We report on scanning capacitance microscopy (SCM) investigations of Fe-doped GaN and nitrogen-doped ZnO layers. Macroscopically, these samples electrically behave in conventional I-V and C-V measurements like semi-insulating or n-type material, respectively. However, in SCM we found local p-type regions surrounded by an n-type matrix instead of homogeneous and uniform layer conductivity. A comparison with topography reveales that these p-type islands with extensions in the micrometer scale exclusively appear in the vicinity of structural defects and grain boundaries. This doping related effect is discussed in terms of selective dopant incorporation at these defects. (c) 2005 Elsevier B.V. All rights reserved.