The Interaction of Water with Solid Surfaces: Fundamental Aspects Revisited

The spectroscopic properties, structure and interconversions of optically active oxygen-deficiency-related point defects in vitreous silica are reviewed. These defects, the E′-centers (oxygen vacancies with a trapped hole or 3-fold-coordinated silicons), different variants of diamagnetic `ODCs' (oxygen-deficiency centers), and their Ge-related analogs play a key role in the fiber-optic Bragg grating writing processes. The controversy surrounding the structural models for the Si- and Ge-related ODCs is discussed and the similarity between the bulk and surface point defects in silica is emphasized. The possible interconversion mechanisms between 2-fold-coordinated Si, neutral oxygen vacancies and E′-centers are discussed. The effects of glassy disorder have a profound effect on defect properties and interconversion processes in silica.

Optically active oxygen-deficiency-related centers in amorphous silicon dioxide

The outstanding properties of SiO2, which include high resistivity, excellent dielectric strength, a large band gap, a high melting point, and a native, low defect density interface with Si, are in large part responsible for enabling the microelectronics revolution. The Si/SiO2 interface, which forms the heart of the modern metal–oxide–semiconductor field effect transistor, the building block of the integrated circuit, is arguably the worlds most economically and technologically important materials interface. This article summarizes recent progress and current scientific understanding of ultrathin (<4 nm) SiO2 and Si–O–N (silicon oxynitride) gate dielectrics on Si based devices. We will emphasize an understanding of the limits of these gate dielectrics, i.e., how their continuously shrinking thickness, dictated by integrated circuit device scaling, results in physical and electrical property changes that impose limits on their usefulness. We observe, in conclusion, that although Si microelectronic devices...

Ultrathin (<4 nm) SiO2 and Si-O-N gate dielectric layers for silicon microelectronics: Understanding the processing, structure, and physical and electrical limits

A nitride semiconductor light-emitting device has an active layer of a single-quantum well structure or multi-quantum well made of a nitride semiconductor containing indium and gallium. A first p-type clad layer made of a p-type nitride semiconductor containing aluminum and gallium is provided in contact with one surface of the active layer. A second p-type clad layer made of a p-type nitride semiconductor containing aluminum and gallium is provided on the first p-type clad layer. The second p-type clad layer has a larger band gap than that of the first p-type clad layer. An n-type semiconductor layer is provided in contact with the other surface of the active layer.

Nitride semiconductor light emitting device

We report the photoreactivity of H−Si(111) with dioxygen and terminally unsaturated hydrocarbons. Illumination of H−Si(111) in air with ultraviolet light of wavelength of 350 nm or shorter produces oxidized silicon; longer wavelengths cause no oxidation. When H−Si(111) is immersed in unsaturated hydrocarbons (1-octene, 1-octadecene, 1-octyne, styrene, and phenylacetylene) that have been deoxygenated, illumination with a Hg lamp results in densely packed hydrocarbon films of molecular thickness. Previous work and the results presented here suggest that the H−Si bond adds across the unsaturated bond in these reactions similar to hydrosilylation reactions known for small-molecule chemistry and produce adsorbates covalently bonded to the surface. We present spectroscopic evidence showing films resulting from terminal acetylenes consist of adsorbates linked to the surface by a vinyl group. We propose that all of these reactions occur through a radical-chain mechanism initiated by the wavelength-dependent photo...

Photoreactivity of Unsaturated Compounds with Hydrogen-Terminated Silicon(111)

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)-- $(2\ifmmode\times\else\texttimes\fi{}1)$ surface, following ${\mathrm{H}}_{2}\mathrm{O}$ exposure and annealing. This reaction, in turn, facilitates the subsequent incorporation of O into the Si backbonds, thereby promoting (local) oxidation.

Initial H 2 O-induced Oxidation of Si(100)- $2×1$

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 $\mathrm{Si}(100)\ensuremath{-}(2\ifmmode\times\else\texttimes\fi{}1)$ surface. We find that initially there is a strong tendency for oxygen to agglomerate on single dimer units at $T\ensuremath{\sim}800\mathrm{K}$. Upon dehydrogenation, a remarkable structural transition is observed, wherein the dangling bonds recombine to form silicon epoxides (three-membered $\mathrm{Si}\char22{}\mathrm{O}\char22{}\mathrm{Si}$ rings). We demonstrate that these epoxides are the thermodynamically favored product in such constrained systems and, consequently, should be preferentially formed at silica interfaces.

Silicon Epoxide: Unexpected Intermediate during Silicon Oxide Formation

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 Si(100)-(2×1) 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 SiO2 films.

Si–H bending modes as a probe of local chemical structure: Thermal and chemical routes to decomposition of H2O on Si(100)-(2×1)

In this paper, we review the pivotal role that defects (in particular vacancy structures) play in driving the H-induced exfoliation of Si. We highlight the central role that infrared spectroscopy has played in delineating the microscopic details of the exfoliation process. We show that when the results of such spectroscopic studies are combined with those obtained using a variety of other experimental probes as well as ab initio quantum chemical cluster calculations, an unambiguous mechanistic picture emerges. Specifically we find that H-terminated vacancy structures drive the formation of internal surfaces into cracks where H2 is then evolved, resulting in the build-up of sufficient internal pressure to cause lift-off of the overlying Si. The role of coimplantation of He is also discussed.

Spectroscopic studies of H-decorated interstitials and vacancies in thin-film silicon exfoliation

The microscopic mechanism of the formation of ultrathin oxides on Si(100) has been investigated using a combination of infrared spectroscopy and ab initio quantum chemical cluster calculations. The 0→2 monolayer oxide films are grown sequentially from the “bottom-up” using repeated water exposures and annealing cycles, with the partial pressure of water ranging from 10−10 to 10 Torr. The resultant films were then compared to the equivalent thicknesses of thermal and native oxide films. In this way, we obtain unprecedented insight into the essential chemical structures formed during the initial oxidation and subsequent layer growth of these technologically relevant films.

Boris Stefanov

Papers

Initial H 2 O-induced Oxidation of Si(100)- \(2×1\)

Silicon Epoxide: Unexpected Intermediate during Silicon Oxide Formation

Si–H bending modes as a probe of local chemical structure: Thermal and chemical routes to decomposition of H2O on Si(100)-(2×1)

Spectroscopic studies of H-decorated interstitials and vacancies in thin-film silicon exfoliation

Mechanistic studies of silicon oxidation