Delineation of structural defects by molten KOH etching is not satisfactory for highly doped n-type Si-face SiC substrates. This difficulty was overcome by converting the substrates to p-type via diffusion of boron, followed by molten KOH etching. Three kinds of typical etch pits were clearly distinguished, corresponding to elementary screw, threading edge, and basal plane dislocations. Comparison of molten KOH etching effects on 4H-SiC samples of different types indicates that molten KOH etching is a combination of chemical and electrochemical processes, during which the preferential and isotropic etchings are competitive, depending on the SiC conductivity type and doping concentration. (C) 2004 The Electrochemical Society.