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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.

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.

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).

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)

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.

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.

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.

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.

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

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.

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.

International audience; The growth and structural properties of GaN/AlN core-shell nanowire heterostructures have been studied using a combination of resonant x-ray diffraction, Raman spectroscopy and high resolution transmission electron microscopy experiments. For a GaN core of 20 nm diameter on average surrounded by a homogeneous AlN shell, the built-in strain in GaN is found to agree with theoretical calculations performed using a valence force field model. It is then concluded that for an AlN thickness up to at least 12 nm both core and shell are in elastic equilibrium. However, in the case of an inhomogeneous growth of the AlN shell caused by the presence of steps on the sides of the GaN core, plastic relaxation is found to occur. Consistent with the presence of dislocations at the GaN/AlN interface, it is proposed that this plastic relaxation, especially efficient for AlN shell thickness above 3 nm, is promoted by the shear strain induced by the AlN inhomogeneity.

International audience; We have performed a real-time in situ x-ray scattering study of the nucleation of GaN nanowires grown by plasma-assisted molecular beam epitaxy on AlN(0001)/Si(111). The intensity variation of the GaN diffraction peak as a function of time was found to exhibit three different regimes: (i) the deposition of a wetting layer, which is followed by (ii) a supralinear regime assigned to nucleation of almost fully relaxed GaN nanowires, eventually leading to (iii) a steady-state growth regime. Based on scanning electron microscopy and electron microscopy analysis, it is proposed that the granular character of the thin AlN buffer layer may account for the easy plastic relaxation of GaN, establishing that three-dimensional islanding and plastic strain relaxation of GaN are two necessary conditions for nanowire growth.

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.

Single crystals of YBa2(Cu1−y, Auy)3O6+x were synthesized during the growth of 123 crytals in Au crucibles. The structure of YBa2(Cu0.967Au0.033)3O6.50 has been determined by X-ray diffraction data taken with an automatic diffractometer equipped with MoKα radiation. The P/4mmm space group was used in the final refinement. The lattice parameters determined by the Gandolfi technique were: α = 3.842(1) A, b = 3.878(1) A, c = 11.765(2) A. Electron diffraction patterns did not exhibit any superstructure reflections or diffuse scattering. Our results confim of those published earlier by Wong-Ng et al. who showed that the Au cations substitute or Cu(1). The interatomic distances compared to those calculatedfrom the Shannon radii show that the Au cations are in the 3+ valence state and have a square coordination. Since the Au cations are isolated and 3-coordinated Cu cations are unusual, the Au incorporation pins down the chain formation along one axis and the orthorhombicity of the samples occurs at lower oxygen stoichiometry. The most important consequences of the Au-doping at a given oxygen content is the shortening of the Cu(2)-O(1) bond which explains why Au-doping has a small effect on Tc. The Au(1)-O(1) distance being larger than Cu(1)-O(1) one induces a pressure effect on Cu(2)-O(1) bond.

We performed heat treatments (800–830 °C) in inert atmosphere followed by oxidation at low temperature (400 °C) on Co-substituted YBa2(Cu1−yCoy)3O6+x, leading to the stabilization of the orthorhombic phase beyond y = .025. For example, orthorhombic symmetry is obtained with y = .06 with twin boundaries spacings as large 1000A. Our STEM data on and between twin walls, and the presence of diffuse crosses around spots of each orthorhombic individual indicate that Co clusters exist also inside the orthorhombic domains. The orthorhombic samples recover the tetragonal symmetry by heating in flowing O2 at 600 °C for 30mn. They present the tweed-like structure and the same type of diffusion as for the native compounds. In order to have a better understanding of the correlations between the structural details and the superconducting properties, ac-suceptibility and dc-magnetization mesurements are being carried out. Our first results show that the middle of the transition of the 6% Co-substituted orthorhombic sample is at lower temperature than that of same sample before heat treatment.

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.

Medium energy ion scattering (MEIS) has been used to measure at the scale of the monolayer the deformation profile of self-organized GaN quantum dots grown on AlN by molecular-beam epitaxy. The effect of capping the GaN dots by a thin layer of AlN has also been studied. It is shown that GaN dots are partially relaxed in every situation. Capping them with AlN has little effect on the basal plane, as expected, but strongly modifies the strain of the upper part of dots. The experimental results are compared with theoretical calculations, allowing one to conclude that GaN quantum dots experience a nonbiaxial strain, which drastically decreases when going from the basal plane up to the apex of the dots.