X-Ray Topographic Observation of Dislocation Contrast in Thin CdS Crystals

Abstract

Dislocations in thin CdS crystals have been observed by x-ray diffraction topography (Laue case). A method with a divergent beam from a point focus has been used for the study of dislocation contrast. Images of individual dislocations observed by this method consist of two lines which seem to be located on either side of their core. The observation shows that dislocation contrast is formed by x rays which are so deviated from the Bragg condition that the x rays are hardly diffracted by the perfect region of the crystal, and that one of the two lines is formed by the incident x rays with larger glancing angles than the Bragg angle and the other with smaller ones. From the comparison of the observed dislocation images and the elastical distortion near the dislocation core, it is inferred that, when a dislocation is observed by a beam deviated from the Bragg condition for the perfect region, an intensity enhancement of diffracted beam is produced at the place near the core where the beam satisfies the Bragg condition. By using a simple model based on this inference, the dislocation contrast and its variation with the dislocation depth are discussed. The signs of Burgers vectors can be determined from the dislocation contrast.