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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.

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.

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.

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 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.

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 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.

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.

7 páginas, 6 figuras, 3 tablas.-- Comunicación presentada al EMRS 2002 Spring Meeting, Symposium S: Micro- and Nano-structured Semiconductors celebrado en Estrasburgo (Francia) del 18 al 21 de Junio de 2002., 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., This work was supported by CICYT projects no. MAT99- 0847 and by the NANOMAT project of the EC growth programme contract no. G5RD-CT-2001-00545. One of the authors (M.G. Proietti) is pleased to acknowledge support from the ‘Centre National pour la recherche scientifique’ for visit during which the research was completed.