Strain, size and composition of InAs Quantum Sticks, embedded in InP, by means of Grazing Incidence X-ray Anomalous Diffraction

We have used x-ray anomalous diffraction to extract the x-ray structure factor of InAs quantum stick-like islands, embedded in InP. The average height of the quantum sticks (QSs), as deduced from the width of the structure factor profile is 2.54nm. The InAs out of plane deformation, relative to InP, is equal to 6.1%. Diffraction Anomalous Fine Structure provides a clear evidence of pure InAs QSs. Finite Difference Method calculations reproduce well the diffraction data, and give the strain along the growth direction. Chemical mixing at interfaces is at most of 1MLComment: 9 pages, 7 figures, submitte

Abrupt GaP/Si hetero-interface using bistepped Si buffer

We evidence the influence of the quality of the starting Si surface on the III-V/Si interface abruptness and on the formation of defects during the growth of III-V/Si heterogeneous crystal, using high resolution transmission electron microscopy and scanning transmission electron microscopy. GaP layers were grown by molecular beam epitaxy on vicinal Si (001). The strong effect of the Si substrate chemical preparation is first demonstrated by studying structural properties of both Si homoepitaxial layer and GaP/Si heterostructure. It is then shown that choosing adequate chemical preparation conditions and subsequent III-V regrowth conditions enables the quasi-suppression of micro-twins in the epilayer. Finally, the abruptness of GaP/Si interface is found to be very sensitive to the Si chemical preparation and is improved by the use of a bistepped Si buffer prior to III-V overgrowth

Structural and morphological evolution of Co on faceted Pt/W(111) surface upon thermal annealing

International audienceThe structural and morphological changes of a 1.1 monolayer (ML) Pt deposit on W(1 1 1) have been investigated in situ, in ultra-high vacuum, as a function of the annealing temperature from 700 to 1340 K, by a combination of grazing incidence X-ray diffraction and grazing incidence small-angle X-ray scattering. Before annealing, the thin Pt layer is two-dimensional and lattice-matched to the W(1 1 1) surface. The faceting of Pt/W(1 1 1) towards nanoscale three-sided pyramids with {2 1 1} facets has been detected from 715 K. At this stage, the pyramids, which have a 5-nm average lateral size, cover nearly perfectly the surface. At higher temperatures, they increase in size. The role of the edge energy in the nanofaceting process is discussed. In addition, 4 MLs Co are deposited at room temperature on the smallest Pt/W pyramids. The obtained three-dimensional Co islands are correlated with the Pt/W nanopyramids and Co is relaxed on Pt/W. At approximately 800 K, a CoPt alloy is formed and becomes better ordered as the annealing temperature increases. At 1100 K, both defaceting and phase separation begin; the CoPt alloy segregates on the W(1 1 1) flat surface, while Co forms an epitaxial layer on the {2 1 1} facets. In addition, in the temperature range of 1100–1200 K, a great majority of {2 1 1} large facets coexist with some {1 1 0} small facets. Finally, the surface becomes flat again at 1250 K

Capping of InAs quantum dots grown on (311)B InP studied by cross-sectional scanning tunneling microscopy

Cross-sectional scanning tunneling microscopy was used to study at the atomic scale the impact of the capping material on the structural properties of self-assembled InAs quantum dots (QDs) grown on a high index (311)B InP substrate. Important differences were found in the capping process when InP or lattice matched InGaAs(P) alloys are used. The QDs capped with InP have a smaller height due to As/P exchange induced decomposition. This effect is not present when InGaAs is used as the capping material. However, in this case a strong strain driven phase separation appears, creating In rich regions above the QDs and degrading the dot/capping layer interface. If the InAs dots are capped by the quaternary alloy InGaAsP the phase separation is much weaker as compared to capping with InGaAs and well defined interfaces are obtained. ©2006 American Institute of Physic

Self-Organized Growth of Nanoparticles on a Surface Patterned by a Buried Dislocation Network

International audienceThe self-organized growth of Co nanoparticles is achieved at room temperature on an inhomogenously strained Ag(001) surface arising from an underlying square misfit dislocation network of 10 nm periodicity buried at the interface between a 5 nm-thick Ag film and a MgO(001) substrate. This is revealed by in situ grazing-incidence small-angle x-ray scattering. Simulations of the data performed in the distorted wave Born approximation framework demonstrate that the Co clusters grow above the dislocation crossing lines. This is confirmed by molecular dynamic simulations indicating preferential Co adsorption on tensile sites

Nitrogen-related intermediate band in P-rich GaNxPyAs1−x−y alloys

Abstract The electronic band structure of phosphorus-rich GaNxPyAs1−x−y alloys (x ~ 0.025 and y ≥ 0.6) is studied experimentally using optical absorption, photomodulated transmission, contactless electroreflectance, and photoluminescence. It is shown that incorporation of a few percent of N atoms has a drastic effect on the electronic structure of the alloys. The change of the electronic band structure is very well described by the band anticrossing (BAC) model in which localized nitrogen states interact with the extended states of the conduction band of GaAsP host. The BAC interaction results in the formation of a narrow intermediate band (E− band in BAC model) with the minimum at the Γ point of the Brillouin zone resulting in a change of the nature of the fundamental band gap from indirect to direct. The splitting of the conduction band by the BAC interaction is further confirmed by a direct observation of the optical transitions to the E+ band using contactless electroreflectance spectroscopy

Manger la chair de son ennemi

International audienceThe goal of this article is to study, through the corpus of Homeric formulas, what might be called a “Homeric Encyclopedia” of food. Most of the formulas provide us with a coherent image of mankind eating bread and cooked animal meat, as opposed to animals eating human flesh and to gods eating nectar and ambrosia. However, some troubling passages show that Achilles, face-to-face with Hector, wishes that he could eat him. Moreover, queen Hecuba in book 24 wishes that she could eat Achilles’ liver. A single passage in the first half of the Iliad shows a similar vow, perhaps ironically attributed by Zeus to Hera, with the rare form βεβρώθοις. We link these passages to two others, where a weapon, namely a spear, is said to be eager for (human) flesh. Though not as common in the texts as the more usual formulas, these forms take part in the formulaic corpus, and evoke a more savage image of the Homeric world, where the desire to eat one’s enemy is not masked by the usual linguistic conventions.À partir des formules homériques se dessine une “Encyclopédie homérique” de la nourriture. La plupart des formules montrent une image cohérente des humains mangeant du pain et de la viande cuite, en opposition aux animaux nourris de chair humaine et aux dieux nourris de nectar et d’ambroisie. Pourtant, certains passages troublants montrent qu’Achille face à Hector fait le vœu de le manger cru. Hécube dans le chant 24 souhaite de manger cru le foie d’Achille. Dans la première partie de l’Iliade, un seul passage montre un vœu comparable, peut-être ironiquement attribué à Héra par Zeus, avec la forme rare βεβρώθοις. Nous relions ces passages à d’autres dans lesquels une arme, une lance, est dite être avide de chair humaine. Même si elles ne sont pas aussi fréquentes dans le texte que celles du premier groupe, ces formes font partie du corpus formulaire, et entraînent une image plus sauvage du monde homérique, dans laquelle le désir de manger son ennemi n’est pas masqué sous les conventions usuelles du langage