We report the impact of both unintentional and intentional nitridation of 6H-SiC(0001)(Si) substrates on the epitaxial growth of GaN by molecular-beam epitaxy. The unintentional nitridation is dependent upon the details of the pregrowth Ga flash-off process used to remove surface oxides and to achieve a specific pregrowth surface reconstruction. The nucleation process and structural and morphological properties of GaN epitaxial layers are strongly influenced by the modifications of the SiC surface induced by the pregrowth preparation process. We found that residual oxygen at the SiC surface, unintentional SiC nitridation, and the formation of cubic GaN grains at the initial nucleation stage strongly decrease as the concentration of Ga used is increased during the flash cleaning. In addition, recent work has shown that the use of a SiN interlayer for GaN epitaxy on various substrates reduces dislocation density. We observe an improvement in the heteroepitaxy of GaN when the SiC surface is intentionally nitridized at low temperature prior to the initiation of growth. (c) 2005 American Vacuum Society.