The radiationless recombination of electron-hole pairs in semiconductors is inherently detrimental to the operation of optoelectronic technologies. Auger recombination, a prominent many-body scattering mechanism, facilitates efficient non-radiative recombination by transferring the released energy and momentum to a third carrier. In this thesis, ultrafast time-resolved two-photon photoemission is used to investigate the action of carriers subject to Auger scattering in two III-V semiconductor material systems, InGaN quantum well light-emitting diodes and bulk GaSb. In InGaN quantum wells, Auger recombination is believed to limit the radiative quantum efficiency at high carrier injection currents. Chapter 3 reports the direct observation of carrier loss from a single InGaN quantum well due to Auger recombination on the picosecond timescale. Selective excitations of the different valence sub-bands reveal that the Auger rate constant decreases by two orders of magnitude as the effective band mass decreases, confirming the critical role of momentum conservation in the Auger process. In Chapter 4, photoemission is used to directly detect Auger electrons as they scatter into high energy and momentum states of the GaSb conduction band. The Auger rate in GaSb is observed to be modulated by a coherent phonon mode at 2 THz, confirming phonon participation in momentum conservation. The commonly assumed Auger rate constant is also found to vary significantly, decreasing by four orders of magnitude as hot electrons cool by ~90 meV. These findings provide quantitative guidance in understanding Auger recombination and in designing a broader range of materials for efficient optoelectronics.

No description available.

L'observatoire Pierre Auger vise à observer les évènements les plus violents de l'univers afin d'en découvrir les origines. Le premier site de cet observatoire, en cours de construction en Argentine, sera constitué de 1600 détecteurs au sol et de 4 télescopes d'observation de la fluorescence dans l'atmosphère. De par sa taille gigantesque, cet observatoire est le plus grand au monde. La présence d'eau en hiver sur une grande partie de la Pampa, et aussi, la grande surface rend l'accès difficile à la majorité des détecteurs. Les conditions difficiles d'environnement (variations thermiques, air salin et humidité) et la nécessité d'un fonctionnement permanent pendant 20 ans, imposent la recherche constante de la fiabilité à toutes les étapes du projet. Après une description nécessaire de l'ensemble de l'observatoire, ce mémoire décrit l'électronique des détecteurs de surface de l'observatoire Pierre Auger et les étapes de sa conception. La fiabilité constitue un problème prédominant de ce mémoire, en particulier en ce qui concerne :<ul><li>La recherche de la fiabilité optimale lors de la conception</li><li>L'évaluation de la fiabilité et la mise en place des stratégies de maintenance</li><li>La gestion de production (mise en place des procédures qualité, choix de l'entreprise, dossier de fabrication...) : de la qualité de production dépend la fiabilité du produit</li><li>Le déverminage qui permet d'éliminer les "défauts de jeunesse"</li><li>La mise au point des équipements de test</li><li>L'étude des performances des détecteurs, sa calibration et les méthodes de sélection des évènements. La bonne connaissance du fonctionnement des détecteurs constitue d'une part une approche système intéressante et d'autre part elle permet une identification rapide des défaillances</li><li>L'intégration, le suivi des cartes sur le terrain et la mise en place d'un retour d'expérience. L'expertise acquise peut profiter avantageusement à de futures expériences.</li></ul>

<p>This master thesis in physics mainly treats Auger spectroscopy of interfaces that has been adhesively bond together with so called controlled delaminating materials (CDM). CDM is a new technology which involves adhesives with the distinctive property that they by the appliance of electricity can be released from a substrate. The reason for using Auger spectroscopy was that it gives a surface sensitive view of the chemical composition of the samples examined and this was believed to give hints of the mechanisms behind loss of adhesion. The samples were so called laminates which is an aluminium foil, CDM adhesive, aluminium foil structure. As expected Auger spectroscopy produced some promising results, especially regarding the breakdown of a certain anion contained into the examined CDM adhesive. This awoke new questions regarding how this anion was decomposed and the idea that it could be due to hydrolysis took form. The by far most important result within this work is that the anion breaks down. This could lead to the formation of hydrofluoric acid if this breakdown in fact is due to hydrolysis and the hydrofluoric acid could then react with the aluminium causing loss of adhesion. This could be the good starting point of a continuing work on CDM.</p>

The interactions of a model synthetic liquid fuel reactant (furan) with a model hydrodeoxygenation catalyst (clean and sulfided single crystal molybdenum) were investigated using the following UHV tools: Auger electron spectroscopy (AES), low energy electron diffraction (LEED), and temperature programed reaction spectroscopy (TPRS). In addition to furan, the reactions of hydrogen, carbon monoxide, ethylene, and propene on clean and sulfided Mo(110) were also examined. All adsorbates exposed to the extremely reactive clean or sulfided Mo(110) surface decomposed, yielding gaseous H��� and surface C. In addition, furan TPRS caused the production of gaseous CO. The presence of background hydrogen caused no major changes in the TPRS of furan or the other adsorbates. Sulfur pre-adsorption caused the chemical shifting of H��� TPRS peaks. Both sulfur and carbon pre-adsorption resulted in the Van der Waal's radius blocking of adsorption sites for all adsorbates studied. / Graduation date: 1997

Graduation date: 1997

This master thesis in physics mainly treats Auger spectroscopy of interfaces that has been adhesively bond together with so called controlled delaminating materials (CDM). CDM is a new technology which involves adhesives with the distinctive property that they by the appliance of electricity can be released from a substrate. The reason for using Auger spectroscopy was that it gives a surface sensitive view of the chemical composition of the samples examined and this was believed to give hints of the mechanisms behind loss of adhesion. The samples were so called laminates which is an aluminium foil, CDM adhesive, aluminium foil structure. As expected Auger spectroscopy produced some promising results, especially regarding the breakdown of a certain anion contained into the examined CDM adhesive. This awoke new questions regarding how this anion was decomposed and the idea that it could be due to hydrolysis took form. The by far most important result within this work is that the anion breaks down. This could lead to the formation of hydrofluoric acid if this breakdown in fact is due to hydrolysis and the hydrofluoric acid could then react with the aluminium causing loss of adhesion. This could be the good starting point of a continuing work on CDM.

Ultrathin graphite films, with thickness from 1-30 atomic layers, are grown on the Si-terminated and C-terminated faces of 6H-SiC and 4H-SiC via thermal desorption of silicon in an ultrahigh vacuum (UHV) chamber or in a high-vacuum RF furnace. Graphite LEED patterns and atom-resolved STM images on graphite films prove that epitaxial growth is achieved on both faces of the SiC substrate. The thickness of graphite films is estimated with modeling the Si:C Auger peak intensities. Through LEED and STM investigations of monolayer graphite grown on the Si-face of SiC(0001) surface, we show the existence of a SiC 6R3*6R3 reconstructed layer between graphite films and the SiC substrate. The complicated LEED patterns can be interpreted partially by the kinematic scattering of the interfacial layer and the 6*6 surface corrugation. Further scanning tunneling spectroscopy (STS) measurements indicate that the graphite films remain continuous over the steps between domains. Carbon nanotubes and carbon nanocaps cover about 40% of the graphitized C-face of SiC. The remaining areas are flat graphite films. Graphite ribbons were made through E-beam lithography. After the lithography process, the graphitic features remain on flat region underneath HSQ residues.

The thesis investigates hot carrier relaxation and carrier recombination mechanism of a InN thin film grown on LAO(LiAlO2) substrate with a ultrafast time-resolved photoluminescence apparatus. Carriers were excited with laser pulses of energy 1.5 eV and of pulsewidth 150 fs from a Ti:sapphire laser. The photoexcited carriers relax excessive energy mostly within 10 ps thorough carrier-LO-phonon interaction. The effective carrier-LO-phonon emission times were estimated 197 to 58 fs in the temperature range from 250 to 35 K. The Shockley-Read-Hall coefficient was found around 0.8 ns-1. The Auger recombination was trivial at 35 K and become significant at 250 K. The fitted radiative recombination was much smaller than the theoretical estimate. Both effective carrier-LO-phonon scattering times and the radiative and nonradiative decay rates of the studied m-plane InN were found to be smaller than those of c-plane InN in other reports.

Die Arbeit befasst sich mit dem Wachstum dünner CrSi2-Schichten auf Si(001). Die Schichtherstellung wurde mittels eines template-Verfahrens in einer MBE-Anlage realisiert. Die Charakterisierung der Schichten erfolgte mittels RBS,AES,LEED,REM,TEM,XRD sowie Widerstandsmessungen.