Ce travail de thèse, réalisé au sein du CEA-LETI, consiste en la décorrélation des différents mécanismes ayant lieu dans une LED à base de GaN par voie de simulation numérique et de caractérisation expérimentale. Dans les chapitres 1 et 2, les théories des différents mécanismes présents dans une diode/LED sont décrites. Dans le chapitre 3, la simulation numérique d’une structure LED VTF ("vertical thin film" en langue anglaise) détermine les mécanismes prédominants dans les différentes gammes de tension. Une étude paramétrique s’ensuit afin d’évaluer les interactions entre les mécanismes.Dans le chapitre 4, les simulations sont effectuées en ajoutant un modèle de dépendance des mobilités des porteurs au champ électrique. En présence de ce modèle, les caractéristiques électro-optiques simulées tendent vers les caractéristiques réelles à haute tension.Les résultats des mesures de vitesse des porteurs dans le GaN-p sont également reportés dans le chapitre 4. En utilisant un motif d’échantillon spécifique et la méthode de mesure quatre points, les résultats suggèrent une tendance à saturer de la vitesse des porteurs, ou une tendance à décroître de la mobilité, à fort champ électrique. Ces résultats renforcent l’hypothèse de simulation utilisée dans le chapitre 4.Les simulations présentées dans les chapitres 3 et 4 permettent de proposer un schéma équivalent d’une LED en décorrélant les différents mécanismes et en ne gardant que les mécanismes dominants. Ce schéma équivalent permettrait, par exemple, à identifier les différents régimes dans une caractéristique électrique d’une LED réelle afin de procéder des améliorations du fonctionnement de la LED.Le chapitre 5 présente une étude par électroluminescence pulsée, une méthode de caractérisation fréquentielle, sur des LED commerciales. L’étude des temps de montée et de descente des signaux électro-optiques, ainsi que l’étude de durée de vie différentielle des porteurs de charge dans une LED, fourniraient des informations supplémentaires concernant l’injection des porteurs dans la LED. / This PhD. works, which was carried out inside CEA-LETI, aims to dissociate the various mechanisms occurring inside a GaN-based LED employing numerical simulation and experimental characterization. In the chapters 1 and 2, various mechanisms occurring inside a diode/LED are theoretically described. In chapter 3, through numerical simulation, the dominant mechanisms as well as their locations in a VTF ("vertical thin film") LED structure are determined for different voltage ranges. A parametric study follows to assess the interactions between the mechanisms.In chapter 4, the simulations are carried out with an additional field-dependent model for charge carrier mobility. With this model enabled, the simulated LED-electrical-and-optical characteristics approximate the real LED characteristics.Carrier-velocity characterization on p-type GaN, using a specific sample structure and the resistivity method, is also shown in chapter 4. It can be inferred from the results that under strong electric-fields, the carrier velocity might saturate, or the carrier mobility might decrease. These results strengthen the hypothesis used for the simulations in this chapter 4.The simulations introduced in the chapters 3 and 4 allow the proposition of an equivalent circuit for a GaN-based LED by dissociating the different mechanisms and retaining the dominant ones. This equivalent circuit could help, for instance, identify the different regimes in a real-LED electrical characteristics in order to improve the LED’s performance.Chapter 5 introduces pulsed electroluminescence, a frequential characterization method, on commercial LEDs. The studies of rise-time and fall-time of electro-optical signals, and the study of the differential lifetime of charge carriers in an LED would provide supplementary information concerning the carrier injection into the LED.

The purpose of this dissertation is to advance the understanding of SrS-based a. c. thin film electroluminescent (ACTFEL) devices. The role of traps in the bulk phosphor layer in the light emission mechanism from ACTFEL devices is studied, characterized and modeled. Experiments were performed to observe the response of the ACTFEL devices to tailored voltage excitations. A physical model was developed to describe the optoelectronic processes taking place in the phosphor; analytical equations were written and numerically simulated to plot the flux and luminance responses of the device to similarly tailored voltage excitations. The voltage excitation parameters such as amplitude, rise times and fall times were varied both experimentally and in simulations and their effect on the opto-electronic response of the device was studied. Thermally stimulated luminance studies were performed to determine critical device parameters. Theoretical predictions matched the experimental data in a qualitative manner. A much improved quantitative accuracy is obtained when the role of dipoles in the EL mechanisms is incorporated into the model.

No description available.

No description available.

Filmes finos de nitreto de silício com excesso de nitrogênio foram depositados sobre silício por sputtering reativo para obter estruturas emissoras de luz. As amostras foram modificadas por implantação iônica para verificar a influência dos dopantes arsênio (As) e boro (B) nos espectros de fotoluminescência (PL). As medidas de PL foram realizadas na faixa de temperatura entre 15-300 K e apresentaram uma emissão entre os comprimentos de onda 370-870 nm. Os dopantes introduziram uma emissão em 725 nm na banda de emissão, principalmente as dopadas com As. Foram realizadas medidas de microscopias para verificar a presença de nanoestruturas assim como a distribuição dos dopantes no material. As imagens de microscopias confirmaram a presença de nanocristais de nitreto de silício nas fases α, β e γ e identificaram a presença do dopante B nas fases cristalinas. O modelo de condução de Pool-Frenkel domina o transporte de portadores, indicando que a condução ocorre pelos níveis intrabandas, característica que definiu o modo que as recombinações radiativas ocorreram. As medidas de eletroluminescência (EL) apresentaram uma emissão centrada nos comprimentos de onda 760 e 880 nm (polarização negativa) e 1010 nm (polarização positiva) revelando diferenças significativas quando comparadas com as medidas de PL. Essa diferença esta associada à maneira como os elétrons populam os níveis intrabanda (excitação óptica para PL e elétrica para EL) que resulta em recombinações radiativas em diferentes comprimentos de ondas. A intensidade dos espectros de EL manifestou uma dependência quase linear com a densidade de corrente para ambas as polarizações. As medidas de EL em campos alternados exibiram um espectro de emissão composto pela soma das bandas obtidas separadamente em cada uma das polarizações. Medidas de EL em diferentes temperaturas (50-300 K) foram realizadas para investigar a influência da temperatura nos processos de recombinação radiativa. A intensidade exibiu uma redução com o aumento da temperatura, devido ao aumento do acoplamento elétron-fônon. / Silicon nitride with excess of nitrogen thin films were deposited on silicon substrate by reactive sputtering in order to obtain light emitting structures. Samples were modified by ion implantation of arsenic (As) and boron (B) to ascertain dopant leverage at photoluminescence (PL) spectra. PL measurements were performed at temperature ranging from 15 K up to 300 K and showed a band emission between wavelength 370 and 870 nm. An emission centered at 725 nm was observed in doped samples; especially in the presence of As. Microscope images showed crystalline structures of α-Si3N4, β-Si3N4 and γ-Si3N4 and confirmed boron dopant in nanocrystalline structures. Pool-Frenkel conduction model dominates electron transport in non-stoichiometric silicon nitride films due to intraband levels, phenomenon that has a huge contribution to electroluminescence (EL) emission. EL signals were composed by two peaks centered at 760 and 880 nm (negative bias – EL-N) and one peak at 1010 nm (positive bias – EL-P). Diffences between PL and EL spectra exhibit a clear dependence on the mode of excitation (photo and current source) on radiative recombination process. EL intensity had almost a linear increase with current density for both polarizations. EL measurements under AC voltage were composed by a superposition of the signals from EL-N and EL-P signals. Photo and electroluminescence measurements were collected at different temperatures (50 to 300 K) in order to investigate the temperature influence on the radiative recombination. The EL intensity was decreasing with temperature increasing, due to electron-phonon interactions.

Bradford Howland, Walter H. Pits, and Robert C. Gesteland. / "September 21, 1962." / Bibliography: p. 28. / Army Signal Corps Contract DA36-039- sc-78108. Dept. of the Army Task 3-99-20-001 and Project 3-99-00-000. Signal Corps Contract DA-SIG-36-039-61-G14. U.S. Air Force Contract AF33(616)-7783. National Institute of Health Grant B-2480(C1)

It has been found that a small magnetic field (<300 mT) can substantial change the electroluminescence, photoluminescence, photocurrent, electrical injection current in nonmagnetic organic semiconductors. It is generally believed that these magnetic field effects (MFE) are related to the spin dependent processes in organic semiconductor. However, the origin of MFE is still not well understood. In this dissertation, we investigate the underlying mechanism for magnetic field effects on electroluminescence (MFEEL) and magnetoresistance (MR) and demonstrate the complete tuning of MFEEL and MR based on our theoretical understanding. We consider MFE arising from magnetic field sensitive intersystem crossing (ISC) and triplet charge reaction. Magnetic field can increase the singlet ratios through ISC, accounting for positive MFEEL. Magnetic field modulated ISC strongly depends on the electron-hole pair separation distance. MFE can be enhanced by increasing the electron hole pair distance through material mixing and interplaying the electric dipole-dipole interaction. Meanwhile, two possible mechanisms corresponding for negative MFEEL: triplet-triplet annihilation and triplet charge reaction are also discussed. The negative MFEEL is achieved through adjusting triplet density charge confinement and exciton/charge ratio, which indicates that triplet charge reaction is a dominate process accountable for negative MFEEL. Significant MR and MFEEL are observed in strong spin orbital coupling iridium complex based OLED device after introducing the non-magnetic insulating blocking PVA layer. A possible mechanism for this new interface induced MR and MFEEL is proposed based on magnetic field perturbed spin-spin interaction at short capture distance of inter-charge carriers. The comparative study of two strong spin orbital coupling materials Ir(ppy)3 and Ir(ppy)2(acac) with different electrical dipole moments indicate the electric dipole-dipole interaction can change MR and MFEEL from short distance capture based regime to long distance intersystem-crossing regime. At last, we demonstrate the fully tuning sign of magnetic field effect on the fluorescence (MFEFEL) and phosphorescence (MFEPEL) by using the ISC, energy transfer and spin-spin interaction. In addition, we demonstrate a giant MFEEL (400%) in electrochemical cells and attribute this giant MFEEL to Lorentz force driven ion transport and Lorentz force dependent diffusion layer thickness through convection.

Charge transport dynamics through molecular scale materials is of common interest to both scientific and engineering disciplines. Putting molecules on nanoscale break junctions is the most straightforward setup to study charge transport dynamics through single molecules. Electromigration process can provide a simple and easy method of forming metallic oxide nanogap junctions. By using silver oxide thin films to form such nanogap junctions, silver nanoclusters (Ag2~Ag8) are also formed in-situ within the junctions. Formed silver nanoclusters strongly and stably electroluminesce under DC, AC, and customized pulse train excitation. By detecting extremely sensitive feedback, i.e. photons, two interesting behaviors of single molecule charge transport dynamics were revealed: 1) asymmetric charge transport and 2) discrete energy level tunneling. The discrete energy level tunneling of field emitted electrons yields negative differential resistance (NDR). Combined with photoconductivity and optical reduction of silver oxide to form silver nanoclusters, junction-asymmetry and NDR can be very useful in both electronic and optoelectronic applications such as on-demand electronics fabrication, single photon sources, and nanoscale photon detectors.

In this thesis, we fabricate the quantum dots (QDs) micro-pillar of metal-coated by E-beam lithography, and analyze the optical and electrical properties of micro-pillar cavity devices. For the sample materials, we use S-K mode to grow 3-layer In0.75Ga0.25As QDs structures sandwiched by up and down Al0.5Ga0.5As cladding layer on GaAs substrate by molecular-beam epitaxy (MBE). 40nm GaAs spacer layers with 2nm p-modulation doping in the central barrier are adopted in this study. The micro-pillar with diameter of 2 m, metal coated on top (p-type) and down (n-type) facet are designed. The good reflectivities of metal contacts provide more energy extraction inside the cavity. We expect the device lasing while the current injection. First, we design the morphology and size of patterns by AutoCAD software. Then, we use e-beam lithography with proper exposure condition to define the patterns, and thermal evaporation to deposit metals. The superfluous metal is lifted off and the defined area metal is served as dry etching mask to transfer the pattern to the dielectric layer and epi-layer. Finally, we use SiO2 layer to prevent current leakage, and the p-n contact on each facet to complete the devices. Micro-pillar samples with/without metal coated are analyzed by micro-PL system. The emission wavelength of 1282nm and the calculated Q-value of 100 are obtained for the sample with metal coated, an increase of 500%. From the EL measurement results, the device of micro-pillar samples with metal coated generate three peaks, 1149nm, 1221nm and 1291nm. Besides, it can efficiently improve the emission intensities. The measured result matched the simulation result.

In this study, the reactive rf magnetron sputtering was used to deposit zinc oxide (ZnO) thin films on SiO2/Si substrate at room temperature. The thermal treatment procedure was carried out to improve the luminescence characteristics of ZnO thin films. The physical characteristics of ZnO thin films with different post annealing process were obtained by the analyses of XRD and SEM. The electron spectroscopy for chemical analysis (ESCA) was used to analyze the chemical states of ZnO thin films. In optical properties, the photoluminescence spectrometer was used to measure the photoluminescence characteristics (PL). According to the results of experiments, the chemical states of ZnO thin films were changed after different post annealing. The photoluminescence characteristics were obtained at different wavelength, and the results indicated that they were affected by the chemical states of ZnO thin films. With 900¢J annealing, the strongest green emission and UV emission intensity can be obtained under the air ambient and the oxygen ambient, respectively. The reason was due to the variation of the proportion of oxygen vacancies and O-Zn bond within the ZnO thin films.