1,721,107 research outputs found
Optical spectroscopy for doped diamond layers
No full textThe present chapter reviews the contributions of optical spectroscopies for the quantitative analysis of shallow donor and acceptor impurities in diamond. Absorption, luminescence and photocurrent spectroscopies are essential for the evaluation of doping properties, the control of which is required for achieving the targeted functionalities and performances of diamond electronic devices. The advantage of these powerful methods lies in their non-destructive analytical nature. The basic concepts and representative results obtained on monocrystalline as well as polycrystalline doped diamond films will be discussed
Optical spectroscopy for doped diamond layers
No full textThe present chapter reviews the contributions of optical spectroscopies for the quantitative analysis of shallow donor and acceptor impurities in diamond. Absorption, luminescence and photocurrent spectroscopies are essential for the evaluation of doping properties, the control of which is required for achieving the targeted functionalities and performances of diamond electronic devices. The advantage of these powerful methods lies in their non-destructive analytical nature. The basic concepts and representative results obtained on monocrystalline as well as polycrystalline doped diamond films will be discussed
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
Cathodoluminescence investigation of diffusion and exciton confinement in ZnO/ZnMgO and diamond 12C/ 13C heterostructures
No full textCe travail de thèse porte sur la diffusion des porteurs de charge en excès dans deux semiconducteurs à large bande interdite: l’alliage ZnMgO et le diamant 13C. Il est basé sur l’étude d’hétérostructures ZnMgO/ZnO/ZnMgO et 13C/12C/13C à puits de collecte ZnO ou 12C. Sur leurs sections transverses et avec la résolution nanométrique en excitation par cathodoluminescence (CL), nous avons étudié l’évolution de l’intensité de l’émission issue du puits en ZnO ou 12C en fonction de la distance entre l’impact de l’excitation et le puits. Cela nous a permis de mesurer directement les longueurs de diffusion effectives dans ZnMgO et le diamant.Dans ZnMgO, la valeur de 55 nm à 300 K, mesurée sur section transverse clivée, est proche de celle du matériau massif. Elle correspond à une diffusion mixte excitons/porteurs libres. Avec l’utilisation de lames minces érodées par faisceau d’ions, une diminution de a été observée jusqu’à 8 nm dans les parties les plus fines. Cet effet est attribué aux recombinaisons non radiatives de surface. Les lames minces apparaissent alors d’un grand intérêt pour améliorer la résolution spatiale des images CL.Dans le diamant, la diffusion excitonique à basse température montre une faible dépendance de avec l’énergie incidente des électrons. Cela indique que ≈ 15 µm à 20 K dans le diamant massif 13C. Une diminution de jusqu’à 3,3 µm à 118 K est observée en fonction de la température.Enfin, nous avons mis en évidence la formation de polyexcitons dans le diamant en augmentant la densité des paires électron-trou, soit par la puissance d’excitation, soit par le confinement spatial des excitons dans des puits de diamant 12C de faible épaisseurs.This work focuses on the determination of the carrier diffusion length in two wide bandgap semiconductors: the ternary alloy ZnMgO and diamond. This determination has been achieved by using of ZnMgO/ZnO/ZnMgO and 13C/12C/13C heterostructures containing ZnO or 12C collecting wells. Their transverse section was scanned by CL spectroscopy with a nanometer scale resolution in excitation. The effective excess carrier diffusion length is deduced from the evolution of the well emission intensity with the distance between the excitation impact and the well.In ZnMgO, the value at 300 K is 55 nm, obtained from a cleaved cross section. It is close to the bulk material diffusion and is attributed to a mixed diffusion of excitons/free carriers. A decrease of down to 8 nm is observed in the thinnest portions of cross sections shaped by focused ion beam (FIB). This effect is attributed to non-radiative surface recombinations. These thin slabs appear of great interest to enhance the spatial resolution of CL images.In diamond, the exciton diffusion at 20 K exhibits a slight dependence on the incident electron energy. This indicates that the exciton diffusion length is around 15 µm in 13C bulk diamond. The values decrease down to 3.3 µm at 118 K.Finally, we highlighted the formation of polyexcitons in diamond by increasing the electron-hole pairs density either by the excitation power, or by the spatial confinement of excitons in thin 12C wells
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
Excitons in lamellar boron nitride : study of the hexagonal and rhombohedral phases and of 2D heterostructures.
No full textLe nitrure de bore hexagonal (hBN) est un semiconducteur lamellaire possédant une large bande interdite de type indirect (> 6 eV). L’autre structure lamellaire du nitrure de bore est rhomboédrique (rBN), mais beaucoup moins connue et étudiée. L’objectif de cette thèse est l’étude des excitons à l’origine de la luminescence de ces deux phases mais aussi d’hétérostructures 2D, où le hBN est utilisé en combinaison d’autres matériaux 2D dans des empilements verticaux.L’étude des propriétés des excitons dans hBN est réalisée sur un cristal de référence synthétisé par haute pression et haute température au Japon. L’énergie de liaison des excitons ainsi que le rendement quantique interne du hBN sont quantitativement évalués par spectroscopie de cathodoluminescence. L’anomalie observée entre absorption et luminescence est résolue avec l’identification du rôle respectif des excitons directs et indirects. À forte excitation, l’efficacité de luminescence de hBN décroit, limitée par une annihilation entre excitons, particulièrement efficace dans ce matériau.Associées à une caractérisation structurale, les signatures spectroscopiques de luminescence et Raman de la phase rhomboédrique sont identifiées. Elles ont permis d’analyser les propriétés de couches minces synthétisées par voie chimique (CVD).La dernière partie de cette thèse porte sur l’étude d’hétérostructures 2D de type hBN/MoX2/hBN où X = S ou Se. Une caractérisation exhaustive des propriétés de luminescence, vibrationnelles et structurales est menée sur l’ensemble des matériaux constituants. Les analyses sont menées à la fois en vue plane et en section transverse grâce à la découpe de lames minces par faisceau d’ions focalisés. Des résultats préliminaires sur la diffusion des excitons et sur les excitons d’interface y sont présentés.Hexagonal boron nitride (hBN) is a lamellar wide indirect bandgap semiconductor (> 6 eV). The other lamellar boron nitride phase is rhombohedral (rBN), but much less known and studied. The goal of this thesis is the study of the excitons source of the luminescence of both phases and of 2D heterostructures, where hBN is used in combination with other 2D materials in vertical stacks.The study of hBN excitons properties is performed on a reference sample synthesized by high pressure and high temperature in Japan. Excitons binding energy as well as hBN internal quantum yield are quantitatively assessed by cathodoluminescence spectroscopy. The observed anomaly between absorption and luminescence is resolved thanks to the identification of the role of direct and indirect excitons respectively. At high excitation, hBN luminescence efficiency decreases limited by exciton-exciton annihilation. This phenomenon is especially efficient in this material.Combined with a structural characterization, the Raman and luminescence spectroscopic signature of the rhombohedral phase are identified. This allowed the analysis of the properties of chemically synthesized thin films (CVD).The last part of this thesis is devoted to the study of a 2D heterostructure hBN/MoX2/hBN where X = S or Se. An exhaustive characterization of the luminescence, vibrational and structural properties is carried out on all the components. Analyses are performed in both flat view and cross-section thanks to the cutting of a thin lamella by focused ion beam. Preliminary results on excitons diffusion and interface excitons are presented
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
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