Excitons in InGaAs Quantum Dots without Electron Wetting Layer States

TL;DR

This paper demonstrates that adding a monolayer of AlAs during InAs quantum dot growth removes the wetting layer, leading to improved optical properties and enabling highly charged excitons without electron wetting layer states.

Contribution

Introducing a modified SK-growth protocol with an AlAs monolayer that eliminates electron wetting layer states in InAs quantum dots.

Findings

Wetting layer states are eliminated by the AlAs monolayer.

Quantum dots exhibit optical linewidths comparable to the best SK QDs.

Highly charged excitons with up to six electrons are achieved.

Abstract

The Stranski-Krastanov (SK) growth-mode facilitates the self-assembly of quantum dots (QDs) using lattice-mismatched semiconductors, for instance InAs and GaAs. SK QDs are defect-free and can be embedded in heterostructures and nano-engineered devices. InAs QDs are excellent photon emitters: QD-excitons, electron-hole bound pairs, are exploited as emitters of high quality single photons for quantum communication. One significant drawback of the SK-mode is the wetting layer (WL). The WL results in a continuum rather close in energy to the QD-confined-states. The WL-states lead to unwanted scattering and dephasing processes of QD-excitons. Here, we report that a slight modification to the SK-growth-protocol of InAs on GaAs -- we add a monolayer of AlAs following InAs QD formation -- results in a radical change to the QD-excitons. Extensive characterisation demonstrates that this additional layer eliminates the WL-continuum for electrons enabling the creation of highly charged excitons where up to six electrons occupy the same QD. Single QDs grown with this protocol exhibit optical linewidths matching those of the very best SK QDs making them an attractive alternative to standard InGaAs QDs.