Suppressing Diffraction-Related Intensity Losses in Transmissive Single-Crystal X-ray Optics

The highest-quality X-ray optics can be made of single-crystal materials such as silicon, germanium, or, even better, diamond. Unfortunately, such X-ray optics have one drawback: diffraction losses or the “glitch effect”. This effect manifests itself as follows: at some energies of X-rays, the intensity of the transmitted beam drops due to the fact that some crystalline planes have satisfied the diffraction condition. Diffraction losses are usually observed in spectroscopic experiments when the energy of the X-rays changes in a certain range. However, this effect might also influence any experiment using X-rays, especially at higher energies. In this paper, we propose a method to overcome the glitch problem in transmissive optics. This is achieved using small rotations of the optical element. We describe the algorithm for “glitch-free” measurements in detail and the theory behind it.

Резонансное отражение света оптической решеткой экситонов, сформированной 100 квантовыми ямами InGaN

Optical properties of a structure with a periodic system of 100 InGaN quantum wells (QWs) separated by non-tunneling GaN barriers have been investigated at room temperature. The structure periodicity corresponded to the Bragg diffraction condition at the frequency of the QW excitons. Numerical modeling using transfer matrices gave a quantitatively accurate fit of the experimental results. The model included the resonance response of A, B, and C excitons in QWs and an optical absorption tail in the barriers and buffer layer. We have determined the radiative and non-radiative broadening of the excitons in the InGaN QWs.

RigakuIntegrate : A New Single Crystal Integration Program

RigakuIntegrate is a new single crystal integration program designed to increase integration accuracy by optimizing the reflection domain to exclude all but relevant elastic scattering. Instead of integrating over a shoebox domain, RigakuIntegrate sums over a tailored detector region that incorporates the known diffraction physics of each reflection as it passes through the Ewald sphere, combined with the known properties of the area detector. The reciprocal space reflections are modelled by ellipsoids that account for the crystal radius, beam crossfire, mosaicity, and wavelength dispersion. In case of laboratory sources separate ellipsoids are assigned to each of the Kα1 and Kα2 components. The passage of the reflection through the diffraction condition is modeled by the intersection of the ellipsoid(s) with the Ewald sphere, resulting in a set of ellipses. This set of intersection ellipses is then projected onto the detector plane along the scattered ray direction. Interaction with the detector sensor is modeled by appropriate convolution, resulting in reflection-specific integration domains over the surface of the detector. Results from crystals ranging in quality from exquisite (a charge density analysis of oxalic acid at 100K using a RAPID IP detector) to marginal (a highly mosaic and split crystal that refines to R1(all data) = 2.6% using a Pilatus detector) will be presented.

Surface diffraction on a ψ-circle diffractometer using the χ-axis geometry

The restricted volume above and below the sample on a six-circle diffractometer can limit the size and complexity of sample environments used in surface diffraction studies. An alternative configuration of the diffractometer, where the sample normal is aligned parallel to the χ axis, allows for ample space above and below the χ circle for instrumentation. The merits of this approach are outlined and angles are derived for the diffraction condition for constant-incident-angle, constant-sample-azimuthal-angle and specular geometries. Using a version of this code written forSPEC(http://www.certif.com/content/spec/), sample specular and nonspecular crystal truncation rods measured from a 5 nm-thick thin film are presented.

3-dimensional imaging of dislocation microstructures by electron beams

ABSTRACTWe review the progress in the electron tomography of dislocation microstructures in the transmission electron microscope (TEM). Dislocation contrast is visible both in conventional TEM and scanning TEM (STEM) modes and, despite the complicated intensity variations, dislocation contrast can be isolated using computational filtering techniques prior to reconstruction. We find that STEM annular dark-field (STEM-ADF) imaging offers significant advantages in terms of dislocation contrast and background artifacts. We present several examples, both in semiconducting and metallic systems, illustrating the properties of 3D dislocations. We present the high-angle triple-axis (HATA) specimen holder where the diffraction condition can be chosen at will and dislocation tomograms of multiple reflections can be combined. 3D dislocations are analyzed in terms of dislocation density and dislocation nodal structures. Several avenues of study are suggested that may exploit the 3D dislocation data.

Investigation of Diffraction Condition and Convergent Probe Effects on Inelastic Mean Free Path Determination by Using a Cone-Shaped Silicon Crystal

Extended abstract of a paper presented at Microscopy and Microanalysis 2006 in Chicago, Illinois, USA, July 30 – August 3, 2006