  We report the spectral imaging in the UV to visible range with nanometer
scale resolution of closely packed GaN/AlN quantum disks in individual
nanowires using an improved custom-made cathodoluminescence system. We
demonstrate the possibility to measure full spectral features of individual
quantum emitters as small as 1 nm and separated from each other by only a few
nanometers and the ability to correlate their optical properties to their size,
measured with atomic resolution. The direct correlation between the quantum
disk size and emission wavelength provides evidence of the quantum confined
Stark effect leading to an emission below the bulk GaN band gap for disks
thicker than 2.6 nm. With the help of simulations, we show that the internal
electric field in the studied quantum disks is smaller than what is expected in
the quantum well case. We show evidence of a clear dispersion of the emission
wavelengths of different quantum disks of identical size but different
positions along the wire. This dispersion is systematically correlated to a
change of the diameter of the AlN shell coating the wire and is thus attributed
to the related strain variations along the wire. The present work opens the way
both to fundamental studies of quantum confinement in closely packed quantum
emitters and to characterizations of optoelectronic devices presenting carrier
localization on the nanometer scale.
