This thesis examines the optical properties of nanoscale silicon and the sensitization of Er with Si. In this context, it predominantly investigates the role of defects in limiting the luminescence of Si nanocrystals, and the removal of these defects by hydrogen passivation. The kinetics of the defect passivation process, for both molecular and atomic hydrogen, are studied in detail. Moreover, the optical absorption of Si nanocrystals and the effect of annealing environment (during nanocrystal synthesis) on the luminescence are investigated. The effect of annealing temperature and hydrogen passivation on the coupling (energy transfer) of Si nanocrystals to optically active centres (Er) is also examined.¶ The electronic structure of silicon-implanted silica slides is investigated through optical absorption measurements. Before and after annealing to form Si nanocrystals, optical absorption spectra from these samples show considerable structure that is characteristic of the particular implant fluence. This structure is shown to correlate with the transmittance of the samples as calculated from the modified refractive index profile for each implant. Due to the high absorption coefficient of Si at short wavelengths, extinction at these wavelengths is found to be dominated by absorption. As such, scattering losses are surprisingly insignificant. To eliminate interference effects, photothermal deflection spectroscopy is used to obtain data on the band structure of Si in these samples. This data shows little variance from bulk Si structure and thus little effect of quantum confinement. This is attributed to the dominance of large nanocrystals in the absorption measurements.¶ The effect of annealing environment on the photoluminescence (PL) from silicon nanocrystals synthesized in fused silica by ion implantation and thermal annealing is studied as a function of annealing temperature and time. Interestingly, the choice of annealing environment (Ar, N2, or 5 % H2 in N2) is found to affect the shape and intensity of luminescence emission spectra, an effect that is attributed both to variations in nanocrystal size and the density of defect states at the nanocrystal/oxide interface.¶ The passivation kinetics of luminescence-quenching defects, associated with Si nanocrystals in SiO2, during isothermal and isochronal annealing in molecular hydrogen are studied by time-resolved PL. The passivation of these defects is modeled using the Generalized Simple Thermal model of simultaneous passivation and desorption, proposed by Stesmans. Values for the reaction-rate parameters are determined for the first time and found to be in excellent agreement with values previously determined for paramagnetic Si dangling-bond defects (Pb type centers) found at planar Si/SiO2 interfaces; supporting the view that non-radiative recombination in Si nanocrystals is dominated by such defects.¶ The passivation kinetics of luminescence-quenching defects during isothermal and isochronal annealing in atomic hydrogen are studied by continuous and time-resolved PL. The kinetics are compared to those for standard passivation in molecular hydrogen and found to be significantly different. Atomic hydrogen is generated using the alneal process, through reactions between a deposited Al layer and H2O or –OH radicals in the SiO2. The passivation and desorption kinetics are shown to be consistent with the existence of two classes of nonradiative defects: one that reacts with both atomic and molecular hydrogen, and the other that reacts only with atomic hydrogen. A model incorporating a Gaussian spread in activation energies is presented that adequately describes the kinetics of atomic hydrogen passivation and dissociation for the samples.¶ The effect of annealing temperature and hydrogen passivation on the excitation cross-section and PL of erbium in silicon-rich silica is studied. Samples are prepared by co-implantation of Si and Er into SiO2 followed by a single thermal anneal at temperatures ranging from 800 to 1100 degrees C, and with or without hydrogen passivation performed at 500 degrees C. Using time-resolved PL, the effective erbium excitation cross-section is shown to increase by a factor of 3, while the number of optically active erbium ions decreases by a factor of 4 with increasing annealing temperature. Hydrogen passivation is shown to increase the luminescence intensity and to shorten the luminescence lifetime at 1.54 micron only in the presence of Si nanocrystals. The implications of these results for realizing a silicon-based optical amplifier are also discussed.

Les calculs de processus représentent les systèmes concurrents par des processus qui s'exécutent en parallèle et s'échangent des messages. Les calculs avec passivation dispose d'un opérateur spécial qui permet de stopper un processus en cours d'exécution. Le processus suspendu peut ensuite être modifié ou transmis avant d'être réactivé. La passivation rend possible la modélisation de défaillances et d'opérations de reconfiguration dynamique. Nous nous intéressons aux équivalences comportementales dans ces calculs. Le comportement d'un processus est donné par un système de transitions étiquetées, qui exhibe les interactions d'un processus avec son environnement. Des relations, appelées bisimilarités, permettent ensuite d'identifier les processus qui ont le même comportement en comparant leurs interactions. Les bisimilarités définies jusqu'ici dans les calculs avec passivation restent trop complexes pour être utilisée en pratique. En outre il n'existe pas de bisimilarité correcte et complète dans le cas faible, c'est-à-dire lorsque les actions internes aux processus ne sont pas observables. Nous étudions ces deux problèmes dans cette thèse. Nous montrons d'abord qu'il est possible de définir une bisimilarité simple à manipuler pour un calcul avec passivation mais sans restriction. En revanche, nous donnons des contre-exemples qui laissent penser qu'il n'est pas possible de faire de même dans les calculs avec passivation et restriction. Nous définissons également un nouveau type de système de transitions étiquetées, qui permet de caractériser la congruence barbue dans le cas faible. Nous appliquons notre technique à différents calculs dont le Kell.

L'objectif de ce travail était d'étudier les interactions entre une protéine modèle, l'albumine de sérum bovin (BSA), et la surface d'un acier inoxydable ferritique, le Fe-17Cr, lors des toutes premières étapes de formation de la couche d'oxyde en solution aqueuse. Différents paramètres ont été testés : potentiel (potentiel de corrosion Ecor et potentiel passif), pH (compris entre 1,3 et 10) et absence/présence d'ions chlorures en solution. Pour atteindre cet objectif, des méthodes électrochimiques (courbes de polarisation, spectroscopie d'impédance électrochimique (SIE)) et la spectroscopie de photoélectrons induits par rayons X (XPS) ont été couplées. En absence de chlorures, les mesures d'impédance révèlent une inhibition de la corrosion par la BSA, à pH 1,3 et Ecor. Les analyses XPS montrent que la couche de BSA adsorbée sur la surface d'acier inoxydable (molécules chimiquement intactes ; épaisseur d'environ 4 nm) n'a d'effet ni sur la composition chimique ni sur l'épaisseur des couches d'oxyde et d'hydroxyde. Par conséquent, l'inhibition de la corrosion mise en évidence à pH 1,3 et Ecor est due à la protéine elle-même et non pas à un changement de composition chimique et/ou d'épaisseur des couches de surface. En présence de chlorures (NaCl 0,5M), la protéine accélèrerait la corrosion localisée du Fe-17Cr passivé à pH 5,5 ; de plus, le potentiel de piqûration Ep est plus faible quand la passivation et l'exposition à la BSA sont simultanées. Le potentiel de piqûration est d'autant plus cathodique que la quantité de protéine adsorbée est importante.

This thesis presents a work which has been devoted to the growth of silicon thin films on crystalline silicon for photovoltaic applications by means of RF PECVD. The primary goal of this work was to obtain an amorphous growth on any c-Si surface in order to provide an efficient passivation, as required in heterojunction solar cells. Indeed, we demonstrated that epitaxial or mixed phase growths, easy to obtain on (100) Si, would lead to poor surface passivation. We proved that growing a few nm thin a-Si1-xCx:H alloy film was an efficient, stable and reproducible way to hinder epitaxy while keeping an excellent surface passivation by the subsequent deposition of a-Si:H films. Process optimization mainly based on Spectroscopic Ellipsometry, Effective lifetime measurements (Sinton lifetime tester) and current-voltage characterization led us to demonstrate that it was possible to obtain a-Si:H/c-Si heterojunction solar cells with stable VOC of 710 mV and FF of 76 % on flat (n) c-Si wafers, with solar cells of 25 cm2 whose metallization was realized by screen-printing technology. This work has also demonstrated the viability of a completely dry process where the native oxide is removed by SiF4 plasma etching instead of the wet HF removal. Last but not least, the epitaxial growth of silicon thin films, undoped and n or p-type doped, on (100)-oriented surfaces has been studied by Spectroscopic Ellipsometry and Hall effect measurements. We have been able to fabricate homojunction solar cells with a p-type emitter as well as p-i-n structures with an undoped epitaxial absorber on a heavily-doped (p) c-Si wafers.

Electronic components, based on current semiconductor technologies and operating in radiation rich environments, suffer degradation of their performance as a result of radiation exposure. Silicon carbide (SiC) provides an alternate solution as a radiation hard material, because of its wide bandgap and higher atomic displacement energies, for devices intended for radiation environment applications. However, the radiation tolerance and reliability of SiC-based devices needs to be understood by testing devices  under controlled radiation environments. These kinds of studies have been previously performed on diodes and MESFETs, but multilayer devices such as bipolar junction transistors (BJT) have not yet been studied. In this thesis, SiC material, BJTs fabricated from SiC, and various dielectrics for SiC passivation are studied by exposure to high energy ion beams with selected energies and fluences. The studies reveal that the implantation induced crystal damage in SiC material can be partly recovered at relatively low temperatures, for damag elevels much lower than needed for amorphization. The implantation experiments performed on BJTs in the bulk of devices show that the degradation in deviceperformance produced by low dose ion implantations can be recovered at 420 oC, however, higher doses produce more resistant damage. Ion induced damage at the interface of passivation layer and SiC in BJT has also been examined in this thesis. It is found that damaging of the interface by ionizing radiation reduces the current gain as well. However, for this type of damage, annealing at low temperatures further reduces the gain. Silicon dioxide (SiO2) is today the dielectric material most often used for gate dielectric or passivation layers, also for SiC. However, in this thesis several alternate passivation materials are investigated, such as, AlN, Al2O3 and Ta2O5. These materials are deposited by atomic layer deposition (ALD) both as single layers and in stacks, combining several different layers. Al2O3 is further investigated with respect to thermalstability and radiation hardness. It is observed that high temperature treatment of Al2O3 can substantially improve the performance of the dielectric film. A radiation hardness study furthermore reveals that Al2O3 is more resistant to ionizing radiation than currently used SiO2 and it is a suitable candidate for devices in radiation rich applications. / QC 20120117

Nanowire heterostructures play a crucial role in nanoscale electronics, i.e., one-dimensional electronics derives benefits from the growth of heterostructures along the nanowire axis. We use first-principles plane-wave calculations within density functional theory with the localized density approximation (LDA) to get information about the structural and electronic properties of bare and hydrogen passivated GaAs/AlAs nanowire heterostructures. We also take into account the reconstruction of the nanowire surfaces. Modeled nanowire heterostructures are constructed using bulk atomic positions along [001] and [111] direction of zinc-blende structures and cutting out wires from this GaAs/AlAs heterostructure crystal with a diameter of 1 nm. We study for the effects of the surface passivation on the band gap and the band offsets for the planar GaAs/AlAs bulk heterostructure system and GaAs/AlAs nanowire heterostructure system. It is possible to control the potential that carriers feel in semiconductor heterostructures. For the planar lattice-matched heterostructures, the macroscopic average of potential of the two materials is constant far from the interface and there is a discontinuity at the interface depending on the composition of the heterostructure. In order to obtain the valence band offset in the heterostructure system, the shift in the macroscopic potential at the interface and the difference between the valence band maximum values of the two constituents must be added. In nanoscale heterostructures, the potential profile presents a more complex picture. The results indicate that while the discontinuity remains close to the planar limit right at the interface, there are fluctuations on the average potential profile beyond the interface developed by the inhomogeneous surface termination, that is, there are variations of the band edges beyond the interface. We report a first-principles study of the electronic properties of surface dangling-bond (SDB) states in hydrogen passivated GaAs/AlAs nanowire heterostructures with a diameter of 1 nm, where the SDB is defined as the defect due to an incomplete passivation of a surface atom. The charge transition levels of SDB states serve as a common energy reference level, such that charge transition level value for group III and V atoms is a constant value and a periodic table atomic property. We have carried out first-principles electronic structure and total energy calculations of aluminum nanowires for a series of different diameters ranging from 3 Angtrom-10 Angstrom, which is cut out from a slab of ideal bulk structure along the [001] direction. First-principles calculations of aluminum nanowires have been carried out within the density-functional theory. We use the norm-conserving pseudopotentials that are shown to yield successful results for ultrathin nanowire regime. Our results show that the number of bands crossing the Fermi level decreases with decreasing wire diameter and all wires studied are metallic.

Underground injection of acid gas has been studied for several decades for oil field applications, such as enhanced oil recovery (EOR), but is now being studied as a solution to climate change. This research aims to simulate underground conditions at injection sites, such as the pilot scale injection site located near the site of a coal fired power facility in the Black Warrior Basin of Alabama. This proposed carbon capture and sequestration (CCS) location would involve injection of liquid CO2 into a carbonaceous saline aquifer. The objective of this study was to investigate carbonate surface treatments that alter the kinetics and mechanism of mineral dissolution resulting from the injection of an acid gas (CO2) into a geologic formation. A variety of mineral coatings were tested in an attempt to preserve mineral integrity under acidic conditions. Surface active chemicals were first tested, including scale inhibitors, followed by a novel acid induced surface treatment that precipitates an inorganic layer on the calcite to preserve the acid soluble mineral. These experiments are the first to investigate the use of scale inhibitors for mineral preservation, although were found ultimately to have little impact on dissolution kinetics. However, anions of moderate to strong acids induced surface coatings that were determined to effectively inhibit dissolution. Additionally, a novel, high pressure flow-through experimental apparatus was developed to simulate pressure and temperature conditions relevant to injection sites. Similar mineralogical studies in the literature have used pressurized, unstirred, batch systems to simulate mineral interactions. Solids with an acid induced surface coating were tested in the high pressure column and no calcium was found to leave the column.

Underground injection of acid gas has been studied for several decades for oil field applications, such as enhanced oil recovery (EOR), but is now being studied as a solution to climate change. This research aims to simulate underground conditions at injection sites, such as the pilot scale injection site located near the site of a coal fired power facility in the Black Warrior Basin of Alabama. This proposed carbon capture and sequestration (CCS) location would involve injection of liquid CO2 into a carbonaceous saline aquifer. The objective of this study was to investigate carbonate surface treatments that alter the kinetics and mechanism of mineral dissolution resulting from the injection of an acid gas (CO2) into a geologic formation. A variety of mineral coatings were tested in an attempt to preserve mineral integrity under acidic conditions. Surface active chemicals were first tested, including scale inhibitors, followed by a novel acid induced surface treatment that precipitates an inorganic layer on the calcite to preserve the acid soluble mineral. These experiments are the first to investigate the use of scale inhibitors for mineral preservation, although were found ultimately to have little impact on dissolution kinetics. However, anions of moderate to strong acids induced surface coatings that were determined to effectively inhibit dissolution. Additionally, a novel, high pressure flow-through experimental apparatus was developed to simulate pressure and temperature conditions relevant to injection sites. Similar mineralogical studies in the literature have used pressurized, unstirred, batch systems to simulate mineral interactions. Solids with an acid induced surface coating were tested in the high pressure column and no calcium was found to leave the column.

Scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and X-ray diffraction (XRD) were used to examine a series of six revolver barrels manufactured by the Colt Patent Arms Manufacturing Company between 1853 and 1863. SEM micrographs revealed a vast range of surface morphologies among the corroded samples. XRD diffraction patterns showed varying levels of magnetite on the blackened samples, but hematite could not be identified. EDS was used to map elemental distribution and quantify elemental abundances on the gun surfaces; further investigation using this technique may reveal more definitive information on whether some elements present were deposited during patination.

In this work, spin-on processes were used to improve front- and rear-side technologies of solar cells to increase efficiencies to >20 %. A limited source diffusion process was developed using phosphoric acid dopant solutions developed in-house. An optimal emitter was obtained to be used in conjunction with screen-printed contacts. This emitter was used to improve the efficiency of conventional full aluminum back surface field solar cells to 19.6 %. A streamlined process was then developed to fabricate high-efficiency dielectric rear passivated cells in a single high temperature step. This process combined the diffusion process described earlier with a spin-on dielectric for rear passivation to achieve solar cell efficiencies of ~20%. Several laser candidates were investigated to improve process reproducibility and throughput. Ultra-violet laser with nanosecond pulse width was identified as the optimal choice. Cell efficiencies of ~20% were reproduced using UV laser for ablation of rear dielectric. This cell design and process were transferred to low-cost low-lifetime commercial grade substrates after identifying the optimal substrate characteristics using modeling.