InxGa1-xN/GaN quantum dot (QD) arrays with high spatial densities have the potential to improve the performance of GaN-based optoelectromic devices. The growth of such QD arrays may be achieved in metal-organic vapour phase epitaxy by pre-dosing the GaN surface with a silicon-bearing precursor. Here, we investigate the mechanism of QD formation in this case, and suggest that for short SiH4 predoses, the variation of island density with predose duration, and the merging of the observed islands with increasing InxGa1-xN/GaN growth time, indicate that the SiH4 is roughening the GaN surface, increasing the number of available nucleation sites, and leading to a change in growth mode. For longer SiH4 precloses, a different mechanism appears to operate. However, we find no evidence for the presence of an SiNx nanomask, but instead note that the observed islands have a distinctive and consistent shape, indicating the influence of strain and surface energy contributions on the growth mode. (c) 2008 Elsevier B.V. All rights reserved.