The optimal threading dislocation density of AlN template for micrometer-thick Al_0.63Ga_0.37N heteroepitaxy

Growth of Si-doped AlN and of Al_0.63Ga_0.37N on high quality AlN templates grown by HVPE has been investigated. The strain state of Si-doped AlN is thickness-dependent due to the surface-mediated dislocation tilt which induces a tensile strain component. At very low dislocation density in the AlN buffer strain relaxation in micrometer-thick Al_0.63Ga_0.37N layers is dominated by generation of additional dislocations. As a result, the final threading dislocation density and the surface roughness increase significantly. For a 1.6 μm Al_0.63Ga_0.37N on 3.4 μm Si-doped AlN bi-layer structure, the optimal threading dislocation density (TDD) of the AlN:Si buffer is estimated to be 7 × 10⁸ cm⁻², where the low TDD can be still transferred from the buffer to the thick AlGaN heterostructure without generation of many new dislocations.