The lattice relaxation of strained superlattices, grown onto thick GaN buffer layers, was investigated by using optical microscopy, X-ray diffraction and photoluminescence spectroscopy. The results were compared with results for strained bulk AlGaN layers. A relaxation process was identified which maintained coherency between AlGaN barriers and GaN wells in the superlattice. This was attributed to the effect of misfit dislocations at the buffer/superlattice interface. The AlGaN barriers also relaxed via cracks which formed beyond a critical Al content and limited the additional strain energy, as compared with a free-standing superlattice, to a maximum value. The cracks relieved tensile plane stresses, and did not put the GaN wells of the superlattice under additional plane compression. This was attributed to the effect of misfit dislocations which nucleated at crack faces and glided into the superlattice at well/barrier interfaces.

Strain Relaxation in AlGaN/GaN Superlattices Grown on GaN. S.Einfeldt, H.Heinke, V.Kirchner, D.Hommel: Journal of Applied Physics, 2001, 89[4], 2160-7