  In molecular beam epitaxy, the spontaneous formation of GaN nanowires on
Si(111) substrates at elevated temperatures is limited by the long incubation
time that precedes nanowire nucleation. In this work, we present three
unconventional growth approaches to minimize the incubation time and thus
facilitate significantly higher growth temperatures (up to 875$^{\circ}$C). We
achieve this by: (i) using III/V flux ratios larger than one to compensate for
Ga desorption, (ii) introducing a two-step growth procedure, and (iii) using an
AlN buffer layer to favor GaN nucleation. The GaN nanowire ensembles grown at
so far unexplored substrate temperatures exhibit excitonic transitions with
sub-meV linewidths and the low-temperature photoluminescence spectra are
comparable to those of state-of-the-art free-standing GaN layers grown by
hydride vapor phase epitaxy.
