Valley Relaxation Of Resident Electrons And Holes In A Monolayer Semiconductor: Dependence On Carrier Density And The Role Of Substrate-Induced Disorder

Using time-resolved optical Kerr rotation, we measure the low-temperature valley dynamics of resident electrons and holes in exfoliated WSe2 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 WSe2 monolayers having different dielectric environments. In a fully encapsulated structure (hBN/WSe2/hBN, where hBN is hexagonal boron nitride), valley relaxation is found to be monoexponential. The valley relaxation time tau(v) is quite long (similar to 10 mu s) at low carrier densities, but decreases rapidly to less than 100 ns at high electron or hole densities greater than or similar to 2 x 10(12) cm(-2). In contrast, in a partially encapsulated WSe2 monolayer placed directly on silicon dioxide (hBN/WSe2/SiO2), carrier valley relaxation is multiexponential at low carrier densities. The difference is attributed to environmental disorder from the SiO2 substrate. Unexpectedly, very small out-of-plane magnetic fields can increase tv, especially in the hBN/WSe2/SiO2 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.