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In this work we investigate the strain state in Ni films belonging to [Ni(tNi)/Cu(tCu)] x N superlattices (with tNi = 2 nm, 3 nm, tCu from 0 to 3 nm and N between 1 and 6) by Extended X-ray Absorption Fine Structure at the Ni K-edge. The strain of these Ni blocks can be up to 80% larger than that obtained for single thin films with the same content of Ni. The dependence of the strain on N indicates that its relaxation is not progressive with N but gets blocked for certain N values. Also, the replacement of a single Ni layer by Cu, at the middle of the Ni block, results in a significant increment of the strain of the nickel layers, which manifests the relevant effect of the introduction of copper. The large strain values observed suggest the presence of blocking mechanisms acting on the propagation of dislocations. We argue that this observation is related to repulsive interactions between dislocations enhanced by nonhomogeneos stress due to presence of the copper layers separating the Ni blocks.

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.

The magnetic domain structure is studied in epitaxial /MgO(0 0 1) films with 0 < x < 30 and thicknesses below 60 nm by magnetic force microscopy. For low gallium content, domains with the magnetization lying in the film plane and domain walls separating micrometric areas are observed. Above x ≈ 20, the magnetic contrast shows a fine corrugation, ranging from 300 to 900 nm, suggesting a ripple substructure with a periodic oscillation of the magnetization. We discuss the presence of a random magnetic anisotropy contribution, that superimposed to the cubic coherent anisotropy, is able to break the uniform orientation of the magnetization. The origin of that random anisotropy is attributed to several factors: coexistence of crystal phases in the films, inhomogeneous distribution of both internal strain and Ga–Ga next nearest neighbor pairs and interface magnetic anisotropy due to the Fe–O bond.

In this work we present a study of the structural properties of Fe100−x Gax (x < 30) films grown by Molecular Beam Epitaxy on Mg0(100). We combine long range and local/chemically selective X-ray probes (X-ray Diffraction and X-ray absorption spectroscopy) together with real space imaging by means of Transmission Electron Microscopy and surface sensitive in situ Reflected High Energy Electron Diffraction. For substrate temperature Ts below 400 °C we obtain bcc films while, for x ≈ 24 and Ts ≥ 400oC the nucleation of the fcc phase is observed. For both systems a Ga anticlustering or local range ordering phenomenon appears. The Ga/Fe composition in the first and second coordination shells of the bcc films is different from that expected for a random Ga distribution and is close to a D03 phase, leading to a minimization of the number Ga-Ga pairs. On the other side, a long-range D03 phase is not observed indicating that atomic ordering only occurs at a local scale. Overall, the epitaxial growth procedure presented in this work, first, avoids the formation of a long range ordered D03 phase, which is known to be detrimental for magnetostrictive properties, and second, demonstrates the possibility of growing fcc films at temperatures much lower than those required to obtain bulk fcc samples.

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.

Understanding and controlling macroscopic quantities directly associated with a random field, such as the coercive field Hc and dislocations in magnetic materials, is important for many applications that include films with perpendicular magnetic anisotropy. Here, using a model system with perpendicular magnetic anisotropy, Cu/Ni/Cu, we show that Hc in double 4 nm thick Ni films is about 0.65 times the value obtained for the single Ni film, a fact that can be understood if the propagation of misfit dislocations is not transmitted to the second magnetic block, implying that its statistical distribution remains unchanged whereas the magnetic driving force increases with the nickel thickness. This interpretation is based on the direct measurement of the in-plane and out-of-plane lattice parameters of the Ni blocks by the extended x-ray absorption fine structure method, a chemically selective technique tuned to probe exclusively the environment of the Ni atoms. With this finding and applying the rigid domain walls model, the Hc ratio between double and single Ni films is calculated, yielding a value of 0.71.

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.

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.

We have performed a quantitative X-ray absorption fine structure analysis of bacteriorhodopsin in purple membrane patches and in lipidic cubic phases regenerated with Mn 2+. Lipidic cubic phases and purple membrane results have been compared, demonstrating that the lipidic cubic phase process does not introduce relevant distortions in the local geometry of the cation binding sites. For both samples, we have observed similarities for Mn 2+ coordination in terms of type, number, and average distances of surrounding atoms, indicating a first coordination shell composed by 6 O atoms, and 3/4 C atoms located in the second coordination shell. © 2011 European Biophysical Societies' Association.

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.

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 (300) 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.

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 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.

When depositing by Molecular Beam Epitaxy a few InAs monolayers onto a (0 0 1) InP substrate, one can obtain an array of self-assembled wires at a nanometer scale. We have performed grazing incidence diffraction anomalous fine structure experiments on capped InAs/InP quantum wires (QWrs) to investigate their structural properties. Elastic strain relaxation has been simulated by the finite differences method.

We have used X-ray absorption spectroscopy to study the structural properties of encapsulated semiconductor quantum wires (QWrs) and quantum dots (QDs), obtained by self-organized molecular beam epitaxy (MBE) growth. Extended X-ray absorption fine structure (EXAFS) spectroscopy gives information on the local microscopic structure of the sample, and allows to determine the strain and composition of the nanostructures. We measured EXAFS at the As K-edge, of the InAs QWrs, and at the In K-edge of the InAs QDs, in glancing-angle geometry (GIXAFS) at the ERSF, keeping the X-ray incidence angle close to the substrate critical angle to enhance the contribution of the thin nanostructured epilayer. The XAFS spectra of QWrs and QDs show a good signal-to-noise ratio that allows quantitative analysis. We have obtained bond distances up to the third coordination shell, and composition. The results show, in agreement with previous anomalous scattering measurements, that contrary to what expected due to diffusion and intermixing of the Group V species, the wires are made essentially of InAs. In the case instead, of the QDs samples a strong In diffusion is observed, in agreement with the well known In tendency to diffuse and segregate towards the surface, in strained InGaAs systems. We have also determined the elastic strain content of the samples.

We have studied self-organized quantum wires of InAs, grown by molecular beam epitaxy onto a InP(0 0 1) substrate, by means of grazing incidence diffraction anomalous fine structure (DAFS). The equivalent quantum wires thickness is 2.5 monolayers. We measured the (4 4 0) and (4 2 0) GIDAFS spectra, at the As K-edge, keeping the incidence and exit angles close to the InP critical angle. The analysis of both the smooth and oscillatory contributions of the DAFS spectrum, provide valuable information about composition and strain inside the quantum wires and close to the interface. We also show preliminary results on InAs wires encapsulated by a 40 Å thick InP capping layer, suggesting the DAFS capability of probing different iso-strain regions of the wires.

We have studied nanostructured samples of InAs/InP(001) by means of Grazing Incidence Diffraction Anomalous Fine Structure. The samples, grown by molecular beam epitaxy, show a periodic corrugation on the surface giving rise to an array of self-assembled quantum wires after deposition of 2.5 monolayers of InAs. We measured the (440) and (420) GIDAFS spectra, at the As K-edge, at incidence and outgoing angles close to the substrate's critical angle. We analysed the anomalous diffraction lineshapes vs. the energy, as well as the oscillatory part of the signal in the extended region above the edge and obtained, for the first time, information about composition and strain inside the quantum wires and close to the interface. Our results suggest possible interfaces.

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.