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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.

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 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.

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)

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.

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.

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.

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; 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.

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

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.

The manganese perovskite system La1-x Ca x MnO3 displays a complex phase diagram of structural, magnetic, and transport properties with varying Ca concentration. At x = 2/3 and at low temperature, the system is antiferromagnetic with Mn4+ and Mn3+ ions occupying special positions in a charge-ordered supperlattice. The charge ordering transition at about 260 K is characterized by the appearance of satellite peaks around certain strong normal lattice reflections. The normal lattice reflections are due to scattering from planes containing Mn4+ and Mn3+ ions in nearly stoichiometric proportion, however the superlattice reflections are due to scattering from planes containing only Mn4+ ions. By measuring Diffraction Anomalous Fine-Structure spectra on a superlattice reflection and its associated normal lattice reflection, it is possible to isolate absorption-like spectra for the two Mn sites. Due to the weak intensity of these superlattice reflections, we were unable to obtain high quality near-edge spectra for the superlattice reflection measured. However, the data offer useful information about the local electronic structures of the two Mn ions.

3 páginas, 7 figuras.-- Póster presentado a la 11ª International Conference on X-ray Absorption Fine Structure (XAFS XI) celebrada en Ako (Japón) del 26 al 31 de Julio del 2000., We have performed Diffraction Anomalous Fine Structure measurements at the As K-edge of self-growth InAs/InP(001) Quantum Wires and InAs/GaAs(001) Quantum Dots. The samples have been grown by Molecular Beam Epitaxy and their equivalent thickness is of 2.5 monolayers. We have measured the (440) and (420) Bragg reflections in glancing-angle scattering geometry, at incidence angles close to the substrate critical angle. We demonstrate the feasibility of the experiment reporting, for the first time, Diffraction Anomalous Fine Structure spectra of such low coverage epitaxial layers, and we show that the analysis of the Diffraction Anomalous Fine Structure lineshape together with the analysis of oscillatory part of the signal, can provide information about composition and strain of the nanostructures., This work was supported by the LEA-MANES European Agreement and by CICYT project No. MAT99-0847.

Diffraction Anomalous Fine Structure (DAFS) and EXAFS measurements have been performed on a [Fe0.7Mn0.3/Ir(100)]40} superlattice at the K-edges of Fe and Mn. Theoretical EXAFS spectra have been refined: a slight difference in the first neighbor distance suggests a non homogeneous distribution of the Mn in the alloy. The smooth features of the DAFS spectra have been modeled to study the composition and the strain profile along the growth axis.

The molecular-beam epitaxy growth of AlN/GaN nanowire superlattices has been studied by using a combination of in situ x-ray diffraction experiments, high-resolution electron-microscopy analysis and theoretical calculations performed in a valence force field approach. It is found that the nanowire superlattices are in elastic equilibrium, in contrast with the two-dimensional case but in line with the predicted increase in the critical thickness in the nanowire geometry.

8 pags, 6 figs, 2 tabs, In the present paper, we aim to show the interest of combining Multiwavelength Anomalous Diffraction (MAD) and Diffraction Anomalous Fine Structure (DAFS) spectroscopy, in grazing incidence, to obtain structural properties (composition, strain and atomic ordering) of semiconductor heterostructures and nanostructures. As an example we report on preliminary results obtained on a series of Ge/Si(001) nano-island samples: pyramides and domes on nominal and prepatterned surfaces. For free standing domes, it is shown that the Ge content strongly depends on the growth condition with a tendency to increase from the bottom to the top of the nano-islands. There is also some indication of atomic ordering in the upper part of the islands. For small, capped pyramids, we show that the Diffraction Anomalous Fine Structure spectroscopy is the unique non destructive method that allows to recover the actual Ge content, the in-plane and out-of-plane strain and to detect atomic ordering. © EDP Sciences and Springer 2009.

In this chapter we provide a detailed view of grazing incidence diffraction anomalous fine structure spectroscopy, which has been shown to be a powerful tool in the study of the structural properties of nanostructures. We give a brief description of resonant elastic X-ray scattering and pay special attention to the major experimental and data analysis issues related to grazing incidence geometry. We show how to combine multiwavelenght anomalous diffraction and diffraction anomalous fine structure spectroscopy, in grazing incidence, to obtain strain and composition of InAs quantum wires and GaN quantum dots. This review also aims to compare extended X-ray absorption fine structure and extended DAFS results on the same samples. It gives a new insight into the complementarity of X-ray diffraction and absorption.

The potential of X-ray scattering techniques using synchrotron radiation is described. It is demonstrated that grazing incidence (GI) conditions make X-rays near surface sensitive, which is mandatory for the characterization of semiconductor nanostructures grown in a self-organized way on or buried near the sample surface. The weak scattering from the nanostructures is thereby enhanced and can further be strengthened by using “anomalous dispersion effects,” also known as resonant scattering, which makes the techniques element sensitive. When combined with in situ studies, the growth of semiconductor nanostructures can be followed in real time. The wealth of results reported in the literature is highlighted by several examples: GI small angle X-ray scattering (GISAXS) of Ge islands growth on Si(001) and the vertical stacking of GaN dots in AlN multilayers. The remaining case studies are dedicated to the combination of GI diffraction (GID) and anomalous scattering using the techniques of iso-strain scattering and multiwavelength anomalous diffraction (MAD). The concluding outlook gives a perspective on the use of the beam coherence which allows for the reconstruction of objects by phase-retrieval algorithms and thereby to accomplish a model-independent characterization of nanostructures with respect to shape, strain and composition. This technique (coherent diffraction imaging (CDI)) is currently under development at several synchrotrons around the world.

The strain state of $\mathrm{Ga}\mathrm{N}∕\mathrm{Al}\mathrm{N}$ quantum dots grown on $6H\text{\ensuremath{-}}\mathrm{Si}\mathrm{C}$ has been investigated as a function of AlN capping thickness by three different techniques. On the one hand, resonant Raman scattering allowed the detection of the ${A}_{1}(\mathrm{LO})$ quasiconfined mode. It was found that its frequency increases with AlN deposition, while its linewidth did not evolve significantly. Available experiments of multiwavelength anomalous diffraction and diffraction anomalous fine structure on the same samples provided the determination of the wurtzite lattice parameters $a$ and $c$ of the quantum dots. A very good agreement is found between resonant Raman scattering and x-ray measurements, especially concerning the in-plane strain state. The results demonstrate the adequacy of Raman scattering, in combination with the deformation potential and biaxial approximations, to determine quantitatively values of strain in GaN quantum dot layers.

The relevance of grazing-incidence anomalous diffraction as a tool to investigate the strain and structure of small-size embedded nano-objects is examined. Multiple scattering effects, originating from the grazing-incidence setup, are analyzed with a special emphasis on the cusp of the diffraction anomalous spectrum and the extended diffraction anomalous fine-structure oscillations. It is shown that even for grazing-incidence angle, a Born approximation treatment is justified for quantum dots (QDs) on top of a thin wetting layer. The discussion focuses on the overgrowth of AlN on top of GaN QDs. Both the in-plane and out-of-plane strains in the dots can be specifically determined, by extracting the Ga partial scattering amplitude from measurements of the scattered intensity along both the in- and out-of-plane directions, close to the $(30\overline{3}0)$ and $(30\overline{3}2)$ reflections, at several energies across the Ga $K$ edge. The study is complemented by the analysis of the local environment of Ga atoms in the dots through the measurement of the fine-structure oscillations in diffraction condition. The oscillations are found almost insensitive to the grazing-incidence multiple-scattering effects. Accordingly, the out-of-plane strain and possible intermixing specifically in the dots can be deduced. The QDs are shown to remain pure GaN all along the capping process. The QDs strain state exhibits a larger strain relaxation than expected from an elastic model, suggesting the presence of a plastic strain relaxation, possibly through dislocations at the vicinity of the QDs. Finally, the influence of the substrate as regards strain relaxation in the QDs is discussed by comparing our results to those we previously obtained for a series of samples grown on AlN/sapphire.

Local chemical composition of InGaN quantum dots grown by molecular‐beam epitaxy on GaN virtual substrates was investigated by x‐ray diffraction anomalous fine‐structure method. Using this approach, we found that the In content increases from 20% at the dot base to 40–50% at the top. From the detailed numerical analysis of the data we were able to reconstruct the local neighborhood of Ga atoms in different positions in the dots, as well as the local elastic relaxation state.

The latest virtual special issue of Journal of Applied Crystallography includes some highlights of the 12th Conference on High-Resolution X-ray Diffraction and Imaging (XTOP). This biennial conference was held in Villard-de-Lans and Grenoble in September 2014, and was organized by the ESRF, Universite Grenoble-Alpes, the CEA and CNRS. A refresher school for PhD students and postdoctoral scientists took place on the first day, with lectures on new diffraction and imaging techniques. The first XTOP conference (‘X-ray Topography and High Resolution Diffraction’) took place in Marseille in 1992. The original topics covered were diffraction topography, double- and triple-crystal diffractometry of epitaxial layers, reflectometry, and the standing wave technique. Many experimental techniques have been added since, which cover today a wide range to reflect the scientific evolution of the XTOP community: coherent imaging, X-ray phase contrast imaging (radiography and microtomography), microdiffraction (monochromatic and Laue), grazing incidence and resonant diffraction, fast (time-resolved) tomography etc. The topics covered now include both synchrotron and laboratory instrumentation, including industrial developments. The materials studied include single crystals and multilayers, nanostructures (metals, oxides, semiconductors), and organic, biological and other natural (sea shells, fossils etc.) samples. A few presentations in 2014 covered advances in the theory of high-resolution X-ray diffraction, and with the occasion of the International Year of Crystallography, Andre Authier gave a presentation on the Early Days of X-ray Diffraction and the Birth of the Dynamical Theory. Among the topics presented during the XTOP conference, the most noteworthy included the development of high temporal and spatial resolution for diffraction and imaging, the use of X-ray coherent beams (including ptychography and X-ray free electron lasers), and applications to nanostructured materials. Following the traditional publication policy of the XTOP conferences, the articles presented in this issue are not selected proceedings but rather original research articles, which were first presented at the XTOP conference. These include laboratory and synchrotron studies of semiconductors, software for fast data analysis or instrumental function simulation, micro-Laue analysis of oxide fuel cells and bent nanowires, microtomography of bone structure, X-ray phase contrast imaging of bacterial endospores, and Bragg imaging of ice under applied stress. Over 200 participants attended the five-day conference, which included six tutorial lectures, 13 invited and 44 contributed presentations, and 144 posters.

Capping of GaN quantum dots with AlN has been studied at the monolayer scale by combining atomic force microscopy, high resolution electron microscopy, and grazing incidence x-ray anomalous diffraction. Consistent with the results provided by these three techniques, it has been demonstrated that, following a wetting of the dots by an AlN layer up to 4 ML coverage, subsequent capping is dominated by a preferential AlN growth in between the dots, eventually resulting in a complete smoothing of AlN. Interdiffusion has been shown to be negligible during this process, which makes the GaN/AlN system unique among semiconductors.

The strain state and the deformation profile of GaN quantum dots embedded in AlN have been measured by high resolution electron microscopy, medium energy ion scattering and grazing incidence X-ray diffraction. The results are compared with theoretical calculations, allowing one to conclude that GaN quantum dots experience a non biaxial strain which drastically decreases when going from the basal plane up to the apex of the dots. We also demonstrate that AlN is distorted in the surroundings of the dots, which provides the driving force for vertical correlation of GaN dots when the AlN spacer between successive planes is thin enough.

The investigation of small-size embedded nanostructures, by a combination of complementary anomalous diffraction techniques, is reported. GaN quantum dots (QD's), grown by molecular beam epitaxy in a modified Stranski-Krastanow mode, are studied in terms of strain and local environment, as a function of the AlN cap layer thickness, by means of grazing-incidence anomalous diffraction. That is, the x-ray photon energy is tuned across the Ga absorption K edge which makes diffraction chemically selective. Measurement of hkl scans, close to the AlN (3030) Bragg reflection, at several energies across the Ga K edge, allows the extraction of the Ga partial structure factor, from which the in-plane strain of GaN QD's is deduced. From the fixed-Q energy-dependent diffracted intensity spectra, measured for diffraction-selected isostrain regions corresponding to the average in-plane strain state of the QD's, quantitative information regarding the composition and out-of-plane strain has been obtained. We recover the in-plane and out-of-plane strains in the dots. The comparison to the biaxial elastic strain in a pseudomorphic layer indicates a tendency to an overstrained regime.

6 pages, 7 figures, 1 table., We report on a general method that takes advantage of the full capability of anomalous diffraction and can be applied to the very interesting and challenging case of small size embedded nanostructures. We study GaN quantum dots (QDs) grown by MBE in the Modified-Stransky–Krastanow regime, and encapsulated by an AlN epilayer. We investigate the QDs 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 absorption Ga K-edge where the Ga atoms scattering power is strongly modified and diffraction becomes chemically selective, giving direct information on composition. hkl-scans close to the (3 0 −3 0) Bragg reflection of AlN at several energies above and below the Ga K-edge, allow to extract the structure factor FA of the Ga atoms which gives the in-plane lattice parameter, a, of GaN. Quantitative analysis of the anomalous edge lineshape and of the oscillatory extended region above the edge give information on the composition and the out-of-plane strain of the dots. A two-step covering mechanism of QDs By AlN is suggested.

We have carried out DAFS experiments at the Ir L3-edge to study the local structure of Ir layers in Ir(100)/Fe superlattices. Our findings agree with the existence of a body centered Fe phase in pseudomorphic regime. These experiments also give information about IrFe interfaces.

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 1ML, Comment: 9 pages, 7 figures, submitted

Diffraction anomalous fine-structure (DAFS) spectroscopy uses resonant elastic X-rays scattering as an atomic, shell and site-selective probe that provides information on the electronic structure and the local atomic environment as well as on the long-range-ordered crystallographic structure. A DAFS experiment consists of measuring the Bragg peak intensities as a function of the energy of the incoming X-ray beam. The French CRG (Collaborative Research Group) beamline BM2-D2AM (Diffraction Diffusion Anomale Multi-longueurs d'Onde) at the ESRF (European Synchrotron Radiation Facility) has developed a state-of-the-art energy scan diffraction set-up. In this article the requirements for obtaining reliable DAFS data are presented and recent technical achievements are reported.

We have investigated the structure of buried interfaces in (111) Au:Ni multilayers. Conventional x-ray diffraction at constant energy as well as anomalous x-ray scattering across the Ni absorption K edge have been used. Whereas the fitting of the spectra at a single energy leads to two different possible interfacial structures, the anomalous diffracted intensity variation unambiguously favors a model with an interfacial concentration gradient at one interface. The multilayer presents an intermixed region extending on six atomic planes around the Ni/Au interface, whereas the Au/Ni interface is chemically abrupt. This structure can explain the unusual stress-strain relation previously reported on this system.

Resonant X ray scattering has been used to investigate charge localization on the octahedral iron atoms in magnetite below and above the Verwey temperature. We have measured the DAFS spectra of the 002 and 006 `forbidden' Bragg reflections permitted by the anisotropy of the iron anomalous scattering factor. We performed ab initio calculations which are in fair agreement with the experiment in the near edge region and demonstrate the sensitivity of the DAFS spectra to tiny structural and electronic changes. No change is observed, in the energy and azimuthal dependences, when the sample is cooled down below the Verwey temperature. Charge ordering can be definitely excluded and different charge localization schemes discarded. Ab initio simulations, performed by using the refined crystallographic structure proposed for the room temperature phase, do not show a good agreement with the experiment in the extended region of the DAFS spectrum. This point is being investigated.

We exploit the crystallographic sensitivity of the Diffraction Anomalous Fine-Structure (DAFS) measurement to separate the fine structure contributions of different atonfic species with closely spaced resonant energies. In BaTiO3 the Ti K edge and Ba Lm edges are separated by 281 eV, or about 8.2 .~-l , thus severely linfiting the information content of the Ti K edge signal. Using the site selectivity of DAFS we can separate the two fine structure spectra using an iterative Kramers-Kronig method, thus extending the range of the Ti K edge spectrum. This technique has application to many rare earth/transition metal compounds, including many magnetic materials of technological significance for which K and L edges overlap in energy.

The structural and optical properties of high temperature-annealed superlattices of GaN quantum dots embedded in AlN barrier have been studied by a combination of X-ray techniques (reciprocal space mapping, multiwavelength anomalous diffraction, and diffraction anomalous fine structure), high resolution transmission electron microscopy, and photoluminescence spectroscopy. Taking advantage of the disentangling of the chemical and structural information provided by the simultaneous use of X-ray absorption and diffraction data obtained in a synchrotron environment, we provide quantitative determination of strain and composition for each different region of the nanostructures. Eventually, it is shown that strain driven dot/barrier intermixing is present, mostly on top of the dots. These observations have been confirmed by high resolution electron microscopy. A blue shift of photoluminescence peak has been furthermore observed and assigned to GaN/AlN intermixing suggesting a new path for engineering the emission wavelength of such heterostructures.

International audience; The production and characterization of ultradense, planarized, and organized silicon nanowire arrays with good crystalline and optical properties are reported. First, alumina templates are used to grow silicon nanowires whose height, diameter, and density are easily controlled by adjusting the structural parameters of the template. Then, post-processing using standard microelectronic techniques enables the production of high-density silicon nanowire matrices featuring a remarkably flat overall surface. Different geometries are then possible for various applications. Structural analysis using synchrotron X-ray diffraction reveals the good crystallinity of the nanowires and their long-range periodicity resulting from their high-density organization. Transmission electron microscopy also shows that the nanowires can grow on nonpreferential substrate, enabling the use of this technique with universal substrates. The good geometry control of the array also results in a strong optical absorption which is interesting for their use in nanowire-based optical sensors or similar devices.

In this paper, we show that combining Multiwavelength Anomalous Diffraction (MAD) and Diffraction Anomalous Fine Structure (DAFS) spectroscopy, in grazing incidence geometry, allows to obtain structural properties (strain and composition) of semiconductor nanostructures. We report results obtained on dome-shaped Ge nano-islands grown on Si(001) surfaces and AlGaN nanowires grown on Si(100). It is shown that, in the case of sharp interfaces, MAD alone can not determine the mean Ge content in the region of the substrate-island interface and needs to be combined with Extented-DAFS measurements.

The strain state of the [11¯00] GaN quantum dots and quantum wires has been studied by a combination of multiwavelength anomalous diffraction and diffraction anomalous fine structure under grazing incidence. The three components of the anisotropic deformation have been independently determined. On one hand, a weak residual strain state along the c-axis, similar for quantum dots and quantum wires, is found. On the other hand, along the [112¯0] direction, quantum wires appear to be more relaxed than quantum dots. Based on high resolution transmission electron microscopy, this is assigned to elastic strain relaxation of GaN governed by either misfit dislocation formation or three-dimensional islanding, depending on the anisotropic strain state of the [11¯00] AlN buffer layer.

Reflection high-energy electron diffraction (RHEED) was used to monitor the strain of GaN/AlN quantum dots (QDs) grown by molecular beam epitaxy. Quantitative and absolute values of the in- and out-of-plane strains of the QDs were determined and compared to reference values, obtained by x-ray diffraction measurements. A very good agreement was found between RHEED and x-ray measurements. The growth and thermal ripening of the dots were analyzed. A progressive strain relaxation was observed during the ripening stage, suggesting a morphology evolution of the dots.

Physical review letters 78(14), 2775 - 2778 (1997). doi:10.1103/PhysRevLett.78.2775, Studies of superlattices are important in order to synthesize 3D metals in a 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 the use of diffraction anomalous fine structure (DAFS) spectroscopy and a crystallographic analysis of the DAFS data to separate the x-ray-absorption fine structure (XAFS)-like information about Fe atoms located at different crystallographic sites of an Fe/Ir(100) superlattice. We obtained the first and the second (in the plane of growth) nearest-neighbor distances of Fe atoms at the Ir-Fe interface., Published by APS, College Park, Md.

This paper reports electron and X-ray diffraction studies along with XAS data of Co-substituted 123 phase. From the geometry and the evolution of diffuse scattering we propose a model based on a Co-cluster formation inducing local orthorhombic/tetragonal deformation in oxidized samples. In these doped samples, Co clusters and YBa2Cu3O6+x domains are juxtaposed. By varying the Co content, the YBa2Cu3O6+x domains size vary and consequently the symmetry of the sample change.Reduction process occur mainly in YBa2Cu3O6+x domains while cobalt coordination is nearly unchanged.