STRUCTURE DES JOINTS DE GRAINS DANS LES SEMICONDUCTEURS

La structure cristallographique locale des joints de grains dans les semiconducteurs comporte certaines particularités : plusieurs types de coincidences existent puisque le groupe d'espace des cubiques diamant contient un élément de translation. De plus les liaisons covalentes tendent à maintenir fa tetracoordination même dans les défauts. Les différents motifs rencontrés dans les dislocations des sous joints peuvent fournir la base des arrangements locaux des joints. Certains motifs particulièrement simples rendent compte des joints à haute symétrie et forte coincidence. Les résultats expérimentaux obtenus par microscopie électronique haute résolution confirment ces structures simples. Cependant il est à prévoir que pour des désorientations ou des plans de joints quelconques, des motifs complexes mettant en jeu des reconstructions soient nécessaires.The grain boundary (GB) local structure in semiconductors has two peculiarities : several coincidence types may exist as the space group of diamond cubic contains a translation. Moreover covalent bonding tends to favour tetrahedral arrangement even in defects. The different patterns observed in subgrain boundaries dislocations should give the basic local structures for general GB. Some of these patterns are particularly simple and can be used in highly symmetrical and coincident twin position. Experimental results obtained by high resoution electron microscopy conf irm this idea. However more complex patterns containing reconstructed bonds may play an important role for more general GB

Analysis of MBE growth and atomic exchange in thin highly strained InAs layers

The aim of this work is to prove the segregation process of indium during the growth of GaAs on InAs layers. A variety of structures has been grown to allow us to determine the width of indium distribution according to the growth process. Strained InAs/GaAs films as monolalayers, multilayers or multiquantum wells have been obtained. The indium segregation during growth has been proved in analyzing the electron images of the films and a new method for avoiding indium segregation in InGaAs/GaAs superlattices has been developed. Concerning the detailed image analyzis using image simulation, distortion measurements and intensity measurements have shown that intensity measurement is the most sensitive way to detect InAs containing layers in a GaAs matrix. It is demonstrated that less than 6% of indium can be detected. The segregation process has been demonstrated on symmetrical structures, and the nominal position of indium was determined with reference to AlAs films. Furthermore, a novel technique, which, by predeposition of indium, controls the segregation itself in order to build abrupt interfaces in the InGaAs/GaAs system, has been validated.Lobjet de ce travail est de prouver l'existence d'une ségrégation de l'indium lors de la croissance de GaAs sur un film d'InAs, et de trouver une technique pour l'éviter. Pour cela les films contraints de InAs/GaAs ont ete obtenus dans différentes conditions de croissance, des films isolés, des multicouches et des multipuits quantiques ont été étudiés. Des images de microscopie électronique haute résolution en transmission ont été analysées en détail afin de déterminer le meilleur moyen pour clairement différencier indium et gallium. La comparaison des résultats obtenus en faisant des simulations d'images, des mesures de distortion et des mesures d'intensité ont montré que les mesures d'intensité sont les plus sensibles pour l'analyse de l'indium. Moins de 6% d'indium peut être détecté. Les résultats obtenus par cette technique ont été complétés par l'étude des propriétés optiques des films. Il a été démontré que la distribution d'indium au delà d'une monocouche est due à une ségrégation en surface lors de la croissance, et qu'une technique de prédéposition permet de l'éviter

STRUCTURE DES JOINTS EN MICROSCOPIE ELECTRONIQUE HAUTE RESOLUTION (MEHR)

La microscopie électronique haute résolution (MEHR) permet dans des conditions géométriques particulières l'observation directe des structures au voisinage des joints de grains. Ces conditions sont limitées aux joints de flexion d'axe commun parallèle à une direction dense du cristal. Cette technique a été employée sur plusieurs macles dans le germanium et le molybdène (Σ41, Σ25, Σ9 et Σ3). Les relaxations primaires et secondaires ont été mises en évidence et les vecteurs de Burgers des primaires mesurés. Une discussion des possibilités et des limites de la méthode est présentée.The high resolution electron microscopy (HREM) allows the direct observation of the grain boundary (GB) microstructure in favorable cases. The G.B. must be pure tilt with a dense axis as a common direction. This technique has been employed on several twins in germanium and molybdenum (Σ41, Σ25, Σ9 and Σ3). Primary and secondary relaxations have been imaged and Burgers vector of primary dislocations measured. Possibilities and limits of the HREM are discussed

STRUCTURE OF THE SILICIDE/Si, SiO2/Si INTERFACE ANALYSED USING HIGH RESOLUTION TRANSMISSION ELECTRON MICROSCOPY

This paper is a kind of over- view of the problems which have to be solved in order to determine an interface structure. The first question is to be able to interpret the micrographs knowing that the dynamical interaction between electron wave and atomic potential is highly dependent on the crystal structure. Differences between centrosymmetric and non-centrosymmetyric crystals are underlined. The example which is treated is ErSi2, and the ErSi2/Si interface is analysed. The second problem is the thickness variation at the interface vicinity ; this is shown to imply contrast variations which can be interpreted as the presence of a thin film between the substrate and the over layer. Finally it is shown how high resolution images are a necessary complement to determine microcrystallite structures, when they are too few and small to be analysed using X-Ray diffraction. The example is SiO2 precipitates grown in amorphous SiO2 during ultra dry oxidation

Epitaxial orientation of β-FeSi2/Si heterojunctions obtained by RTP chemical vapor deposition

In recent years the semiconducting phase of iron silicide β-FeSi 2 has attracted interest. Promising applications of a great deal of β-FeSi 2/Si heterojunctions are reported in semiconductor technology due to the 0.89 eV direct band gap of β-FeSi2. Most of the papers devoted to this material present three different deposition modes, i.e. MBE, Solid Phase Epitaxy (SPE) and Reactive Deposition Epitaxy (RDE). The epitaxy of very thin layers of β-FeSi2 has already been reported on (111) and (001) silicon substrates. This paper presents an original application of Chemical Vapor Deposition (CVD) for the growth of β-FeSi2 using Rapid Thermal Processing (RTP). The results presented here mainly concern the epitaxial orientations and the morphology of β-FeSi2 on silicon. The different epitaxial relationships are experimentally distinguished by the use of transmission electron diffraction (TED) and microscopy (TEM). Thick β-FeSi2 layers (> 100 nm) have been selectively grown by RTP chemical vapor deposition on patterned (111) and (001) silicon wafers and under different experimental conditions. They are polycrystalline with large grains (about 1 μm) and mainly epitaxial. The main epitaxial relationship found is (220) β-FeSi2 // (111) Si named type B in the literature. An important result is the flatness of the interface under each β-FeSi 2 grain which presents large areas (about 50 nm) without any monoatomic step. This result seems to be an advantage of the promising chemical vapor deposition process used which minimizes the interdiffusion processes at the interface

How would nanostructures emerge in stressed multilayers?

We study theoretically the emergence of nanostructures in stressed multilayers. A non-linear amplitude expansion provides us with a simple argument on how the nanostructures emerging at the interfaces of the materials should behave. The criterion is simply expressed in terms of the relative elastic hardness of the solids. This study should serve as a guide to design new nano-architectures, a problem of much current interest

Filtering of electron images of crystal defects

The interpretation of electron micrographs becomes easier after filtering of the photographic and electronic noise. The filtering process can be optical or numerical. The use of mask in the Fourier space must not disturb too much the spectrum of the information of interest. The influence of four mask shapes is studied on two nearly similar test objects : atomic columns near the core of germanium crystal dislocations. The optimal mask shape is applied to a real electron micrograph of a high resolution electron image of dislocation.Pour rendre l'interprétation des images obtenues au microscope électronique plus aisée, il est nécessaire de filtrer le bruit photographique et électronique. Le filtrage peut être soit optique soit numerique. Cependant l'utilisation des masques ne doit pas perturber le spectre de Fourier du signal au point d'altérer l'image des défauts cristallins observés. On présente diverses formes de masque et leur influence sur la restitution de l'image d'objets tests. Enfin un filtrage numérique est applique à une image de colonnes atomiques autour d'une dislocation obtenue réellement au microscope électronique