In the global context of increasing oil prices and public concern regarding the safety of nuclear plants, renewable forms of energy are called upon to play a major role in tomorrow’s energy market. Among the various forms of renewable energies, solar power holds the greatest potential for development. Despite the constant improvement of photovoltaic technologies over the past few year, these technologies are rapidly approaching the theoretic performance limits. New ideas and materials are required to overcome this bottleneck and to take full advantage of solar power. With a band-gap energy spanning the full solar spectrum, and an absorption coefficient ten times higher than competing materials, indium gallium nitride alloys are amongst the most promising solar-cell materials. Nevertheless, fundamental issues related to the fabrication and doping of InGaN alloys still hamper the development of InGaN-based photovoltaics. In the present thesis, conducted within the framework of the ANR project NewPVonGlass, the growth of InGaN alloys suitable for photovoltaics using metalorganic vapor-phase epi- taxy (MOVPE) is studied. A combination of several cutting-edge characterization tools is employed to determine the fundamental mechanism that govern the growth of InGaN. Based on the results of this study, an innovative procedure that allows the growth of hig-quality InGaN epitaxial layers is demonstrated and is used for the fabrication of InGaN-based solar cells.

In the present study, the formation and the evolution of lamellar structures in different Al-Ag alloys were investigated by transmission electron microscopy (TEM). Plates of the hexagonal [gamma] phase form semi-coherently on the {111} planes of the face centered cubic lattice of the alloy after the formation of Guinier-Preston zones. Guinier-Preston zones are metastable coherent preprecipitates which are silver rich in the aluminum-rich Al-Ag alloys. The decomposition of aluminum rich Al-Ag alloys, particularly the sequence of the later stages of precipitate formation was studied. With scanning electron microscopy and high-resolution transmission electron microscopy the development of the [gamma] phase was investigated. Samples cut from different Al-Ag alloys were homogenized at temperatures between 530 degrees C to 560 degrees C to obtain a single phase f.c.c solid solution. The samples were then quenched to room temperature, followed by heat treatments at temperatures between 140 degrees C and 220 degrees C for varying lengths of times. While Guinier-Preston zones increase in diameter with increasing aging duration, silver rich platelets of the [gamma] phase form. The [gamma] phase is the next metastable phase in the decomposition sequence before finally the [gamma] phase transforms to the stable silver-rich phase, termed [gamma]. For samples with silver contents above 12 at.% a parallel lamellar alignment of fine [gamma] plates and Alrich matrix is found after extended heat treatments. For all alloys with Ag concentrations below 12 at.% individual [gamma] plates are found on all four possible (111) planes of the [alpha] matrix. A method is presented to calibrate the medium-magnification high-angle annular dark-field contrast in scanning transmission electron microscopy. This calibration allows for the quantitative measurement of plate thicknesses from high-angle annular dark-field scanning transmission electron micrographs of Ag2Al plates inclined to the electron beam. Results from these measurements are in good agreement with direct bright-field micrographs of plates viewed edge-on.

Ces travaux de doctorat concernent le développement de la tomographie électronique appliquée à la nano-caractérisation tridimensionnelle de dispositifs à semi-conducteurs et de matériaux pour la micro et la nanoélectronique. Les contributions les plus significatives de ces travaux sont (i) l'exploration et l'application de différents modes de contraste en microscopie électronique à transmission (TEM) pour des applications spécifiques liées au semi-conducteurs et (ii) l'investigation de nouvelles pistes pour améliorer encore la résolution spatiale, en particulier en adaptant les schémas d'acquisition en tomographie. Le TEM en balayage (STEM), basé sur des mesures annulaires aux forts angles et en champ sombre (HAADF) a été mis en œuvre pour observer des dopants dont le numéro atomique est typiquement largement supérieur à celui de la matrice (en silicium), et nous avons combiné le TEM résolu en énergie (EFTEM) dans un régime de faible perte d'énergie des électrons avec les techniques de tomographie afin de reconstruire les spectres de perte d'énergie locaux, en chaque voxel. La tomographie double-axe a été expérimentalement mise en œuvre pour améliorer la résolution spatiale, et le potentiel de la tomographie à axe multiple a été démontré, grâce aux simulations. Enfin, des algorithmes de reconstruction basés sur la minimisation de la variation totale ont été appliqués à la tomographie électronique. Les analyses effectuées comprennent les transistors triple-grille, les nanofils III-V, les capacités à base de nanofils de silicium et le silicium sur-dopé au sélénium, un matériau utilisé pour des applications optoélectroniques. / In this thesis electron tomography is developed and applied as a tool for three-dimensional nanoscale characterization of semiconductor materials and devices. The major contributions of this thesis are the exploration and application of transmission electron microscopy (TEM) contrast techniques for specific semiconductor applications and the exploration of routes towards improving spatial resolution, in particular by adapting tomographic acquisition schemes. As contrast techniques we apply high-angle annular dark-field (HAADF) scanning TEM (STEM) for investigations of heavy dopants in a lighter environment and we combine spectral low-loss energy-filtered TEM (EFTEM) with tomography and explore the features of reconstructed low-loss spectra. For resolution improvement we experimentally apply dual-axis electron tomography and investigate the potential of multiple-axis tomography based on simulations. Furthermore reconstruction algorithms based on totalvariation minimization are applied to electron tomography. Samples investigated in this work include tri-gate transistors, III-V nanowire heterostructures and silicon nanowire based capacitors as well as selenium-hyperdoped silicon, a material for optoelectronic applications.

No description available.

Transmission Electron Microscopy (TEM) is a reliable tool for chemical and structural studies of nanostructured systems. The shape, size and volumes of nanoparticles on surfaces play an important role in surface chemistry. As nanostructured surfaces become increasingly important for catalysis, protective coatings, optical properties, detection of specific molecules, and many other applications, different techniques of TEM can be used to characterize the properties of nanoparticles on surfaces to provide a path for predictability and control of these systems. This dissertation aims to provide fundamental understanding of the surface chemistry of Electroless Metallization onto Polymeric Surfaces (EMPS) through characterization with TEM. The research focuses on a single EMPS system: deposition of Ag onto the cross-linked epoxide "SU8", where Au nanoparticles act as nucleation sites for the growth of Ag nanoparticles on the polymer surface. TEM cross sections were analyzed to investigate the morphology of the Au nanoparticles and to determine the thicknesses of the Ag nanoparticles and of the Ag layers. A method for the direct measurement of the volume and thickness of nanomaterials has been developed in the project using High-Angle Annular Dark-Field (HAADF) Scanning Transmission Electron Microscopy (STEM). The morphology of Au and Ag NPs has been studied to provide reliable statistics for 3-D characterization. Deposition rates have been obtained as a function of metallization conditions by measuring the composition and thickness of the metal for EMPS. In the present work a calibration method was used to quantify the sensitivity of the HAADF detector. For thin samples a linear relationship of the HAADF signal with the thickness of a material is found. Cross-sections of multilayered samples provided by Triquint Semiconductors, FL, were analyzed as calibration standards with known composition in a TECNAI F30 transmission electron microscope to study the dependence of the HAADF detector signal on sample thickness and temperature. Dynamical diffraction processes play an important role in electron scattering for larger sample thicknesses. The HAADF detector intensity is not linearly dependent on sample thicknesses for thick samples. This phenomenon involves several excitation processes including Thermal Diffuse Scattering (TDS) which depends on temperature-dependent absorption coefficients. Multislice simulations have been carried out by Python programming using the scattering parameters (2) available in the literature. These simulations were compared with experimental results. Wedge-shaped Focused Ion Beam (FIB) samples were prepared for quantitative HAADF-STEM intensity measurements for several samples and compared with these simulations. The discrepancies between the simulated and experimental results were explained and new sets of absorptive parameters were calculated which correctly account for the HAADF-STEM contrasts. A database of several pure elements is compiled to illustrate the absorption coefficients and fractions of scattered electrons per nanometer of the sample. In addition, the wedge-shaped FIB samples were used for studying the HAADF-STEM contrasts at an interface of a high- and a low-density material. The use of thick samples reveals an increased signal at the interfaces of high- and low-density materials. This effect can be explained by the transfer of scattered electrons from the high density material across the interface into the less-absorbing low-density material. A ballistic scattering model is proposed here for the HAADF-STEM contrasts at interfaces of thick materials using Python. The simulated HAADF-STEM signal is compared with experimental data to showcase the above phenomenon. A detailed understanding of the atomic number contrast in thick samples is developed based on the combination of experimental quantitative HAADF-STEM and simulated scattering. This approach is used to describe the observed features for Ag deposition on SU8 polymers.

<p>Heteroepitaxial films of group III-antimonide-based semiconductor compounds on Si are amongst the most appealing candidates for solar applications because of the well-established Si platform and also for offering band-gap energies beyond the silicon road map. Nonetheless, high lattice mismatch between GaSb and Si as well as ambiguous nucleation of GaSb on Si are major drawbacks in manufacturing of heteroepitaxial GaSb/Si films because they can generate various defects in films. Atomic-level detection of these defects and delving into their origin, orientation, distribution, propagation, and interaction with each other will therefore provide an insight into inhibiting their formation or reducing their severity. State-of-the-art aberration-corrected transmission electron microscopes have marked a new era in the investigation of interfaces and defects. With sub-angstrom electron probes in scanning transmission electron microscopes, it is possible to pinpoint the individual atomic columns at interfaces and defects.</p> <p>In this thesis, GaSb epilayers grown with molecular beam epitaxy on Si substrates were studied through aberration-corrected scanning transmission electron microscopy. The strain-relief mechanism of the epitaxial GaSb through formation of interfacial misfit dislocations was investigated and the strain distribution in the vicinity of dislocation cores as well as epitaxial layer was analyzed. The specific atomic-number dependent contrast mechanism of the high-angle annular dark-field technique enabled the unprecedented direct observation of anti-phase boundaries, the extended defects of highest interest in polar-on-nonpolar growths. This observation unraveled the ambiguity of nucleation of GaSb at interface regardless of preferential deposition of atomic species during growth procedure. The growth of GaSb at the initial stage of deposition was further investigated to understand the role of an AlSb buffer layer and growth mechanism of GaSb precisely. This investigation showed that AlSb and GaSb epilayers occur by Volmer-Weber growth mode and AlSb islands provide energetically favorable nucleation sites for GaSb film. Furthermore, taking advantage of atomic-resolution detection capability of high-angle annular dark-field in scanning transmission electron microscopy a novel mechanism of strain relief through multiple twining resulting in a lattice-registered growth of GaSb on Si(211) was elucidated. This contribution demonstrates that aberration-corrected scanning transmission electron microscopy provides profound insight into the polar-on-nonpolar growth which can be exploited to suppress the formation of structural defects.</p> / Doctor of Philosophy (PhD)

Ce travail vise à utiliser des surfaces d'alumine nanostructurées pour guider la croissance et l'organisation de particules métalliques (Ag, Au et AgxAu1-x), et à les tester en tant que substrats SERS-actifs robustes et réutilisables. Nous avons utilisé la spectrophotométrie pour la caractérisation des propriétés optiques résultantes, l'ellipsométrie spectroscopique pour l'extraction des indices optiques et la microscopie électronique en transmission pour les caractérisations structurales. La diffusion Raman exaltée de surface (SERS) a été utilisée pour la détection de molécules de bipyridine adsorbées sur la surface des échantillons, en collaboration avec l’Institut des Matériaux Jean Rouxel de Nantes. Nous étudions d'abord des systèmes de nanoparticules monométalliques et bimétalliques afin de comprendre les modes de croissance de telles assemblées. Nous montrons que des arrangements différents de nanoparticules bimétalliques sont obtenus selon la séquence de dépôt utilisée et qu'un alliage est obtenu à l'issue de recuits ex situ sous vide. Les propriétés optiques en champ proche et lointain de nanoparticules d'alliage AgxAu1-x noyées dans une matrice d'Al2O3 sont comparées numériquement à celles des métaux purs, par la méthode de calcul des différences finies dans le domaine temporel. Les résultats indiquent que l’amplification du champ pour les nanoparticules de métal pur est plus élevée que pour les nanoparticules d’alliage. Enfin, les expériences SERS menées sur un système dichroïque de nanoparticules d’Ag plus ou moins couplées montrent que l'on peut obtenir un signal SERS intense avec des nanoparticules recouvertes. / The aim of this work is to use nanostructured alumina surfaces to guide the growth and to optimize the organization of metallic particles (Ag, Au and AgxAu1-x), and to test those systems as reusable SERS-active substrates. We used spectrophotometry to characterize the resulting optical properties, spectroscopic ellipsometry for the determination of the optical index and transmission electron microscopy for the structural characterizations. Surfaced-enhanced Raman spectroscopy (SERS) was used for the detection of adsorbed bipyridine molecules on the sample surface, in collaboration with the Institut des Matériaux Jean Rouxel at Nantes. We first study systems consisting of monometallic and bimetallic nanoparticles in order to understand the growth modes of such particle assemblies. A particular attention is paid to the influence of the sequential deposition of Au and Ag on the structural and optical properties. We show that different arrangements of bimetallic nanoparticles are obtained according to the deposition sequence used and that an alloy is obtained after ex situ annealing under vacuum. The near-field and far-field optical properties of AgxAu1-x nanoparticle alloys embedded in an alumina matrix are compared numerically by the finite difference time domain method, with those of pure metal nanoparticles. Our results indicate that pure metal nanoparticles exhibit a greater field enhancement than alloy nanoparticles. Finally, SERS experiments conducted with a dichroic system made of coupled Ag nanoparticles show that an intense SERS signal can be obtained with coated nanoparticles.

L'objectif de ce travail est d'une part de comprendre et contrôler la formation de rides périodiques nanométriques produites par pulvérisation ionique de films minces diélectriques. D'autre part, ces surfaces nanostructurées sont utilisées pour guider la croissance et l'organisation de nanoparticules d'argent. Ces systèmes anisotropes sont caractérisés par une position spectrale de la résonance plasmon de surface dépendant de la polarisation de la lumière incidente. Nous étudions d'abord par AFM et GISAXS l'influence des conditions de pulvérisation (angle d'incidence et énergie des ions, température, flux, fluence) sur la morphologie des rides (période, amplitude, ordre, ...). Les paramètres pertinents pour le contrôle de la morphologie sont identifiés ainsi qu'une partie des mécanismes physiques mis en jeu. Ensuite, nous étudions par HAADF-STEM l'influence des conditions de croissance (angle d'incidence du flux métallique, degré d'organisation des rides) sur les propriétés structurales des nanoparticules d'argent. Nous montrons que la croissance préférentielle des nanoparticules le long des rides est favorisée par des effets d'ombrage, ce qui conduit à la formation de chaînes linéaires de même période que les rides sous-jacentes et au sein desquelles les nanoparticules sont plus ou moins alignées et allongées. Cela se traduit par une anisotropie optique en champ lointain variable due à la polydispersité des distances interparticules (inférieures à quelques nanomètres) ainsi que des phénomènes de couplage en champ proche plus ou moins importants. Ces structures peuvent trouver des applications en spectroscopie Raman exaltée de surface (SERS). / On the one hand, the aim of this work is to understand and control the formation of periodic nanometric ripples produced by ion sputtering of dielectric thin films. On the other hand, these nanostructured surfaces are used to guide the growth and organization of silver nanoparticles. These anisotropic systems are characterized by a surface plasmon resonance whose spectral postion is dependent on the polarization of the incident light. We first study the influence of different ion beam sputtering parameters (the ions incidence angle and energy, temperature, energy, flux, fluence) on the ripple morphology (period, amplitude, order, ...) by AFM and GISAXS. The relevant parameters for the control of the ripple morphology are identified as well as some of the physical mechanisms involved. Then, we study the influence of the growth conditions on the structural properties of the nanoparticles (metal deposition angle, ripples pattern quality) by HAADF-STEM. We show that the preferential growth along the ripples is promoted by shadowing effects, thus leading to the formation of linear chains with period similar to the underlying ripples and with more or less elongated and aligned nanoparticles. This results in a far-field tunable optical anisotropy arising from polydisperse interparticle gaps (less than a few nanometers) as well as from more or less strong near-field coupling phenomena. These structures could offer potential for surface enhanced Raman spectroscopy (SERS) applications.

L'objectif de ce travail est d'ajuster la position spectrale de la résonance plasmon de surface de nanoparticules de métaux nobles dispersées dans une matrice diélectrique en contrôlant leur morphologie et leur organisation spatiale. Nous montrons que lorsque la croissance est réalisée sur des substrats plans, il est possible de jouer sur la nature de la matrice pour modifier le rapport d'aspect hauteur sur diamètre H/D des particules, et donc leur réponse optique. Des analyses structurales quantitatives menées par microscopie électronique en transmission à balayage (HAADF-STEM) font apparaître que H/D est une fonction décroissante du diamètre D, indépendamment de la quantité de métal déposé. Afin de comprendre ces effets de la matrice sur les propriétés structurales et optiques des particules, différentes études (influence du métal et de la quantité déposés, présence d'une couche tampon, influence des conditions d'élaboration, vitesse de recouvrement des particules, ... ) sont menées et des simulations numériques des spectres du facteur de transmission sont réalisées en intégrant dans un modèle de Yamaguchi les paramètres structuraux issus de l'analyse HAADF. Une autre approche consiste à utiliser des surfaces d'alumine nanostructurées afin d'induire une organisation surfacique des particules et ainsi entraîner une anisotropie de leurs propriétés optiques. Nous montrons que selon la géométrie de dépôt utilisée (incidence normale, incidence rasante, orientation et angle d'incidence du flux atomique), il est possible par effet d'ombrage de sélectionner le type de facettes sur lesquelles la croissance a lieu, et par conséquent d'organiser les nanoparticules d'argent sur la surface sous forme de bandes ou de chaînes linéaires, dont les propriétés optiques présentent une dépendance à la polarisation de la lumière incidente.

Ces travaux de doctorat concernent le développement de la tomographie électronique appliquée à la nano-caractérisation tridimensionnelle de dispositifs à semi-conducteurs et de matériaux pour la micro et la nanoélectronique. Les contributions les plus significatives de ces travaux sont (i) l'exploration et l'application de différents modes de contraste en microscopie électronique à transmission (TEM) pour des applications spécifiques liées au semi-conducteurs et (ii) l'investigation de nouvelles pistes pour améliorer encore la résolution spatiale, en particulier en adaptant les schémas d'acquisition en tomographie. Le TEM en balayage (STEM), basé sur des mesures annulaires aux forts angles et en champ sombre (HAADF) a été mis en œuvre pour observer des dopants dont le numéro atomique est typiquement largement supérieur à celui de la matrice (en silicium), et nous avons combiné le TEM résolu en énergie (EFTEM) dans un régime de faible perte d'énergie des électrons avec les techniques de tomographie afin de reconstruire les spectres de perte d'énergie locaux, en chaque voxel. La tomographie double-axe a été expérimentalement mise en œuvre pour améliorer la résolution spatiale, et le potentiel de la tomographie à axe multiple a été démontré, grâce aux simulations. Enfin, des algorithmes de reconstruction basés sur la minimisation de la variation totale ont été appliqués à la tomographie électronique. Les analyses effectuées comprennent les transistors triple-grille, les nanofils III-V, les capacités à base de nanofils de silicium et le silicium sur-dopé au sélénium, un matériau utilisé pour des applications optoélectroniques.