  Strain relaxation of thick InGaN layers was studied in order to develop
technology of InGaN templates for deposition of InGaN Quantum Wells (QWs) and
InGaN layers of high-In-content. In this paper, we show that InGaN layers grown
on misoriented (0001)-GaN substrates relax by preferential activation of
certain glide planes for misfit dislocation formation. Substrate misorientation
changes resolved shear stresses, affecting the distribution of misfit
dislocations within each dislocation set. We demonstrate that this mechanism
leads to an anisotropic strain as well as a tilt of the InGaN layer with
respect to the GaN substrate. It appears that these phenomena are more
pronounced in structures grown on substrates misoriented toward <11-20>
direction than corresponding structures with <-1100> misorientation. These
features would influence the properties of the overgrown InGaN QWs and should
be taken into consideration during designing structures grown on relaxed InGaN
templates. We reveal that the lattice of partially relaxed InGaN has a
triclinic deformation, thus requiring advanced XRD analysis. The presentation
of just a single asymmetric reciprocal space map commonly practiced in the
literature can lead to misleading information regarding the relaxation state of
partially relaxed wurtzite structures.
