Oxygen-excess amorphous SiO2 with 18O-labeled interstitial oxygen molecules

Exchange between oxygen molecules embedded in amorphous SiO₂ (interstitial O₂) and oxygen atoms in the a-SiO₂ network is found to be remarkably slow at 500 °C. Thermal loading of ¹⁸O₂ at this temperature yields a-SiO₂ containing ¹⁸O-labeled interstitial O₂ whose ¹⁸O fraction is as high as ~ 90%. The ¹⁸O fraction of interstitial O₂ in this sample is quickly decreased by thermal annealing at or above 700 °C because of the oxygen exchange accompanied by the release of ¹⁶O from the a-SiO₂ network. This finding indicates that the oxygen exchange starts at much lower temperatures than indicated by previous works, based on monitoring of the isotopic composition of oxygen atoms in the a-SiO₂ network.     

Electron beam poling in amorphous Ge-doped H:SiO2 films

Amorphous Ge-doped H:SiO₂ films on silica, deposited by matrix-distributed electron cyclotron resonance – plasma enhanced chemical vapor deposition, were irradiated with an electron beam while varying the dose. Using the Maker fringe method, second-harmonic generation was measured in the irradiated regions of the films. With a current of 5 nA, and an acceleration voltage of 25 kV for 25 s, a Ge-doped H:SiO₂ film (3.8 at.% Ge) showed a maximum second-order nonlinearity of d₃₃ = 0.0005 pm/V. In contrast, a H:SiO₂ film with a smaller Ge content (1.0 at.% Ge), showed a large SHG: d₃₃ = 0.06 pm/V when irradiated for 15 s. The second-harmonic generation in the films is caused by a frozen-in electric field induced by charge implantation from the electron beam. The strength of the electric field is determined by two conditions: the trapping centers (numbers, depth) and the remaining conductivity under large electric field.     

Structural and luminescence properties of amorphous SiO2 nanoparticles

We report an experimental study on the photoluminescence band peaked at 2.7 eV (blue band) induced by thermal treatments in nanometric amorphous SiO₂. In particular the emission dependence on the nanometric particles size as a function of their mean diameter from 7 nm up to 40 nm is investigated. We found that the emission amplitude increases on decreasing the particle diameter, showing a strong correlation between the blue band and the nanometric nature of the particles. By Raman spectroscopy measurements it is evidenced that the SiO₂ nanoparticles matrix is significantly affected by the reduction of size. Basing on the shell-like model, these findings are interpreted assuming that the defects responsible for the photoluminescence are localized on a surface shell of the particles and not simply on their surface. In addition it is found that the generation efficiency of these defects depends on the structural properties of the SiO₂ matrix in the surface shell.     

Luminescence of polymorph crystalline and glassy SiO2, GeO2: A short review

Studies of SiO₂ and GeO₂ crystals with α-quartz and rutile structures were performed during last two decades. The goal of such studies was comparison of properties with those of glassy modifications of these crystals. Luminescence of oxygen deficient centers in these glassy materials was found to resemble the luminescence of the rutile-type modification rather than α-quartz modification. In α-quartz, similar luminescence centers appear after damaging irradiation by electron beam at low temperatures (<60 K) or at ambient temperatures after gamma or neutron irradiation.     

Vacuum-ultraviolet absorption of interstitial O2 and H2O molecules in SiO2 glass

Vacuum-ultraviolet (VUV) absorption of O₂ and H₂O molecules incorporated in interstitial voids in SiO₂ glass by thermal annealings was examined. Interactions of the interstitial molecules with the surrounding SiO₂ glass network lead to a redshift of the VUV absorption band of interstitial O₂, while a blueshift of that of interstitial H₂O, both accompanied by an increase in the intensity of the absorption bands. The Coulomb repulsion, London dispersion, and hydrogen bonding are the main interactions responsible for the modification of the VUV absorption bands.     

RHEED investigations of unusual crystalline states in amorphous SiO2 films

Microcrystalline regions of silicon dioxide with structures unobserved in the macro-state are studied by Reflection High Energy Electron Diffraction (RHEED). Their formation can be understood by considering the existence of several possible relatively stable crystalline states of SiO₂, with free energies higher than those of the known silicon oxides. SiO₂ with structures and lattices identical to the structures and the lattices of the CaF₂-, of the rutile-type GeO₂, and of α-PbO₂ have been identified. Considerations for the state of silicon in some of these structures are also included.     

From amorphous phase separations to nanostructured materials in sol–gel derived ZrO2:Eu3+/SiO2 and ZnO/SiO2 composites

ZrO₂:Eu³⁺–SiO₂ and ZnO–SiO₂ composites have been synthesized by a sol–gel method by using a specific gelation and drying procedure. In the two cases we were able to produce large and transparent monolithic samples. Microstructural properties of these materials were investigated by thermo-differential and thermo-gravimetric analysis, X-ray diffraction, transmission electron microscopy and small angle X-ray scattering. The existence of a miscibility gap in both systems results in the formation of nanocomposites where crystallized zirconia or amorphous zinc oxide nanoparticles are dispersed in a silica glass matrix. These two kinds of nanocomposites are potential high efficiency luminescent materials because the nanoparticles size is easily controlled by the annealing conditions.     

The electronic structure of SiO2: A review of recent spectroscopic and theoretical advances

Experimental and theoretical progress in understanding the electronic structure of SiO₂ is critically reviewed. Relatively basic introductions to both the experimental and theoretical methods are provided.     

Effective oxygen charge in SiO2 and its dependence on the bond angle of oxygen

It is not clear until now, whether the oxygen bond angle and the effective oxygen charge in SiO₂ are correlated or not. This paper reviews various theoretical and experimental facts concerning some special features of the atomic structure and of electronic properties of SiO₂. They lead to contrary conclusions about the correlation between the effective oxygen charge and the oxygen bond angle in distinct SiO₂ forms. Results basing on quantum-chemical investigations are compared with conclusions drawn from the dielectric theory and from tight-binding calculations of electronic states. A critical valuation of the different methods with special consideration of solid-state investigations yield the result that the effective oxygen charge increases with increasing bond angle.     

Fictive temperature measurement of amorphous SiO2 films by IR method

The structure and properties of amorphous materials, in general, change with their thermal history. This is usually explained using the concept of fictive temperature, i.e., the temperature at which the super-cooled liquid state turned into a glassy state. In earlier studies, a simple IR method was used to determine the fictive temperature of silica glasses, both bulk and fiber. In the present study the applicability of the same technique for thin amorphous silica films on silicon was examined. It was found that the IR absorption as well as reflection peak wavenumber of the silica structural band can be used to determine the fictive temperature of amorphous silica films on silicon with an unknown thermal history. Specifically, IR absorbance spectra of an amorphous silica film of thickness greater than 0.5 μm grown on silicon can be taken before and after etching a thin surface layer of 20–30 nm and the peak wavenumber of the difference signal can be compared with the pre-determined calibration curve to convert the peak wavenumber to the fictive temperature. For a film thicker than ∼2 μm, IR reflection peak wavenumber can be converted directly to the fictive temperature of the film by using the calibration curve.     

Crystallization of amorphous Al2O3–SiO2 precursors doped with nickel

The crystallization of amorphous diphasic Al₂O₃–SiO₂ precursors doped with nickel has been studied by differential scanning calorimetry (DSC), XRD diffraction (XRD) and scanning and transmission electron microscopy (SEM, TEM) equipped with energy dispersive X-ray spectroscopy (EDS). Diphasic gels with constant atomic ratio (Al + Ni)/Si = 3:1, where 0, 1, 2 and 3 at.% of aluminum were replaced by nickel, have been prepared by hydrolyzing of TEOS in aqueous solution of aluminum nitrate. Crystallization of Ni-containing γ-Al₂O₃ preceded the crystallization of Al–Si spinel. Activation energy of 603 ± 16 kJ mol⁻¹ for crystallization of Ni-containing γ-Al₂O₃ was obtained in non-isothermal conditions. Ni-incorporated γ-Al₂O₃ transforms gradually with the temperature increase into Ni aluminate spinel, while Al–Si spinel reacts with amorphous silica forming mullite at about 1200 °C. Rietveld structure refinement of phases present in the samples annealed at 1600 °C and SEM-EDS and TEM-EDS analyses of related phases have shown that nickel predominantly crystallizes as NiAl₂O₄, but small amount of nickel is incorporated in mullite structure, as well as, dissolved in the glassy phase of the system.     

Reactivity of SiCl and SiF groups in SiO2 glass with mobile interstitial O2 and H2O molecules

Reactions of common network-bound halogens in synthetic SiO₂ glass, SiCl and SiF groups, with interstitial O₂ and H₂O molecules incorporated by thermally annealing were studied. It was found that the chemical properties are distinctly different between SiCl and SiF groups. SiCl groups react with interstitial O₂ and H₂O to form interstitial Cl₂ and HCl, respectively. In contrast, formation of interstitial F₂ and HF due to the reaction of SiF groups with interstitial O₂ and H₂O is not observed. The reactivity of SiCl and SiF groups is in accord with the properties and thermodynamic data of their respective analogous compounds, SiCl₄ and SiF₄.     

Sintering process of amorphous SiO2 nanoparticles investigated by AFM,IR and Raman techniques

We report an experimental investigation on the effects of thermal treatments at different temperatures (room—1270 K) and for different duration (0–75 h) on amorphous silica nanoparticles (fumed silica) in powder tablet form. Three types of fumed silica are considered, comprising nearly spherical particles of 40 nm, 14 nm and 7 nm mean diameter. The experimental techniques used here are Raman and infrared absorption (IR) spectroscopy together with atomic force microscopy (AFM). Raman and IR spectra indicate that the structure of nanometer silica particles is significantly different with respect to that of a bulk silica glass. In particular, the main differences regard the positions of the IR band peaked at about 2260 cm^?1, the Raman R-band peaked at about 440 cm^?1 and the intensity of the D1 and the D2 Raman lines, related to the populations of 4- and 3-membered rings, respectively. Our data also indicate that, under thermal treatments, the structure of fumed silica samples is significantly changed, gradually relaxing towards that pertaining to ordinary bulk silica. These changes are interpreted here on the basis of the morphological information provided by the AFM measurements and assuming a two-shell structure for the fumed silica primary particles.     

Growth of amorphous SiO2 nano-wires on pre-oxidized silicon substrate via chemical vapor deposition

Without an additional silicon source, amorphous SiO₂ nano-wires were grown on the pre-oxidized silicon substrate with the assistance of Ni-based catalyst under ambient pressure. The as-grown amorphous SiO₂ nano-wires were characterized by X-ray diffraction, scanning electron microscopy, high resolution transmission electron microscopy (HRTEM) and selected area diffraction. The micro-region chemical composition investigation on the as-grown amorphous SiO₂ nano-wires was carried out using X-ray energy dispersion spectroscopy (EDS) on the HRTEM. The present work focuses on the formation of atomic H on the surface of pure zinc powder by introducing moisture (N₂ + H₂O) into the furnace at high temperature. The growth mechanism has been discussed and attributed to the vapor–liquid–solid (VLS) mechanism instead of the adopted solid–liquid–solid (SLS) mechanism owing to the observed evidence of an etching reaction of atomic H at the SiO₂ buffer layer and/or that of H at the Si substrate to form a gaseous hydro-silicon radical (SiH_x) that is then transported to the growth sites. The intrinsic luminescent behavior of the amorphous SiO₂ nano-wires in the range of 350–430 nm was also reported and discussed. These results provide an alternative and simple procedure for nanostructures growth, which will be helpful to understand the growth mechanism of one dimensional SiO₂ nanostructures.     

Transparent K2OTiO2P2O5SiO2 monolithic gels and inorganic amorphous solids through incomplete hydrolysis

Sol–gel derived transparent glasses are of technological interest because of its precisely controlled composition for multicomponent glasses at low temperature processing. The present work demonstrates a new and simple methodology for preparing transparent multicomponent oxide gels by incomplete hydrolysis of alkoxides. Through this processing, a small quantity of organic agencies resulted from incomplete hydrolysis of alkoxides self-disperses in inorganic oxide network, and thus control the formation of the monolith gel free of cracks. Specially, K₂OTiO₂P₂O₅SiO₂ gel monoliths have been synthesized through this route. The gels transformed into transparent K₂OTiO₂P₂O₅SiO₂ inorganic amorphous solids after heat treatment above 450 °C. This approach could be applied to many other multicomponent oxides.     

Effect of acid,water and alcohol ratios on sol-gel preparation of antireflective amorphous SiO2 coatings

Varying amounts of nitric acid catalyst, water and ethyl alcohol were used in the preparation of SiO₂ sols by hydrolysis and condensation reactions of tetraethyl orthosilicate in a one step acid catalysis process. Hydrolysis of TEOS was followed by FT-IR analyses. Size of SiO₂ particles was seen to vary in 8–41 nm range with respect to changing HNO₃ and water amounts in the sols. Gelation occurred in some systems. Surfaces of films were examined by FESEM and AFM, after coating on glass substrates by dip coating. Thicknesses of the films were measured to be in the range of 80–120 nm. 5.6 ± 0.2% point increase in light transmittance was obtained when HNO₃/TEOS (mol/mol) ratio of 4.74 × 10^? 4 and H₂O/TEOS (mol/mol) ratio of 9.08 were utilized. Sols were found to be stable for months and coatings prepared after 45 days still provided 5.2 ± 0.2% point increase in light transmittance.     

Electronic and vibrational states of oxygen and sulfur molecular ions inside implanted SiO2 films

The paper presents the results of a low-temperature time-resolved photoluminescence (PL) investigation of thin SiO₂ films implanted with either oxygen or sulfur ions. Both PL and PLE spectra of the samples demonstrate vibronic oscillations which were attributed to the ground and excited states of sulfur- and oxygen-related molecular species. Vibrational frequencies and anharmonicity parameters calculated from the experimental data agree well with known literature. The most intensive excitation is observed in the spectral region of silica matrix excitons. It is established that such high-energy irradiation leads to non-bridging oxygen hole center creation as well as to the luminescence of molecular species. The results obtained on silica film modification could be of interest for future planar waveguides and thin oxide structure design.     

A new hydro-fluorinated amorphous silicon produced by using intermediate species SiF2

Intermediate species SiF₂, mixed with H₂, is used instead of SiF₄ as the material gas for a hydro-fluorinated amorphous silicon (a-Si:F:H). The deposition rate can be easily increased up to about 20 Å/sec without apparent degradation of photo-conductive property by using intermediate species, and also, Si-F or Si-H single bond configuration becomes dominant although there apparently exist Si-H₂ or Si-F₂ higher order bonds in the film produced by using SiF₄.