| AVS 55th International Symposium & Exhibition | |
| Surface Science | Tuesday Sessions |
| Session SS2-TuA |
| Session: | Dynamics and Novel Probes |
| Presenter: | D. Krix, University of Duisburg-Essen, Germany |
| Authors: | D. Krix, University of Duisburg-Essen, Germany K. Huba, University of Duisburg-Essen, Germany H. Nienhaus, University of Duisburg-Essen, Germany |
| Correspondent: | Click to Email |
Non-adiabatic energy dissipation, e.g. the creation of electron-hole pairs, plays an important role in the understanding of chemical reactions at metal surfaces. For a large number of gas-metal reactions it is possible to measure hot charge carriers produced while exposing the metal to reactive gases as chemicurrents. The method is based on depositing thin metal films on a semiconducting substrate in order to form a rectifying electrical device - i.e. Schottky diodes. Hot electrons and holes may be ballistically injected into the semiconductor for energies larger than the intrinsic barrier at the interface. Thermalising in the bulk of the semiconductor the current can not flow back into the metal other than through the measuring circuit. Therefore large area, nanometer thick Pd films were grown on wet chemically prepared, hydrogen terminated, Boron doped p-Si(001) substrates. The palladium diodes are used as a platform for further deposition of alkali metal layers. Palladium was chosen for its low barrier height ( 0.38 eV ) on p-doped silicon, making the diodes most sensitive to low energetic hot holes while retaining excellent device characteristics. This barrier is also considerably lower than that of pure alkali-Si diodes – 0.58 eV for potassium. Chemicurrent, Auger spectroscopy, as well as Kelvin probe data are presented for the adsorption of molecular oxygen and molecular hydrogen on Pd and K/Pd surfaces at low temperatures ( T = 120 K). During the oxidation with molecular oxygen the chemicurrents show as at least two distinct maxima. Varying the initial alkali coverage changes the relative contribution of the maxima to the chemicurrent. The overall charge detected increases monotonically with potassium coverage. After the deposition of about two monolayers of potassium a saturation is seen of about 1μAs/cm2. Increasing the Pd film thickness results in a strong exponential attenuation of the observed charge. This is strong support for the identification of the detected currents as due to hot hole generation at the surface. The data give an estimate of the probability to generate a hot hole in a reaction event of approximately 1%.