A Comparison of the Effects of Systematic Reflections on Extinction Contours in Bright‐ and Dark‐Field Images of Wedge Crystals

Abstract

AbstractThe dependence of the fringe spacing of extinction contours on crystal orientation has been studied in bright‐field images of wedge crystals of silicon. Crystal orientations were chosen so that either the (111) or (220) set of systematic reflections was excited. Comparison of the results obtained with extinction contours in (111) and (220) dark‐field images shows that when effects of systematic reflections are weak, as in the (220) case, there are no significant differences between bright‐and dark‐field contours. When effects of systematic reflections are strong however, as in the (111) case, bright‐ and dark‐field extinction contours are markedly different. Thus, for example, at certain crystal orientations the brightfield intensity profiles show the complex structure characteristic of the effects of multiple reflections whereas the dark‐field images, for the same diffracting conditions, are two‐beam in character.