1,721,153 research outputs found
Blue photoluminescence activated by surface states in GaN grown by molecular beam epitaxy
Get PDFWe have studied the broad blue band, which emerges in the photoluminescence(PL)spectrum of c-plane GaN layers after etching in hot H3PO4 and subsequent exposure to air. This band exhibited a 100 meV blueshift with increasing excitation intensity and a thermal quenching with activation energies of 12 and 100 meV. These observations led us to suggest that surface states may be formed on etchedsurfaces and cause bandbending, which leads to a shift in transition energy with excitation. The blue PL is related to transitions from the shallow donors filled with nonequilibrium electrons to the surface states, which capture the photogenerated holes. The observed irreversible bleaching of the blue luminescence may be attributed to the metastable nature of the surface states or to the oxygen desorption
Recombination at surface states in GaN
Get PDFWe have studied radiative and nonradiative recombination at surface states in GaN, including as-grown samples and those treated with acids or bases. The surface states manifested
themselves in two ways: (i) a reversible increase of the photoluminescence (PL) intensity after ultraviolet (UV) illumination in vacuum and (ii) appearance of new PL bands after treatment
with acid or base and subsequent exposure to air. It has been established that the GaN surface physi-sorbs species from air (presumably oxygen) which induce surface states acting as
nonradiative recombination centers. It has been found that nonradiative recombination of photogenerated carriers via surface states comprises more than 70% of the recombination in
some GaN samples. Another type of the surface state, which participates in radiative recombination, has been found in GaN samples with Ga polarity after brief etching of the surface
with hot acid or base and subsequent exposure to air. In such samples, a broad PL band emerges in the blue region of the spectrum at low temperatures. The blue band has been attributed to transitions of photogenerated electrons from donors in the near-surface depletion region to the surface states introduced by the above-mentioned procedure. The changes in the GaN surface
caused by etching were examined by atomic force microscopy. In some samples the blue band appeared even when no evidence of the layer etching was found except for quite shallow etch pits formed at dislocation sites. The emerging blue band can be related to the surface states formed on the a-planes of etch pits
Photoluminescence from structural defects in GaN
Get PDFWe studied optical manifestation of structural defects in unintentionally doped GaN. A series of sharp peaks (labeled
as Yi with i=1,2,y,11) was observed in the low-temperature photoluminescence spectrum in the photon energy range
between 2.6 and 3.46 eV. We attribute the majority of these peaks to excitons bound to yet unidentified structural
defects. A preliminary transmission electron microscopy study in one of the samples exhibiting strong Yi lines revealed
numerous inclusions, presumably inversion domains, which may be responsible for at least some of the Yi lines
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
"Electrical Properties, Reliability Issues, and ESD Robustness of InGaN-Based LEDs" in III-Nitride Based Light Emitting Diodes and Applications
No full textThis chapter describes the physical mechanisms that may induce the degradation of InGaN-based LEDs. Before going into a detailed description of the main degradation processes, we will give a brief overview on the electrical (current-voltage) characteristics of LEDs, to provide the basic information which is useful to understand the electrical characterization data reported in the subsequent part of the chapter. On the basis of literature data, we will analyze the main mechanisms responsible for current conduction, with focus on tunneling mechanisms, reverse-current conduction, and on the origin of the high ideality factors, always present in InGaN LEDs. After this introductory section, we will discuss the main physical mechanisms that may limit the lifetime of GaN-based LEDs submitted to short and long-term operation. More specifically, we will focus on the following mechanisms: (i) the degradation of the blue semiconductor chip, due to the generation of non-radiative recombination centers, and to the degradation of the p-type semiconductor and contacts; (ii) the degradation mechanisms that affect the performance of the package and phosphors, which are typically activated at high temperature levels; and (iii) the failure of LEDs submitted to ElectroStatic Discharges (ESD)
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
- …
