Prediction of a Strain Induced Conduction Band Minimum in Embedded Quantum Dots

TL;DR

This paper predicts that embedding InP quantum dots in GaP induces a strain-related conduction state, causing the fundamental optical transition to be indirect regardless of dot size, contrasting with free-standing dots.

Contribution

It reveals a strain-induced conduction state in embedded quantum dots that alters their optical transition nature, a phenomenon absent in free-standing dots.

Findings

Embedded dots exhibit a strain-induced conduction state at the interface.

The fundamental transition remains indirect for all sizes due to strain effects.

Free-standing dots do not show this strain-related conduction state.

Abstract

Free standing InP quantum dots have previously been theoretically and experimentally shown to have a direct band gap across a large range of experimentally accessible sizes. We demonstrate that when these dots are embedded coherently within a GaP barrier material, the effects of quantum confinement in conjunction with coherent strain suggest there will be a critical diameter of dot (60A), above which the dot is direct, type I, and below which it is indirect, type II. However, the strain in the system acts to produce another conduction state with an even lower energy, in which electrons are localized in small pockets at the interface between the InP dot and the GaP barrier. Since this conduction state is GaP X_1c-derived and the highest occupied valence state is InP, Gamma-derived, the fundamental transition is predicted to be indirect in both real and reciprocal space (``type II'') for all dot sizes. This effect is peculiar to the strained dot, and is absent in the free-standing dot.