Sub-meV linewidth of excitonic luminescence in single GaN nanowires: Direct evidence for surface excitons

We investigate the low-temperature photoluminescence of GaN nanowires grown catalyst free on Si(111). For single nanowires dispersed on Si(111), we observe excitonic transitions with linewidths below 300 mu eV and at energies clearly above the donor-bound exciton in the bulk. We show that these transitions are due to donor-bound excitons close to the surface. The broadening of about 3 meV observed for the nanowire ensemble is shown to be a natural consequence of the energy dispersion of bound-exciton states as a function of their distance from the surface.