Temperature Dependence of Excitonic Auger Recombination in Excitonic-Complex-Free Monolayer WS₂ by Considering the Auger Broadening Effect

Monolayer transition metal dichalcogenides (TMDs) have been extensively studied for their optoelectronic properties and application to sizable and flexible light-emitting devices. However, even at moderate exciton densities, their light emitting capability is severely limited by strong exciton-exciton annihilation (EEA) through nonradiative Auger recombination. Consequently, it is essential to understand the fundamental EEA dynamics to achieve a higher quantum yield. Numerous studies on EEA in TMDs, however, do not properly consider so-called Auger broadening, resulting in underestimated values for the Auger coefficient. Also, Auger coefficients measured at low temperatures were seriously affected by the presence of excitonic complex such as trions and biexcitons, yielding less confident results. In order to study EEA in an ideal excitonic system, we transferred monolayer WS₂ on a gold substrate with hBN encapsulation, in which excitons persist as the main species from room temperature to 3 K via suppression of the formation of excitonic complex by metal proximity. To account for the effect of Auger broadening, we numerically solved the rate equation for the precise estimation of the Auger coefficient as a function of temperature by considering the laser pulse width and by spatially averaging the inhomogeneous exciton density. The measured Auger coefficient is about 2.4 cm²s^-1at room temperature, but it significantly drops down to 0.14 cm²s^-1at 3 K. We found that the Auger coefficient <i>A</i> in monolayer WS₂ can be expressed as a function of temperature (<i>T </i>) in the form of A = αT+β where α = 0.00054cm²s^-1K^-1and β = 0.1371 cm²s^-1, which is indeed consistent with a theoretical model predicted for direct and exchange processes, respectively. We believe that our results provide more profound understanding of the Auger processes in monolayer TMDs, which potentially provide a guide for enhancing the quantum yield of the TMD materials.