Gaussian-2 (G2) theory is combined with isodesmic bond separation reaction energies to yield accurate thermochemistry for larger molecules. For a test set of 40 molecules composed of H, C, O, and N, our method yields enthalpies of formation, ΔHf0(298 K), with a mean absolute deviation from experiment of only 0.5 kcal/mol. This is an improvement of a factor of three over the deviation of 1.5 kcal/mol seen in standard G2 theory.

Surface infrared absorption spectroscopy and density functional cluster calculations are used to definitively demonstrate that the Si-Si dimer bond is the target for the initial insertion of oxygen into the Si(100) -͑231͒ surface, following H 2 O exposure and annealing. This reaction, in turn, facilitates the subsequent incorporation of O into the Si backbonds, thereby promoting (local) oxidation.[S0031-9007(97)04218-X]

Infrared absorption spectroscopy and density functional cluster calculations are used to identify the intermediate oxide structures formed by high temperature annealing of the water-exposed Si͑100͒-͑2 3 1͒ surface. We find that initially there is a strong tendency for oxygen to agglomerate on single dimer units at T ϳ 800 K. Upon dehydrogenation, a remarkable structural transition is observed, wherein the dangling bonds recombine to form silicon epoxides (three-membered Si-O-Si rings). We demonstrate that these epoxides are the thermodynamically favored product in such constrained systems and, consequently, should be preferentially formed at silica interfaces. [S0031-9007(98)07445-6] PACS numbers: 81.65. Mq, 31.15.Ar, 68.35.Ja, 78.55.Ap Understanding the formation and evolution of SiO (defect) structures at silica surfaces and Si͞SiO 2 interfaces is of prime importance due both to the utilization of ultrathin oxide ͑ϳ10 Å͒ films as the gate dielectric layer in state-of-the-art semiconductor devices [1] and to the production of ultralow loss silica-based fiber for long distance optical communications. The detailed structural understanding of such interfaces poses a formidable scientific challenge due to the lack of long-range order and critical dependence on a wide array of production parameters. Therefore, a comprehensive knowledge of the fundamental physical and chemical properties of such systems must commence with the characterization of related systems that are more reproducible and amenable to control. In accordance with this prescribed approach, a wide variety of experimental [2] and theoretical [3] studies of such model systems have been undertaken over the past decade, but have singularly failed to provide a definitive mechanistic picture. For example, previous theoretical studies have focused primarily on a few simple Si-O structures that may result from the insertion of a single oxygen into the Si͑100͒-͑2 3 1͒ surface, with little exploration of the wider group of multiply oxidized structures that are subsequently formed during oxidation.Similarly, experimental studies have typically employed techniques [e.g., ultraviolet photoemission spectroscopy (UPS), x-ray photoemission spectroscopy (XPS)] and conditions that do not permit identification of the relevant discrete (sub)oxide structures formed. Recently, a combined infrared and theoretical study of the initial water-induced oxidation of Si͑100͒-͑2 3 1͒ [4] has shown that oxygen is first inserted into the dimer bonds and then into the Si backbonds, although the formation of the "real" multiply oxidized Si structures (that are the constituent subunits of extended layer growth) could not be delineated, as the spectral signatures of these oxide species were below the accessible range ͑,1000 cm 21 ͒.In this work, we have implemented a novel spectroscopic configuration that allows access to a broad spectral range ͑550 4000 cm 21 ͒ with submonolayer sen-sitivity to all vibrational components. We have primarily focused on the H 2 O:Si͑100͒-͑2 3 1͒ system a...

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Articles you may be interested inThe thermal chemistry of saturated layers of acetylene and ethylene on Ni(100) studied by in situ synchrotron xray photoelectron spectroscopy Surface infrared spectroscopy and density functional cluster calculations are used to study the thermal and atomic hydrogen-induced decomposition of water molecules on the clean Si͑100͒-͑2ϫ1͒ surface. We report the first observation of the Si-H bending modes associated with the initial insertion of oxygen into the dimer and backbonds of a silicon dimer. We find that, while one and two oxygen-containing dimers are formed almost simultaneously during the thermal decomposition of water on this surface, atomic H can be used to drive the preferential formation of the singly oxidized dimer. This work highlights the sensitivity of Si-H bending modes to the details of local chemical structure in an inhomogeneous system, suggesting that the combined experimental and theoretical approach demonstrated herein may be extremely useful in studying even more complex systems such as the hydrogenation of defects in SiO 2 films.