We investigated the origin of morphological instability in 2 μm thick Al 0.6 Ga 0.4 N/AlN heteroepitaxy. The primary morphology was driven by the residual epitaxial strain, forming hill-like morphologies via surface diffusion. The secondary morphology was driven by the interaction between the primary morphology and dislocation clusters in the epitaxial layers. The difference in the local growth rate yields volcano-like morphologies centering on deep pits. Insertion of multi-stack superlattice transition layers between AlGaN and GaN effectively suppressed the secondary morphologies by simultaneously pre-relaxing the template and filtering treading dislocations.