Ga-assisted catalyst-free growth mechanism of GaAs nanowires by molecular beam epitaxy

The mechanisms of Ga-assisted GaAs nanowires grown by molecular beam epitaxy are addressed. The axial and radial growth rates as a function of the Ga rate and As pressure indicate that on the opposite of what is observed in thin film epitaxy, the growth rate of the nanowires is arsenic limited. As a consequence, the axial growth rate of the wires can be controlled by the ${\mathrm{As}}_{4}$ pressure. Additionally, due to the small ${\mathrm{As}}_{4}$ pressure leading to nanowire growth, the deposition on the facets is very slow, leading to a much lower radial growth rate. Finally, we present a model that is able to accurately describe the presented observations and predicts a maximum length of nontapered nanowires of $40\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{m}$.