Microscopic Mechanisms of the Formation, Relaxation and Recombination of Excitons in Two-Dimensional Semiconductors
Licentiate thesis, 2019
Monolayers of Transition Metal Dichalcogenides (TMDs) present a giant leap forward towards the realization of semiconductor devices with atomic scale thickness. As a natural consequence of their two-dimensional character TMDs exhibit a reduced dielectric screening, leading to the formation of unusually stable excitons, i.e. Coulomb-bound electron-hole pairs. Excitons dominate the optical response as well as the ultrafast dynamics in TMDs. As a result, a microscopic understanding of excitons, their formation, relaxation and decay dynamics becomes crucial for a technological application of TMDs. A detailed theoretical picture of the internal structure of excitons and their scattering channels allows for a controlled manipulation of TMD properties enabling an entire new class of light emitters and detectors.
The aim of this thesis is to investigate the many-particle processes governing the ultrafast dynamics of excitons. The focus is to provide a sophisticated picture of exciton-phonon and exciton-photon interaction mechanisms and the impact of dark exciton states starting from the formation of bound excitons out of a free electron-hole gas up to the eventual radiative decay of bright and dark exciton populations. Based on an equations-of-motion approach for the density matrix of an interacting electron, phonon and photon system, we simulate the dynamics of excitons in TMDs across the full Rydberg-like series of bright and dark states. Our theoretical model allows us to predict fundamental relaxation time scales as well as spectral features accessible in multiple spectroscopic experiments, such as absorption, photoluminescence and ultrafast pump-probe. In particular we predict intriguing features appearing in the terahertz absorption spectrum during the formation of excitons as well as distinct -so far unexplained- low temperature luminescence features stemming from phonon-assisted recombinations of dark excitons.
density matrix formalism