Quasiparticle dynamics at oxide surfaces: Electrons, Excitons and Polarons
Event details
| Date | 12.12.2013 |
| Hour | 16:30 › 17:30 |
| Speaker |
Dr A. Julia Staehler Department of Physical Chemistry Fritz Haber Institute of the Max Planck Society Berlin |
| Location | |
| Category | Conferences - Seminars |
Transparent conducting oxides are promising compounds for the application in optoelectronic devices and increasingly used as transparent electrodes. Both, ZnO and SrTiO3 (STO), exhibit a large band gap (> 3.2 eV) and high carrier mobilities and are, therefore, suitable candidates. The carrier and excitons dynamics at the surface of these materials crucially determine the functionality of any interface formed for optoelectronic application.
We study the ultrafast quasiparticle dynamics in and below the ZnO and STO conduction band using femtosecond time-resolved two-photon photoelectron (2PPE) spectroscopy. Above band gap excitation (hvpump = 4.19 eV) enables the investigation of the hot electron relaxation by electron-phonon scattering with an excess energy of 0.8 eV down to the Fermi level EF. We furthermore show for ZnO that surface exciton formation creates additional density of states below EF already a few hundred femtoseconds after excitation. These quasiparticle formation dynamics are almost independent of temperature and absent at the STO surface, which exhibits a dense charge accumulation layer (CAL) that screens the electron-hole Coulomb interaction. We furthermore tune the charge density of the CAL at the ZnO surface by hydrogen termination and thereby adjust the number of surface excitons formed. The remarkable stability of this surface species with lifetimes exceeding hundreds of picoseconds is enabled by downward surface band bending, shifting the exciton deep into the bulk band gap (DEbdg = 250 meV).
We study the ultrafast quasiparticle dynamics in and below the ZnO and STO conduction band using femtosecond time-resolved two-photon photoelectron (2PPE) spectroscopy. Above band gap excitation (hvpump = 4.19 eV) enables the investigation of the hot electron relaxation by electron-phonon scattering with an excess energy of 0.8 eV down to the Fermi level EF. We furthermore show for ZnO that surface exciton formation creates additional density of states below EF already a few hundred femtoseconds after excitation. These quasiparticle formation dynamics are almost independent of temperature and absent at the STO surface, which exhibits a dense charge accumulation layer (CAL) that screens the electron-hole Coulomb interaction. We furthermore tune the charge density of the CAL at the ZnO surface by hydrogen termination and thereby adjust the number of surface excitons formed. The remarkable stability of this surface species with lifetimes exceeding hundreds of picoseconds is enabled by downward surface band bending, shifting the exciton deep into the bulk band gap (DEbdg = 250 meV).
Practical information
- General public
- Free
Organizer
- Dr Frank van Mourik
Contact
- Frank van Mourik
[email protected]