Structural Evaluation of Interfaces by Electron Diffraction
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would produce a streak whose length would increase smoothly with exposure time. As the exposure time increases, the image of the matrix reflection and its streaklike asymmetric extension would both extend in a coupled manner. 200 KeV electron beam
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Fig. 1 Fig. 2 Measurement of absolute intensities are very difficult for at least two reasons. The radius of the Ewald sphere for diffraction of high-energy electrons is quite large, such that the attainment of a pure two-beam condition is highly unlikely. Secondly, extinction distances in most materials are rather short (< 100O) for the lower order reflections, leading to double diffraction and diffuse scattering even in thin specimens. These effects are competitive; making the crystal thinner decreases double diffraction but will lengthen the matrix relrods, increasing the chances for partial excitation of other reflections. We view the requirement for knowing the absolute intensities to be more restrictive than necessary, and shall demonstrate that knowledge of the relative intensities is sufficient to begin understanding the source of these diffraction streaks. MEASUREMENT OF DIFFRACTED ELECTRON INTENSITY. The usual means for recording diffracted intensity in an electron microscope is electron image film, which converts electron intensity into optical density. The difficulty with this approach is that the central region of the matrix reflection will saturate the film when the exposure is set for the weaker streak, thus preventing any measurement of the optical density. We solved this problem by recording the same diffraction pattern at a variety of exposure times, such that at the shortest time the film at the central region was not overexposed. The concept is illustrated in Fig. 2, where the electron dose (proportional to the diffracted intensity) is plotted vs distance from [004] in the kl-plane. The three profiles, labeled tj, t., and t., correspond to electron doses on the film for three different exposure times. The band of dosages accommodated by the film before saturation is indicated at the left on the vertical axis. Schematic representations of the images of the diffracted intensity for each corresponding exposure time are shown below,
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the blackened region denoting film saturation. Note that the length and width of the reflection increase with increasing exposure time, and that the image dimension can be defined at any intensity level within the range accommodated by the film. In order to convert measured optical density levels into equivalent intensity levels, the relationship between optical density and electron dose must be known. This relationship was obtained by measuring the optical density of uniformly exposed Kodak Estar SO-163 electron image film as a function o
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