Kinematical and dynamical contrast of dislocations in thick GaN substrates observed by synchrotron-radiation X-ray topography under six-beam diffraction conditions

Dislocations in a thick ammonothermal GaN substrate were investigated using synchrotron-radiation X-ray topography (SR-XRT) under six-beam diffraction conditions. The high brilliance of the synchrotron source enabled the observation of the super-Borrmann effect, which markedly enhanced the anomalous transmission of X-rays through the 350~$μ$m-thick crystal. Systematic variation of the deviation angle~$Δω$ revealed a clear transition from kinematical to dynamical diffraction, consistent with theoretical predictions based on dynamical diffraction theory. By selectively exciting five equivalent two-beam diffraction conditions near the six-beam configuration, the Burgers vectors of individual threading edge dislocations (TEDs) were determined according to the $g\cdot b$ invisibility criterion. The measured dislocation image widths agreed well with calculated values derived from the extinction distance and $|g\cdot b|$ dependence, confirming that most dislocations possess Burgers vectors containing an $a$-type component of $\frac{1}{3}\langle 11\bar{2}0\rangle$ or $\frac{2}{3}\langle 11\bar{2}0\rangle$. These results demonstrate that SR-XRT under multibeam diffraction provides a powerful, nondestructive method for quantitative dislocation analysis in thick GaN crystals, offering valuable insights into defect structures critical for high-performance GaN-based electronic devices.