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Search Results: 1 - 10 of 2725 matches for " Guy Feuillet "
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MOCVD growth mechanisms of ZnO nanorods
Guillaume Perillat-Merceroz,Pierre-Henri Jouneau,Guy Feuillet,Robin Thierry,Milan Rosina,Pierre Ferret
Physics , 2011, DOI: 10.1088/1742-6596/209/1/012034
Abstract: ZnO is a promising material for the fabrication of light emitting devices. One approach to achieve this goal is to use ZnO nanorods because of their expected high crystalline and optical quality. Catalyst free growth of nanorods by metalorganic chemical vapour deposition (MOCVD) was carried out on (0001) sapphire substrates. Arrays of well-aligned, vertical nanorods were obtained with uniform lengths and diameters. A thin wetting layer in epitaxy with the sapphire substrate is formed first, followed by pyramids and nanorods. The nucleation of nanorods occurs either directly at the interface, or later on top of some of the pyramids, suggesting various nucleation mechanisms. It is shown that crystal polarity plays a critical role in the growth mechanism with nanorods of Zn polarity and their surrounding pyramids with O polarity. A growth mechanism is proposed to explain that most threading dislocations lie in the wetting layer, with only a few in the pyramids and none in the nanorods.
Structural recovery of ion implanted ZnO nanowires
Guillaume Perillat-Merceroz,Fabrice Donatini,Robin Thierry,Pierre-Henri Jouneau,Pierre Ferret,Guy Feuillet
Physics , 2012, DOI: 10.1063/1.4704697
Abstract: Ion implantation is an interesting method to dope semiconducting materials such as zinc oxide provided that the implantation-induced defects can be subsequently removed. Nitrogen implantation followed by anneals under O2 were carried out on zinc oxide nanowires in the same conditions as in a previous study on bulk ZnO [J. Appl.Phys. 109, 023513 (2011)], allowing a direct comparison of the defect recovery mechanisms. Transmission electron microscopy and cathodoluminescence were carried out to assess the effects of nitrogen implantation and of subsequent anneals on the structural and optical properties of ZnO nanowires. Defect recovery is shown to be more effective in nanowires compared with bulk material due to the proximity of free surfaces. Nevertheless, the optical emission of implanted and annealed nanowires deteriorated compared to as-grown nanowires, as also observed for unimplanted and annealed nanowires. This is tentatively attributed to the dissociation of excitons in the space charge region induced by O2 adsorption on the nanowire surface.
Strain relaxation by dislocation glide in ZnO/ZnMgO core-shell nanowires
Guillaume Perillat-Merceroz,Robin Thierry,Pierre-Henri Jouneau,Pierre Ferret,Guy Feuillet
Physics , 2012, DOI: 10.1063/1.4704927
Abstract: Plastic relaxation of the misfit stress in core-shell semi-conducting nanowires can lead to structural defects, detrimental to applications. Core-shell Zn{0.7}Mg{0.3}O/ZnO quantum well heterostructures were deposited on ZnO nanowires. Strain along the a and c axes of the wurtzite structure is relaxed through the glide of dislocation half-loops from the free surfaces, within pyramidal and prismatic planes. Some half-loops are closed up in the barriers to accommodate the misfit at two consecutive interfaces of the quantum well stack. Dislocations are also observed within the nanowire core: contrary to two-dimensional structures, both the core and the shell can be plastically relaxed.
Core-shell multi-quantum wells in ZnO / ZnMgO nanowires with high optical efficiency at room temperature
Robin Thierry,Guillaume Perillat-Merceroz,Pierre-Henri Jouneau,Pierre Ferret,Guy Feuillet
Physics , 2013, DOI: 10.1088/0957-4484/23/8/085705
Abstract: Nanowire-based light-emitting devices require multi-quantum well heterostructures with high room temperature optical efficiencies. We demonstrate that such efficiencies can be attained through the use of ZnO/Zn(1-x)MgxO core shell quantum well heterostructures grown by metal organic vapour phase epitaxy. Varying the barrier Mg concentration from x=0.15 to x=0.3 leads to the formation of misfit induced dislocations in the multi quantum wells. Correlatively, temperature dependant photoluminescence reveals that the radial well luminescence intensity decreases much less rapidly with increasing temperature for the lower Mg concentration. Indeed, about 54% of the 10K intensity is retained at room temperature with x=0.15, against 2% with x=0.30. Those results open the way to the realization of high optical efficiency nanowire-based light emitting diodes.
Oxygen vacancy and EC - 1 eV electron trap in ZnO
Gauthier Chicot,Pierre Muret,Julien Pernot,J. L. Santailler,Guy Feuillet
Physics , 2014, DOI: 10.1088/0022-3727/47/46/465103
Abstract: Fourier transform deep level transient spectroscopy has been performed between 80 K and 550 K in five n-type ZnO samples grown by different techniques. The capture cross section and ionization energy of four electron traps have been deduced from Arrhenius diagrams. A trap 1 eV below the conduction band edge is systematically observed in the five samples with a large apparent capture cross section for electrons (1.6 $\pm$ 0.4 x 10 - 13 cm2) indicating a donor character. The assignment of this deep level to the oxygen vacancy is discussed on the basis of available theoretical predictions.
Compared growth mechanisms of Zn-polar ZnO nanowires on O-polar ZnO and on sapphire
Guillaume Perillat-Merceroz,Robin Thierry,Pierre-Henri Jouneau,Pierre Ferret,Guy Feuillet
Physics , 2013, DOI: 10.1088/0957-4484/23/12/125702
Abstract: Controlling the growth of zinc oxide nanowires is necessary to optimize the performances of nanowire-based devices such as photovoltaic solar cells, nano-generators, or light-emitting diodes. In this view, we investigate the nucleation and growth mechanisms of ZnO nanowires grown by metalorganic vapor phase epitaxy either on O-polar ZnO or on sapphire substrates. Whatever the substrate, ZnO nanowires are Zn-polar, as demonstrated by convergent beam electron diffraction. For growth on O-polar ZnO substrate, the nanowires are found to sit on O-polar pyramids. As growth proceeds, the inversion domain boundary moves up in order to remain at the top of the O-polar pyramids. For growth on sapphire substrates, the nanowires may also originate from the sapphire / ZnO interface. The presence of atomic steps and the non-polar character of sapphire could be the cause of the Zn-polar crystal nucleation on sapphire, whereas it is proposed that the segregation of aluminum impurities could account for the nucleation of inverted domains for growth on O-polar ZnO.
Formation and annealing of dislocation loops induced by nitrogen implantation of ZnO
Guillaume Perillat-Merceroz,Patrice Gergaud,Pascal Marotel,Stéphane Brochen,Pierre-Henri Jouneau,Guy Feuillet
Physics , 2011, DOI: 10.1063/1.3537917
Abstract: Although zinc oxide is a promising material for the fabrication of short wavelength optoelectronic devices, p-type doping is a step that remains challenging for the realization of diodes. Out of equilibrium methods such as ion implantation are expected to dope ZnO successfully provided that the non-radiative defects introduced by implantation can be annealed out. In this study, ZnO substrates are implanted with nitrogen ions, and the extended defects induced by implantation are studied by transmission electron microscopy and X-ray diffraction (XRD), before and after annealing at 900^{\circ}C. Before annealing, these defects are identified to be dislocation loops lying either in basal planes in high N concentration regions, or in prismatic planes in low N concentration regions, together with linear dislocations. An uniaxial deformation of 0.4% along the c axis, caused by the predominant basal loops, is measured by XRD in the implanted layer. After annealing, prismatic loops disappear while the density of basal loops decreases and their diameter increases. Moreover, dislocation loops disappear completely from the sub-surface region. XRD measurements show a residual deformation of only 0.05% in the implanted and annealed layer. The fact that basal loops are favoured against prismatic ones at high N concentration or high temperature is attributed to a lower stacking fault energy in these conditions. The coalescence of loops and their disappearance in the sub-surface region are ascribed to point defect diffusion. Finally, the electrical and optical properties of nitrogen-implanted ZnO are correlated with the observed structural features.
Two new species of Dilkea subgenus Dilkea (Passifloraceae) from Loreto, Peru
Christian Feuillet
PhytoKeys , 2011, DOI: 10.3897/phytokeys.2.722
Abstract: Two new species of Dilkea subgenus Dilkea (Passifloraceae) are described from Loreto, Peru. Dilkea hebes Feuillet, sp. nov., has leaves with elliptic to oblanceolate blades that are dull adaxially, and spherical fruits with thick walls; D. nitens Feuillet, sp. nov., has leaves with narrow–ovate blades that are shiny adaxially, and fruits with an apical cone and thin walls. Both species have been collected in Loreto (Peru). A key to the species of subgenus Dilkea is provided.
On the flow-level stability of data networks without congestion control: the case of linear networks and upstream trees
Mathieu Feuillet
Computer Science , 2010, DOI: 10.1007/s11134-011-9265-7
Abstract: In this paper, flow models of networks without congestion control are considered. Users generate data transfers according to some Poisson processes and transmit corresponding packet at a fixed rate equal to their access rate until the entire document is received at the destination; some erasure codes are used to make the transmission robust to packet losses. We study the stability of the stochastic process representing the number of active flows in two particular cases: linear networks and upstream trees. For the case of linear networks, we notably use fluid limits and an interesting phenomenon of "time scale separation" occurs. Bounds on the stability region of linear networks are given. For the case of upstream trees, underlying monotonic properties are used. Finally, the asymptotic stability of those processes is analyzed when the access rate of the users decreases to 0. An appropriate scaling is introduced and used to prove that the stability region of those networks is asymptotically maximized.
Orogenesis: Cause of Sedimentary Formations  [PDF]
Guy Guy Berthault
Open Journal of Geology (OJG) , 2013, DOI: 10.4236/ojg.2013.32B005

Experiments on stratification discussed here have revealed the mechanical nature of lamination as well as the role of turbulent current as agent of stratification. They challenge Steno’s principle that superposed strata are successive sedimentary layers. They show that relative chronology should not be referred to as “stages” but as “sequences” of series. The rock formation studied by Lalomov shows that the duration of sedimentation could be considerably shorter than indicated by the Geological Time Scale. The latter scale corresponds to large marine transgressions and regressions that can result from the shift of polar axis following such major orogeneses as the Caledonian, Hercynian and Alpine.

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