Diameter evolution of selective area grown Ga-assisted GaAs nanowires

TL;DR

This paper introduces a two-step molecular beam epitaxy method for selective area growth of GaAs nanowires, enabling precise control over diameter and shape, including untapered morphologies, through combined modeling of droplet dynamics and vapour-solid growth.

Contribution

The study develops a novel two-step growth process and a comprehensive model to control nanowire diameter and shape, advancing nanowire shape engineering capabilities.

Findings

Achieved thin, untapered GaAs nanowires with 45 nm diameter.

Developed a model combining droplet dynamics and vapour-solid growth.

Demonstrated control over nanowire morphology for improved doping applications.

Abstract

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.