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The very first steps of the formation of O- and Zn-polar ZnO nanowires grown by chemical bath deposition are studied using in situ synchrotron-based XRD with ex situ electron microscopy, casting a new light on the physicochemical processes at work.

International audience; We perform quantitative analysis of the X-ray absorption data taken in situ during the earliest cycles of the ZnO atomic layer deposition on atomically flat InGaAs (001) surfaces. As deposition progresses, we observe a transition from an amorphous structure to a nanocrystalline one. The former retains much of the characteristics of a ZnO crystal in the Zn coordination shell, while the latter shows atomic ordering up to at least the third neighbor shell of Zn atoms, despite the absence of Bragg X-ray diffraction peaks. We show that the different chemical preparation of the substrate surface affects the ZnO local structure and that, counterintuitively, a stronger short-range order is obtained in the nanostructures characterized by lower local order at the interface. We propose a model that accounts for these findings.

In the present study we combined, in the same synchrotron x-ray experiment, reciprocal space mapping, multiwavelength anomalous diffraction and diffraction anomalous fine structure, to determine the strain, crystallographic polarity, alloy composition and ordering at the atomic scale in [0001]-oriented AlGaN nanowires grown by molecular beam epitaxy on GaN nanowire bases. The information that we obtained was averaged over a macroscopic ensemble of NWs. We found from the diffraction anomalous fine structure that there were an isotropic increased number of Ga-Ga pairs in the Ga next nearest coordination shell (cation sublattice) with respect to what is expected for the AlGaN alloy composition determined by anomalous diffraction. This significant deviation from random alloy atomic distribution is present whatever the AlN molar fraction and growth conditions. Our results are consistent with nanoscale composition fluctuations expected from both alloy disorder or kinematically driven spontaneous ordering, both effects being suspected to account for the physical properties of AlGaN ternary alloys., MB was supported by GANEX laboratory of excellence (ANR-11-LABX-0014). Financial support for this work by ANR project ANR-15-CE24-0006 is gratefully acknowledged. We thank the French CRG and SOLEIL commitees for beamtime allocation (proposal number 20151044) and facilities placed at our disposal at beamline BM2-D2AM, and in particular N Boudet and N Blanc, for their help with the experimental setup. MGP acknowledges the Spanish MICINN Grants No. MAT2015-66726-R, and DGA Grant No. E10-17D and Fondo Social Europeo.

International audience; We report on the initial stages of ZnO Atomic Layer Deposition (ALD) on In 0.53 Ga 0.47 As (InGaAs), studied by monitoring the ZnO film thickness in situ with spectroscopic ellipsometry. Using diethylzinc (DEZn) and water, at a substrate temperature equal to 120 • C, we found the presence of two different ZnO growth regimes prior to the steady growth: a slow ZnO nucleation on InGaAs, 0.005 nm.cy −1 (growth delay), then a substrate inhibited growth of type II. Increasing the DEZn injection time, the growth delay shortened from 30 cycles down to 3 cycles, concomitantly the steady growth rate increased from 0.18 to 0.23 nm.cy −1. The DEZn residence time and pressure increase during the first ALD cycle, allowing to suppress the growth delay, instead, no change is observed when performing the same experiment with water. Atomic Force Microscopy (AFM) images showed that the InGaAs surface roughened after the first cycle with a long DEZn pulse and residence time. The rough surface is likely at the origin of the growth delay elimination.

TiO 2 Atomic Layer Deposition (ALD) is used in microelectronics due to its ability to produce conformal thin films whose thickness is controlled at the sub-nanometer scale. Tetrakis(dimethylamido)titanium(IV) and water are frequently used precursors for TiO 2 ALD, although there are still some differences in growth behavior in the literature. In this parametric study, the growth of TiO 2 was controlled in situ by Multi-Wavelength Ellipsometry in combination with ex situ thickness measurements by X-ray Reflectometry. The injection and purge times were optimized to reach self-saturation on the surface. We put in evidence two regions of the Growth Per Cycle (GPC) as a function of substrate temperature: GPC decreases from 0.8 A cy − 1 to 0.5 A cy − 1 as the temperature rises from 50 °C to 200 °C, and then GPC increases from 0.6 A cy − 1 to 0.9 A cy − 1 as the temperature rises from 250 °C to 350 °C. There is no evidence of an ALD window - the temperature region where the growth rate is the same. The stoichiometry of the layer grown at 200 °C, determined by X-ray Photoelectron Spectroscopy, is TiO 2 (1 Ti atom per 2 O atoms). The 200 °C as-grown sample becomes crystalline (Anatase crystals) after annealing 3 h in air at 400 °C, or at 600 °C. The initial growth stages of TiO 2 were studied by Multi-Wavelength Ellipsometry and Atomic Force Microscopy. Regarding TiO 2 ALD on (100)Si substrate (with a native SiO 2 ), we observed a substrate enhanced growth which turned into steady-state ALD beyond 20–30 cycles. Additionally, TiO 2 ALD on (001)In 0.53 Ga 0.47 As substrate demonstrates substrate inhibited growth of type 2 (islands formation) which turned into TiO 2 steady-state ALD beyond 20–30 cycles. The results of TiO 2 ALD are compared with those of the existing literature.

International audience; This study reports the influence of the magnesium incorporation into cuprous oxide (Cu2O) on its transformation in cupric oxide (CuO). Thermal treatments under oxidizing conditions are performed on undoped and magnesium-doped cuprous oxide thin films, Cu2O and Cu2O:Mg respectively, deposited by aerosol-assisted metal organic chemical vapour deposition. The oxidation kinetics of these films shows a slower rate in the Cu2O:Mg system, since the complete oxidation into CuO occurs at a higher temperature when compared to undoped Cu2O. The increased stability of Cu2O:Mg can be explained by the inhibition of the formation of split copper vacancies, the defect most frequently associated with the CuO nucleation. Annealing treatments performed on Cu2O thin films provide new insights on the dopant influence on the mechanism to generate simple and split copper vacancies as well as the transformation of Cu2O into CuO.

InGaAs is one of the III-V active semiconductors used in modern high-electron-mobility transistors or high-speed electronics. ZnO is a good candidate material to be inserted as a tunneling insulator layer at the metal-semiconductor junction. A key consideration in many modern devices is the atomic structure of the hetero-interface, which often ultimately governs the electronic or chemical process of interest. Here, a complementary suite of in situ synchrotron X-ray techniques (fluorescence, reflectivity and absorption) as well as modeling is used to investigate both structural and chemical evolution during the initial growth of ZnO by atomic layer deposition (ALD) on In0.53Ga0.47As substrates. Prior to steady-state growth behavior, we discover a transient regime characterized by two stages. First, substrate-inhibited ZnO growth takes place on InGaAs terraces. This leads eventually to the formation of a 1 nm-thick, two-dimensional (2D) amorphous layer. Second, the growth behavior and its modeling suggest the occurrence of dense island formation, with an aspect ratio and surface roughness that depends sensitively on the growth condition. Finally, ZnO ALD on In0.53Ga0.47As is characterized by 2D steady-state growth with a linear growth rate of 0.21 nm cy-1, as expected for layer-by-layer ZnO ALD.

Hafnia-based resistive memories technology has come to maturation and acceded to the market of nonvolatile memories. Nevertheless, the physical mechanisms involved in resistive switching are not yet fully understood and the numerous ab initio simulations studies have few many atomic-scale experimental counterparts. In this study we investigate the oxygen migration mechanism from an amorphous HfO2 layer to the Ti cap layer at a local scale before and after a thermal treatment. X-ray absorption spectroscopy at the Ti K edge and Hf LIII edge has been performed on samples as-deposited and annealed in Ar at 400∘C to mimic the back-end-of-line thermal budget (BEOL) of CMOS technology. The short-range Ti and Hf environments have been determined, showing that annealing promotes the migration of O from HfO2 to Ti, the amount of which is quantified. This provokes an expansion and an increase of atomic disorder in the Ti lattice. The nature of the oxygen gettering mechanism by the Ti metal is understood by comparing samples with increasing Ti-capping thickness. We show that the Ti getter effect has to be activated by thermal treatment and that the O diffusion takes place in a region of a few nanometers close to the Ti/HfO2 interface. Therefore, the thermal budget history and the Ti cap-layer thickness determine the oxygen vacancy content in the HfO2 layer, which in turn controls the electrical properties, especially the forming operation., R.V. was supported by the LabEx MINOS ANR-10-LABX55-01. We acknowledge the French CRG (ESRF) and ESRF for granting beam time at beamline BM30-FAME (proposal number MA2383). M.G.P. acknowledges the support of project MAT 2015-66726-R of the Spanish Ministry of Science and Innovation and is grateful to the Diputación General de Aragón for granting support in the frame of the Researchers Mobility program (Grant No. 224-183).

ZnO thin films are interesting for applications in several technological fields, including optoelectronics and renewable energies. Nanodevice applications require controlled synthesis of ZnO structures at nanometer scale, which can be achieved via atomic layer deposition (ALD). However, the mechanisms governing the initial stages of ALD had not been addressed until very recently. Investigations into the initial nucleation and growth as well as the atomic structure of the heterointerface are crucial to optimize the ALD process and understand the structure–property relationships for ZnO. We have used a complementary suite of in situ synchrotron x-ray techniques to investigate both the structural and chemical evolution during ZnO growth by ALD on two different substrates, i.e., SiO2 and Al2O3, which led us to formulate an atomistic model of the incipient growth of ZnO. The model relies on the formation of nanoscale islands of different size and aspect ratio and consequent disorder induced in the Zn neighbors’ distribution. However, endorsement of our model requires testing and discussion of possible alternative models which could account for the experimental results. In this work, we review, test, and rule out several alternative models; the results confirm our view of the atomistic mechanisms at play, which influence the overall microstructure and resulting properties of the final thin film.

The polarity in ZnO nanowires is an important issue since it strongly affects surface configuration and reactivity, nucleation and growth, electro-optical properties, and nanoscale-engineering device performances. However, measuring statistically the polarity of ZnO nanowire arrays grown by chemical bath deposition and elucidating its correlation with the polarity of the underneath polycrystalline ZnO seed layer grown by the sol-gel process represents a major difficulty. To address that issue, we combine resonant x-ray diffraction (XRD) at Zn K-edge using synchrotron radiation with piezoelectric force microscopy and polarity-sensitive chemical etching to statistically investigate the polarity of more than 107 nano-objects both on the macroscopic and local microscopic scales, respectively. By using high temperature annealing under an argon atmosphere, it is shown that the compact, highly c-axis oriented ZnO seed layer is more than 92% Zn-polar and that only a few small O-polar ZnO grains with an amount less than 8% are formed. Correlatively, the resulting ZnO nanowires are also found to be Zn-polar, indicating that their polarity is transferred from the c-axis oriented ZnO grains acting as nucleation sites in the seed layer. These findings pave the way for the development of new strategies to form unipolar ZnO nanowire arrays as a requirement for a number of nanoscale-engineering devices like piezoelectric nanogenerators. They also highlight the great advantage of resonant XRD as a macroscopic, non-destructive method to simultaneously and statistically measure the polarity of ZnO nanowire arrays and of the underneath ZnO seed layer.

The core–shell geometry is a strong tool for inducing and controlling strains in nano-objects in order to tune their optoelectronic properties. We synthesized and characterized core–shell nanostructures by depositing a non-epitaxial silicon nitride shell around germanium nanowires. Scanning electron microscopy as well as energy dispersive X-ray spectroscopy confirms the structural integrity of the heterostructures, and grazing incidence X-ray diffraction measurements reveal the presence of a radial tensile strain in the Ge nanowires. A control of this strain is then demonstrated up to 0.3% by adjusting the SiNx shell thickness versus Ge nanowire diameter. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in Appl. Phys. Rev. 3, 041303 (2016) and may be found at https://doi.org/10.1063/1.4963919., The polar nature of the wurtzite crystalline structure of GaN and ZnO results in the existence of a spontaneous electric polarization within these materials and their associated alloys (Ga,Al,In)N and (Zn,Mg,Cd)O. The polarity has also important consequences on the stability of the different crystallographic surfaces, and this becomes especially important when considering epitaxial growth. Furthermore, the internal polarization fields may adversely affect the properties of optoelectronic devices but is also used as a potential advantage for advanced electronic devices. In this article, polarity-related issues in GaN and ZnO are reviewed, going from theoretical considerations to electronic and optoelectronic devices, through thin film, and nanostructure growth. The necessary theoretical background is first introduced and the stability of the cation and anion polarity surfaces is discussed. For assessing the polarity, one has to make use of specific characterization methods, which are described in detail. Subsequently, the nucleation and growth mechanisms of thin films and nanostructures, including nanowires, are presented, reviewing the specific growth conditions that allow controlling the polarity of such objects. Eventually, the demonstrated and/or expected effects of polarity on the properties and performances of optoelectronic and electronic devices are reported. The present review is intended to yield an in-depth view of some of the hot topics related to polarity in GaN and ZnO, a fast growing subject over the last decade.

International audience; The growth of zinc oxide thin films by atomic layer deposition is believed to proceed through an embryonic step in which three-dimensional nanoislands form and then coalesce to trigger a layer-by-layer growth mode. This transient initial state is characterized by a poorly ordered atomic structure, which may be inaccessible by X-ray diffraction techniques. In this work, we apply X-ray absorption spectroscopy in situ to address the local structure of Zn after each atomic layer deposition cycle, using a custom-built reactor mounted at a synchrotron beamline, and we shed light on the atomistic mechanisms taking place during the first stages of the growth. We find that such mechanisms are surprisingly different for zinc oxide growth on amorphous (silica) and crystalline (sapphire) substrate. Ab initio simulations and quantitative data analysis allow the formulation of a comprehensive growth model, based on the different effects of surface atoms and grain boundaries in the nanoscale islands, and the consequent induced local disorder. From a comparison of these specttoscopy results with those from X-ray diffraction reported recently, we observe that the final structure of the zinc oxide nanolayers depends strongly on the mechanisms taking place during the initial stages of growth. The approach followed here for the case of zinc oxide will be of general interest for characterizing and optimizing the growth and properties of more complex nanostructures.

The aim of this paper is to illustrate the use of Multi-Wavelength Anomalous Diffraction (MAD) and Diffraction Anomalous Fine Structure (DAFS) spectroscopy for the study of structural properties of semiconductor nanostructures. We give a brief introduction on the basic principles of these techniques providing a detailed bibliography. Then we focus on the data reduction and analysis and we give specific examples of their application on three different kinds of semiconductor nanostructures: Ge/Si nanoislands, AlN capped GaN/AlN Quantum Dots and AlGaN/AlN Nanowires. We show that the combination of MAD and DAFS is a very powerful tool to solve the structural problem of these materials of high technological impact. In particular, the effects of composition and strain on diffraction are disentangled and composition can be determined in a reliable way, even at the interface between nanostructure and substrate. We show the great possibilities of this method and give the reader the basic tools to undertake its use.

After discussing the GaN NW nucleation issue, we will present the structural properties of axial and radial (i.e. core/shell) GaN/AlN NW heterostructures and adress the issue of critical thickness during the growth of such heterostructures. Next, we will present the growth of InGaN NWs on a GaN NW base. It will be shown that the morphology and structural properties of the InGaN NW sections depend on the In content: for high In content a flat top is observed and plastic relaxation is occuring, with mismatch dislocations formed at the InGaN/GaN interface. By contrast, for In content below 25% InGaN NWs exhibit a pencil-like shape assigned to a purely elastic strain relaxation process, without formation of dislocations. Such a strain relaxation process was found to be associated with the spontaneous formation of an InGaN/GaN core-shell heterostructure. As concerns optical properties, it will be shown that the purely elastic relaxation process is associated with a marked In clustering. As a consequence, for samples with a 17% nominal In content, data were found to exhibit a S-shape characteristic of localization, with a minimum around 110K. By contrast, for a 40% In content PL data suggest that localization is much weaker. Finally, a growth model of InGaN/GaN heterostructures will be proposed.

International audience; We present a new beamline of Synchrotron SOLEIL dedicated to the study of thin films, nanostructures, and advanced materials via X-ray diffraction and spectroscopy in the energy range 1.4–12 keV. This range covers most of the absorption edges of interest in the fields of semiconductors and functional oxides. In order to meet the increasing demand of advanced real-time characterization of nanoscale materials, the beamline optics and instrumentation have been designed with remarkable dynamic characteristics. SIRIUS presently ends in two experimental stations used for in situ X-ray characterization: a baby chamber and a chemical reactor, both mounted on a large seven-circle diffractometer. The rector is dedicated to atomic layer deposition and metal organic chemical vapor deposition of oxide materials. The third end-station, an in-vacuum diffractometer, will be operative by the end of 2016. SIRIUS offers several synchrotron radiation techniques which can be performed simultaneously or quasi-simultaneously on the same sample. We show here some examples of the first in situ results obtained at the beamline.

International audience; A complementary suite of in situ synchrotron X-ray techniques is used to investigate both structural and chemical evolution during ZnO growth by atomic layer deposition. Focusing on the first 10 cycles of growth, we observe that the structure formed during the coalescence stage largely determines the overall microstructure of the film. Furthermore, by comparing ZnO growth on silicon with a native oxide with that on Al2O3(001), we find that even with lattice-mismatched substrates and low deposition temperatures, the crystalline texture of the films is dependent strongly on the nature of the interfacial bonds.

We have studied in detail changes in the strain state of GaN/AlN quantum dots during the capping process. μ-Raman scattering experiments allowed the detection of a resonant mode which provided information on the evolution of strain with capping. Simultaneously, Multiwavelength Anomalous Diffraction (MAD) and Diffraction Anomalous Fine Structure (DAFS) experiments were performed on the same samples, providing the independent determination of the wurtzite lattice parameters a and c. The remarkable agreement between Raman and X-ray data stands out the suitability of polar vibrational modes for the determination of strain in nanostructures. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

International audience

5 pages, 2 figures, 1 table.-- PACS 61.10.Eq, 61.10.Ht, 61.10.Nz, 61.46.+ w, 68.65.Cd, 68.65.Hb, We report on a general method that takes advantage of the full capability of anomalous diffraction and can be applied to the challenging case of small size embedded nanostructures. We study in situ and ex situ GaN Quantum Dots (QDs) grown by Plasma Assisted Molecular Beam Epitaxy (PAMBE), and encapsulated by an AlN epilayer. We investigate the QD strain and composition that are related to size, morphology, and cap layer thickness by means of anomalous diffraction in grazing incidence. The X-ray energy is tuned across the Ga K-edge where the Ga atoms scattering power is strongly modified and diffraction becomes chemically selective, giving direct information on composition. Quantitative analysis of the oscillatory extended region above the edge gives information on composition and out-of-plane strain of the dots.

We report on the study of strain, size and composition of small-size encapsulated semiconductor nanostructures. We show that the partial structure factor of As atoms in InAs stick-like nanostructures (quantum sticks), embedded in InP, can be directly extracted from grazing incidence anomalous X-ray diffraction maps at the As K-edge. We have recovered the average height and strain of the islands and determined their composition. The average height of the quantum sticks (QSs), as deduced from the width of the structure factor profile is 2.54 nm. The InAs out of plane deformation, relative to InP, is equal to 6.1%. Fixed-Q anomalous diffraction spectra, measured at the As K-edge, in grazing incidence provide clear evidence of pure InAs QSs. This is confirmed by the analysis of the diffraction anomalous fine structure (DAFS) that also gives a direct way to recover the strain accomodation inside the quantum sticks. Finite difference method calculations reproduce well the diffraction data. Chemical mixing at interfaces is at most 1 ML. This paper shows that ultimate application of anomalous diffraction and DAFS together with reciprocal space maps is a powerful method to sudy the structural properties of nanostructures.

This work elucidates the exact chemical nature of the interfaces in (1 1 1) Au–Ni multilayers grown by molecular beam epitaxy. We performed an X-ray investigation combining fitting of symmetric X-ray diffraction (XRD) spectra, and anomalous X-ray scattering analysis. We unambiguously show that the interfaces Au/Ni are abrupt, whereas the Ni/Au ones extend on five planes.

Resonant x-ray scattering was used to investigate electronic fluctuations of the octahedral iron atoms in magnetite. We measured the (002) and (006) ``forbidden'' x-ray diffraction reflections permitted by the anisotropy of the iron anomalous scattering factor. The energy and azimuthal angle dependencies of these reflections, and the polarization analysis, are shown and discussed. The results clearly show $p$ and $d$ iron empty states ordering in magnetite at room temperature. Moreover, the octahedral iron atoms are electronically equivalent in a time scale lower than ${10}^{\ensuremath{-}16}\phantom{\rule{0ex}{0ex}}\mathrm{sec}$. Therefore, magnetite should be considered as an itinerant magnet and not as a fluctuating mixed valence material.

When two or more absorption edges in a material are sufficiently close in energy, extended-x-ray-absorption-fine-structure spectroscopy is of limited utility as the usable data range above the lower-energy edge is truncated by the higher-energy edge. Energy or wavelength discriminating detection methods may fail to resolve fluorescence lines which are very close in energy. In this paper we present a solution to this problem using the resolution in momentum transfer of diffraction anomalous fine structure (DAFS) to separate the fine- structure signals from elements with closely spaced fluorescence lines. We demonstrate our technique by isolating the titanium edge signal from DAFS measurements of ${\mathrm{BaTiO}}_{3}.$

(1999). Separating overlapping x-ray-absorption edges using diffraction anomalous fine-structure. Synchrotron Radiation News: Vol. 12, No. 3, pp. 30-33.

We present the first successful attempt at calculating cluster full-potential x-ray absorption near-edge structure (XANES) spectra, based on the finite difference method. By fitting XANES simulations onto experimental spectra we are able to perform electron population analysis. The method is tested in the case of Ti $K$-edge absorption spectrum in ${\mathrm{TiO}}_{2}$, where the amount of charge transfer between Ti and O atoms and of the screening charge on the photoabsorber is obtained taking into account both dipolar and quadrupolar transitions.

Under the terms of the Creative Commons Attribution 3.0 Unported License., In this work, we investigate the local atomic structure of defect-free homogeneous and self-organized core-shell structure nanowires by means of X-ray Absorption Fine Structure (XAFS) Spectroscopy at the In LIII and K edges and Multiwavelength Anomalous Diffraction. The results are interpreted by comparison of the experimental data with X-ray absorption calculations carried out with ab initio structural models. Extended-XAFS data analysis at In K-edge shows an anisotropic In distribution in the second nearest neighbors pointing out to a deviation from randomness in In distribution for the core-shell sample. © 2014 AIP Publishing LLC., M.G.P. acknowledges the support of project MAT2009-03074 of Spanish Ministry of Science and Innovation and is grateful to the Diputación General de Aragón for granting support in the frame of the Researchers Mobility program (Grant No. 224-183). C.L., G.T., B.D., and H.R. acknowledge the French ANR for support (Fidel project ANR 2011 NANO 02902).

The effect of built-in strain on III-V epitaxial semiconductors has been investigated by Diffraction Anomalous Fine Structure (DAFS). We study two different systems in a different strain regime: a Strained Layer Superlattice of (GaP) 2 (InP) 3 grown on a GaAS(001) substrate, and a single epilayer of GaAs 1 - x P x , (x=0.225), also grown on a GaAs (001) substrate. In the first case the strain is accommodated by plastic deformation of the lattice, while in the second one it is partially relaxed by dislocations generation. The bond distances for the Ga-P and Ga-As pairs are obtained showing how they are affected by strain. The Ga-As pair shows to be softer' than the Ga-P pair, i.e. more available to accommodate strain by bond deformation, in good agreement with previous results obtained by different techniques. The DAFS provide a unique tool of studing systems that are out of the reach of the other X-ray techniques.

Superlattice studies are important to synthesise 3d metals in new crystalline structure which may exhibit exotic magnetic properties. The challenge is to relate these properties to the details of the chemical gradient and the local strain. We report here on the use of Diffraction Anomalous Fine Structure (DAFS) spectroscopy to separate the XAFS-like information about Fe atoms located at different crystallographic sites of an Fe/Ir(100) superlattice. We obtained the Fe nearest neighbor distances at the Ir-Fe interfaces by the use of a new crystallographic-based analysis of the DAFS data.

International audience; We report on the growth of AlxGa1-xN nanowires by plasma-assisted molecular beam epitaxy for x in the 0.3-0.8 range. Based on a combination of macro- and micro-photoluminescence, Raman spectroscopy, x-ray diffraction and scanning electron microscopy experiments, it is shown that the structural and optical properties of AlGaN NWs are governed by the presence of compositional fluctuations associated with strongly localized electronic states. A growth model is proposed, which suggests that, depending on growth temperature and metal adatom density, macroscopic composition fluctuations are mostly of kinetic origin and are directly related to the nucleation of the AlGaN nanowire section on top of the GaN nanowire base which is used as a substrate.

International audience; A detailed investigation of the mechanisms related to the low temperature oxidation of nanocrystalline magnetite thin films into maghemite is presented. Despite strong differences in the functional properties of these two phases, structural similarities make it difficult to distinguish between them, and to quantify the oxidation process, particularly in the case of nanostructured polycrystalline layers. Contrary to the case of bulk materials or monocrystalline films and particles, the oxidation processes in nanocrystalline thin film have only scarcely been studied. In this work, structural and optical techniques, including X-ray diffraction (XRD), EXAFS/X-ray absorption near edge structure, FTIR, and Raman scattering, have been used to estimate the oxidation rate of magnetite. The overall oxidation reaction rates are discussed in the framework of two limiting cases corresponding to intra grain diffusion and to grain boundary diffusion. SIMS profiling and electrical measurements were also carried out to better assess the oxidation quantification in order to conclude on the predominant oxidation mechanisms in this heterogeneous material. We propose a qualitative model for the structure, in terms of insulating zone distribution, for partially oxidized films. (C) 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4772714]

Anomalous x‐ray diffuse scattering is used to study the short‐range order (SRO) and lattice distortions in the electron‐type superconductor, Nd1.85Ce0.15CuO4−y. The intensity measurements were made at two energies below the Ce LIII absorption edge; the difference data greatly simplified the analysis to involve only atom pairs including Ce. The data sets were analyzed using a kinematic diffraction equation expanded up to the second order in atomic displacements. The fitted SRO parameters indicate the formation of Ce‐Ce atom pairs along the 〈001〉 directions. For superconductivity to occur in this and other doped cuprate compounds, the size of the dopant free regions in the basal plane may have to exceed the superconducting coherence length.

International audience; Nucleation of GaN nanowires grown by plasma-assisted molecular beam epitaxy is studied through a combination of two in situ tools: grazing incidence x-ray diffraction and reflection high energy electron diffraction. Growth on bare Si(111) and on AlN/Si(111) is compared. A significantly larger delay at nucleation is observed for nanowires grown on bare Si(111). The difference in the nucleation delay is correlated to a dissimilarity of chemical reactivity between Al and Ga with nitrided Si(111).

We applied Grazing Incidence Diffraction Anomalous Fine Structure to the study of the structure of Ge dome-shaped nanoislands, grown by Molecular Beam Epitaxy on Si (0 0 1) substrates at a temperature of 650 °C. We determined the vertical composition of the islands showing the presence of a strong Ge/Si intermixing that is nearly constant from bottom to top. In particular, an abrupt change is found at the substrate interface where the composition switches from pure Si to Ge 0.6Si 0.4. The analysis of the Diffraction Anomalous Fine Structure oscillations of the spectra is crucial to obtain the true composition profile. We performed atomistic simulations to investigate the role of the strained substrate underneath the dome on the diffraction results and to quantify the resolution of our method. Anomalous Diffraction spectra and Diffraction Anomalous Fine Structure oscillations have been simulated for a real size and real shape cluster including faceting, giving a more detailed data interpretation and understanding of the Ge-Si intermixing mechanism. © 2011 Elsevier B.V. All rights reserved., MGP and NAK acknowledge support of Spanish Ministry of Science and Education project MAT2008-03074, J.Z. and G.B. the FWF, Vienna (SFB025).

International audience; no abstract

International audience; Silicon nanowires were grown by gold-catalyzed chemical vapor deposition in the vapor liquid solid mode. In this paper, we use grazing incidence X-ray diffraction and multiwavelength anomalous diffraction to investigate the presence of gold and its effects on silicon nanowire strain after different chemical and physical treatments. We especially focus on the efficiency of the "thermal oxidation-oxide etching" cycle to remove the gold contamination. Analysis reveals a decrease of the contamination and a "core-shell like" spatial distribution of gold, but also shows that this technique is not efficient enough to remove all traces of gold from the nanowires.

The local order in a single crystal of YBa2Cu2.53Co0.47O7.13 has been studied with anomalous diffuse x-ray scattering. (For such a Co concentration the compound is nonsuperconducting.) Intensity measurements were carried out at two energies below the Co edge. The difference data could then be expressed in terms of the local structure around a Co atom. The short-range order parameters (α's) indicate that the Co and Cu atoms are nearly randomly distributed on the Cu1 sites. The estimated size of the Co-free “domains” is 5-7 Å. The first neighbor in-plane Co-Co distances are significantly shortened, indicating that the Co atoms are displaced from their average positions. The data also show a significant decrease of the Co-O1 distance, leading to an increase of the Cu2-O1 distance. The lengthening of the Cu2-O1 distance implies a lowering of the Cu2 formal valence. The Co substitution also affects the in-plane Cu2 positions. The present study shows that the Cu-O2 structural coherence is altered on a scale smaller than the superconducting coherence length. As far as the superconductivity is concerned, the Cu2 valence remains one of the most important parameters in determining the superconducting properties of the Co-doped 123 compounds. On the other hand, there is some evidence that in order for superconductivity to occur in this and other doped cuprate compounds, the size of dopant-free regions in the basal plane may have to exceed the superconducting coherence length.

We have investigated in detail the structural properties of the system YBa2 CuPin1−y Coy)3O6+x with different values of x and y. X-ray diffraction shows that the Co doping induces an orthorhombic-to-tetragonal transition at y≈0.025. Electron diffraction and electron microscopy indicate that for y

International audience

Scattering maps from strained or disordered nano-structures around a Bragg reflection can either be computed quickly using approximations and a (Fast) Fourier transform, or using individual atomic positions. In this article we show that it is possible to compute up to 4.10^10 $reflections.atoms/s using a single graphic card, and we evaluate how this speed depends on number of atoms and points in reciprocal space. An open-source software library (PyNX) allowing easy scattering computations (including grazing incidence conditions) in the Python language is described, with examples of scattering from non-ideal nanostructures., Comment: 7 pages, 4 figures

The growth of GaN nanowires by means of plasma assisted molecular beam epitaxy directly on Si(1 1 1) has been investigated as a function of temperature. Statistical analysis of scanning electron microscopy pictures taken for different growth temperatures has revealed that density, diameter, length and length dispersion of nanowires were strongly dependent on temperature. Length dispersion, in particular, was found to be significant at high temperature. These features have been assigned to the different duration of the nucleation process with temperature, namely to the dependence with temperature of the time necessary for the size increase of the three-dimensional precursors up to a critical value.

Strain fields and composition gradient in epitaxial Ge dome-shaped islands grown on Si(001) substrates are determined by combining grazing-incidence multiwavelength anomalous diffraction and grazing-incidence diffraction anomalous fine-structure spectroscopy. This unique combination, together with numerical X-ray diffraction calculations carried out in the frame of the distorted-wave Born approximation with a structural model obtained by molecular dynamics, is shown to probe unambiguously the Ge composition even in the difficult region close to the island/substrate interface and to elucidate the vertical compositional profile inside the dome-shaped islands. A rather abrupt chemical composition profile close to the interface and a slight increase of the average Ge composition from the base to the top are observed for islands grown at a temperature of 650 °C. These experiments provide unique fundamental knowledge on the mechanism of Si-Ge intermixing during island growth, clarifying and quantifying the effects of growth conditions and morphology on the structural local properties of the nano-islands. © 2011 Europhysics Letters Association., MGP and NAK acknowledge the support of Spanish Ministry of Science and Education project MAT2008-03074, JZ and GB the support of FWF, Vienna (SFB205).

International audience; In this paper, we measure experimentally the strains in silicon nanowires using Grazing Incidence X-ray Diffraction. Silicon nanowires have been grown with a gold-catalytic method and have a crystalline character. Bragg's diffraction peak is measured and analyzed. A lattice mismatch parameter between the silicon substrate and silicon nanowires is found and accurately measured. The influence of the presence of the gold catalyst in the wires is investigated, and reveals a small decrease of the lattice mismatch parameter when the gold catalyst is removed. A residual strain is measured and a possible origin is proposed. An estimation of the surface and bulk stress is calculated and presented. (C) 2011 Elsevier B.V. All rights reserved.

We studied the Cu substitution by Co in ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{6+\mathit{x}}$. The samples have been analyzed by extended x-ray-absorption fine structure (EXAFS) and x-ray absorption near-edge structure (XANES) techniques in the oxidized and the reduced forms. We determined the Co cations average coordination and valence. Structural models for the arrangement around the cobalt atoms are proposed. The cobalt valence is ${3}^{+}$ in both oxidized and reduced forms. XANES experiments showed that the coordination polyhedra around the cobalt cations are not centrosymmetrical. EXAFS spectra showed that the cobalt cations are displaced from the (000) position and are surrounded by 5 oxygen atoms (4 at 1.82 \AA{} and 1 at 2.02 \AA{}). Both the EXAFS and XANES show that the difference between these two sets of Co-O distances decreases for reduced compounds. This corresponds to a slight increase of the site symmetry. Tetrahedral, pyramidal, and highly distorted octahedral coordinations for the cobalt cations are proposed. They take into account all these results obtained for oxidized and reduced compounds. Our results corroborate the assumption that the Co cations in the ${\mathrm{YBa}}_{2}$(${\mathrm{Cu}}_{1\mathrm{\ensuremath{-}}\mathit{y}}$${\mathrm{Co}}_{\mathit{y}}$${)}_{3}$${\mathrm{O}}_{6+\mathit{x}}$ compounds are in the intermediate spin state with a mixing of high-spin ${\mathrm{Co}}^{3+}$ in the tetrahedral site and low-spin ${\mathrm{Co}}^{3+}$ in the distorted octahedral site.