EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

Transition metal dichalcogenide (TMD) monolayers are direct-bandgap semiconductors with two valleys in their band structure. The broken inversion symmetry of the lattice gives rise to optical selection rules that enable valley-selective excitation of electrons using circularly polarized light. A strong Coulomb interaction results in the subsequent formation of neutral and charged excitons (trions), which form a chemical equilibrium governed by the net charge density. The valley polarization of both exciton species is determined by the ratio between the intervalley scattering time and the lifetime. We use chemical doping to drive the conversion of excitons into trions in WS$_2$ monolayers at room temperature and study the resulting valley polarization via photoluminescence measurements. We show that the doping causes the emission to become dominated by trions with a strong valley polarization associated with rapid non-radiative recombination. Simultaneously, the enhanced conversion of excitons into trions results in strongly quenched but highly valley-polarized exciton emission. We use a rate equation model to explain the observed valley polarization in terms of the doping-controlled exciton-trion equilibrium. Our results shed light on the important role of exciton-trion conversion on valley polarization in monolayer TMDs.