Assessment of orientation and extinction contrast contributions to the direct dislocation image

Superscrew dislocations in SiC have been investigated using synchrotron top ography in order to probe the details of the direct dislocation image at a magnified level. In view of the fact that the conventional extinction contr ast theory cannot fully explain the contrast features of superscrew disloca tions, a simplified orientation contrast model is developed. This model ena bles more accurate simulations of the observed dislocation images, and the results obtained from this model indicate that, in the low absorption case, the direct dislocation image comprises a significant orientation contrast contribution arising from the overlap and separation of the inhomogeneously diffracted x-rays with continuously varying directions. A new insight into the general contrast formation mechanisms of direct dislocation images on x-ray topographs is thus presented.