Dislocations in a thick ammonothermal GaN substrate were investigated by synchrotron-radiation x-ray topography (SR-XRT) performed under six-beam diffraction conditions. The use of high-brilliance synchrotron radiation enabled the observation of the super-Borrmann effect, which significantly enhanced the anomalous transmission of x rays through the 350-μm-thick crystal. The evolution of dislocation contrast with varying deviation angle Δω revealed a transition from kinematical to dynamical diffraction, in agreement with theoretical predictions based on the dynamical theory. By selectively generating five equivalent two-beam diffraction conditions near the six-beam configuration, the Burgers vectors of individual threading edge dislocations were identified according to the g⋅b invisibility criterion. The measured linewidths of the dislocation images were consistent with calculated values derived from the extinction distance and |g⋅b| dependence, confirming that most dislocations had a Burgers vector containing a a-type component of 13⟨112¯0⟩ or 23⟨112¯0⟩. These results demonstrate that SR-XRT under multi-beam diffraction provides a powerful nondestructive approach for quantitative dislocation analysis in thick GaN crystals, offering valuable insight into defect structures relevant to high-performance GaN-based electronic devices.
Yao et al. (Wed,) studied this question.