BCA Symposium Considers Diffraction from Nearly Perfect Crystals
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BCA Symposium Considers Diffraction from Nearly Perfect Crystals The British Crystallography Association (BCA) and Institute of Physics Physical Crystallography Group organized a Symposium on "Diffraction from Nearly Perfect Crystals" during the Solid State Physics Conference in Bristol, December 16-18, 1987. The Symposium comprised three sessions, all on slightly different themes and all proving very successful. Mary Halliwell (British Telecom Research Laboratories) organized the scientific program and Andrew Lang arranged the local requirements for the sessions. The first session was based on studies on Si. Joachin Schneider (Hahn-Meitner Institute) presented his work on y-ray diffraction. The typical wavelength is 0.039 Ä and a typical sample will be 1 cm thick and 10 cm in diameter. The advantages of this are the low absorption, no anomalous dispersion and the extinction is easy to handle, giving rise to an indefinite number of Pendellö'sung fringes allowing accurate measures of thicknesses and structure factors. The absence of multiple scattering is checked by the complementary nature of the transmitted and diffracted rocking curves. Schneider determined the strain fields arising from Si02 precipitates in Si samples by the slope in the average reflectivity vs. thickness line by rilting the crystal. The strain fields from these Clusters appear to extend over a few thousand angstroms, and are not fully understood. Peter Halfpenny (Hirst R.C.) gave a good account of topography on processed Si and gave examples of defects caused by mechanical, elastic and thermomechanical stresses. This he related to the original material and the processing. Keith Bowen et al. (Warwick University) used double crystal rocking curves to assess residual saw damage in Si wafers. The work showed that low energy ion beam milling can result in an acceptable surface; no improvement was found by an initial etch of the surface or by pursuing the ion beam milling beyond a certain depth removal. Mary Halliwell et al. (British Telecom) presented studies on Si/SiGe superlattice structures, including the comparison of the modulation period determined by x-ray diffraction, Raman, and TEM. The first two agreed well, but the TEM results differed for low strain levels because of the sampling volume to dislocation ratio, although it did show the dislocation distribution from the relaxation process.
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In a colorful talk, Andrew Lang (Bristol) threw much light on the study of defects in diamond. He summarized the optical Classification of these materials in relation to nitrogen defects and described the effects of natural and artificial radiation on diamond, indicating their potential as lasing materials. Mixed habit growth was studied by "spike" x-ray topography (on the SRS) and by cathodoluminescence. This showed the formation of platelet-like "laths," which was verified by TEM. In a second example, the formation of "negative crystals" or "voidites" due to the absence of a platelet was found by TEM. Very recent EELS work suggests that these consis
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