Dresden 2014 – scientific programme

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O: Fachverband Oberflächenphysik

O 100: Surface Dynamics II

O 100.3: Talk

Friday, April 4, 2014, 11:00–11:15, PHY C 213

Electronic structure and exciton formation dynamics at the ZnO(1010) surface — •Jan-Christoph Deinert, Daniel Wegkamp, Michael Meyer, Clemens Richter, Julia Stähler, and Martin Wolf — Fritz-Haber-Institut der MPG, Abt. Physikalische Chemie, Faradayweg 4-6, 14195 Berlin

Zinc oxide (ZnO) is a promising electrode material for optoelectronic applications. It is transparent to visible light (band gap of 3.4 eV), natively n-type doped and abundant. Notably, the electronic properties of its interfaces with - possibly functional - organic molecules are not well understood, e. g. only recently the effect of ZnO metallization by hydrogen adsorption and formation of a surface charge accumulation layer (SCA) has been elucidated in detail. We employ a pump-probe technique to gain insight into the dynamics of excited carriers at hydrogen-terminated ZnO surfaces: time-resolved two-photon photoelectron spectroscopy (2PPE). Above band gap excitation at the (1010) surface leads to the population of states in the conduction band (CB). These states decay on a few ten fs timescale via LO phonon scattering, where scattering rates show no distinct temperature dependence. At energies below the bulk CB, however, the excited electron dynamics deviate strongly from that ultrafast behaviour. We observe exciton formation within two ps after excitation, 50 meV below the Fermi energy. These surface excitons exhibit lifetimes exceeding several hundred ps. Remarkably, the signature of the surface excitons is quenched by increasing the charge density in the SCA. This is due to enhanced screening as the charge density approaches the Mott limit.