  In this paper, we describe the design and characterization of 400-nm-long (88
periods) AlxGa1-xN/AlN (0 < x < 0.1) quantum dot superlattices deposited on
self-assembled GaN nanowires for application in electron-pumped ultraviolet
sources. The optical performance of GaN/AlN superlattices on nanowires is
compared with the emission of planar GaN/AlN superlattices with the same
periodicity and thickness grown on bulk GaN substrates along the N-polar and
metal-polar crystallographic axes. The nanowire samples are less sensitive to
nonradiative recombination than planar layers, attaining internal quantum
efficiencies (IQE) in excess of 60% at room temperature even under low
injection conditions. The IQE remains stable for higher excitation power
densities, up to 50 kW/cm2. We demonstrate that the nanowire superlattice is
long enough to collect the electron-hole pairs generated by an electron beam
with an acceleration voltage VA = 5 kV. At such VA, the light emitted from the
nanowire ensemble does not show any sign of quenching under constant electron
beam excitation (tested for an excitation power density around 8 kW/cm2 over
the scale of minutes). Varying the dot/barrier thickness ratio and the Al
content in the dots, the nanowire peak emission can be tuned in the range from
340 to 258 nm. Keywords: GaN, AlN, nanowire, ultraviolet
