Academic literature on the topic 'Space charge doping'

Space charge accumulation in polymer dielectrics may lead to serious electric field distortion and even insulation failure during long-term operations of power equipment and electronic devices, especially under conditions of high temperature and direct current electric stress. The addition of nanoparticles into polymer matrices has been found effective in suppressing space charge accumulation and alleviating electric field distortion issues. Yet, the underlying mechanisms of nanoparticle doping remain a challenge to explore, especially from multi-dimensional composite insights. Here, a two-dimensional bipolar charge transport model with consideration of interface zones between organic/inorganic phases is proposed for the investigation into space charge behaviors of nanodielectrics. To validate the effectiveness and feasibility of the model, pulsed electroacoustic experiments are performed on epoxy/nano-MgO composites with different doping ratios of nanoparticles. Experimental observations match well with simulation anticipations, i.e., higher doping ratios of nanoparticles below the percolation threshold exhibit better capabilities to inhibit space charge accumulation. The deep traps (∼1.50 eV) generated in the interface zones are demonstrated to capture free charges, forming a reverse electric field in the region adjacent to electrodes and impeding the space charge migration toward the interior of the composite. This model is anticipated to provide theoretical insight for understanding space charge characteristics in polymer nanodielectrics and computing charge dynamics in extreme conditions where experiments are challenging to perform.

The methods of description of semiconduc tor-insulator interface characteristics based on process change of MIS type structure was considered. By using Maple Software, the calculations of quantities of inversion layer charge , total charge of semiconductor, inversion layer width and SCR semiconductor total width were m ade. Also, dependence theses quantities from doping level, temperature and surface potential were obtained

Mott and Gurney point out1, for defect-free semiconductors, I-V curve deviates from linear Ohmic type to nonlinear space-charge limited behavior at high electric field. A surprising large magnetoresistance (MR) has been reported in space-charge limited region by Delmo2-4recently. In present work, I-V and MR curves of silicon samples with different doping concentration are measured. It is observed that I-V curve enters into space charge region at lower voltage in heavily doped samples, however, space-charge limited current is absent in lightly doped samples. Two samples show different types of MR curve. In heavily doped samples, 8% MR is acquired at 3kG and the value of MR increases linearly up to 17%, while MR increases slowly up to 11% in lightly doped samples. It is believed that the dopant and trap in N-type silicon has a strong influence on the space-charge limited current and MR.

Micropillar arrays with radial p–n junctions are attractive for photovoltaic applications, because the light absorption and carrier collection become decoupled. The main challenge in manufacturing radial p–n junctions is achieving shallow (dopant depth <200 nm) and heavy doping (>1020 cm−3) that will allow the formation of a quasi-neutral region (QNR) and space charge region (SCR) in its tiny geometry. This experimental study investigates an approach that allows shallow and heavy doping in silicon micropillars. It aims to demonstrate that silicon dioxide (SiO2) can be used to control the dopant penetration depth in silicon micropillars.

Electrical properties of silicon doped AlN nanowires grown by plasma assisted molecular beam epitaxy were investigated by means of temperature dependent current–voltage measurements. Following an Ohmic regime for bias lower than 0.1 V, a transition to a space-charge limited regime occurred for higher bias. This transition appears to change with the doping level and is studied within the framework of the simplified theory of space-charge limited current assisted by traps. For the least doped samples, a single, doping independent trapping behavior is observed. For the most doped samples, an electron trap with an energy level around 150 meV below the conduction band is identified. The density of these traps increases with a Si doping level, consistent with a self-compensation mechanism reported in the literature. The results are in accordance with the presence of Si atoms that have three different configurations: one shallow state and two DX centers.

In this work, we study the emergence of polarization doping in AlxGa1−xN layers with graded composition from a theoretical viewpoint. It is shown that bulk electric charge density emerges in the graded concentration region. The magnitude of the effect, i.e., the relation between the polarization bulk charge density and the concentration gradient is obtained. The appearance of mobile charge in the wurtzite structure grown along the polar direction was investigated using the combination of ab initio and drift-diffusion models. It was shown that the ab initio results can be recovered precisely by proper parameterization of drift-diffusion representation of the complex nitride system. It was shown that the mobile charge appears due to the increase of the distance between opposite polarization-induced charges. It was demonstrated that, for sufficiently large space distance between polarization charges, the opposite mobile charges are induced. We demonstrate that the charge conservation law applies for fixed and mobile charge separately, leading to nonlocal compensation phenomena involving (i) the bulk fixed and polarization sheet charge at the heterointerfaces and (ii) the mobile band and the defect charge. Therefore, two charge conservation laws are obeyed that induces nonlocality in the system. The magnitude of the effect allows obtaining technically viable mobile charge density for optoelectronic devices without impurity doping (donors or acceptors). Therefore, it provides an additional tool for the device designer, with the potential to attain high conductivities: high carrier concentrations can be obtained even in materials with high dopant ionization energies, and the mobility is not limited by scattering at ionized impurities.

Traditional inorganic semiconductors can be electronically doped with high precision. Conversely, there is still conjecture regarding the assessment of the electronic doping density in metal-halide perovskites, not to mention of a control thereof. This paper presents a multifaceted approach to determine the electronic doping density for a range of different lead-halide perovskite systems. Optical and electrical characterization techniques, comprising intensity-dependent and transient photoluminescence, AC Hall effect, transfer-length-methods, and charge extraction measurements were instrumental in quantifying an upper limit for the doping density. The obtained values are subsequently compared to the electrode charge per cell volume under short-circuit conditions ([Formula: see text]), which amounts to roughly 1016 cm−3. This figure of merit represents the critical limit below which doping-induced charges do not influence the device performance. The experimental results consistently demonstrate that the doping density is below this critical threshold (∼1012 cm−3, which means ≪ [Formula: see text]) for all common lead-based metal-halide perovskites. Nevertheless, although the density of doping-induced charges is too low to redistribute the built-in voltage in the perovskite active layer, mobile ions are present in sufficient quantities to create space-charge-regions in the active layer, reminiscent of doped pn-junctions. These results are well supported by drift–diffusion simulations, which confirm that the device performance is not affected by such low doping densities.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2009.
Includes bibliographical references (p. 161-170).
In the search for new materials for energy conversion and storage technologies such as solid oxide fuel cells, nano-ionic materials have become increasingly relevant because unique physical and transport properties that occur on the nanoscale may potentially lead to improved device performance. Nanocrystalline cerium oxide, in particular, has been the subject of intense scrutiny, as researchers have attempted to link trends in electrical conductivity with the properties of space charge layers within the material. In this thesis, efforts designed to intentionally modify the space charge potential, and thus the space charge profiles and the macroscopic conductivity, are described.Nanocrystalline CeO2 thin films with a columnar microstructure were grown by pulsed laser deposition. A novel heterogeneous doping technique was developed in which thin NiO and Gd203 diffusion sources were deposited on the ceria surface and annealed in the temperature range of 7008000C in order to diffuse the cations into the ceria layer exclusively along grain boundaries. Time-offlight secondary ion mass spectrometry (ToF-SIMS) was utilized to measure the diffusion profiles. A single diffusion mechanism, identified as grain boundary diffusion, was observed. Using the constant source solution to the diffusion equation, grain boundary diffusion coefficients on the order of 10-15 to 10-13 cm2/s were obtained for Ni, as well as Mg diffusion emanating from the underlying substrate. Microfabricated Pt electrodes were deposited on the sample surface, and electrical measurements were made using impedance spectroscopy and two-point DC techniques. The asdeposited thin films displayed a total conductivity and activation energy consistent with reference values in the literature. After in-diffusion, the electrical conductivity decreased by one order of magnitude. Novel electron-blocking electrodes, consisting of dense yttria-stabilized zirconia and porous Pt layers were fabricated in order to deconvolute the ionic and electronic contributions to the total conductivity. In the as-deposited state, the ionic conductivity was determined to be pO2-independent, and the electronic conductivity displayed a slope of -0.30. The ionic transference number in the as-deposited state was 0.34.
(cont.) After annealing either with or without a diffusion source at temperatures of 700-8000C, both the ionic and electronic partial conductivities decreased. The ionic transferene numbers with and without a diffusion source were 0.26 and 0.76, respectively. Based on the existing framework of charge transport in polycrystalline materials, carrier profiles associated with the Mott-Schottky and Gouy-Chapman models were integrated in order to predict conductivity values based on parameters such as grain size and the space charge potential. Mott-Schottky profiles with a space charge potential of 0.44V were used to describe the behavior of the ceria thin films in the as-deposited state. It is proposed that annealing at temperatures of 700TC and above resulted in segregation of acceptor impurity ions to the grain boundary, resulting in GouyChapman conditions. The best fit to the annealed data occurred for a space charge potential of 0.35 V: a decrease of approximately 90 mV from the as-deposited state. In addition, a high-conductivity interfacial layer between the CeO2 and substrate was detected and was determined to influence samples with no surface diffusion source to a greater degree than those with NiO or Gd203.
by Scott J. Litzelman.
Ph.D.

La modulation de la densité de charge est un aspect important de l'étude de les transitions de phase électroniques ainsi que des propriétés électroniques des matériaux et il est à la base de plusieurs applications dans la micro-électronique. L'ajustement de la densité des porteurs de charge (dopage) peut être fait par voie chimique, en ajoutant des atomes étrangers au réseau cristallin du matériau ou électrostatiquement, en créant un accumulation de charge comme dans un Transistor é Effet de Champ. Cette dernier m ethode est réversible et particuliérement appropriée pour les matériaux bidimensionnels (2D) ou pour des couches ultra-minces. Le Dopage par Charge d'Espace est une nouvelle technique inventée et développée au cours de ce travail de thèse pour le dopage electrostatique de matériaux déposés sur la surface du verre. Une charge d'espace est créée à la surface en provoquant le mouvement des ions sodium présents dans le verre sous l'effet de la chaleur et d'un champ électrique extérieur. Cette espace de charge induit une accumulation de charge dans le matériau déposé sur la surface du verre, ce qui peut être supérieure à 10^14/cm^2. Une caractérisation détaillée faite avec mesures de transport, effet Hall, mesures Raman et mesures de Microscopie a Force Atomique (AFM) montrent que le dopage est réversible, bipolaire et il ne provoque pas des modifications chimiques. Cette technique peut être appliquée a des grandes surfaces, comme il est montré pour le cas du graph ene CVD. Dans une deuxiéme partie le dopage par espace de charge est appliqué à des couches ultra-minces (< 40 nm) de ZnO_(1-x). Le résultat est un abaissement de la résistance par carré de 5 ordres de grandeur. Les mesures de magnéto-transport faites à basse température montrent que les électrons dop es sont confinés en deux dimensions. Une transition remarquable de la localisation faible à l'anti-localisation est observée en fonction du dopage et de la température et des conclusions sont tirées à propos des phénoménes de diffusion qui gouverne le transport électronique dans des diff erentes conditions dans ce matériau
Carrier modulation is an important parameter in the study of the electronic phase transitions and the electronic properties of materials and at the basis for many applications in microelectronics. The tuning of charge carrier density (doping) can be achieved chemically, by adding foreign atoms to the crystal structure of the material or electrostatically, by inducing a charge accumulation like in a Field Eect Transistor device. The latter method is reversible and particularly indicated for use in two dimensional (2D) materials or ultra-thin films. Space Charge Doping is a new technique invented and developed during this thesis for the electrostatic doping of such materials deposited on a glass surface. A space charge is created at the surface by causing sodium ions contained in glass to drift under the Eect of heat and an external electric field. This space charge in turn induces a charge accumulation in the material deposited on the glass surface which can be higher than 10^14/cm^2. Detailed characterization using transport, Hall effect, Raman and AFM measurements shows that the doping is reversible, ambipolar and does not induce chemical changes. It can be applied to large areas as shown with CVD graphene. In a second phase the space charge doping method is applied to polycrystalline ultra-thin films (< 40 nm) of ZnO_(1-x). A lowering of sheet resistance over 5 orders of magnitude is obtained. Low temperature magneto-transport measurements reveal that doped electrons are confined in two dimensions. A remarkable transition between weak localization and anti-localization isobserved as a function of doping and temperature and conclusions are drawn concerning the scattering phenomena governing electronic transport under different conditions in this material

La modulation de la densité de charge est un aspect important de l'étude de les transitions de phase électroniques ainsi que des propriétés électroniques des matériaux et il est à la base de plusieurs applications dans la micro-électronique. L'ajustement de la densité des porteurs de charge (dopage) peut être fait par voie chimique, en ajoutant des atomes étrangers au réseau cristallin du matériau ou électrostatiquement, en créant un accumulation de charge comme dans un Transistor é Effet de Champ. Cette dernier m ethode est réversible et particuliérement appropriée pour les matériaux bidimensionnels (2D) ou pour des couches ultra-minces. Le Dopage par Charge d'Espace est une nouvelle technique inventée et développée au cours de ce travail de thèse pour le dopage electrostatique de matériaux déposés sur la surface du verre. Une charge d'espace est créée à la surface en provoquant le mouvement des ions sodium présents dans le verre sous l'effet de la chaleur et d'un champ électrique extérieur. Cette espace de charge induit une accumulation de charge dans le matériau déposé sur la surface du verre, ce qui peut être supérieure à 10^14/cm^2. Une caractérisation détaillée faite avec mesures de transport, effet Hall, mesures Raman et mesures de Microscopie a Force Atomique (AFM) montrent que le dopage est réversible, bipolaire et il ne provoque pas des modifications chimiques. Cette technique peut être appliquée a des grandes surfaces, comme il est montré pour le cas du graph ene CVD. Dans une deuxiéme partie le dopage par espace de charge est appliqué à des couches ultra-minces (< 40 nm) de ZnO_(1-x). Le résultat est un abaissement de la résistance par carré de 5 ordres de grandeur. Les mesures de magnéto-transport faites à basse température montrent que les électrons dop es sont confinés en deux dimensions. Une transition remarquable de la localisation faible à l'anti-localisation est observée en fonction du dopage et de la température et des conclusions sont tirées à propos des phénoménes de diffusion qui gouverne le transport électronique dans des diff erentes conditions dans ce matériau
Carrier modulation is an important parameter in the study of the electronic phase transitions and the electronic properties of materials and at the basis for many applications in microelectronics. The tuning of charge carrier density (doping) can be achieved chemically, by adding foreign atoms to the crystal structure of the material or electrostatically, by inducing a charge accumulation like in a Field Eect Transistor device. The latter method is reversible and particularly indicated for use in two dimensional (2D) materials or ultra-thin films. Space Charge Doping is a new technique invented and developed during this thesis for the electrostatic doping of such materials deposited on a glass surface. A space charge is created at the surface by causing sodium ions contained in glass to drift under the Eect of heat and an external electric field. This space charge in turn induces a charge accumulation in the material deposited on the glass surface which can be higher than 10^14/cm^2. Detailed characterization using transport, Hall effect, Raman and AFM measurements shows that the doping is reversible, ambipolar and does not induce chemical changes. It can be applied to large areas as shown with CVD graphene. In a second phase the space charge doping method is applied to polycrystalline ultra-thin films (< 40 nm) of ZnO_(1-x). A lowering of sheet resistance over 5 orders of magnitude is obtained. Low temperature magneto-transport measurements reveal that doped electrons are confined in two dimensions. A remarkable transition between weak localization and anti-localization isobserved as a function of doping and temperature and conclusions are drawn concerning the scattering phenomena governing electronic transport under different conditions in this material

Le diagramme de phase des supraconducteurs à haute température critique en fonction du dopage et de la température a été étudié de manière intensive avec une variation chimique du dopage. Le dopage chimique peut provoquer des changements structurels et du désordre, masquant les effets intrinsèques. Alternativement, des échantillons ultra-minces dopés électrostatiquement peuvent être utilisés à travers des dispositifs de type transistors à effet de champ (FET). Cependant, nombreux défis technologiques sont à affronter lorsque des supraconducteurs à haute température sont concernés. Dans cette thèse nous surmontons ces obstacles en utilisant des techniques développées dans notre laboratoire et nous nous concentrons sur le supraconducteur à haute température BSCCO-2212 dont le diagramme de phase n'a jamais été étudié par effet électrostatique. Notamment, nous fabriquons des dispositifs supraconducteurs de BSCCO-2212 de haute qualité et utilisons une méthode électrostatique originale appelée dopage de charge d'espace, et mesurons les caractéristiques de transport de 330~K à basse température. Nous extrayons les paramètres et les températures caractéristiques sur une grande plage de dopage et établissons un diagramme de phase complet pour les échantillons BSCCO-2212 d'épaisseur 1~u.c. en fonction du dopage, de la température et du désordre. Nous identifions aussi la plage critique de dopage où une transition de phase quantique est prédite. Enfin, nous examinons de près la transition supraconductrice dans la limite de la bi-dimensionnalité. Les dispositifs BSCCO-2212 à 1 maille cristalline d'épaisseur. Les fluctuations et les effets extrinsèques sont décrits par des formalismes théoriques appropriés et le caractère bidimensionnel de la transition supraconductrice de BSCCO-2212 est analysé
The phase diagram of hole-doped high critical temperature superconductors as a function of doping and temperature has been intensively studied with chemical variation of doping. Chemical doping can provoke structural changes and disorder, masking intrinsic effects. Alternatively, electrostatically doped ultra-thin samples can be used through Field-Effect Transistor (FET) devices. The electrostatic modulation of charge carrier density in 2D materials is an elegant and clean approach that presents many technological challenges when high temperature superconductors are concerned. In this thesis we overcome these technological obstacles by using proprietary techniques developed in our laboratory for the study of 2D materials, and we focus on the high temperature superconductor BSCCO-2212, whose phase diagram has so far never been studied via electrostatic effect. Notably we fabricate ultra-thin high quality superconducting BSCCO-2212 devices and use an original electrostatic method called space charge doping to measure transport characteristics from 330~K to low temperature. We extract parameters and characteristic temperatures over a large doping range and establish a comprehensive phase diagram for one-unit-cell-thick BSCCO-2212 samples as a function of doping, temperature and disorder. We also identify the critical doping range where a quantum phase transition is predicted. Finally we take a closer look at the superconducting transition in the two dimensional limit. Fluctuations and extrinsic effects are accounted for using appropriate theoretical formalism and the two dimensional character of the superconducting transition of BSCCO-2212 is analysed

Ce travail allie les propriétés singulières des matériaux 2D à une technique innovante utilisée pour modifier les propriétés électroniques des films ultra-minces pour proposer une nouvelle technologie permettant de réaliser le dispositif électronique le plus simple, la diode. Tout d'abord, nous identifions les matériaux semi-conducteurs pouvant être fabriqué en couches ultra-minces. Deuxièmement, nous utilisons une technique appelée dopage par charge d'espace développée dans notre groupe pour le dopage n ou p des matériaux. Enfin, nous obtenons les caractéristiques de diode des dispositifs. Le manuscrit commence par une revue des matériaux. Dans la famille des matériaux 2D, notre choix s'est porté sur un semi-conducteur en couches III-VI avec une bande interdite directe : InSe. Nous avons aussi choisi un type de matériau très différent, le CdO polycristallin qui n'est pas lamellaire et n'a pas une bande interdite directe, mais qui est facile à fabriquer sous forme de film ultra-mince avec une grande mobilité de porteurs. Après des expériences préliminaires, nous avons choisi InSe et fabriqué des dispositifs de films ultra minces de InSe. Nous avons développé en parallèle deux géométries pour la diode p-n. Nous avons pu obtenir un redressement pour chaque géométrie, ce qui implique que notre approche de dopage par charge d'espace a réussi à produire un dopage différencié spatialement dans chaque dispositif. Nous discutons des caractéristiques I-V obtenues et les limitations inhérentes aux dispositifs (chauffage local, hystérèses) et suggérons des améliorations afin d'obtenir un fonctionnement plus efficace et stable dans le cadre des perspectives de cette thèse
This work combines the singular properties of 2D materials with an innovative technique used for changing the electronic properties of ultra-thin films to propose a new technology for making the simplest bipolar electronic device, the diode. Firstly we identify semiconducting materials which can be fabricated in ultra-thin layers. Secondly, we use a proprietary technique called Space Charge Doping developed in our group for doping the material, either n or p. Finally, we obtain diode characteristics from the device. The manuscript begins with a review of different materials and properties. In the family of 2D materials, our choice was a III-VI layered semiconductor with a direct bandgap: InSe. We also chose a completely different kind of material, polycrystalline CdO, which is neither layered nor has a direct bandgap but is easy to fabricate in the ultra-thin film form and has high carrier mobility. After preliminary experiments, we chose InSe and fabricated devices of ultra-thin, few atomic layer InSe thin films. We chose to develop in parallel two different geometries for the p-n junction diode. We were able to obtain rectifying behavior for each geometry implying that our space charge doping approach was successful in producing microscopically, spatially differentiated doping in each device. We discuss the obtained I-V characteristics and the inherent limitations of the devices (local heating, hysteresis) and suggest improvements for future experiments and ways of obtaining more efficient and stable functioning and geometry as part of the perspectives of this thesis

Organic semiconductors are a new key technology for large-area and flexible thin-film electronics. They are deposited as thin films (sub-nanometer to micrometer) on large-area substrates. The technologically most advanced applications are organic light emitting diodes (OLEDs) and organic photovoltaics (OPV). For the improvement of performance and efficiency, correct modeling of the electronic processes in the devices is essential. Reliable characterization and validation of the electronic properties of the materials is simultaneously required for the successful optimization of devices. Furthermore, understanding the relations between material structures and their key characteristics opens the path for innovative material and device design. In this thesis, two material characterization methods are developed, respectively refined and applied: a novel technique for measuring the charge carrier mobility μ and a way to determine the ionization energy IE or the electron affinity EA of an organic semiconductor. For the mobility measurements, a new evaluation approach for space-charge limited current (SCLC) measurements in single carrier devices is developed. It is based on a layer thickness variation of the material under investigation. In the \"potential mapping\" (POEM) approach, the voltage as a function of the device thickness V(d) at a given current density is shown to coincide with the spatial distribution of the electric potential V(x) in the thickest device. On this basis, the mobility is directly obtained as function of the electric field F and the charge carrier density n. The evaluation is model-free, i.e. a model for μ(F, n) to fit the measurement data is not required, and the measurement is independent of a possible injection barrier or potential drop at non-optimal contacts. The obtained μ(F, n) function describes the effective average mobility of free and trapped charge carriers. This approach realistically describes charge transport in energetically disordered materials, where a clear differentiation between trapped and free charges is impossible or arbitrary. The measurement of IE and EA is performed by characterizing solar cells at varying temperature T. In suitably designed devices based on a bulk heterojunction (BHJ), the open-circuit voltage Voc is a linear function of T with negative slope in the whole measured range down to 180K. The extrapolation to temperature zero V0 = Voc(T → 0K) is confirmed to equal the effective gap Egeff, i.e. the difference between the EA of the acceptor and the IE of the donor. The successive variation of different components of the devices and testing their influence on V0 verifies the relation V0 = Egeff. On this basis, the IE or EA of a material can be determined in a BHJ with a material where the complementary value is known. The measurement is applied to a number of material combinations, confirming, refining, and complementing previously reported values from ultraviolet photo electron spectroscopy (UPS) and inverse photo electron spectroscopy (IPES). These measurements are applied to small molecule organic semiconductors, including mixed layers. In blends of zinc-phthalocyanine (ZnPc) and C60, the hole mobility is found to be thermally and field activated, as well as increasing with charge density. Varying the mixing ratio, the hole mobility is found to increase with increasing ZnPc content, while the effective gap stays unchanged. A number of further materials and material blends are characterized with respect to hole and electron mobility and the effective gap, including highly diluted donor blends, which have been little investigated before. In all materials, a pronounced field activation of the mobility is observed. The results enable an improved detailed description of the working principle of organic solar cells and support the future design of highly efficient and optimized devices
Organische Halbleiter sind eine neue Schlüsseltechnologie für großflächige und flexible Dünnschichtelektronik. Sie werden als dünne Materialschichten (Sub-Nanometer bis Mikrometer) auf großflächige Substrate aufgebracht. Die technologisch am weitesten fortgeschrittenen Anwendungen sind organische Leuchtdioden (OLEDs) und organische Photovoltaik (OPV). Zur weiteren Steigerung von Leistungsfähigkeit und Effizienz ist die genaue Modellierung elektronischer Prozesse in den Bauteilen von grundlegender Bedeutung. Für die erfolgreiche Optimierung von Bauteilen ist eine zuverlässige Charakterisierung und Validierung der elektronischen Materialeigenschaften gleichermaßen erforderlich. Außerdem eröffnet das Verständnis der Zusammenhänge zwischen Materialstruktur und -eigenschaften einen Weg für innovative Material- und Bauteilentwicklung. Im Rahmen dieser Dissertation werden zwei Methoden für die Materialcharakterisierung entwickelt, verfeinert und angewandt: eine neuartige Methode zur Messung der Ladungsträgerbeweglichkeit μ und eine Möglichkeit zur Bestimmung der Ionisierungsenergie IE oder der Elektronenaffinität EA eines organischen Halbleiters. Für die Beweglichkeitsmessungen wird eine neue Auswertungsmethode für raumladungsbegrenzte Ströme (SCLC) in unipolaren Bauteilen entwickelt. Sie basiert auf einer Schichtdickenvariation des zu charakterisierenden Materials. In einem Ansatz zur räumlichen Abbildung des elektrischen Potentials (\"potential mapping\", POEM) wird gezeigt, dass das elektrische Potential als Funktion der Schichtdicke V(d) bei einer gegebenen Stromdichte dem räumlichen Verlauf des elektrischen Potentials V(x) im dicksten Bauteil entspricht. Daraus kann die Beweglichkeit als Funktion des elektrischen Felds F und der Ladungsträgerdichte n berechnet werden. Die Auswertung ist modellfrei, d.h. ein Modell zum Angleichen der Messdaten ist für die Berechnung von μ(F, n) nicht erforderlich. Die Messung ist außerdem unabhängig von einer möglichen Injektionsbarriere oder einer Potentialstufe an nicht-idealen Kontakten. Die gemessene Funktion μ(F, n) beschreibt die effektive durchschnittliche Beweglichkeit aller freien und in Fallenzuständen gefangenen Ladungsträger. Dieser Zugang beschreibt den Ladungstransport in energetisch ungeordneten Materialien realistisch, wo eine klare Unterscheidung zwischen freien und Fallenzuständen nicht möglich oder willkürlich ist. Die Messung von IE und EA wird mithilfe temperaturabhängiger Messungen an Solarzellen durchgeführt. In geeigneten Bauteilen mit einem Mischschicht-Heteroübergang (\"bulk heterojunction\" BHJ) ist die Leerlaufspannung Voc im gesamten Messbereich oberhalb 180K eine linear fallende Funktion der Temperatur T. Es kann bestätigt werden, dass die Extrapolation zum Temperaturnullpunkt V0 = Voc(T → 0K) mit der effektiven Energielücke Egeff , d.h. der Differenz zwischen EA des Akzeptor-Materials und IE des Donator-Materials, übereinstimmt. Die systematische schrittweise Variation einzelner Bestandteile der Solarzellen und die Überprüfung des Einflusses auf V0 bestätigen die Beziehung V0 = Egeff. Damit kann die IE oder EA eines Materials bestimmt werden, indem man es in einem BHJ mit einem Material kombiniert, dessen komplementärer Wert bekannt ist. Messungen per Ultraviolett-Photoelektronenspektroskopie (UPS) und inverser Photoelektronenspektroskopie (IPES) werden damit bestätigt, präzisiert und ergänzt. Die beiden entwickelten Messmethoden werden auf organische Halbleiter aus kleinen Molekülen einschließlich Mischschichten angewandt. In Mischschichten aus Zink-Phthalocyanin (ZnPc) und C60 wird eine Löcherbeweglichkeit gemessen, die sowohl thermisch als auch feld- und ladungsträgerdichteaktiviert ist. Wenn das Mischverhältnis variiert wird, steigt die Löcherbeweglichkeit mit zunehmendem ZnPc-Anteil, während die effektive Energielücke unverändert bleibt. Verschiedene weitere Materialien und Materialmischungen werden hinsichtlich Löcher- und Elektronenbeweglichkeit sowie ihrer Energielücke charakterisiert, einschließlich bisher wenig untersuchter hochverdünnter Donator-Systeme. In allen Materialien wird eine deutliche Feldaktivierung der Beweglichkeit beobachtet. Die Ergebnisse ermöglichen eine verbesserte Beschreibung der detaillierten Funktionsweise organischer Solarzellen und unterstützen die künftige Entwicklung hocheffizienter und optimierter Bauteile

EDELWEISS est une expérience de détection directe de matière noire froide non-baryonique sous forme de particules massives et faiblement interagissantes (connues sous l'acronyme de WIMPs), qui constituent actuellement les candidats les plus populaires pour rendre compte de la masse manquante de l'Univers. Dans ce but, EDELWEISS utilise des bolomètres de germanium opérés à température cryogénique (20 mK environ) dans le Laboratoire Souterrain de Modane (LSM) à la frontière franco-italienne. En particulier, depuis 2008, un nouveau type de détecteur est en fonctionnement, équipé d'électrodes concentriques pour optimiser le rejet des évènements de surface (détecteurs à grilles coplanaires). Cette thèse se décompose en plusieurs axes de recherche. Tout d'abord, nous avons réalisé des mesures concernant la collecte des charges dans les cristaux. Les lois de vitesse des porteurs (électrons et trous) ont été déterminées dans le germanium à 20 mK dans la direction <100>, et une étude complète de la répartition des charges a été menée, avec une évaluation de l'anisotropie du transport et de la diffusion transverse des porteurs. Ces résultats permettent d'avoir une meilleure compréhension du fonctionnement interne des détecteurs d'Edelweiss. Ensuite, des études portant sur l'amélioration des performances ont été effectuées. Nous avons en particulier permis d'optimiser la procédure de régénération des cristaux et améliorer le rejet passif des évènements de surface (β). Le volume utile de détection des détecteurs a été évalué en utilisant les raies de deux radio-isotopes activés cosmiquement, le 68Ge et le 65Zn. Enfin, une étude exhaustive portant sur l'étude des spectres à basse énergie a été menée, ce qui permet de mettre au point une méthode d'analyse systématique pour la recherche de WIMPs de basse masse dans EDELWEISS
EDELWEISS is a direct non-baryonic cold dark matter detection experiment in the form of weakly interacting massive particles (also known as WIMPs), which currently constitute the most popular candidates to account for the missing mass in the Universe. To this purpose, EDELWEISS uses germanium bolometers at cryogenic temperature (20 mK approximately) in the Underground Laboratory of Modane (LSM) at the French-Italian border. Since 2008, a new type of detector is operated, equipped with concentric electrodes to optimize the rejection of surface events (coplanar-grid detectors). This thesis work is divided into several research orientations. First, we carried out measurements concerning charge collection in the crystals. The velocity laws of the carriers (electrons and holes) have been determined in germanium at 20 mK in the <100> orientation, and a complete study of charge sharing has been done, including an evaluation of the transport anisotropy and of the straggling of the carriers. These results lead to a better understanding of the inner properties of the EDELWEISS detectors. Then, studies relating to the improvement of the performances were carried out. In particular, we have optimized the space-charge cancellation procedure in the crystals and improved the passive rejection of surface events (β). The fiducial volume of the detectors has been evaluated using two X-ray lines from cosmically activated radionuclides: 68Ge and 65Zn. Lastly, an exhaustive study of the low energy spectra has been carried out, which makes it possible to develop a systematic analysis method for the search of low-mass WIMPs in EDELWEISS

Organic semiconductors are a new key technology for large-area and flexible thin-film electronics. They are deposited as thin films (sub-nanometer to micrometer) on large-area substrates. The technologically most advanced applications are organic light emitting diodes (OLEDs) and organic photovoltaics (OPV). For the improvement of performance and efficiency, correct modeling of the electronic processes in the devices is essential. Reliable characterization and validation of the electronic properties of the materials is simultaneously required for the successful optimization of devices. Furthermore, understanding the relations between material structures and their key characteristics opens the path for innovative material and device design. In this thesis, two material characterization methods are developed, respectively refined and applied: a novel technique for measuring the charge carrier mobility μ and a way to determine the ionization energy IE or the electron affinity EA of an organic semiconductor. For the mobility measurements, a new evaluation approach for space-charge limited current (SCLC) measurements in single carrier devices is developed. It is based on a layer thickness variation of the material under investigation. In the \"potential mapping\" (POEM) approach, the voltage as a function of the device thickness V(d) at a given current density is shown to coincide with the spatial distribution of the electric potential V(x) in the thickest device. On this basis, the mobility is directly obtained as function of the electric field F and the charge carrier density n. The evaluation is model-free, i.e. a model for μ(F, n) to fit the measurement data is not required, and the measurement is independent of a possible injection barrier or potential drop at non-optimal contacts. The obtained μ(F, n) function describes the effective average mobility of free and trapped charge carriers. This approach realistically describes charge transport in energetically disordered materials, where a clear differentiation between trapped and free charges is impossible or arbitrary. The measurement of IE and EA is performed by characterizing solar cells at varying temperature T. In suitably designed devices based on a bulk heterojunction (BHJ), the open-circuit voltage Voc is a linear function of T with negative slope in the whole measured range down to 180K. The extrapolation to temperature zero V0 = Voc(T → 0K) is confirmed to equal the effective gap Egeff, i.e. the difference between the EA of the acceptor and the IE of the donor. The successive variation of different components of the devices and testing their influence on V0 verifies the relation V0 = Egeff. On this basis, the IE or EA of a material can be determined in a BHJ with a material where the complementary value is known. The measurement is applied to a number of material combinations, confirming, refining, and complementing previously reported values from ultraviolet photo electron spectroscopy (UPS) and inverse photo electron spectroscopy (IPES). These measurements are applied to small molecule organic semiconductors, including mixed layers. In blends of zinc-phthalocyanine (ZnPc) and C60, the hole mobility is found to be thermally and field activated, as well as increasing with charge density. Varying the mixing ratio, the hole mobility is found to increase with increasing ZnPc content, while the effective gap stays unchanged. A number of further materials and material blends are characterized with respect to hole and electron mobility and the effective gap, including highly diluted donor blends, which have been little investigated before. In all materials, a pronounced field activation of the mobility is observed. The results enable an improved detailed description of the working principle of organic solar cells and support the future design of highly efficient and optimized devices.:1. Introduction 2. Organic semiconductors and devices 2.1. Organic semiconductors 2.1.1. Conjugated π system 2.1.2. Small molecules and polymers 2.1.3. Disorder in amorphous materials 2.1.4. Polarons 2.1.5. Polaron hopping 2.1.6. Fermi-Dirac distribution and Fermi level 2.1.7. Quasi-Fermi levels 2.1.8. Trap states 2.1.9. Doping 2.1.10. Excitons 2.2. Interfaces and blend layers 2.2.1. Interface dipoles 2.2.2. Energy level bending 2.2.3. Injection from metal into semiconductor, and extraction 2.2.4. Excitons at interfaces 2.3. Charge transport and recombination in organic semiconductors 2.3.1. Drift transport 2.3.2. Charge carrier mobility 2.3.3. Thermally activated transport 2.3.4. Diffusion transport 2.3.5. Drift-diffusion transport 2.3.6. Space-charge limited current 2.3.7. Recombination 2.4. Mobility measurement 2.4.1. SCLC and TCLC 2.4.2. Time of flight 2.4.3. Organic field effect transistors 2.4.4. CELIV 2.5. Organic solar cells 2.5.1. Exciton diffusion towards the interface 2.5.2. Dissociation of CT states 2.5.3. CT recombination 2.5.4. Flat and bulk heterojunction 2.5.5. Transport layers 2.5.6. Thin film optics 2.5.7. Current-voltage characteristics and equivalent circuit 2.5.8. Solar cell efficiency 2.5.9. Limits of efficiency 2.5.10. Correct solar cell characterization 2.5.11. The \"O-Factor\" 3. Materials and experimental methods 3.1. Materials 3.2. Device fabrication and layout 3.2.1. Layer deposition 3.2.2. Encapsulation 3.2.3. Homogeneity of layer thickness on a wafer 3.2.4. Device layout 3.3. Characterization 3.3.1. Electrical characterization 3.3.2. Sample illumination 3.3.3. Temperature dependent characterization 3.3.4. UPS 4. Simulations 5.1. Design of single carrier devices 5.1.1. General design requirements 5.1.2. Single carrier devices for space-charge limited current 5.1.3. Ohmic regime 5.1.4. Design of injection and extraction layers 5.2. Advanced evaluation of SCLC – potential mapping 5.2.1. Potential mapping by thickness variation 5.2.2. Further evaluation of the transport profile 5.2.3. Injection into and extraction from single carrier devices 5.2.4. Majority carrier approximation 5.3. Proof of principle: POEM on simulated data 5.3.1. Constant mobility 5.3.2. Field dependent mobility 5.3.3. Field and charge density activated mobility 5.3.4. Conclusion 5.4. Application: Transport characterization in organic semiconductors 5.4.1. Hole transport in ZnPc:C60 5.4.2. Hole transport in ZnPc:C60 – temperature variation 5.4.3. Hole transport in ZnPc:C60 – blend ratio variation 5.4.4. Hole transport in ZnPc:C70 5.4.5. Hole transport in neat ZnPc 5.4.6. Hole transport in F4-ZnPc:C60 5.4.7. Hole transport in DCV-5T-Me33:C60 5.4.8. Electron transport in ZnPc:C60 5.4.9. Electron transport in neat Bis-HFl-NTCDI 5.5. Summary and discussion of the results 5.5.1. Phthalocyanine:C60 blends 5.5.2. DCV-5T-Me33:C60 5.5.3. Conclusion 6. Organic solar cell characteristics: the influence of temperature 6.1. ZnPc:C60 solar cells 6.1.1. Temperature variation 6.1.2. Illumination intensity variation 6.2. Voc in flat and bulk heterojunction organic solar cells 6.2.1. Qualitative difference in Voc(I, T) 6.2.2. Interpretation of Voc(I, T) 6.3. BHJ stoichiometry variation 6.3.1. Voc upon variation of stoichiometry and contact layer 6.3.2. V0 upon stoichiometry variation 6.3.3. Low donor content stoichiometry 6.3.4. Conclusion from stoichiometry variation 6.4. Transport material variation 6.4.1. HTM variation 6.4.2. ETM variation 6.5. Donor:acceptor material variation 6.5.1. Donor variation 6.5.2. Acceptor variation 6.6. Conclusion 7. Summary and outlook 7.1. Summary 7.2. Outlook A. Appendix A.1. Energy pay-back of this thesis A.2. Tables and registers
Organische Halbleiter sind eine neue Schlüsseltechnologie für großflächige und flexible Dünnschichtelektronik. Sie werden als dünne Materialschichten (Sub-Nanometer bis Mikrometer) auf großflächige Substrate aufgebracht. Die technologisch am weitesten fortgeschrittenen Anwendungen sind organische Leuchtdioden (OLEDs) und organische Photovoltaik (OPV). Zur weiteren Steigerung von Leistungsfähigkeit und Effizienz ist die genaue Modellierung elektronischer Prozesse in den Bauteilen von grundlegender Bedeutung. Für die erfolgreiche Optimierung von Bauteilen ist eine zuverlässige Charakterisierung und Validierung der elektronischen Materialeigenschaften gleichermaßen erforderlich. Außerdem eröffnet das Verständnis der Zusammenhänge zwischen Materialstruktur und -eigenschaften einen Weg für innovative Material- und Bauteilentwicklung. Im Rahmen dieser Dissertation werden zwei Methoden für die Materialcharakterisierung entwickelt, verfeinert und angewandt: eine neuartige Methode zur Messung der Ladungsträgerbeweglichkeit μ und eine Möglichkeit zur Bestimmung der Ionisierungsenergie IE oder der Elektronenaffinität EA eines organischen Halbleiters. Für die Beweglichkeitsmessungen wird eine neue Auswertungsmethode für raumladungsbegrenzte Ströme (SCLC) in unipolaren Bauteilen entwickelt. Sie basiert auf einer Schichtdickenvariation des zu charakterisierenden Materials. In einem Ansatz zur räumlichen Abbildung des elektrischen Potentials (\"potential mapping\", POEM) wird gezeigt, dass das elektrische Potential als Funktion der Schichtdicke V(d) bei einer gegebenen Stromdichte dem räumlichen Verlauf des elektrischen Potentials V(x) im dicksten Bauteil entspricht. Daraus kann die Beweglichkeit als Funktion des elektrischen Felds F und der Ladungsträgerdichte n berechnet werden. Die Auswertung ist modellfrei, d.h. ein Modell zum Angleichen der Messdaten ist für die Berechnung von μ(F, n) nicht erforderlich. Die Messung ist außerdem unabhängig von einer möglichen Injektionsbarriere oder einer Potentialstufe an nicht-idealen Kontakten. Die gemessene Funktion μ(F, n) beschreibt die effektive durchschnittliche Beweglichkeit aller freien und in Fallenzuständen gefangenen Ladungsträger. Dieser Zugang beschreibt den Ladungstransport in energetisch ungeordneten Materialien realistisch, wo eine klare Unterscheidung zwischen freien und Fallenzuständen nicht möglich oder willkürlich ist. Die Messung von IE und EA wird mithilfe temperaturabhängiger Messungen an Solarzellen durchgeführt. In geeigneten Bauteilen mit einem Mischschicht-Heteroübergang (\"bulk heterojunction\" BHJ) ist die Leerlaufspannung Voc im gesamten Messbereich oberhalb 180K eine linear fallende Funktion der Temperatur T. Es kann bestätigt werden, dass die Extrapolation zum Temperaturnullpunkt V0 = Voc(T → 0K) mit der effektiven Energielücke Egeff , d.h. der Differenz zwischen EA des Akzeptor-Materials und IE des Donator-Materials, übereinstimmt. Die systematische schrittweise Variation einzelner Bestandteile der Solarzellen und die Überprüfung des Einflusses auf V0 bestätigen die Beziehung V0 = Egeff. Damit kann die IE oder EA eines Materials bestimmt werden, indem man es in einem BHJ mit einem Material kombiniert, dessen komplementärer Wert bekannt ist. Messungen per Ultraviolett-Photoelektronenspektroskopie (UPS) und inverser Photoelektronenspektroskopie (IPES) werden damit bestätigt, präzisiert und ergänzt. Die beiden entwickelten Messmethoden werden auf organische Halbleiter aus kleinen Molekülen einschließlich Mischschichten angewandt. In Mischschichten aus Zink-Phthalocyanin (ZnPc) und C60 wird eine Löcherbeweglichkeit gemessen, die sowohl thermisch als auch feld- und ladungsträgerdichteaktiviert ist. Wenn das Mischverhältnis variiert wird, steigt die Löcherbeweglichkeit mit zunehmendem ZnPc-Anteil, während die effektive Energielücke unverändert bleibt. Verschiedene weitere Materialien und Materialmischungen werden hinsichtlich Löcher- und Elektronenbeweglichkeit sowie ihrer Energielücke charakterisiert, einschließlich bisher wenig untersuchter hochverdünnter Donator-Systeme. In allen Materialien wird eine deutliche Feldaktivierung der Beweglichkeit beobachtet. Die Ergebnisse ermöglichen eine verbesserte Beschreibung der detaillierten Funktionsweise organischer Solarzellen und unterstützen die künftige Entwicklung hocheffizienter und optimierter Bauteile.:1. Introduction 2. Organic semiconductors and devices 2.1. Organic semiconductors 2.1.1. Conjugated π system 2.1.2. Small molecules and polymers 2.1.3. Disorder in amorphous materials 2.1.4. Polarons 2.1.5. Polaron hopping 2.1.6. Fermi-Dirac distribution and Fermi level 2.1.7. Quasi-Fermi levels 2.1.8. Trap states 2.1.9. Doping 2.1.10. Excitons 2.2. Interfaces and blend layers 2.2.1. Interface dipoles 2.2.2. Energy level bending 2.2.3. Injection from metal into semiconductor, and extraction 2.2.4. Excitons at interfaces 2.3. Charge transport and recombination in organic semiconductors 2.3.1. Drift transport 2.3.2. Charge carrier mobility 2.3.3. Thermally activated transport 2.3.4. Diffusion transport 2.3.5. Drift-diffusion transport 2.3.6. Space-charge limited current 2.3.7. Recombination 2.4. Mobility measurement 2.4.1. SCLC and TCLC 2.4.2. Time of flight 2.4.3. Organic field effect transistors 2.4.4. CELIV 2.5. Organic solar cells 2.5.1. Exciton diffusion towards the interface 2.5.2. Dissociation of CT states 2.5.3. CT recombination 2.5.4. Flat and bulk heterojunction 2.5.5. Transport layers 2.5.6. Thin film optics 2.5.7. Current-voltage characteristics and equivalent circuit 2.5.8. Solar cell efficiency 2.5.9. Limits of efficiency 2.5.10. Correct solar cell characterization 2.5.11. The \"O-Factor\" 3. Materials and experimental methods 3.1. Materials 3.2. Device fabrication and layout 3.2.1. Layer deposition 3.2.2. Encapsulation 3.2.3. Homogeneity of layer thickness on a wafer 3.2.4. Device layout 3.3. Characterization 3.3.1. Electrical characterization 3.3.2. Sample illumination 3.3.3. Temperature dependent characterization 3.3.4. UPS 4. Simulations 5.1. Design of single carrier devices 5.1.1. General design requirements 5.1.2. Single carrier devices for space-charge limited current 5.1.3. Ohmic regime 5.1.4. Design of injection and extraction layers 5.2. Advanced evaluation of SCLC – potential mapping 5.2.1. Potential mapping by thickness variation 5.2.2. Further evaluation of the transport profile 5.2.3. Injection into and extraction from single carrier devices 5.2.4. Majority carrier approximation 5.3. Proof of principle: POEM on simulated data 5.3.1. Constant mobility 5.3.2. Field dependent mobility 5.3.3. Field and charge density activated mobility 5.3.4. Conclusion 5.4. Application: Transport characterization in organic semiconductors 5.4.1. Hole transport in ZnPc:C60 5.4.2. Hole transport in ZnPc:C60 – temperature variation 5.4.3. Hole transport in ZnPc:C60 – blend ratio variation 5.4.4. Hole transport in ZnPc:C70 5.4.5. Hole transport in neat ZnPc 5.4.6. Hole transport in F4-ZnPc:C60 5.4.7. Hole transport in DCV-5T-Me33:C60 5.4.8. Electron transport in ZnPc:C60 5.4.9. Electron transport in neat Bis-HFl-NTCDI 5.5. Summary and discussion of the results 5.5.1. Phthalocyanine:C60 blends 5.5.2. DCV-5T-Me33:C60 5.5.3. Conclusion 6. Organic solar cell characteristics: the influence of temperature 6.1. ZnPc:C60 solar cells 6.1.1. Temperature variation 6.1.2. Illumination intensity variation 6.2. Voc in flat and bulk heterojunction organic solar cells 6.2.1. Qualitative difference in Voc(I, T) 6.2.2. Interpretation of Voc(I, T) 6.3. BHJ stoichiometry variation 6.3.1. Voc upon variation of stoichiometry and contact layer 6.3.2. V0 upon stoichiometry variation 6.3.3. Low donor content stoichiometry 6.3.4. Conclusion from stoichiometry variation 6.4. Transport material variation 6.4.1. HTM variation 6.4.2. ETM variation 6.5. Donor:acceptor material variation 6.5.1. Donor variation 6.5.2. Acceptor variation 6.6. Conclusion 7. Summary and outlook 7.1. Summary 7.2. Outlook A. Appendix A.1. Energy pay-back of this thesis A.2. Tables and registers

In this chapter, the basic principles of the origins of transport levels and bands, charge conduction in disordered materials, and injection from contacts are introduced. Charge transport in organics is fundamentally different than in inorganic semiconductors due to narrow transport bands that, in general, lead to charge transport via hopping, resulting in carrier mobilities that are at most only a few cm²/V s. Processes of charge injection leading to space charge limited transport that defines the current vs. voltage characteristics of the materials are discussed. Methods of measuring mobility, background charge densities, and quantifying charge recombination are described. Doping of organics using both molecular and atomic species to modify their conductivity is also considered. The theory of transport in energetically and structurally disordered films is developed. The chapter closes by describing, from first principles, the theory of conduction over organic and organic/inorganic semiconductor heterojunctions that are used in almost all organic photonic devices.

Ribbon beams of heavy ions have advantages over cylindrical beams, including higher space-charge limits. History of use goes from Calutrons, Freeman and Bernas ion sources, to the first ion implanters in the 1970s. In the 1990s, 300 mm uniform parallel mass-analyzed ribbon beams were developed to enable precise doping by mechanically scanning a substrate through the ion beam in one dimension. Ion species included the primary dopants boron, phosphorus and arsenic, but many others are also used. Such sources can produce currents of heavy ions with linear current densities at the source of the order of 10 mA/cm, but these sources are limited in the beam breadth they can produce. Broader beams are used for flat-panel display manufacture. A new linear source design combines a modified Penning trap with magnetic cusp confinement system, allowing extension of linear sources to meter scale beam breadths, maintaining around 10 mA/cm linear current density. Magnetic analysis of such beams has required new developments because the weight of conventional dipole magnets increases very steeply as the pole gap is increased. A new magnetic configuration has been developed to address this issue, reducing the potential weight of meter-scale analyzed systems by an order of magnitude.

The chapter aims at introducing students to the basic of electronic wavefunctions and energy bands in semiconductors. The Bloch mode is the key point, with the band edges and the bandgap. The Fermi-Dirac statistics based on the Fermi level is introduced to account for the population with the help of the relevant density-of-states. Doping is explained. The optical transitions, their rule and their dynamics are outlined. The focus is then more on heterojunctions, notably double heterojunctions and quantum wells, circumventing the textbook description of space charges, as they are suppressed in injected devices. Quantum wells are discussed in relation with laser diodes anr their threshold current, showing why the separate confinement heterostructure (SCH) combining a guide with quantum wells, stands as an early nanophotonic device design. The confinement factor of the optical mode relates this chapter to that on guided modes. Silicon and III-V semiconductors are contrasted throughout.

Doping or n-i-p-i superlattices are promising materials for tunable light sources. Long-period GaAs doping superlattices have exhibited wide tunability in the photoluminescence (PL) peak energy versus excitation intensity,1 but photopumped lasing has been observed only at high excitation, where excess carriers completely screen the superlattice space-charge potential.2 By contrast, short-period superlattices possess a smaller degree of luminescence tunability yet exhibit larger oscillator strengths because of the greater overlap between electron and hole wave functions in the n- and p-type layers, respectively. A desirable goal is to achieve lasing at carrier densities below the value that completely screens the superlattice potential, thereby permitting tunability of the radiation. We have investigated the limits of tunability in uniformly doped short-period GaAs doping superlattices and present results of low-temperature photoluminescence.

In a semiconductor doping or n-i-p-i superlattice, the periodic variation of impurities introduces a space-charge-induced superlattice potential which modifies the bulk electronic band structure and allows tailoring of the optical properties. We propose that short-period doping superlattices are suitable for the enhancement of a third- order optical susceptibility arising from electrons in nonparabolic conduction subbands. The advantages of doping superlattices are the ability to simply engineer the superlattice potential profile, thus giving control of miniband dispersion, and to provide free carriers to occupy these subbands. Room temperature electronic and nonlinear optical properties are calculated and optimized for uniformly doped and planar doped superlattice geometries. Compensated calculations are used to determine the superlattice potential profile which gives an optimally nonparabolic first conduction subband. We then self-consistently calculate the resulting optical nonlinearity for noncompensated n-type superlattices possessing this same potential shape. We show that small modulations of the superlattice potential lead to small minigaps and large subband nonparabolicities; a twentyfold improvement in the third-order optical susceptibility over bulk GaAs is predicted. Optical characterization and preliminary measurements for uniformly doped GaAs doping superlattices are discussed.

The time-resolved reflectivity from a series of variously prepared GaAs interfaces has been observed. GaAs n-type wafers of 2 × 1016 cm-3 doping density were used. The surfaces were prepared in the following ways: growth of a thermal oxide, growth of a photochemically passivated oxide, and growth of an epitaxial ZnSe overlayer using the technique of laser-assisted MOCVD. Reflectivity measurements were performed using a CPM laser (620 nm, pulsewidth <100 fsec) in a standard pump-probe geometry. The time dependence of the reflectivity within the first few picoseconds is highly dependent on surface preparation. This variation of surface treatment is linked to a variation of both the density of states present at the interface and a variation in the space-charge layer on the GaAs side of the interface. Characterization of the GaAs spacecharge layer by contactless optical probing techniques, as well as additional experiments on III–V device structures enable us to make some initial correlations between the time dependence of the optical reflectivities observed and the properties of the GaAs space-charge layer.

1. Introduction Photorefractive gratings are most often produced by illumination of an electro-optic crystal with laser light below the band gap and the absorbed photons induce photoexcitation and redistribution of carriers within mid-gap trap centers [11. The photorefractive response time is longer or equal to the time needed to photoexcite a large enough number of earners to create the space-charge field [2], To increase the speed of response, beside using higher light intensities, also a large absorption constant a of the crystal may be helpful. By doping, a can be increased only in a limited range. In contrast, large absorption constants are observed without doping at the band edge of any photorefractive material. In this region, the absorbed photons can induce interband phototransitions of electrons. The creation of photorefractive gratings by photoexcitation over the band gap has been demonstrated recently in multiple quantum well (MQW) devices [3-6]. However, the thickness of such multilayer structures is limited because of the epitaxial growth process and reasonable diffraction efficiencies are obtained only making use of resonant nonlinear effects at the band-edge. This limits the read-out wavelength range to a width of 10-20 nm and the optical interaction length to few µm. The use of oxide crystals with large linear electro-optic coefficients instead of MQW’s overcomes this problem because the read-out can be done at any wavelength in the visible or infrared. Even though the thickness of the photorefractive grating depends on the absorption constant and on the light intensity of the writing beams, the interaction length L can be increased to the crystal size by propagating the non-absorbed readout beams parallel to the crystal surface.

A new generation of Alkaline earth sulfides (MgS, CaS, and BaS) doped with rare-earth ions have been identified by the University of Montpellier as the very high sensitivity of these phosphors, the short time constant of the luminescence and the perfectly separated spectra enable many applications in real time and online dosimetry. The online detecting technology of optically stimulated luminescent (OSL) radiation dosimeter main make use of the OSL characteristics of doping the alkaline-earth metal sulphides, makes the material into the thin films for storing energy from Ionizing radiation, the excitation light through optical fibers reached the where under radiation-field, with a sensitive detection device to read out the radiation dose from storing the OSL material, obtains a novel technology of radiation dose measurement. In the previous works, the dosimeter benefits from a printed circuit board mount. Both the sensor and the electronics are exposed to radiation, the problem of the radiation induced damage is supposedly being addressed. In both cases, the use of optical fibers can provide an elegant solution. Optical fibers offer a unique capability for remote monitoring of radiation in difficult-to-access and hazardous locations. Optical fiber can be located in radiation hazardous areas and optically interrogated from a safe distance. Hence, optical fiber dosemeters are immune to electrical and radio-frequency interference that can seriously degrade the performance of remote electronic dosimeters. In this paper, a novel remote optical fiber radiation dosimeter is described. The optical fiber dosimeter takes advantage of the charge trapping materials CaS:Ce, Sm and SrS:Eu, Sm that exhibit optically stimulated luminescence (OSL). The range of the dosimeter is from 0.01 to 1000Gy. The optically stimulated luminescent (OSL) radiation dosimeter technically surveys a wide dynamic measurement range and a high sensitivity. The equipment is relatively simple and small in size, and has low power consumption. This device is suitable for measuring the space radiation dose; it also can be used in high radiation dose condition and other dangerous radiation occasions.

Doping superlattices, also called nipi superlattices, consist of thin alternating n- and p-doped layers of semiconductor material. This periodic doping profile leads to a periodic electric potential perpendicular to the layers. Potential wells for electrons are in the n-type layers, while potential wells for the holes are in the p-type layers. The periodic potential creates an effective band gap in real space, smaller in energy than the direct gap, which is a function of the density of confined carriers, the impurity doping density, and the thickness of the layers. When mobile electrons and holes are generated, they become spatially separated by the electric potential, screen the oppositely charged ionized dopants in their respective wells, and increase the effective band gap. The resulting tunability of electronic states and optical absorption characteristics as a function of carrier density have been investigated theoretically by Döhler and Ruden.1,2

We use a novel, nondegenerate, polarization-sensitive, transient-grating technique1 to monitor the picosecond dynamics of the photorefractive effect in undoped CdTe and InP:Fe at 960 nm. The technique circumvents the limited temporal resolution of the two-beam coupling geometry by using a time-delayed third probe pulse (with a duration of <5 psec) to read the gratings written in the semiconductor. The technique also exploits the crystal symmetry of zincblende semiconductors by using an optically induced anisotropy in the crystal index of refraction2 to separate the photorefractive gratings from the stronger, co-existing instantaneous bound-electronic and freecarrier gratings. In both semiconductors, the photorefractive effect is associated with the Dember field between mobile electron-hole pairs, in contrast to the more conventional photorefractive space-charge field connected with the separation of a mobile carriers species from a stationary, but oppositely charged, mid-gap state. In the undoped CdTe sample, which possesses no optically-active mid-gap levels, the electron-hole pairs are produced by two-photon absorption of 1.3 eV photons across the 1.44 eV band-gap of the semiconductor. The resultant ~1 eV excess carrier energy, which allows hot carrier transport to dominate the initial formation of the space-charge field, causes up to an order of magnitude enhancement in the photorefractive effect on picosecond timescales. After the carriers have cooled and the initial overshoot in the space-charge field has decayed, the photorefractive effect is observed to decay as the Dember field is destroyed by ambipolar diffusion of the electron-hole pairs across the grating period. In InP:Fe on the other hand, the electron-hole pairs are produced predominantly by direct single-photon band-to-band absorption into the band-tail of the semiconductor (band-gap ~1.35 eV), since the iron dopant only dominates the linear absorption at longer wavelengths. This means that the carriers are generated with little excess energy. Consequently, no hot carrier enhancement of the photorefractive effect was observed, and once formed, the Dember space-charge field decayed directly by ambipolar diffusion.

Laser induced modification and damage in transparent materials (e.g., glass) has attracted considerable interest and been studied since the advent of high power pulsed lasers. Especially while using a femtosecond laser as the irradiation source, the tight focusing and nonlinear nature of the absorption make it possible to confine the absorption to the focal volume inside the bulk of the material, allowing for micromaching in extremely small region. In this talk, we introduce the progress in space-selective modification of glass by using femtosecond laser. We show that some fundamental processes, including valence state change of dopant, decomposition of cluster, element redistribution, phase transition and nanocrystallization, can occur in the femtosecond laser irradiation region inside glass. As a result, the optical response of the modified glass can be controlled. For examples, the luminescence properties of main group ions doped glass can be tuned and tunable luminescence can be achieved. The microstructures with multicolor luminescence can be induced. The results suggest that space-selective modification of glass by using femtosecond laser can be applied to fabricate various types of 3D active microstructures.

<div class="section abstract"><div class="htmlview paragraph">Li-ion batteries are commonly used in Electric Vehicles (EVs) due to its high-power density and higher life cycle performance. Individual cells in such battery packs may sometimes lead to thermal runaway conditions under the effect of localized heat generation and faults. Battery liquid cooling methods are normally being employed to resolve this problem with limitations of limited temperature operating range and difficulty in reaching the intricate spaces between the cells. Introducing phase change material (PCM) can mitigate these limitations. The present study deals with a detailed numerical study of a single (Li-ion) cell in ANSYS Fluent using multi-scale multi dimension (MSMD) - Newman, Tiedenann, Gu and Kim (NTGK) model. The single cell model is investigated for the evaluation of its temperatures at varying air velocity surrounding the cell at higher C-rating (load) values. It was observed that the maximum cell surface temperatures were as 322.6, 319.8, 318.1, 316.9, 314.4 and 313.4 K corresponding to 0.5, 1.0, 1.5, 2.0, 2.5 and 3.0 m/s for 1.5C discharge rating. Further, the study also involved the investigation of composite PCM based on Paraffin and Vaseline (50:50; CPCM) for improving the cooling performance. It was found that the doping of 2% of Cu in CPCM resulted in ~8°C lower cell surface temperature.</div></div>