Influence of Pitch on the Morphology and Luminescence Properties of Self-Catalyzed GaAsSb Nanowire Arrays

We report on the influence of hole pattern pitch lengths in the silicon oxide mask and specific nanowire (NW) locations on the morphology and luminescence properties of self-catalyzed GaAsSb NW arrays grown by molecular beam epitaxy. Due to stronger competition for the limited amount of Ga adatoms, the GaAsSb NWs in the center of arrays with short pitch lengths possess a smaller catalyst droplet contact angle than that of the NWs at the array edge. This smaller contact angle leads to a reduction in the collection of group V flux, bringing about shorter NWs in the center. For pitch lengths beyond the diffusion length of Ga adatoms on the mask, the GaAsSb NWs are taller with larger contact angles than in the case with short pitch lengths. Considering that Sb has a longer diffusion length on the side facets of the NWs than that of As, a reduction/increase of the contact angle will bring about an increase/reduction in the Sb/As ratio of the group-V fluxes collected by the catalyst droplets. By performing micro-photoluminescence (μ-PL) measurements on the GaAsSb NW arrays at the center of the array for different pitch lengths, a red shift of the μ-PL spectra was found with a decrease in pitch length. Our findings demonstrate that the Ga diffusion-induced contact angle difference is the main cause for the variations in NW morphology and composition with different pitch lengths and NW locations in the array, which provides guidance to optimize the design of NW array devices for advanced optoelectronic applications.