Bloch waves and weak-beam imaging of crystals
Heinrich, Helge ; Kostorz, Gernot
In: Microscopy, 2000, vol. 49, no. 1, p. 61-65
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- The influence of the number of diffracted beams on weak-beam contrast simulations of thickness contour lines and dislocation images is investigated. For large deviation parameters sg→ thickness contour lines from two-beam simulations are similar to those from many-beam simulations. In many-beam simulations of wedge-shaped bent samples extra thickness contour lines appear at locations with (g→, 3g→). These extra lines occur between the imaging condition (g→,-g→) and (g→, 3g→). Therefore, in the case of a more symmetrical imaging condition many-beam simulations are mandatory. In bent samples the contributions of different Bloch waves to weak-beam images change as a function of the imaging conditions (g→, xg→). Near (g→, 3.5 g→) two Bloch waves dominate. In the case of x <3 two other Bloch waves with different wavelengths are most important for the image contrast. The ‘classical' (g→, 3g→) weak-beam condition is not suitable to determine signs and magnitudes of Burgers vectors from terminating thickness contour lines. Higher deviation parameters sg→ are necessary, especially for dense dislocation arrangements