The origin of threading dislocations in GaN epitaxial layers grown onto a sapphire (00▪1) substrate was investigated using moiré fringes from plan-view transmission electron microscopy. The studied samples were nucleation layers deposited at 560C for times ranging from 20 to 180s. This initial stage growth gives rise to islands which were randomly rotated and relaxed with misfit dislocations. The islands that start to coalesce from 60  s growth time keep this random orientation and this led to the bending of 60° misfit dislocations in the interface plane to form a-type threading dislocations inside low angle boundaries.

On the Origin of a-Type Threading Dislocations in GaN Layers. Y.B.Kwon, J.H.Je, P.Ruterana, G.Nouet: Journal of Vacuum Science & Technology A, 2005, 23[6], 1588-91