High-temperature growth of GaN nanowires by molecular beam epitaxy: toward the materials quality of bulk GaN
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.