Equilibrium shape and dislocation nucleation in strained epitaxial nanoislands

TL;DR

This study uses atomistic simulations to analyze equilibrium shapes, shape transitions, and dislocation nucleation in small strained epitaxial islands, revealing new shape classifications and nucleation mechanisms beyond continuum predictions.

Contribution

It provides a detailed atomistic phase diagram of island shapes and uncovers spontaneous dislocation nucleation mechanisms at island edges.

Findings

Four generic equilibrium island shapes identified.

Dislocations nucleate spontaneously at island edges above critical size.

Island shapes and dislocation behaviors depend on size, misfit, and substrate attraction.

Abstract

We study numerically the equilibrium shapes, shape transitions and dislocation nucleation of small strained epitaxial islands with a two-dimensional atomistic model, using simple interatomic pair potentials. We first map out the phase diagram for the equilibrium island shapes as a function of island size (up to N = 105 atoms) and lattice misfit with the substrate and show that nanoscopic islands have four generic equilibrium shapes, in contrast with predictions from the continuum theory of elasticity. For increasing substrate-adsorbate attraction, we find islands that form on top of a finite wetting layer as observed in Stranski-Krastanow growth. We also investigate energy barriers and transition paths for transitions between different shapes of the islands and for dislocation nucleation in initially coherent islands. In particular, we find that dislocations nucleate spontaneously at the edges of the adsorbate-substrate interface above a critical size or lattice misfit.