Enormous progresses regarding the growth of group-III nitrides were achieved in the past years and commercial InGaN-based light emitting diodes and laser diodes have been realised. However, kinetic growth effects such as phase separation and segregation governing epitaxial growth of InGaN quantum wells are still not understood well enough.

In InGaN/GaN the local lattice parameter in growth direction can be used to obtain the local In-concentration. However, recent investigations e.g. by Smeeton et al. showed that InGaN layers are extremely sensitive to the electron beam in the microscope and that strain fluctuation form after a couple of minutes. Therefore, methods of composition determination that require acquisition of defocus series or precise adjustment of imaging conditions at the area under investigation cannot be used. In contrast, imaging conditions are needed that require minimum adjustment and which allow an accurate but sufficiently quick measurement of the strain distribution in the specimen.

In our contribution, we will show that strain state analysis in In_xGa_1-xN/GaN using a three- or two-beam condition can lead to substantial artefacts. We evaluated simulated images in dependence of specimen thickness, specimen orientation and objective lens defocus. We observed that the measurement is in agreement with the true strain profile for certain conditions only. An analysis of error sources revealed, that artefacts are mainly caused by a combination of delocalization and the composition dependence of the phases of the beams contributing to the image formation. Due to the delocalization effect, interference of the undiffracted beam with an 0002 beam works best. The chemical shift of the phases is minimized by using the 000+2 beam and a strong excitation of 000+4. Images simulated for these conditions taking into account lattice plane bending and strain using finite-element calculations revealed a good agreement of evaluated and true strain profiles.

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