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In nanostructures made of a mixture of bulk-immiscible metallic species, the alloy formation down to the atomic scale is a crucial and debated point. We report on the first experimental evidence of an amorphous metallic phase in Au-Co thin films and 2D array of nanostructures, that is constituted by a fine mixing of single-metal (sub)-nm domains, as shown by experiments coupling short- and long range- order characterization techniques, as X-ray Absorption Spectroscopy-XAS, X-ray Diffraction-XRD, Diffraction Anomalous Fine Structure-DAFS. Despite the mixing does not reach the atomic scale, the extended Au-Co interface can entail about half of atoms, and is responsible for the previously measured magnetic moment of Au in these systems. This amorphous nanomixed phase coexists with a minor fraction of fcc Au x Co 1 - x nanocrystals, preferentially oriented with the 1 1 1 crystallographic planes parallel to the film surface. 2D patterned Au-Co films with very similar structure can be easily obtained, but with smaller and randomly oriented nanocrystals. The thermal stability of the system (amorphous and crystalline) is limited to below 250 °C. At higher temperatures an extended decomposition occurs and Au and fcc Co nanocrystals coexist.

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.

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

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

Impressive changes in the transport and ferromagnetic properties of Co-doped ZnO thin films have been obtained by postgrowth hydrogen irradiation at temperatures of 400 °C. Hydrogen incorporation increases the saturation magnetization by one order of magnitude (up to ∼1.50 μB/Co) and increases the carrier density and mobility by about a factor of two. In addition to the magnetic characterization, the transport and structural properties of hydrogenated ZnO:Co have been investigated by Hall effect, local probe conductivity measurements, micro-Raman, and X-ray absorption spectroscopy. Particular care has been given to the detection of Co oxides and metal Co nanophases, whose influence on the increase in the transport and ferromagnetic properties can be excluded on the ground of the achieved results. The enhancement in ferromagnetism is directly related to the dose of H introduced in the samples. On the contrary, despite the shallow donor character of H atoms, the increase in carrier density n is not related to the H dose. These apparently contradictory effects of H are fully accounted for by a mechanism based on a theoretical model involving Co-VO (Co-O vacancy) pairs. © 2016 American Chemical Society.

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

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

Extended X-ray absorption fine structure (EXAFS) measurements have been carried out on CaSO4:Dy phosphors at the Dy L3 edge with synchrotron radiation. The data have been analysed to find out the Dy–S and Dy–O bond lengths in the neighborhood of the Dy atoms. Measurements have been carried out over several samples thermally annealed for different cycles at 400 °C in air for 1 h and the change in bond lengths in samples with increasing number of annealing cycles have been studied by analyzing the EXAFS data.

We report N-K–edge x-ray absorption near-edge spectra of a set of InxGa1−xN alloy epilayers with 0.36 < x < 0.87, including linear dichroism effects. Careful experimental and data treatment procedures lead to consistent variations of spectral features with In concentration and sample orientation with respect to the direction of linear polarization of the x-ray beam. Insight into the origin of spectral features is provided by a combination of ab initio simulations of the equilibrium structure and atom-by-atom spectral simulations. The relation between the spectral lineshape and the variations in the composition of the first coordination shell and the corresponding local structural distortions is discussed.

The surface of ferromagnetic Zn1xCoxO wurtzite epilayers has been studied by coupling atomic force microscopy and advanced x-ray spectroscopy. We found that, even in high-quality epilayers, the formation of Co clusters and iso-space-group Co-rich regions can take place at the sample surface while the bulk maintains random Co distribution. Comparing structural characterization with magnetometry, we show that these surface modifications are not at the origin of the magnetic properties of the material. Quite the reverse, ferromagnetic behavior is enhanced in the sample characterized by the less defective surface.

Postgrowth hydrogen incorporation in In-rich InxGa1−xN (x>0.4) alloys strongly modifies the optical and structural properties of the material: A large blueshift of the emission and absorption energies is accompanied by a remarkable broadening of the interatomic-distance distribution, as probed by synchrotron radiation techniques. Both effects vanish at a finite In-concentration value (x ∼ 0.5). Synergic x-ray absorption measurements and first-principle calculations unveil two different defective species forming upon hydrogenation: one due to the high chemical reactivity of H, the other ascribed to mere lattice damage. In the former species, four H atoms bind to as many N atoms, all nearest-neighbors of a same In atom. The stability of this peculiar complex, which is predicted to behave as a donor, stems from atomic displacements cooperating to reduce local strain.

We have used an experimental strategy that, combining nuclear reaction analysis and Rutherford backscattering spectrometry both in random and channeling geometry, allowed an accurate quantification of the total amount of N in InxGa1−xNyAs1−y/GaAs and GaNyAs1−y/GaAs epitaxial systems (0.038

In an attempt to clarify the origin of the limited electrical activity of Fe implanted in InP we report an X-ray absorption spectroscopy investigation. Fe implantation was performed with a recently developed procedure involving dynamical annealing which is able to increase the electrically active Fe concentration by at least a factor of 100 with respect to previous methods. We have found that Fe is always bonded to P atoms and is never present in the metallic or oxide form. The local structural parameters suggest the formation upon annealing of a disordered Fe–P compound, which may be at the origin of the limited electrical activity.

We investigated the formation mechanism of Sn and Sb nanoclusters in thin SiO2 films by X-ray absorption spectroscopy (XAS). Sn and Sb nanoclusters have been formed in 85 and 22 nm thick SiO2 layers, respectively, by 80 keV/1×1016 cm−2 Sn and 10 keV/5×1015 cm−2 Sb ion implantation followed by thermal treatments. XAS analyses provided unique information on the local environment of Sn and Sb ions in SiO2 with respect to different annealing conditions. Sn and Sb atoms are mainly coordinated with oxygen in the as-implanted samples. Annealing at different temperatures (800–1100 °C) and in different atmospheres (N2 or Ar+7%H2) leads to the formation of metallic clusters or oxidized ones. Even when the metallic phase is detected, a fraction of Sn or Sb atoms still remains dissolved in the matrix or forms small oxide clusters. Annealings at high temperature (1100 °C) for a short time (30 s) or in Ar+7%H2 atmosphere (even if at lower temperature) are more effective for the formation of metallic clusters and for reducing the oxidized atom percentage in the matrix.

High temperature (>200 °C) Fe ion implantation in combination with a post-implantation annealing treatment is used to introduce a high active Fe concentration, well above the solubility limit, in n-InP. Current–voltage (I–V) characteristics and photo-induced current transient spectroscopy (PICTS) experiments reveal that a strong increase of the Fe2+ concentration is reached with respect to the equilibrium values, with a limited fraction (of the order of 10%) of the implanted Fe being activated and a strong dependence of the Fe2+ concentration on the substrate doping density. Proton induced X-ray emission spectroscopy (PIXE) and X-ray absorption fine structure (XAFS) measurements were carried out in order to analyze the local structural environment of the implanted Fe atoms in the host InP lattice. The results indicate that most of the Fe atoms resides on non-substitutional sites after annealing, most likely in Fe–P agglomerates. These structures, however, do not show any detrimental effects on the Fe2+-related electrical and optical properties.

We have investigated the structure of nitrogen-hydrogen complexes in ${\text{GaAs}}_{1\ensuremath{-}y}{\text{N}}_{y}$ and ${\text{In}}_{x}{\text{Ga}}_{1\ensuremath{-}x}{\text{As}}_{1\ensuremath{-}y}{\text{N}}_{y}$ dilute nitride alloys by performing x-ray absorption near-edge structure spectroscopy (XANES). We simulated the spectra based on first-principles calculations of the most recent defective structures proposed in the literature for hydrogenated materials. The comparison between the experimental data and simulations allows us to clarify that the core of the defect is a complex with ${\text{C}}_{2v}$ structure in the neutral charge state, in agreement with the expansion of the lattice parameter measured by x-ray diffraction. Our results are compatible with the presence of H satellites bound to neighboring Ga atoms but not with complexes involving more than two H atoms bound to the same N. Nevertheless, we were not able to determine uniquely the number of H satellites, which may depend on growth conditions. Strain related to the epitaxial growth has a very little effect on the XANES spectra.

The determination of the local structure of dopants in semiconductors is of paramount importance to understand their influence on the optical and transport properties; this knowledge is of great value since it can lead to insightful materials design. Many commonly used dopants are low atomic number elements, the K-edge of which lie in the soft X-ray region. Application of X-ray absorption spectroscopy, the technique of choice for local structural investigations, to the study of dopants in this energy range has been limited in the past, mainly due to the very small fluorescence yields (3x10-3 for N); fluorescence detection is mandatory in order to guarantee sufficient bulk sensitivity. By taking advantage of the high brilliance of the X-ray beam emitted by undulators on ELETTRA and using a dedicated window-less Ge fluorescence detector, we have been able to overcome this limitation; we have performed a series of experiments in this field on the branch line of the ALOISA beamline, some of which are here briefly described. The method we have used involves recording X-ray absorption near edge spectra (XANES) and their structural interpretation in the framework of multiple scattering theory, using the FEFF code. The clusters used for the analysis were based on density-functional theory simulations. ZnO is a wide band gap, naturally n-type semiconductor with great promise for optoelectronic applications; the main obstacle yet to be overcome is p-type doping. Nitrogen, the most promising candidate currently being pursued as a dopant, has been predicted to preferentially incorporate into the ZnO lattice in the form of a N2 molecule at an O site when a plasma source is used, leading to compensation rather than p-type doping. We demonstrate this to be incorrect and that in fact, N incorporates substitutionally at O sites where it is expected to act as an acceptor. Using high resolution spectra in the region of the pi* transition and comparing to those of molecular nitrogen, we also detect the formation of molecular nitrogen upon annealing at the moderately low temperature of 800 °C. These results suggest that effective p-type doping of ZnO with N may be possible only for low-temperature growth processes. N-dilute GaAsN alloys belong to a novel class of semiconductors with fascinating physical properties. Indeed, a small amount of nitrogen incorporation in GaAs leads to a counterintuitive and large band gap reduction, along with a decrease of the lattice parameter. Even more surprisingly, both electronic and structural changes can be reversed fully and in a tunable manner by hydrogen incorporation. We have investigated the atomic geometry of N-H complexes in hydrogenated GaAsN and we have demonstrated that dihydrogen-nitrogen complexes with C2v symmetry, in which H breaks two N-Ga bonds on the same N atom replacing them by two N-H ones, are the most abundant species in hydrogenated GaAsN. This finding contradicts all previous predictions of “in-line” N-H2 complexes (in which H breaks only one N-Ga bond and the two H atoms bind N and Ga, respectively) as the predominant species, and accounts for many other physical properties. These investigations illustrate that high quality fluorescence-yield XANES spectra for dilute low-Z atoms can now be recorded and that combining structural and spectral simulations, precious information on the three-dimensional structure of dopants and defects can be obtained.

We investigated the local structure around N and Sb atoms in GaAsSbN/GaAs epilayers as a function of growth conditions and annealing time via soft and hard x-ray absorption spectroscopies in order to find out if short range ordering (SRO) in the group-V sublattice is present. SRO is one of the potential origins of the huge blueshift of the band gap observed upon annealing in these materials. By combining a Sb K- and L- and N K-edge x-ray absorption fine structure spectroscopy analysis, we demonstrate that neither strong Sb clustering nor preferential Sb-N association is possible, and that Sb atoms see a random number of N next nearest neighbors except for growth temperatures smaller than 400 degrees C, for which Sb-N neighbors in the type-V sublattice are in excess with respect to statistical disorder. On the other hand, the evolution of SRO around N anions (breaking of nitrogen pairs and randomization) can play a role in the annealing-induced band gap blueshift. Varying growth conditions and concentration modifies the band gap but, surprisingly, it does not affect the position of the conduction band minimum when Sb is incorporated.

Dilute nitrides, in particular In$_x$Ga$_{1-x}$As$_{1-y}$N$_y$ are very interesting alloys because of their actual and potential applications in the field of telecommunications and photovoltaic. A full knowledge of the local structure around each kind of atom, in particular the local ordering, is of fundamental importance because it is predicted to affect the electronics properties of these alloys. In this work we investigated the local environment of N and In by X-Ray Absorption Fine Structure, paying particular attention to demonstrate that N is substitutional to As and preferentially links In atoms

GaAsN alloys belong to a class of semiconductors with fascinating physical properties. Indeed, a small amount of nitrogen incorporation in GaAs leads to a counterintuitive and large band-gap reduction, and to an unexpected sudden increase in the effective mass of electrons. Even more surprisingly, both electronic and structural changes can be reversed fully and in a tunable manner by hydrogen incorporation. In this paper, we combine x-ray absorption spectroscopy at the nitrogen edge with ab initio simulations to investigate the atomic geometry of N-H complexes in hydrogenated GaAsN. In this way, we provide experimental evidence that dihydrogen-nitrogen complexes with C2v symmetry are the most abundant species in hydrogenated GaAsN. This finding contradicts previous predictions of “in-line” N-H2 * complexes as the predominant species, and accounts for recent infrared absorption experiments.

We address in this paper the issue of the effects of hydrogenation on the local structure of InxGa1−xAs1−yNy quantum wells by combining In K-edge x-ray absorption and Ga K-edge x-ray diffraction anomalous fine structure experiments. We found that the cation-As bond lengths in hydrogenated samples are systematically longer than the values predicted by a valence force field model corrected for the epitaxial strain. We interpret this bond lengths stretching as a local effect of the formation of NuH complexes very recently predicted by theoretical calculations. By analyzing the Debye-Waller factor of the GauAs bond length distribution, we observed that hydrogenation removes the static disorder induced by N incorporation in GaAs; this effect is due to the unique characteristics of the N substitutional anion and to the breaking of the ionic GauN bonds upon hydrogenation.

We present a combined experimental and theoretical study of the local structure of a dilute nitride alloy, GaAs$_{1-y}$N$_y$. Experimental results obtained by X-ray Absorption Spectroscopy have been compared with first-principles density-functional supercell calculations and with the predictions of different Valence Force Field models. Both experiments and calculations find that inclusion of N induces static disorder in the Ga-As bond length distribution. An increase in the Ga-As bond length with N concentration is found; this is due to the competing effects of the decrease of the free lattice parameter and of tensile strain due to pseudomorphic growth. A good agreement between the experimental and theoretical determination of the bond length expansion is found. We discuss the performance of the Valence Force Field models in predicting the bond length expansion.

We used a synergic Co-edge X-ray absorption spectroscopy (XAS) and density functional theory calculations approach to perform a study of defects which could account for the room temperature ferromagnetism of ZnCoO, an oxide of great potential interest in semiconductor spintronics. Our results suggest that a key role is played by specific defect complexes in which O vacancies are located close to the Co atoms. Extended defects such as Co clusters have a marginal function, although we observe their formation at the epilayer surface under certain growth conditions. We also show preliminary results of the study of hydrogen-induced defects in ZnCoO epilayers deliberately hydrogen irradiated via a Kaufman source. Hydrogen was in fact predicted to mediate a ferromagnetic spin-spin interaction between neighboring magnetic impurities.

The field of dilute magnetic semiconductors (DMS) is gaining increasing interest for the potential applications in spintronic devices, in which the spin degree of freedom of the charge carriers can be used to enhance the capabilities of conventional electronics, combining semiconducting and magnetic properties. Compounds based on ZnO, in particular ZnCoO, are especially appealing among DMS since they exhibit ferromagnetism at room temperature; however the origin of ferromagnetism in ZnCoO is still controversial. In this work, we use a synergic Co-edge X-ray absorption spectroscopy (XAS) and density functional theory calculations approach to perform a study of different kind of defects which could account for the magnetic behavior of ZnCoO. Our results suggest that a key role is played by specific defect complexes in which O vacancies are located close to the Co atoms1. Extended defects such as Co clusters have a marginal function, although we observe their formation at the epilayer surface under certain growth conditions2. Finally, we show preliminary results of the study of hydrogen-induced defects in ZnCoO epilayers deliberately hydrogenated via a Kaufman source. H is predicted to mediate a ferromagnetic spin-spin interaction between neighboring magnetic impurities. Co-edge XAS is supported by XRD, AFM, SEM, and VSM characterization.

Postgrowth hydrogen incorporation in In-rich In${}_{x}$Ga${}_{1\ensuremath{-}x}$N ($xg0.4$) alloys strongly modifies the optical and structural properties of the material: A large blueshift of the emission and absorption energies is accompanied by a remarkable broadening of the interatomic-distance distribution, as probed by synchrotron radiation techniques. Both effects vanish at a finite In-concentration value ($x$ \ensuremath{\sim} 0.5). Synergic x-ray absorption measurements and first-principle calculations unveil two different defective species forming upon hydrogenation: one due to the high chemical reactivity of H, the other ascribed to mere lattice damage. In the former species, four H atoms bind to as many N atoms, all nearest-neighbors of a same In atom. The stability of this peculiar complex, which is predicted to behave as a donor, stems from atomic displacements cooperating to reduce local strain.

We report N-K-edge x-ray absorption near-edge spectra of a set of InxGa1-xN alloy epilayers with 0.36

Postgrowth hydrogen incorporation in In-rich InxGa1-xN (x > 0.4) alloys strongly modifies the optical and structural properties of the material: A large blueshift of the emission and absorption energies is accompanied by a remarkable broadening of the interatomic-distance distribution, as probed by synchrotron radiation techniques. Both effects vanish at a finite In-concentration value (x similar to 0.5). Synergic x-ray absorption measurements and first-principle calculations unveil two different defective species forming upon hydrogenation: one due to the high chemical reactivity of H, the other ascribed to mere lattice damage. In the former species, four H atoms bind to as many N atoms, all nearest-neighbors of a same In atom. The stability of this peculiar complex, which is predicted to behave as a donor, stems from atomic displacements cooperating to reduce local strain.

The surface of ferromagnetic Zn1−xCoxO wurtzite epilayers has been studied by coupling atomic force microscopy and advanced x-ray spectroscopy. We found that, even in high-quality epilayers, the formation of Co clusters and iso-space-group Co-rich regions can take place at the sample surface while the bulk maintains random Co distribution. Comparing structural characterization with magnetometry, we show that these surface modifications are not at the origin of the magnetic properties of the material. Quite the reverse, ferromagnetic behavior is enhanced in the sample characterized by the less defective surface.

We investigate the local structure of ferromagnetic Zn(1-x)Co(x)O epilayers by coupling polarization-dependent x-ray absorption spectroscopy and ab initio calculations of selected defect structures. We give clear evidence of the presence of oxygen vacancies, located close to the Co atoms in a specific complex configuration. We also establish the upper concentration limit of metallic parasitic nanophases and their contribution to magnetism. Our results lead to the conclusion that oxygen vacancies play an important role in originating the high temperature ferromagnetism of Zn(1-x)Co(x)O.

We investigate the local structure of ferromagnetic Zn(1-x)Co(x)O epilayers by coupling polarization-dependent x-ray absorption spectroscopy and ab initio calculations of selected defect structures. We give clear evidence of the presence of oxygen vacancies, located close to the Co atoms in a specific complex configuration. We also establish the upper concentration limit of metallic parasitic nanophases and their contribution to magnetism. Our results lead to the conclusion that oxygen vacancies play an important role in originating the high temperature ferromagnetism of Zn(1-x)Co(x)O.

We addressed the issue of bismuth heteroantisite defects (BiGa) in GaAs1-xBix/GaAs epilayers by coupling x-ray absorption spectroscopy at the bismuth edge with density functional theory calculations of the defect structure. Calculations predict a large relaxation of the Bi-As interatomic distances when Bi atoms substitute Ga, however we found no experimental evidence of it. Quantitative analysis of the x-ray absorption spectra allows us to establish a maximum concentration limit for Bi_Ga, which corresponds to about 5% of the total Bi atoms. Bi_Ga do not account for the modifications in the spectra previously attributed to short range ordering.

We investigate the local structure of ferromagnetic Zn1-xCoxO epilayers by coupling polarization-dependent x-ray absorption spectroscopy and ab initio calculations of selected defect structures. We give clear evidence of the presence of oxygen vacancies, located close to the Co atoms in a specific complex configuration. We also establish the upper concentration limit of metallic parasitic nanophases and their contribution to magnetism. Our results lead to the conclusion that oxygen vacancies play an important role in originating the high temperature ferromagnetism of Zn1-xCoxO.

We address the compositional evolution of the structure of ${\text{GaAs}}_{1\ensuremath{-}x}{\text{Bi}}_{x}$ thin films at different scale lengths by combining x-ray absorption spectroscopy, atomic force microscopy, and x-ray diffraction. We find that Bi short range ordering, observed for $xl3\mathrm{%}$, drastically vanishes for $x\ensuremath{\ge}5.4\mathrm{%}$. The recovery of random anion distribution goes along with the formation of Bi droplets at the sample surface while bulk maintains high crystalline quality. These structural changes go with the anomalous behavior of optical and electronic properties. In particular, we find that the decrease in emission intensity in the samples corresponds to the concentration point where Bi short range ordering is lost, hence the preservation of nonrandom distribution turns to be a key point in devices design.

We present an investigation of magnetic and structural properties of Co-doped ZnO (ZCO) film grown by pulsed laser deposition at different dopant concentrations (cCo). X-ray diffraction patterns show that the films are single phase and exhibit ferromagnetism (FM) above room temperature (RT) with coercive fields up to 700 Oe. X-ray absorption fine-structure spectroscopy (XAFS) at the Co edge suggests that in films grown below 600°C dopant clustering involve less than 10% of the Co atoms in the alloy. In samples grown at higher temperature a larger fraction of Co atoms is involved in the formation of small metallic clusters. The experimental work has been accompanied by preliminary first-principles Density Functional Theory calculations.

In this paper we investigate the incorporation of hydrogen in ${\text{Ga}}_{1\ensuremath{-}x}{\text{Mn}}_{x}\text{As}$ for samples with $x\ensuremath{\approx}0.005$ grown by metal-organic vapor-phase epitaxy and with $0.03lxl0.05$ grown by low-temperature molecular-beam epitaxy. The anisotropic electron-paramagnetic-resonance (EPR) signal observed for the paramagnetic ${\text{Ga}}_{1\ensuremath{-}x}{\text{Mn}}_{x}\text{As}$ samples with $x\ensuremath{\approx}0.005$ after hydrogenation is characteristic for ${\text{Mn}}^{2+}$ substitutional on the Ga site. Contributions of crystal fields to the EPR signal indicative of Mn-H complexes with H atoms near the Mn are negligibly small. The relative volume increase in a single ${\text{Mn}}_{\text{Ga}}\text{-As}$ atom pair upon hydrogenation $\ensuremath{\Delta}{V}_{\text{H}}/{V}_{\text{Mn-As}}\ensuremath{\approx}0.14\ifmmode\pm\else\textpm\fi{}0.07$ as deduced from a comparison of the high-resolution x-ray diffraction $2\ensuremath{\Theta}/\ensuremath{\Omega}$ scans of as-grown and hydrogenated samples with $0.03lxl0.05$ is expected for Mn-H complex formation. However, the accuracy of this measurement is not sufficient to draw unambiguous conclusions about the specific nature of the Mn-H configuration. Extended x-ray absorption fine-structure (EXAFS) analysis and x-ray absorption near-edge spectroscopy (XANES) on samples with $0.03lxl0.05$ show no indication for bond-centered Mn-H complexes as determined from a detailed comparison of the EXAFS Fourier transforms and the XANES spectra with the simulations. The overwhelming structural evidence of these techniques therefore points to comparatively large distances between the Mn and the H atoms at least in ${\text{Ga}}_{1\ensuremath{-}x}{\text{Mn}}_{x}\text{As}$ films with Mn concentrations above 0.005, which would be the case for either complexes with the hydrogen atom in the antibonding position or for compensation via isolated interstitial hydrogen.

We use x-ray absorption spectroscopy to investigate the local structure around Bi atoms in GaAs1�xBix layers grown on GaAs as a function of Bi concentration in order to detect short-range order. We find that static disorder in the Bi next-nearest-neighbor interatomic distances dramatically increases when the Bi concentration is increased. At 1.2% Bi concentration, the Bi atoms are randomly distributed whereas at 1.9%, they tend to form next-nearest-neighbor pairs. When the Bi concentration rises to 2.4%, our results suggest that some of the Bi atoms form small Bi clusters. Such strong deviations from a random distribution are likely to play an important role in the occurrence of the giant optical bowing recently measured in this alloy.

We investigate the structure of the austenite phase in CuZnAl shape memory alloys by a combined x-ray absorption and diffraction analysis. Ab initio simulations of the near Zn-edge x-ray absorption coefficient allow us to directly discard the hypothesis of a DO3 superstructure. At the same time, we give evidence of the existence of an ordered structure (B2-like) different from the L2(1) one recently proposed by neutron diffraction. However, some partial L2(1) ordering is present at room temperature. This superstructure develops and recovers order when increasing the temperature above 400 K. (C) 2008 American Institute of Physics.

We investigated the local structure around N and Sb atoms in $\mathrm{Ga}\mathrm{As}\mathrm{Sb}\mathrm{N}∕\mathrm{Ga}\mathrm{As}$ epilayers as a function of growth conditions and annealing time via soft and hard x-ray absorption spectroscopies in order to find out if short range ordering (SRO) in the group-V sublattice is present. SRO is one of the potential origins of the huge blueshift of the band gap observed upon annealing in these materials. By combining a Sb $K$- and $L$- and N $K$-edge x-ray absorption fine structure spectroscopy analysis, we demonstrate that neither strong Sb clustering nor preferential Sb-N association is possible, and that Sb atoms see a random number of N next nearest neighbors except for growth temperatures smaller than $400\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$, for which Sb-N neighbors in the type-V sublattice are in excess with respect to statistical disorder. On the other hand, the evolution of SRO around N anions (breaking of nitrogen pairs and randomization) can play a role in the annealing-induced band gap blueshift. Varying growth conditions and concentration modifies the band gap but, surprisingly, it does not affect the position of the conduction band minimum when Sb is incorporated.

We address in this paper the issue of the effects of hydrogenation on the local structure of In{sub x}Ga{sub 1-x}As{sub 1-y}N{sub y} quantum wells by combining In K-edge x-ray absorption and Ga K-edge x-ray diffraction anomalous fine structure experiments. We found that the cation-As bond lengths in hydrogenated samples are systematically longer than the values predicted by a valence force field model corrected for the epitaxial strain. We interpret this bond lengths stretching as a local effect of the formation of N-H complexes very recently predicted by theoretical calculations. By analyzing the Debye-Waller factor of the Ga-As bond length distribution, we observed that hydrogenation removes the static disorder induced by N incorporation in GaAs; this effect is due to the unique characteristics of the N substitutional anion and to the breaking of the ionic Ga-N bonds upon hydrogenation.

GaAsN alloys belong to a class of semiconductors with fascinating physical properties. Indeed, a small amount of nitrogen incorporation in GaAs leads to a counterintuitive and large band-gap reduction, and to an unexpected sudden increase in the effective mass of electrons. Even more surprisingly, both electronic and structural changes can be reversed fully and in a tunable manner by hydrogen incorporation. In this paper, we combine x-ray absorption spectroscopy at the nitrogen edge with ab initio simulations to investigate the atomic geometry of $\mathrm{N}\ensuremath{-}\mathrm{H}$ complexes in hydrogenated GaAsN. In this way, we provide experimental evidence that dihydrogen-nitrogen complexes with ${C}_{2\mathrm{v}}$ symmetry are the most abundant species in hydrogenated GaAsN. This finding contradicts previous predictions of ``in-line'' $\mathrm{N}\ensuremath{-}\mathrm{H}_{2}{}^{*}$ complexes as the predominant species, and accounts for recent infrared absorption experiments.

Dilute nitrides, in particular InxGa1-xAs1-yNy are very interesting alloys because of their actual and potential applications in the field of telecommunications and photovoltaics. A full knowledge of the local structure around each kind of atom, in particular the local ordering, is of fundamental importance because it is predicted to affect the electronics properties of these alloys. In this work we investigated the local environment of N and In by X-Ray absorption Fine Structure (XAFS). By exploiting the polarization dependency of XAFS we were able to demonstrate that N is fully substitutional to As. The lineshapes of the N K-edge XANES spectra are in fact identical, setting different grazing angles, which is a fingerprint of the cubic geometry of the N site. This information has been exploited to generate atomic clusters that very well simulate the experimental N K-edge XANES in the Full Multiple Scattering approach and to elaborate a multi-shell fit for XAFS data at the In K-edge. We concluded that N preferentially links In atoms, even if this short range ordering is by far weaker than the predicted one.

We describe the use of X-ray absorption spectroscopy (XAS) with synchrotron radiation to study the local structure in dilute nitrides. After a brief description of the advantages of XAS to probe local atomic arrangements in semiconductor alloys and nanostructures we focus our attention on (InGa)(AsN). We discuss data which demonstrate that atomic ordering (in the form of an excess of In-N over Ga-N bonds) is present, but is significantly weaker than predicted; also we show that the experimental values for the bond lengths are in agreement with recent models which take into account strain due to pseudomorphic growth.

We report on the structural investigation of the atomic environment of Fe impurities introduced in InP by high-temperature ion implantation. The lattice location of the implanted Fe atoms and its evolution upon annealing treatments have been investigated by means of proton-induced x-ray emission and Rutherford backscattering spectrometry in channeling conditions. X-ray absorption spectroscopy measurements have been performed in order to study the Fe local structure. The results of these measurements have been correlated to those obtained with other structural characterization techniques, as transmission electron microscopy and high-resolution x-ray diffraction, and provide an overall picture of the Fe incorporation mechanisms at the atomic level. It has been found that the high-temperature implantation process favors the incorporation of Fe in high-symmetry sites. Conversely, the point defect flux occurring during high-temperature annealing controls the kick-out of the Fe atoms from substitutional locations, leading to the formation of Fe-P complexes.

Short-range ordering in the form of an excess of In-N bonds with respect to the random case has been recently predicted for the dilute nitride alloy In x Ga 1 - x As 1 - y N y . This ordering induces a blue shift of the optical band gap and could represent a fundamental materials limitation. In this report we provide a quantitative determination of the effect of annealing on the short-range ordering in In x Ga 1 - x As 1 - y N y , using x-ray-absorption spectroscopy and state-of-the-art analysis methods. We find that in annealed samples short-range ordering is weak, one order of magnitude smaller than predicted.

We study the effect of hydrogen incorporation on the lattice properties of In xGa12xAs12yNy /GaAs heterostructures. The band gap widening observed in the photoluminescence spectra of hydrogenated GaAs12yNy and InxGa12xAs12yNy is accompanied by a lattice expansion along the growth direction, as measured by x-ray diffraction. At the same time, far-infrared spectroscopy reveals that a Ga-N local vibrational mode at ;472 cm 21 disappears upon hydrogen irradiation. All these effects are reversed upon hydrogen removal from the hydrogenated samples by thermal annealing. Finally, first-principles calculations indicate that a same dihydrogen complex is responsible for both the band gap reopening and the lattice expansion of hydrogenated InxGa12xAs12yNy .

(InGa)(AsN) quantum wells are particularly interesting for applications in fiber optics based communications, since they permit to make lasers with emission at 1.3 and 1.55 μm, with good temperature stability. The aim of this work is to study the local environment of In by means of X-ray absorption fine structure. In particular, the site ordering around In will be investigated in order to understand the structural origin of the non-linear optical and electronic proprieties which characterize this alloy. Despite the low N concentration (⩽5.2%) a significant contribution of In–N bonds to the signal is detected. The data provide evidence of a preferential distribution of N atoms around indium, as recently predicted via Monte Carlo simulations. Hydrogenation further enhances the increase in the number of In–N over In–As bonds.

Short-range ordering in the form of an excess of In-N bonds with respect to the random case has been recently predicted for the dilute nitride alloy InxGa1-xAs1-yNy. This ordering induces a blue shift of the optical band gap and could represent a fundamental materials limitation. In this report we provide a quantitative determination of the effect of annealing on the short-range ordering in InxGa1-xAs1-yNy, using x-ray- absorption spectroscopy and state-of-the-art analysis methods. We find that in annealed samples short-range ordering is weak, one order of magnitude smaller than predicted.