No description available.

No description available.

Ternary InGaN and AlGaN alloys have been sought after for the application of various optoelectronic devices spanning a large spectral range between the deep ultraviolet and infrared, including light-emitting diodes, and laser diodes. Their non-ideal alloy mixing, and differences in bond energy and in adatom diffusion are established as the cause for various types of nanoscale compositional inhomogeneity commonly observed in nitride thin films. Growth in a nanowire geometry can overcome the phase separation, surface segregation, and chemical ordering by providing enhanced strain relaxation of the large lattice mismatch at the free surfaces. In this dissertation, the spectral and spatial luminescence distributions of ternary III-N alloy nanowire heterostructures are investigated and correlated to structural and chemical properties with scanning transmission electron microscopy. Quantitative elemental mapping of InGaN/GaN dot-in-a-wire structures using electron energy-loss spectroscopy revealed compositional non-uniformity between successive quantum dots. Local strain mapping of the heterostructure showed a dependence of the incorporation of indium on the magnitude of the out-of-plane compressive strain within the underlying GaN barrier layer. Cathodoluminescence spectroscopy on individual nanowires presented diverse emission properties, nevertheless, the In-content variability could be directly correlated to the broad range of peak emission energies. Atomic-level chemical ordering within the InGaN was then reported, and attributed to the faceted growth surface in nanowires that promotes preferential site incorporation by In-atoms that allows for better strain relaxation. Distinct atomic-scale alloy inhomogeneities were also investigated in AlGaN nanowires, which evidenced spatial localization of carriers taking place at the resulting energy band fluctuations. A high spectral density of narrow emission lines arose from such compositional modulations, whose luminescence behaviours exhibit a dependence on the nature of the compositional fluctuations from which they originate. / Thesis / Doctor of Philosophy (PhD)

<p>Repolarization inhomogeneity is arrhythmogenic. QT dispersion (QTd) is an easily accessible ECG-variable, related to the repolarization and shown to carry prognostic information. It was originally thought to reflect repolarization inhomogeneity. Lately, arguments have been risen against this hypothesis. Other measures of inhomogeneity are being investigated, such as nondipolar components from principal component analysis (PCA) of the T-wave. In all here described populations, continuous 12-lead ECG was collected during the initial hours of observation and secondary parameters used for description of a large number of ECG-recordings.</p><p>Paper I studied QTd in 548 patients with chest pain with a median number of 985 ECG-recordings per patient. Paper II explored a spatial aspect of QTd in 276 patients with unstable coronary artery disease. QTd and a derived localized ECG-parameter were compared to angiographical measures. QTd, expressed as the mean value during the observation was a powerful marker of risk. It was however not effective in identifying high-risk patients. Variations in QTd contained no additional prognostic information. In unstable coronary artery disease, QTd was increased by a mechanism unrelated to localization of the disease.</p><p>Two relevant conditions for observing repolarization inhomogeneity might occur with conduction disturbances and during initial course of ST-elevation myocardial infarction (STEMI). Paper III compared the PCA-parameters of the T-wave in 135 patients with chest pain and conduction disturbance to 665 patients with normal conduction. Nondipolar components were quantified by medians of the nondipolar residue (TWRabsMedian) and ratio of this residue to the total power of the T-wave (TWRrelMedian). Paper IV described the changes in the nondipolar components of the T-wave in 211 patients with thrombolyzed STEMI. TWRabsMedian increased with increasing conduction disturbance and contained a moderate amount of prognostic information. In thrombolyzed STEMI, TWRabsMedian was elevated and has an increased variability. A greater decrease in absolute TWR during initial observation was seen in patients with early ST-resolution. Nondipolar components do however not reflect identical ECG-properties as the ST-elevation and their change does not occur at the same time.</p>

Despite the vast myriad of applications and the long way it has come, MRI is still a relatively new field of knowledge with much prospect for more advancement and expansion. This work is mainly concerned with two gradient echo imaging methods which are directly or indirectly related to blood vessel imaging as well as iron depiction in the human brain. In each case, new methods are introduced that overcome existing limitations. For blood vessel imaging, 3D Time-of-Flight (TOF) MR angiography (MRA) with its known capability to image arteries as well as veins was implemented at 3.0 T. At this field strength, the significant RF profile variability due to RF inhomogeneity is a liability for circle-of-Willis imaging in the human brain that was overcome by introducing a new means to counter the RF effects through increased slope of the ramped pulse. In addition a new method is introduced for TOF MRA with two-in-one arterial and venous 3D TOF imaging to overcome the significant scan time overhead of a traditional second venous scan and for cutting down RF power utilization. Using this method, total scan time could be reduced by as much as 46% and specific absorption rate (SAR) due to spatial saturation could be reduced by as much as 92%. For iron sensitive imaging, Susceptibility Weighted Imaging (SWI) was developed at 4.7 T. The phase SWI method was used to visualize lesions in Multiple Sclerosis (MS) patients and was experimentally compared to the visibility on standard T2 weighting with results demonstrating visualization of new lesions, with 18% of total lesions exclusively visible on SWI. A new approach to 3D imaging was also introduced to enable accurate oblique SWI scanning while overcoming the current restriction to axial imaging to produce correct phase effects for oblique imaging. New results from oblique phase imaging were presented and the phase measurements from key brain structures were successfully validated against images obtained by the current standard of axial imaging.

Repolarization inhomogeneity is arrhythmogenic. QT dispersion (QTd) is an easily accessible ECG-variable, related to the repolarization and shown to carry prognostic information. It was originally thought to reflect repolarization inhomogeneity. Lately, arguments have been risen against this hypothesis. Other measures of inhomogeneity are being investigated, such as nondipolar components from principal component analysis (PCA) of the T-wave. In all here described populations, continuous 12-lead ECG was collected during the initial hours of observation and secondary parameters used for description of a large number of ECG-recordings. Paper I studied QTd in 548 patients with chest pain with a median number of 985 ECG-recordings per patient. Paper II explored a spatial aspect of QTd in 276 patients with unstable coronary artery disease. QTd and a derived localized ECG-parameter were compared to angiographical measures. QTd, expressed as the mean value during the observation was a powerful marker of risk. It was however not effective in identifying high-risk patients. Variations in QTd contained no additional prognostic information. In unstable coronary artery disease, QTd was increased by a mechanism unrelated to localization of the disease. Two relevant conditions for observing repolarization inhomogeneity might occur with conduction disturbances and during initial course of ST-elevation myocardial infarction (STEMI). Paper III compared the PCA-parameters of the T-wave in 135 patients with chest pain and conduction disturbance to 665 patients with normal conduction. Nondipolar components were quantified by medians of the nondipolar residue (TWRabsMedian) and ratio of this residue to the total power of the T-wave (TWRrelMedian). Paper IV described the changes in the nondipolar components of the T-wave in 211 patients with thrombolyzed STEMI. TWRabsMedian increased with increasing conduction disturbance and contained a moderate amount of prognostic information. In thrombolyzed STEMI, TWRabsMedian was elevated and has an increased variability. A greater decrease in absolute TWR during initial observation was seen in patients with early ST-resolution. Nondipolar components do however not reflect identical ECG-properties as the ST-elevation and their change does not occur at the same time.

No description available.

L'imagerie par résonance magnétique (IRM) à haut champ offre un bénéfice certain en rapport signal-sur-bruit. De ce fait, les imageurs à 3T sont souvent utilisés en pratique clinique.Cependant, à haut champ, les images d'IRM sont entachées de pertes de signal et de contraste liées à la baisse de la longueur d’onde radiofréquence (RF) en deçà des dimensions de l'objet irradié. A 3T, où la longueur d'onde est de 30 cm environ dans les tissus humains, de tels artéfacts sont fréquents en imagerie abdominale, des seins ou encore des cuisses, ce qui peut expliquer la difficulté que rencontre l'IRM à haut champ à s'imposer comme référence dans les hôpitaux. Les imageurs 3T les plus récents disposent en général d'un système de transmission parallèle à deux canaux RF. Chaque canal peut en principe émettre des formes d'impulsion RF indépendantes. En pratique, sur la plupart des systèmes IRM cliniques, la méthode dite de shim RF statique est utilisée. Les différents canaux transmettent la même forme d’onde,en ajustant amplitude et phase de sorte à entraîner des motifs d’interférences pour contrer les inhomogénéités, mesurées au préalable sur le patient: cartes de champ RF et éventuellement de champ statique. Cette méthode fonctionne d’autant mieux qu’un grand nombre de canaux est disponible, mais montre ses limites lorsqu’il s’agit d’homogénéiser l’excitation sur un grand champ de vue. La méthode des points kT, développée à NeuroSpin pour l’IRM cérébrale à ultra-haut champ (7T) utilise une alternance d’impulsions RF rectangulaires et de gradients de champ statique pour moduler à dessein l’aimantation des spins et ainsi homogénéiser l’excitation malgré un champ RF inhomogène. Les impulsions ainsi créées sont d’autant plus efficaces qu’elles peuvent exploiter la transmission parallèle (huit canaux à 7T). Dans cette thèse, les points kT sont employés à 3T avec pour objectif d’en démontrer l’intérêt et l’applicabilité en routine clinique. Dans un premier temps, nous décrivons des modifications apportées à l’algorithme de conception de points kT et à la cartographie de champ statique permettant d’adapter la technique à l’imagerie du corps – foie, seins – où la présence des poumons et de la graisse entraîne des contraintes supplémentaires par rapport au cerveau.Dans un second temps, plusieurs études cliniques sont exposées. La première concerne l’IRM du sein en pondération T₂ sur un imageur à canal d’émission unique. Elle met en évidence que le mode d’émission par défaut fonctionne correctement et n’est que peu amélioré par les points kT. Une deuxième étude se penche sur l’imagerie dynamique du foie avec injection de produit de contraste, avec deux canaux. Des analyses quantitatives et qualitatives sont menées sur un grand nombre de patients pour comparer le shim RF statique avec les points kT. Ces derniers améliorent très nettement les images obtenues chez certains patients« difficiles », permettant ainsi d’offrir une qualité d’examen et de diagnostic plus homogène sur l’ensemble de la population. Enfin,une nouvelle technique est présentée, intitulée SmartPulse, qui s’appuie sur le concept d’impulsions universelles, développé à NeuroSpin pour l’imagerie du cerveau, et dont le principe est de concevoir des impulsions de type points kT qui, pour une application donnée, soient efficaces sur toute la population et permettent de se passer de calibration. En divisant la population en catégories pour lesquelles des impulsions différentes sont conçues,et en utilisant un algorithme de classification par apprentissage automatique, SmartPulse étend la portée des impulsions universelles au corps, et en particulier à l’abdomen, où la variabilité morphologique est importante. Par ces travaux de thèse, nous espérons donner un nouveau souffle à la gestion des inhomogénéités RF en routine clinique à 3T, et apporter des éléments permettant à terme de démocratiser l’imagerie des gros organes à ultra-haut champ. / High field magnetic resonance imaging (MRI) is often used in clinical practice, for the high signal-to-noise ratio it offers.However, at high field, the radiofrequency (RF) wavelength used for imaging is shorter, which can induce loss of signal and contrast when it is close to or shorter than the dimensions of the irradiated objects. At 3T, RF wavelength is about 30 cm in human tissues,and such artefacts are frequently observed in the abdomen, as well as in the thighs or in the breasts. This is certainly one of the main reasons why high field MRI has failed to establish itself as the gold standard in hospital, where 1.5T scanners are more frequent.Recent 3T scanners usually come with a two-RF-channel parallel transmission setup: in principle, each channel can transmit completely independent waveforms. However, this technology is not exploited fully in practice, as only the static RF shimming is implemented: a single waveform is used, with adjusted amplitude and phase on each channel. This allows to create interference patterns, calculated to counteract transmission inhomogeneities measured beforehand in the patient (RF and possibly static field).This method works best when many channels are available, but shows its limits when good homogeneity is expected over a large field of view. The kT-points method, developed at CEA-NeuroSpin for brain imaging at ultra-high field (7T) relies on a succession of short rectangular RF pulses interleaved with static gradient “blips” to modulate magnetisation at will, thus producing homogeneous excitation in spite of an imperfect RF field. Those composite pulses are even more effective as they can take advantage of parallel transmission (eight channels at 7T). In this thesis, the kT-points technique is applied at 3T. The objective is to demonstrate its usefulness and its compatibility with a clinical routine workflow. First, several changes made to the kT-points pulse design algorithm and to static field mapping in order to adapt them to body imaging (liver, breasts) are described. Indeed, the presence of lungs and fat add further constraints compared to the brain. Then, several clinical studies are detailed. The first one regards T₂-weighted breast MRI on a single-channel scanner. It shows that in that case the default transmit mode is satisfactory,and only slightly improved by kT-points. A second study focuses on T₁-weighted dynamic contrast-enhanced imaging of the liver,with two transmit channels. Static RF shimming and kT-points were compared on a large cohort. For some “difficult” patients,acquisitions were quantitatively and qualitatively better with kTpoints,which therefore offer a more uniform diagnostic quality among the population. Finally, a novel method is proposed:SmartPulse. It is based on the universal pulse concept, developed in NeuroSpin for brain imaging, whose principle is to design pulses (e.g. kT-points) for a given application, that homogenise excitation in the whole population, and not only for one subject.Thus, there is no more need for cumbersome mapping and inline pulse design. SmartPulse extends the range of universal pulses to body imaging, by adequately clustering the population, designing different pulses for each cluster, and applying a machine learning classifier to assign the most appropriate pulse to any new subject.Proof of concept was undertaken in abdominal imaging, whereinter-subject variability is considerable. We hope this thesis will give a new outlook on RF inhomogeneity handling in routine 3T MRI, and in the long run will help making body imaging moreaccessible at high and ultra-high field.

Interest in wide bandgap semiconductors such as silicon carbide (SiC), gallium nitride (GaN), gallium oxide (Ga 2 O 3 ) and diamond has increased due to their ability to deliver high power, high switching frequency and low loss electronic devices for power conversion applications. To meet these requirements, semiconductor material defects, introduced during growth and fabrication, must be minimized. Otherwise, theoretical limits of operation cannot be achieved. In this dissertation, the non-ideal current- voltage (IV) behavior of GaN-based Schottky diodes is discussed first. Here, a new model is developed to explain better the temperature dependent performance typically associated with a multi-Gaussian distribution of barrier heights at the metal-semiconductor interface [Section 3.1]. Application of this model gives researches a means of understanding not only the effective barrier distribution at the MS interface but also its voltage dependence. With this information, the consequence that material growth and device fabrication methods have on the electrical characteristics can be better understood. To show its applicability, the new model is applied to Ru/GaN Schottky diodes annealed at increasing temperature under normal laboratory air, revealing that the origin of excess reverse leakage current is attributed to the low-side inhomogeneous barrier distribution tail [Section 3.2]. Secondly, challenges encountered during MOCVD growth of low-doped GaN drift layers for high-voltage operation are discussed with focus given to ongoing research characterizing deep-level defect incorporation by deep level transient spectroscopy (DLTS) and deep level optical spectroscopy (DLOS) [Section 3.3 and 3.4]. It is shown that simply increasing TMGa so that high growth rates (>4 µm/hr) can be achieved will cause the free carrier concentration and the electron mobilities in grown drift layers to decrease. Upon examination of the deep-level defect concentrations, it is found that this is likely caused by an increase in 4 deep level defects states located at E C - 2.30, 2.70, 2.90 and 3.20 eV. Finally, samples where the ammonia molar flow rate is increased while ensuring growth rate is kept at 2 µm/hr, the concentrations of the deep levels located at 0.62, 2.60, and 2.82 eV below the conduction band can be effectively lowered. This accomplishment marks an exciting new means by which the intrinsic impurity concentration in MOCVD-grown GaN films can be reduced so that >20 kV capable devices could be achieved. / Doctor of Philosophy / We constantly rely on electronics to help assist us in our everyday lives. However, to ensure functionality we require that they minimize the amount of energy lost through heat during operation. One contribution to this inefficiency is incurred during electrical power conversion. Examples of power conversion include converting from the 120 V wall outlet to the 5 V charging voltage used by cellphones or converting the fluctuating voltage from a solar panel (due to varying sun exposure) to the 120 V AC power found in a typical household. Electrical circuits can be simply designed to accomplish these conversions; however, consideration to every component must be given to ensure high efficiency. A popular example of an electrical power conversion circuit is one that switches the input voltage on and off at high rates and smooths the output with an inductor/capacitor network. A good analogy of this process is trying to create a small stream of water from a fire hydrant which can either be off or on at full power. Here we can use a small cup but turning the fire hydrant on and trying to fill the cup will destroy it. However, if the fire hydrant is pulsed on and off at very short intervals (1 µs), its possible to fill the cup without damaging it or having it overflow. Now, under ideal circumstances if a small hole is poked in the bottom of the cup and the interval of the fire hydrant is timed correctly, a small low power stream of water is created without overflowing the cup and wasting water. In this analogy, a devices capable of switching the stream of water on and off very fast would need to be implemented. In electrical power conversion circuits this device is typically a transistor and diode network created from a semiconducting material. Here, similar to the fire hydrant analogy, a switch would need to be capable of holding off the immense power when in the off position and not impeding the powerful flow when in the on position. The theoretical limit of these two characteristics is dependent on the material properties of the switch where typically used semiconductors include silicon (Si), silicon carbide (SiC), or gallium nitride (GaN). Currently, GaN is considered to be a superior option over Si or SiC to make the power semiconductor switching device, however research is still required to remove non-ideal behavior that ultimately effects power conversion efficiency. In this work, we first examine the spurious behavior in GaN-based Schottky diodes and effectively create a new model to describe the observed behavior. Next, we fabricated Ru/GaN Schottky diodes annealed at different temperatures and applied the model to explain the room-temperature electrical characteristics. Finally, we grew GaN under different conditions (varying TMGa and ammonia) so that quantum characteristics, which have been shown to affect the overall ability of the device, could be measured.

Ferroelectric and ferromagnetic materials have been extensively studied for potential applications in sensors, actuators and transducers. Highly electrostrictive (1-x)Pb(Mg_1/3Nb_2/3)-xPbTiO₃ (PMN-xPT) and highly magnetostrictive Fe-xat.%Ga are two such novel materials. Both materials systems have chemical disorders and structural inhomogeneity on a microscale, giving rise to an interesting diversity of crystal structures and novel macroscopic physical properties, which are dependent on thermal and electrical histories of the crystals. In this thesis, I have to investigated phase transformations in these two systems under thermal and field (electric/magnetic) histories, using x-ray and neutron scattering techniques. In PMN-xPT crystals, x-ray and neutron diffractions were performed along the different crystallographic orientations and for different thermal and electrical histories. Various intermediate monoclinic (M) phases that structurally “bridge” the rhombohedral (R) and tetragonal (T) ones across a morphtropic phase boundary (MPB) have been observed. Systematic investigations of (001) and (110) electric (E) field-temperature phase diagrams of PMN-xPT crystals have demonstrated that the phase stability of PMN-xPT crystals is quite fragile: depending not only on modest changes in E (≤ 0.5kV/cm), but also on the direction along which E is applied. Structurally bridging monoclinic Mc or orthorhombic (O) phases were found to be associated with the T phase, whereas the monoclinic Ma or Mb phases bridged the Cubic (C) and R ones. In addition, neutron inelastic scattering was performed on PMN-0.32PT to study the dynamic origin of the MPB. Data were obtained between 100 and 600 K under various E applied along the cubic [001] direction. The lowest frequency zone-center, transverse optic phonon was strongly damped and softened over a wide temperature range, but started to recover on cooling into the T phase at the Curie temperature (TC). Comparisons of my findings with prior ones for PMN and PMN-0.60PT suggest that the temperature dependence and energy scales of the soft mode dynamics in PMN-xPT are independent of PT concentration below the MPB, and that the MPB may be defined in composition space x when TC matches the temperature at which the soft mode frequency begins to recover. High-resolution x-ray studies then showed that the C–T phase boundary shifted to higher temperatures under E by an expected amount within the MPB region: suggesting an unusual instability within the apparently cubic phase at the MPB. In Fe-xat.%Ga alloys, the addition of Ga atoms into the b.c.c. α-Fe phase also results in diversity of crystal structures and structural inhomogeneity, which are likely the source of its unusual magneto-elastic properties. I have carefully investigated decomposition of Fe-xat.%Ga alloys subjected to different thermal treatments by x-ray and neutron diffraction for 12 < x < 25. Quenching was found to suppress the formation of a DO₃ structure in favor of a high-temperature disordered bcc (A2) one. By contrast, annealing produced a two-phase mixture of A2 + DO₃ for 14 < x < 20 and a fully DO₃ phase for x = 25. A splitting of the (2 0 0) and (0 0 2) Bragg peaks observed along the respective transverse directions indicated that Fe-xat.%Ga –crystals' are composed of several crystal grain orientations (or texture structures), which are slightly tilted with respect to each other. In order to investigate the local structural distortions and heterogeneities, neutron diffuse scattering was performed on Fe-x%Ga alloys for different thermal conditions. Diffuse scattering around a (100) superlattice reflection was found for 14 < x < 22 in the furnace-cooled condition, indicative of short-range ordered DO₃ nanoprecipitates in an A2 matrix. This diffuse intensity had an asymmetric radial contour and an off-centering. Analysis (x=19) revealed two broad peaks with c/a–1.2: indicating that the DO₃-like nanoprecipitates are not cubic, but rather of lower symmetry with a large elastic strain. The strongest diffuse scattering was observed for x=19, which correspondingly had maximum magnetostriction: indicating a structural origin for enhanced magnetostriction. / Ph. D.