Schematic model of oxidation reaction on . The open and gray circles indicate the Si and O atoms, respectively. The initial product at is of the ad-ins type which transforms to the structure at . It was proposed that the successive reaction with further produces the structure (Refs. 23 and 26).
EELS spectra of as a function of the exposure to taken at . The exposures are (a) 0, (b) 0.01, (c) 0.02, (d) 0.03, (e) 0.07, (f) 0.1, and (g) . The inset shows the inspect of the Si–O stretch region taken with another sample at . The exposures are 0.01, 0.02, 0.04, 0.06, 0.08, 0.1, and from the bottom to the top. The loss intensities are normalized to the elastic peak intensities.
(a) An EELS spectrum of exposed to at . The 80- and losses are characteristic of the ad-ins product. (b) The surface was subsequently heated to . The peak disappears and two peaks are observed at 81 and , which are ascribed to the product. The corresponding spectrum taken with , which exhibits isotope-shifted peaks at 80 and . (c) The surface after annealing [spectrum (b)] was further exposed to . The 80-, 128-, and peaks are characteristic of the secondary product.
Isotope-labeled EELS spectra of the secondary oxidation products prepared by way of the two-step reaction: The clean surface was exposed to [(a) and (b)] or [(c) and (d)], and subsequently heated to . Then the surface was further exposed to [(a) and (c)] or [(b) and (d)]. The Si–O stretch mode is associated with the postexposed isotope. The Si–O–Si asymmetric stretch modes show energy shifts depending on the isotopic combinations due to dynamical couplings.
The calculated energies of the Si–O–Si asymmetric stretch modes as a function of the interaction parameter . The isotopic combinations are (a) , (b) , (c) , and (d) . The schematic structures are depicted in the inset, where the open, gray, and black circles indicate Si, , and atoms, respectively. The solid and dashed curves correspond to the in-phase and out-of-phase modes, respectively. For (a) and (d), the out-of-phase modes are dipole forbidden and thus are not presented. The experimental energies are shown by the dots, and the parameters , , and were determined by the least-squares fitting.
Schematic of the Si–O–Si asymmetric stretch vibrations of the [(a) and (b)] and [(c) and (d)] structures. The large open circles indicate Si atoms, and the small gray and black circles indicate and atoms, respectively. The normal modes are represented by the [(a) and (c)] in-phase and [(b) and (d)] out-of-phase motions of the oxygen atoms. The arrows are depicted with its length proportional to the dynamic dipole moment induced by the excitations.
A summary of experimental energies (meV) with and mode assignments for the structures shown in Fig. 1.
Energies of the Si–O–Si asymmetric stretch modes observed in the isotope-labeled experiments and the results of the least-squares fitting (meV). The procedure indicates that the clean surface was exposed to , heated to , and subsequently exposed to .
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