| AVS 55th International Symposium & Exhibition | |
| Surface Science | Tuesday Sessions |
| Session SS-TuM |
| Session: | Dynamics at Surfaces |
| Presenter: | H. Nienhaus, University of Duisburg-Essen and CeNIDE, Germany |
| Authors: | H. Nienhaus, University of Duisburg-Essen and CeNIDE, Germany K. Huba, University of Duisburg-Essen, Germany D. Krix, University of Duisburg-Essen, Germany |
| Correspondent: | Click to Email |
The oxidation of reactive metal surfaces may lead to the emission of photons (surface chemiluminescence) and of electrons into vacuum (exoemission). This is due to the highly non-adiabatic character of the reaction. The exoemission signals are strongly influenced by the change of work function with oxidation and are, therefore, difficult to interpret. Likewise, thin-film electronic devices, e.g. metal semiconductor contacts have been used to detect the internal exoemission by measuring chemicurrents. The internal Schottky barrier acts a high pass energy filter for hot charge carriers. The barrier is much lower than the work function and stays constant during oxidation. Hence, the reaction kinetics may be studied by recording the chemicurrent transients as has been successfully demonstrated for the oxidation of Mg.1 The interaction of oxygen molecules with alkali metal surfaces is a prototype for charge transfer reactions with strong non-adiabatic energy dissipation. To study the internal exoemission large area K/p-Si(001) Schottky diodes are prepared by evaporation of ultrathin K layers on hydrogen terminated Si(001) surfaces in the thickness range between 2 and 30 monolayers. The metal film growth at low temperatures is monitored by Kelvin probe and Auger spectroscopy. The interface properties are characterized by current-voltage measurements revealing excellent rectifying properties of the diodes. The reverse currents are extremely low and a homogeneous barrier height of approximately 0.56 eV is determined using thermionic emission theory. When the diodes are exposed to molecular oxygen a strong chemicurrent signal is observed. The current increases with exposure time, exhibits a maximum and levels off with large exposures. This behavior indicates a nucleation-and-growth type of oxide formation. The total charge detected in the diode depends on the potassium film thickness in the thin-film regime which allows for an estimate of the oxidation depth.
1 S. Glass, H. Nienhaus, Phys. Rev. Lett. 93 (2004) 168302.