Les excitons spatialement indirects, en tant que bosons composites, sont des candidats prometteurs pour l'exploration des systèmes corrélés à N-corps. Ils possèdent une dipôle électrique intrinsèque et une variété de spin 4 fois dégénérée, et devraient former un condensat de Bose-Einstein au-dessous de quelques Kelvins. De récents résultats théoriques montrent que cette condensation doit se produire au sein des états optiquement noirs. Néanmoins les interactions peuvent créer un couplage cohérent vers une population brillante, rendant ainsi accessible la détection du condensat par le biais de sa photoluminescence. Nos expériences portent sur un gaz froid d'excitons indirects dans un double puits quantique. Les excitons sont photo-générés par une excitation laser et confinés dans un piège électrostatique. Nous avons observé une réduction de la population d'excitons brillants pour un gaz de densité fixe à basses températures. Ceci contraste fortement avec le comportement attendu d'un gaz froid soumis à la statistique de Maxwell-Boltzmann. Ces résultats expérimentaux sont confirmés par un modèle phénoménologique montrant que la condensation dans les états noirs est compatible avec le noircissement anormal observé. Une réduction de la température pourrait en principe amplifier ces signatures, cependant dans le GaAs l'interaction exciton-phonon permettant le refroidissement est fortement réduite pour des températures inférieures au Kelvin. Nous avons donc développé une technique permettant le contrôle in-situ du confinement des excitons indirects sans échauffement du gaz, ouvrant ainsi la voie à l'exploration du refroidissement évaporatif des excitons. / Spatially indirect excitons, being composite bosons, are attractive candidates to explore correlated many-body systems. They possess an inherent electric dipole and a four-fold spin manifold. Indirect excitons are expected to form a BEC below a few Kelvins. Recent theoretical results show this condensation must occur in optically dark states. Interactions, however, can lead to a coherent coupling to a bright population, rendering the condensate accessible through its PL. Here we report on a cold gas of indirect excitons in coupled quantum wells. indirect excitons are photo-generated through pulsed laser excitation. Indirect excitons are confined in an electrostatic traps. Thus, we are able to observe an anomaluos depletion of the bright state population for a fixed gas density at lower bath temperatures. This stands in stark contrast to the expected classical behavior of a cold gas of indirect excitons obeying Maxwell-Boltzmann statistics. The experimental results are confirmed by a phenomenological model showing that condensation into the dark state is compatible with the observed anomalous darkening. Reducing the gas temperature should reinforce these signatures. However, in GaAs exciton-phonon interaction is strongly reduced for sub-Kelvin temperatures. We have thus developed a technique to control the indirect excitons confinement in-situ. Our method does not increase the gas temperature and thus paves the way towards the exploration of evaporative cooling for indirect excitons.

In this thesis, I will present and discuss research performed on InGaN/GaN multiple quantum well (QW) structures. The results of which were taken using photoluminescence (PL) spectroscopy and PL time decay spectroscopy. In the first two experimental chapters, I report on the effects of QW growth methodology on the optical properties of c-plane InGaN/GaN QWs. I compare structures grown using the single temperature (1T), quasi-two temperature (Q2T), temperature bounced (T-bounced) and two temperature (2T) QW growth methodologies. The T-bounced and 2T structures are observed to have gross well width fluctuations (GWWF), where the QW width varies from 0 to 100 % created when the QWs are exposed to a temperature ramp. Whereas, the 1T and Q2T structures have continuous QWs with only one or two monolayer well width fluctuations. The structures with GWWFs are observed to have a larger room temperature internal quantum efficiency (RT-IQE) at low excitation conditions i.e. below efficiency droop compared to those without. The larger RT-IQE is ascribed to several factors which include an increased radiative recombination rate, increased thermal activation energy of non-radiative recombination and reduced defect density of the QWs. The effect of barrier growth temperature is also investigated. No clear trend is observed between barrier growth temperature and RT-IQE.In the last experimental chapter I report on studies of carrier localisation in InGaN/GaN QWs using resonant PL spectroscopy. The effect of carrier localisation on the independently localised electrons and holes are investigated and the resonant PL spectrum is studied in detail. The InGaN/GaN QW structure is observed to exhibit an effective mobility edge at 12 K where delocalised carriers are created above a particular excitation energy. The emission from the resonantly excited localised states which are accompanied by the emission of a longitudinal optical phonon (resonant LO feature) is investigated as a function of temperature and excitation energy. The integrated PL intensity of the resonant LO feature is observed to quench rapidly with temperature up to around 45 K, independent of excitation energy. The integrated PL intensity of the resonant LO feature is fitted to an Arrhenius model and a thermal activation energy of ∼ 1(±1) meV is extracted. This activation energy is speculated to be consistent with the localisation energy of electrons.

No description available.

In this thesis experimental samples of multiple quantum wells in the InGaN/GaN structures will be compared using methods of laser spectroscopy. In particular, the optical properties of the samples will be investigated. The samples were prepared under different conditions; therefore one of the aims is to compare them. The knowledge of the influence of preparation enables utilization not only for fundamental research, but also for the construction of radiation sources or scintillation detectors. Measurements of absorption and photoluminescence will be carried out and their dynamic properties will be measured as well. There will be examined the effect of different excitation power and different excitation wavelength on the intensity of photoluminescence. From dynamic properties there will be examined the effect of different excitation wavelength on the lifetime of the absorption and how does temperature influence the lifetime of the photoluminescence. Individual quantities will be compared amongst samples and their suitability for further applications will be discussed.

QC 20101206

[pt] Este trabalho apresenta a caracterização por fotoluminescência de amostras com camadas de arseneto de índio depositadas em substratos de arseneto de gálio. O objetivo é estudar a transição entre os modos bidimensional e tridimensional no crescimento epitaxial em reator MOVPE, com a formação de pontos quânticos autoorganizados pelo método Stranski-Krastanov e a subsequente aplicação da técnica de indium flush. São analisados espectros de fotoluminescência, simulações dos níveis de energia, imagens de microscopia de força atômica e microscopia eletrônica de transmissão para a obtenção de informações sobre densidade, tamanho, uniformidade e energia das transições radiativas dos pontos quânticos. Imagens de microscopia eletrônica de transmissão revelam a qualidade das interfaces e a espessura das camadas. / [en] This work presents the photoluminescence characterization of indium arsenide layers deposited on gallium arsenide substrates. The objective is to analyze the transition in growth mode, from two-dimensional to tri-dimensional in MOVPE, with the formation of self-assembled quantum dots by Stranski-Krastanov method and subsequent use of the indium flush technique. Photoluminescence spectra, quantum well and quantum dot simulations, atomic force microscopy and transmission electron microscopy images are analyzed in order to obtain information about quantum dot size, density, uniformity and electronic transitions. Transmission electron microscopy images reveal the interfaces quality and layers thicknesses.

<p>  </p> <p>Research that seeks to understand and develop the growth of III-nitride materials by molecular beam epitaxy (MBE) is beneficial to a broad range of the device community. MBE and the III-nitrides have been used to develop transistors, diodes, electroacoustic devices, solar cells, LEDs, LDs, intersubband devices, and quantum-cascade lasers. In this work we focus on the growth of III-nitride materials specifically for applications in near-infrared intersubband (NIR ISB) optical devices, however all this work is broadly applicable. </p> <p><br></p> <p>We begin by investigating the reduced indium incorporation in non-polar m-plane InGaN films. We find that InGaN grown on m-plane GaN has an effective activation energy for thermal decomposition of 1 eV, nearly half that reported for similar c-plane films. We produce high quality m-plane In0.16Ga0.84N and utilize it in AlGaN/InGaN devices designed for near-infrared ISB absorption measurements. We continue this work by exploring the growth of low-temperature AlGaN, necessary for these devices. We find that the utilization of an indium surfactant during low-temperature AlGaN growth enhances adatom diffusion, resulting in smoother surface morphologies, sharper interfaces, and reduced defects within the material. This growth method also prevents the anomalous suppression of the AlGaN growth rate, which we link to a reduction in the formation of high-aluminum containing defects. These investigations result in the demonstration of an Al0.24Ga0.76N/In0.16Ga0.84N heterostructure with a conduction band offset large enough to enable NIR ISB transitions.</p> <p><br></p> <p>Lastly, we explore the novel material ScAlN. This material’s large bandgap, large spontaneous polarization, ferroelectricity, and ability to be lattice matched to GaN at ~18% scandium composition make it an ideal candidate for a variety of devices, including NIR ISB devices. We investigate the reported temperature dependence of ScAlN’s <em>c</em>-lattice constant and confirm this dependence is present for high growth-temperature ScxAl1-xN with 0.11 < x < 0.23. We find that this temperature dependence is no longer present below a certain composition-dependent growth temperature. This finding, coupled with observations that samples grown at lower temperatures exhibit lower defect densities, smoother surfaces, and homogeneous chemical compositions suggest that high growth temperatures lead to defect generation that may cause the observed change in lattice parameters. We demonstrate lattice-matched, 50 repeat Sc0.18Al1-xN/GaN heterostructures with ISB absorption in excess of 500 meV with FWHM as little as 45 meV. </p>

Understanding the many-body behaviors of interacting electron systems remains one of the central topics in condensed matter physics. Novel correlated phases coupled to lattice symmetry, topological orders and hidden geometrical degrees of freedom could be induced and modulated by external electric or magnetic fields. Extensive attention have been drawn to these research directions which are of significant interests for both fundamental understanding and practical applications of many-body electron systems. In this dissertation I report optical spectroscopic studies on the Coulomb coupling, phase interplay and geometric fluctuations of interacting two-dimensional electron systems. The research provides a key approach to engineering many-body ground states and offers critical insights into their underlying nature. Electric potential or magnetic field modulations are applied to the electrons hosted in semiconductor quantum wells. Through lateral superlattice nanopatterning, we fabricate semiconductor artificial graphene where resonant inelastic light scattering is employed to characterize the engineered band structures. Flat bands hosting van Hove singularities are directly observed by optical emission. Coulomb coupling between electrons with diverging density of states are found to have significant impacts on the energies and line-shapes of the optical spectra. The results demonstrate a novel and tunable platform to explore intriguing many-body physics. External magnetic fields have been known to trigger a rich phase diagram in interacting two-dimensional electron systems, encompassing phenomena such as the fractional quantum Hall effect. The phase interplay gives rise to domain textures in the bulk of electron systems and affects the dispersion of collective excitations. We probe impacts of domain textures on low-lying neutral excitations through doubly resonant inelastic light scattering. We demonstrate that large domains of quantum fluids can support well-defined long-wavelength modes which could be interpreted by theories for uniform phases. Equipped with ultra-high mobility quantum wells and circularly polarized light scattering techniques, we resolve the spin of long-wavelength magnetoroton modes and provide characteristic evidence of the chiral graviton at Landau level filling factor $\nu= ⅓ fractional quantum Hall state. The results offer the first experimental evidence of geometrical degrees of freedom in the fractional quantum Hall effect.

The application of epitaxially grown, III-V semiconductor-based nanostructures to the development of electro-optical and all-optical switches is investigated through the fabrication and testing of integrated photonic devices designed using multimode interference (MMI) waveguides. The properties and limitations of the materials are explored with respect to the operation of those devices through electrical carrier injection and optical pumping. MMI waveguide geometry was employed as it offered advantages such as a very compact device footprint, low polarization sensitivity, large bandwidth and relaxed fabrication tolerances when compared with conventional single-mode waveguide formats. The first portion of this dissertation focuses on the characterization of the materials and material processing techniques for the monolithic integration of In0.15Ga0.85As/GaAs self-assembled quantum dots (SAQD) and InGaAsP/InGaAsP multiple quantum wells (MQW). Supplemental methods for post-growth bandgap tuning and waveguide formation were developed, including a plasma treatment process which is demonstrated to reliably inhibit thermally induced interdiffusion of Ga and In atoms in In0.15Ga0.85As/GaAs quantum dots. The process is comparable to the existing approach of capping the SAQD wafer with TiO2, while being simpler to implement along-side companion techniques such as impurity free vacancy disordering. Study of plasma-surface interactions in both wafer structures suggests that the effect may be dependent on the composition of the contact layer. The second portion of this work deals with the design, fabrication, and the testing of MMI switches which are used to investigate the limits of electrical current control when employing SAQD as the active core material. A variable power splitter based on a 3-dB MMI coupler is used to analyze the effects of sub-microsecond electrical current pulses in relation to carrier and thermal nonlinearities. Electrical current controlled switching of the variable power splitter and a tunable 2 x 2 MMI coupler is also demonstrated. The third part of this dissertation explores the response of In0.15Ga0.85As/GaAs SAQD waveguide structures to photogenerated carriers. Also presented is a simple, but effective, design modification to the 2 x 2 MMI cross-coupler switch that allows control over the carrier distribution within the MMI waveguide. This technique is combined with selective-area bandgap tuning to demonstrate a compact, working, all-optical MMI based switch.

Semiconductor based micro- and nano-structures grown in a systematic and controlled way using selective area growth are emerging as a promising route toward devices for integrated optical circuitry in optoelectronics and photonics field. This dissertation focuses on the experimental investigation of the nonlinear optical effects in selectively grown gallium nitride micro-pyramids that act as optical cavities, zinc oxide submicron rods and indium gallium nitride multiple quantum well core shell submicron tubes on the apex of GaN micro pyramids that act as optical antennae. Localized spatial excitation of these low dimensional semiconductor structures was optimized for nonlinear optical light (NLO) generation due to second harmonic generation (SHG) and multi-photon luminescence (MPL). The evolution of both processes are mapped along the symmetric axis of the individual structures for multiple fundamental input frequencies of light. Effects such as cavity formation of generated light, electron-hole plasma generation and coherent emission are observed. The efficiency and tunability of the frequency conversion that can be achieved in the individual structures of various geometries are estimated. By controlling the local excitation cross-section within the structures along with modulation of optical excitation intensity, the nonlinear optical process generated in these structures can be manipulated to generate coherent light in the UV-Blue region via SHG process or green emission via MPL process. The results show that these unique structures hold the potential to convert red input pulsed light into blue output pulsed light which is highly directional.