Island formation in heteroepitaxial growth

TL;DR

This paper investigates the transition from layer-by-layer to island growth in heteroepitaxial thin films using kinetic Monte Carlo simulations and scaling theories, revealing how substrate binding strength influences growth modes.

Contribution

It introduces a minimal lattice model and scaling approach to analyze the LBL-ISL transition, providing insights into the kinetic and thermodynamic factors involved.

Findings

The LBL-ISL transition is driven by substrate binding strength.

Second layer nucleation is weak on top of monolayer islands.

Transition occurs in the partial wetting regime with slow convergence.

Abstract

Island formation in strain-free heteroepitaxial deposition of thin films is analyzed using kinetic Monte Carlo simulations of two minimal lattice models and scaling approaches. The transition from layer-by-layer (LBL) to island (ISL) growth is driven by a weaker binding strength of the substrate which, in the kinetic model, is equivalent to an increased diffusivity of particles on the substrate compared to particles on the film. The LBL-ISL transition region is characterized by particle fluxes between layers 1 and 2 significantly exceeding the net flux between them, which sets a quasi-equilibrium condition. Deposition on top of monolayer islands weakly contributes to second layer nucleation, in contrast with the homoepitaxial growth case. A thermodynamic approach for compact islands with one or two layers predicts the minimum size in which the second layer is stable. When this is linked to scaling expressions for submonolayer island deposition, the dependence of the ISL-LBL transition point on the kinetic parameters qualitatively matches the simulation results, with quantitative agreement in some parameter ranges. The transition occurs in the equilibrium regime of partial wetting and the convergence of the transition point upon reducing the deposition rate is very slow and practically unattainable in experiments.