Interplay of quantum confinement and strain effects in type I to type II transition in Ge/Si core-shell nanocrystals

TL;DR

This study explores how quantum confinement and strain influence the electronic properties and band alignment transitions in Ge/Si core-shell nanocrystals, revealing size-dependent type I and type II configurations.

Contribution

It provides detailed insights into the conditions under which Ge/Si nanocrystals transition from type I to type II band alignment, considering size, strain, and core-shell interactions.

Findings

Si(core)/Ge(shell) nanocrystals always show type II offset.

Small Ge(core)/Si(shell) nanocrystals exhibit type I offset.

Larger Ge(core)/Si(shell) nanocrystals can be engineered for type II offset.

Abstract

The electronic properties of hydrogenated, spherical, Si/Ge and Ge/Si core-shell nanocrystals with a diameter ranging from 1.8 to 4.0 nm are studied within Density Functional Theory. Effects induced by quantum confinement and strain on the near-band-edge states localization, as well as the band-offset properties between Si and Ge regions, are investigated in detail. On the one hand, we prove that Si(core)/Ge(shell) nanocrystals always show a type II band-offset alignment, with the HOMO mainly localized on the Ge shell region and the LUMO mainly localized on the Si core region. On the other hand, our results point out that a type II offset cannot be observed in small (diameter less than 3 nm) Ge(core)/Si(shell) nanocrystals. In these systems, quantum confinement and strain drive the near-band-edge states to be mainly localized on Ge atoms inducing a type I alignment. In larger Ge(core)/Si(shell) nanocrystals, instead, the formation of a type II offset can be engineered by playing with both core and shell thickness. The conditions that favor the transition from a type I to a type II alignment for Ge(core)/Si(shell) nanocrystals are discussed in detail.