Morphological evolution in a strained-heteroepitaxial solid droplet on a rigid substrate: Dynamical simulations

TL;DR

This study uses dynamical simulations to explore how strained solid droplets evolve into island structures on a substrate, revealing the formation of wetting layers and multiple islands depending on strain and size.

Contribution

It introduces a physico-mathematical model that self-consistently simulates droplet evolution without assuming equilibrium contact angles.

Findings

Wetting layers of sub-nanometer thickness form during island development.

Multiple islands emerge at high strain or droplet volume thresholds.

The model captures spontaneous morphological transitions driven by surface diffusion.

Abstract

A systematic study based on the self-consistent dynamical simulations is presented for the spontaneous evolution of flat solid droplets (bumps), which are driven by the surface drift diffusion induced by the capillary and mismatch stresses, during the development of the Stranski-Krastanow island morphology on a rigid substrate. The physico-mathematical model, which bases on the irreversible thermodynamics treatment of surfaces and interfaces with singularities (Ogurtani, T.O., J. Chem. Phys. 124, 144706, 2006) furnishes us to have auto-control on the otherwise free-motion of the triple junction contour line between the substrate and the droplet without presuming any equilibrium dihedral contract (wetting) angles at the edges. During the development of Stranski-Krastanow islands through the mass accumulation at the central region of the droplet via surface drift-diffusion with and/or without growth, the formation of an extremely thin wetting layer is observed. . This wetting layer has a thickness of a fraction of a nanometer and covers not only the initial computation domain but also its further extension beyond the original boundaries. Above a certain threshold level of the mismatch strain and/or the size (i.e. volume) of the droplets, which depends on the initial physicochemical data and the aspect ratio (i.e., shape) of the original droplet, the formation of the multiple islands separated by the shallow wetting layers is also observed