  We use line-of-sight quadrupole mass spectrometry to monitor the spontaneous
formation of GaN nanowires on Si during molecular beam epitaxy. We find that
the temporal evolution of nanowire ensembles is well described by a double
logistic function. The analysis of the temporal evolution of nanowire ensembles
prepared under a wide variety of growth conditions allows us to construct a
growth diagram which can be used to predict the average delay time that
precedes nanowire formation.
