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We observe unusually narrow donor-bound exciton transitions (0.4 meV) in thephotoluminescence spectra of GaN nanowire ensembles grown on Si(111) substratesat very high (> 850 degrees Celsius) temperatures. The spectra of these samplesreveal a prominent transition of excitons bound to neutral Si impurities whichis not observed for samples grown under standard conditions. Motivated by theseexperimental results, we investigate theoretically the impact ofsurface-induced internal electric fields on the binding energy of donors by acombined Monte Carlo and envelope function approach. We obtain the ranges ofdoping and diameter for which the potential is well described using the Poissonequation, where one assumes a spatially homogeneous distribution of dopants.Our calculations also show that surface donors in nanowires with a diametersmaller than 100 nm are ionized when the surface electric field is larger thanabout 10 kV/cm, corresponding to a doping level higher than 2 x 10^16 cm^-3.This result explains the experimental observation: since the (D+,X) complex isnot stable in GaN, surface-donor-bound excitons do not contribute to thephotoluminescence spectra of GaN nanowires above a certain doping level, andthe linewidth reflects the actual structural perfection of the nanowireensemble.

Sub-meV linewidth in GaN nanowire ensembles: absence of surface excitons due to the field-ionization of donors