Dislocation Climb in AlN Crystals Grown at Low-Temperature Gradients Revealed by 3D X-ray Diffraction Imaging

The dislocation evolution in a cross-section a-plane cut through a sublimation-grown aluminum nitride (AlN) crystal grown with low-temperature gradients and subsequent low thermal stress is investigated with different X-ray diffraction imaging methods. Exploiting the so-called weak-beam contrast using monochromatic X-rays in combination with suitable three-dimensional (3D) interpretation and reconstruction allows the identification of individual dislocations as well as tracing their progression in the crystal volume, even in the considerably strained interface region. It is particularly striking that the laterally grown crystal volume is dislocation-free. The dislocation densities in the seed and the bulk volume are similar (1 x 103 cm-2), but while the dislocations in the seed are randomly arranged, the dislocations in the bulk volume show a uniform line shape, indicating a common mechanism of dislocation movement. Since the dislocation slings in the bulk do not lie in slip planes, it can be concluded that the lateral movement does not result from dislocation glide, but from impurity-driven climb of dislocations during growth. The absence of slip can be explained by the low-temperature gradients and the subsequent low thermal stress below the critical resolved shear stress (CRSS).

Automated summary

The evolution of dislocations in a sublimation-grown aluminum nitride crystal with low-temperature gradients and low thermal stress is investigated using different X-ray diffraction imaging methods. Individual dislocations and their progression in the crystal volume, including the strained interface region, are identified through weak-beam contrast imaging and 3D interpretation. The absence of slip in the laterally grown crystal volume suggests that the movement of dislocations is driven by impurities rather than dislocation glide, due to low-temperature gradients and low thermal stress below the critical resolved shear stress (CRSS).