  The three dimensional carrier confinement in GaN nanodiscs embedded in
GaN/AlGaN nanowires and its effect on their photoluminescence properties is
analyzed for Al concentrations between x = 0.08 and 1. Structural analysis by
high resolution transmission electron microscopy reveals the presence of a
lateral AlGaN shell due to a composition dependent lateral growth rate of the
barrier material. The structural properties are used as input parameters for
three dimensional numerical simulations of the confinement which show that the
presence of the AlGaN shell has to be considered to explain the observed
dependence of the emission energy on the Al concentration in the barrier. The
simulations reveal that the maximum in the emission energy for x ~ 30% is
assigned to the smallest lateral strain gradient and consequently the lowest
radial internal electric fields in the nanodiscs. Higher Al-concentrations in
the barrier cause high radial electric fields that can overcome the exciton
binding energy and result in substantially reduced emission intensities.
Effects of polarization-induced axial internal electric fields on the
photoluminescence characteristics have been investigated using nanowire samples
with nanodisc heights ranging between 1.2 nm and 3.5 nm at different Al
concentrations. The influence of the quantum confined Stark effect is
significantly reduced compared to GaN/AlGaN quantum well structures which is
attributed to the formation of misfit dislocations at the heterointerfaces
which weakens the internal electric polarization fields.
