1,720,958 research outputs found
Effect of Si doping on the structural and the optical properties in high-quality AlxGa1-xN/GaN heterostructures grown by metalorganic chemical vapor deposition
No full textAlxCa1-xN/GaN films were grown on sapphire substrate by using rnetalorganic chemical vapor deposition, and their structural, electrical, and optical properties were systematically investigated by using atomic force microscopy, high-resolution X-ray diffraction (HRXRD), Hall measurements, and photoluminescence (PL). The strain states and the mosaic properties of the AlGaN and GaN layers were studied by using the (10(.)5) asymmetric reciprocal space maps (RSMs). From the RSMs, we confirmed that the crystal lattice of the Al0.13Ga0.89N film was fully strained and that the crystal quality was good. From the PL and the HRXRD data, we observed that a. certain range of Si doping could cause a small variation of Al incorporation during the growth of the AlGaN film, resulting in a variation in the Al content and the crystal quality of the AlGaN layer. From the results, we conclude that moderate Si-doping improves the optical and the structural properties of Al0.11Ga0.89N films.
Anomalous temperature dependence of optical emission in visible-light-emitting amorphous silicon quantum dots
No full textWe have investigated optical properties of amorphous silicon quantum dots (a-Si QDs) embedded in a silicon nitride grown on (100) silicon substrates by plasma-enhanced chemical vapor deposition. The photoluminescence (PL) emission colors of red, orange, and blue were shown for the a-Si QDs with average dot sizes of 5.8, 2.4, and 1.3 nm, respectively. The increase in the PL peak energy with decreasing QD size was attributed to the quantum confinement effect in a-Si QDs. An anomalous temperature dependence of the PL intensity and PL peak energy was observed for a-Si QDs. The visible-light PL emission from a-Si QDs is most efficient at near room temperature, and the PL spectral shape and peak energy are almost independent of temperature, which are most favorable for light-emitting device applications. From time-resolved PL experiments, an enhancement of phonon-assisted indirect radiative recombination was proposed to explain the anomalous temperature dependence. (C) 2003 American Institute of Physics.
Comparison of carrier dynamics in GaN quantum dots and GaN quantum wells embedded in low-Al-content AlGaN waveguides
No full textComparative analysis of the carrier dynamics of GaN quantum dot (QD) and GaN quantum well (QW) separated confinement heterostructures (SCHs) with low-Al-content AlGaN waveguide layers is reported. A redshift (blueshift) of QD (wetting layer) emission is found with respect to QW emission, as expected from the thickness hierarchy of these objects. The influence of nonradiative processes on QD emission in QD SCH is dramatically reduced compared to the case of QW SCH. It is concluded that GaN QDs in low-Al-content AlGaN matrix are robust localization centers and that the carrier dynamics is seriously affected by the built-in internal field effect. (c) 2006 American Institute of Physics.
Depth-resolved optical studies of excitonic and phonon-assisted transitions in ZnO epilayers
No full textThe authors report depth-resolved optical properties of excitonic and phonon-assisted transitions in ZnO epilayers by photoluminescence (PL) and cathodoluminescence. A weaker free exciton (FX) emission than its first longitudinal optical phonon replica (FX-1LO) is observed at elevated temperatures (T > 150 K) for interior area, while a stronger FX than FX-1LO is seen at all temperatures for top surface area of the sample. The authors exclude out a possible self-absorption process by PL excited at back surface of the sample. Therefore, the authors conclude that the different intensity ratios of FX and FX-1LO depending on the sample depth are strongly associated with extrinsic features of ZnO. (c) 2006 American Institute of Physics.
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
Get PDF“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
Get PDFWe 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
Get PDFWe 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
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