78 research outputs found

    PEE7: THE BURDEN OF AGE-RELATED MACULAR DEGENERATION—RESULTS OF A COHORT STUDY IN TWO FRENCH REFERRAL CARE CENTERS

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    ÉTUDE PAR ELLIPSOMÉTRIE DE FILMS PHYSISORBÉS SUR GRAPHITE

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    L'adsorption du Xénon sur la face de base (0001) du graphite, dans le domaine de la monocouche atomique et plus (cinq) est étudiée par Ellipsométrie et les résultats sont comparés à ceux obtenus par Volumétrie, D. E. L. et diffusion Auger. Nous avons testé plusieurs approches théoriques combinant l'effet ellipsométrique et le degré de recouvrement. En employant deux grandeurs expérimentales connues, la polarisabilité de l'atome libre de Xénon, la distance proche voisine Xe-Xe, nous avons calculé les constantes diélectriques de la couche adsorbée. Un bon accord est obtenu entre les valeurs observées et calculées avec la théorie macroscopique ellipsométrique et la théorie de Sivukhin (2e ordre).Xenon's adsorption on the basal face (0001) of graphite, in the monoatomic layer range and more (five) was studied by means of ellipsometry and the results were compared with volumetric adsorption, LEED and Auger diffusion measurements. We have tested several theorical approaches relating the ellipsometric effect to surface coverage. Using two experimentally known quantities, the polarizability of a free xenon atom, and the nearest-neighbour distance Xe-Xe, we have calculated the dielectric constants for the adsorbed layer. Good agreement was obtained, observed and calculated values with macroscopic ellipsometric theory and Sivukhin's microscopic theory (2nd order)

    ELLIPSOMETRIC STUDY OF THE CLEAVED GERMANIUM (111) SURFACE AS A FUNCTION OF TEMPERATURE

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    La mĂ©thode ellipsomĂ©trique, relativement facile Ă  utiliser, est trĂšs sensible Ă  toute modification et perturbation pouvant se produire sur une surface cristalline. Dans le cas des semiconducteurs (Ge,Si), la perturbation de la surface peut avoir diffĂ©rentes origines. Typiquement, les effets de tempĂ©rature conduisent Ă  des modifications de structure de surface (reconstruction) de Ge (111) 2 x 8 (ou Si (111) 7 x 7). Nous avons Ă©tudiĂ© optiquement la transition de surface 2 x 1 → 2 x 8 du Germanium en fonction de la tempĂ©rature (300 K<T<700 K). D'une part, nous avons montrĂ© que la variation avec la tempĂ©rature de l'Ă©nergie de la bande interdite Eg a une forte influence sur les constantes optiques de surface des semiconducteurs. Le bon accord entre les mesures ellipsomĂ©triques effectuĂ©es en fonction de la tempĂ©rature et celles obtenues par ailleurs Ă  longueur d'onde variable, montre que les transitions optiques correspondent aux mĂȘmes bandes d'Ă©nergie dans le domaine de tempĂ©rature considĂ©rĂ©. D'autre part, des mesures optiques fines concernant la transition 2 x 1 → 2 x 8 ont Ă©tĂ© interprĂ©tĂ©es en fonction de la modification de la constante diĂ©lectrique d'une couche de transition. Cette variation, transposĂ©e Ă  la conductivitĂ© Ă©lectrique de surface, a Ă©tĂ© comparĂ©e Ă  la variation dĂ©terminĂ©e lors de mesures Ă©lectriques statiques.The ellipsometric method, relatively easy to handle, is very sensitive to any modification and perturbation which may occur on a crystalline surface. In the case of semiconductors (Ge, Si), the surface perturbation may have different origins. Typically, temperature effects lead to surface structural modifications (reconstruction) of Ge (111) 2 x 8 (or Si (111) 7 x 7). We have studied optically the 2 x 1 → 2 x 8 surface transition of Germanium as a function of temperature (300 K<T<700 K). On the one hand we have shown that the variation with temperature of the Gap energy Eg has strong influence on the surface optical constants of the semiconductors. The good agreement between the ellipsometric measurements performed as a function of temperature and those obtained elsewhere with variable wavelengths show that the optical transitions give the same energy bands in the temperature domain investigated. On the other hand fine optical measurements concerning the 2 x 1 → 2 x 8 transition have been interpreted in terms of the modification of the dielectric constant of a transition layer. This variation transposed to surface electrical conductivity has been compared with the variation determined upon static electrical measurements

    Experimental results from spectroscopic ellipsometry on the (7 X 7)SiO 11) surface reconstruction: dielectric function determination

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    International audienceThe dielectric function of the (7 X 7) Si(l11) surface has been directly determined in UHV conditions by ellipso~etric measurements on a Si(111) clean surface. The dielectric function obtained from ellipsometric spectra near the pseudo-Brew-ster angle (PB) has been calculated by means of a three-phase model (bulk, (7 X 7) transition layer and vacuum) in which optical properties of bulk Si are deduced from literature data and the thickness of the (7 X 7) transition layer is taken from the DAS model. Our calculation allows us to confirm the metallic character of the (7 X 7) transition layer and to point out the previously found surface-state transitions at 2.6 and 3.4 eV and perhaps at 1.6 eV. Furthermore, a surface-state transition has also been confirmed at 3 eV. Ellipsometric results depend on the crystallographic direction. This optical anisotropy is ascribed to some roughness anisotropy induced by steps
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