Effects on structural characteristics of growth temperature and Si doping in GaN thin films grown on high temperature GaN was investigated. GaN/GaN homojunction structure was grown via metalorganic chemical vapour deposition at intermediate temperature (IT, 840degreesC) with various Si doping concentrations. V-shaped pits are observed on the surface of the undoped GaN grown at IT by atomic force microscopy analysis. Two beam images by transmission electron microscopy show that only screw- and mixed-type component of dislocations contributes to pit formation. Moderate Si doping reduces pit density but the over-doping of Si deteriorates the IT-GaN property by forming pits again. Not all pits are related with threading dislocations but some of them are observed in the dislocation free area of high Si-doped GaN. (C) 2003 Elsevier B.V. All rights reserved.