Nucleation-Dependent Surface Diffusion in Anisotropic Growth of III-V Nanostructures

Vertical growth rate of highly anisotropic III-V nanostructures, including the vapor-liquid-solid or catalyst-free nanowires and quasi-one-dimensional nanomembranes obtained by selective area epitaxy, is largely influenced by surface diffusion of group III adatoms. Here, we present a growth model which is based on the self-consistent calculation of the diffusion flux depending on the nucleation rate on the top facet. It is shown that the magnitude and even the direction of surface diffusion flux is controlled by the position and shape-dependent mononucleation of two-dimensional islands on the top facet of the structures, and is sensitive to the V/III flux ratio during growth. Group III adatoms diffuse from the sidewalls to the top of the structures if the nucleation-mediated growth rate is larger than the direct vapor flux of group III atoms. This "positive" diffusion results in superlinear evolution of height with time. When the nucleation-mediated growth rate is smaller than the direct flux of group III atoms, the direction of the diffusion flux is reversed to "negative", resulting in sublinear increase of height with time. Positive diffusion usually occurs in symmetrical III-V nanowires, while negative diffusion suppresses selective area growth of elongated GaAs nanomembranes with tapered sidewalls on their short sides. The model sheds more light on the general growth properties of anisotropic nanostructures and should be useful for morphological design of such structures in different epitaxy techniques.