF students were 21.4 points higher than their non-PF counterparts when controlling for prior math achievement (HSTP math), socioeconomic status (SES), and ethnicity/race.