Theory of Reactive Adsorption on Si(100)
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magnitude come from basis set effects and the DFT method itself. The conclusions of this work are not affected by errors of this order. RESULTS Bare Surface Structure The Si(100) surface displays a 2x1 reconstruction with surface atoms paired up into rows of "dimers." It is now generally agreed that the dimers have an asymmetric structure, being tilted with one atom of the dimer higher than the other. This structure is illustrated in the cluster model of Figure 1. This structure is slightly more stable than a symmetric structure. The stabilization energy is predicted to be no more than 0.2 eV, though the precise value depends on details of the theory (a brief review of the literature on this subject is included in Ref. 4). Using larger cluster models or periodic boundary conditions generally results in even greater asymmetry than shown in Figure 1.Many transition states display a similar asymmetry. Tilting the surface dimers causes an asymmetry in the surface charge distribution, so that the "up" atom has a higher electron density than the "down" atom. Mulliken charges for the two atoms illustrating this effect are shown in Figure la. An important consequence of this charge asymmetry is that the two atoms in a dimer are chemically inequivalent. The electron-deficient "down" atom can be expected to react as an electrophile, preferring to interact with the electronrich regions of incident molecules. On the other hand, the electron-rich "up" atom of the dimer is expected to react as a nucleophile, preferring to interact with electron-poor regions of incident molecules. The asymmetry appears in the highest occupied molecular orbital (HOMO) of the bare surface (Figure lb), where the two dangling bond orbitals have different spatial extents. The asymmetry increases with the tilting angle. Thus, tilting the surface dimers establishes two chemically inequivalent sites, even on well-ordered surfaces of this elemental semiconductor. The consequences of this effect are illustrated in the following examples.
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Figure 1. a) The Si 9H12 cluster model of the bare surface b) The HOMO of the bare surface. Hydrogen (Hz2 Hydrogen dissociates with a very low probability (10-) to form Si-H bonds. The reaction mechanism for adsorption and desorption of H2 remains controversial. Two reviews of the issues involved have recently been published."' Some workers have suggested that reaction is much more likely at defect sites. For present purposes, it is valuable examine the
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reaction path that involves the majority sites since it establishes a comparison with the mechanisms for reactions of other adsorbates at these sites. The transition state structure calculated with the cluster model is shown in Figure 2a. The energy of this configuration is 1.0 eV above the energy of the isolated H2 and bare surface, consistent with a small sticking probability. A more detailed discussion and comparisons to other calculations (which find similar transition state structures) are given in Ref. 3. At the transition state, the H2 molecule is loc
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