In this thesis, I present an excitonic approach to treating the coherent dynamics of optically generated charge carriers in semiconductor nanostructures. The main feature of this approach is that it includes exchange interactions and phase space filling effects, which have generally been omitted in previous excitonic treatments of coherent dynamics, so that it can go beyond the low excitation limit. In contrast to the well-known semiconductor Bloch equations, this approach treats intraband correlations without factorization. The excitonic formalism and the obtained excitonic equations are shown to be particularly advantageous in systems where bound excitons dominate the optical response and where intraband correlations play a central role. To demonstrate the application of the excitonic approach, we simulate the coherent carrier dynamics of an optically-excited, updoped AlGaAs superlattice in the presence of a terahertz pulse, where 1s excitonic states as well as higher in-plane excited states are included. We find that gain coefficients greater than 20/cm can be achieved over a tuning range of 3-11THz and that due to the coherent cascading of the carriers down the excitonic Wannier-Stark ladder, the gain coefficients have much higher gain saturation fields than comparable two-level systems. To investigate the effects of phase space filling and exchange interaction on exciton dynamics, we then apply the excitonic formalism to a simple model of a quantum ring as well as a realistic model of a quantum well. For the quantum ring, we have obtained numerical results regarding exciton population and interband polarization. We also compared our excitonic approach to the semiconductor Bloch equations in detail using this simple model. For the quantum well, in addition to the investigation of exciton dynamics, we propose and examine several approximations that can make our excitonic dynamic equations very efficient. The excitonic formalism presented in this thesis is an efficient approach that can be applied in a wide range of systems, which makes it a potential alternative to the standard miconductor Bloch equations for many systems where the intraband correlations are crucial. / Thesis (Ph.D, Physics, Engineering Physics and Astronomy) -- Queen's University, 2008-12-01 18:21:17.181

xiv, 114 p. A print copy of this thesis is available through the UO Libraries. Search the library catalog for the location and call number. / Electron spin coherence can arise through a coherent superposition of two spin states in the conduction band of a semiconductor and can persist over remarkably long time and length scales. The robust nature of electron spin coherence makes it an excellent model system for exploring coherent quantum phenomena in semiconductors. This dissertation presents both spectral- and time-domain nonlinear optical studies of electron spin coherence through Λ-type three-level systems in two- and zero-dimensional semiconductor systems. The spectral domain study focuses on the experimental realization of electromagnetically induced transparency (EIT), a phenomenon that exploits destructive interference induced by the spin coherence. Coherent Zeeman Resonance (CZR), a precursor to EIT, is demonstrated in two 2D systems, a GaAs mixed-type quantum well (MTQW) and a modulation doped CdTe quantum well (QW). For these studies, Λ-type three-level systems are formed via dipole coupling of a trion to two electron spin states. The CZR response can be described qualitatively by effective density matrix equations. In addition, effects of manybody Coulomb interactions on CZR are investigated by varying the electron density in the MTQW via optical carrier injection. Time-domain studies based on transient differential transmission (DT) are carried out to explore the excitation, manipulation, and detection of electron spin coherence and to better understand how manybody interactions affect coherent nonlinear optical processes in semiconductors. While electron spin coherence can be formed and detected via resonant excitation of excitons or trions, a surprising observation is that injecting excitons into the 2D electron gas in a modulation doped CdTe QW can significantly alter the oscillatory nonlinear response of the electron spin coherence, while the response remains qualitatively unchanged when trions are injected. These behaviors are attributed to an interplay between manybody effects and carrier heating generated by trion formation from excitons. Finally, donor-bound electrons in GaAs are used as a model of localized electron spins. Spin decoherence of order 10 ns, limited by nuclear hyperfine interactions, is observed. Electron spin rotation induced by a nearly resonant laser pulse is also observed, opening the door for further work on mitigating electron spin decoherence time through optical spin echoes. / Adviser: Hailin Wang

Conductivity of semicoductor nanostructures has its maximum in terahertz spectral range. Linear response is described in reasonable detail. With high intensity light sources it is also needed to be concerned with nonlinear response. In this thesis, there is firstly described already existing quantum model of linear conductivity which is using perturbation theory. This model is then extended by adding another perturbations, getting us quantum model for calculating nonlinear conductivity of arbitrary order. Model is then applied for calculation of third order nonlinear conductivity spectra for cubic nanocrystal. There is described spectra for varying parametres, such as tempera- ture, volume, electron density or scattering rate. In the end it is compared with semiclassical compuptation using Monte-Carlo simu- lation. 1

No description available.

Les nanocristaux colloïdaux sont des fluorophores semi-conducteurs de taille nanométrique. Fluorescents à température ambiante et synthétisés par voie chimique, ces nanoémetteurs représentent d'excellents candidats pour divers domaines d'application tels que l'éclairage, le marquage biologique ou le photovoltaïque.Mon travail expérimental, s'inscrit dans le développement de ces nanoémetteurs via deux approches différentes : leur couplage avec des nanostructures plasmoniques à 4K et la mise en place de régime d'émission collective.Dans un premier temps, nous avons étudié le couplage de nancoristaux individuels avec des films d'or plan dans le but de réduire les pertes par effet Joule. Tout d'abord, nous nous sommes intéressés à l'influence de la température sur ce couplage. A partir d'expériences de photoluminescence et de simulations numériques effectuées par Gérard Colas des Francs du LCIB, nous avons pu mesurer et simuler une forte diminuation de l'accélération du temps caractéristique d'émission des nanocristaux lorsque la température passe de 300K à 4K. Cette diminution est directement liée à une importante diminution des pertes ohmiques des couches d'or. L'efficacité quantique des nanocristaux est ainsi augementée d'un facteur 3. Ensuite, nous avons étudié l'influence de la cristallinité de l'or sur ce couplage. A nouveau une forte réduction de l'accélération de l'émission des nanocristaux a été mesurée sur or cristallin en comparaison avec des couches d'or amorphe. Ces mesures laissent de nouveau présager une réduction des pertes par effet Joule ainsi qu'une augmentation d'au moins un facteur deux de l'efficacité quantique des nanocristaux.Dans un second temps, nous avons effectué les premières caractérisations d'agrégats de nanocristaux enrobés dans une coquille de silice. A température ambiante, nous avons mis en évidence la présence d'interaction de type FRET entre les nanocristaux émettant dans le bleu et les nanocristaux émettant dans le rouge. Cette interaction permet ainsi une accélération de l'émission globale des agrégats. A 4K, nous avons observé une modification de la dynamique d'émission des agrégats avec l'apparition de deux échelles de temps différentes. Pour les temps courts, la dynamique d'émission est accélérée et est régie par la recombinaison de l'exciton. Pour les temps longs, la dynamique d'émission est régie par une loi de puissance traduisant ainsi l'apparition de temps caractéristiques d'émission extrêmement longs. / Colloidal semiconductor nanocrystals are fluorescent semiconductors with a nanometric size. Bright at room temperature and chemically synthesized, nanocrystals are interesting candidates for differents applications as lighting, biological labeling or photovoltaic.My experimental work, is part of the development of these emitters by two differents approaches : coupling with plasmonic structures at 4,K and formation of collective emission.First, we studied the coupling between single nanocrystals and a flat gold film in order to decrease the optical losses. To begin we studied the influence of the temperature. With some photoluminescence measurements and some simulations, we show significant decrease of the enhancement of the photoluminescence decay rate at 4,K. This reduction is linked to the decrease of optical losses. Then, we studied the influence of crystallinity of gold. We show again an important reduction of enhancement of the photoluminescence decay rate with crystalline gold compared to amorphous layer.Second, we investigated the optical properties of compact nanocrystal clusters encapsulated in a silica shell. At room temperature, we observed an enhancement of the photoluminescence decay rate through Förster resonance energy transfer (FRET). At 4K, we measured an important variation of the emission dynamic with emergence of two times scales. At short time scale, emission is accelerated and governed by the exciton recombination. At long time scale, the decay is governed by power law showing the emergence of long-lived states.

The goal of this Thesis is to study the electronic and optical properties of semiconductor nanostructures by employing different theories. The work present in this Thesis is divided into three parts. Part I is devoted to the effective-mass theory and its several applications. A general description of the effective mass theory and several ways of solving the effective-mass Schrodinger equation with an emphasis on the potential morphing method are given in the first chapter. In the following few chapters, we apply these theories in many realistic systems for the study of many properties. They include: i) the binding energy of hydrogentic donor impurity in semiconductor quantum dots under the influence of static electric field and/or magnetic field, ii) the linear and nonlinear optical properties associated with intraband transitions in semiconductor quantum dots, core shell quantum dots and quantum-dot-quantum-ring systems. Part II is devoted to the pseudopotential theory and its several applications. The background theories primarily regarding to the empirical pseudopotential method and configuration interaction approach are described in the first chapter. In the following few chapters, we employ these theories for the study of the electronic and optical properties of many nanostructures of group II-VI materials. The optical properties studied herein include the band gap, Stokes shift, exciton fine structure, optical polarization and absorption spectra. Part III is devoted to the appendix, where twelve published papers are presented. / Στόχος της παρούσας διατριβής είναι η μελέτη των ηλεκτρονικών και οπτικών ιδιοτήτων νανοδομών ημιαγωγών κάνοντας χρήση κατάλληλων υπολογιστικών μεθόδων και τεχνικών. Η διατριβή χωρίζεται σε τρία μέρη. Το πρώτο μέρος εστιάζει στην θεωρία της ενεργούς μάζας (Effective-mass Theory) και τις εφαρμογές της. Στο πρώτο κεφάλαιο παρουσιάζεται το απαραίτητο θεωρητικό υπόβαθρο και δίνεται μία συνοπτική περιγραφή των συνηθέστερων μεθόδων επίλυσης της μονοηλεκτρονιακής εξίσωσης του Schrodinger,δίνοντας ιδιαίτερη έμφαση στην μέθοδο μορφοποίησης δυναμικού (Potential Morphing Method). Στα επόμενα κεφάλαια του πρώτου μέρους οι τεχνικές και μέθοδοι που περιγράφηκαν χρησιμοποιούνται για την μελέτη κρίσιμων ιδιοτήτων και παραμέτρων σε νανοσυστήματα ημιαγωγών. Μεταξύ αυτών είναι: i) η ενέργεια δέσμευσης υδρογονοειδών προσμίξεων τύπου δότη υπό την επίδραση στατικού ηλεκτρικού ή/και μαγνητικού πεδίου, ii) γραμμικές και μη γραμμικές οπτικές ιδιότητες που συνδέονται με intraband μεταβάσεις εντός ζώνης σε κβαντικές τελείες ημιαγωγών, κβαντικές τελείες με δομή πυρήνα-φλοιού και σε μεικτά συστήματα κβαντικής τελείας – κβαντικού δακτυλίου. Το δεύτερο μέρος εστιάζει στην θεωρία των ψευδοδυνάμικών και τις εφαρμογές της. Αρχικά παρουσιάζεται το απαραίτητο θεωρητικό υπόβαθρο της μεθόδου εμπειρικών ψευδοδυναμικών (Empirical Pseudopotential Method) καθώς επίσης και της μεθόδου αλληλεπίδρασης διαμορφώσεων (Configuration Interaction). Στην συνέχεια, οι προαναφερθείσες τεχνικές εφαρμόζονται στην μελέτη των ηλεκτρονικών και οπτικών ιδιοτήτων σε μία πληθώρα νανοδομών ημιαγωγών II-VI. Μεταξύ των ιδιοτήτων αυτών είναι: το ενεργειακό χάσμα, η μετατόπιση Stokes, η λεπτή δομή των εξιτονίων, η οπτική πόλωση και τα φάσματα απορρόφησης. Το τρίτο μέρος της διατριβής περιλαμβάνει το παράρτημα, στο οποίο παρατίθενται οι δώδεκα δημοσιευμένες εργασίες.

Ce travail porte sur l'étude d'hétérostructures de semiconducteurs II-VI à alignements de bande de type II, en particulier sous forme de superréseaux. Il s'agit d'un système qui peut être prometteur pour une application photovoltaïque, et c'est dans cette optique qu'est orienté ce travail. Une première partie traite ainsi d'une réflexion conceptuelle sur l'apport des interfaces de type II au photovoltaïque.Nous présentons ensuite une étude sur la croissance de CdSe et ZnTe par épitaxie par jets moléculaires, sur différents substrats. Ces matériaux sont particulièrement intéressants et adaptés pour cette application car ils ont un gap direct, quasiment le même paramètre de maille, un alignement de bandes de type II, et le CdSe une bande interdite compatible avec le spectre solaire. Mais en contrepartie il s'agit de semiconducteurs binaires qui n'ont aucun atome en commun, de sorte que la croissance d'échantillons avec des épaisseurs précises à la monocouche près constitue un vrai défi. Pour cette raison nous avons procédé à une étude fine des interfaces grâce à des analyses de diffraction de rayons X et de microscopie en transmission, qui nous permet de conclure sur la nature chimique des atomes à proximité des interfaces.Vient ensuite une étude poussée de spectroscopie sur les effets des interfaces de type II sur les porteurs de charges, à travers leur énergie et cinétique de recombinaison. Nous avons développé un modèle analytique qui permet d'ajuster précisément toutes les caractéristiques observées en relation avec ces interfaces, et qui témoigne d'un mécanisme de séparation des charges très efficace. Nous montrons par la suite que ces effets observés sont des caractéristiques intrinsèques de toutes les interfaces de type II, indépendamment des matériaux et des structures, et que ceux-ci nous permettent d'extraire avec précision les valeurs des décalages de bandes entre différents matériaux à alignement de type II. / This work focuses on the study of II-VI semiconductor heterostructures with type II band alignments, especially in the form of superlattices. This is a system that can be promising for photovoltaic applications, and my work is presented in this perspective. Thus the first part deals with a conceptual reflection on the contribution of type II interfaces for photovoltaics.In a second step I present a study on the growth of CdSe and ZnTe by molecular beam epitaxy on various substrates. These materials are particularly interesting and suitable for this application because they have a direct bandgap, are almost lattice-matched, present a type II band alignment, and CdSe shows a bandgap compatible with the solar spectrum. But in return these are binary semiconductors which have no atoms in common, so that the growth of samples with specific thicknesses close to the monolayer is challenging. For this reason we conducted a detailed study at the interfaces through analysis of X-ray diffraction and transmission electron microscopy, which allows us to conclude on the chemical nature of the atoms near the interfaces.This is followed by a detailed spectroscopy study on the effects of type II interfaces on the charge carriers through their energy and kinetics of recombination. We have developed an analytical model that allows to precisely adjust all the features observed in relation to these interfaces, and shows a very efficient charge separation mechanism. We show later that these effects are inherent characteristics of all interfaces of type II, regardless of materials and structures, and that they allow us to accurately extract the values of band offsets between different materials with type II band alignments.

The motivation of this thesis is to understand the physical properties of semiconductor quantum dots (QDs) and to get insight on the basic physics of charge separation in composites made from QDs with graphene oxide (GO)/organic semiconductors. The flexion phonon interactions is one of fundamental issues in solid state physics, which has a significant effect on both electrical and optical properties of solid state materials. This thesis investigates the physical properties of aqueous grown QDs through exciton-phonon coupling and non-radiative relaxation of excited carriers which have been carried out by temperature dependent photoluminescence spectroscopy. Several e orts have been made in order to understand the basic physics of photo induced charge separation in the hybrid systems made from QDs with graphene oxide and organic semiconductors. Investigations on the photoconductivity of the devices made from these hybrid composites have been carried out keeping the motive of its application in nanotechnology. This thesis work is presented in six chapters inclusive of summary and directions for future work. Chapter 1 discusses the background knowledge and information of the general properties of semiconductor nanostructures, QDs and their hybrid nanocomposites. Chapter 2 deals with the sample preparation and experimental techniques used in this thesis. Chapter 3 elaborates the exciton-phonon scattering and nonradiative relaxations of excited carriers in visible emitting cadmium telluride QDs with help of temperature and size dependent photoluminescence. Chapter 4 presents the investigations on time resolved photoluminescence dynamics and temperature dependent photoluminescence properties of near infrared (NIR) emitting mercury cadmium telluride (CdHgTe). Chapter 5 discusses the importance of NIR emitting silver sulphide (Ag2S) QDs and gives insight of nonradiative recombinations through defect/trap states. Chapter 6 investigates the excited state interactions between CdHgTe QDs and GO. Chapter 7 focuses on the understanding of basic physics of charge separation/transfer between poly (3hexylthiophene) and Ag2S QDs. Chapter 1: Semiconductor nanostructures have attracted significant scientific attention due to their fundamental physical properties and technological interests. Quasi zero dimensional nanocrystals or quantum dots (QDs) have shown unique optical and electrical properties compared to its bulk counterpart. These QDs show discrete energy levels due to the quantum confinement effect hence known as arti cial atoms. Large surface to volume ratio in these QDs is expected to play a crucial role in determing the photo-physical properties. Temperature dependent photoluminescence is a powerful tool for understanding the role of the large surface area on exciton recombination process in QDs. Inorganic QDs combined with different materials like graphene oxide or organic semiconductors forms an exciting class of synthetic materials which integrates the properties of organic and inorganic semiconductors. It is quite important to understand the basic physics of electronic interactions in these composites for its future application in many elds. Chapter 2: Synthesis of the inorganic QDs, graphene oxide, composites and fabrication of devices is an important and integral part of this thesis. Hydrothermal and three necked ask technique is adopted to get highly dispersible colloidal quantum dots in solvents. Synthesis of graphene oxide from graphite through oxidation and ultrasonication has been carried out to obtain homogenous dispersed graphene oxide in water. Structural properties have been studied by techniques like X ray diffraction, Raman spectroscopy, X ray photoelectron spectroscopy and high resolution transmission electron microscopy. Morphological properties are studied by atomic force microscopy and transmission electron microscopy. Optical properties are investigated by absorption spectroscopy, steady state and time resolved photoluminescence spectroscopy. Photoconductivity characteristics are analyzed to understand the basics of enhanced current in the various devices made from QDs composites. Chapter 3:Investigations on exciton phonon coupling and nonradiative relaxations in various sizes of visible light emitting cadmium telluride (CdTe) QDs size have been presented. Due to the large surface area, QDs are prone to have defect/trap states which can affect the exciton relaxation. Hence, understanding the role of such defect/trap states on photoluminescence is very essential for achieving the optimum optical properties. Temperature dependent (15 300 K) photoluminescence has been used to understand nonradiative relaxation of excited carriers. Thermally activated processes and multiple phonons scattering is thoroughly investigated to understand the quenching of photoluminescence with temperature. The strength of exciton-phonon coupling is investigated which determines the variation in energy bandgap of QDs with temperature. Role of exciton phonon scattering is also discussed to understand the basic physics of photoluminescence line width broadening in QDs. Chapter 4 and 5: This part of thesis focuses on the size and temperature pho-toluminescence properties of near infra red emitting ternary alloyed CdHgTe and Ag2S QDs. Near infrared emitting semiconductor quantum dots (QDs) have attracted significant scientific and technological interests due to their potential applications in the fields of photosensor, solar energy harvesting cells, telecommunication and biological tissue imaging etc. Structural and photophysical properties of CdHgTe QDs have been analyzed by high resolution transmission electron microscopy, X rayphotoelectron microscopy, photoluminescence decay kinetics and low temperature photoluminescence. Investigations on the nonradiative recombinations through trap/defects states and exciton phonon coupling are carried out in colloidal Ag 2S QDs which emits in the range of 1065 1260 nm. Particularly, the photoluminescence quenching mechanism with increasing temperature is analyzed in the presence of multiple nonradiative relaxation channels, where the excited carriers are thermally stimulated to the surface defect/trap states of QDs. Chapter 6 and 7: The aim of these chapters is to understand the basic physics of photo induced charge separation in the hybrid systems made from the inorganic QDs with graphene oxide and organic semiconductors. In chapter 6, CdHgTe QDs are decorated on graphene oxide sheets through physisorption. The excited state electronic interactions have been studied by optical and electrical characterizations in these CdHgTe QDs GO hybrid systems. In chapter 7, investigations are carried out for understanding the basic physics of charge separation in the composites of Ag2S QDs and poly (3hexylthiophene 2,5 diyl)(P3HT). These composites of inorganic organic materials are made by simple mixing with help of ultrasonication technique. Steady state and time resolved photoluminescence measurements are used as powerful technique to gain insight of energy/charge transfer process between P3HT and Ag2S QDs. Furthermore, investigations have been carried out on the photoconductivity of the devices made from these hybrid composites keeping the motive of its application in nanotechnology. Chapter 8: The conclusions of the work presented in this thesis are coherently summarized in this chapter. Thoughts and prospective for future directions are also summed up.

Terahertz conductivity spectra contain information on charge transport mechanisms and charge confinement on nanometer distances. In this thesis, we make a substantial progress in understanding of terahertz conductivity in several regimes. First, we theoretically investigate linear terahertz conductivity of confined electron gas: while the spectra of degenerate electron gas exhibit geometrical resonances, the response in non-degenerate case smears into a single broad resonance due to the wide distribution of charge velocities. Then, we theoretically and experimentally analyze various TiO2 nanotube layers: their linear charge transport properties strongly depend on the fabrication process, which influences the internal structure of the nanotube walls. In the main part of the thesis, we develop a framework for evaluation of the nonlinear terahertz response of semiconductor nanostructures based on microscopic Monte-Carlo calculations. The nonlinear regime is highly non-perturbative even in moderate fields as illustrated by efficient high harmonics generation. We investigate measurable nonlinear signals for various semiconductor nanostructures; metallic nanoslits filled with nanoelements are the most promising for the experimental observation of terahertz nonlinearities. These nonlinearities per unit charge are...

In recent years, there has been growing interests on II-VI semiconductor nanostructures, which are suitable for applications in electronics and optoelectronic devices such as solar cells, UV lasers, sensors, light emitting diodes and field emission displays. II-VI semiconductor nanostructures with different morphologies such as wires, belts, rods, tubes, needles, springs, tetrapods, plates, hierarchical structures and so on, have been widely grown by vapor transport methods. However the process conditions used for the growth of nanostructures still remains incompatible for device fabrication. The realization of practical nanoscale devices using nanostructured film depends mainly on the availability of low cost and lower processing temperatures to manufacture high purity nanostructures on a variety of substrates including glass and polymer. In this thesis work, studies have been made on the growth and characterization of II-VI semiconductor nanostructures prepared at room temperature, under high vacuum, without employing catalysts or templates. (i) ZnO nanostructured films with different morphology such as flowers, needles and shrubs were deposited at room temperature on glass and polymer substrates by plasma assisted reactive process. (ii) Zn/ZnO core/shell nanowires were grown on Si substrates under optimized oxygen partial pressure. Annealing of this core shell nanowire in high vacuum resulted in the formation of ZnO nanocanals. (iii) ZnS and ZnSe nano and microstructures were grown on Si substrates under high vacuum by thermal evaporation. The morphology, structural, optical properties and composition of these nano and microstructures were investigated by XRD, SEM, TEM, Raman, PL and XPS. The growth mechanism behind the formation of the different nanostructures has been explained on the basis of vapour-solid (VS) mechanism.