1,721,152 research outputs found
Electronic properties of hydrogen exposed III-V semiconductor surfaces
No full textA critical review of experimental results aiming at determining the atomic geometry and the electronic properties of the hydrogenated (110) surfaces of III-V semiconductor compounds is given. Results deal mainly with the prototype GaAs(110) surface. Experimental results include photoemission, electron energy loss, metastable deexcitation spectroscopy, Auger electron spectroscopy, photoelectron diffraction and grazing incidence X-ray diffraction data. A unified picture for the hydrogenated surface can be derived. It is characterized by an almost nonrelaxed substrate (or even counter-relaxed), absence of gap states and, up to one monolayer of coverage, by a surface order and an almost preserved substrate stoichiometry. Higher exposures induce surface etching with changes of Ga to As concentration ratio
Metastable He deexcitation at semiconductor interfaces
No full textA review is given of the application of metastable deexcitation spectroscopy (MDS) to the study of the interface formation between semiconductors and different materials. In particular we present an overview of the results obtained on nanostructured interfaces, where strain and reaction between the substrate and the overlayer atoms drive the growth mode and the morphology of the system. As prototypical examples we discuss the growth of CaF, on silicon and rare earths (Yb, Er) on silicon and gallium arsenide. The mechanisms and chemical reactions which bring to interface formation are examined on the basis of MDS results and their comparison with photoemission
Dielectric properties of the Si(111)2×1 surface: Optical constants and the energy-loss spectrum
No full textWe present a method of computation of the optical constants of the surface layer of a solid, starting from a differential reflectivity spectrum. The differential reflectivity was obtained by comparing the overall reflectivity of the same sample in the case of a clean and oxidized surface. The method assumes sharp interfaces at the vacuum surface and at the surface bulk side. The physical properties of the bulk and of the surface layer are described by an energy-dependent dielectric constant. This method is applied to the Si(111)2×1 surface. The surface optical constants are derived in the (0.3-4.0)-eV range. With the use of the same model and the calculated optical constants, the electron-energy-loss spectrum of the Si(111)2×1 surface has been calculated. The agreement with the experiment is good. This is assumed as a proof of the reliability of the optical constants. In addition the comparison of the calculated energy-loss spectrum and the experiment allows the explanation of the apparent disagreement between optical and energy-loss experimental data in the near-infrared range. We demonstrate the different nature between the energy-loss peak and the optical one, the first due to the excitation of the two surface interface modes of the Si(111)2×1 surface layer and the second to an interband transition
FISICA A
No full textIl testo rappresenta un manuale di introduzione alla Fisica classica per studenti di corsi di laurea scientifici di primo livello, ingegneria in particolare. Esso presenta una quadro essenziale ma completo, che partendo dai fondamenti, copre le aree della meccanica e dell'elettromagnetismo classici. Partendo dalla scrittura dell’equazione di Newton, introducendo l’origine delle forze ed a seguire fornendo il concetto di campo vengono man mano composti gli strumenti concettuali e formali con cui si imposta e si risolve il problema del moto sotto l’azione di forze. Lungo tale costruzione, risalto viene dato alle proprietà dei campi in quanto tali, giungendo alla scrittura delle equazioni di Maxwell in forma locale
Efficiency of gratings in the conical diffraction mounting for an EUV time-compensated monochromatorProceedings of SPIE
No full textRecent measurements of the absolute diffraction efficiency of plane gratings in the conical diffraction mounting (in which the light approaches the grating in the plane parallel to the direction of the grooves) are presented. Three gratings have been tested at the beamline BEAR (Elettra Synchrotron, Trieste) in the 10-130 nm region, showing a peak efficiency as high as 70%. The aim of these measurements is the use of two gratings in the conical diffraction mounting for the realization of a high-throughput time-compensated monochromator for the spectral selection of high-order harmonic radiation produced by the interaction between an ultrashort laser pulse (less than 100 fs) and a gas jet. The monochromatic and ultrashort pulse at the monochromator exit can be used for the injection of a Free Electron Laser. The theory of the time-compensation with gratings will be briefly resumed, the design of the monochromator will be presented, and the results of the measurements at BEAR will be discussed
Surface electronic properties by metastable deexcitation spectroscopy
No full textMetastable deexcitation spectroscopy (MDS) is used to probe the surface electronic properties of semiconductor surfaces and different interfaces with semiconductors. Results are compared to photoemission data and calculations. The following case studies are treated: clean GaAs(110), clean Ge(111)c(2 x 8), Sb/GaAs(110), H:GaAs(110), Yb/GaAs(110), Yb/Si(100) and Ge(111) surface incomplete melting. For each system, MDS spectral features are related to surface valence band (VB) states
Performances and stability of Se/Si multilayers with barrier layers for wavelengths around 46 nm
No full textWe present an experimental study of aging and thermal stability of Sc/Si multilayers deposited by magnetron sputtering. These multilayers have been characterized by using hard X-ray grazing incidence reflectometry at 0.154 nm and synchrotron radiation reflectometry at near normal incidence. The reflectivity was found to be stable after one year. A maximum reflectivity of 46% has been measured at 46 nm. However a 20% relative decrease of the reflectivity have been observed after one hour thermal annealing at 200°C. In order to improve thermal stability, we studied two different barriers layers (B4C and ScN). We compare the decrease of peak reflectivity and its wavelength shift after one hour annealing at 200°C under argon atmosphere. The best result was observed with the design using 0.3 nm B4C barrier layers. A relative decrease of 2% of the reflectivity peak has been observed with this design as compared to a 20% decrease without barrier layers
Optical properties of the clean and slowly oxidized surface of silicon
No full textThe optical absorption of an UHV cleaved (111) surface of Si has been investigated as a function of the exposure to oxygen. Data concerning the disappearing of surface states with oxidation are reported. The sticking coefficient for oxygen of a clean surface of Si is calculated from optical absorption data
Hydrogen chemisorption on III-V semiconductor surfaces
No full textA review of the experimental data and theoretical results dealing with the fundamental aspects of the hydrogenation of III-V semiconductor surfaces is presented. The collected material covers the production available in the specialised literature in this field over the last 30 years. The whole body of surface science experimental and theoretical tools were exploited. According to the investigated physical properties the paper is divided into six sections including experimental, surface atomic geometry, vibrational properties, electronic properties and adsorption and desorption. The most extended part of the collected material deals with the (100) and (110) GaAs surfaces, InP surfaces were extensively studied as well, though at a reduced extent. As a general result the hydrogenation of GaAs can be taken as a case study and the consequent picture of general validity in the interpretation of the hydrogenation of all the III-V surfaces. (C) 2003 Elsevier Science B.V. All rights reserved
Effect of an Electric Field on the Optical Gap of Ge(111) 2 × 1 Surface States
No full textResults of electric field modulation of internal reflections in cleaved Ge(111) 2 × 1 surfaces are presented. They can be interpreted on the basis of a buckled model of the reconstructed surface with raised (lowered) atoms negatively (positively) charged. The value of the force constant for normal displacement of surface ions (1.1 × 105 dyn cm-1) is estimate
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