SFB 616


Project a1

 Project A1:

Electron-Hole Pairs in Metal Surfaces Excited
by Chemical Reactions and Inelastic Scattering

 When chemical or kinetic energy is deposited on metal surfaces by reactions or inelastic scattering the dissipation may occur non-adiabatically, i.e., hot electrons and holes may be excited in the metal surface. To detect the ultrashort living hot charge carriers, thin-film electronic devices may be used. The principle of detection is demonstrated in Fig. 1 for n-type metal-semiconductor contacts (Schottky diodes). The metal film thickness is in the range of the mean free path of the excited charge carriers. The difference between the Fermi level EF and the conduction band minimum is the Schottky barrier height. If electron-hole pairs are generated at the surface, the hot electron may travel ballistically through the thin metal film, traverse the Schottky barrier, and is detected as a chemicurrent in the diode. The barrier height is typically 0.5 eV in case of Si.

 The detected currents are proportional to the reaction rate at the surface. Hence, the observed chemicurrent transients represent the kinetics of the respective surface reaction. This is shown in the semi-logarithmic plot of Fig. 2 for various chemicurrent traces. When Ag/Si diodes are exposed to atomic hydrogen an exponential decay of the chemicurrent and a constant current for long exposure times are observed. It is attributed to spontaneous adsorption of the atoms and abstraction of adsorbed hydrogen. The chemicurrent during oxidation of a Mg film on p-Si(111) shows a distinct maximum which is due to a nucleation and growth of oxide. Clean Mg surfaces are rather inert to molecular oxygen but as soon as oxide islands exist the oxidation process is promoted. The reaction of NO molecules with Ag surfaces results in a chemicurrent transient with two maxima. The first is attributed to exothermic molecular adsorption and the second is due to the intermolecular reaction of neighboring NO molecules on the surface leading to an oxygen atom and a desorbing N2O molecule.

To prove that the chemicurrent effect is in fact explained by a non-adiabatic energy dissipation, the currents were recorded at Ag/Si diodes upon exposure to atomic hydrogen and deuterium, respectively. As displayed in Fig. 3, the chemicurrent efficiency is approximately four times larger with atomic hydrogen than with D atoms. Hence, the currents are not induced by a thermal process since the deposited adsorption energy is independent of the isotope mass.

 The diodes may also be used to detect chemiluminescence, i.e., photons generated by the reaction process. In Fig. 4 the maximum chemicurrent in Mg/Si diodes during oxidation is plotted as a function of the metal film thickness. For thin films the current is rapidly attenuated according to an exponential function with an attenuation constant of 5.4 nm. With thicker films a second maximum is observed which is less attenuated with a constant of 50 nm and may be attributed to photons reabsorbed in the diode. Hence, hot charge carriers and photons may be distinguished in the device by varying the film thickness.

 Current and future experiments try to clarify the excitation mechanisms by investigating
 - the internal exoemission with very exothermic reactions like O2 or Cl2 with alkali metals,
 - the hot charge carrier distribution by varying the homogeneous Schottky barrier height,
 - the chemovoltage during reactions on microscopic metal islands.

In addition, variations of device characteristics upon gas exposures are studied in detail.

 The project has close collaborations with other projects: A2, A3, A4, A6, B5, B7, C1



Poster Overview

A1 Krix et al.
PDF (1.6 MB)

Begehung 2008

Internationaler Workshop 2008
A1 Krix et al.
 PDF (1.0 MB)

Begehung 2005

Begehung 2005
A1 Nienhaus et al.
 PDF (1.0 MB)