A systematic investigation of radiative recombination in GaN nanowires: The influence of nanowire geometry and environmental conditions

Nanowires intrinsically exhibit a large surface area, which makes them sensitive to physical and chemical interactions with their environment. Here, we investigate the surface recombination at m-plane side walls of selective area-grown GaN nanowires on Si (111) subjected to different environmental conditions. In contrast to the stable photoluminescence observed from c-plane surfaces of films, nanowires exhibit a distinct time-dependent photoluminescence quenching by over 90% within the time scale of seconds in the presence of air or dissociated liquids. This quenching is most pronounced for 50 nm diameter nanowires with interwire spacings larger than 500 nm due to internal electric field and external light field distributions. Ion- and pH-sensitive measurements, in combination with an externally applied voltage, allow the assignment of this effect to anions from the surroundings to accumulate at the nonpolar GaN side walls of the UV-exposed GaN nanowires. The decay times of the luminescence signal follow the dynamics of valence band holes, which deplete GaN surface states and positively charge the nanowire surfaces. This, in turn, induces the buildup of a capacitive anion shell around the nanowires, leading to an enhanced nonradiative surface recombination of photo-generated charge carriers from the GaN nanowire. In the absence of UV light, a recovery of the photoluminescence signal within tens of minutes indicates the dissolution of the anionic shell via charge balancing. The impact of light-induced electronic and ionic charge redistribution on photocarrier recombination represents an important mechanism of function for GaN nanowire-based devices, ranging from sensors to photocatalysts.