paper

Valley relaxation of resident electrons and holes in a monolayer semiconductor: Dependence on carrier density and the role of substrate-induced disorder

arXiv:2103.05752 · doi:10.1103/PhysRevMaterials.5.044001

Abstract

Using time-resolved optical Kerr rotation, we measure the low temperature valley dynamics of resident electrons and holes in exfoliated WSe $_{2}$ monolayers as a systematic function of carrier density. In an effort to reconcile the many disparate timescales of carrier valley dynamics in monolayer semiconductors reported to date, we directly compare the doping-dependent valley relaxation in two electrostatically-gated WSe $_{2}$ monolayers having different dielectric environments. In a fully-encapsulated structure (hBN/WSe $_{2}$ /hBN, where hBN is hexagonal boron nitride), valley relaxation is found to be monoexponential. The valley relaxation time $I ¨ _{v}$ is quite long ( $\sim$ 10~ $I^¼$ s) at low carrier densities, but decreases rapidly to less than 100~ns at high electron or hole densities $≳$ 2 $\times 1 0^{12}$ ~cm $^{- 2}$ . In contrast, in a partially-encapsulated WSe $_{2}$ monolayer placed directly on silicon dioxide (hBN/WSe $_{2}$ /SiO $_{2}$ ), carrier valley relaxation is multi-exponential at low carrier densities. The difference is attributed to environmental disorder from the SiO $_{2}$ substrate. Unexpectedly, very small out-of-plane magnetic fields can increase $I ¨ _{v}$ , especially in the hBN/WSe $_{2}$ /SiO $_{2}$ structure, suggesting that localized states induced by disorder can play an important role in depolarizing spins and mediating the valley relaxation of resident carriers in monolayer transition metal-dichalcogenide semiconductors.

11 pages, 5 figures