The coordination state and valence state of selected transition metal ions in SiO2-B2O3 xerogels

A series of SiO₂-B₂O₃ xerogels with changing SiO₂/B₂O₃ mol% and doped with selected transition metal ions was prepared. These mixed oxide materials contained copper, nickel, cobalt, manganese, chromium and vanadium ions coordinated to oxygen donor atoms in water and OH groups. Extensive studies of the transition metal complexes in the xerogels by such spectral techniques as diffuse reflectance (UV-vis), electron paramagnetic resonance and fluorescence spectroscopies show that there exist Cu(II) in the coordination environment of D_4h symmetry, Ni(II) in octahedral coordination sphere, Co(II) in both tetrahedral and octahedral environments, Mn(II) preferably in the O_h coordination and Mn(III) in pseudo-octahedral sphere; then octahedrally coordinated Cr(III) ions occur in coupled pairs or clusters and V(IV) as VO²⁺ ions exist in distorted (C_4v) octahedral surrounding.     

Characterisation of ALCVD Al2O3-ZrO2 nanolaminates, link between electrical and structural properties

Al₂O₃ and ZrO₂ mixtures for gate dielectrics have been investigated as replacements for silicon dioxide aiming to reduce the gate leakage current and reliability in future CMOS devices. Al₂O₃ and ZrO₂ films were deposited by atomic layer chemical vapor deposition (ALCVD) on HF dipped silicon wafers. The growth behavior has been characterized structurally and electrically. ALCVD growth of ZrO₂ on a hydrogen terminated silicon surface yields films with deteriorated electrical properties due to the uncontrolled formation of interfacial oxide while decent interfaces are obtained in the case of Al₂O₃. Another concern with respect to reliability aspects is the relatively low crystallization temperature of amorphous high-k materials deposited by ALCVD. In order to maintain the amorphous structure at high temperatures needed for dopant activation in the source drain regions of CMOS devices, binary Al/Zr compounds and laminated stacks of thin Al₂O₃ and ZrO₂ films were deposited. X-ray diffraction and transmission electron microscope analysis show that the crystallization temperature can be increased dramatically by using a mixed oxide approach. Electrical characterization shows orders of leakage current reduction at 1.1-1.7 nm of equivalent oxide thickness. The permittivity of the deposited films is determined by combining quantum mechanically corrected capacitance voltage measurements with structural analysis by transmission electron microscope, X-ray reflectivity, Rutherford backscattering, X-ray photoelectron spectroscopy, and inductively coupled plasma optical emission spectroscopy. The k-values are discussed with respect to formation of interfacial oxide and possible silicate formation.     

Structure of partially reduced xPbO (1 − x)SiO2 glasses: combined EXAFS and MD study

We have studied the structure of partially reduced lead-silicate glasses using combined EXAFS (extended X-ray absorption fine structure) and MD (molecular dynamics) methods. The analysis was performed for glasses of x[(1 − p)Pb pPbO] (1 − x)SiO₂ composition, x = 0.3, 0.5, 0.7, where parameter (1 − p) describes the degree of reduction, i.e. the content of the granular metallic phase, appearing as the result of the reduction process (e.g. annealing in hydrogen atmosphere). In the EXAFS experiment (1 − p) was expressed via the time of reduction realized at 400 °C (1.5 h, 24 h, 70 h), whereas in the MD simulations it was determined precisely by using proper numbers of particles (corresponding to (1 − p) = 0.0, 0.25, 0.5, 0.75 and 1.0). In the paper we describe in detail the local structure around lead atoms and its changes in the function of glass composition and reduction degree. The tendency for agglomeration of Pb⁰ into clusters, the formation of the granular metallic phase, and continuity of silica and lead oxide subnetworks are discussed. A good agreement between EXAFS-extractcd and MD-extracted parameters of the short-range structure encouraged us to preform a medium-range order analysis, based on the MD simulations only. Moreover, combining the EXAFS and MD methods we could correlate the reduction time (technological parameter) with the degree of reduction (1 − p) and the actual state of the granular structure. The latter relation may be useful for controlled production of reduced glasses of pre-requcstcd physical properties.     

Structural investigations of copper doped B2O3-Bi2O3 glasses with high bismuth oxide content

Raman and infrared spectroscopy have been employed to investigate the 99.5%[xB₂O₃(1−x)Bi₂O₃]0.5%CuO glasses with different Bi/B nominal ratios (0.07?x?0.625) in order to obtain information about the competitive role of B₂O₃ and Bi₂O₃ in the formation of the glass network. The glass samples have been prepared by melting at 1100 °C and rapidly cooling at room temperature. In order to relax the structure, to improve the local order and to develop crystalline phases the glass samples were kept at 575 °C for 10 h. The influence of both Bi₂O₃ and CuO on the vitreous B₂O₃ network as well as the local order changes around bismuth and boron atoms in as prepared and heat treated samples was studied. Structural modifications occurring in heat treated samples compared to the untreated glasses have been observed.     

Multinuclear NMR study of phosphosilicate gels derived from POCl3 and Si(OC2H5)4

Solid state ¹H, ²⁹Si and ³¹P MAS NMR have been used to investigate the microstructure of phosphosilicate gels prepared by a modified sol-gel method involving hydrolysis of silicon precursors in a solely aqueous environment at 50 °C. Gels with molar compositions 5, 10, 20 and 30 mol% P₂O₅ in P₂O₅-SiO₂ were studied. After drying to 400 °C the gels have very similar structures formed by a siloxane framework containing silanol groups and trapped molecules of orthophosphoric acid together with a very small amount, of pyrophosphoric acid. Unlike the gel samples previously synthesized by the hydrolysis of the silicon precursor in alcoholic solution at room temperature, the co-polymerization of phosphorus and silicon is much reduced. Although co-polymerization increases with phosphorus content, it still represents less than 50% of the phosphorus in the 30 mol% P₂O₅ gel. Furthermore there is no evidence for six-coordinated silicon in the glassy matrix.     

Cathodoluminescence of Ge, Si, and O implanted SiO2 layers and the role of mobile oxygen in defect transformations

Thermally grown SiO₂ layers of thickness d=500 nm have been implanted by Ge⁺, Si⁺, and O⁺ ions of energy 350, 150, and 100 keV, respectively, and a uniform implantation dose of D_i=5×10¹⁶ ions/cm². Thus the implantation profiles are expected with a concentration maximum of nearly 4 at.% at the half-depth d_m≅250 nm of the SiO₂ layers. After thermal annealing to 900 °C for 1 h in dry nitrogen or vacuum the typical violet luminescence band (λ=400 nm) of the Ge⁺ implanted centers is increased more than 200-fold and the Ge luminescent center depth profile is shifted from about 250 to 170 nm towards the surface as determined by cathodoluminescence (CL) depth profiling. Implanting oxygen increases the red band (λ=650 nm) but does not affect the blue band (λ=460 nm). Silicon surplus increases the amplitude of the blue (B) luminescence, but reduces the amplitude of the red (R) one. Studying the irradiation dose dependence of these blue and red bands we have established defect kinetics in SiO₂ including six main defects and precursors, including the non-bridging oxygen hole center for the red luminescence, the twofold-coordinated silicon as the oxygen deficient center ODC(2) for the blue luminescence and the mobile oxygen as the main transmitter between precursors and the radiation induced defects. The kinetics are described by a set of eight differential equations which predict the dose dependence of the CL.     

High-temperature XAFS study of solid and molten SrCl2

The local structure of solid and molten SrCl₂ was investigated by X-ray absorption fine structure technique. According to the curve fitting analysis with assessment of the anharmonic vibration effect, the nearest Sr²⁺-Cl⁻ distance and the coordination number are 2.99 ± 0.01 Å and 6.6 ± 0.2 in molten state. XAFS functions calculated from molecular dynamics simulation results were compared with the experimental XAFS data.     

Structure and crystallization of potassium titanium phosphate glasses containing B2O3 and SiO2

Transparent glasses composition of which can be expressed by the formula: (100−x) · (K₂O · 2TiO₂ · P₂O₅) · x(K₂O · 2B₂O₃ · 7SiO₂), where x=5, 10, 15 and 20 mol% (KTP-xKBS), were obtained by melt quenching technique. The structure and crystallization behavior of these glasses have been examined by Fourier transform infrared spectroscopy, differential thermal analysis and X-ray diffraction. In spite of their nominal composition, the studied glasses exhibit a similar oxygen polyhedra distribution. However, significant differences were found in the trigonal BO₃ units amount. During DTA runs all the examined glasses devitrify in two steps. In the former, very small crystals of an unknown crystalline phase are produced. In KTP-5KBS and KTP-10KBS glasses anatase phase was also detected. Attempts were made in order to identify the unknown phase (UTP) for which a AB₃(XO₄)₂(OH)₆ Crandallite-type structure was proposed where the A, B and X sites were occupied by K, Ti and/or Al, and P, respectively. In the second devitrification step the crystallization of the KTiOPO₄ phase occurs while the UTP phase previously formed disappears. Isothermal heat treatments performed at temperature just above T_g have allowed one to obtain transparent crystal-glass nanocomposites, formed by crystalline nanostructure of the UTP phase uniformly dispersed in the amorphous matrix.     

Sol-gel preparation and optical properties of SiO2-Ta2O5 glass

Tantalum-doped silica glass was fabricated by the sol-gel process in order to obtain a glass with a high refractive index for optical use. A crack-free, clear glass rod was successfully prepared from a low-density gel and used as the core material for fabricating optical fibers. Transmission loss in the fabricated fibers was high, in the range of 10³-10⁴ dB/km, which may be caused by coloration due to the multivalency of tantalum; however, the loss was reduced by nearly one order of magnitude by heat treatment at 800 °C, that is, to 75 dB/km at a wavelength of 0.8 μm.     

Density, viscosity and surface tension of 50RO-50P2O5 (R: Mg, Ca, Sr, Ba, and Zn) glass melts

The density, surface tension and viscosity of the 50RO-50P₂O₅ (R: Mg, Ca, Sr, Ba and Zn) glass melts have been measured over the range, 1073-1623 K. The effects of R cations on these properties have been investigated. The density of the melt was found to increase in the order, R: Mg<Ca<Zn<Sr<Ba, with increasing molar weight of cation. The surface tension in the temperature range of 1373-1473 K increased approximately with cation in the order, R: Zn<Ba<Mg<Ca<Sr. The viscosity and the negative, temperature coefficient of surface tension increased in the order, R: Ba<Sr<Ca<Zn<Mg. All melts exhibited negative temperature coefficients of surface tension. The effect of Mg and Zn cations on the properties were different to that for Ba, Sr, and Ca cations and this is discussed using bulk glass data published in a previous report. The features of Mg and Zn metaphosphate glass melts, that is high values of viscosity, and temperature coefficient of surface tension, are related to the small Oxygen Coordination Number of the cations (=4) when compared with those of Ca, Sr, Ba metaphosphate glass melts.     

The effect of alumina on the Sn/Sn redox equilibrium and the incorporation of tin in Na2O/Al2O3/SiO2 melts

Glasses with the basic compositions 10Na₂O · 10CaO · xAl₂O₃ · (80 − x)SiO₂ (x=0, 5, 15, 25) and 16Na₂O · 10CaO · xAl₂O₃ · (74 − x)SiO₂ (x=0, 5, 10, 15, 20) doped with 0.25-0.5 mol% SnO₂ were studied using square-wave-voltammetry at temperatures in the range from 1000 to 1600 °C. The voltammograms exhibit a maximum which increases linearly with increasing temperature. With increasing alumina concentration and decreasing Na₂O concentration the peak potentials get more negative. Mössbauer spectra showed two signals attributed to Sn²⁺ and Sn⁴⁺. Increasing alumina concentrations did not affect the isomer shift of Sn²⁺; however, they led to increasing quadrupole splitting, while in the case of Sn⁴⁺ both isomer shift and quadrupole splitting increased. A structural model is proposed which explains the effect of the composition on both the peak potentials and the Mössbauer parameters.     

Anomalous amorphous SiO2 films

Anomalous SiO₂ films have been prepared by sputtering Si in a mixture of Ar-10% O₂ at 77 K. The same sputtering conditions at room temperature yield normal SiO₂ which means that the anomaly is produced by the low temperature deposition. The anomaly reveals itself in several physical properties. The density of the anomalous SiO₂ is 1.72 as compared with 2.20 for bulk and the dielectric constant is about 50% larger than bulk and with a much stronger temperature dependence. The infrared (ir) spectrum of the anomalous SiO₂ is only slightly different from bulk SiO₂ but esr experiments reveal about 3 × 10¹⁸ spins cm which do not exist in bulk SiO₂. These anomalous films are extremely stable: upon heating only a small amount of oxygen (1 part in 10⁵) evolves at 440°C but the density and IR spectrum remain unchanged up to 1300°C. Annealing at 1500°C completely removes the ESR signal and returns the ir spectrum and the density to that of cristobalite. An electron diffraction and transmission electron microscopy study reveals that the anomalous SiO₂ films consist of essentially bulk like SiO₂ clusters about 250 Å in diameter separated by a low density network. The low density network undoubtedly contains unbound O atoms and the SiSi bonds which give rise to the esr signal. The structural model can account for all the anomalous properties.     

Sintering process of glasses in the system Na2O-B2O3-SiO2

In this work, the sintering process of different glasses in the system Na₂O-B₂O₃-SiO₂ has been studied. The studied compositions are suitable for sealing the gas manifolds of molten carbonate fuel cells. Sealing glasses are usually applied on the surfaces to be sealed using powder glass mixed with an organic medium. The agglomerant elimination and the sintering of the glass powder take place during the thermal treatment. Three different particle sizes of glass powder and different sintering temperatures and times have been used to reveal the influence of the specific surface area and viscosity on sintering. The control of these parameters allows optimization of the sealing conditions of the glasses. Dense materials have been characterized as well as the sintering mechanism. Two processes take place during the thermal treatment: the sintering process and the quartz crystallisation. Both processes act in opposite directions on the glass densification. Crystallisation is the dominant process at long times and high temperatures. The viscous flow Scherer model has been adequately applied to the experimental data.     

Oscillator strength of the infrared absorption band near 1080 cm in SiO2 films

It has been well known that the absorption maximum of the peak near 1080 cm⁻¹ in amorphous SiO₂ films shifts continuously with variation of thickness and properties such as stress. This is a first report on the oscillator strength of the absorption against frequency at the absorption maximum. SiO₂ films on silicon wafers were prepared by thermal growth in either dry O₂ or an O₂/H₂ mixture or liquid-phase deposition in HF saturated with silica gel. The oscillator strength continuously decreased from 1×10⁻⁴ down to 1×10⁻⁵ with the frequency shift from 1099 to 1063 cm⁻¹.     

A molecular dynamics study on vibration spectra of a-SiO2 surface

The charge equilibration (QEq) method was used in the molecular dynamics study of the vibration spectra on the a-SiO₂ surface. The vibrational density of states (VDOS) of a silica glass surface and partial contribution of Si and O atoms were compared with those obtained from the fixed-charge (FQ) model. The VDOS in each of the samples has two groups of vibrational bands, i.e., wide-lower-frequency band (WB) between 0-25 THz and narrow-higher-frequency band (NB) between 25 and 40 THz, which is similar to the bulk VDOS obtained experimentally. The QEq surface exhibits a peak and two shoulders while two peaks are found for the FQ surface. At the surface, an excess peak (at 1-2 THz) was observed by substracting the Debye VDOS from the calculated VDOS, which is regarded as the experimentally observed the boson peak (BP). The position of the BP is shorter than that in the case of the bulk. In the FQ surface, the VDOS consists of narrower bands, while in the QEq sample, wider bands exist due to charge variation.     

Molecular dynamics simulation of La2O3-Na2O-SiO2 glasses. II. The clustering of La cations

Clustering of high-field strength rare-earth ions in silicate glasses has been experimentally observed for a wide range of concentrations. Clustering has also been observed by molecular dynamics (MD) computer simulations over a range 1-10 mol% in soda silicate glasses. Although there have been numerous experimental studies, atomic-level details of the mechanisms that lead to clustering remain unclear. Coupling experiment with MD simulations is essential to uncovering the factors that lead to clustering. In this work, MD computer simulations are used to verify that clustering found in previous MD simulations is not an artifact of the simulation method. This work also provides clues as to the mechanism of atomic-level clustering.     

Dependence of nanocrystalline SnO2 particle size on synthesis route

Very fine SnO₂ powders were produced by (a) slow and (b) forced hydrolysis of aqueous SnCl₄ solutions and (c) hydrolysis of tin(IV)-isopropoxide dissolved in isopropanol (sol-gel route) and then characterized by X-ray powder diffraction, Fourier transform infrared and laser Raman spectroscopies, TEM and BET. The XRD patterns showed the presence of the cassiterite structure. As found from XRD line broadening the crystallite sizes of all powders were in the nanometric range. TEM results also showed that the sizes of SnO₂ particles in all powders are in nanometric range. Very fine SnO₂ powders showed different features in the FT-IR spectra, depending on the route of their synthesis. The reference Raman spectrum of SnO₂ showed four bands at 773, 630, 472 and 86 (shoulder) cm⁻¹, as predicted by group theory. Very fine SnO₂ powders showed additional Raman bands, in dependence on their synthesis. The broad Raman band at 571 cm⁻¹ was ascribed to amorphous tin(IV)-hydrous oxide. The additional Raman bands at 500, 435 and 327 cm⁻¹ were recorded for nanosized SnO₂ particles produced by forced hydrolysis of SnCl₄ solutions. However, these additional Raman bands were not observed for nanosized SnO₂ particles produced by slow hydrolysis of SnCl₄ solution or the sol-gel route. The aggregation effects of nanosized particles were considered in the interpretation of the Raman band at 327 cm⁻¹. The method of low frequency Raman scattering was applied for SnO₂ particle size determination. On the basis of these measurements it was concluded that the size of SnO₂ particles was also in the nanometric range and that, the sol-gel particles heated to 400 °C consisted of several SnO₂ crystallites.     

Structural and physical properties of a novel TeO2-BaO-SrO-Ta2O5 glass system for photonic device applications

A detailed study on a novel TeO₂-BaO-SrO-Ta₂O₅ glass system developed for photonic device applications is reported in this paper. The glass transition and crystallization temperatures could be selected by varying the Ta₂O₅ content in this glass system. This glass system is found to have good thermal stability among tellurite glasses. Raman spectroscopy has been used as a tool to analyze the structural details of this technologically important glass system. In addition to the TeO₄ trigonal bipyramid and TeO₃ trigonal pyramid structural units, glasses in this system revealed the presence of an additional Raman band attributed to TaO₆ octahedra. The Raman bandwidth of the present glasses are broader compared to the conventional tellurite glasses by 35%. The influence of a gradual addition of the modifier oxides on the coordination geometry of tellurium atoms has been elucidated. Unlike the other tellurite glasses, even at higher modifier concentrations the TeO₄ structural units dominate in the glass network compared to TeO₃ trigonal pyramids. The ratio of TeO₄/TeO₃ structural units was discussed for different series of glass compositions.