  Several of the key issues of planar (Al,Ga)N-based deep-ultraviolet light
emitting diodes could potentially be overcome by utilizing nanowire
heterostructures, exhibiting high structural perfection and improved light
extraction. Here, we study the spontaneous emission of GaN/(Al,Ga)N nanowire
ensembles grown on Si(111) by plasma-assisted molecular beam epitaxy. The
nanowires contain single GaN quantum disks embedded in long (Al,Ga)N nanowire
segments essential for efficient light extraction. These quantum disks are
found to exhibit intense emission at unexpectedly high energies, namely,
significantly above the GaN bandgap, and almost independent of the disk
thickness. An in-depth investigation of the actual structure and composition of
the nanowires reveals a spontaneously formed Al gradient both along and across
the nanowire, resulting in a complex core/shell structure with an Al deficient
core and an Al rich shell with continuously varying Al content along the entire
length of the (Al,Ga)N segment. This compositional change along the nanowire
growth axis induces a polarization doping of the shell that results in a
degenerate electron gas in the disk, thus screening the built-in electric
fields. The high carrier density not only results in the unexpectedly high
transition energies, but also in radiative lifetimes depending only weakly on
temperature, leading to a comparatively high internal quantum efficiency of the
GaN quantum disks up to room temperature.
