Joint de grains dans le silicium et suite du nombre d'argent

International audienceA scheme is proposed to solve the structure of incommensurate interfaces, starting from high-resolution images of electron microscopy, supplemented by adapted simulation techniques and complemented by theoretical tools. Direct silicon bonding is a way to produce artificial interfaces, in particular incommensurate ones. We focus on a technology-driven tilt grain boundary in silicon. While the Fibonacci sequence, linked to the golden ratio, is a prototype of the quasicrystalline structures, a silver-ratio sequence allows us to analyze this incommensurate interface. The four-fold coordination of the Si atoms is kept at the interface.Une procédure est proposée pour résoudre la structure d'interfaces incommensurables, en partant d'images de microscopie électronique de haute résolution, en complétant avec des techniques de simulation adaptées et en parachevant avec des outils théoriques. Le collage de plaques de silicium est une manière de créer des interfaces artificielles, en particulier de type incommensurable. Nous nous concentrons sur un joint de grains de flexion dans le silicium, joint ayant un intérêt technologique. Alors que la suite de Fibonacci, liée au nombre d'or, est un prototype des structures quasi-cristallines, la suite du nombre d'argent nous permet d'analyser cette interface incommensurable. La tétravalence des atomes de silicium est conservée à l'interface

Quantitative Scanning Transmission Electron Microscopy–High-Angle-Annular Dark-Field Study of the Structure of Pseudo-2D Sb2Te3 Films Grown by (Quasi) Van der Waals Epitaxy

peer reviewedScanning transmission electron microscopy (STEM) techniques are used to improve the understanding of out-of-plane oriented Sb2Te3 thin films deposited by sputtering on SiO2 and Si substrates. Nanobeam precession electron diffraction, energy-dispersive X-ray spectroscopy, and high-angle-annular dark-field imaging show that the presence of 1–2 atomic planes of Te on top of the substrate is a crucial factor for successful growth of such films, which can be achieved by optimizing cosputtering of Te and Sb2Te3 targets. The formation of an actual van der Waals (vdW) gap between the substrate and the first Sb2Te3 quintuple layer allows for vdW epitaxy. This gap is larger than those separating Te planes in the pseudo-2D Sb2Te3 structure. HAADF image analysis provides detailed information on the atomic arrangement such as interplanar distances, vdW gaps, and Debye–Waller coefficients, all these with a few pm precision. For the anisotropic atomic displacements, a new methodology is introduced based on the statistical analysis of atomic column positions that provides information on the low-frequency phonon modes. Ab initio calculations are used to support our results. Overall, this study provides quantitative STEM tools particularly well suited for nonperiodic pseudo-2D materials, such as Sb2Te3/GeTe superlattices

Solving difficult structures with electron diffraction

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Combining 2 nm Spatial Resolution and 0.02% Precision for Deformation Mapping of Semiconductor Specimens in a Transmission Electron Microscope by Precession Electron Diffraction

International audiencePrecession electron diffraction has been used to provide accurate deformation maps of a device structure showing that this technique can provide a spatial resolution of better than 2 nm and a precision of better than 0.02%. The deformation maps have been fitted to simulations that account for thin specimen relaxation. By combining the experimental deformation maps and simulations, we have been able to separate the effects of the stressor and recessed sources and drains and show that the Si3N4 stressor increases the in-plane deformation in the silicon channel from 0.92 to 1.52 +/- 0.02%. In addition, the stress in the deposited Si3N4 film has been calculated from the simulations, which is an important parameter for device design

The addition of strain in uniaxially strained transistors by both SiN contact etch stop layers and recessed SiGe sources and drains

International audienceSiN contact etch stop layers (CESL) and recessed SiGe sources/drains are two uniaxial strain techniques used to boost the charge carriers mobility in p-type metal oxide semiconductor field effect transistors (pMOSFETs). It has already been shown that the electrical performances of the devices can be increased by combining both of these techniques on the same transistor. However, there are few experimental investigations of their additivity from the strain point of view. Here, spatially resolved strain mapping was performed using dark-field electron holography (DFEH) on pMOSFETs transistors strained by SiN CESL and embedded SiGe sources/drains. The influence of both processes on the strain distribution has been investigated independently before the combination was tested. This study was first performed with non-silicided devices. The results indicated that in the channel region, the strain induced by the combination of both processes is equal to the sum of the individual components. Then, the same investigation was performed after Ni-silicidation of the devices. It was found that in spite of a slight reduction of the strain due to the silicidation, the strain additivity is approximately preserved. Finally, it was also shown that DFEH can be a useful technique to characterize the strain field around dislocations

Measurement of pulsatile motion with millisecond resolution by MRI.

We investigated a technique based on phase-contrast cine MRI combined with deconvolution of the phase shift waveforms to measure rapidly varying pulsatile motion waveforms. The technique does not require steady-state displacement during motion encoding. Simulations and experiments were performed in porcine liver samples in view of a specific application, namely the observation of transient displacements induced by acoustic radiation force. Simulations illustrate the advantages and shortcomings of the methods. For experimental validation, the waveforms were acquired with an ultrafast ultrasound scanner (Supersonic Imagine Aixplorer), and the rates of decay of the waveforms (relaxation time) were compared. With bipolar motion-encoding gradient of 8.4 ms, the method was able to measure displacement waveforms with a temporal resolution of 1 ms over a time course of 40 ms. Reasonable agreement was found between the rate of decay of the waveforms measured in ultrasound (2.8 ms) and in MRI (2.7-3.3 ms)

Non-radiative recombination centres in InGaN/GaN nanowires revealed by statistical analysis of cathodoluminescence intensity maps and electron microscopy

International audienceAbstract The methodology of statistical analysis of cathodoluminescence intensity mappings on ensembles of several hundreds of InGaN/GaN nanowires used to quantify non-radiative recombination centres was validated on InGaN/GaN nanowires exhibiting spatially homogeneous cathodoluminescence at the scale of single nanowires. Cathodoluminescence intensity variations obeying Poisson's statistics were assigned to the presence of randomly incorporated point defects acting as non-radiative recombination centres. Additionally, another type of non-radiative recombination centres, namely extended defects leading to spatially inhomogeneous cathodoluminescence intensity at the scale of single InGaN/GaN nanowires are revealed by high resolution scanning transmission electron microscopy, geometrical phase analysis and two-beam diffraction conditions techniques. Such defects are responsible for deviations from Poissons’s statistics, allowing one to achieve a rapid evaluation of the crystallographic and optical properties of several hundreds of nanowires in a single cathodoluminescence intensity mapping experiment

Influence of milling on structural and microstructural properties of cerium oxide: Consequence of the surface activation on the dissolution kinetics in nitric acid

International audienceCeria (CeO2) is known as a refractory oxide for dissolution in nitric acid, since the leaching reaction is thermodynamically unfavorable, except when it is complexed by nitrates but with very slow kinetics. To enhance dissolution, surface activation was achieved using high-energy milling. With the mechanically-activated cerium oxide, leaching in nitric acid reached 36%. The mechanical activation of the solid caused structural and microstructural changes (particle size, specific surface area, crystallite size, lattice strain, defects…). After one hour, the cleavage induced by energetic milling generated two populations: nanoparticles and grains containing defects like dislocations. Beside crystallite size and micro-strain evaluation using X-ray diffraction, cerium oxidation state was measured by Electron Energy-Loss Spectroscopy (EELS) analyses while linear defects were pictured by Transmission Electron Microscopy (TEM) observations. On one hand, it was found that the nanoparticles formed during milling process greatly enhance the dissolution reaction by the creation of Ce3+ thin layers of a few nanometer depth on their surfaces. On the other hand, it is shown that dislocations represent another way to increase the kinetics by activation energy. In conclusion, dissolution rate's growth can be due to different parameters like the leaching of the smallest particles, the presence of reduced oxidation state on nanoparticles and some highly reactive sites concentrating structural defects such as dislocation nodes. Finally, as ceria is also well known to be a safe analogue of PuO2, especially for dissolution studies, a solution for improving the dissolution of ceria would probably also be useful for dissolving the oxides rich in Pu

High-precision deformation mapping in finFET transistors with two nanometre spatial resolution by precession electron diffraction

International audienc

Attribution of the 3.45 eV GaN nanowires luminescence to inversion domain boundaries

International audienceUsing correlated experiments on single nanowires (NWs) by microphotoluminescence (mu-PL) and high resolution scanning transmission electron microscopy, we attribute the 3.45 eV luminescence of GaN NWs grown by plasma assisted molecular beam epitaxy (PA-MBE) to the presence of prismatic inversion domain boundaries (pIDBs). This attribution is further strengthened by a recent publication demonstrating the observation of pIDBs in PA-MBE grown GaN NWs. A statistical study of the presence of 3.45 eV lines in NWs PL spectra allows to estimate the ratio of single NWs nucleating with a pIDB to be 50% in the sample under scrutiny. (C) 2015 AIP Publishing LLC