TEM and AFM data show that a significant reduction of threading dislocations in heteroepitaxial GaN/Al2O3 grown by MOCVD has been achieved. The reduction has been obtained by growth interruption followed by annealing in silane (SiH4). Density of threading dislocations in the GaN layer above the silane-exposed surface decreased to 5 x 10(7) cm(-2) in comparison to 10(9) cm(-2) in the layer below this surface. TEM data showed the existence of pyramidal pits at the silane-exposed surface. They were overgrown by the subsequent GaN layer. The presence of these pits indicates that the GaN surface was selectively etched during the silane flow. These pits were sites where dislocations drastically changed propagation direction from parallel to the c-axis to horizontal. Horizontal propagation of dislocations above the surface treated by silane (where formation of SiN was expected) suggests that the GaN layer in this region was grown in the lateral epitaxial overgrowth mode. EDX measurements performed at the interface between the SiH4-treated GaN layer and the subsequently grown GaN did not show any presence of Si. Therefore, it is believed that the dislocation reduction is related to the lateral overgrowth above the pits and not to the formation of a SiN interlayer. (C) 2004 Elsevier B.V. All rights reserved.