  (a+c) dislocations in hexagonal materials are typically observed to be
dissociated into partial dislocations. Edge (a+c) dislocations are introduced
into (0001) nitride semiconductor layers by the process of plastic relaxation.
As there is an increasing interest in obtaining relaxed InGaN buffer layers for
the deposition of high In content structures, the study of the dissociation
mechanism of misfit (a+c) dislocations laying at the InGaN/GaN interface is
then crucial for understanding their nucleation and glide mechanisms. In the
case of the presented plastically relaxed InGaN layers deposited on GaN
substrates we observe a trigonal network of (a+c) dislocations extending at the
interface with a rotation of 3 degrees from <1-100> directions. High resolution
microscopy studies show that these dislocations are dissociated into two
Frank-Shockley 1/6<2-203> partial dislocations with the I1 BSF spreading
between them. Atomistic simulations of a dissociated edge (a+c) dislocation
revealed a 3/5 atom ring structure for the cores of both partial dislocations.
The observed separation between two partial dislocations must result from the
climb of at least one of the dislocations during the dissociation process,
possibly induced by the mismatch stress in the InGaN layer.
