[en] An extensive structural investigation of InAs ultrathin epitaxial films of different thickness (0.18 nm≤t_InAs≤0.77 nm) buried in a GaAs host matrix is presented. The results show a transition from a perfect coherently grown epitaxial structures to a 3D nucleation mode with the increasing of the InAs thickness beyond 0.81 nm. 10 refs., 3 figs., 1 tab

[en] In this work, the authors investigate the structural properties of (GaIn)(AsN)/GaAs multiple quantum wells (MQW) grown at low temperature by metalorganic vapor phase epitaxy. The structural properties, in particular the In- and N-incorporation, the lattice strain (strain modulation), the structural perfection of the metastable (GaIn)(AsN) material system and the structural quality of the (GaIn)(AsN)/GaAs interfaces are investigated by means of high-resolution x-ray diffraction, transmission electron microscopy (TEM), and secondary ion mass spectrometry. They demonstrate that (GaIn)(AsN) layers of high structural quality can be fabricated up to lattice mismatches of 4%. The experiments reveal that N and In atoms are localized in the quaternary material and no evidences of In-segregation or N-interdiffusion could be found. TEM analyses reveal a low defect density in the highly strained layers, but no clustering or interface undulation could be detected. High-resolution TEM images show that (GaIn)(AsN)/GaAs interfaces are slightly rougher than GaAs/(GaIn)(AsN) ones

[en] The Materials Science beamline at the Swiss Light Source saw 'first light' in Spring 2001 and is presently undergoing commissioning and pilot experiment operation. In this article we concentrate on one of the three MS beamline stations, the Powder Diffractometer, and report on resolution tests and first experimental results. Using the high-resolution multi-crystal analyzer detector, full widths at half maximum (FWHM) smaller than 0.01 deg. were measured with standard powders and FWHMs smaller than 0.003 deg. were measured with a single Si(111) crystal and a very low-divergent incident beam. With the same detector, first residual stress measurements on low pressure plasma sprayed coatings were performed. Dynamic electric field-induced strain effects in relaxor perovskite crystals and the influence of water-soluble polymers on the early hydration of cement were studied using a fast read-out multistrip solid-state detector

[en] In this work, we present a generalized dynamical theory for strained multilayers within the Laue formalism valid for x-ray Bragg diffraction. In the generalized theory we take into account all the four solutions of the secular equation, the asymptotic sphericity of the dispersion surface, the difference between electric and displacement fields, and the boundary conditions of continuity at the heterointerfaces for the tangential components of the electric and magnetic fields. Only the following approximations are made: to consider a two-beam case, and to neglect quadratic terms of the dielectric susceptibility. Our general equations give the correct x-ray reflectivity in the whole angular range between 0 and π/2, i.e., also very far from the Bragg peaks. Great differences between the predictions of the conventional and generalized theories are obtained in several cases, for important features of the rocking curves, like angular position, peak intensity, and full width at half maximum of superlattice satellite or epitaxial layer peaks. We discuss in detail the following cases for which only the generalized theory describes correctly the x-ray scattering: coplanar diffraction (i) very far from the Bragg condition, (ii) near to the Bragg condition for strong asymmetric reflections, and (iii) for highly mismatched heterostructures. copyright 1997 The American Physical Society

[en] In this work investigate the lateral periodicity of symmetrically strained (GaIn)As / GaAs / Ga(PAs) / GaAs superlattices by means of X-ray scattering techniques. The multilayers were grown by metalorganic vapour phase epitaxy on (001)GaAs substrates, which were intentionally off-oriented towards the [011[ direction. The substrate off-orientation and the strain distribution was found to affect the structural properties of the superlattices inducing the generation of laterally ordered macro steps. Several high-resolution triple-crystal reciprocal space maps, which were recorded for different azimuth angles in the vicinity of the (004) Bragg diffraction and contour maps of the specular reflected beam collected in the vicinity of the (000) reciprocal lattice point, are reported and discussed. The reciprocal space maps clearly show a two-dimensional periodicity in the direction of the surface normal and to a lateral periodicity in a crystallographic direction coinciding with the miscut orientation. The distribution and correlation of the vertical as well as of the lateral interface roughness was investigated by specular reflectivity and diffuse scattering measurements. The results show that the morphology of the roughness is influenced by the off-orientation angle and can be described by a 2-dimensional waviness

No abstract available

[en] The increasing scientific and technological interest in nanoparticles has raised the need for fast, efficient, and precise characterization techniques. Powder diffraction is a very efficient experimental method, as it is straightforward and nondestructive. However, its use for extracting information regarding very small particles brings some common crystallographic approximations to and beyond their limits of validity. Powder pattern diffraction calculation methods are critically discussed, with special focus on spherical particles with log-normal distributions, with the target of determining size distribution parameters. A 20-nm CeO₂ sample is analyzed as an example

[en] A review article about phase retrieval problem in X-ray phase contrast imaging is presented. A simple theoretical framework of Fresnel diffraction imaging by X-rays is introduced. A review of the most important methods for phase retrieval in free-propagation-based X-ray imaging and a new method developed by our collaboration are shown. The proposed algorithm, Combined Mixed Approach (CMA) is based on a mixed transfer function and transport of intensity approach, and it requires at most an initial approximate estimate of the average phase shift introduced by the object as prior knowledge. The accuracy with which this initial estimate is known determines the convenience speed of algorithm. The new proposed algorithm is based on the retrieval of both the object phase and its complex conjugate. The results obtained by the algorithm on simulated data have shown that the obtained reconstructed phase maps are characterized by particularly low normalized mean square errors. The algorithm was also tested on noisy experimental phase contrast data, showing a good efficiency in recovering phase information and enhancing the visibility of details inside soft tissues.

[en] In this work the authors report on a structural study performed by specular reflectivity, diffuse scattering and grazing angle X-ray diffraction measurements of new Langmuir-Blodgett multilayers of a perylene-3, 4, 9, 10-tetracarboxyldiimide derivative (PTCDI-Opent) and of a long chain-ammonium salt of 12-phosphomolybdate (PM₁₂ O₄₀^3-). The periodic arrangement of the multilayers along the growth direction as well as the lateral crystalline in-plane ordering is investigated. Their results indicate a good structural quality of these complex organic molecules. Precisely, the specular reflectivity spectra exhibit very well defined Kiessig fringes implying a uniform thickness of both the investigated structures. Moreover, both films show correlated interfaces and a lateral structural arrangement of the multilayers in domains (70-149 _A). A fairly good structural ordering within these domains is found

[en] This paper reports on the striking correlation between nanosize mosaic domain walls in YBCO films and 1D rows of parallel Josephson junctions, determining the J_cvs.B curves. From X-ray data analysis, it results that the average ''hidden'' domain wall, faceted at a nanometric scale, is almost mimicking the Josephson Junction (JJ) 1D array. The assumption that the JJs and the domain-wall arrays are coincident, enables to find out the particular scaling field, making the J_cvs.B curves independent of temperature. This scaling field can be interpreted in terms of the Josephson nature of the transport current across these particular patterns in the intermediate temperature range. By means of our model it is also possible to calculate two asymptotic behaviors of the pinning force as a function of field, for low and high fields, respectively. These behaviors are punctually repeated by the experimental results in the same asymptotic limit, so that two corresponding vortex regimes are clearly pointed out. All results can be interpreted by concluding that in the intermediate temperature range, the strong pinning observed in high quality YBCO films is due to the Josephson Junctions average patterns. These patterns are the counterpart related to the transport mechanisms of ''hidden'' structural nano-domains. (orig.)