Conventionally, metal halide lamps were struck by voltages higher than those required for breaking down the electrodes to ensure successful ignition. These high ignition voltages may hurt the electrodes to some extent, leading to a shorter lamp lifecycle. In practice, the breakdown voltage can be affected by the dark current which occurs when a voltage is applied on lamp before the electrodes have been broken down. A lamp model to account for the dark current is derived from the test results. Three ignition schemes with single-pulse, multiple pulses and step voltage are used for describing the effect of the dark current on the breakdown voltage. Experimental results exhibit that the breakdown voltage can be lowered by applying a higher dark current or allotting more times of dark current to the lamp. The investigation provides useful information for the design of the ignition circuit.

Title from PDF of title page. / Document formatted into pages; contains 71 pages. / Thesis (M.S.E.E.)--University of South Florida, 2003. / Includes bibliographical references. / Text (Electronic thesis) in PDF format. / ABSTRACT: The CdTe solar cell is a leading candidate for cost-effective thin-film solar cells having demonstrated small area cell effciencies of 16.4%. A Key issue associated with CdTe thin film photovoltaic modules is the analysis of degradation behavior of the device. The analysis is complicated as changes due to degradation may be reversible. Solar cell measurement techniques were used to understand the changes in device parameters after light soaking for 1000 hours. An automated measurement setup was implemented as part of this thesis work. The main objective of this thesis was to study the effect of CdCl&esc;b2&esc;s heat treatment on the device stability. The temperature for this heat treatment was varied from 360oC to 400oC. Cells were stressed under illumination at both short circuit and open circuit conditions. It was found that the increase CdCl&esc;b2&esc;s heat treatment temperature slowed down the degradation rate.This was true for both short and open circuit stress conditions. Also short circuit stress condition slowed down the degradation of the device when compared with the open circuit condition. It became evident that the recombination current mainly got affected when the device was said to be degraded. / System requirements: World Wide Web browser and PDF reader. / Mode of access: World Wide Web.

Neste trabalho, foram estudados os modelos mais comuns para a descrição da corrente de escuro em fotodetectores baseados em poços quânticos (QWIPs). Foram também realizadas as alterações necessárias para tornar estes modelos independentes de ajustes experimentais, possibilitando assim a otimização dos dispositivos antes de sua fabricação. Estas simulações foram comparadas com os dados experimentais de amostras desenvolvidas em nosso laboratório para avaliar a qualidade do sistema de aquisição de curvas I x V recém-instalado, bem como dos dispositivos desenvolvidos. Analisando os resultados experimentais e teóricos, foi possível ainda identificar os diferentes regimes de transporte em cada temperatura e tensão aplicada. / In this work, we analyzed the most common models of the literature aiming to describe the dark current in quantum well based photodetectors (QWIPs), making the necessary changes to make them independent of experimental data, and enabling thus the optimization of the devices before they are fabricated. These simulations were compared to experimental data of sample grown in our laboratory to evaluate the quality of the I-V curve acquisition system recently installed, as well as the performance of the new devices being produced. By analyzing the experimental and theoretical results, it was still possible to identify the different transport regimes at each temperature and applied voltage.

Neste trabalho, foram estudados os modelos mais comuns para a descrição da corrente de escuro em fotodetectores baseados em poços quânticos (QWIPs). Foram também realizadas as alterações necessárias para tornar estes modelos independentes de ajustes experimentais, possibilitando assim a otimização dos dispositivos antes de sua fabricação. Estas simulações foram comparadas com os dados experimentais de amostras desenvolvidas em nosso laboratório para avaliar a qualidade do sistema de aquisição de curvas I x V recém-instalado, bem como dos dispositivos desenvolvidos. Analisando os resultados experimentais e teóricos, foi possível ainda identificar os diferentes regimes de transporte em cada temperatura e tensão aplicada. / In this work, we analyzed the most common models of the literature aiming to describe the dark current in quantum well based photodetectors (QWIPs), making the necessary changes to make them independent of experimental data, and enabling thus the optimization of the devices before they are fabricated. These simulations were compared to experimental data of sample grown in our laboratory to evaluate the quality of the I-V curve acquisition system recently installed, as well as the performance of the new devices being produced. By analyzing the experimental and theoretical results, it was still possible to identify the different transport regimes at each temperature and applied voltage.

The CdTe solar cell is a leading candidate for cost-effective thin-film solar cells having demonstrated small area cell effciencies of 16.4%. A Key issue associated with CdTe thin film photovoltaic modules is the analysis of degradation behavior of the device. The analysis is complicated as changes due to degradation may be reversible. Solar cell measurement techniques were used to understand the changes in device parameters after light soaking for 1000 hours. An automated measurement setup was implemented as part of this thesis work. The main objective of this thesis was to study the effect of CdCl2 heat treatment on the device stability. The temperature for this heat treatment was varied from 360oC to 400oC. Cells were stressed under illumination at both short circuit and open circuit conditions. It was found that the increase CdCl2 heat treatment temperature slowed down the degradation rate.This was true for both short and open circuit stress conditions. Also short circuit stress condition slowed down the degradation of the device when compared with the open circuit condition. It became evident that the recombination current mainly got affected when the device was said to be degraded.

Cette thèse concerne l’étude des photodétecteurs à base de matériauxsemi-conducteurs organiques (OPDs) sensibles à la lumière verte. Ces travaux sedivisent en cinq parties distinctes. Tout d’abord, une recherche bibliographique suiviedes notions fondamentales sur les matériaux et le fonctionnement des dispositifsorganiques est présentée. Ensuite, un travail sur la méthodologie concernant lafabrication de bancs expérimentaux et sur la métrologie garantissant l’exactitude desdonnées expérimentales sont effectués. Les méthodes de fabrication etd’optimisation, notamment le dépôt par sérigraphie, sont également présentées. Lapartie expérimentale concerne l’étude de l’origine du courant d’obscurité dans lesstructures organiques à base des matériaux donneur et accepteur d’électrons, laconception et la caractérisation d’un photodétecteur organique à l’état de l’art dontles performances optoélectroniques sont proches des dispositifs inorganiques baséssur la technologie silicium. Enfin, l’étude des mécanismes de dégradation d’uncapteur organique est présentée mettant en avant le rôle de l’oxygène et l’humidité. / This thesis deals with the study of photodetectors based on organicsemiconductor materials (OPDs) that are sensitive to green light. There are five partsto this study. First, a bibliographic study following the fundamentals of organicmaterials and the working principle of organic photodetectors is presented. Then, anextended study on the methodology and metrology is detailed, which was carried outin order to design and fabricate new optoelectronic instruments that are able tocharacterize organic devices accurately. Fabrication and optimization steps oforganic photodetectors are detailed. The experimental section concerns the study onthe origin of the dark current in organic devices based on electron donor/acceptorsystems. A choice of materials is discussed and a full characterisation of state-of-theartorganic photodetectors is presented in detail. The measured performances wereclose to the those of inorganic sensors based on silicon technology. Finally, a studyof degradation mechanisms is presented which highlights the role of oxygen andmoisture.

<p>Quantum Dot Infrared Photodetectors (QDIPs) are important alternatives to conventional infrared photodetectors with high potential to provide required detector performance, such as higher temperature operation and multispectral response, due to the 3-D quantum confinement of electrons, discrete energy levels, and intrinsic response to perpendicular incident light due to selection rules. However, excessive dark current density, which causes QDIPs to underperform theoretical predictions, is a limiting factor for the advancement of QDIP technologies. The purpose of this dissertation research is to achieve a better understanding of dopant incorporation into the active region of QDIPs, which is directly related to dark current control and spectral response. From this dissertation research, doping related dipole fields are found to be responsible for excessive dark current in QDIPs. </p><p>InAs/GaAs QDIPs were grown using solid source molecular beam epitaxy (MBE) with different doping conditions. The QDIPs were optically characterized using photoluminescence and Fourier transform infrared (FT-IR) spectroscopy. Devices were fabricated using standard cleanroom fabrication procedures. Dark current and capacitance measurements were performed under different temperature to reveal electronic properties of the materials and devices. A novel scanning capacitance microscopy (SCM) technique was used to study the band structure and carrier concentration on the cross section of a quantum dot (QD) heterostructure. In addition, dark current modeling and bandstructure calculations were performed to verify and better understand experimental results.</p><p>Two widely used QDIP doping methods with different doping concentrations have been studied in this dissertation research, namely direct doping in InAs QD layer, and modulation doping in the GaAs barrier above InAs QD layer. In the SCM experiment, electron redistribution has been observed due to band-bending in the modulation-doping region, while there is no band-bending observed in directly doped samples. A good agreement between the calculated bandstructure and experimental results leads to better understanding of doping in QD structures. The charge filling process in QDs has been observed by an innovative polarization-dependent FT-IR spectroscopy. The red-shift of QD absorbance peaks with increasing electron occupation supports a miniband electronic configuration for high-density QD ensembles. In addition, the FT-IR measurement indicates the existence of donor-complex (DX) defect centers in Si-doped QDIPs. The existence of DX centers and related dipole fields have been confirmed by dark current measurements to extract activation energies and by photocapacitance quenching measurements. </p><p>With the understanding achieved from experimental results, a further improved dark current model has been developed based on the previous model originally established by Ryzhii and improved by Stiff-Roberts. In the model described in this dissertation, two new factors have been considered. The inclusion of background drift current originating from Si shallow donors in the low bias region results in excellent agreement between calculated and measured dark currents at different temperatures, which has not been achieved by previous models. A very significant effect has been observed in that dark current leakage occurs due to the dipole field caused by doping induced charge distribution and impact-ionized DX centers. </p><p>Last but not least, QDIPs featuring the dipole interface doping (DID) method have been designed to reduce the dark current density without changing the activation energy (thus detection wavelength) of QDIPs. The DID samples involve an InAs QD layer directly-doped by Si, as well as Be doping in the GaAs barrier on both sides of the QD layer. The experimental result shows the dark current density has been significantly reduced by 104 times without any significant change to the corresponding activation energy. However, the high p-type doping in the GaAs barrier poses a challenge in that the Fermi level is reduced to be well below the QD energy states. High p-type doping is reported to reduce the dark current, photocurrent and the responsivity of the devices. </p><p>To conclude, it is significant to identify to effect of Si-induced defect centers on QDIP dark currents. The subsequent study reveals doping induced dipole fields can have significant effects on QDIP device performance, for example, causing charge leakage from QDs and reducing activation energy, thereby increasing dark current density. The DID approach developed in this work is a promising approach that could help address these issues by using controlled dipole fields to reduce dark current density without changing the minimum detectable energy of QDIPs.</p> / Dissertation

The most promising technology for radiography is active matrix flat panel imaging systems (AMFPI). However, AMFPI systems are relatively expensive in comparison with conventional computed radiography (CR) systems. Therefore for general radiography applications low cost systems are needed, especially in hospitals and healthcare systems of the developing countries. The focus of this research is the fabrication and characterization of a low cost amorphous silicon metal-semiconductor-metal photodetector as a photosensitive element in a AMFPI systems. Metal-Semiconductor-Metal photodetectors (MSM-PD) are attractive as sensors due to their ease of fabrication and compatibility with thin film transistor fabrication process primarily because there is no p+ doped layer in comparison with conventional p-i-n photodiodes. We have reported low dark current lateral a-Si MSM-PD (lower than 20pA/mm2 ) with responsivity of 280mA/W and EQE of 65 percent to green light ( l = 525nm). These improvement are achieved by introduction of a PI blocking layer and operating the device at high electric field (15 V/µm). This new structure eliminates the need of p+ and n+ layers which makes this structure fully compatible with the a-Si:H TFT fabrication process and consequently a low cost flat panel imager. Further, in this study we have investigated the effect of the spacing and width of the comb structure in the proposed lateral a-Si MSM-PD to determine the best configuration. Moreover, a-Si MSM-PD with PI blocking layer shows a linear behaviour to the photon flux in the wide range of 200nW/cm2 - 300µW/cm2 intensity of the incoming light. In comparison to vertical p-i-n structures, the reported MSM lateral device shows gains in terms of dynamic range, ease of fabrication (no p+ layer) without any deterioration in EQE and responsivity. This results are promising and encourage the development of a-Si lateral MSM-PD for indirect conversion large area medical imaging applications and especially low cost flat.

La taille des pixels des capteurs d’image CMOS approche aujourd’hui lemicron. Dans ce contexte, le courant d’obscurité reste un paramètrecritique. Il se superpose au courant photogénéré en affectant la qualité del’image par l’apparition de pixels blancs. La contamination métalliqueintroduite au cours du procédé de fabrication joue un rôle prépondérant dansla création des défauts à l’origine de ce courant d’obscurité. Cette étude apermis d’établir les seuils de dangerosité de différents élémentsmétalliques sur la technologie imageur. L’origine de contaminationsaccidentelles a été identifiée lors de crises de rendement. Pour cela, untravail sur les techniques de détection a été mené par µPCD, DLTS, pompagede charge, SIMS, TEM et photoluminescence. La spectroscopie de courantd’obscurité (DCS), particulièrement efficace dans ce contexte, a étédéveloppée pour l’identification de contaminations en or, tungstène etmolybdène, avec des limites de détection qui atteignent 108 à 1010 at/cm3.Nous observons la quantification du courant d’obscurité et étudionsl’amplification du champ électrique sur le taux de génération afin demodéliser les pics de courant d’obscurité obtenus. Le comportement decertains métaux dans le silicium est précisé par ces expériences, et nousévaluons l’efficacité de piégeage de plusieurs substrats imageur. Ce travailconduit à la mise en place de protocoles de contrôle de la contaminationmétallique en salle blanche. / Pixels size of CMOS image sensors is now decreasing towards one micron. Inthat context, dark current is a critical parameter. It superimposes with thecurrent generated by photons and affects the image quality with whitepixels. The metallic contamination introduced during the fabrication processplays an important role in the generation of defects that induce this darkcurrent. This study has allowed to determine dangerousness thresholds ofseveral metals on the imager technology. The origin of some accidentalcontaminations has been identified during yield crisis. Some work withdetection techniques has been performed with µPCD, DLTS, charge pumping,SIMS, TEM and photoluminescence. Dark current spectroscopy (DCS),particularly adapted to this situation, has been developped for theidentification of gold, tunsgten and molybdenum contaminations, withdetection limits that reach 108 to 1010 at/cm3. We have observed the darkcurrent quantization and studied the electric field enhancement ofgeneration rate to model the dark current peaks obtained. The behavior ofsome metals in silicon is confirmed by these experiments and we haveevaluated the getter efficiency of different substrates for image sensors.This work has lead to the application of protocols for the metalliccontamination control in clean room.

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