Effect of beam energy and surface temperature on the dissociative adsorption of H₂ on Si(001)

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Michael Dürr, Markus B. Raschke, and Ulrich Höfer
The Journal of Chemical Physics 111, 10411 (1999).
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Dissociative adsorption of H₂ from a high-flux supersonic molecular beam on flat and vicinal Si(001) surfaces was investigated by means of optical second harmonic generation (SHG). The initial sticking coefficients for terrace adsorption varied between 10^-8 and 10^-4. They revealed a strongly activated dissociation process, both with respect to the kinetic energy of the incident molecules (70 meV ≤ E_kin ≤ 380 meV) and the surface temperature (440K ≤ T_s ≤ 670 K). The results indicate that dynamical distortions of Si surface atoms can lower the effective adsorption barriers from 0.8±0.2 eV to almost negligible values. Previously proposed defect-mediated processes can be ruled out as a major adsorption channel.