Electron dynamics in graphene in the presence of an electrical field

Licentiatavhandling, 2017

Graphene as atomically thin two-dimensional material exhibits remarkable optical and electronic properties that suggest its technological application in novel optoelectronic devices, such as graphene-based lasers and photodetectors. The linear electronic bandstructure and the vanishing band gap at the Dirac point open up new relaxation channels, such as Auger scattering. Here, optically excited carriers can eciently bridge the valence and the conduction band, which might result in an increase of the number of charge carriers (electrons and holes), i.e. by absorbing a single photon one can create multiple electron-hole pairs through internal scattering. This many-particle process is called carrier multiplication (CM) and has a large technological potential. In the presence of an electric eld, carriers become accellerated in the momentum space depleting the region around the Dirac point and providing optimal conditions for Auger scattering and CM. To investigate ultrafast phenomena characterizing the carrier dynamics in graphene, we develop a microscopic approach based on the density matrix formalism and the semiconductor Bloch equations, which provides microscopic access to the timeand momentum resolved carrier dynamics in the presence of an electric eld. The aim of the thesis is to investigate the many-particle processes behind the ultrafast electron dynamics in graphene. The focus lies on understanding the dynamics in the presence of an electrical eld and in particular providing a microscopic foundation for the photoconduction eect, which is crucial for the application of graphene as an ultrafast photodetector. The highlight of the thesis is the proposal of a very ecient dark carrier multiplication in the presence of an electrical eld. While scattering processes are generally considered to reduce the eld-induced current, we have revealed that in graphene Auger processes give rise to a signicant current enhancement via dark CM. Furthermore, we have investigated the interplay of optical excitation, many-particle scattering and eld-induced acceleration of carriers resulting in asymmetric scattering processes and generation of photocurrents.