Morphology of epitaxial core-shell nanowires

TL;DR

This paper investigates the stability of epitaxial core-shell nanowires against various perturbations, revealing how geometric parameters influence morphology and explaining experimental observations in Si-Ge systems.

Contribution

It provides a comprehensive analysis of morphological stability considering different growth regimes and identifies key parameters for stable shell growth and helical nanowire formation.

Findings

Small core radius promotes stability.

Rapid shell growth leads to stable morphologies.

Intrinsic mechanisms can induce helical structures.

Abstract

We analyze the morphological stability against azimuthal, axial, and general helical perturbations for epitaxial core-shell nanowires in the growth regimes limited by either surface diffusion or evaporation-condensation surface kinetics. For both regimes, we find that geometric parameters (i.e., core radius and shell thickness) play a central role in determining whether the nanowire remains cylindrical or its shell breaks up into epitaxial islands similar to those observed during Stranski-Krastanow growth in thin epilayers. The combination of small cores and rapid growth of the shell emerge as key ingredients for stable shell growth. Our results provide an explanation for the different core-shell morphologies reported in the Si-Ge system experimentally, and also identify a growth-induced intrinsic mechanism for the formation of helical nanowires.