Decay dynamics and exciton localization in large GaAs quantum dots grown by droplet epitaxy

TL;DR

This study explores the decay dynamics and exciton localization in large GaAs quantum dots grown by droplet epitaxy, revealing strong confinement and localized excitons contrary to giant oscillator strength predictions.

Contribution

It provides new insights into the optical properties and exciton behavior of large droplet-epitaxy GaAs quantum dots, highlighting strong confinement and localization effects.

Findings

Quantum efficiency of about 75% in the quantum dots

Excitons are strongly confined and localized in potential minima

Temperature reduces the effective transition strength due to thermal excitation

Abstract

We investigate the optical emission and decay dynamics of excitons confined in large strain-free GaAs quantum dots grown by droplet epitaxy. From time-resolved measurements combined with a theoretical model we show that droplet-epitaxy quantum dots have a quantum efficiency of about 75% and an oscillator strength between 8 and 10. The quantum dots are found to be fully described by a model for strongly-confined excitons, in contrast to the theoretical prediction that excitons in large quantum dots exhibit the so-called giant oscillator strength. We attribute these findings to localized ground-state excitons in potential minima created by material intermixing during growth. We provide further evidence for the strong-confinement regime of excitons by extracting the size of electron and hole wavefunctions from the phonon-broadened photoluminescence spectra. Furthermore, we explore the temperature dependence of the decay dynamics and, for some quantum dots, observe a pronounced reduction in the effective transition strength with temperature. We quantify and explain these effects as being an intrinsic property of large quantum dots owing to thermal excitation of the ground-state exciton. Our results provide a detailed understanding of the optical properties of large quantum dots in general, and of quantum dots grown by droplet epitaxy in particular.