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Adsorption Dynamics of Molecular Hydrogen on Silicon

film30.gifThe reaction dynamics of molecular hydrogen with silicon surfaces is strongly influenced by surface structure and lattice distortions. Sufaces diffusion and recombinative desorption experiments showed that the covalent nature of hydrogen bonding on silicon surfaces leads to high diffusion barriers and to desorption kinetics that strongly depend on surface reconstruction. Dissociative adsorption exhibits a pronounced increase in reaction rate with surface temperature and demonstrates the decisive role of the lattice degrees of freedom in the reaction dynamics on semiconductor surfaces.






russ1bThe lattice distortions are the basis of the model of phonon-assisted sticking first proposed by Brenig et al. The essence is shown in the 2-dim. model-potential in the figure to the left. The hydrogen molecule approaching the Si surface experiences a high barrier resulting in a low sticking probability. With increasing surface temperature, more of the vibrational states are occupied and the lattice is more often in the favorable configuration where the hydrogen molecule experiences a lower potential barrier and is consequently leading to a higher sticking coefficient.
In the desorption process the molecules do not gain kinetic energy because most of the energy released is stored in lattice vibration.



Experimental Procedure

hads2bThe high sensitivity of SHG to hydrogen adsorption is exploited to in situ measure the H coverage on silicon surfaces. The red dots in the left hand side figure show the decrease of the SHG signal with the adsorption of molecular hydrogen. Measuring the hydrogen pressure (indicated as a blue line), the flux of molecules on the sample and therefore the sticking coefficient can be calculated. Click here for the experimental setup.




Experimental Results

jcp3cUsing molecular beam techniques and second-harmonic generation as the probing method, we investigated the dependence of the sticking coefficient of molecular hydrogen on kinetic energy and the surface temperature Ts. Results are shown in the figure on the left hand side. The strong dependence on the kinetic energy can be described by s-shaped adsorption functions with a common mean adsorption barrier of about 0.8 eV. With higher surface temperature a wider range of barriers is accessible, resulting in broader adsorption functions and an Arrhenius law for the initial sticking coefficient s0=A exp(-Ea/kTs) at fixed kinetic energy. The resulting activation energy of Ea=0.7 eV for the Si(001) surface gives convincing evidence for the strong influence of the Si lattice on the reaction dynamics.


The current work in our group deals with static distortions of the lattice (steps, pre-adsorbed hydrogen) and their influence on the adsorption dynamics. By this, detailed informations on the adsorption mechanism are obtained.


P. Bratu, K. L. Kompa, and U. Höfer
Optical second-harmonic investigations of H2 and D2 adsorption on Si(100)2×1:
the surface temperature dependence of the sticking coefficient

Chem. Phys. Lett. 251, 1-7 (1996). Abstract Reprint (PDF) (© Elsevier)

U. Höfer
Nonlinear optical investigations of the dynamics of hydrogen interaction with silicon surfaces
Appl. Phys. A 63, 533-47 (1996). Abstract Reprint (PDF) (© Springer)

M. Dürr, M. B. Raschke, and U. Höfer
Effect of beam energy and surface temperature on the dissociative adsorption of H2 on Si(001)
J. Chem. Phys. 111, 10411-4 (1999). Abstract Reprint (PDF) (© AIP)

Hydrogen on Silicon - Worldwide Links

John J. Boland Johns Hopkins University, Baltimore, USA
Wilhelm Brenig TU München, Germany
Emily A. Carter University of California, Los Angeles, USA
Douglas Doren University of Delaware, Newark, USA
Steven M. George University of Colorado, Boulder, USA
Philippe Guyot-Sionnest University of Chicago, USA
Eckart Hasselbrink University of Essen, Germany
Tony F. Heinz Columbia University, New York, USA
Ulrich Höfer Philipps-Universität Marburg, Germany
John D. Joannopoulos Massachusetts Institute of Technology, Cambridge, USA
Kenneth D. Jordan University of Pittsburgh, USA
Efthimios Kaxiras Harvard University, Cambridge, USA
Kurt W. Kolasinski University of Birmingham, UK
Alan Cooper Luntz Odense University, Denmark
Jens K. Nørskov Technical University of Denmark, Lyngby, Denmark
Matthias Scheffler Fritz-Haber-Institut, Berlin, Germany
John T. Yates University of Pittsburgh, USA

Zuletzt aktualisiert: 27.05.2015 · armbrusn

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