Atomically thin two-dimensional semiconductors are of strong current interest fo applications as well as for fundamental studies. For opto-electronic applications like displays and photovoltaics, transition-metal-dichalcogenides (TMDs) are an appealing system, as they combine great physical strength with high carrier mobility and an direct optical band gap. To determine the opto-electronic properties under device operation conditions, it is important to consider both the optical properties as well as the relaxation dynamics of excited carriers.
In these two-dimensional atomically thin semiconductors, the Coulomb interaction is known to be much stronger than in quantum wells of conventional semiconductors like GaAs, as witnessed by the 50 times larger exciton binding energy. The question arises, whether this directly translates into equivalently faster carrier-carrier Coulomb scattering of excited carriers. We answer this question via a microscopic approach, taking into account the dynamics over the full brillouin zone. The other main source of carrier kinetics is the interaction of the excited carriers with phonons, that has been shown experimentally to cause efficient carrier scattering in TMDs. To analyze carrier-phonon scattering processes of electrons and holes on a microscopic footing, we use a quantum-kinetic description based on non-equilibrium Green’s function techniques. This allows, in a first step, for the description of polarons, which are quasi-particles of the carrier-phonon interaction. In a second step, the quasi-particles are used for the determination of the carrier kinetics. Such quantum-kinetic models can also incorporate non-Markovian effects, i.e. dependences of the time evolution at the actual time on integrals over the past of the time evolution, that are often important on ultra-short timescales.
We analyze the optical properties and carrier dynamics for electrons and holes in MoS2 due to the interaction via the carrier-carrier Coulomb interaction as well as due to carrier-phonon interaction. Ultrafast carrier relaxation on a timescale faster than 100fs is demonstrated and the resulting optical properties are discussed.