Negative diffusion of excitons in quasi-two-dimensional systems

TL;DR

This paper explains the observed negative diffusion of excitons in quasi-two-dimensional systems as a result of trapping and delayed release, which affects the interpretation of exciton density profiles.

Contribution

It introduces and compares two models that account for negative exciton diffusion by considering trapping and delayed release effects.

Findings

Negative diffusion arises from trapping and delayed release of excitons.

The effect is amplified by higher recombination and exciton-exciton interactions.

Profiles show narrowing due to unregistered trapped excitons.

Abstract

We show how two different mobile-immobile type models explain the observation of negative diffusion of excitons reported in experimental studies in quasi-two-dimensional semiconductor systems. The main reason for the effect is the initial trapping and a delayed release of free excitons in the area close to the original excitation spot. The density of trapped excitons is not registered experimentally. Hence, the signal from the free excitons alone includes the delayed release of not diffusing trapped particles. This is seen as the narrowing of the exciton density profile or decrease of mean-squared displacement which is then interpreted as a negative diffusion. The effect is enhanced with the increase of recombination intensity as well as the rate of the exciton-exciton binary interactions.