Intrinsic Donor-Bound Excitons in Ultraclean Monolayer Semiconductors

TL;DR

This paper discovers and characterizes intrinsic donor-bound excitons in ultraclean monolayer WSe2, revealing their unique optical properties and potential for quantum applications in 2D semiconductors.

Contribution

It reports the identification of dark donor-bound excitons with long lifetimes and specific optical behaviors in ultraclean monolayer WSe2, a novel finding in 2D semiconductor physics.

Findings

Donor-bound excitons exhibit long population and polarization lifetimes.

These excitons are only observed with electron doping and are linked to specific phonons.

Resonant excitation controls their optical orientation and emission.

Abstract

The monolayer transition metal dichalcogenides are an emergent semiconductor platform exhibiting rich excitonic physics with coupled spin-valley degree of freedom and optical addressability. Here, we report a new series of low energy excitonic emission lines in the photoluminescence spectrum of ultraclean monolayer WSe2. These excitonic satellites are composed of three major peaks with energy separations matching known phonons, and appear only with electron doping. They possess homogenous spatial and spectral distribution, strong power saturation, and anomalously long population (> 6 ${\mu}$s) and polarization lifetimes (> 100 ns). Resonant excitation of the free inter- and intra-valley bright trions leads to opposite optical orientation of the satellites, while excitation of the free dark trion resonance suppresses the satellites photoluminescence. Defect-controlled crystal synthesis and scanning tunneling microscopy measurements provide corroboration that these features are dark excitons bound to dilute donors, along with associated phonon replicas. Our work opens opportunities to engineer homogenous single emitters and explore collective quantum optical phenomena using intrinsic donor-bound excitons in ultraclean 2D semiconductors.