Diffraction contrast effects from stacking faults with phase angle ?

Abstract
The appearance of multiple fringes at stacking faults with phase angle π, observed by transmission electron microscopy, is investigated. Examples of this case have been observed at prismatic faults in hexagonal AIN crystals. In this case anomalous absorption effects are small and the theory without absorption is applicable. For α=π thickness extinction contours are not visible as an undulation in the background intensity inside the fault image as happens for the more usual case α=±2π/3. Examples of this behaviour are given. The method can be used to distinguish the cases α=π and α=±2π/3. The case α=π with strong anomalous absorption is also considered. No clear examples of this have yet been observed. It is shown that no strong fringes are expected in this case. The physical explanation in terms of the two crystal Bloch waves is discussed.

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