Study of the driving force for the self-assembly of heterojunction quantum dots (zero D molecules) using finite element analysis

摘要

The formation and control of quantum dots have been the focus of considerable research. This research is largely motivated by the unique properties zero dimensional structure possess. During heteroepitaxy, quantum dots form through a self-assembly process in order to reduce the strain energy of the heteroepitaxial material while increasing the surface area. Using this procedure, quantum dots composed of many different materials have been formed on various substrates or buffer layers. These structures have been classified into two basic groups (type I and type II) based on how the dot band structure lines up relative to the surrounding barrier band structure. In the case of a type I quantum dot the dot band gap is contained within the energy range of the band gap of the barrier material. This arrangement is typical for In_xGa_(1-x)As on GaAs and InP. The other type of quantum dot, referred to as type II, indicates that the quantum dot band gap straddles the conduction band (or valence band) of the barrier layer, which is the case of GaSb or InP on GaAs. With this alignment, the hole (or electron) states are located in the quantum dot material while the other state is held in barrier region in the vicinity of the quantum dot through electrostatic interactions. Recently, the authors have shown that quantum dots structures can be formed through the self-assembly process that allow the zero-dimensional region to be composed of multiple materials which produce heterojunctions within the quantum dot structure. In these structures, the authors refer to the initial material as a quantum dot and the second material to be a quantum crown. In this article, they discuss various characteristics of these structures which can be utilized in the production of devices. In addition, they study the variation in strain energy for the nucleation and growth of the crown material on an existing quantum dot structure through finite element analysis. They find that even for compositions which have a significant difference in lattice constant from the existing quantum dot materials, an enhancement exists for nucleating the crown material on the existing quantum dot structure. In addition, by carefully selecting the composition (i.e., lattice constant) of the quantum crown material, the optimum location for nucleation can be controlled.