Silicon epitaxy below 200°C: Towards thin crystalline solar cells

International audienceLow temperature plasma processes provide a toolbox for etching, texturing and deposition of a wide range of materials. Here we present a bottom up approach to grow epitaxial crystalline silicon films (epi-Si) by standard RF-PECVD at temperatures below 200°C. Booth structural and electronic properties of the epitaxial layers are investigated. Proof of high crystalline quality is deduced from spectroscopic ellipsometry and HRTEM measurements. Moreover, we build heterojunction solar cells with intrinsic epitaxial absorber thickness in the range of a few microns, grown at 175 °C on highly doped (100) substrates, in the wafer equivalent approach. Achievement of a fill factor as high as 80 % is a proof that excellent quality of epitaxial layers can be produced at such low temperatures. While 8.5 % conversion efficiency has already been achieved for a 3.4 µm epitaxial silicon absorber, the possibility of reaching 15 % conversion efficiency with few microns epi-Si is discussed based on a detailed opto-electrical modeling of current devices

Using a cognitive architecture to examine what develops

Different theories of development propose alternative mechanisms by which development occurs. Cognitive architectures can be used to examine the influence of each proposed mechanism of development while keeping all other mechanisms constant. An ACT-R computational model that matched adult behavior in solving a 21-block pyramid puzzle was created. The model was modified in three ways that corresponded to mechanisms of development proposed by developmental theories. The results showed that all the modifications (two of capacity and one of strategy choice) could approximate the behavior of 7-year-old children on the task. The strategy-choice modification provided the closest match on the two central measures of task behavior (time taken per layer, r = .99, and construction attempts per layer, r = .73). Modifying cognitive architectures is a fruitful way to compare and test potential developmental mechanisms, and can therefore help in specifying “what develops.

ISML: an interface specification meta-language

In this paper we present an abstract metaphor model situated within a model-based user interface framework. The inclusion of metaphors in graphical user interfaces is a well established, but mostly craft-based strategy to design. A substantial body of notations and tools can be found within the model-based user interface design literature, however an explicit treatment of metaphor and its mappings to other design views has yet to be addressed. We introduce the Interface Specification Meta-Language (ISML) framework and demonstrate its use in comparing the semantic and syntactic features of an interactive system. Challenges facing this research are outlined and further work proposed

Case-based learning: Predictive features in indexing

Interest in psychological experimentation from the Artificial Intelligence community often takes the form of rigorous post-hoc evaluation of completed computer models. Through an example of our own collaborative research, we advocate a different view of how psychology and AI may be mutually relevant, and propose an integrated approach to the study of learning in humans and machines. We begin with the problem of learning appropriate indices for storing and retrieving information from memory. From a planning task perspective, the most useful indices may be those that predict potential problems and access relevant plans in memory, improving the planner's ability to predict and avoid planning failures. This “predictive features” hypothesis is then supported as a psychological claim, with results showing that such features offer an advantage in terms of the selectivity of reminding because they more distinctively characterize planning situations where differing plans are appropriate.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/46928/1/10994_2004_Article_BF00993173.pd

Enterocolitis necrotizante en recién nacidos ingresados en el Servicio de Neonatología del Hospital Escuela "Carlos Roberto Huembes" en el período comprendido de Enero 2011 a Diciembre 2013

La enterocolitis necrotizante en el recién nacido presenta un amplio espectro de manifestaciones clínicas, caracterizándose principalmente por la tríada de distensión abdominal, sangramiento gastrointestinal y neumatosis intestinal. A pesar del avance en el cuidado intensivo neonatal, persiste como una enfermedad grave, que afecta habitualmente al recién nacido pretérmino, especialmente de muy bajo peso

ANISOTROPY IN SULPHUR DOPING OF GaAs GROWN BY V.P.E.

Les résultats présentés ont été obtenus en utilisant une source de soufre élémentaire pour doper les couches épitaxiales de GaAs réalisées dans un réacteur de croissance en phase vapeur par la méthode au trichlorure d'arsenic. Quatre échantillons présentant des orientations cristallines (100), (110), (111) A, (111) B sont introduits simultanément dans le tube de réaction et subissent les mêmes conditions de croissance. En fonction de la température du thermostat de soufre, la concentration en porteurs libres laisse apparaître une importante variation de l'incorporation du dopant avec la nature de la face cristalline. Toutefois, la concentration maximale en porteurs libres ne dépend pas de celle-ci et vaut environ 5.1018 at.cm-3 pour les conditions utilisées. L'importance des phénomènes de transport à l'interface cristal-vapeur est mis en lumière, et plus particulièrement celui de tous les paramètres modifiant le recouvrement de la surface et la vitesse de croissance, dont le rôle est important dans l'incorporation des impuretés.The doping behaviour of sulphur during the vapour growth of GaAs using the GaAs/AsCl3/H2 method is investigated for the following orientations of the substrate : (100), (110), (111) A, (111) B. A linear relationship is found between the measured electron concentration n and the sulphur partial pressure up to a saturation limit for n at about 5.1018 at.cm-3. This value of the effective solubility limit for S is found to be the same for the four different substrate orientations. However, the strong dependence of the effective distribution coefficient with the cristalline orientation puts forward the importance of the surface and adsorbed layer structure in the rate limiting step for dopant incorporation

LOW PRESSURE VAPOUR PHASE EPITAXY OF GaAs –THE GROWTH RATE LIMITING PROCESSES

La croissance de GaAs par la méthode au trichlorure d'arsenic fait intervenir comme étapes limitantes, des phénomènes de volume (transfert des espèces vers l'interface, etc...) et des réactions de surface (adsorption, incorporation, désorption, etc...). La cinétique des réactions de surface a été modélisée, mais celle-ci ayant été paramétrée à partir de résultats obtenus à 1atmosphère, le résultat tient compte implicitement des effets du transfert de masse par diffusion dans la phase vapeur. L'abaissement de la pression de travail, en augmentant le coefficient de diffusion des molécules, contribue à diminuer l'importance des phénomènes de volume au profit des réactions de surface. Les mesures de cinétique de croissance à basse pression ont ainsi permis de confirmer le modèle théorique mais également de mettre en évidence l'importance du transfert de masse sur la limitation de la vitesse de croissance. A partir d'un modèle simple de "couche limite", qui rend compte des effets du transfert par diffusion, et d'un modèle de cinétique de surface paramétré sur les résultats obtenus à faible pression, nous avons pu analyser les effets conjugués du flux diffusionnel et de la cinétique de surface. Cette modélisation permet d'atteindre les "vraies" conditions à l'interface cristal-vapeur (pressions partielles, sursaturation, recouvrement de la surface.. ) et d'étudier leurs fluctuations en fonction des paramètres expérimentaux. Ce type d'approche est appliqué à l'optimisation des conditions opératoires en vue de réaliser des dépôts homogènes en épaisseur et en dopage.A simple model is developped to study the competition between mass transfer and surface kinetics during the growth of GaAs in a chloride process. This more realisitic model provides an useful tool to investigate the "real" conditions at the growth interface (partial pressures, supersaturation, surface coverage...), and their variations with the process parameters fluctuations. Our treatment is applied to the optimization of the growth parameters for a better thickness and dopant uniformity in the grown layers

Problems and techniques of text analysis

Includes bibliographical references (p. 145-150)Research reported herein was supported in part by the National Institute of Education. US-NIE-C-400-76-011