Characterization of prismatic slip in PVT-grown AlN crystals

The most frequently observed deformation mechanism in wurtzite aluminum nitride (AlN) grown by the physical vapor transport (PVT) method is basal plane slip. However, prismatic slip also takes place in such crystals. In this study, prismatic slip is observed in a 50 mm commercial AlN substrate wafer, leading to a sixfold symmetric BPD pattern in the wafer. Prismatic dislocations cross slip on to basal plane and multiply there, increasing the dislocation density of the wafer. A radial thermal model is established to explore the origin of the unevenly distributed dislocations. The unevenly distributed resolved shear stress across the whole radial area of the boule during PVT growth caused by radial thermal gradients offers driving force for the activation of the three sets of prismatic slip systems, producing nonuniform dislocation distribution in the boule and thus the wafer. Both experimental observations and theoretical calculations of the critical resolved shear stress (CRSS) correlate well with the model results, revealing that controlling radial thermal gradients is critical for lowering dislocation density and enhancing the crystal quality using PVT growth method.