Up to now the structural characterization of non-perfect epitaxial thin films always needs a series of different X-ray reflection measurements to determine strain and relaxation and also crystal quality. This is a very time consuming procedure for layer evaluation. In 1937 the appearance of Umweganregungen was discovered by Renninger – a method in which X-ray multiple diffraction for forbidden or quasi forbidden reflections is studied mostly for single crystals. Their intensity shows a strong dependence not only on the size of a crystal but also to an increasing degree on structural quality. In a Renninger scan a crystal is rotated about the normal to a set of diffracting planes while the diffraction from these planes is measured. The higher the intensity of the Umweganregungen is the higher the quality of the crystals. In this paper we will present the use of this old technique in a new context which will allow structural assessment of thin GaN layers doped with Chromium by just one measurement.
This method we applied to the MOVPE growth optimization of GaN:Cr layers to assess the best growth temperature and the influence of the dopant Chromium on GaN growth. The Bragg angle of the exactly (0001) reflection of GaN was used. To confirm the results additionally the conventional X-ray evaluation methods were carried out.
With both approaches we come to the same conclusion: an intermediate growth temperature and the incorporation of Cr are advantageous for the crystal quality. This report will give a comparative overview on both structural investigation methods, their advantages and limitations. It will be shown that Renninger scans are promising for future layer evaluation using only one measurement.

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