Particularly high coherence of the x-ray beam is associated, on the ID19 beamline at ESRF, with the small angular size of the source as seen from a point of the sample (0.1-1 µrad). This feature makes the imaging of phase objects extremely simple, by using a `propagation' technique. The physical principle involved is Fresnel diffraction. Phase imaging is being simultaneously developed as a technique and used as a tool to investigate light natural or artificial materials introducing phase variations across the transmitted x-ray beam. They include polymers, wood, crystals, alloys, composites or ceramics, exhibiting inclusions, holes, cracks, ... . `Tomographic' three-dimensional reconstruction can be performed with a filtered back-projection algorithm either on the images processed as in attenuation tomography, or on the phase maps retrieved from the images with a reconstruction procedure similar to that used for electron microscopy. The combination of diffraction (`topography') and Fresnel (`phase') imaging leads to new results.
The behaviour of a (001) slice of initially single-domain rubidium titanyl arsenate (RbTiOAsO4, RTA) crystal, when prepared with a periodic Ag electrode of period 38 µm, as for periodic poling in nonlinear optics, is investigated for applied voltages of up to ±1.5 kV. The method of investigation is by synchrotron x-ray section topography with electric fields applied in situ, while under white-beam x-ray illumination at the ID19 topography beamline of the ESRF, Grenoble. An increasing expansion of the width of section topographs is observed with increasing voltage resulting from a corresponding bending of the lattice planes in the near-surface region, with angles ranging between 4–200 µrads. This behaviour is explained by the formation of a Schottky barrier, which results from a semiconductor–metal contact interaction between RTA and the Ag film, in the near-surface region beneath the high voltage electrode. This restricts the depth of the electric field to a near-surface depletion layer. The actual bending of the planes is by the electrostrictive strain that acts only in the depletion layer where the field is non-zero.
4H silicon carbide as-grown ingots were investigated by diffraction
imaging using synchrotron radiation. The white beam section topographs
obtained for various sample geometries allowed us to reveal structural
imperfections before slicing the bulky ingots to the thin wafers used as
electronic device substrates. The systematic investigation indicated that the
observed inclusions of different polytypes in 4H-SiC ingots are correlated
with the 8° off-axis orientation of the seed. These inclusions, formed
at the beginning of the crystal growth, provoke planar defects that propagate
along the main vertical axis of the cylindrical crystal. New findings
permitted us to understand the inclusion formation with the aim to increase
the useful volume.
The surface deformation and atomic-level distortions associated with crystal structural matching at ferroelectric inversion domain walls are investigated in periodically poled potassium titanyl phosphate (KTP) crystals. A deformation, of the order of 10 −8 m in scale and having the periodicity of the domains, is observed at the surfaces by optical interferometry. It is discussed in terms of the piezoelectric effect. The matching of the crystal structures at the domain walls is studied by combining the hard x-ray Fresnel phase-imaging technique with Bragg diffraction imaging methods ('Bragg-Fresnel imaging') and using synchrotron radiation. Quantitative analysis of the contrast of the Bragg-Fresnel images recorded as a function of the propagation distance is demonstrated to allow the determination of how the domains are matched at the atomic (unit cell) level, even though the spatial resolution of the images is on the scale of micrometres. The atom P(1) is determined as the linking atom for connecting the inversion domains across the wall in KTP crystals for domain walls forced by electric-field processing to be parallel to (100). In addition to this, it is shown that a shift of 1/2(a + b) between atoms in the original and inverted structures is introduced as a result of the domain inversion operation.
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