Analysis of Dislocation Contrast in Synchrotron Grazing-incidence X-ray Topographs and Ray-tracing Simulation in Off-axis 4H-SiC Crystals

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The contrast of dislocations lying on the basal plane in off-axis 4H-SiC crystals, such as basal plane dislocations, deflected threading screw and mixed dislocations are simulated using a more sophisticated ray-tracing simulation method, which is based on the orientation contrast mechanism. The effects of surface relaxation combined with X-ray absorption are both considered in this simulation method in order to further evaluate the factors contributing to the dislocation contrast formation. Comparison of the simulated results with the actual grazing-incidence topographic images of the dislocations reveals that the dislocation contrast is dominated by surface relaxation effect, which mainly occurs for diffractions near the crystal surface. Depth analysis shows that the diffracted X-ray beam from regions below the dislocation contribute additional contrast to the image formation. With consideration of X-ray absorption, the simulated contrast of the dislocation gradually weakens as the diffraction position gets deeper into the crystal. Detailed discussion on each type of dislocation on the basal plane including inclined dislocation contrast is presented in this paper. The full BPD loop contrast is also simulated and thus the Burgers vectors of all types of BPDs can be determined by comparing with the simulated results.