Mercuric Iodide in its tetragonal form has received a lot of attention for many years as a prospective room temperature X-ray and y-ray detector. Its basic properties are well suited for this purpose. Its wide band gap of 2.1eV contributes to a high dark resistivity of 1012ohm-cm or higher. A high atomic number of its constituent atoms (Hg-80, I -53) and a density of 6.3g/cm3 result in its efficient interaction with incident X-ray or y-ray radiation. Single crystalline mercuric iodide has been thoroughly studied and successfully utilized in commercial radiation detectors. But with the urgent need for large area ,low cost efficient X-ray detectors, focus has now shifted towards the development and understanding of the properties of thin film Polycrystalline Mercuric iodide detectors. Such detectors also have the advantage of being most suited for direct X-ray detection i.e. a direct conversion of incident X rays into electric signals which are then used to obtain an equivalent image in digital X-ray imaging. They also can be used in applications where a scintillator intermediate is used to generate visible light from incident high energy photons.Therefore it is important to study their optical response in order to understand and evaluate their Optical Properties. The present work focuses on obtaining the Optical response of the thin film Mercuric iodide photoconductive detectors .These films were grown on TEC-15 LOF glass with a Tin Oxide (SnO2) coating on it, which acts as a growth surface for the films and also functions as the front contact of the detector.Palladium which is sputtered on top of this film acts as the back contact. There are a total of seven contacted devices on each film sample and each device has been tested for its optical response in terms of Spectral Response and I-V characteristics in both light and dark conditions.

Photodetectors are used in many applications such as digital and thermal cameras or in solar panels. They can also be designed to detect the omnipresent high-energy radiation/particles, and for radiation imaging in biomedical applications. Novel nanostructures offer significant advantages compared to traditional designs for the realization of fast, sensitive, compact and cheap sensors and efficient solar cells. Examples of such nanostructures include quantum dots (QDs), quantum wells (QWs) and NW arrays. This thesis is devoted to experimental investigations of effects of high-energy (1 MeV) protons on the optical and electrical performance of InP/InAsP NW-based QDiscs-in wire photodetectors. The proton-induced degradation of the optical performance has been studied by means of Fourier Transform Infrared (FTIR) photocurrent spectroscopy. The spectrally resolved photocurrent (PC) and current-voltage (I-V) characteristics were measured at low temperature (5 K and 77K) and at room temperature (300K) before and after 1 MeV proton irradiation under vacuum conditions with fluences ranging from 1.0×1012–3.0×1013 cm-2. The particle radiation exposure has been done in the Ion Beam Accelerator at the Department of Nuclear Physics Department at Lund University. Considering both PC and I-V characteristics, it was found that the devices were sensitive to all proton irradiation at all fluences. In general, the PC intensity significantly increased after radiation for all fluences, however, a week after exposure the PC and dark current gradually recovered. At 3×1012 p/cm2 fluence level, it was figured out that photocurrent which attributed to QDiscs disappeared for a couple of days after exposure, however, over time and gradually, those started to manifest again even at low and room temperatures, causing radiation-induced changes in device parameters to be time-dependent; however, it was not recorded any signals related to QDiscs at fluence of 3×1013 p/cm2. Substantial changes in the dark I-V characteristics, as well as increases in the dark current, are observed after irradiation. The influence of proton irradiation on light and dark current characteristics also indicated that NW structures are a good potential candidate for radiation harsh-environment applications. It was also observed a significant increase in dark current after the radiation for all devices, however, by applying the voltage to the photodetectors, the PC and I-V characteristics gradually being to diminish, which may be attributed to an annealing process.

This report determines the influence of spectral beam splitting on the temperature, maximum power and efficiency of a polycrystalline silicon cell under concentrated light. The PV cell was exposed to wavelengths ranging between 450 nm – 1000 nm. It was found that spectral beam splitting results in a temperature 11 °C lower than the PV cell that was exposed to the full spectrum after one hour. Additionally, it was also found that spectral beam splitting improves the efficiency of the PV cell by 2.1% at 980 W·m-2 and cell temperature of 25 °C. A study into the effect of light intensity on the efficiency showed that the efficiency increases between 580 W·m-2 – 680 W·m-2, after which the efficiency decreases up to 1380 W·m-2. Furthermore, it was found that the reason for the decrease in the efficiency was the decrease in the fill factor which is caused by the decrease in the shunt resistance. A comparison between the PV cell under the filtered spectrum and the full spectrum, showed that the PV cell exhibits a similar trend in efficiency as light intensity increases. However, the efficiency difference is initially at approximately 3% between 580 W·m-2 and 780 W·m-2, thereafter, the efficiency difference decreases to approximately 2 %. Based on these results, it has been recommended that further research be carried out to understand how wavelengths influence the band gaps of PV cells as the light intensity increases. / Dissertation (MEng)--University of Pretoria, 2018. / Chemical Engineering / MEng / Unrestricted

A numerical model have been developed in order to describe and achieve deeper understanding of experimentally obtained I-V curves from Cu2O/ZnO p-n heterojunctions for potential use as future solar cell material. The model was created using the simulation software COMSOL Multiphysics® and their semiconductor module. To experimentally study the samples two approaches were taken: (1) macro-electrical measurements and (2) local I-V measurements using conductive AFM. The final model is one-dimensional, time dependent and with the ability to study photovoltaic effects of samples with different layer thickness at different voltage ramping speeds and different light irradiance. The model is also able to study the effects of using different contact materials by treating the contacts as ideal Schottky contacts. The dynamic behavior of a Cu_2O/ZnO heterojunction was studied by considering the systems response to a voltage step and the effect of changing the voltage ramping speed. The output from the step response, the current as a function of time, is varying a short time after a step has occurred before settling on to a steady value. The response also shows an overshoot of the current in the direction of the voltage step and the final steady value depends on whether the junction is conducting or not. The effects of this behavior on the shape of the I-V curves are witnessed when studying the different voltage ramping speeds. The voltage is ramped from 2 V to -2 V and back again for different speeds (V/s). The I-V curves have different shapes when sweeping the voltage in different directions and the magnitude increases with increasing speed. The photovoltaic effects were studied by applying different light irradiances. The behavior of the model agrees well with the theory for an ideal diode solar cell. An investigation was done of how the work function of the metal in contact with the Cu_2O affects the shape of the I-V curve under dark and illuminated conditions. The metal work function was changed from 4.5 eV to 6.5 eV in steps of 0.4 eV and does not affect the shape of the I-V curves much in dark after increasing it above 4.5 eV. The effects are more visible under illuminated conditions where a "step"-behavior appears for the lower values of the work function. Only one of the physical samples show a noticeable light effect. The macro-electrical measurement on this sample is compared with simulated results and are in qualitative agreement with each other. The agreement between the local electrical measurements and the simulated results is not as good with the current model.

Computational Fluid Dynamics (CFD) and predictive models are presented in this dissertation that illustrates the detailed electrical characteristics, and the current-voltage (i-v) relationship throughout the preheating process of premixed methane-oxygen (CH4-O2) oxyfuel cutting flame subject to electric bias voltages. As such, the equations describing combustion, electrochemical transport for charged species, and potential are solved through a commercially available finite-volume CFD code. The reactions of the methane-oxygen (CH4 – O2) flame were combined with the GRI 3.0 mechanism and a 25-species reduced mechanism, respectively, and additional ionization reactions that generate three chemi-ions, H3O+, HCO+, and e– , to describe the chemistry of ions in flames. The electrical characteristics such as ion migrations and ion distributions are investigated for a range of electric potential, V ∈ [−10V, +10V ]. Since the physical flame is comprised of twelve Bunsen-like conical flame, inclusion of the third dimension imparts the resolution of fluid mechanics and the interaction among the individual cones. As for developing the predictive models, four different supervised machine learning (ML) algorithms, decision tree (DT), random forest (RF), K-nearest neighbors (KNN), and artificial neural network (ANN), were employed to predict the i-v relationship. An experimental dataset of ≈ 10050 was utilized where a 60:20:20 split was adopted, allocating 60% for training, 20% for validation, and 20% for testing. It was concluded that charged 'sheaths' are formed at both torch and workpiece surfaces, subsequently forming three distinct regimes in the i-v relationship. The i-v characteristics obtained have been compared to the previous experimental study for premixed flame. In this way, the overall model generates a better understanding of the physical behavior of the oxyfuel cutting flames, along with a more validated i-v characteristics. Such understanding might provide critical information towards achieving an autonomous oxyfuel cutting process. / Doctor of Philosophy / Oxyfuel flame cutting is a century-old technique having widespread applications in heavy industries, including, but not limited to, building construction, defense, shipyards, etc. However, the mechanized oxyfuel cutting process has never benefited from the degree of autonomy due to contemporary sensing technologies' limitations at high-temperature working conditions. As a result, an experienced labor force is required to operate the system, thereby lowering the efficacy associated with this cutting process. A potential solution to this problem is motivated by preliminary measurements demonstrating that electrical events called 'ion currents' associated with the flame itself can reliably indicate vital process states. Provided that an autonomous process is achieved, this work could realize reliable cost-effective control of the oxyfuel cutting process, a capability of great interest to many core US industries involved in construction, and major equipment manufacture for defense and energy applications. Critical parameters (standoff, F/O ratio, flow rate, etc.) must be detected during operation to ensure an autonomous oxyfuel cutting process. The motivation stems from the fact that by measuring such co-dependence between critical parameters and electrical characteristics through a data acquisition unit (DAQ) and power supply, the shortcomings of sensing suites in a harsh operating environment can be compromised. Experimental data in the literature indicated the current-voltage (i-v) relationship with different critical parameters of oxyfuel flame to be the salient electrical characteristic in the preheating process when cutting steel. A comprehensive two-dimensional computational simulation using StarCCM+ only with the reduced combustion chemical mechanism with ion-exchange reactions has already been completed to elucidate the experimental results and to investigate the electrical characteristics such as ion migrations and ion distributions. Nonetheless, the findings exhibit some magnitude of differences compared to the experimental results. Thereby to further improve the results and better understand the underlying physics, further computational models using ANSYS FLUENT are proposed herein, having the reduced surface chemical mechanism considered. In addition, predictive models were developed based on machine learning (ML) algorithms. Four supervised ML algorithms - decision tree (DT), random forest (RF), Knearest neighbors (KNN), and artificial neural network (ANN) - were adopted to predict the current-voltage (i-v) relationship at different process states. ML offers a more data-driven, adaptable, and scalable approach to prediction compared to traditional methods. Its ability to handle large, noisy, and complex data makes it especially powerful for tasks that are challenging for conventional analytical techniques. The results of this study illustrate the detailed electrical characteristics of premixed methane-oxygen (CH4 – O2) oxyfuel cutting flame subject to an electric field, for both the computational fluid dynamics (CFD) and ML models. Since the physical flame is comprised of twelve Bunsen-like conical flame, inclusion of the third dimension will impart the resolution of fluid mechanics and the interaction among the individual cones. Moreover, the chemical activity at the work surface will also be considered, however, with a substantial simplification of the three-dimensional model as a cost. The overall model will generate a better understanding of the physical behavior of the oxyfuel cutting flames, along with a more validated currentvoltage (i-v) relationship. Consequently, this relationship could then be embedded into a control algorithm to detect the critical process parameters that may facilitate a step towards achieving an autonomous oxyfuel cutting process.

Le travail de thèse effectué porte sur la caractérisation électrique et la modélisation du gaz d’électrons à deux dimensions (2D) dans les dispositifs MOS-HEMT à base de l’hétérojonction AlGaN/AlN/GaN. Ces dispositifs ont un fort potentiel pour les applications d'électronique de puissance. Ce travail de recherche se place en soutien aux efforts de recherche pour l’élaboration des épitaxies GaN sur Si et pour les filières technologiques HEMT sur GaN. Il s'agit de comprendre précisément le fonctionnement du gaz d'électrons 2D et ses propriétés de transport électronique. Une nouvelle méthodologie a été développée pour identifier le dopage résiduel de la couche GaN, lequel est un paramètre important des substrats GaN et était par ailleurs difficile à évaluer. Un deuxième axe de recherche a consisté à proposer des techniques de mesure fiables ainsi qu’une modélisation des propriétés de transport du gaz d'électrons 2D. Dans ce cadre, des mesures split-CV et effet Hall ont été réalisées en fournissant pour chacune d’elles un protocole expérimental adéquat, avec un montage innovant pour les mesures effet Hall. Ce travail expérimental a été enrichi par une modélisation des propriétés du transport du 2DEG basée sur le formalisme de Kubo-Greenwood. Enfin, dans un dernier axe de recherche, un aspect plus général visant la compréhension en profondeur de l’électrostatique de l’empilement de la grille de nos GaN-MOS-HEMT a été proposé. Il est basé sur la caractérisation électrique C-V, la modélisation et l’extraction des paramètres. Le modèle développé a permis de souligner l'impact des charges surfaciques de polarisation et des défauts sur la tension de seuil des MOS-HEMT. Ce modèle a également permis d’estimer une valeur de la déformation dans les couches GaN épitaxiées sur un substrat Silicium. / This thesis aims at studying the electrical characterization and modelling of two-dimensional (2D) electron gas in MOS-HEMT devices based on the hetero-junction AlGaN/AlN/GaN. These devices are very promising candidates for power electronics applications. This research work provides the production team with detailed data on phenomena affecting GaN material. The goal is to understand precisely how 2D electron gas works and evaluate its electronic transport properties. A new methodology has been developed to identify residual doping of the GaN layer. This method was developed in order to answer a real need to know this doping to determine the quality of the epitaxial GaN layer. The second research priority was to provide reliable measurement techniques and modelling of the transport properties of 2D electron gas. Within this framework, the split-CV and Hall effect measurements were carried out by providing for each of them a suitable experimental protocol, with an innovative set-up for Hall effect measurements. In addition, this experimental work was supported by modelling the transport properties of 2DEG based on Kubo-Greenwood's formalism. Finally, a more general aspect aimed at an in-depth understanding of the electrostatic stacking of the GaN-MOS-HEMT gate. It is based on C-V electrical characterization, modelling and parameter extraction. The model developed made it possible to highlight the impact of polarization surface charges and defects on the threshold voltage of MOS-HEMT. This model also contributed to the estimation of the value of deformation in epitaxial GaN layers on a Silicon substrate.

Face aux sources d’énergie épuisables et polluantes, la production d’électricité par énergie solaire à concentration photovoltaïque (CPV) connaît depuis 2010 une croissance significative. Les systèmes ayant les plus hauts rendements (< 29%) sont constitués de trackers à deux axes combinés à des modules à modules (CPV) allant de 300x à 2500x, intégrant des cellules PV à triple jonction, dont l’efficacité mesurée en laboratoire est supérieure à 35%. Les pertes des systèmes sont dues aux conditions de fonctionnement extérieures fluctuantes, parmi lesquelles : la température, le contenu spectral du flux lumineux, la taille variable du disque solaire et la précision du suivi soumis à des limites physiques et des charges de vents variables. L’industrie tente de pallier à ces difficultés par des dispositifs à concentrations optique à angle d’acceptance (AA) large. Mais la caractérisation en laboratoire de tout dispositif doit être complétée par des tests en conditions externes. Ce projet consiste en la conception et la réalisation d’un banc d’essai extérieur de caractérisation de modules CPV, sous conditions réelles et variables, au moyen de tests I-V. Les données sont archivées sur une base de données, et prétraitées pour analyses des résultats. Les objectifs de caractérisation ont été atteints, soit le positionnement ponctuel précis en azimut et en élévation lors d’un test I-V, la répétition de cette étape selon des protocoles ajustables. Six dispositifs CPV à large AA ont été caractérisés en même temps (plus de 146 000 lectures). Des suggestions sont faites pour corriger les problèmes rencontrés. Des questions de recherches ultérieures sont posées, l’ensemble confirmant la pertinence de ce type de banc d’essai.