Pure Silica Films Research Articles

Broadband Anti-Reflective Coating Based on Plasmonic Nanocomposite.

We report on the fabrication, the characterization, and the optical simulation of a gold–silica nanocomposite and present its integration into a broadband anti-reflective coating (ARC) for a silicon substrate. The two-layer ARC consists of a nanocomposite (randomly distributed gold cluster in a silica matrix) and a pure silica film. We capitalize on the large refractive index of the composite to impose an abrupt phase change at the interface of the coating to diminish the light reflection from the substrate through the ultrathin nanocoating. The average reflectivity of the silicon can be reduced by such a coating to less than 0.1% in the entire visible spectrum. We experimentally and numerically prove that percolated nanocomposites with an overall thickness of 20 nm can provide anti-reflectivity up to near infrared (NIR). The ARC bandwidth can be shifted more than 500 nm and broadened to cover even the NIR wavelength by changing the volume filling fraction of the gold clusters. The angular sensitivity of thin ultrathin antireflective coating is negligible up to 60°. The present ARC could find applications in thermo-photovoltaics and bolometers.

Preparation of superhydrophobic silica film on Mg–Nd–Zn–Zr magnesium alloy with enhanced corrosion resistance by combining micro-arc oxidation and sol–gel method

To improve the corrosion resistance, a superhydrophobic silica film on Mg–3.0Nd–0.2Zn–0.4Zr (wt.%, NZ30K) magnesium alloy was successfully prepared by the combination of micro-arc oxidation (MAO) and sol–gel method using tetraethoxysilane (TEOS) and methyltriethoxysilane (MTES) as precursors. The MAO layer with highly porous surface was formed as an intermediate layer to ensure the excellent adhesion of superhydrophobic silica film to the substrate. The parameters of the sol–gel process such as molar ratios of TEOS/C2H5OH and MTES/TEOS had significant effects on the surface morphologies and water-repellent properties of the silica films. A superhydrophobic silica film with static contact angle of 151° was obtained as the molar ratios of TEOS/C2H5OH and MTES/TEOS were 1/30 and 1/2 respectively. It is shown that the superhydrophobic silica film can significantly improve the corrosion resistance of NZ30K magnesium alloy by nearly three orders of magnitude by analysis of the potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) tests in 3.5wt.% NaCl solution. 168h immersion test further demonstrates that the superhydrophobic silica film can effectively provide long-term corrosion protection for NZ30K magnesium alloy. In addition, the superior corrosion protection performance of the superhydrophobic silica film compared with that of the pure silica film without the addition of MTES can be attributed to the good coverage of hydrophobic methyl groups on its surface, which could effectively decrease the direct contact surface to corrosive medium.

Sensitized broadband near-infrared luminescence from bismuth-doped silicon-rich silica films

Developing Si compatible optical sources has attracted a great deal of attention owing to the potential for forming inexpensive, monolithic Si-based integrated devices. In this Letter, we show that ultra broadband near-IR (NIR) luminescence in the optical telecommunication window of silica optical fibers was obtained for Bi-doped silicon-rich silica films prepared by a co-sputtering method. Without excess Si, i.e., Bi-doped pure silica films, no luminescence was observed in the NIR range. A broad Bi-related NIR photoluminescence appears when excess Si was doped in the Bi-doped silica. The luminescence properties depended strongly on the amount of excess Si and the annealing temperature. Photoluminescence results suggest that excess Si acts as an agent to activate Bi NIR luminescence centers and also as an energy donor to transfer excitation energy to the centers. It is believed that this peculiar structure might find some important applications in Si photonics.

Eu3+ DOPED SILICA FILM AS LUMINESCENT DOWN-SHIFTING LAYER FOR CRYSTALLINE Si SOLAR CELLS

Eu3+ doped silica films have been prepared by sol–gel method and employed as luminescent down-shifting layer on the front side of a crystalline Si solar cell to improve their conversion efficiency. Measurements under standard test conditions (AM1.5, 100 mW/cm2) show the conversion efficiency of Si solar cell with silica film containing Eu 3+ is improved 9.5% maximally as compared to the Si solar cell with pure silica film. However, high Eu 3+ concentration is not encouraged because concentration quenching effect will decrease the efficiency of the solar cell.

A soft Plasma Enhanced-Chemical Vapor Deposition process for the tailored synthesis of SiO 2 films

The availability of soft synthetic processes for the preparation of SiO 2 films with tailored features plays a key role for several technological applications, from optics to electronics, from surface modifications to barrier coatings. In such context, this work presents the development and optimization of a Plasma Enhanced-Chemical Vapor Deposition route towards high-purity SiO 2 films at near room temperature. Depositions were performed from Ar and Ar–O 2 plasmas using tetramethoxysilane as precursor, devoting particular attention to the interplay between film properties and process parameters (RF- (Radio Frequency-) power, total pressure, O 2/(Ar + O 2) ratio and precursor vaporization temperature). Real-time information on the growth process was gained by Laser Reflection Interferometry, while the chemical composition and bonding structure of the obtained layers were analyzed by Fourier Transform Infrared Spectroscopy and X-ray Photoelectron Spectroscopy. Scanning Electron and Atomic Force Microscopies were adopted to analyze the surface and cross-sectional film morphology. The system structural and optical properties were investigated by means of Glancing Incidence X-Ray Diffraction and UV–Vis spectroscopy. Finally, nanoindentation measurements were performed to investigate film hardness. Under optimized conditions, very pure silica films, characterized by a remarkable optical transparency and favorable mechanical properties, were obtained even at RF-power values as low as 5 W and in the absence of O 2 in the plasma.

Second-order optical nonlinearity in thermally poled multilayer germanosilicate thin films

The Maker fringe method and second-harmonic light microscopy were applied to investigate the second-order optical nonlinearity induced in thermally poled silica thin films. It was found that the nonlinearity in Ge-doped film was more than three times that in pure silica film. Results from the second-harmonic microscopy revealed that, in the Ge-doped silica thin films with varying Ge-doping concentration along the thin film depth, the nonlinearity exhibited a multilayer distribution with peaks located at positions with highest contrast in Ge doping profile.

Stability of Mesoporous Oxide and Mixed Metal Oxide Materials under Biologically Relevant Conditions

The dynamic behavior of nanoscale mesoporous oxide materials exposed to aqueous solutions under biologically relevant conditions is shown to be highly dependent on composition, porosity, and calcination temperature. Dynamic processes were followed as a function of exposure on thin oxide films amenable to environmental ellipsometry porosimetry for the analysis of mechanical strength and pore size distributions as a function of exposure. Additionally, X-ray photoelectron spectroscopy was used for the elucidation of compositional changes as a function of exposure. Combined, this approach gives the first detailed, quantitative information of the degradation of nanoscale oxide materials under biologically relevant conditions. This approach also shows the utility of using film geometry as a convenient model system for the study of dynamic properties, as films are amenable to sensitive ellipsometric characterization. Pure silica films underwent a rapid degradation, occurring on the time scale of hours, while silica films mixed with 10% or less of zirconia or alumina were significantly more stable. These mixed metal oxide films showed structural changes on two time scales, undergoing a rapid partial degradation followed by a stabilization of the structure as the composition of the films evolved toward a depleted silica state. The time scales of these two processes were on the order of hours and days, respectively, and could be tuned by varying the composition and the calcination temperature of the films. These time scales are especially relevant to the culture and growth of mammalian cells and for drug release applications. Titania materials were shown to be stable under all conditions studied, making them suitable candidates for applications where the scaffold functions as a permanent support. These results yield unprecedented levels of detail on the kinetics of degradation and the dynamic structural and compositional changes occurring in these nanostructured materials.

Probing the microenvironment of an oligo-(p-phenylene vinylene) derivative encapsulated in polymer-impregnated sol-gel silica matrix

Polymer-impregnated silica sol-gel composite materials are hosts for organics in advanced optics application. An oligo-phenylene vinylene derivative 4,4′-(1,4-phenylene dithenylene)-bis-(N-methyl pyridinium iodide) (OPVD) was introduced to poly(hydroxyethyl methacrylate) (PHEMA)-impregnated silica composite film by the sol-gel process. By comparing the X-ray diffraction, UV-visible spectra, steady-state and time-resolved fluorescence spectra of OPVD in three solid matrices (PHEMA/silica composite film, pure PHEMA film, and pure silica film), the similar results of PHEMA/silica composite film and pure PHEMA film demonstrate that the OPVD is primarily surrounded by a PHEMA-like environment in the composite matrix. The model of such structure is presented, which is useful for further understanding and optimizing of properties of doped sol-gel materials.

Thickness, Morphology and Structure of Sol-Gel Hybrid Films: II—The Role of the Solvent

The influence of the solvent on the thickness, morphology and structure of silica-polytetrahydrofuran hybrid films, prepared by spin coating, has been analysed. The inorganic precursor, tetraethylorthosilicate, was hydrolysed under acid catalysis, the hydrolysis molar ratio being 4. Polymers of average molecular weight (M n ) 650 and 2900 were incorporated in the initial colloidal solutions, in a low concentration (organic/inorganic molar ratio 0.01). Two solvents were compared: ethanol, protic, and tetrahydrofuran, aprotic and a little less polar. The thickness and surface texture parameters of the films were determined by profilometry, their morphology characterized by SEM and their structure studied by FTIR. It is shown that the solvent has no effect on the molecular structure of the films, but strongly influences the surface texture and the morphology of both pure silica and hybrid films. The solutions prepared in tetrahydrofuran present shorter gelation times (t G ) and allow the deposition of good quality films almost up to the gelation point (to a reduced time, t/t G , of ∼0.9). The films are thinner than those prepared from corresponding ethanolic solutions at the same reduced ageing times. For pure silica films, tetrahydrofuran is the best choice, since it reduces the fractured region on the edge of the substrate. For hybrid films, this effect is achieved by the polymer and tetrahydrofuran is responsible for a higher arithmetical mean roughness. Therefore, ethanol becomes the preferable solvent.

Pure Silica Zeolite Films as Low-k Dielectrics by Spin-On of Nanoparticle Suspensions

The preparation of ultra low-k zeolite films with high mechanical strength by a simple spin-on process is demonstrated. Continuous, smooth, thin zeolite silicalite films with a dielectric constant of 2.1 and elastic modulus of 16–18 GPa are obtained by this process, which is simple and compatible with the requirements of the semiconductor manufacturers. Polishing experiments showed that the film is potentially compatible with chemical mechanical polishing (CMP). The described pure silica film could be an attractive candidate for ultra-low-k materials for future generation microprocessors.

Silica coating on plastics by liquid phase deposition (LPD) method

It has been reported in our previous works on liquid phase deposition (LPD) method that uniform, dense and pure silica film could be deposited from supersaturated hexafluorosilicic acid aqueous solution at near room temperature. According to our recent work, it was found that silica film could be deposited even on several kinds of polar plastics by optimizing the pre-treatment condition. Further work revealed that non-polar plastics such as cycloolefin polymer could be coated by the LPD method. The adhesion of the deposited silica with the plastic substrates was good enough at the initial stage and even after the durability test. Several merits over ordinal coating methods are considered for this silica film on plastics and many applications such as optical lens, liquid crystal display devices are proposed here.

Phase separation in SiO2–TiO2 gel and glassy films studied by atomic force microscopy and transmission electron microscopy

An investigation of phase separation phenomena in gel and glassy thin films of silica–titania, with TiO2 contents of 20 and 40 mol%, has been carried out by atomic force microscopy (AFM) and transmission electron microscopy (TEM). The thin films were prepared by spin coating of a precursor sol on silicon wafers. Both the TEM measurements (carried out on scrapped thin film flakes) and the AFM measurements (carried out on films coated on the silicon substrates) for samples with different heat treatments suggest that spinodal-like structural inhomogeneities occur in these samples, unlike the corresponding observations in pure silica films, which are known to be homogeneous. Changes in the microstructure of the films have been noticed with the thermal treatment, in agreement with earlier x-ray photoemission studies. The finer characteristic dimensions of the phase separated regions reveal that silica–titania samples prepared by sol-gel processing exhibit a more intimate mixing of the phases.

Synthesis and luminescence properties of organic–inorganic hybrid thin films doped with Eu(III)

In this study, silica-based transparent organic–inorganic hybrid films were prepared by the sol–gel method. Tetraethoxysilane and 3-(trimethoxysilyl)propyl methacrylate were used as the inorganic and organic compounds, respectively. Lanthanide complexes [Eu(phen)2]Cl3 were incorporated into the organically modified silicates (ORMOSIL) and the luminescence properties of the resultant hybrid films (ORMOSIL:[Eu(phen)2]Cl3) were characterized. The relative quantum efficiency was observed higher and the lifetimes were longer in hybrid films than those in pure silica films. Furthermore, thermal stability of hybrid films incorporating various concentration of Eu(III) complex was studied.

Preparation and properties of a humidity sensor based on LiCl-doped porous silica

The importance of humidity control has been recognized in many fields, such as the electronics industry, food manufacturing processes, hospitals and, in everyday life [1]. A wide variety of ceramic, polymeric and composite sensors are being produced to serve these applications. With the development of silicon micromachining and on-chip integration technology, miniaturization of sensors is increasingly required. Recent progress in ceramic deposition technologies, mainly by the continuous advancement of microelectronics, provides new possibilities for the improvement of ceramic active elements in the field of chemical sensors. The sol–gel method is very attractive for the preparation of very uniform ceramic powders with high purity and a very homogeneous distribution of the components on the atomic scale. Also several ceramic oxides have recently been studied in thin film form prepared by the sol–gel process [2, 3]. This method starts with liquid precursors and converts the liquid into useful solids through chemical reactions at low temperatures. Some research on sol–gel-derived silica films has been done and the results provide some useful guidelines for obtaining successful films [4]. The sol–gel silica films were possibly used as humidity sensors because of their large pores, but had a high intrinsic resistance [5]. So, in this report, LiCl is employed as a dopant to modify the humidity sensitivity of the porous silica thin film. In addition, preparation by the dip-coating method and properties of the materials are described. Reagent grade tetraethyl orthosilicate (TEOS), ethyl alcohol (C2H5OH) and deionized water (H2O) were used to prepare a silica sol solution. The optimized composition of the solution is TEOS: C2H5OH:H2O ˆ 1:4:4(mol). LiCl was dissolved in deionized water to form another solution. These two solutions were mixed by magnetic stirring for 30 min. Dilute hydrochloric acid (HCl) at above 1% in molar ratio was added into the mixed solution as a catalyst when the TEOS hydrolized. Four specimens, with the proportions of TEOS:LiCl ˆ 100:0, 90:10, 80:20, and 70:30 (mol), respectively, were studied in this report. Al2O3 plates, with a Ag– Pd integrated electrode, were used as substrates. After cleaning, the substrates were immersed in the solution for 15 s and then withdrawn at a rate of 2 cm miny1. Coated samples were air-dried for a least 24 h and then calcined in air at over 400 8C. The thickness of the silica thin films with or without LiCl is 0.1–0.2 μm. The microstructural characteristics of the films were studied by using an Amray 100 type scanning electron microscope. Several saturated salt aqueous solutions were used to provide ambient atmospheres with humidities from 11 to 95%RH. The temperature was kept at 25 8C throughout the measurement process. The resistance of the samples at different relative humidities were measured with a HP 4274A impedance analyser. Fig. 1 indicates the typical microstructure of the silica film with 20% LiCl treated at 400 8C. It shows that the film is very uniform. At the same time, it can be observed that the sol–gel-derived silica films have a lot of pores. This result suggests that the films could be suitable for adsorbing water under ambient atmospheric conditions, that is to say, they are sensitive to relative humidity. Fig. 2 shows the humidity sensitivity of the films at different %RH at 1 KHz. It is found that the silica films with LiCl have a lower impedance than the pure silica ones, and the impedance of the dried gel films (not treated at an elevated temperature) is less than that of those films calcined at 400 8C. With increasing LiCl content, the conductance of the films increases by one to three orders of magnitude at different humidity regions. The impedance–humidity characteristic of pure silica films indicate that the gel sample is sensitive to humidity at the higher %RH region while that treated at 400 8C shows no sensitivity. Because the silica gel has many surface Si–OH hydroxyl groups [6], which adsorb water vapour physically through hydrogen bonding, this physisorbed water layer is localized by hydrogen bonding of a single water molecule to two surface hydroxyls, and a proton may be transferred from a

Pure and fluorine-doped silica films deposited in a hollow cathode reactor for integrated optic applications

Silica films have been deposited in a high density hollow cathode plasma deposition system from silane and oxygen gas mixtures. Additions of carbon tetrafluoride (CF4) were used to fluorine dope the silica. The deposited films were characterized by means of Fourier transform infrared (FTIR) spectroscopy, wavelength dispersive x-ray spectroscopy, chemical etch rate (P etch), stress and refractive index measurements. The pure silica films, though deposited at a high rate (over 1500 Å/min), exhibit a P-etch rate only 1.3 times that of thermal oxide. The refractive index of the as-deposited silica is higher than that of thermal oxide, but reduces to the thermal oxide value after high-temperature (1000 °C) annealing. Based on the thickness change measurements, the higher refractive index was attributed to a higher density of the deposited silica due to a smaller Si–O–Si bond angle, as supported by FTIR data. Fluorine doping results in a reduction in film stress by a factor of 4 over pure silica, as well as a reduction in OH content from about 1 at. % in pure silica to below the FTIR detection limit (0.1 at. %). The refractive index initially decreases with CF4 flow rate, concomitant with an increase in fluorine content, but then rises above the refractive index of pure silica. This increase has been found to be due to the deposition of silicon-rich oxide at the higher CF4 flow rates, which is attributed to an increasingly oxygen deficient discharge resulting from oxygen consumption by the dissociation products of CF4.

Preparation of B2O3-P2O5-SiO2 coating films by the sol-gel method

ThexB2O3 · (20-x) P2O5 · 80SiO2 (in mol%) glass films withx=0, 10 and 20 have been prepared from metal alkoxides by carrying out the coating in a dry atmosphere. These coating films have shown a larger value of load at scratch and a smaller shrinkage during heat-treatment by replacing P2O5 in the films with B2O3. It has been found that B2O3 more effectively reduces the glass transition temperature of SiO2 glass than P2O5. The concentrations of sodium ions, which migrated from soda-lime-silica glass substrates during the film formation, were higher in phosphosilicate and borophosphosilicate films than in borosilicate and pure silica films. This finding should be ascribed to the gettering effects of phosphorus for sodium ions.