Thermodynamic driving force in the formation of hexagonal-diamond Si and Ge nanowires

TL;DR

This paper investigates the thermodynamic factors influencing the formation and stability of hexagonal-diamond Si and Ge nanowires, revealing surface energy advantages and kinetic barriers that enable metastable phase growth.

Contribution

It combines first-principles calculations, interatomic potentials, and geometrical modeling to analyze the stability and growth conditions of hexagonal-diamond nanowires and core-shell structures.

Findings

Hexagonal-diamond phase has lower surface energy than cubic in Si and Ge nanowires.

A critical radius for stable hexagonal shells is identified, lower than experimental values.

Metastability is facilitated by kinetic barriers, allowing growth beyond thermodynamic stability limits.

Abstract

The metastable hexagonal-diamond phase of Si and Ge (and of SiGe alloys) displays superior optical properties with respect to the cubic-diamond one. The latter is the most stable and popular one: growing hexagonal-diamond Si or Ge without working at extreme conditions proved not to be trivial. Recently, however, the possibility of growing hexagonal-diamond group-IV nanowires has been demonstrated, attracting attention on such systems. Based on first-principle calculations we show that the surface energy of the typical facets exposed in Si and Ge nanowires is lower in the hexagonal-diamond phase than in cubic ones. By exploiting a synergic approach based also on a recent state-of-the-art interatomic potential and on a simple geometrical model, we investigate the relative stability of nanowires in the two phases up to few tens of nm in radius, highlighting the surface-related driving force and discussing its relevance in recent experiments. We also explore the stability of Si and Ge core-shell nanowires with hexagonal cores (made of GaP for Si nanowires, of GaAs for Ge nanowires). In this case, the stability of the hexagonal shell over the cubic one is also favored by the energy cost associated with the interface linking the two phases. Interestingly, our calculations indicate a critical radius of the hexagonal shell much lower than the one reported in recent experiments, indicating the presence of a large kinetic barrier allowing for the enlargement of the wire in a metastable phase.