Recently efforts are focused on growing by HVPE and by high pressure methods quasi-bulk thick epitaxial layers on GaN single crystals, MOCVD- and HVPE-grown templates. Using these technologies reduced dislocation density is achieved (as compared to that characteristic for standard hetero-epitaxial GaN layers) but still remarkable number of defects is present, including both dislocations and chemical non-homogeneities.

This communication will summarize the results of the study of chemical non-homogeneities which occur in thick homo- and hetero-epitaxial GaN layers. During the HVPE growth the macroscopic growth direction is changed at the edges of templates and inside the pinholes which results in higher carrier concentration. In the central part of the growing layers large area non-homogeneities also develop despite that the growth proceeds in the [0001] direction in macroscopic scale. Selective photo-etching is used for revealing these extended defects which show up as areas of different etch rate, i.e. different carrier concentration. The results were confirmed using cathodoluminescence and quantitative micro-Raman method [1]. The non-homogeneities were studied in GaN layers grown in different laboratories and they show markedly different morphological characteristics.

It will be shown that carrier concentration may vary up to two orders of magnitude across such material which considerably influences the structural quality of the subsequently grown homo-epitaxial layers and may have damaging impact to the device structures.

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