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ItemDirect correlation of microtwin distribution with growth face morphology of CVD diamond films by a novel TEM technique(Georgia Institute of Technology, 1994-06) Wang, Z. L. (Zhong Lin) ; Bentley, James ; Clausing, R. E. ; Heatherly, L. ; Horton, L. L.A thick as-grown diamond film was examined directly by conventional transmission electron microscopy (TEM) without thinning, and the important microstructures near the growth surface were characterized. Specimen preparation for TEM involved simply fracturing the film; some of the diamond grains located on the specimen edge were thin enough to be directly examined by TEM. The 3-D topography of the diamond grains located at the intersection of the growth and the fracture surfaces was obtained using secondary electron images, so that the 2-D projected grain geometry could be derived easily to help interpret the TEM images. A diamond film grown with a <001> texture and having grains 2–3 µm diameter with {001} facets parallel to the substrate and four inclined {111} facets was examined. Grains with fracture surfaces that intersected the top (001) facet, grains with fractures that intersected only {111} facets, and unfractured grains were studied. It was found that the core volume bounded by the (001) top facet and its projected column defined by orthogonal internal {110} were free from microtwins, but contained a few dislocations. The remaining volume around this core, bounded by {111} facets (or grain boundaries) and the internal {110}, was filled with microtwins. The microtwins were not merely at the {111} surfaces. Our results reveal a growth mechanism in which microtwins are formed as material is added to {111} but not {001}. The formation of microtwins in CVD diamond is thus clearly associated with growth on {111} surface facets.
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ItemEnergy-filtered HREM images of valence-loss electrons(Georgia Institute of Technology, 1991-12) Bentley, James ; Wang, Z. L. (Zhong Lin)A theory is proposed to include the effects of valence excitations in electron image simulations for high-resolution electron microscopy (HREM) based on the single inelastic scattering model. Under the small thickness approximation, this general theory reduces to the simplified theory of perfectly delocalized inelastic scattering model, in which the image can be considered to be an incoherent sum of those incident electrons of different energies weighted by the intensity distribution in the electron energy-loss spectrum from the area where the pattern was taken. The main effect of valence-loss is to introduce a focus shift due to chromatic aberration, resulting in contrast variation (or reversal) of the image. The generalization of this theory for simulations of interface images with considering surface and interface plasmon excitations is given. Calculations for GaAs surface profile images are demonstrated to show the effect of inelastic localization.
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ItemOptimum experimental conditions for quantitative surface microanalysis by reflection electron energy-loss spectroscopy(Georgia Institute of Technology, 1991-04) Bentley, James ; Wang, Z. L. (Zhong Lin)Experimental conditions for obtaining high quality core-shell ionization edges in reflection electron energy-loss spectroscopy (REELS) are investigated. Under the (600) specular- "mirror" reflection conditions and using the relative ionization cross-section measured from a MgO thin foil in the transmission geometry for collection semi-angle β = 1.2mrad, the chemical composition of MgO (100) surfaces is determined to be NO/NMg = 1.5 ± 0.15. This value is not significantly affected by varying the resonance diffraction conditions near the [001] zone axis, under which the spectra were acquired. An incorrect apparent composition will result if channeling effects along the [011] zone axis are not considered properly. Surface microanalysis is limited by the accuracy of the core-shell effective ionization cross-section (EICS), which depends not only on the property of a single atom but also on the dynamical elastic and inelastic scattering and channeling processes of electrons. An experimental method is outlined by which to measure the relative EICS from a thin foil specimen in the transmission case under the equivalent resonance conditions as in reflection geometry.
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ItemIn-situ observations of high temperature surface processes on α-alumina bulk crystals(Georgia Institute of Technology, 1991) Wang, Z. L. (Zhong Lin) ; Bentley, JamesReflection electron microscopy (REM) was applied to image in-situ the dynamic changes of atomic-height steps on the surfaces of a-alumina bulk crystals heated to high temperatures. Atomic diffusion, desorption and adsorption processes on cleaved a-alumina (012) surfaces were directly observed at temperatures of 1470 to 1670 K. The surface started to show visible structural changes at 1470-1520 K. The main surface process appears to be atomic desorption, which creates large, flat vacancy-type terraces on the surface.