Optical and x-ray diffraction studies on the incorporation of carbon as a dopant in cubic GaN

We performed optical and x-ray diffraction experiments on carbon doped cubic-GaN samples, deposited by plasma-assisted molecular beam epitaxy on (001) GaAs substrates, for various carbon concentrations. The samples were studied by Raman, photoluminescence, and photoluminescence excitation spectroscopies. These techniques give some insight into the mechanism of carbon incorporation in the material. Detailed analysis of these spectra leads to a picture in which carbon initially enters into N vacancies producing a marked improvement in the crystalline properties of the material. At higher concentrations it also begins to enter interstitially and form C complexes, with a consequent decrease of crystalline quality. This increase and later decrease of crystalline quality of our samples with the addition of C were also detectable in x-ray diffraction scans. A model calculation of the localized vibrations of the C atom in the GaN lattice allows for the interpretation of a feature in the Raman spectrum of some samples, which reinforces this view.681

Near band-edge optical properties of cubic GaN

We used photoluminescence, photoluminescence excitation spectroscopy (PLE) and photoreflectance (PR) to study the optical properties of thin films of cubic GaN, deposited by plasma-assisted molecular beam epitaxy on a GaAs (001) substrate. Our results show a clear step-like absorption edge, resulting from the merging of the free exciton with the continuum. Quantitative values for the absorption-edge energy and lifetime broadening are obtained. The dependence of the latter on temperature, as well as some features of the PR spectrum, reveal that the cubic material still presents residual strain and distortions. A secondary absorption-edge due to hexagonal inclusions is also observed in the PLE spectra. (C) 2003 Elsevier Science Ltd. All rights reserved.1254173220520

Photoreflectance studies of optical transitions in cubic GaN grown on GaAs(001) substrates

The optical properties of cubic GaN epitaxial layers were investigated by modulated photoreflectance (PR) and photoluminescence in the temperature interval from 5 to 300 K. The epilayers were grown on GaAs(001) substrates by molecular beam epitaxy using a nitrogen RIF-activated plasma source. The PR spectra show a transition which is well fitted using the third-derivative functional form of the unperturbed dielectric function, which we interpret as band-to-band transition. Our results allow determination of the temperature dependence of the main gap of c-GaN and give insights into the residual strain in the film, as well as allow us to estimate the binding energy of the complex formed by an exciton bound to a neutral acceptor. (C) 2003 Elsevier Science B.V. All rights reserved.2524169920821

Nuclear high-ionisation outflow in the Compton-thick AGN NGC 6552 as seen by the JWST mid-infrared instrument

status: publishe

JWST MIRI/MRS in-flight absolute flux calibration and tailored fringe correction for unresolved sources

status: publishe

Incorporação das ciências sociais na produção de conhecimentos sobre trabalho e saúde Incorporation of the social sciences in the production of knowledge about work and health

Este artigo apresenta uma revisão bibliográfica sobre a influência das ciências sociais para a superação de concepções reducionistas de relação trabalho-saúde, nas duas últimas décadas. Trata-se de um tipo de diagnóstico da produção científica, no qual destacam-se: as contribuições para a caracterização da saúde do trabalhador, como campo de conhecimento e de intervenção, e para a análise da política e das práticas das instituições públicas; as abordagens compreensivas e as questões de gênero. Efetuou-se um levantamento de artigos de periódicos indexados e de dissertações e teses de pós-graduação. Foram consultados: o banco de dissertações e teses da Coordenação de Aperfeiçoamento do Ensino Superior e do Instituto Brasileiro de Informação em Ciência e Tecnologia; a Biblioteca Virtual em Saúde da Biblioteca Regional de Medicina e o site do Scientific Electronic Library On-line. Constatou-se a predominância de estudos sobre temáticas específicas e determinadas categorias de trabalhadores, em contraposição a tentativas de abordagens totalizadoras. Apesar dos notáveis avanços em termos de conhecimento, existe carência significativa de investigações sobre segmentos da população trabalhadora que apresentam maior vulnerabilidade social.<br>This article presents a bibliographical revision on the influence of the social sciences to overcome the reducing conceptions of relationship work-health, in the last two decades. It is a type of diagnosis of the scientific production, in which outstanding aspects are: the contributions for the characterization of the worker’s health, as knowledge field and of intervention, and for the analysis of the politics and of the practices of the public institutions; the comprehensive approaches and gender questions. It was conducted an assessment of indexed journals and masters degree dissertations and doctoral thesis. It was consulted: the bank of dissertations and thesis organized by the Coordination of Improvement of the Higher Education and for the Brazilian Institute of Information in Science and Technology; the Virtual Library in Health of the Regional Library of Medicine and Scientific Electronic site On-line Library. It was verified the predominance of studies on specific themes and certain categories of workers, in opposition to attempts of totalizing approaches. In spite of the outstanding progresses in knowledge terms and of enlargement of study objects, significant lack of investigations exists on segments of the working population that present larger social vulnerability