Core-shell nanowire LEDs are light emitting devices which, due to a high aspect ratio, have low substrate sensitivity, allowing the possibility of low defect density GaN light emitting diodes. Current growth techniques and physical non-idealities make the production of high conductivity p-type GaN for the shell region of these devices difficult. Due to the structure of core-shell nanowires and the difference in conductivity between ntype and p-type GaN, the full junction area of a core-shell nanowire is not used efficiently. To address this problem, a series of possible doping profiles are applied to the core of a simulated device to determine effects on current crowding and overall device efficiency. With a simplified model it is shown that current crowding has a possible dependence on the doping in the core in regions other than those directly in contact with the shell. The device efficiency is found to be improved through the use of non-constant doping profiles in the core region with particularly large efficiency increases related to profiles which modify portions of the core not in contact with the shell

No description available.

Spontaneous parametric down conversion (SPDC) – incoherent light scattering – is one of the main entangled photons source applied in quantum optics experiments. The tradition to pump SPDC by laser radiation was established from the very first SPDC experiments in 1968. The aim of this thesis was experimentally to investigate the ability to generate an SPDC pumping by both temporal and spatially incoherent radiation - a high-power blue LED. Weak SPDC signals were registered with high sensitivity CCD cameras, photons coincidences were detected with photon counters. The theoretical simulations were performed in parallel with experiments. Therefore, mathematical simulation code was written in order to estimate the SPDC power distribution and simulate photon coincidence experiment changing the properties of pump beam and detection system. Experimental results reveal that incoherent light sources can be good alternative for the laser systems in order to generate average quality biphoton fields especially in those experiments in which low biphoton field coherency would be advantage. The main advantages of the incoherent sources over laser systems are low cost, simple production technology and the huge commercial variety of different wavelength sources. / Parametrinė fluorescencija (PF) – nekoherentinė šviesos sklaida – yra vienas pagrindinių susietųjų fotonų šaltinių taikomų kvantinės optikos eksperimentuose. Nuo pat pirmųjų PF eksperimentinių tyrimų 1968 metais įsigalėjo tradicija šį reiškinį žadinti išimtinai lazerine spinduliuote. Šios disertacijos tikslas – eksperimentiškai ištirti galimybę generuoti PF tiek laikiškai, tiek ir erdviškai nekoherentine spinduliuote – didelės galios šviesos diodu. Atliekant tyrimus didelio jautrio CCD kamera buvo registruojami silpni PF signalai, pavienių fotonų skaitliukais buvo registruojami fotonų sutapimai,. Lygiagrečiai eksperimentiniams tyrimams buvo atliekami teoriniai skaičiavimai. Šiuo tikslu buvo parašytas matematinio modeliavimo programinis kodas, skirtas įvertinti PF erdvinį galios pasiskirstymą bei modeliuoti fotonų sutapimų eksperimentą, keičiant kaupinimo pluošto ir detekcijos sistemos savybes. Šio darbo rezultatai atskleidžia, kad nekoherentiniai šaltiniai gali būti puiki alternatyva lazerinėms sistemoms siekiant žadinti vidutinės kokybės dvyninius laukus, ypatingai tokiose tyrimų srityse, kuriose mažas dvyninio lauko koherentiškumas būtų didžiulis privalumas. Pagrindiniai nekoherentinių šaltinių pranašumai prieš lazerines sistemas: maža kaina, paprasta gamybos technologija ir didžiulė komercinė skirtingo bangos ilgio šaltinių įvairovė.

Parametrinė fluorescencija (PF) – nekoherentinė šviesos sklaida – yra vienas pagrindinių susietųjų fotonų šaltinių taikomų kvantinės optikos eksperimentuose. Nuo pat pirmųjų PF eksperimentinių tyrimų 1968 metais įsigalėjo tradicija šį reiškinį žadinti išimtinai lazerine spinduliuote. Šios disertacijos tikslas – eksperimentiškai ištirti galimybę generuoti PF tiek laikiškai, tiek ir erdviškai nekoherentine spinduliuote – didelės galios šviesos diodu. Atliekant tyrimus didelio jautrio CCD kamera buvo registruojami silpni PF signalai, pavienių fotonų skaitliukais buvo registruojami fotonų sutapimai,. Lygiagrečiai eksperimentiniams tyrimams buvo atliekami teoriniai skaičiavimai. Šiuo tikslu buvo parašytas matematinio modeliavimo programinis kodas, skirtas įvertinti PF erdvinį galios pasiskirstymą bei modeliuoti fotonų sutapimų eksperimentą, keičiant kaupinimo pluošto ir detekcijos sistemos savybes. Šio darbo rezultatai atskleidžia, kad nekoherentiniai šaltiniai gali būti puiki alternatyva lazerinėms sistemoms siekiant žadinti vidutinės kokybės dvyninius laukus, ypatingai tokiose tyrimų srityse, kuriose mažas dvyninio lauko koherentiškumas būtų didžiulis privalumas. Pagrindiniai nekoherentinių šaltinių pranašumai prieš lazerines sistemas: maža kaina, paprasta gamybos technologija ir didžiulė komercinė skirtingo bangos ilgio šaltinių įvairovė. / Spontaneous parametric down conversion (SPDC) – incoherent light scattering – is one of the main entangled photons source applied in quantum optics experiments. The tradition to pump SPDC by laser radiation was established from the very first SPDC experiments in 1968. The aim of this thesis was experimentally to investigate the ability to generate an SPDC pumping by both temporal and spatially incoherent radiation - a high-power blue LED. Weak SPDC signals were registered with high sensitivity CCD cameras, photons coincidences were detected with photon counters. The theoretical simulations were performed in parallel with experiments. Therefore, mathematical simulation code was written in order to estimate the SPDC power distribution and simulate photon coincidence experiment changing the properties of pump beam and detection system. Experimental results reveal that incoherent light sources can be good alternative for the laser systems in order to generate average quality biphoton fields especially in those experiments in which low biphoton field coherency would be advantage. The main advantages of the incoherent sources over laser systems are low cost, simple production technology and the huge commercial variety of different wavelength sources.

PROPERTIES OF LIGHT EMITTING DIODES FOLLOWING COBALT-60 IRRADIATION &Ouml / zcan, Safak M.S., Department of Physics Supervisor: Prof. Dr. ibrahim G&uuml / nal September 2004, 71 pages The main purpose of this study is to investigate the effects of gamma radiation on the properties of the light emitting diodes. GaP and GaAsP LEDs are used in the study. It is observed that the exposure of a light emitting diode affects its various properties. A cobalt-60 gamma-cell is used to irradiate the selected light emitting diodes. For the different total doses of gamma pre-irradiation and post-irradiation I-V characteristics and spectral responses are recorded. The capacitance characteristics are measured at 1MHz at room temperature. Gamma ray bombardment of these LEDs results in reduction of electroluminescent intensity and increase in forward current up to levels tested. In GaP diodes dominant current transport mechanism has found to be effected by irradiation. No noticeable change is observed in the series resistances. The impurity density remains same in the green LED and increases in the red one due to the irradiation, which is deduced from the C-V characteristics. Both the circuit designers and the users should be aware of these effects in order to reach a reliable application for these components in a radiation environment.

Organic electronic devices are receiving growing interest because of their potential to employ lightweight, low-cost materials in a flexible architecture. Typically, indium tin oxide (ITO) is utilized as the transparent positive electrode in these devices due to its combination of high transmission in the visible spectrum and high electrical conductivity. However, ITO may ultimately hinder the full market integration of organic electronics due to its increasing cost, the limited availability of indium, lack of mechanical flexibility, and sustainability with regards to the environment and material utilization. Therefore, alternatives for ITO in organic electronics are currently being pursued. Transparent electrodes comprised of single wall carbon nanotubes (SWNTs) are an appealing choice as a surrogate because of the extraordinary electrical and mechanical properties these 1-D structures posses. As such, the research presented in this dissertation has been conducted to advance the goal of manufacturing SWNT networks with transparent electrode properties that meet or exceed those of ITO. To this end, SWNT films were characterized with regard to the collective and individual optoelectronic properties of the SWNTs that comprise the network. Specifically, corroborative theoretical and experimental observations were employed to expand the understanding of how the optoelectronic properties of polydisperse and monodisperse SWNT networks are enhanced and sustained through chemical treatment and subsequent processing. In addition, the impact of interfacial electrical contact resistance between SWNT electrodes and metallic fingers often used in photovoltaic system applications was elucidated. In summary, the research presented in this dissertation can be leveraged with present state of the art in SWNT films to facilitate future SWNT electrode development.

Research in the field of Photonics is in part, directed at the application of light-emitting materials based on silicon platforms. In this work silicon wafers are modified by carbon ion implantation to incorporate silicon carbide, a known light-emitting material. Ion beam synthesis treatments are applied with implant energy of 20 keV, and ion fluences of 3, 5 and 10 × 1016 ions/cm2 at both ambient temperature and high temperature (400 °C). The samples are annealed at 1000 °C, after implantation. The carbon ion implanted silicon is characterized using Raman and Fourier transform infrared spectroscopic techniques, grazing-incidence X-ray diffraction, transmission electron microscopy and electron energy loss spectroscopy. The materials are observed to have a multilayer structure, where the ambient temperature implanted materials have an amorphous silicon layer, and an amorphous silicon layer with carbon-rich, nanoscale inclusions. The high temperature implanted materials have the same layers, with an additional polycrystalline Si layer at the interface between the implanted layer and the target substrate and the amorphous Si layer with SiC inclusions is reduced in thickness compared to the ambient temperature samples. The carbon-rich inclusions are confirmed to be SiC, with no evidence of carbon clusters in the materials observed using Raman spectroscopy. The carbon ion-implanted material is used to fabricate Schottky diodes having a semitransparent gold contact at the implanted surface, and an aluminum contact on the opposite side. The diodes are tested using current-voltage measurements between -12 and +15 V. No reverse breakdown is observed for any of the diodes. The turn-on voltages for the ambient temperature implanted samples are 2.6±0.1 V, 2.8±0.6 V and 3.9±0.1 V for the 3, 5 and 10 × 1016 ions/cm2 samples, respectively. For the high temperature implanted samples, the turn-on voltages are 3.2±0.1 V, 2.7±0.1 V, and 2.9±0.4 V for the implanted samples with same fluences. The diode curves are modeled using the Shockley equation, and estimates are made of the ideality factor of the diodes. These are 188±16, 224.5±5.8, and 185.4±9.2 for the ambient temperature samples, and 163.6±6.3, 124.3±5.3, and 333±12 for the high temperature samples. The high ideality factor is associated with the native oxide layer on the silicon substrate and with the non-uniform, defect-rich implanted region of the carbon ion implanted silicon. Red-orange visible light emission from the diodes is observed with voltage greater than the turn-on voltage applied across the diodes. The luminescence for the ambient temperature samples is attributed to porous silicon, and amorphous silicon. The high temperature implanted samples show luminescence associated with porous silicon, nanocrystalline silicon carbide, and defects in silicon related to ion implantation. The luminescent intensity observed for the ambient temperature samples is higher than for the high temperature samples. The dominant luminescence feature in the carbon ion-implanted silicon material is porous silicon, which is described by quantum confinement of excitons in silicon.

The efficacy, reliability and versatility of the light emitting diode (LED) can outcompete most established light source technologies. However, they are particularly sensitive to high temperatures, which compromises their efficacy and reliability, undermining some of the technology s key benefits. Consequently, effective thermal management is essential to exploit the technology to its full potential. Thermal management is a well-established subject but its application in the relatively new LED lighting industry, with its specific constraints, is currently poorly defined. The question this thesis aims to answer is how can LED thermal management be achieved most effectively? This thesis starts with a review of the current state of the art, relevant thermal management technologies and market trends. This establishes current and future thermal management constraints in a commercial context. Methods to test and evaluate the thermal management performance of a luminaire system follow. The defined test methods, simulation benchmarks and operational constraints provide the foundation to develop effective thermal management strategies. Finally this work explores how the findings can be implemented in the development and comparison of multiple thermal management designs. These are optimised to assess the potential performance enhancement available when applied to a typical commercial system. The outcomes of this research showed that thermal management of LEDs can be expected to remain a key requirement but there are hints it is becoming less critical. The impacts of some common operating environments were studied, but appeared to have no significant effect on the thermal behaviour of a typical system. There are some active thermal management devices that warrant further attention, but passive systems are inherently well suited to LED luminaires and are readily adopted so were selected as the focus of this research. Using the techniques discussed in this thesis the performance of a commercially available component was evaluated. By optimising its geometry, a 5 % decrease in absolute thermal resistance or a 20 % increase in average heat transfer coefficient and 10 % reduction in heatsink mass can potentially be achieved . While greater lifecycle energy consumption savings were offered by minimising heatsink thermal resistance the most effective design was considered to be one optimised for maximum average heat transfer coefficient. Some more radical concepts were also considered. While these demonstrate the feasibility of passively manipulating fluid flow they had a detrimental impact on performance. Further analysis would be needed to conclusively dismiss these concepts but this work indicates there is very little potential in pursuing them further.

Cette thèse porte sur la réalisation de composants photoniques à base de fils de nitrures III-V. Les fils de GaN non-catalysés ont été élaborés de manière auto-assemblée par épitaxie en phase vapeur aux organométalliques (MOVPE) sur saphir. Un des axes de ce travail a porté sur la croissance organisée de ces fils à travers un réseau d’ouvertures défini par lithographie et gravure d’une couche de SiNx. Nous avons étudié en particulier l’influence des paramètres de croissance (température, pression, ratio V/III) et du motif sur l’homogénéité de la croissance sélective. Ces fils ont servi de substrat pour la croissance d’hétérostructures radiales cœur-coquille InGaN/GaN.D’autre part, la croissance, la fabrication et les propriétés physiques de trois types de composant ont pu être étudiées :-Des cellules solaires à fils uniques. Nous avons comparé l’efficacité de conversion de deux types d’hétérostructures : des coquilles épaisses d’In0.1Ga0.9N et des coquilles à 15 et 30 puits quantiques In0.18Ga0.82N/GaN. Après optimisation du contact électrique sur la coquille p-GaN, un rendement maximal de 0,33 % a été obtenu avec des fils à 30 puits quantiques sous éclairement équivalent à 1 soleil (AM1.5G). Le seuil d’absorption mesuré par spectroscopie de photocourant varie entre 400 et 440 nm.- Une plateforme émetteur-détecteur. Le système, qui fonctionne à 400 nm, comprend deux fils de GaN à hétérostructure radiale InGaN/GaN positionnés sur le même substrat et couplés par un guide d’onde en SiNx. La caractérisation électrique du dispositif a mis en évidence une durée de commutation inférieure à 0,25 s sans photocourant persistant.- Des diodes électroluminescentes (LED) flexibles. Ces diodes qui émettant dans le visible (400-470 nm) ont été réalisées en se basant sur une approche hybride organique/inorganique. Les fils émetteurs à puits quantiques InGaN/GaN sont encapsulés dans une matrice organique de PDMS puis détachés de leur substrat de croissance. Les contacts sont réalisés à partir de nanofils d’argent qui présentent l’avantage d’être à la fois flexibles, transparents et conducteurs. A partir de ce procédé, une LED bicolore flexible a été réalisée en combinant des émetteurs bleus et « verts ». / This thesis reports on the realization of photonic devices based on nitride wires. Self-assembled GaN wires were grown without catalyst by metal-organic vapor phase epitaxy (MOCVD) on sapphire substrates. Part of this work focused on the selective area growth of GaN wires through a dielectric SiNx mask with regular arrays of holes defined by lithography and dry etching. We studied the influence of the growth conditions (temperature, pressure, V/III ratio) and pattern geometry on the homogeneity of the selective area growth. These wires were used as templates for the growth of core-shell InGaN/GaN heterostructures. In addition, the growth, microfabrication process and properties of three types of devices were studied:- Single wire solar cells. We compared the efficiency of two type of heterostructures: shells composed of thick In0.1Ga0.9N layers and In0.18Ga0.82N/GaN quantum wells. After optimization of the electrical contact on the p-GaN shell, a maximal conversion efficiency of 0,33 % was obtained on single GaN wires with a shell of 30 quantum wells under 1 sun illumination (AM1.5G). Photocurrent spectroscopy revealed that the wire absorption edge varied between 400 and 440 nm.- An integrated photonic platform. The system, that operates around 400 nm, is composed of two GaN wires with radial InGaN/GaN heterostructures positioned on the same substrate and coupled with a SiNx waveguide. The electrical characterization of the platform revealed a switching speed inferior to 0.25 s without persistent photocurrent.- Flexible light emitting diodes (LED). The LED fabrication is based on a dual approach which associates inorganic InGaN/GaN emitters (400-470 nm) and a polymer. The wires are encapsulated in a PDMS matrix before being detached from their native substrate. Electrical contacts are made with silver nanowires which are flexible, highly conductive and transparent in the visible range. Based on this procedure a two-color LED was realized by stacking a blue and a “green” LED.

Made available in DSpace on 2014-06-11T19:25:31Z (GMT). No. of bitstreams: 0 Previous issue date: 2006-12-18Bitstream added on 2014-06-13T20:53:29Z : No. of bitstreams: 1 biscuola_ma_me_rcla.pdf: 2829290 bytes, checksum: 4f1c3c8a080dce0360b3e6c8d278b78c (MD5) / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) / As novas tecnologias para a construção de telas e displays têm permitido reduzir o consumo de energia, o peso e a espessura, como também melhorar a eficiência e flexibilidade se comparado aos antigos monitores de CRTs e até mesmo aos atuais displays de cristal líquido. Dentre estas novas tecnologias destacam-se as que utilizam materiais orgânicos (OLED) ou orgânicos poliméricos (PLED) não somente por suas possibilidades de aplicações industriais, mas também por permitirem o desenvolvimento das ciências básica e aplicada. Seguindo uma linha alternativa, propomos, neste trabalho, um novo compósito híbrido, resultante da combinação de uma fase polimérica e uma fase inorgânica, que, do nosso ponto de vista também poderá contribuir para o desenvolvimento dessa nova geração de displays e telas. Este compósito é formado pela blenda de um polímero condutor dopado, a PANI (polianilina) ou POMA (poli(o-metóxi anilina)), e um polímero isolante, o P(VDF-TrFE) poli(vinilideno-co-trifluoretileno), na qual micro partículas de Zn2SiO4:Mn (SZF) foram adicionadas. Para o desenvolvimento desse novo compósito, estudamos cada um de seus elementos constituintes, notadamente quanto ao seu modo de preparo, características morfológicas e propriedades elétricas. Como resultado final, obtivemos dispositivos constituídos de uma única camada do compósito híbrido depositado por espalhamento sobre um substrato de ITO/FTO, acrescido, ainda, de um eletrodo de alumínio depositado por evaporação. Para a caracterização destes dispositivos foram realizadas medidas de condutividade elétrica, luminescência (L) e fotoluminescência (PL). Os compósitos PANI/P(VDF-TrFE)/SZF 05/95/80 e POMA/P(VDF-TrFE)/SZF 25/75/80 com condutividade elétrica da ordem de 10-3 S/m foram os que melhor se adequaram para a construção dos dispositivos. Os espectros de L e PL apresentaram um pico em ? = 538... . / New technologies recently applied to the fabrication of organic screens and displays have allowed the manufacture of lighter, thinner monitors in comparison to either the conventional cathodic rays tube (CRTs) or the liquid crystal displays (LCDs). In addition, displays based on organic materials (the so-called organic light emitting displays (OLEDs and the polymeric light emitting displays - PLEDs) may exhibit improved efficiency and flexibility. The organic displays have attracted attention of a number of research groups aimed at investigating the potential applications of these new devices in the electronic industry. In this study, a new hybrid composite comprising an inorganic phase dispersed in a polymeric matrix was investigated as a possible candidate material for displays fabrication. The continuous, organic phase was a blend containing a conducting polymer, viz. polyaniline (PANI) or its derivative poly(o-methoxyaniline) (POMA) and an insulating polymer, P(VDF-TrFE) (Poly(viny1idene fluoride- trifluorethylene)). Micro particles of Zn2SiO4:Mn (SZF) was added to the polymeric phase as the luminescent, active material. Luminescent devices were fabricating using a conventional trilayer architecture in which a thin film of the hybrid composite was deposited between two conducting electrodes. As the anode electrode we used either Indium-TinOxide (ITO) or Fluorine-Tin-Oxide (FTO)-covered glass plates. Evaporated aluminum was used as the cathode electrode. The material/device characterization was carried out using electric conductivity, luminescence (L) and photoluminescence (PL) measurements. PANI/P(VDFTrFE)/ SZF (05/95)/80 and POMA/P(VDF-TrFE)/SZF (25/75)/80 composites compositions presented the best performance, with electric conductivity of ca. 10-3 S/m. Luminescence and photoluminescence measurements revealed that both devices exhibited an emission band centered at 538 nm... (Complete abstract, click electronic address below).