The optimal threading dislocation density of AlN template for micrometer-thick Al0.63Ga0.37N heteroepitaxy

•0.15% excessive compressive strain was found in AlN regrown on HVPE AlN buffer.•Dislocation nucleation and secondary morphologies was observed in thick AlGaN.•Compensation by Si-doped AlN is ineffective when the dislocation density is low.•Optimal dislocation density of a 1.6 μm n-AlGaN was estimated to be 7 × 108 cm−2.