Hot-Electron Transport through Interfaces and in Metal Films
Project management at the University of Würzburg:
Transport of excited carriers through interfaces is of fundamental interest in a broad range of different fields, such as optoelectronics, biophotonics and surface chemistry. Excitation of carriers with ultrashort laser pulses allows investigating dynamic processes at the interface in the time domain. However, the time resolved spectroscopy of carrier dynamics at buried interfaces is up to now limited to the application of purely optical methods (Second Harmonic Generation, Four-Wave-Mixing, Transient Absorption..) which are not directly sensitive to carrier transport phenomena. Here we apply time-resolved two-photon tunneling current spectroscopy and two-photon photoemission spectroscopy to investigate the transport of excited carriers in metal-insulator-metal contacts. Both techniques provide state selective information on excited carrier transport phenomena in heterogeneous systems. This substantially broadens the scope for time resolved spectroscopy of buried interfaces.
Two-photon excitation of electrons in a metal electrode and subsequent tunneling of these electrons through a barrier offers new possibilities to investigate the dynamic properties of interface states. Femtosecond laser pulses (1.5 eV, 25 fs) generate a non-equilibrium electron distribution in the top electrode of a metal-insulator-metal contact (Ag-Al2O3 -Al). The effective barrier height of 3.9 eV excludes internal photoemission, i.e. transmission over the barrier, even via a two-photon excitation. A pump-probe technique can be applied to investigate the electron dynamics at the metal-insulator interface. The bulk or interface states are populated by the first laser pulse and the second delayed pulse excites the electrons further, giving them a chance to tunnel through the barrier. This time-resolved two-photon tunnelling current spectroscopy reveals the existence of an unoccupied intermediate state located at the Ag-Al2O3 interface with a lifetime of about 35 fs and a dephasing time of 10 fs. The dephasing time is long compared to the dephasing time of electrons in bulk metals. Therefore we conclude that we study the ultrafast dynamics of an unoccupied state located at the Ag-Al2O3 interface.
The experiments are performed in collaboration with D. Diesing and A. Otto from the University of Düsseldorf
Projekt period: from 06.2002 to 05.2002
DFG ,Granting date: 11.04.2002