Robustness of momentum-indirect interlayer excitons in MoS₂/WSe₂ heterostructure against charge carrier doping

Abstract

Monolayer transition-metal dichalcogenide (TMD) semiconductors exhibit strong excitonic effects and hold promise for optical and optoelectronic applications. Yet, electron doping of TMDs leads to the conversion of neutral excitons into negative trions, which recombine predominantly nonradiatively at room temperature. As a result, the photoluminescence (PL) intensity is quenched. Here we study the optical and electronic properties of a MoS₂/WSe₂ heterostructure as a function of chemical doping by Cs atoms performed under ultrahigh vacuum conditions. By PL measurements, we identify two interlayer excitons and assign them to the momentum-indirect Q-Γ and K-Γ transitions. The energies of these excitons are in very good agreement with ab initio calculations. We find that the Q-Γ interlayer exciton is robust to the electron doping and is present at room temperature even at a high charge carrier concentration (∼10¹³ cm⁻²). Submicrometer angle-resolved photoemission spectroscopy (μ-ARPES) reveals a charge transfer from deposited Cs adatoms to both the upper MoS₂ and the lower WSe₂ monolayers without changing the band alignment. This leads to a small (∼10 meV) energy shift of interlayer excitons. Robustness of the momentum-indirect interlayer exciton to charge doping opens up an opportunity for using TMD heterostructures in light-emitting devices that can work at room temperature at high densities of charge carriers.