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

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 ${\tau }_v$ is quite long ($\sim$10~$\mu$s) at low carrier densities, but decreases rapidly to less than 100~ns at high electron or hole densities $\gtrsim$2 $\times 10^{12}$~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 ${\tau }_v$, 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

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• Ab initio Ultrafast Spin Dynamics in Solids
• Many-body exciton and inter-valley correlations in heavily electron-doped WSe${}_2$ monolayers
• Twist angle dependent interlayer transfer of valley polarization from excitons to free charge carriers in WSe${}_2$/MoSe${}_2$ heterobilayers
• Optical detection of long electron spin transport lengths in a monolayer semiconductor
• Predicting Phonon-Induced Spin Decoherence from First Principles: Colossal Spin Renormalization in Condensed Matter
• Ultrafast Momentum-resolved Hot Electron Dynamics in the Two-dimensional Topological Insulator Bismuthene
• Spin-valley relaxation dynamics of Landau-quantized electrons in MoSe${}_2$ monolayer
• Dynamic Valley Polarization in Moire Quantum Dots
• Rapid spin depolarization in the layered 2D Ruddlesden Popper perovskite (BA)(MA)PbI
• Spin relaxation of localized electrons in monolayer MoSe${}_2$: importance of random effective magnetic fields
• Spin/Valley pumping of resident electrons in WSe2 and WS2 monolayers
• Exchange-enhanced spin-orbit splitting and its density dependence for electrons in monolayer transition metal dichalcogenides
• Energy shifts and broadening of excitonic resonances in electrostatically-doped semiconductors
• Spin dependent charge transfer in MoSe2/hBN/Ni hybrid structures
• Trapping the carrier in the spin-locked MoS2 atomic valley by absorption of chiral L-cysteine
• Valley polarization fluctuations, bistability, and switching in two-dimensional semiconductors
• Predicting and understanding diffusion lengths and lifetimes in solids via a many-body \textit{ab initio} method: The role of coupled dynamics
• Unprecedented Spin-Lifetime of Itinerant Electrons in Natural Graphite Crystals
• Contribution to Excitonic Linewidth from Free Carrier-Exciton Scattering in Layered Materials: The example of h-BN