101 research outputs found
MOCVD growth mechanisms of ZnO nanorods
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
Strain relaxation by dislocation glide in ZnO/ZnMgO core-shell nanowires
4 pagesInternational audiencePlastic 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
International audienceNanowire-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
Compared growth mechanisms of Zn-polar ZnO nanowires on O-polar ZnO and on sapphire
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
EBSD investigation of SiC for HTR fuel particles
Xavier Bourrat : Present Address ISTO - CNRS-Université d'OrléansInternational audienceElectron back-scattering diffraction (EBSD) can be successfully performed on SiC coatings for HTR fuel particles. EBSD grain maps obtained from thick and thin unirradiated samples are presented, along with pole figures showing textures and a chart showing the distribution of grain aspect ratios. This information is of great interest, and contributes to improving the process parameters and ensuring the reproducibility of coating
Formation and annealing of dislocation loops induced by nitrogen implantation of ZnO
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.Comment: 8 page
Composition Analysis of III-Nitrides at the Nanometer Scale: Comparison of Energy Dispersive X-ray Spectroscopy and Atom Probe Tomography
International audienc
Ultrastructure of macromolecular assemblies contributing to bacterial spore resistance revealed by in situ cryo-electron tomography
Bacterial spores owe their incredible resistance capacities to molecular structures that protect the cell content from external aggressions. Among the determinants of resistance are the quaternary structure of the chromosome and an extracellular shell made of proteinaceous layers (the coat), the assembly of which remains poorly understood. Here, in situ cryo-electron tomography on lamellae generated by cryo-focused ion beam micromachining provides insights into the ultrastructural organization of Bacillus subtilis sporangia. The reconstructed tomograms reveal that early during sporulation, the chromosome in the forespore adopts a toroidal structure harboring 5.5-nm thick fibers. At the same stage, coat proteins at the surface of the forespore form a stack of amorphous or structured layers with distinct electron density, dimensions and organization. By analyzing mutant strains using cryo-electron tomography and transmission electron microscopy on resin sections, we distinguish seven nascent coat regions with different molecular properties, and propose a model for the contribution of coat morphogenetic proteins
Plastid thylakoid architecture optimizes photosynthesis in diatoms
Photosynthesis is a unique process that allows independent colonization of the land by plants and of the oceans by phytoplankton. Although the photosynthesis process is well understood in plants, we are still unlocking the mechanisms evolved by phytoplankton to achieve extremely efficient photosynthesis. Here, we combine biochemical, structural and in vivo physiological studies to unravel the structure of the plastid in diatoms, prominent marine eukaryotes. Biochemical and immunolocalization analyses reveal segregation of photosynthetic complexes in the loosely stacked thylakoid membranes typical of diatoms. Separation of photosystems within subdomains minimizes their physical contacts, as required for improved light utilization. Chloroplast 3D reconstruction and in vivo spectroscopy show that these subdomains are interconnected, ensuring fast equilibration of electron carriers for efficient optimum photosynthesis. Thus, diatoms and plants have converged towards a similar functional distribution of the photosystems although via different thylakoid architectures, which likely evolved independently in the land and the ocean.ISSN:2041-172
Characterization of the Microstructure of HgCdTe with p-Type Doping
International audienc
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