Global ETD Search

341	Ultraschnelle Ladungsträger- und Gitterdynamik in GaN- und GaAs-basierten Übergittern Mahler, Felix 20 April 2021 (has links) In dieser Dissertation wird zum einen die ultraschnelle Ladungsträgerkinetik in einem Galliumnitrid (GaN)-basierten Übergitter, zum anderen die piezoelektrische Elektron-Phonon-Wechselwirkung kohärenter zonengefalteter Phononen in Galliumarsenid (GaAs)-basierten Übergittern behandelt. Mittels spektral- und zeitaufgelöster Photolumineszenzmessungen an einem n-dotierten GaN/Al0,18Ga0,82N Übergitter mit Parametern ähnlich derer in optoelektronischen Bauelementen wurde die defektbedingte Ladungsträgerkinetik untersucht, die innerhalb von ca. 150 ps durch den Einfang in tiefe, nichtstrahlende Rekombinationszentren beeinflusst wird. Die Untersuchung einer Passivierung mit Siliziumnitrid zur Verhinderung von Degradationseffekten zeigte ein stabiles optisches Langzeitverhalten bei gleichzeitiger Zunahme nichtstrahlender Defekte. Ferner wurde mit spektral aufgelöster Anrege-Abfrage-Spektroskopie eine Einfangkinetik auf einer Zeitskala von 150 - 200 fs in Defektzustände nahe der Übergitterbandkante gemessen, gefolgt von der Abkühlung der Ladungsträger durch Phononemission innerhalb weniger Pikosekunden bei Raumtemperatur und 35 ps bei 5 K. Kohärente zonengefaltete Phononen wurden mit Anrege-Abfrage-Spektroskopie an zwei AlAs/GaAs-Übergittern untersucht, die in [100]-, bzw. [111]-Richtung gewachsen wurden. Dies ermöglicht die (gezielte) Untersuchung der piezoelektrischen Elektron-Phonon-Kopplung, da diese für longitudinal-akustischen Phononen nur in der [111]-Probe existiert. Die Amplitude kohärenter Phononen mit einem Wellenvektor von q=0 in der [111]-Probe fällt verglichen mit denen in der [100]- und der [111]-Probe mit q≠0 signifikant schneller ab. Kohärente Phononen verursachen in der [111]-Probe bei q=0 ein makroskopisches piezoelektrisches Feld, welches Ladungsträger beschleunigt, die durch Reibung kohärente Phononen dämpfen. Bei hohen Ladungsträgerdichten unterdrückt die Abschirmung der induzierten piezoelektrischen Felder diese zusätzliche Dämpfung. / In this dissertation, the ultrafast carrier dynamics in a gallium nitride (GaN)-based superlattice as well as the piezoelectric electron-phonon-coupling of coherent zone-folded phonons in gallium arsenide (GaAs)-based superlattices are addressed. Using spectrally and time-resolved photoluminescence experiments on an exemplary n-doped GaN/Al0.18Ga0.82N superlattice with parameters similar to those in optoelectronic devices, we investigated the defect-related carrier kinetics, that are affected by trapping in saturable nonradiative recombination centers on time scales of ~150 ps. The investigation of a passivation with silicon nitride to prevent degradation effects show a long-term optical stability with a concomitant increase in non-radiative defect densities. Furthermore, spectrally resolved pump-probe spectroscopy was used to measure trapping kinetics into defect states near the conduction band minimum on a time scale of 150 – 200 fs. These kinetics are followed by carrier cooling through phonon emission within a few picoseconds at room temperature and within 35 ps at 5 K. Coherent zone-folded phonons were studied with pump-probe spectroscopy on two AlAs/GaAs superlattices grown in [100] and [111] direction, respectively. This allows the specific investigation of the piezoelectric electron-phonon interaction, since this exists for longitudinal acoustic phonons only in the [111] sample. The amplitude of coherent phonons with a wave vector of q=0 in the [111] sample decays significantly faster than in the [100] and the [111] samples with q≠0. Coherent phonons in the [111] sample cause a macroscopic piezoelectric field to which the photogenerated electron-hole plasma couples. Friction of the accelerated carriers provides the additional damping mechanism. High carrier densities screen the induced piezoelectric field, thus reducing the damping mechanism via the piezoelectric interaction. kohärente Phononen Übergitter Galliumnitrid Galliumarsenid Defektspektroskopie Ultrakurzzeit-Spektroskopie Zeitaufgelöste Photolumineszenz Gallium nitride Gallium arsenide Superlattice Coherent phonons Defect spectroscopy Short pulse spectroscopy Time-resolved photoluminescence 530 Physik ddc:530
342	Le procédé HVPE pour la croissance de nanofils semiconducteurs III-V / The HVPE process for the growth of III-V semiconductor nanowires Lekhal, Kaddour 18 February 2013 (has links) Cette thèse est consacrée à l’étude de l’outil d’épitaxie HVPE (Hydride Vapour Phase Epitaxy) pour la synthèse avec et sans catalyseur de nanofils semiconducteurs GaN et GaAs. Une étude systématique de l’influence des conditions expérimentales sur la croissance des fils de GaN est effectuée, afin de démontrer la faisabilité de cette croissance sur la surface des substrats saphir plan-c et silicium sans aucun traitement de la surface préalablement à la croissance. Nous avons démontré la croissance par VLS-HVPE, de nanofils de GaN de diamètres constants de 40 à 200 nm, de longueurs supérieures à 60 μm et présentant des qualités optique et cristallographique remarquables. Pour les nanofils de GaAs, la stabilité, inédite, de la phase cubique zinc-blende pour des diamètres de 10 nm a été démontrée par le procédé de croissance VLS-HVPE sur des longueurs de quelques dizaines de micromètres. Les mécanismes de croissance sont discutés à partir des diagrammes de phase et de la physique de la croissance HVPE qui met en oeuvre des précurseurs gazeux chlorés. Pour les semiconducteurs III-V, cette étude permet d’envisager des applications liées aux nanofils longs qui jusque là n’étaient exploitées que pour le silicium. Ces travaux montrent que dans le contexte des Nanosciences, la HVPE, outil épitaxial à fortes vitesses de croissance, mérite une audience élargie, et peut s’inscrire comme un outil complémentaire efficace aux procédés MOVPE et MBE pour le façonnage contrôlé de la matière à l’échelle nanométrique. / This thesis is devoted to the study of HVPE (Hydride Vapour Phase Epitaxy) method of growing GaN and GaAs nanowires with and without catalyst. A systematic study of the influence of the growth conditions on GaN formation was performed in order to demonstrate the feasibility of this growth on c-plane sapphire and silicon substrates without preliminary treatment of the surface. We have demonstrated by VLS-HVPE the growth of the GaN nanowires with constant diameters of 40 to 200 nm and of length up to 60 μm, while they possess remarkable optical and crystal quality. The newly observed stability of the zinc blende structure for GaAs nanowires with diameters of 10 nm has been described by the VLS-HVPE process, for lengths of few tens of micrometers. The growth mechanisms are discussed based on the phase diagram and the physics of near-equilibrium HVPE using chloride precursors. For III-V semiconductors, the study allows us to consider applications related to long nanowires that, at present, are used only for silicon. This work shows that in the context of Nanoscience, the fast growth HVPE method deserves a wider audience and thus could be considered as an effective complementary tool to MOVPE and MBE processes for the controlled shaping of matter on the nanoscale. Nanofils Nitrure de gallium (GaN) Arséniure de gallium (GaAs) VSS (Vapor-Solide- Solide) Hydride Vapour Phase Epitaxy (HVPE) Nanowires Gallium nitride (GaN) Gallium arsenide (GaAs) VSS (Vapour-Solid-Solid)
343	Novel tools for ultrafast spectroscopy Jarvis, Thomas William 06 February 2012 (has links) Exciton dynamics in semiconductor nanostructures are dominated by the effects of many-body physics. The application of coherent spectroscopic tools, such as two-dimensional Fourier transform spectroscopy (2dFTS), to the study of these systems can reveal signatures of these effects, and in combination with sophisticated theoretical modeling, can lead to more complete understanding of the behaviour of these systems. 2dFTS has previously been applied to the study of GaAs quantum well samples. In this thesis, we outline a precis of the technique before describing our own experiments using 2dFTS in a partially collinear geometry. This geometry has previously been used to study chemical systems, but we believe these experiments to be the first such performed on semiconductor samples. We extend this technique to a reflection mode 2dFTS experiment, which we believe to be the first such measurement. In order to extend the techniques of coherent spectroscopy to structured systems, we construct an experimental apparatus that permits us to control the beam geometry used to perform four-wave mixing reflection measurements. To isolate extremely weak signals from intense background fields, we extend a conventional lock-in detection scheme to one that treats the optical fields exciting the sample on an unequal footing. To the best of our knowledge, these measurements represent a novel spectroscopic tool that has not previously been described. / text Spectroscopy Ultrafast spectroscopy Ultra-fast spectroscopy Exciton Exciton dynamics Exciton optics Excitons Four-wave mixing Four wave mixing GaAs Gallium Arsenide Quantum well Semiconductor Acousto-optic Acousto optic Acousto-optic modulation Agile frequency Direct digital synthesis Lock-in detection Lock in detection Two-dimensional Two dimensional Fourier transform Multi-dimensional Multi dimensional Coherent Dephasing Relaxation Many body Many-body Physics Correlation Plasmon Surface plasmon Polariton Surface Hybrid Coupling Mode Grating Nanostructure Nano-structure Nano structure Reflection Mode geometry Transmission Variable Angle tuning Tunable Tuned Angle-tuning Angle-tunable Beam controllable Control

Page generated in 0.0525 seconds