Preparation and Characterizations of TiO2/Organically Modified Silane Composite Materials Produced by the Sol-Gel Method

TiO₂/organically modified silane (ORMOSIL) composite materials produced by the sol-gel method were studied for optical waveguide applications. High optical quality waveguiding films on different substrates, including silicon, gallium arsenide, silica/silicon substrates, and microscope glass slides, were prepared from high titanium content (0.2 molar) ÿ-glycidoxypropyltrimethoxysilane at low temperature. Scanning electron microscopy (SEM), atomic force microscopy (AFM), differential thermal analysis (DTA), thermal gravimetric analysis (TGA), Fourier transform infrared spectroscopy (FT-IR) and X-ray photoelectron spectroscopy (XPS) have been used to investigate the optical and structural properties of the composite films. The TGA/DTA results showed that the organic compounds in the film would tend to decompose in the temperature range from 200°C to 500°C. SEM and AFM results showed that a dense and porous-free composite material film could be obtained at the heat treatment temperature of 100°C. It was also shown that ORMOSIL is integrated in the glass, providing low shrinkage and high cracking resistance. The propagation loss properties of the composite films were also investigated. About 1.1 dB/cm propagation loss of the planar waveguide film was obtained at the wavelength of 633 nm.     

Sol-Gel Derived Titania/γ-Glycidoxypropyltrimethoxysilane and Methyltrimethoxysilane Hybrid Materials for Optical Waveguides

We report here on titania/organically modified silane hybrid materials produced by the sol-gel technique for optical waveguide applications. Acid catalyzed solutions of -glycidoxypropyltrimethoxysilane and methyltrimethoxysilane mixed with tetrapropylorthotitanate have been used as precursors for the hybrid materials. Waveguide films with a thickness about 1.3-m have been prepared on a silicon substrate by a single-coating process and low-temperature heat treatment. Atomic force microscopy (AFM), thermal gravimetric analysis (TGA), UV-visible spectroscopy (UV-VIS), and Fourier transform infrared (FTIR) spectroscopy have been used to investigate the optical and structural properties of these waveguide films. The results have shown that dense, pore-free, and highly transparent films can be obtained by low temperature heat treatment. The planar waveguide propagation loss of the hybrid films has also been measured.     

Structural Changes in Sol-Gel Derived SiO2 and TiO2 Films by Exposure to Water Vapor

Structural changes in sol-gel derived thin films by exposure to water vapor were investigated using Fourier transform infrared absorption spectroscopy, ellipsometry and x-ray diffraction. We found that SiO₂ gel films were densified with a decrease in OH groups by the exposure at 60°–180°C. The shrinkage and the peak frequency of ₄ (TO) for the exposed films were comparable to those heated in a dry atmosphere at temperatures above 500°C. However, OH groups in the films were not completely removed by the water exposure. A subsequent annealing above 300°C changed the structure of the water-exposed SiO₂ films with condensation of the remaining OH groups. Although the exposed SiO₂ gel films were amorphous, TiO₂ gel films were transformed to anatase with a decrease in OH groups by the treatments at 80°–180°C.     

Preparation and Characterization of Germanium Sulfide Based Sol-Gel Planar Waveguides

Germanium sulfide based glass films have been deposited by spin-coating onto single crystal silicon wafers and silica glass disks, using the reaction between GeCl₄, either pure or doped with of 5 mol% of SbCl₃, dissolved in toluene, with H₂S. The films, heat-treated under different conditions, were characterized by X-ray diffraction, infrared spectroscopy, X-ray photoemission spectroscopy, mechanical profilometry and ellipsometry. Oxide contamination was found in these films, but this was reduced or even eliminated by a heat-treatment in H₂S gas, at 270°C. A maximum film thickness of 1.3 m was achieved and the refractive indices of the films were in the range of 1.9–2.8 at 633 nm. Propagated light was observed by butt-coupling, for waveguides deposited on silica glass disks, at = 633 nm. Optical losses in the range of 1.1–1.9 dB/cm were measured at this wavelength, for different regions of different planar waveguides. The present method has achieved reasonably low loss and low levels of oxide contamination, which are promising for active applications.     

Structural and Optical Properties of Sol-Gel Derived Aluminosilicate Planar Waveguides Doped with Er3+ Ions

Er3+ doped-aluminosilicate thin films were prepared on silica and silica/Si substrates by the sol-gel process and dip-coating. The sol-gel aluminosilicate planar waveguides were prepared from silicon and aluminium alkoxides. Their structural characterization has been carried out by Raman spectroscopy, Atomic Force and Scanning Electron Microscopies. The results indicated that these films present an amorphous structure until an annealing temperature of 900°C, while at temperatures higher than 1000°C, crystallization occurs. An estimate of microcrystallite sizes using Raman spectroscopy is given, which agrees with data from scanning electron microscopy. The optical properties have been investigated by Fluorescence spectroscopy in the visible region.     

Preparation and Characterization of Sol-Gel Derived Y2O3 Thin Films

Sol-gel derived Y2O3 thin films have been prepared on platinum coated silicon wafers and fired to temperatures ranging from 400°C to 750°C. Multiple coats were used to obtain films up to 0.5 m thick with an intermediate firing of 400°C between coatings. Top Pt electrodes were sputtered to form monolithic capacitors. These films exhibited a dielectric constant of 18 and a leakage current of 10–11–10–7 A/cm2, making them attractive candidates for high dielectric constant dielectric films in high density DRAMs.     

Fine Patterning of Transparent, Conductive SnO2 Thin Films by UV-Irradiation

Finely patterned transparent, conductive SnO₂ thin films have been prepared. UV-light from a high-pressure mercury lamp was irradiated through a mask on the precursor films prepared from SnCl₂ with acetyl acetone in the ambient atmosphere, and this irradiation led to the change of solubility of the films in alkaline solution. Patterns with a width of about 3 to 50 m and thickness of about 0.1 m were formed with a pitch of about 2 to 20 m. The resistivity of the films heat-treated at 500°C after UV irradiation was about 1 × 10^–2 cm, which was almost the same resisitivity for the films heat-treated at 500°C without UV irradiation.     

SrBi2Nb2O9 Ferroelectric Powders and Thin Films Prepared by Sol-Gel

Pure SrBi2Nb2O9 powders and thin films were obtained using sol-gel synthesis from mixtures of niobium ethoxide, bismuth and strontium 2-ethylhexanoates. Powders crystallized for 2 hours at 700°C had grain sizes of about 100–150 nm. SrBi2Nb2O9 thin films were prepared by spin-coating a stable precursor solution onto Si/SiO2/TiO2/Pt substrates. Crystallization of pure SrBi2Nb2O9 phase occurred at about 500–550°C. Randomly oriented 0.3 m-thick crack-free films were obtained after 10 successive depositions and heating at 700°C for 2 hours. P-E hysteresis loops have confirmed the ferroelectric behavior of the films: they show a remanent polarization of 2.5 C/cm2 (5 V, 8 ms). No fatigue was observed up to 109 full switchings.     

High performance distributed CPW phase shifters with etched BST thin films on Ф 3″ LaAlO3 substrates

The design, fabrication and microwave properties of distributed coplanar waveguide (CPW) phase shifters using etched Ba_0.6Sr_0.4TiO₃ (BST) thin films on Ф 3″ LaAlO₃ (100) substrates were investigated. The BST thin films employed in the circuits are deposited by RF magnetron sputtering, and then annealed at 800 °C for 30 min in air. BST thin films parallel-plate capacitors were fabricated by photolithography and etching process. At 10 kHz and 600 kV/cm electric field, the dielectric tunability, remanent polarization (2P_r) and the coercive electric field (2E_C) of BST film were 28.7%, 2.265 μC/cm² and 38.8 kV/cm, respectively. The loss tangent was 0.005 at zero electric field. The CPW phase-shifter designed was subsequently fabricated by optimum BST thin films and thickened top electrodes. At 21.3 GHz and 35 V, 360° phase shift was achieved, the insertion loss was −8.5 dB, the ?gure-of-merit (FOM) was 42.4°/dB, and the return loss was −12.1 dB.     

Crystallization of Silica Xerogels: A Study by Raman and Fluorescence Spectroscopy

Pure and europium doped silica xerogels were annealed at 1050°C to obtain full densification, and at 1300°C to induce crystallization. Raman spectroscopy, time resolved selective luminescence and lifetime measurements were performed on glassy and crystallized samples. We discuss the differences between the Raman spectra of the xerogel annealed at 1050°C and those of a commercial silica. The typical Raman structures of -crystobalite are evident for the 1300°C annealed samples, but a glassy phase coexists, indicating an incomplete crystallization. Fluorescence measurements give information on the environment of the Eu³⁺ ions in the glassy and crystallized sample.     

Mesoporous Nb2O5 Films: Influence of Degree of Crystallinity on Properties

Nanocrystalline Nb₂O₅ films were prepared by an extended sol-gel method. The synthesis is based on the hydrolysis of a modified Nb-alkoxide precursor. Reaction of the modified precursor (Nb(OEt)₅ + 2 2,4-pentanedione) with water in ethanol leads to a homogeneous hydrolyzed solution, which is stable against precipitation of niobium oxide after evaporation of the ethanol and in the whole pH-range investigated (1–10). Autoclaving leads to amorphous gels, from which homogeneous nanocrystalline niobium oxide films of up to 15 m can be made. During annealing crystalline phases are first observed above 500°C with fully crystalline films of orthorhombic T-phase Nb₂O₅ attained at 600°C. The microstructural, crystallographic, optical and photoelectrical properties of the films were characterized by means of SEM, XRD, UV-VIS spectroscopy and surface photovoltage spectroscopy, respectively.     

Characterization of iron-silicon thin films by means of electron diffraction and X-ray spectroscopy

Iron-silicon thin films have been characterized by means of analytical transmission electron microscopic methods. Under certain conditions — composition and annealing temperature — these films exhibit thermoelectric behavior. In particular, the morphology and phase formation which results from annealing of these films, and doping with oxygen and nitrogen, are of interest. The thermoelectric phase -FeSi₂ is formed at temperatures above 500°C. This phase is transformed into electrically conducting phases at about 1000°C. A small oxygen content does not influence this crystallization process. If the oxygen content is higher than 15 atom-% the electrically conducting phases exist even at 500°C. The presence of a small nitrogen content inhibits the formation of the -FeSi₂ phase. The development of silicon and iron nitrides is possible.     

Microstructural and Dielectric Characterization of Sol-Gel Derived Silicon Oxycarbide Glass Sheets

Thin (50—1000 m) silicon oxycarbide glass sheets were synthesized by the pyrolysis of gel sheets obtained from a methyl-modified silica sol containing colloidal silica under inert atmosphere between 900 and 1450°C. The microstructure of these glass sheets was investigated with the help of high resolution scanning and transmission electron microscopy (HR-SEM and HR-TEM), X-ray diffraction and Raman spectroscopy and their dielectric properties were determined. The surface morphology as observed with HR-SEM did not exhibit a notable temperature dependence. HR-TEM studies showed that the glass sheets sintered up to 1200°C are amorphous, whereas those sintered at 1450°C contain uniformly dispersed crystallites of SiC and graphite. X-ray diffraction studies were found in agreement with the HR-TEM results. Raman spectroscopy showed that free carbon is present as an amorphous phase till a temperature of 1000°C, whereas at temperatures 1200°C, the presence of graphitic carbon was observed. Silicon oxycarbide glass sheets heat treated at temperatures up to 1200°C, showed a dielectric constant between 4.1 ± 0.11 and 4.6 ± 0.15 in the frequency range from 75 kHz to 5 MHz, with corresponding losses between 0.0008 and 0.1100. Such silicon oxycarbide glass sheets sintered at 1200°C could find an application as substrates for electronic packaging.     

Preparation and Characterization of TiO2 Thin Films by Sol-Gel Method

Titanium dioxide (TiO₂) thin films have been deposited on silicon and glass substrates by the sol-gel process using titanium iso-propoxide [Ti(O-i-C₃H₇)₄]. The bond configuration of the TiO₂ thin films was analyzed by using FTIR in the wavenumber range from 400 to 4000 cm^–1. The spectral transmittance of as-deposited TiO₂ films deposited on fused silica glass was measured in the wavelength range from 200 to 900 nm. X-ray diffraction measurements were performed to determine the crystallinity of the TiO₂ films. As-deposited films were amorphous. As the film was annealed at higher temperature, the structure was transformed from amorphous to the anatase crystalline state. The chemical composition of the deposited film was investigated using X-ray photoelectron spectroscopy (XPS). The films are essentially stoichiometric with carbon as the dominant impurity on the surface. Raman spectra show the characteristic of TiO₂ anatase phase. The electrical properties of the TiO₂ films were measured using capacitance-voltage (C-V) and current-voltage techniques. From C-V measurements, the dielectric constants were calculated to be approximately 26 for the as-deposited films and 75–82 for films annealed at 700°C in different atmosphere. For the as-deposited samples, the breakdown voltage was 2.7 MV/cm, and for an electric field of 1 MV/cm, the leakage current was 5 × 10^–5 A/cm² and the resistivity was 2.2 × 10¹⁰ -cm.     

Sol-gel processing and properties of cerium doped Barium Strontium Titanate thin films

A stock solution sol-gel based method for making Barium Strontium Titanate (BST) thin films has been developed. A modified titanium alkoxide was combined with a barium and/or strontium inorganic salt in methoxyethanol and ethylene glycol to form the solution. The effect of chemistry on the stability of this BST solution is discussed. The crystallization temperature of 700–725°C for rapid thermally processed films dropped by 100°C using cerium doping. The permittivity for undoped films was 250 and doping by 3 at. % Ce increased the dielectric constant by 20%. A remanent polarization of approximately 0.5 C/cm² and coercive field of 28 kV/cm were measured for the undoped films. The leakage current densities were <10 nA/cm² at E=60 kV/cm and improved for cerium concentrations up to 3 at. %. The charge storage density was 50 fF/m² at 200 kV/cm and the DC breakdown voltage was 300 kV/cm for Ce doped films.     

Fabrication and Characterization of Sol-Gel GeO2-SiO2Erbium-Doped Planar Waveguides

GeO2-SiO2 sol-gel planar waveguides doped with Er were deposited by spinning on silica substrates. P2 O5 or Al2O3 were used as co-dopants to improve erbium dissolution in the GeO2-SiO2 matrix. Multilayer amorphous films were obtained at 600 or 700°C.Er ions in the planar waveguide pumped at 980 nm showed fluorescence features around 1530 nm. Narrow fluorescence spectra (20 nm) and long lifetimes (6 ms) were found in P2O5 co-doped samples, whereas Al2O3 co-doping gave wider spectra (50 nm) with slightly lower lifetimes (5 ms). The quenching concentration in the Al2O3 co-doped samples was 0.9 mol% Er.Heat treatments in CCl4 improve the active properties and the addition of Yb enhances the pump absorption efficiency.     

Effects of Thermal Treatment on the Structure and Properties of SiO2-TiO2 Gel Films on Silicon Substrates

Solution derived thin films of molar composition 90SiO₂-10TiO₂ deposited on c-Si substrates, for use as planar waveguides are characterized in terms of thickness and refractive index, by ellipsometry, as a function of isothermal heat treatments. The structural evolution of the films with thermal treatment is followed by infrared spectroscopy. The estimated average Si–O–Si intertetrahedral bond angle increases from 137°, at room temperature, to 144°, after heating at 900°C. The films should be annealed at T > 800°C, in order to release the residual stresses.     

Structural and Photoluminescence Behaviors of Nano-Structure Thin Film and Bulk Silica Gel Derived Glasses

Nano-structure bulk and thin film silica gel derived glasses were prepared by sol-gel technique. Both samples were derived from the same precursor and subjected to the same heat-treatment regime. Structural information about prepared samples are obtained by analyzing the XRD patterns and TEM micrographs. The bulk samples phase changes from amorphous to -crystoballite at higher temperature (1300°C) than that in the thin film (500°C). The crystallite size depends to a large extent on the heat-treatment temperature. Bulk sample heat treated at 1400°C was as small as 10.4 nm. Thin film samples show higher response to heat-treatment temperature than the bulk samples, where the film is denser, has smaller pores and seems more homogeneous at lower temperature than bulk sample as revealed by SEM. The observed Raman spectra for bulk and thin film samples are in accordance with that of the -crystoballite. The Raman peak intensity is higher for thin film than bulk samples. The photoluminescence PL measurements for bulk samples show a broad intense peak at 532 nm combined with three weak peaks at longer wavelength 587, 635 and 666 nm. The PL peak intensity shows a reasonable decrease with increasing the heat-treatment temperature while the peak position shifted slightly to a lower wavelength. While the thin film samples show a unique peak at wavelength = 523 nm. The appearance of PL bands are interpreted on the light of non-bridged oxygen hole center as well as the structure defects.     

Sol-Gel Preparation and Photoelectrochemical Properties of Fe2TiO5 Thin Films

Polycrystalline Fe₂TiO₅ films were prepared on nesa silica glass substrates by the sol-gel method, and their photoanodic properties were measured in a three-electrode wet cell with an aqueous buffer solution of pH = 7. Gel films were crystallized into Fe₂TiO₅ when fired at 500°C. The photoanodic current significantly increased when the films were fired at 700°C, and then decreased with increasing firing temperature. Thicker films obtained by repeating the gel film deposition and firing showed smaller photocurrent, and the 50 nm thick film prepared via non-repetitive deposition exhibited the maximum photocurrent. Although the photoresponse was extended to wavelengths near 500 nm, the maximum quantum yield was as low as 0.12 at a wavelength of 340 nm.     

Synthesis and Characterization of the β-BaB2O4 Phase Obtained by the Polymeric Precursor Method

A modified polymeric precursor method based on the Pechini process was used to synthesize -BaB₂O₄ (-BBO) crystalline phase. D-sorbitol (C₆H₁₄O₆) was used as a polymerizing agent to avoid the loss of boron during the samples' calcination and crystallization. The -BBO stoichiometric crystalline phase was only obtained when sorbitol was added to the solution. The results of Raman spectroscopy show that the amorphous phase is only completely eliminated when the samples are heat-treated at 750°C for 20 h. Thin films of -BBO phase displaying a preferred orientation were obtained when crystallized at 750°C for 2 h and deposited on sapphire substrate.