  Electron irradiation of GaN nanowires in a scanning electron microscope
strongly reduces their luminous efficiency as shown by cathodoluminescence
imaging and spectroscopy. We demonstrate that this luminescence quenching
originates from a combination of charge trapping at already existing surface
states and the formation of new surface states induced by the adsorption of C
on the nanowire sidewalls. The interplay of these effects leads to a complex
temporal evolution of the quenching, which strongly depends on the incident
electron dose per area. Time-resolved photoluminescence measurements on
electron-irradiated samples reveal that the carbonaceous adlayer affects both
the nonradiative and the radiative recombination dynamics.
