Step by step capping and strain state of GaN/AlN quantum dots studied by grazing incidence diffraction anomalous fine structure

The investigation of small size embedded nanostructures, by a combination of complementary anomalous diffraction techniques, is reported. GaN Quantum Dots (QDs), 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 photons energy is tuned across the Ga absorption K-edge which makes diffraction chemically selective. Measurement of \textit{hkl}-scans, close to the AlN (30-30) 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 QDs is deduced. From the fixed-Q energy-dependent diffracted intensity spectra, measured for diffraction-selected iso-strain regions corresponding to the average in-plane strain state of the QDs, quantitative information regarding composition and the 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 over-strained regime.Comment: submitted to PR

Antiferromagnetic ordering in a 90 K copper oxide superconductor

Using elastic neutron scattering, we evidence a commensurate antiferromagnetic Cu(2) order (AF) in the superconducting (SC) high-$\rm T_c$ cuprate $\rm YBa_2(Cu_{1-y}Co_y)_3O_{7+\delta}$ (y=0.013, $\rm T_c$=93 K). As in the Co-free system, the spin excitation spectrum is dominated by a magnetic resonance peak at 41 meV but with a reduced spectral weight. The substitution of Co thus leads to a state where AF and SC cohabit showing that the CuO$_2$ plane is a highly antiferromagnetically polarizable medium even for a sample where T$_c$ remains optimum.Comment: 3 figure

Ab initio study of bilateral doping within the MoS2-NbS2 system

We present a systematic study on the stability and the structural and electronic properties of mixed molybdenum-niobium disulphides. Using density functional theory we investigate bilateral doping with up to 25 % of MoS2 (NbS2) by Nb (Mo) atoms, focusing on the precise arrangement of dopants within the host lattices. We find that over the whole range of considered concentrations, Nb doping of MoS2 occurs through a substitutional mechanism. For Mo in NbS2 both interstitial and substitutional doping can co-exist, depending upon the particular synthesis conditions. The analysis of the structural and electronic modifications of the perfect bulk systems due to the doping is presented. We show that substitutional Nb atoms introduce electron holes to the MoS2, leading to a semiconductor-metal transition. On the other hand, the Mo doping of Nb2, does not alter the metallic behavior of the initial system. The results of the present study are compared with available experimental data on mixed MoS2-NbS2 (bulk and nanoparticles).Comment: 7 pages, 6 figure

Structural properties of Ge/Si(001) nano-islands by diffraction anomalous fine structure and multiwavelength anomalous diffraction

8 pags, 6 figs, 2 tabsIn 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

Local structure study about Co in YBa2_2(Cu1−x_{1-x}Cox_x)3_3O7−δ_{7-\delta} thin films using polarized XAFS

We have studied the local structure around Co in YBa$_2$(Cu$_{1-x}$Co$_x$)$_3$O$_{7-\delta}$ thin films with three different concentrations: x=0.07, 0.10, 0.17, and in a PrBa$_2$(Cu$_{1-x}$Co$_x$)$_3$O$_{7-\delta}$ thin film of concentration x=0.05 using the X-ray Absorption Fine Structure (XAFS) technique. Data were collected at the Co $K$-edge with polarizations both parallel and perpendicular to the film surface. We find that the oxygen neighbors are well ordered and shortened in comparison with YBCO Cu-O values to 1.80 \AA{} and 1.87 \AA{} in the $c$-axis and $ab$-plane, respectively. A comparison of further neighbors in the thin film and powder data show that these peaks in the film are suppressed in amplitude relative to the powder samples, which suggests there is more disorder and/or distortions of the Co environment present in the thin films.Comment: 14 pages; To be submitted to Phys. Rev.

SIRIUS at Synchrotron SOLEIL: a beamline suitable for in situ x-ray characterization of the ALD of advanced materials

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SIRIUS: A new beamline for in situ X-ray diffraction and spectroscopy studies of advanced materials and nanostructures at the SOLEIL Synchrotron

International audienceWe 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