The properties of semi-insulating thick films of Zn-doped GaN grown by hydride vapor phase epitaxy have been studied using cathodoluminescence, transmission electron microscopy and electron holography techniques. We have mapped the depletion region in the vicinity of threading dislocations and we show that it is associated with sharp drops in cathodoluminescence (CL) intensity. The monochromatic CL images showed considerable intensity variations across the sample surface. These long-range variations in luminescence intensity were also found to be related to potential fluctuations in the regions between dislocations. Such variations could be due to surface states caused by polarization effects of dislocations and/or various surface treatments. © 2002 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.