Diameter evolution of selective area grown Ga-assisted GaAs nanowires

We present a novel two-step approach for the selective area growth (SAG) of GaAs nanowires (NWs) by molecular beam epitaxy which has enabled a detailed exploration of the NW diameter evolution. In the first step, the growth parameters are optimized for the nucleation of vertically-oriented NWs. In the second step, the growth parameters are chosen to optimize the NW shape, allowing NWs with a thin diameter (45 nm) and an untapered morphology to be realized. This result is in contrast to the commonly observed thick, inversely tapered shape of SAG NWs. We quantify the flux dependence of radial vapour-solid (VS) growth and build a model that takes into account diffusion on the NW sidewalls to explain the observed VS growth rates. Combining this model for the radial VS growth with an existing model for the droplet dynamics at the NW top, we achieve full understanding of the diameter of NWs over their entire length and the evolution of the diameter and tapering during growth. We conclude that only the combination of droplet dynamics and VS growth results in an untapered morphology. This result enables NW shape engineering and has important implications for doping of NWs.