Polar charge induced self-assembly: An electric effect that causes non-isotropic nanorod growth in wurtzite semiconductors

TL;DR

This paper uncovers a novel non-isotropic growth mode in wurtzite semiconductors caused by internal electric fields and thermally-excited charges, explaining asymmetric nanorod formation.

Contribution

It introduces a new understanding of how built-in electric fields influence crystal growth, revealing a previously unknown mechanism in polar semiconductor nanostructures.

Findings

Internal electric fields cause non-uniform charge screening.

Formation of symmetric inverted polarity domains limits lateral growth.

Explains asymmetric nanorod growth in InN and ZnO.

Abstract

Crystals grow by gathering and bonding of atoms to form an ordered structure. Typically, the growth is equally probable in all crystalline directions, but sometimes, it is not, as is the case of nanowire growth. Nanowire growth is explained, in most cases, by the presence of liquid metal droplets that mediate between an incoming flux of atoms and a substrate or an existing crystal nucleus, while defining the lateral dimension. Here, we report and explain a previously unknown mode of non-isotropic crystal growth observed in two wurtzite semiconductors, InN and ZnO. Being of polar structure, wurtzite crystals possess a built-in internal electric field. Thermally-excitied charges screen the built-in electric field during growth in a non-uniform, yet symmetric, manner, causing the formation of symmetric domains of inverted polarity. These domains limit the lateral expansion of the crystal, inducing a fiber growth mode. The mechanism described here elucidates previously unexplained phenomena in the growth of group III-nitrides on sapphire, emphasizing the need to consider the effects of built-in electric fields in the growth of polar semiconductors.