1,720,970 research outputs found
Coincident electron channeling and cathodoluminescence studies of threading dislocations in GaN
No full textWe combine two scanning electron microscopy techniques to investigate the influence of dislocations on the light emission from nitride semiconductors. Combining electron channeling contrast imaging and cathodoluminescence imaging enables both the structural and luminescence properties of a sample to be investigated without structural damage to the sample. The electron channeling contrast image is very sensitive to distortions of the crystal lattice, resulting in individual threading dislocations appearing as spots with black–white contrast. Dislocations giving rise to nonradiative recombination are observed as black spots in the cathodoluminescence image. Comparison of the images from exactly the same micron-scale region of a sample demonstrates a one-to-one correlation between the presence of single threading dislocations and resolved dark spots in the cathodoluminescence image. In addition, we have also obtained an atomic force microscopy image from the same region of the sample, which confirms that both pure edge dislocations and those with a screw component (i.e., screw and mixed dislocations) act as nonradiative recombination centers for the Si-doped c-plane GaN thin film investigated
Role of defects in ultra-high gain in fast planar tin gallium oxide UV-C photodetector by MBE
Get PDFWe report ultra-high responsivity of epitaxial (SnxGa1-x)2O3 (TGO) Schottky UV-C photodetectors and experimentally identified the source of gain as deep-level defects, supported by first principles calculations. Epitaxial TGO films were grown by plasma-assisted molecular beam epitaxy on (-201) oriented n-type β-Ga2O3 substrates. Fabricated vertical Schottky devices exhibited peak responsivities as high as 3.5×104 A/W at -5 V applied bias under 250 nm illumination with sharp cutoff shorter than 280 nm and fast rise/fall time in milliseconds order. Hyperspectral imaging cathodoluminescence (CL) spectra were examined to find the mid-bandgap defects, the source of this high gain. Irrespective of different tin mole fractions, the TGO epilayer exhibited extra CL peaks at the green band (~2.20 eV) not seen in β-Ga2O3 along with enhancement of the blue emission-band (~2.64 eV) and suppression of the UV emission-band. Based on hybrid functional calculations of the optical emission expected for defects involving Sn in β-Ga2O3, VGa–Sn complexes are proposed as potential defect origins of the observed green and blue emission-bands. Such complexes behave as acceptors that can efficiently trap photogenerated holes and are predicted to be predominantly responsible for the ultra-high photoconductive gain in the Sn-alloyed Ga2O3 devices by means of thermionic emission and electron tunneling. Regenerating the VGa–Sn defect complexes by optimizing the growth techniques, we have demonstrated a planar Schottky UV-C photodetector of the highest peak responsivity
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
GaN_Dislocations_1 EBSD Example data
No full text<p>EBSD Example data from a GaN sample, measured by Naresh Gunasekar. Data conversion by Aimo Winkelmann.</p>
Electron channelling contrast imaging of nitride semiconductor thin films
Get PDFThis work reports on the development of a scanning electron microscopy based technique known as electron channelling contrast imaging for characterising extended defects in nitride semiconductors. Extended defects in nitride semiconductors act as scattering centres for light and charge carriers and thus limit the performance of optoelectronic devices. Developing the capability to rapidly analyse extended defects, namely threading dislocation and stacking fault densities without any sample preparation represents a real step forward in the development of more efficient nitride-based semiconductor devices. In electron channelling contrast imaging, changes in crystallographic orientation, or changes in lattice constant due to local strain, are revealed by changes in grey scale in an image constructed by monitoring the intensity of backscattered electrons as an electron beam is scanned over a suitably oriented sample. Extremely small orientation changes are detectable, enabling small angle tilt and rotation boundaries and extended defects to be imaged. Images with a resolution of tens of nanometres are obtainable with electron channelling contrast imaging. Vertical threading dislocations are revealed as spots with black-white contrast in electron channelling contrast imaging. A simple geometric procedure was developed which exploits the differences observed in the direction of this black-white contrast for screw, edge, and mixed dislocations for two electron channelling contrast images acquired from two symmetrically equivalent crystal planes. By using this procedure, an order of magnitude reduction in the time required to obtain quantitative information on dislocations is envisaged compared to the presently available techniques. The use of electron channelling contrast imaging to reveal and characterise basal plane stacking faults and partial dislocations in m-InGaN thin films and quantifying threading dislocations in InAlN thin films is also demonstrated. Preliminary work on combining electron channelling contrast imaging and cathodoluminescence imaging has been demonstrated for the first time to understand the effect of threading dislocations on light emission characteristics in nitride semiconductors.This work reports on the development of a scanning electron microscopy based technique known as electron channelling contrast imaging for characterising extended defects in nitride semiconductors. Extended defects in nitride semiconductors act as scattering centres for light and charge carriers and thus limit the performance of optoelectronic devices. Developing the capability to rapidly analyse extended defects, namely threading dislocation and stacking fault densities without any sample preparation represents a real step forward in the development of more efficient nitride-based semiconductor devices. In electron channelling contrast imaging, changes in crystallographic orientation, or changes in lattice constant due to local strain, are revealed by changes in grey scale in an image constructed by monitoring the intensity of backscattered electrons as an electron beam is scanned over a suitably oriented sample. Extremely small orientation changes are detectable, enabling small angle tilt and rotation boundaries and extended defects to be imaged. Images with a resolution of tens of nanometres are obtainable with electron channelling contrast imaging. Vertical threading dislocations are revealed as spots with black-white contrast in electron channelling contrast imaging. A simple geometric procedure was developed which exploits the differences observed in the direction of this black-white contrast for screw, edge, and mixed dislocations for two electron channelling contrast images acquired from two symmetrically equivalent crystal planes. By using this procedure, an order of magnitude reduction in the time required to obtain quantitative information on dislocations is envisaged compared to the presently available techniques. The use of electron channelling contrast imaging to reveal and characterise basal plane stacking faults and partial dislocations in m-InGaN thin films and quantifying threading dislocations in InAlN thin films is also demonstrated. Preliminary work on combining electron channelling contrast imaging and cathodoluminescence imaging has been demonstrated for the first time to understand the effect of threading dislocations on light emission characteristics in nitride semiconductors
- …
