Additional image peaks in the high resolution imaging of dislocations

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Abstract
Various high resolution methods of imaging dislocations in electron microscopy are discussed and a new technique is considered. It is shown that an assumption made in these methods, namely that one dislocation core gives rise to one image peak, is not necessarily correct. In the weak-beam technique and in the high-resolution bright-field technique it is shown that an undissociated dislocation can, under certain conditions, give rise to two image peaks, the second peak arising from the dislocation core region. This ‘core peak’ is typically about 20 Å from the normal image peak in the weak-beam technique, and about 40 Å from the image dip in the high-resolution bright-field method. Hence care must be taken in the interpretation of weak-beam or high-resolution bright-field images of apparently dissociated dislocations having separations of about 20 or 40 Å, respectively. The core peak is very narrow, about 4 Å wide, and its contrast is substantially reduced using a highly convergent incident beam. Hence it would be detected only in very high resolution imaging using reasonably parallel illumination. Weak-beam images of an undissociated dislocation using a second-order reflection can have two or even three image peaks unless the deviation parameter is very large. A new method of imaging, called elastic diffuse imaging, is discussed. In this method the objective aperture is placed between Bragg spots in order to collect electrons elastically scattered from highly disordered regions of crystals. It is suggested that this method should yield detailed information on dislocation core structures.

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