We studied the initial growth of Si-doped GaN (GaN:Si) epilayers grown under both N- and Ga-rich conditions. Upon Si doping, the surface polarity changed from N- to Ga-polarity. The surface diffusion kinetics of the Ga adatoms of the GaN:Si epilayers depended strongly on the Ga/N flux ratio. GaN:Si films with good crystal quality were obtained for a Ga/N flux ratio slightly larger than 1. The dislocation density decreased about one order of magnitude, while the stacking fault and cubic phase density near the interfacial region increased. The main types of dislocations in the undoped GaN were mixed and edge dislocations. In the GaN:Si, the main dislocations were pure-edge dislocations. The dislocation-density reduction in the GaN:Si may have been due to a low density of mixed dislocations in the presence of a high density of stacking faults and cubic phase. (C) 2002 Elsevier Science B.V. All rights reserved.