RDF Analysis of Ion-Amorphized SiO 2 and SiC from Electron Diffraction using Post-Specimen Scanning in the Field-Emissio
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Mat. Res. Soc. Symp. Proc. Vol. 504 © 1998 Materials Research Society
q- 4 , slightly faster than for X-rays at high angles. Nonetheless, energy-filtered electron diffraction (EFED) is the only practical way to investigate very small volumes, because of the much stronger interaction of the electron with matter, and can provide information about the spatial variation of atom correlations unavailable from X-ray or neutron diffraction methods, spatially averaging over much larger volumes. EFED still appears to be the only method by which information about changes in intermediate-range order can be assessed in the small sample volumes associated with amorphization using the focused electron or ion-beams required to effect the amorphization fluences. EXPERIMENTAL In the HB603 FEG-STEM instrument used for our studies, two methods of generating a selectedarea diffraction pattern are possible. In the first, the incident electron beam is rocked about the scanned area to define the scattering angle with respect to the entrance aperture of the electron spectrometer. This is the mode in which our earlier EFED data were generated [1,2], but spherical aberration of the objective lens causes offsets in the effective area from which the data are collected [3]. Since the samples typically have non-uniform thickness, the result is that data collected at different angular distances originate from areas of differing thickness, effectively precluding practical data reduction for data collected at large scattering angles (q > 16 nmI). In the second [4], the incident beam is kept stationary and post-specimen scan coils, available in the HB603, are used to sweep the diffraction pattern across the spectrometer entrance aperture. The spherical aberration manifests itself in this mode in a defocusing of the PEELS spectrometer, but enough adjustment is available to compensate. In this way, we have recently been able to extend the collection angle to at least q = 25 nm- 1 (Fig. 1), comparable to the range (and corresponding RDF peak resolution) available in X-ray diffraction [5]. Current operation of the HB603 in the EFED mode replaces the PIN-diode array of the PEELS with an optical slit, demagnified by lenses in the PEELS system to define an 8 eV energy window centered on zero energy loss into a photomultiplier detector operating in analog mode. The arrangement effectively converts the system from parallel to serial detection and is described in detail elsewhere [6]. A future implementation may employ spectrum imaging for EFED when the associated software becomes available. Energy discrimination cannot remove the low-loss but high-angle electron-phonon scattering, which comprises the principal background limiting angular collection range, so EFED at cryogenic temperatures is presently being explored. Energy filtered diffraction-pattern line scans I(q) are calibrated against a crystalline reference, which in our case is a polycrystalline gold sample, and ideally are corrected for multiple elastic scattering and scattering from any thin amor
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