For obtaining low defect density GaN substrates with controllable electronic properties for high performance electronic and optoelectronic devices, quasi-bulk growth by hydride vapour phase epitaxy (HVPE) on ammonothermal grown GaN substrates appears to be a feasible approach . Synchrotron X-ray topography (white beam and monochromatic beam) has been employed to characterize dislocation configurations in such GaN substrates that were grown by HVPE on ammonothermal GaN substrate, and then slicing the substrate. Threading dislocations and basal plane dislocations (BPDs) are observed in these GaN substrates. Compared with GaN grown on sapphire and silicon carbide substrates , HVPE GaN on ammonothermal GaN  is obviously of higher crystalline quality with lower defect densities. Threading screw dislocation (TSD) density was measured to be about 880 cm-2
while threading edge dislocation (TED) density was about an order higher (~ 5824 cm-2
). The distribution of BPDs which are induced by deformation, was found to be highly non-uniform with most regions of the wafer nearly BPD-free as well as high densities concentrated near one edge. Also, besides the surface features, like the surface scratches, arrays of threading dislocations can be seen from the transmission and grazing incidence topographs, which likely originate from scratches on the surface of the ammonothermal substrate. Correlation of the defect distributions with the HVPE growth process and their implication for devices will be discussed.
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 Tian. Y, et al. Direct growth of freestanding GaN on C-face SiC by HVPE. Sci. Rep. 5, 10748; doi: 10.1038/srep10748 (2015).
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