Contrast mechanisms in electron-channelling patterns and lattice-defect images obtained with a scanning electron microscope

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Abstract
The yield of back-scattered electrons in bulk crystalline specimens has been formulated as a function of the incident beam direction. In this theory, the initial inelastic scattering of primary electrons results from the excitation of phonons and core electrons, while subsequent inelastic scatterings are simulated by a Monte-Carlo method that ignores the diffraction effect. Based on this theory, the contrast of electron-channelling patterns can be classified into two types: line- and band-type contrasts. The yield for a silicon specimen is calculated numerically over several detection ranges at specimen tilt angles of 0°, 45° and 60°. It is shown that line-type contrast increases with increasing angle of specimen tilt, while band-type contrast decreases with increasing angle. Both contrasts are conveyed by primary electrons which, initially, are inelastic-scattered at small angles. In this case, the excitation of phonons and core electrons is the prime cause of band- and line-type contrasts respectively. Changes in the yield caused by an inclined stacking fault have been calculated in terms of the present theory. The maximum change occurs in an incident beam direction deviating slightly from the Bragg position where the channelling patterns exhibit a line-type contrast at oblique incidence. The relationship between the direction and the reflection used is also given. The contrast mechanisms in the channelling patterns and fault images are then discussed, including the importance of small-angle inelastic scattering.

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