Second-layer nucleation in coherent Stranski-Krastanov growth of quantum dots

TL;DR

This study investigates the nucleation process of second-layer islands during coherent Stranski-Krastanov growth, revealing that nucleation occurs at island edges rather than centers, with specific critical nucleus shapes and sizes.

Contribution

It provides new insights into the preferred nucleation sites and shapes of second-layer nuclei in epitaxial growth, enhancing understanding of quantum dot formation.

Findings

Nucleation favors island edges over centers.

Critical nuclei are rectangular or square with specific atom counts.

Nucleation involves atomic rows forming at edges.

Abstract

We have studied the monolayer-bilayer transformation in the case of the coherent Stranski-Krastanov growth. We have found that the energy of formation of a second layer nucleus is largest at the center of the first-layer island and smallest on its corners. Thus nucleation is expected to take place at the corners (or the edges) rather than at the center of the islands as in the case of homoepitaxy. The critical nuclei have one atom in addition to a compact shape, which is either a square of i*i or a rectangle of i*(i-1) atoms, with i>1 an integer. When the edge of the initial monolayer island is much larger than the critical nucleus size, the latter is always a rectangle plus an additional atom, adsorbed at the longer edge, which gives rise to a new atomic row in order to transform the rectangle into the equilibrium square shape.