Why the strain relaxation by V-pit defects is not visible in X-ray reciprocal space maps?

What is it about?

V-pit is a very common defect in InGaN/GaN layers. The V-pits form, because they decrease the strain energy accumulated in the epilayer. Theoretical calculations confirm that, but the X-ray reciprocal space maps (RSM) measured on real samples show something unexpected: the position of the InGaN peak is the same as for a fully strained layer. We used FEM simulations of the strain field together with numerical calculations of X-ray RSMs to explain why the InGaN peak does not show the strain relaxation. The results are supported by experimental data from our own series of samples.

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