1,721,118 research outputs found
Optical and morphological properties of stain-etched porous silicon films for anti-reflection coatings of photovoltaic devices
No full textPorous Silicon was formed on mono- and poly-crystalline Si substrates by stain etching in aqueous HF/HNO3 solutions. Optical properties of the resulting films have been investigated by reflectance spectroscopy and have been related to process parameters, e.g. HF, HNO3 concentration and etching time. Film morphology has been investigated by High Resolution Electron Microscopy (HREM). Porous Si films allowed to reduce surface reflectance to under 2% in 550-700 nm wavelength range. Efficient anti-reflection coatings were developed and used in large area solar cells: photovoltaic conversion efficiency higher than 12% was obtained by 12.8×12.8 cm2 polycrystalline devices under standard AM1.5G simulated sunlight
Analysis of the electrical activation data in thermally annealed implanted Al/4H–SiC systems: A novel approach based on cooperativity
No full textThe dependence on the annealing temperature, Tann, of the Al asymptotic substitutional fraction φ∞ in implanted silicon carbide (4H–SiC), is addressed from a statistical mechanical point of view. Concepts relating to the cooperative nature of atomic motion in (extremely) viscous liquids or in plastic crystals are applied for the first time in describing Si ⇄ Al substitution and the tendency of the system towards a regularization of its crystalline structure during annealing. The worked out model, although inspired by Al/4H–SiC literature data, can be applied to similar systems with the due changes. The slope ΔEs of the Arrhenian lnφ∞ vs. 1/kBTann dependence encompasses both the energy that would characterize a single Si ⇄ Al ideal substitution in a regular lattice, and a negative contribution associated to the irregularity of the lattice at the substitution sites. The early evolution of the substitutional fraction φ(t;Tann) in implanted systems during annealing is then considered. The slope of lnφ vs. 1/kBTann at short annealing times, when the asymptotic state is not yet reached, provides information about the average energy barrier ΔEw characterizing on average the preliminary molecular rearrangements preceding the Si ⇄ Al substitution. Values of ΔEs in the order of 1 eV or a bit less are envisaged and worked out from φ∞-fittings, while ΔEw∼3 eV is found from the analysis of incomplete annealing. The theory applied in this work represents a reference for a more complete description of the structural evolution of implanted systems
Electron scattering by spatially correlated DX charges
No full textElectron scattering by a spatially correlated system of DX charges has been described using the formalsim of composition waves. The matrix element for the scattering rate is given through an I(Q) interference function (Q=scattering vector) containing pair correlation functions ɛ(p) defined at the p lattice vectors. The ɛ(p) are able to describe long-range as well as short-range order and they are simply related to short-range order correlation functions given in the literature. The method is developed for scattering centers having equal charges; the case of positively and negatively charged impurities present together is briefly discussed. A comparison between the two extreme cases of randomly distributed scattering centers and of centers arranged in a superlattice suggests, for intermediate cases, an I(Q) given by an array of Gaussian shaped functions with common dispersion σ, centered on the reciprocal nodes of a virtual superlattice. On this basis, experimental mobility data for Si-doped Al0.25Ga0.75As samples prepared by molecular beam epitaxy have been analyzed and discussed. Data refer to isothermal electron capture transients into DX centers, as well as to steady-state measurements taken for different free electron densities under a persistent photoconductivity regime. It has been confirmed that the initial stage of the capture process takes place together with increasing order in the scattering center distribution (decreasing σ), whereas the contrary happens during the final stage (increasing σ)
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Porous silicon nanocracking
No full textWe report on the collapse of porous silicon on the nanometer scale observed during a study of freeze dried low doped p-type samples. During the drying process highly porous layers, about 4 mu m in depth, changed their colour from an initial bright brown to a well defined green at the end of the procedure. However, the mirror aspect of the sample was kept. This phenomenon which results from a change of the optical path (nd) cannot uniquely be attributed to a refractive index variation. Simulation of reflectivity spectra shows that compared to the expected values, the thickness is lower and the density of material higher. An analysis of the profile shows that the volume can be reduced by as much as a factor of six. X-ray and TEM observations have confirmed these data and have shown that these samples have lost their crystallinity and are completely amorphous. In situ observations shows that this compression occurs during the sublimation or evaporation phase. During this phase, if the sample is wetted again the initial colour of the sample can be recovered as an effect of the elasticity of the PS structure. We attributed the origin of this compaction to nanoscopic cracking due to capillary effects, as in the case of the well known macroscopic cracking. (C) 2000 Elsevier Science S.A. All rights reserved
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Contrast of small SiX particles in silicon by computed HREM images
No full textThe contrast in HREM images of small spherical coherent precipitates in silicon, having a hypothetical sphalerite structure and an SiX composition, is computed by the multislice method; X represents a dopant (or impurity) atom, ranging from boron up to platinum. The particles are assumed to have a diameter of 2 nm and to exert different amounts of in situ strain ε on the silicon matrix. The ideal case of no misfit (ε = 0) is also considered. For in situ strains 0 < ε < 2%, the maximum contrast, corresponding to the optimum defocus, depends mainly on the atomic number of the element X. For larger strains, which can occur in some cases if the Si[BOND]X bond length in the SiX particle is determined by the covalent radius of the impurity atom X, the strain contrast dominates over the structure factor contrast
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