  We investigate the origin of the fast recombination dynamics of bound and
free excitons in GaN nanowire ensembles by temperature-dependent
photoluminescence spectroscopy using both continuous-wave and pulsed
excitation. The exciton recombination in the present GaN nanowires is dominated
by a nonradiative channel between 10 and 300 K. Furthermore, bound and free
excitons in GaN NWs are strongly coupled even at low temperatures resulting in
a common lifetime of these states. By solving the rate equations for a coupled
two-level system, we show that one cannot, in practice, distinguish whether the
nonradiative decay occurs directly via the bound or indirectly via the free
state. The nanowire surface and coalescence-induced dislocations appear to be
the most obvious candidates for nonradiative defects, and we thus compare the
exciton decay times measured for a variety of GaN nanowire ensembles with
different surface-to-volume ratio and coalescence degrees. The data are found
to exhibit no correlation with either of these parameters, i. e., the
dominating nonradiative channel in the GaN nanowires under investigation is
neither related to the nanowire surface, nor to coalescence-induced defects for
the present samples. Hence, we conclude that nonradiative point defects are the
origin of the fast recombination dynamics of excitons in GaN nanowires.
