  Controlling the growth of zinc oxide nanowires is necessary to optimize the
performances of nanowire-based devices such as photovoltaic solar cells,
nano-generators, or light-emitting diodes. In this view, we investigate the
nucleation and growth mechanisms of ZnO nanowires grown by metalorganic vapor
phase epitaxy either on O-polar ZnO or on sapphire substrates. Whatever the
substrate, ZnO nanowires are Zn-polar, as demonstrated by convergent beam
electron diffraction. For growth on O-polar ZnO substrate, the nanowires are
found to sit on O-polar pyramids. As growth proceeds, the inversion domain
boundary moves up in order to remain at the top of the O-polar pyramids. For
growth on sapphire substrates, the nanowires may also originate from the
sapphire / ZnO interface. The presence of atomic steps and the non-polar
character of sapphire could be the cause of the Zn-polar crystal nucleation on
sapphire, whereas it is proposed that the segregation of aluminum impurities
could account for the nucleation of inverted domains for growth on O-polar ZnO.
