Mechanism of O 2 Molecule Adsorption and Subsequent Oxidation of Dimers on Si(001) Surfaces
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Fig.1 Computational model molecular system for the oxidation reaction by (a) a molecular oxygen; Si9H12+02 and by (b) an atomic oxygen; Si9H12+O. Large and small solid spheres denote silicon and hydrogen atoms and open spheres denote oxygen atoms, respectively. Si(001) surface have been performed using an Si9H12+02 model molecular system [Fig.l(a)]. The surface part of this model, Si9H12, consists of two dimer silicon atoms and seven underlayer silicon atoms. Twelve hydrogen atoms terminate the outer bonds to make the model compound usable for molecular orbital calculations. Ri represents a distance between a Si plane composed of four second-layer Si atoms and the lower 0 atom and R2 represents the bond length of 02 molecule. An Si9H12+O model molecular system [Fig.l(b)] was used for the investigation of oxidation reaction of symmetric dimers by atomic oxygen. Ab initio Hartree-Fock method has been employed for the determination of the lowest potential energy reaction path [10]. The potential energy change along the reaction path was exactly obtained by the total energy calculations in the Moller-Plesset perturbation theory [10] including the electron correlations. The basis set is a standard split valence type functions; 321G [11]. The positions of two Si dimer atoms and an 02 molecule have been optimized at any
point on the reaction path whereas the movement of the under-layer Si atoms and all hydrogen atoms are restricted to keep the crystal structure. RESULTS Since the electronic ground states of a symmetric Si dimer and 02 molecule were confirmed to be the spin singlet state [12] and the triplet state [13], respectively, the total system should be the triplet state before the reaction. From some preliminary calculations considering the incident direction of 02 molecule, it was found that the dissociation of 02 molecule and the subsequent oxidation of dimers proceed when 02 molecule was vertically inserted into a Si dimer bond. In the lowest energy reaction path, the spin triplet state was kept during the reaction. The potential energy curve along the lowest energy reaction path is shown in Fig.2. The zero level of this potential curve is determined by the value at the infinite separation of a dimer and an 02 molecule. It can be recognized that there exist the shallow minimum (a), the transition state (b), and the final products (c) along the reaction coordinate. The curve in the inset of Fig.2 expresses how the distance Ri and R2 changes along the reaction Oath. In the beginning of the reaction, the distance Ri decreases while the distance R2 remains unchanged. This means that 02 molecule approaches the Si dimer as a molecule. When 94
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(a) Shallow Minimum
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Fig.2 The potential energy curve along the lowest energy reaction path for the oxidation by 02 molecule. The abscissa represents the distance from the transition state measured in the mass-weighted 3N-6 di
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