Excitation-induced transition to indirect band gaps in atomically thin transition metal dichalcogenide semiconductors

TL;DR

This study explores how high carrier densities in monolayer TMDCs cause band-gap renormalization and exciton dissociation, leading to a transition from direct to indirect band gaps, impacting their optical and electronic properties.

Contribution

It predicts excitation-induced direct-to-indirect band gap transitions in monolayer TMDCs, a novel insight into their electronic structure under high excitation conditions.

Findings

Carrier density reduces exciton binding energies.

Excitation causes band-gap shrinkage and excitonic redshifts.

Two TMDCs undergo a transition from direct to indirect band gaps.

Abstract

Monolayers of transition metal dichalcogenides (TMDCs) exhibit an exceptionally strong Coulomb interaction between charge carriers due to the two-dimensional carrier confinement in connection with weak dielectric screening. High densities of excited charge carriers in the various band-structure valleys cause strong many-body renormalizations that influence both the electronic properties and the optical response of the material. We investigate electronic and optical properties of the typical monolayer TMDCs MoS$_2$, MoSe$_2$, WS$_2$ and WSe$_2$ in the presence of excited carriers by solving semiconductor Bloch equations on the full Brillouin zone. With increasing carrier density, we systematically find a reduction of the exciton binding energies due to Coulomb screening and Pauli blocking. Together with excitation-induced band-gap shrinkage this leads to redshifts of excitonic resonances up to the dissociation of excitons. As a central result, we predict for all investigated monolayer TMDCs that the $\Sigma$-valley shifts stronger than the K-valley. Two of the materials undergo a transition from direct to indirect band gaps under carrier excitation similar to well-known strain-induced effects. Our findings have strong implications for the filling of conduction-band valleys with excited carriers and are relevant to transport and optical applications as well as the emergence of phonon-driven superconductivity.