In modern low pressure plasma discharges, the electron mean free path often exceeds the device dimensions. Under such conditions the electron velocity distribution function may significantly deviate from Maxwellian, which strongly affects the discharge properties. The description of such plasmas has to be kinetic and often requires the use of numerical methods. This thesis presents the study of kinetic effects in inductively coupled plasmas and Hall thrusters carried out by means of particle-in-cell simulations. The important result and the essential part of the research is the development of particle-in-cell codes. <p>An advective electromagnetic 1d3v particle-in-cell code is developed for modelling the inductively coupled plasmas. An electrostatic direct implicit 1d3v particle-in-cell code EDIPIC is developed for plane geometry simulations of Hall thruster plasmas. The EDIPIC code includes several physical effects important for Hall thrusters: collisions with neutral atoms, turbulence, and secondary electron emission. In addition, the narrow sheath regions crucial for plasma-wall interaction are resolved in simulations. The code is parallelized to achieve fast run times. <p>Inductively coupled plasmas sustained by the external RF electromagnetic field are widely used in material processing reactors and electrodeless lighting sources. In a low pressure inductive discharge, the collisionless electron motion strongly affects the absorption of the external electromagnetic waves and, via the ponderomotive force, the density profile. The linear theory of the anomalous skin effect based on the linear electron trajectories predicts a strong decrease of the ponderomotive force for warm plasmas. Particle-in-cell simulations show that the nonlinear modification of electron trajectories by the RF magnetic field partially compensates the effects of electron thermal motion. As a result, the ponderomotive force in warm collisionless plasmas is stronger than predicted by linear kinetic theory. <p>Hall thrusters, where plasma is maintained by the DC electric field crossed with the stationary magnetic field, are efficient low-thrust devices for spacecraft propulsion. The energy exchange between the plasma and the wall in Hall thrusters is enhanced by the secondary electron emission, which strongly affects electron temperature and, subsequently, thruster operation. Particle-in-cell simulations show that the effect of secondary electron emission on electron cooling in Hall thrusters is quite different from predictions of previous fluid studies. Collisionless electron motion results in a strongly anisotropic, nonmonotonic electron velocity distribution function, which is depleted in the loss cone, subsequently reducing the electron wall losses compared to Maxwellian plasmas. Secondary electrons form two beams propagating between the walls of a thruster channel in opposite radial directions. The secondary electron beams acquire additional energy in the crossed external electric and magnetic fields. The energy increment depends on both the field magnitudes and the electron flight time between the walls. <p>A new model of secondary electron emission in a bounded plasma slab, allowing for emission due to the counter-propagating secondary electron beams, is developed. It is shown that in bounded plasmas the average energy of plasma bulk electrons is far less important for the space charge saturation of the sheath than it is in purely Maxwellian plasmas. A new regime with relaxation oscillations of the sheath has been identified in simulations. Recent experimental studies of Hall thrusters indirectly support the simulation results with respect to the electron temperature saturation and the channel width effect on the thruster discharge.

No description available.

L’espace est un milieu hostile pour les équipements embarqués à bord des satellites. Les importants flux d’électrons qui les bombardent continuellement peuvent pénétrer à l’intérieur de leurs composants électroniques et engendrer des dysfonctionnements. Leur prise en compte nécessite des outils numériques 3D très performants, tels que des codes de transport d’électrons utilisant la méthode statistique de Monte-Carlo, valides jusqu’à quelques eV. Dans ce contexte, l’ONERA a développé, en partenariat avec le CNES, le code OSMOSEE pour l’aluminium. De son côté, le CEA a développé, pour le silicium, le module basse énergie MicroElec dans le code GEANT4. L’objectif de cette thèse, dans un effort commun entre l’ONERA, le CNES et le CEA, est d’étendre ces codes à différents matériaux. Pour ce faire, nous avons choisi d’utiliser le modèle des fonctions diélectriques, qui permet de modéliser le transport des électrons à basse énergie dans les métaux, les semi-conducteurs et les isolants. La validation des codes par des mesures du dispositif DEESSE de l’ONERA, pour l’aluminium, l’argent et le silicium, nous a permis d’obtenir une meilleure compréhension du transport des électrons à basse énergie, et par la suite, d’étudier l’effet de la rugosité de la surface. La rugosité, qui peut avoir un impact important sur le nombre d’électrons émis par les matériaux, n’est habituellement pas prise en compte dans les codes de transport, qui ne simulent que des matériaux idéalement plats. En ce sens, les résultats de ces travaux de thèse offrent des perspectives intéressantes pour les applications spatiales. / Space is a hostile environment for embedded electronic devices on board satellites. The high fluxes of energetic electrons that impact these satellites may continuously penetrate inside their electronic components and cause malfunctions. Taking into account the effects of these particles requires high-performant 3D numerical tools, such as codes dedicated to electrons transport using the Monte Carlo statistical method, valid down to a few eV. In this context, ONERA has developed, in collaboration with CNES, the code OSMOSEE for aluminum. For its part, CEA has developed for silicon the low-energy electron module MicroElec for the code GEANT4. The aim of this thesis, in a collaborative effort between ONERA, CNES and CEA, is to extend those two codes to different materials. To describe the interactions between electrons, we chose to use the dielectric function formalism that enables to overcome of the disparity of electronic band structures in solids, which play a preponderant role at low energy. From the validation of the codes, for aluminum, silver and silicon, by comparison with measurements from the experimental set-up DEESSE at ONERA, we obtained a better understanding of the transport of low energy electrons in solids. This result enables us to study the effect of the surface roughness. This parameter, which may have a significant impact on the electron emission yield, is not usually taken into account in Monte Carlo transport codes, which only simulate ideally flat materials. In this sense, the results of this thesis offer interesting perspectives for space applications.

No description available.

Les hétérostructures hybrides organique-inorganique présentent des propriétés intéressantes, notamment pour des applications dans le domaine de l’électronique et de la spintronique. Notre intérêt s’est porté particulièrement sur la réalisation d’hétérostructures de type Métal/Monocouche organique/Semiconducteur, dont l’étape de reprise de top-contact métallique reste actuellement un verrou majeur à la réalisation de telles jonctions. L’expérience de l’équipe sur des hétérostructures de type MOS (Métal/Oxyde/Semiconducteur), ainsi que les différentes techniques de surface et de transport disponibles au laboratoire, sont appliquées ici à l’étude de ces hétérostructures hybrides. En particulier, la Microscopie à Emission d’Electrons Balistiques (BEEM) permet d’étudier localement les propriétés électroniques des hétérostructures, avec une résolution spatiale nanométrique. A partir du système Au/GaAs(001) bien connu au laboratoire, nous avons intercalé une monocouche d’alcanethiols à l’interface, pour former des hétérostructures de type Au/Alcanethiols/GaAs(001), entièrement préparées sous ultra-vide. Lors du dépôt d’or à température ambiante, les images BEEM ont révélé des interfaces hétérogènes, avec des zones où le peigne moléculaire est court-circuité ou non par le métal. Une analyse quantitative en spectroscopie BEEM des zones non court-circuitées a mis en évidence des signatures particulières, avec une première contribution associée au passage tunnel des électrons à travers le peigne moléculaire, et une seconde contribution, à plus haute énergie, révélant l’existence de nouveaux canaux de conduction associés à l’existence d’états inoccupés dans la monocouche organique. Les effets de l’épaisseur du métal déposé, de la longueur de chaîne des molécules organiques, ainsi que du groupe terminal de la chaîne organique, ont été discutés. Afin d’améliorer le dépôt du contact métallique, un dispositif expérimental original a permis de déposer l’or sur le substrat refroidi, sur lequel une couche tampon de Xénon est condensée (méthode BLAG : Buffer Layer Assisted Growth). L’analyse BEEM de ces hétérostructures a révélé ici des interfaces homogènes, sans pénétration du métal. Des signatures spectroscopiques similaires aux zones non court-circuitées précédentes ont été mises en évidence. Une étude complète de ces hétérostructures préparées par la méthode BLAG a été réalisée via des mesures de transport à l’échelle macroscopique (J(V) et C(V)), ainsi que des mesures de photoémission par rayonnement synchrotron. Ces mesures ont confirmé le caractère reproductible des jonctions formées, avec des hauteurs de barrière en accord avec celles déterminées par BEEM. / In molecular electronics and spintronics, top-contact metal electrode deposition on organic molecular monolayer (OML)/semiconductor hybrid heterostructures is still a critical issue, leading to metal penetration through the molecules and monolayer’s damage. The experimental set-ups available in the lab and the team’s experience in inorganic-inorganic heterostructures are here applied to hybrid organic-inorganic heterostructures. In particular, the Ballistic Electron Emission Microscopy (BEEM), a technique derived from Scanning Tunneling Microscopy (STM), allows to study electronic properties of such heterostructures, at a nanometer scale. Starting from the well-known Au/GaAs(001) Schottky contact, we here intercalate an alkanethiols monolayer, in order to obtain Au/Alkanethiols/GaAs(001) heterostructures, fully grown in ultra-high vacuum environment. In the case of room-temperature metal deposition, BEEM imaging reveals domains which are short-circuited or not by the metal. A quantitative analysis of non-short-circuited interfaces is realized by BEEM in spectroscopy mode. Particular fingerprints are obtained, with a first component related to electron tunnel transport through the monolayer, and a second component, at higher energy, related to first unoccupied states of the molecular layer reachable for electrons. The effects of metal thickness, molecular chain length and terminal group are discussed. In order to minimize the degree of gold penetration, an alternative top-contact deposition method is used, based on buffer-layer assisted growth (BLAG). BEEM studies on these heterostructures reveal homogeneous interfaces without metal penetration, and similar spectroscopic fingerprints. Complementary studies at macroscopic scale (J(V) and C(V) transport measurements and photoemission by synchrotron radiation) confirm the reproducible character of the junctions with barrier height values similar to the ones obtained by BEEM.

Quantum dots (QDs) are a semiconductor nanostructure in which a small island of one type of semiconductor material is contained within a larger bulk of a different one. These structure are interesting for a wide range of applications, including highly efficient LASERs, high-density novel memory devices, quantum computing and more. In order to understand the nature of QDs, electrical characterisation techniques such as capacitance-voltage (CV) profiling and deep-level transient spectroscopy (DLTS) are used to probe the nature of the carrier capture and emission processes. This is limited, however, by the nature of QD formation which results in a spread of sizes which directly affects the energy structure of the QDs. In this work, I sought to overcome this by using Si substrates patterned with a focused ion beam (FIB) to grow an array of identically-sized Ge dots. Although I was ultimately unsuccessful, I feel this approach has great merit for future applications.In addition, this thesis describes several unusual characteristics observed in InAs QDs in a GaAs bulk (grown by molecular beam epitaxy-MBE). Using conventional and Laplace DLTS, I have been able to isolate a single emission transient. I further show an inverted relation between the emission rate and the temperature under high field (emissions increase at lower temperatures). I attribute this to a rapid capture to and emission from excited states in the QD. In addition, I examine a metastable charging effect that results from the application of a sustained reverse bias and decreases the apparent emission rate from the dots. I believe this to be the result of a GaAs defect with a metastable state which acts as a screen, inhibiting emission from the dots due to an accumulation of charge in the metastable state. These unusual characteristics of QDs require further intensive work to fully understand. In this work I have sought to describe the phenomena fully and to provide hypotheses as to their origin.

<div>Developments in modern electronics drive device design to smaller scale and higher electric fields and currents. Device size reductions to microscale and smaller have invalidated the assumption of avalanche formation for the traditional Paschen’s law for predicting gas breakdown. Under these conditions, the stronger electric fields induce field emission driven microscale gas breakdown; however, these theories often rely upon semi-empirical models to account for surface effects and the dependence of gas ionization on electric field, making them difficult to use for predicting device behavior a priori.</div><div>This dissertation hypothesizes that one may predict a priori how to tune emission physics and breakdown conditions for various electrode conditions (sharpness and surface roughness), gap size, and pressure. Specifically, it focuses on experiments to demonstrate the implications of surface roughness and emitter shape on gas breakdown for microscale and nanoscale devices at atmospheric pressure and simulations to extend traditional semi-empirical representations of the ionization coefficient to the relevant electric fields for these operating conditions.</div><div>First, this dissertation reports the effect of multiple discharges for 1 μm, 5 μm, and 10 μm gaps at atmospheric pressure. Multiple breakdown events create circular craters to 40 μm deep with crater depth more pronounced for smaller gap sizes and greater cathode surface roughness. Theoretical models of microscale breakdown using this modified effective gap distance agree well with the experimental results.</div><div>We next investigated the implications of gap distance and protrusion sharpness for nanoscale devices made of gold and titanium layered onto silicon wafers electrically isolated with SiO2 for gas breakdown and electron emission at atmospheric pressure. At lower voltages, the emitted current followed the Fowler-Nordheim (FN) law for field emission (FE). For either a 28 nm or 450 nm gap, gas breakdown occurred directly from FE, as observed for microscale gaps. For a 125 nm gap, emission current begins to transition toward the Mott-Gurney law for space-charge limited emission (SCLE) with collisions prior to undergoing breakdown. Thus, depending upon the conditions, gas breakdown may directly transition from either SCLE or FE for submicroscale gaps.</div><div>Applying microscale gas breakdown theories to predict this experimental behavior requires appropriately accounting for all physical parameters in the model. One critical parameter in these theories is the ionization coefficient, which has been determined semi-empirically with fitting parameters tabulated in the literature. Because these models fail at the strong electric fields relevant to the experiments reported above, we performed particle-in-cell simulations to calculate the ionization coefficient for argon and helium at various gap distances, pressures, and applied voltages to derive more comprehensive semi-empirical relationships to incorporate into breakdown theories.</div><div>In summary, this dissertation provides the first comprehensive assessment of the implications of surface roughness on microscale gas breakdown, the transition in gas breakdown and electron emission mechanisms at nanoscale, and the extension of semi-empirical laws for ionization coefficient. These results will be valuable in developing theories to predict electron emission and gas breakdown conditions for guiding nanoscale device design.</div>

No description available.

<p dir="ltr">Calculating space-charge limited current (SCLC) is a critical problem in plasma physics and intense particle beams. Accurate calculations are important for validation and verification of particle-in-cell (PIC) simulations. The theoretical assessment of SCLC is complicated by the nonlinearity of the Poisson equation when combined with the energy balance and continuity equations. This dissertation provides several theoretical tools to convert the nonlinear Poisson equation into a corresponding linear differential equation, which is then solved for numerous geometries of practical interest.</p><p dir="ltr">The first and second chapters briefly summarize the application of variational calculus (VC) to solve for one-dimensional (1D) SCLC in cylindrical and spherical diode geometries by extremizing the current in the gap. Next, conformal mapping (CM) is presented to convert the concentric cylindrical diode geometry into a planar geometry to obtain the same SCLC solution as VC. In the next chapter, SCLC is determined for several geometries with curvilinear electron flow that cannot be solved using VC because the Poisson equation cannot be written easily. We then map a hyperboloid tip onto a plane to form a non-Euclidean disk (Poincaré disk). These mappings on to Poincaré disk are utilized to solve for SCLC in tip-to-tip and tip-to-plane geometries. Lie symmetries are then introduced to solve for SCLC with nonzero monoenergetic injection velocity, recovering the solutions for concentric cylinders, concentric spheres, tip-to-plane, and tip-to-tip for zero injection velocity. We then extend the SCLC calculations to account for any geometry in multiple dimensions by using VC and vacuum capacitance. First, we derive a relationship between the space-charge limited (SCL) potential and vacuum potential that holds for any geometry. This relationship is utilized to obtain exact closed-form solutions for SCLC in two-dimensional (2D) and three-dimensional (3D) planar geometries considering emission from the full surface of the cathode. PIC simulations using VSim were performed that agreed with the SCLC in 2D diode with a maximum error of 13%. In the final chapters, we extend these multidimensional SCLC calculations to nonzero monoenergetic emission. The SCLC in any orthogonal diode in any number of dimensions is obtained by relating it to the vacuum capacitance. The current in the bifurcation regime is also derived from first-principles from vacuum capacitance. The simulations performed in VSim agreed with the theory with a maximum error of 7%.</p><p dir="ltr">These mathematical techniques form a set of powerful tools that extend prior studies by yielding exact and approximate SCLC in numerous nonplanar and multidimensional diode geometries, thereby not requiring expensive and time-consuming PIC simulations. While more experiments are required to benchmark the validity of these calculations, these results may ultimately prove useful by providing a rapid first-principles approach to determine SCLC for many geometries that can be used to assess the validity of PIC simulations and facilitate multiphysics simulations.</p>

Le travail de cette thèse porte sur l’étude expérimentale de la diffusion d’atomes d’hydrogène sur des surfaces et sous incidence rasante. L’énergie des atomes et des ions varie de quelques centaines d’eV à quelques keV, tandis que les surfaces étudiées sont des isolants et des semi-métaux. En particulier on a étudié la formation de l’ion H- sur du graphite pyrolytique dit HOPG (highly oriented pyrolytic graphite) et sur une surface de LiF(001). Pour ce dernier système, nous avons étudié en détail la diffraction d’atomes H° et d’ions H+. Ces expériences ont été réalisées sur un montage expérimental utilisant un faisceau pulsé et permettant de détecter en coïncidence les particules diffusées et les électrons secondaires. L’ensemble permet de connaître la charge finale de la particule diffusée, sa perte d’énergie, son angle de diffusion, le tout en corrélation avec la statistique et l’énergie des électrons émis.Le résultat de ce travail a révélé que la diffraction persiste dans le régime inélastique. En effet, nous observons un motif de diffraction après la neutralisation de proton sur la surface de LiF(001). Un modèle est proposé pour expliquer ces résultats qui semblent en contradiction avec ceux publiés par le groupe de H. Winter sur la diffraction d’atomes d’hydrogène sur cette même surface. Concernant la formation d’ion négatif sur HOPG, nous avons mis en évidence un taux de H- (~10%) sur une surface propre. C’est le plus haut taux de H- jamais observé avec ce type d’expérience en incidence rasante. C’est encore plus élevé qu’avec des isolants ioniques, ces derniers donnant un taux déjà 10 fois plus grand que celui observé sur métaux propres. Ces résultats confirment l’efficacité du graphite à convertir des ions et des atomes en ions négatifs. En exploitant les données fournies par la technique des coïncidences, nous avons pu élucider le mécanisme à l’œuvre dans cette conversion. Du fait de la structure électronique particulière de HOPG, avec une bande interdite projetée dans la direction Gamma, seuls les électrons localisés sigma contribuent à la formation de l’ion négatif, donnant au HOPG un caractère isolant du point de vue de la capture électronique. Les électrons pi contribuant de manière efficace à la perte d’énergie par collisions binaires, donnant de ce point de vue au HOPG son caractère métallique. / In this thesis, we have investigated experimentally the scattering of hydrogen atoms and ions on solid surfaces at grazing incidence. The projectile energy ranges from several hundred eV to few keV. The formation of H- ions is studied on highly oriented pyrolytic graphite (HOPG) surface; and surface diffraction is carried out on LiF(001) surface with H° and H+ particle scattering. Both experiments were performed in the same experimental setup (see Figure 1.2 and 2.1)—with grazing scattering geometry and a PSD (position sensitive detector) located downstream to record scattered particles. For charge state analysis a set of electrostatic plates is inserted between sample and PSD. During the experiment, coincident measurement technique is used to identify the energy loss associated to 0, 1, 2…electrons emission. Clear evidence of diffraction with inelastic scattering by proton on LiF(001) has been obtained, which has not been observed before. Indeed, the group of H. Winter reported that no diffraction exists with inelastic scattering of H° on LiF(001). However, according to our result, a coherence scattering factor still exists even though the electron capture by the proton is an inelastic process. For negative ion formation on HOPG surface, we report here the highest fraction of H- (~10%) measured in grazing scattering experiments; it is larger than those obtained on ionic insulators, the latter being typically 10 times larger than those measured on clean metals. These results confirm the high yields of negative hydrogen ions from graphite reported in the literature. Electron emission and energy loss of scattered beam have also been deciphered via coincidence measurement. Due to the special structure of HOPG, two kinds of electron emissions (σ and π-band electron) and energy losses (cycles and metal-like energy loss) have been measured. Furthermore, the total electron emission on HOPG with insulator-like behavior and total energy loss with metal-like are the most representative property of HOPG which have been first presented in this thesis.