Modélisation de la microstructure des grains dans le silicium multicristallin pour le photovoltaïque

L'objectif de ce travail est d'approfondir et de mieux comprendre les mécanismes responsables de la formation et de la croissance de la structure des grains dans le silicium multicristallin pour des applications photovoltaïques. Lors de la solidification du silicium multicristallin, la sélection des grains, le contrôle de la distribution de leur taille et leur direction de croissance sont des paramètres importants pour obtenir un matériau de bonne qualité et homogène. Ces paramètres influencent directement le rendement de conversion des cellules photovoltaïques, au travers de la capture et de la recombinaison des porteurs de charges et des interactions avec les impuretés. La structure de grains dans le silicium photovoltaïque évolue au cours de la solidification : des grains vont disparaître, d'autres vont apparaître, d'autres vont grossir pour donner au final une structure composée de gros grains, de petits grains dénommés grits', de joints de grains, et de macles. Il est donc important de comprendre les relations entre les différents paramètres du procédé industriel et leur influence sur les phénomènes physico-chimiques qui se produisent lors de la croissance afin de pouvoir influer sur la structure de grains dans le silicium, et de prévoir ses propriétés. Dans une première étape, nous avons établi un modèle de développement des grains basé sur le type de croissance (facettée, rugueuse ou mixte), la cinétique de ces divers types de croissances, le phénomène de maclage et la sélection des grains, dont nous montrons qu'ils sont, avec la germination initiale, à l'origine de la taille et de la structure des grains. Ensuite, nous proposons une approche de modélisation numérique de l'évolution de la structure des grains au cours de la solidification. Cette méthode se base sur l'analyse dynamique bidimensionnelle du joint de grains au niveau de la ligne triple grain-grain-liquide (rugueuse, facettée) tout en prenant en compte les phénomènes produits à l'échelle macroscopique (le champ de température local) et microscopique (la cinétique des grains). Le modèle résulte du couplage thermique et des mécanismes cinétiques de croissance. Nous avons donc développé un modèle numérique de croissance des grains en 2 dimensions et nous l'avons introduit dans le code 2D-MiMSiS qui se déroule en 2 étapes : Premièrement, le calcul en régime transitoire de la solidification macroscopique d'un lingot de silicium nous permet d'obtenir le champ thermique dans le lingot et la position précise de l'interface solide-liquide à différents instants ainsi que sa vitesse, son orientation (sa forme) et les gradients de température dans le liquide et le solide. Deuxièmement, la modélisation de la croissance est basée sur la description géométrique des joints de grains qui dépend de la cinétique des grains qui les bordent. Elle suit des critères dépendants de la morphologie (rugueuse ou facettée) de l'interface. Elle s appuie sur le réseau d'isothermes du calcul thermique sans l'influencer dans un premier temps. Un des objectifs de ce modèle est de faire varier différents paramètres du procédé et d'en mesurer l'impact sur la structure cristalline finale. Des résultats de calculs 2D sont présentés et discutés par rapport à l'expérience.The objective of this work is to explore and better understand the mechanisms responsible for the formation and growth of the grain structure in polycrystalline silicon for photovoltaic applications. During the solidification of polycrystalline silicon for the selection of the grain, control the distribution of their size and direction of growth are important parameters to obtain a material of good quality and homogeneous. These parameters directly influence the conversion efficiency of solar cells, through the capture and recombination of charge carriers and interactions with impurities. Grain structure in silicon photovoltaic evolves during solidification: Grain will disappear, others will appear, others will grow to give the final structure composed of large grains, small grains called 'grits' grain boundaries and twins. It is therefore important to understand the relationship between the parameters of the industrial process, the physico-chemical phenomena that occur during the growth and structure of grains in the silicon to predict its properties. In a first step, we established a model of development based on the grain growth type (faceted, rough or mixed), the kinetics of the various growths, the phenomenon of twinning and the selection of grains, we show that they are, with the initial germination, originally of the size and structure of the grains. Then, we propose an approach to numerical modeling of the evolution of lala grain structure during solidification. This method is based on the two-dimensional dynamic analysis of the grain boundary at the triple line grain-grain-liquid (rough, faceted) taking into account the phenomena produced at the macroscopic scale (the local temperature field) and microscopic (kinetic grain). The resulting model of the thermal coupling mechanisms and growth kinetics. We have developed a numerical model of grain growth in two dimensions, and we have introduced in the 2D-code MiMSiS which takes place in two steps: First, the calculation of transient macroscopic solidification of an ingot of silicon allows us to obtain the temperature field in the ingot and the precise position of the solid-liquid interface at different times as well as its speed, direction ( form) and the thermal gradients in the liquid and the solid. Second, the growth model is based on the geometrical description of grain boundary which depends on the kinetics of grain that border. It follows dependent criteria of the rough morphology or faceted interface. It relies on a network of insulated thermal calculation without influence in the first place. One objective of this model is to vary the process parameters and to measure their impact on the final crystalline structure. 2D calculation results are presented and discussed in relation to the experience.SAVOIE-SCD - Bib.électronique (730659901) / SudocGRENOBLE1/INP-Bib.électronique (384210012) / SudocGRENOBLE2/3-Bib.électronique (384219901) / SudocSudocFranceF

Dewetting During Crystal Growth of (Cd,Zn)Te:In under Microgravity

The phenomenon of "Dewetting" during crystal growth has been observed in several microgravity experiments for different semiconductor crystals. The results of these experiments showed an improvement of the material quality due to the contact-less growth of the crystals. A number of crystal growth techniques have been used to grow CZT. The most widely used is the growth from the melt by the Bridgman method. However the crucible, which is generally made of carbon-layered silica glass, causes a number of problems: solid-liquid interface curvature, spurious nucleation of grains and twins, thermal stresses during the cooling of the crystal. This work is concentrated on the growth of high resistivity (Cd,Zn)Te:In (CZT) crystals by using the phenomenon of dewetting and its application in the processing of CZT detectors. Two Cd0.9Zn0.1Te:In crystals were grown under microgravity on the Russian FOTON satellite in the Polizon facility in September 2007. One crystal was grown under a rotating magnetic field during the phase of homogenization to destroy the typical tellurium clusters in the melt. The other crystal was superheated with 20 K above the melting point. A third crystal has been grown on the ground in similar thermal conditions. Inspection of the surface of the space grown crystals gave the evidence of successful dewetting during the crystal growth. The influence of the dewetting on the material properties is shown by the results of optical and electrical characterization methods. Finally, CZT detectors have been processed from the grown part of the different crystals. The influence of dewetting on their performance will be studied by means of the detector measurements with X- and Gamma-ray sources

Use of Growth-Rate/Temperature-Gradient Charts for Defect Engineering in Crystal Growth from the Melt

International audienceAs the requirements in terms of crystal defect/quality and production yield are generally contradictory, it is necessary to develop methods in order to find the best compromise for the growth conditions of a given crystal. Simple growth-rate/temperature-gradient charts are a possible tool in this respect. After the recall of the classical analytical equations useful for describing the process and defect engineering, a simple pedagogic case explains the building and use of such charts. The more complex application to the directional casting of photovoltaic Si necessitated the development of new physical models for twinning and equiaxed growth. This allowed plotting charts that proved useful for industrial applications. The conclusions discuss the drawbacks and advantages of the method. It finally proves to be a pedagogic tool for teaching crystal growth engineering

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Modeling effects of solute concentration in Bridgman growth of cadmium zinc telluride

International audienceNumerical modeling is used to investigate the effect of solute concentration on the melt convection and interface shape in Bridgman growth of Cd1-xZnxTe (CZT). The numerical analysis is compared to experimental growth in cylindrical ampoules having a conical tip performed by Komar et al. (2001) 151. In these experiments, the solidification process occurs at slow growth rate (V = 2.10(-7) m/s) in a thermal field characterized by a vertical gradient G(T) = 20 K/cm at the growth interface. The computations performed by accounting the solutal effect show a progressive damping of the melt convection due to the depleted Zn at the growth interface. The computed shape of the crystallization front is in agreement with the experimental measurement showing a convex-concave shape for the growth through the conical part of the ampoule and a concave shape of the interface in the cylindrical region. The distribution of Zn is nearly uniform over the crystal length except for the end part of the ingots. The anomalous zinc segregation observed in some experiments is explained by introducing the hypothesis of incomplete charge mixing during the homogenization time which precedes the growth process. When the crystallization is started in ampoules having a very sharp conical tip, the heavy CdTe is accumulated at the bottom part of the melt, giving rise to anomalous segregation patterns, featuring very low zinc concentration in the ingots during the first stage of the solidification. (C) 2016 Elsevier B.V. All rights reserved

Etude expérimentale et thermodynamique du procédé de démouillage appliqué aux semiconducteurs

Le procédé par démouillage est un procédé Bridgman vertical modifié qui offre l'avantage de supprimer les contacts solide-creuset grâce à la formation d'un espacement au cours de la croissance cristalline de matériaux semiconducteurs. Ce procédé permet de réduire significativement la densité des défauts cristallins (dislocations et nucléation parasite). Il dépend essentiellement des phénomènes capillaires mis en oeuvre, et particulièrement des propriétés de mouillage du liquide sur les parois du creuset. Au cours de cette thèse, nous avons montré expérimentalement que le démouillage résulte du déplacement d'un ménisque stable le long des parois lors de la solidification d'antimoniures d'indium et de gallium (InSb et GaSb) dans des creusets fermés en verre de silice. Des calculs d'équilibre thermodynamique ont été effectués afin d'expliquer, dans cette configuration précise, l'influence de la pollution chimique sur le procédé.The dewetting process is a modified Vertical Bridgman process that allows avoiding solid-crucible contacts thanks to the formation of a gap during the crystal growth of semiconductor materials. This process reduces drastically the density of crystalline defects (dislocations and secondary nucleation). It depends mainly on the capillary phenomena and especially on the wetting properties of the melt on the crucible walls. In this work, we have shown experimentally that the dewetting results from the displacement of a stable meniscus along the walls during the solidification of the indium and gallium antimonides in sealed fused silica crucibles. Equilibrium thermodynamics computations have been carried out in order to explain the role of the chemical pollution in this experimental configuration.GRENOBLE1-BU Sciences (384212103) / SudocSudocFranceF

Effet d'un champ magnétique alternatif sur la solidification Bridgman des alliages semi-conducteurs concentrés

La croissance des alliages semi-conducteurs concentrés homogénes représente une tache difficile à cause de la ségrégation des éléments au cours du processus de solidification. Dans le cadre de cette thése. nous avons étudié une méthode originale. basée sur les effets du champ magnétique alternatif, destinée à réduire la ségrégation chimique et à éviter l'amortissement solutal dans les alliages GalnSb concentrés. Une bobine placée autour de l'échantillon et parcourue par un courant alternatif génére dans le liquide, près de l'interface de solidification, un mouvement convectif susceptible d'améliorer l'homogénéisation du liquide. Les phénomène engendrés lors de la solidification Bridgman verticale sous champ magnétique alternatif ont été analysés en variant plusieurs paramètres: la concentration de l'alliage Ga(1-x)lnxSb, l'intensité du champ magnétique alternatif et la position de l'interface solide/liquide dans la spire.GRENOBLE1-BU Sciences (384212103) / SudocSudocFranceF

Influence of rotating magnetic fields on THM growth of CdZnTe crystals under microgravity and ground conditions

International audienceThe influence of rotating magnetic fields (RMF) on species transport and interface stability during the growth of Cd0.96Zn0.04Te:In crystals by using the traveling heater method (THM), under microgravity and terrestrial conditions, is numerically investigated. The numerical results are compared to ground and space experiments. The modeling of THM under ground conditions shows very deleterious effects of the natural convection on the morphological stability of the growth interface. The vertical flow transports the liquid of low Te concentration from the dissolution interface to the growth interface, which is consequently destabilized. The suppression of this flow, in low-gravity conditions, results in higher morphological stability of the growth interface. Application of RMF induces a two flow cell pattern, which has a destabilizing effect on the growth interface. Simulations performed by varying the magnetic field induction in the range of 1-3 mT show optimal conditions for the growth with a stable interface at low strength of the magnetic field (B = 1 mT). Computations of indium distribution show a better homogeneity of crystals grown under purely diffusive conditions. Rotating magnetic fields of B = 1 mT induce low intensity convection, which generates concentration gradients near the growth interface. These numerical results are in agreement with experiments performed during the FOTON M4 space mission, showing good structural quality of Cd0.96Zn0.04Te crystals grown at very low gravity level. Applying low intensity rotating magnetic fields in ground experiments has no significant influence on the flow pattern and solute distribution. At high intensity of RIVIF (H = 50 mT), the buoyancy convection is damped near the growth front, resulting in a more stable advancing interface. However, convection is strengthening in the upper part of the liquid zone, where the How becomes unsteady. The multi cellular unsteady flow generates temperature oscillations, having deleterious effects on the growth process. (c) 2015 Elsevier B.V. All rights reserved