Kolloquiumsvortrag: Ultrafast dynamics of exciton formation at the ZnO (10-10) surface
Kiel, Leibnizstraße 13, Hans-Geiger-Hörsaal
Dr. Julia Stähler
(Fritz Haber Institute of the Max Planck Society)
Ultrafast dynamics of exciton formation at the ZnO(10-10) surface
We study the ultrafast quasiparticle dynamics in and below the ZnO conduction band using femtosecond time-resolved two-photon photoelect-ron spectroscopy. Above band gap excitation (hpump = 4.19 eV) enables the investigation of the hot electron relaxation by electron-phonon scatte-ring with an excess energy of 0.8 eV down to the Fermi level EF. It is shown that surface exciton (SX) formation creates additional states below EF already a few hundred femtoseconds after excitation. These quasipar-ticle formation dynamics are independent of temperature, while the decay is accelerated by heating. We furthermore tune the excitation density across the Mott limit and thereby reduce the SX formation due to transient carrier-carrier screening. Lastly, hydrogen titration proves the remarkable stability of this subsurface species. First results on energy level alignment and carrier dynamics in organic thin films on ZnO(10-10) will be dis-cussed.