Quantum-dot nucleation in strained-layer epitaxy: minimum-energy pathway in the stress-driven 2D-3D transformation

TL;DR

This paper investigates the energy pathways of monolayer to bilayer island transformation in strained-layer epitaxy, revealing different behaviors under compression and expansion, with implications for growth modes in materials.

Contribution

It provides a detailed analysis of the minimum-energy pathways for 2D-3D transformation in strained epitaxy, highlighting the influence of stress state on nucleation behavior.

Findings

Compressed overlayers show nucleation-like energy behavior.

Expanded overlayers exhibit abrupt energy collapse near transformation completion.

Growth in expanded overlayers is less probable kinetically.

Abstract

The transformation of monolayer islands into bilayer islands as a first step of the overall two-dimensional to three-dimensional (2D-3D) transformation in the coherent Stranski-Krastanov mode of growth is studied for the cases of expanded and compressed overlayers. Compressed overlayers display a nucleation-like behavior: the energy accompanying the transformation process displays a maximum at some critical number of atoms, which is small for large enough values of the misfit, and then decreases gradually down to the completion of the transformation, non-monotonically due to the atomistics of the process. On the contrary, the energy change in expanded overlayers increases up to close to the completion of the transformation and then abruptly collapses with the disappearance of the monoatomic steps to produce low-energy facets. This kind of transformation takes place only in materials with strong interatomic bonding. Softer materials under tensile stress are expected to grow predominantly with a planar morphology until misfit dislocations are introduced, or to transform into 3D islands by a different mechanism. It is concluded that the coherent Stranski-Krastanov growth in expanded overlayers is much less probable than in compressed ones for kinetic reasons.