  We investigate the nature of excitons bound to I1 basal-plane stacking faults
[(I1;X)] in GaN nanowire ensembles by continuous-wave and time-resolved
photoluminescence spectroscopy. Based on the linear increase of the radiative
lifetime of these excitons with temperature, they are demonstrated to exhibit a
two-dimensional density of states, i. e., a basal-plane stacking fault acts as
a quantum well. From the slope of the linear increase, we determine the
oscillator strength of the (I1;X) and show that the value obtained reflects the
presence of large internal electrostatic fields across the stacking fault.
While the recombination of donor-bound and free excitons in the GaN nanowire
ensemble is dominated by nonradiative phenonema already at 10 K, we observe
that the (I1;X) recombines purely radiatively up to 60 K. This finding provides
important insight into the nonradiative recombination processes in GaN
nanowires. First, the radiative lifetime of about 6 ns measured at 60 K sets an
upper limit for the surface recombination velocity of 450 cm/s considering the
nanowires mean diameter of 105 nm. Second, the density of nonradiative centers
responsible for the fast decay of donor-bound and free excitons cannot be
higher than 2x10^16 cm^-3. As a consequence, the nonradiative decay of
donor-bound excitons in these GaN nanowire ensembles has to occur indirectly
via the free exciton state.
