1,720,992 research outputs found
Atomistic Models of Amorphous Semiconductors
No full textCrystalline silicon is probably the best studied material, widely used by the semiconductor industry. The subject of this thesis is an intriguing form of this element namely amorphous silicon. It can contain a varying amount of hydrogen and is denoted as a-Si:H. It completely lacks the neat long range order of the crystal, yet its structure is not random. Almost all silicon atoms have four neighbors and the average bond angle is identical to the tetrahedral angle in the crystal. Order is thus preserved over several bond lengths. The motivations to study a-Si:H are two-fold. Firstly some of its properties are different from the crystalline form and we do not understand them completely. For example, the electronic properties degrade after exposure to intense light, but can be recovered reversibly by heat treatment. The microscopic process of this is not known. Secondly, research on a-Si:H is motivated by its applications. These are mostly large area devices such as liquid crystal displays and solar cells. The latter are in use already today, the former are waiting to be widely used in future. Amorphous semiconductors can be deposited over large areas from vapor. On the other hand, the size of c-Si devices is limited by the much smaller size of the wafers. The production of a-Si:H is also cheaper and consumes less energy. Unlike its crystalline counterpart a-Si:H has a direct band gap, leading to an increased light absorption. Consequently, a-Si:H solar cells are ~ 1000 times thinner than c-Si cells, resembling more a foil than a semiconductor device. The methods used in the thesis are computational, largely relying on algorithms and powerful computers. The structural models are atomistic, where the interaction between electrons and nuclei is treated on the level of Density Functional Theory. This is a first-principles methods, meaning that it does not use any adjustable parameters. The chemical bonding, even of complex structures is described accurately. Calculation of total energies and forces allows us to find equilibrium structures and perform molecular dynamics calculations. The models of a-Si:H are prepared by cooling a melt to room temperature. This method resembles the preparation of glasses. We find that the structure is strongly in influenced by the cooling rate. Using slower cooling rates we improved existing models that contained excessive strain and a high defect concentration. Using a cooling rate of ~ 0.02 K/fs we were even able to prepare small defect-free models. The structure was in good agreement with available neutron scattering data. Calculated density of states shows a pronounced band gap. After the generation of structural models we turn our attention to defects. Defects in an amorphous solid are defined as atoms that deviate from the normal coordination. We find 3-fold and 5-fold coordinated Si atoms and 2-fold coordinated H atoms. We focus only on the 3-fold coordinated Si, also called the dangling bond (DB), that is believed to be the major defect in a-Si:H. We have calculated formation of the DB defect in the negative, neutral and positive charge state. By averaging over 25 distinct DB models we find a considerable spread in the energies of 0.2 eV. Another related property of a defect is its correlation energy U. A positive value of U means that we have to invest energy to add an extra electron to the defect. The size and sign of U are still a subject of controversy. On average we find a positive U value of 0.1 eV. Four models, however, have a negative correlation energy, suggesting large relaxations in the defect structure. Amorphous silicon readily forms compounds with nitrogen and carbon. We have investigated silicon-rich nitride (a-SiN:H) at two different densities of 2.0 and 3.0 g/cm3. Features in the pair-distribution functions can be related to "square structures". These are planar structures consisting of two Si in opposite corners of a square and two N in the remaining corners. The dense phase shows signs of phase separation into silicon and stoichiometric nitride. Both valence and conduction band edges are dominated by Si states. This is corroborated by the fact that by increasing the nitrogen content the band gap of the nitride can be varied from 1.8 to 5.3 eV. Recently there has been a considerable interest in man-made materials. Examples are multilayers (ML) formed by two semiconductors with a different band gap. By adjusting the thickness of the small band gap material (the well) one can tune the band gap of the ML due to quantum confinement effects. This concept is well established in crystalline semiconductors. The existence of quantum confinement in amorphous structures is, however, being still debated. Using models prepared previously we have constructed a model of a silicon/nitride ML. This allowed us to study confinement effects directly without using transport or optical measurements that can obscure the observations. Comparing our model to an experimental system with the same composition gave almost identical band gaps. This confirmed the existence of quantum confinement in a amorphous multilayer. The calculation of band offsets between the materials revealed that there is almost no barrier for the electrons and the confinement originated solely from holes.Electrical Sustainable EnergyElectrical Engineering, Mathematics and Computer Scienc
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
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
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
Author-wise bibliometric analysis based on entropy.
No full textAuthor-wise bibliometric analysis based on entropy.</p
Author Under Sail The Imagination of Jack London, 1893-1902
No full textIn Author Under Sail, Jay Williams offers the first complete literary biography of Jack London as a professional writer engaged in the labor of writing. It examines the authorial imagination in London's work, the use of imagination in both his fiction and nonfiction, and the ways he defined imagination in the creative process in his business dealings with his publishers, editors, and agents. In this first volume of a two-volume biography, Williams traverses the years 1893 to 1902, from London's "Story of a Typhoon" to The People of the Abyss. The Jack London who emerges in the pages of Author Under Sail is a writer whose partnership with publishers, most notably his productive alliance with George Brett of Macmillan, was one of the most formative in American literary history. London pioneered many author models during the heyday of realism and naturalism, blurring the boundaries of these popular genres by focusing on absorption and theatricality and the representation of the seen and unseen. London created an impassioned, sincere, and extremely personal realism unlike that of other American writers of the time. Author Under Sail is a literary tour de force that reveals the full range of London as writer, creative citizen, and entrepreneur at the same time it sheds light on the maverick side of machine-age literature.Intro -- Title Page -- Copyright Page -- Dedication -- Contents -- Acknowledgments -- Introduction -- 1. Spirit Truth -- 2. From Absorption to Theatricality and Back Again -- 3. "I Will Build a New Present" -- 4. Sons as Authors -- 5. Fathers as Publishers -- 6. The Daughter as Author -- 7. Lovers as Authors -- 8. At Sea with the Family -- 9. Yellow News, Yellow Stories -- 10. The Return Home -- Notes -- Bibliography -- Index -- About Jay WilliamsIn Author Under Sail, Jay Williams offers the first complete literary biography of Jack London as a professional writer engaged in the labor of writing. It examines the authorial imagination in London's work, the use of imagination in both his fiction and nonfiction, and the ways he defined imagination in the creative process in his business dealings with his publishers, editors, and agents. In this first volume of a two-volume biography, Williams traverses the years 1893 to 1902, from London's "Story of a Typhoon" to The People of the Abyss. The Jack London who emerges in the pages of Author Under Sail is a writer whose partnership with publishers, most notably his productive alliance with George Brett of Macmillan, was one of the most formative in American literary history. London pioneered many author models during the heyday of realism and naturalism, blurring the boundaries of these popular genres by focusing on absorption and theatricality and the representation of the seen and unseen. London created an impassioned, sincere, and extremely personal realism unlike that of other American writers of the time. Author Under Sail is a literary tour de force that reveals the full range of London as writer, creative citizen, and entrepreneur at the same time it sheds light on the maverick side of machine-age literature.Description based on publisher supplied metadata and other sources.Electronic reproduction. Ann Arbor, Michigan : ProQuest Ebook Central, YYYY. Available via World Wide Web. Access may be limited to ProQuest Ebook Central affiliated libraries
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