SiO2 Thin Films Research Articles

Improving Temperature-Sensing Performance of Photonic Crystal Fiber via External Metal-Coated Trapezoidal-Shaped Surface

This article describes a photonic crystal fiber (PCF) temperature sensor that utilizes a flat, metal-coated trapezoidal surface. The PCF is made up of two layers of elliptical air holes and a polished trapezoidal surface that allows temperature sensing. An external sensing approach is used to deposit a thin silver layer on the reflective surface, while a thin SiO2 film acts as an oxidation-resistant coating. The top elliptical air hole serves as the interface for energy transformation from the core-guided mode to the surface plasmon-polariton (SPP) mode. Simulations carried out using the finite element method indicate that the proposed SPR-PCF temperature sensor can achieve a maximum temperature sensitivity and resolution of up to 5200 pm/°C and 0.01923 °C, respectively, across a temperature range of 10 to 60 °C. This research has significant potential for sensor design and real-time temperature remote sensing applications.

Plasma-enhanced atomic layer deposited SiO2 enables positive thin film charge and surface recombination velocity of 1.3 cm/s on germanium

The excellent field-effect passivation provided by aluminum oxide (Al2O3) on germanium surfaces relies on the high negative fixed charge present in the film. However, in many applications, a neutral or a positive charge would be preferred. Here, we investigate the surface passivation performance and the charge polarity of plasma-enhanced atomic layer deposited (PEALD) silicon oxide (SiO2) on Ge. The results show that even a 3 nm thick PEALD SiO2 provides a positive charge density (Qtot, ∼2.6 × 1011 cm−2) and a relatively good surface passivation (maximum surface recombination velocity SRVmax ∼16 cm/s). When the SiO2 thin film is capped with an ALD Al2O3 layer, the surface passivation improves further and a low midgap interface defect density (Dit) of ∼1 × 1011 eV−1 cm−2 is achieved. By varying the SiO2 thickness under the Al2O3 capping, it is possible to control the Qtot from virtually neutral (∼2.8 × 1010 cm−2) to moderately positive (∼8.5 × 1011 cm−2) values. Consequently, an excellent SRVmax as low as 1.3 cm/s is obtained using optimized SiO2/Al2O3 layer thicknesses. Finally, the origin of the positive charge as well as the interface defects related to PEALD SiO2 are discussed.

Impact of annealing and soft template concentration on optical and sensing properties of wet-deposited SiO2 thin films

The present study demonstrates deposition of porous SiO2 thin films using combination of sol-gel, soft-templating and spin-coating methods. The recipe for Si-sol using TEOS (tetraethylorthosilicate) as a precursor was optimized in order transparent and stable sol to be obtained. Commercially available triblock co-polymer Pluronic 123 was considered as a soft template and its concentration and volume fraction to sol were varied. For achieving the final composition of silica films, different stepwise annealing regimes were implemented where the temperatures and duration of each step were optimized. Optical properties of films were calculated using nonlinear curve fitting of measured reflectance spectra in UV-VIS spectral range. Surface morphology and structure of the films were characterized by Transmission Electron Microscopy (TEM) and Selected Area Electron Diffraction (SAED). The porosity and optical sensing ability of the films were tested by measurement of their reflectance spectra prior to and during exposure to acetone vapors which served as probe molecules. The influence of soft template composition and annealing regimes on SiO2 properties were revealed and application of studied films as sensitive media for opto-chemical sensing was discussed.

Fabrication of a porous SiO2 thin film with an ultralow refractive index for anti-reflective coatings

Anti-reflective (AR) coatings are used in various optical products, such as lenses, filters, and transparent conductive films. High-performance multilayer AR coatings can be achieved by reducing the refractive index of the top layer. Preventing light scattering by forming finely structured film is also important for a high-performance antireflection film. In this study, porous SiO2 thin films with ultralow refractive indices were prepared using a simple sol–gel method. A high-quality porous SiO2 thin film with an ultralow refractive index of 1.17 bearing a fine porous structure was fabricated by carefully selecting the solvent and base species. Observation and simulation of the film structure revealed the factors responsible for the low scattering of light and the ultralow refractive index exhibited by the film. Hydrophobic treatment of the film was conducted to reduce the fluctuation of optical performance caused by changes in the refractive index owing to the water adsorbed. Therefore, porous SiO2 thin films can be used in the lenses and filters of various optical instruments.Graphical abstractA transparent film with low refractive index was obtained. The film has porous structure, consisting of silica particles of a few nm in size. The pores size was found to be a few nm by image analysis. Thin films with low refractive index and low scattering for optics were successfully prepared by a simple sol–gel method.

Investigation of TiO2 Deposit on SiO2 Films: Synthesis, Characterization, and Efficiency for the Photocatalytic Discoloration of Methylene Blue in Aqueous Solution.

TiO2-SiO2 thin films were created on Corning glass substrates using a simple method. Nine layers of SiO2 were deposited; later, several layers of TiO2 were deposited, and their influence was studied. Raman spectroscopy, high resolution transmission electron spectroscopy (HRTEM), an X-ray diffractometer (XRD), ultraviolet-visible spectroscopy (UV-Vis), a scanning electron microscope (SEM), and atomic force microscopy (AFM) were used to describe the sample's shape, size, composition, and optical characteristics. Photocatalysis was realized through an experiment involving the deterioration of methylene blue (MB) solution exposed to UV-Vis radiation. With the increase of TiO2 layers, the photocatalytic activity (PA) of the thin films showed an increasing trend, and the maximum degradation efficiency of MB by TiO2-SiO2 was 98%, which was significantly higher than that obtained by SiO2 thin films. It was found that an anatase structure was formed at a calcination temperature of 550 °C; phases of brookite or rutile were not observed. Each nanoparticle's size was 13-18 nm. Due to photo-excitation occurring in both the SiO2 and the TiO2, deep UV light (λ = 232 nm) had to be used as a light source to increase photocatalytic activity.

Potential‐free sodium‐induced degradation of silicon heterojunction solar cells

AbstractSilicon‐based photovoltaic (PV) modules suffer from potential‐induced degradation (PID) caused by sodium (Na) permeation, which is present in large quantities in soda‐lime glass. Here, we report that Na atoms can decrease the performance of amorphous/crystalline silicon heterojunction (SHJ) solar cells without the help of a voltage bias. The three degradation stages are investigated in this work. First, H2O molecules open channels for Na transport in the transparent conductive oxide (TCO), while the device performance remains almost unchanged. Next, when Na atoms reach the boron‐doped hydrogenated amorphous silicon (p‐a‐Si:H), the field passivation is poisoned, leading to a great decline in the fill factor (FF), whereas the open‐circuit voltage (Voc) only slightly declines. Finally, Na atoms further diffuse into the intrinsic a‐Si:H layer and c‐Si surface, resulting in a substantial decrease in Voc. These findings have important implications for the installation of SHJ solar modules in Na‐abundant environments. As a feasible solution, we demonstrate that a compact SiO2 thin film can efficiently prevent H2O molecules from penetrating into the TCO layer and therefore guarantee a long‐term stable operation of SHJ solar cells.

Evaluating direct detection detectors for short-range order characterization of amorphous materials by electron scattering

The introduction of direct electron detectors (DEDs) to transmission electron microscopy has set off the ‘resolution revolution’, especially for cryoTEM low-dose imaging of soft matter.In comparison to traditional indirect electron detectors such as Charged-Coupled Devices (CCD), DEDs show an improved modulation transfer function (MTF) and detective quantum efficiency (DQE) across all spatial frequencies, as well as faster frame rates which enable single electron counting.The benefits of such characteristics for imaging, spectroscopy and electron holography have been demonstrated previously. However, studies are lacking on the application of DEDs for localized characterization of short- to medium- range-order (SRO, MRO) in amorphous materials using electron scattering.Therefore, we evaluate the performance of a Monolithic Active Pixel Sensor DED for the characterization of SRO and MRO in nanoscale volumes of amorphous materials, using SiO2 and Ta2O5 thin films as test cases. The performance of the detector is compared systematically to electron scattering measurements recorded on an indirect detector (CCD) using 200 keV electrons and electron doses starting at approximately 500e−Å2 . In addition, the effects of sample cooling and energy-filtering on the measured SRO of the oxides were investigated.We demonstrate that the performance of the DED resulted in improved SRO characterization in comparison to that obtained from the CCD measurements. The DED enabled to achieve a larger measured maximal scattering vector, ∼16.51Å compared to ∼151Å, for the CCD. Furthermore, an improved signal-to-noise ratio of approximately two-fold was observed across all spatial frequencies for both 200 keV and 80 keV electrons. These improvements are shown to result from the superior DQE of the DED. Consequently, the DED measurements enabled to determine the coordination numbers of atomic bonds more accurately.We expect that further benefits of the DED for S/MRO characterization will be highlighted for ultra- sensitive materials that cannot withstand electron doses above several e−Å2 .

Experimental and Monte-Carlo study of double-hump electron emission yield curves of SiO2 thin films

In this work, we have made experimental measurements of multiple-hump total electron emission yield (TEEY) curves on SiO2 thin films. A Monte-Carlo electron transport model, published in Gibaru et al., J. Electron Spectrosc. Relat. Phenom. 261, 147265 (2022), has been developed to analyze the physical reasons of such atypical behavior. It is shown that the multiple-hump TEEY curves of thin dielectric layers are due to internal recombination effects. However, such kind of phenomenon is demonstrated to be strongly correlated to the incident current density. This analysis reveals that the double-hump TEEY curves observed commonly on insulators are also most probably a measurement artifact, tied to the operating parameters of the electron gun. A careful choice of experimental parameters can eliminate this artifact, by using a constant current density that is also low enough to limit recombination effects.

UV-enhanced environmental charge compensation in near ambient pressure XPS

In this work, we discuss the possibility of improving charge neutralization in near ambient pressure X-ray photoelectron spectroscopy by co-irradiating the sample with He I photons of 21.2 eV. This UV-enhanced neutralization of charges is a variation of the so-called environmental charge compensation, which uses the electrons produced by the photoionization of the ambient gas to neutralize the positive charges built at the sample surface. Adding an additional ionization source generates more charges at the sample but also larger amounts of electrons available for neutralization. The final surface charge equilibrium depends on different aspects of the experiment, such as the sample composition and geometry, the total ionization cross sections of the gas compared to the surface materials, the gas used, the luminosity and spot size of the sources used for photoionization, and the energy of the electrons present in the gas phase. Here we illustrate the efficiency of the UV-enhanced neutralization using three different dielectric samples with different geometries (a porous SiO2 monolith with an irregular surface, a flat mica sample, and a thin SiO2 film deposited onto a Si substrate), different X-ray spot sizes, and two different gases (N2 and Ar). The effect of biasing on the efficiency of the sample surface to attract electrons produced in the gas phase is also discussed.

Comparative study of liquid crystal variable retarder with rubbed polyimide and SiO2 thin films by glancing angle deposition

A comparative study on the optical, morphological and laser-induced damage characteristics of rubbed polyimide, SiO2 thin films by glancing angle deposition, and the corresponding liquid crystal (LC) retarders, is presented in this paper. SiO2 inorganic thin films were prepared with a deposition angle of 45[Formula: see text], and traditional polyimide (PI) films were prepared by spin coating and rubbing. The experimental results indicate that the SiO2 thin films have a good transmittance and resistance to laser damage. The transmittance of the SiO2 thin films was above 92% in the wavelength range 400–1200 nm, and that of the corresponding LC retarder was approximately 90% in the wavelength range 800–1200 nm. The laser-induced damage threshold of the SiO2 thin films reached 32.6 J/cm2, and that of the corresponding LC retarder was 1.84 J/cm2. These results are better than those of the PI alignment films. SiO2 thin films have potential for use in the LC optical devices for laser applications.

Local Field Enhancement Due to the Edge States of Nanoplasmonic Crystal

In this paper, we describe the effect of local electric field enhancement due to the existence of photonic edge states in a nanoplasmonic crystal. These states can be excited by a normal incident wave and are localized along the continuous line defect, where translational symmetry is broken. The nanoplasmonic crystal is formed by a triangular lattice of SiO2 cylinders on top of SiO2 and Ag thin films. Numerical simulations demonstrated that edge modes that are concentrated around a defect in a nanophotonic crystal may result in 12 field enhancements of the electric field for the incident plane wave of wavelength 545 nm. This effect can be employed for improving the sensitivity of surface-enhanced Raman scattering (SERS) spectroscopy, increasing the Purcell factor of quantum systems and improving the efficiency of higher harmonic generation.

Anti-reflective and high-transmittance optical films based on nanoporous silicon dioxide fabricated from templated synthesis

Herein, we report anti-reflective and high-transmittance optical films based on nanoporous silicon dioxide (SiO2) fabricated from templated synthesis. Amorphous mesoporous polystyrene (PS) thin-film templates were prepared by solvent nano-crystallization induced phase separation technique. The mesoporous PS films were used as templates for the preparation of nanoporous SiO2 thin films through the sol-gel method followed by the removal of the template at high temperatures. The nanoporous SiO2 thin films were well-characterized by Field-Emission Scanning Electron Microscopy (FE-SEM), and N2 sorption isotherms for framework size, morphology, surface area, and pore volume respectively. The average pore size for disordered nanoporous SiO2 and calculated porosities are 23 nm and 0.60 respectively. The amorphous nanoporous SiO2 was evaluated for optical reflectance and transmittance in the visible light spectrum and found to have reflectance as low as 6.9% and transmittance as high as 96.5%.

Polarized angle-resolved spectral reflectometry for real-time ultra-thin film measurement.

We propose a polarized, angle-resolved spectral (PARS) reflectometry for simultaneous thickness and refractive-index measurement of ultra-thin films in real time. This technology acquires a two-dimensional, angle-resolved spectrum through a dual-angle analyzer in a single shot by radially filtering the back-focal-plane image of a high-NA objective for dispersion analysis. Thus, film parameters, including thickness and refractive indices, are precisely fitted from the hyper-spectrum in angular and wavelength domains. Through a high-accuracy spectral calibration, a primary PARS system was built. Its accuracy was carefully verified by testing a set of SiO2 thin films of thicknesses within two µm grown on monocrystalline-Si substrates against a commercial spectroscopic ellipsometer. Results show that the single-shot PARS reflectometry results in a root-mean-square absolute accuracy error of ∼1 nm in film thickness measurement without knowing its refractive indices.

Photoluminescence of SiO2:Eu and SiON:Eu films: Color selection through activating emissions from SiOx, Eu3+, and Eu2+

Photoluminescence (PL) from Eu-doped SiO2 (SiON) thin films sputter-deposited in O2, H2O vapor, or a mixture of O2 and N2 gases was characterized by excitation at a wavelength of 325 nm. The SiO2 films deposited at 500 °C with O2 contained small amount of α-quartz nanocrystals. The primary condition for either or both of the Eu3+ and Eu2+ emissions to appear was Eu content between 1 and 3 at.%, and 2.5 at.% gave the maximum emission intensity of Eu3+. The PL spectra showed only the Eu3+ emission when the films were deposited at room temperature (RT) and subsequently post annealed in an O2 atmosphere. Changing the annealing atmosphere from O2 to a vacuum and annealing at temperatures above 600 °C resulted in an intense broad violet emission signal peaking at 390 nm along with the Eu3+ emission signal. This violet emission originated from reduced SiOx species. When deposited at RT, similar temperature-dependent emission properties were obtained regardless of the reactive gas (H2O vapor or a mixture of O2 and N2). Complete transformation of Eu3+ into Eu2+ occurred when deposition was carried out at temperatures between 400 and 500 °C; a blue emission at around 467 nm was predominant from the SiO2:Eu2+ films. Deposition in the O2 and N2 gas mixture at elevated temperatures produced SiON:Eu2+ films, from which an intense white emission band peaking at 525 nm and ranging between 400 and 650 nm was observed.

Evaluation of photocatalytic activity, water contact angle, and annealing for TiO2 thin films deposited with mixed WOX–SiO2 thin films using radiofrequency sputtering

A mixture of hydrophilic silicon dioxide (SiO2) and visible-light-reactive tungsten oxide (WOX) has the potential to improve the photocatalytic activity of conventional titanium dioxide (TiO2). This study deposits mixed WOX–SiO2 thin films on TiO2 surfaces by controlling the composition of WOX:SiO2 using radiofrequency sputtering to improve photocatalytic activity and hydrophilicity. The photocatalytic activity is evaluated via the degradation of a methylene blue solution, and hydrophilicity is measured using the water contact angle. In addition, the effect of annealing is determined at 400 °C after deposition. The optical bandgap decreases as the composition of WOX increases and subsequently anneals. The XRD measurements show that polycrystalline monoclinic WO3 peaks appear after annealing when the composition of the mixed WOX–SiO2 thin films only consists of WOX. In contrast, monoclinic WO3 (200) appears after adding SiO2. Atomic force microscopy images show that the grain size decreases as the SiO2 content increases. Moreover, the photocatalytic activity of the mixed WOX–SiO2 thin films improves after annealing. In particular, the mixed WOX–SiO2 thin films that are deposited at a sputter power of WOX:SiO2 = 100:50 W demonstrate a remarkable improvement in photocatalytic activity. Furthermore, the water contact angle of the mixed WOX–SiO2 thin films decreases as the SiO2 content increases and after annealing. This proposed approach can be used for high-performance photocatalytic materials and be widely applied for the fabrication of various semiconducting devices.

Two-step GISAXS characterization of NiSi2 nanoplates and Ni nanocrystals embedded in a silicon wafer covered with a silica thin film

Here, an experimental grazing-incidence small-angle X-ray scattering (GISAXS) study of the nanostructure of a sample composed of a Si(001) wafer covered by a Ni-doped SiO2 thin film and thermally treated at high temperature is described. Previous studies indicated that this type of composite contains Ni nanocrystals mainly inside the thin film and NiSi2 nanoplates buried in the Si wafer. To achieve accurate determinations of low-resolution structural parameters of the nanoparticles derived from the experimental results (shape, volume and size), two GISAXS patterns were recorded. The first pattern was produced by the Si wafer covered by a nanoporous Ni-doped SiO2 thin film and the second pattern was recorded after removal of the SiO2 thin film by chemical etching. By using the procedure of best fitting of a modeled isotropic GISAXS intensity to a set of 1D GISAXS patterns recorded before thin-film removal, the average radius and radius dispersion of a size polydisperse set of spherical Ni nanocrystals were determined. The GISAXS pattern recorded after removal of the SiO2 thin film was almost completely due to the scattering intensity produced by oriented NiSi2 nanoplates located inside the Si wafer. By fitting a theoretical function for the X-ray scattering intensity produced by oriented nanoplates to a set of experimental 1D GISAXS patterns, the maximum diameters and average thickness of the hexagonal NiSi2 nanoplates were determined. The GISAXS intensity pattern produced by the sample after thin-film removal is essentially anisotropic (with only a weak isotropic contribution from Ni nanocrystals), thus allowing for precise quantitative determinations of the relevant sizes of the NiSi2 nanoplates.

Heterostructure Films of SiO2 and HfO2 for High-Power Laser Optics Prepared by Plasma-Enhanced Atomic Layer Deposition

Absorption losses and laser-induced damage threshold (LIDT) are considered to be the major constraints for development of optical coatings for high-power laser optics. Such coatings require paramount properties, such as low losses due to optical absorption, high mechanical stability, and enhanced damage resistance, to withstand high-intensity laser pulses. In this work, heterostructures were developed by sub-nanometer thin films of SiO2 and HfO2 using the plasma-enhanced atomic layer deposition (PEALD) technique. Thin-film characterization techniques, such as spectroscopic ellipsometry, spectrophotometry, substrate curvature measurements, X-ray reflectivity, and Fourier transform infrared spectroscopy, were employed for extracting optical constants, residual stress, layer formation, and functional groups present in the heterostructures, respectively. These heterostructures demonstrate tunable refractive index, bandgap, and improved optical losses and LIDT properties. The films were incorporated into antireflection coatings (multilayer stacks and graded-index coatings) and the LIDT was determined at 355 nm wavelength by the R-on-1 method. Optical absorptions at the reported wavelengths were characterized using photothermal common-path interferometry and laser-induced deflection techniques.

Bipolar Switching Properties of GdOx:SiO2 Thin Film Resistive Random Access Memory Using Co-Sputtering Technology

Using two kinds of targets (gallium and silicon dioxide) and the rf magnetron sputtering deposited technique, GdOx:SiO2 thin film RRAM devices were deposited on TiN/Si substrate to form a metal–insulator–metal (MIM) structure. In addition, different oxygen concentrations and rf sputtering power parameters were prepared for the GdOx:SiO2 thin films. Decrease of the defects and oxygen vacancies of the GdOx:SiO2 thin films were used and repaired by rapid thermal annealing technology. Indium tin oxide (ITO) as the top electrode on the GdOx:SiO2 thin film was prepared by the physical vapor deposition (PVD) method, and ITO/GdOx:SiO2/TiN/Si structures of the GdOx:SiO2 thin films’ RRAM devices were also made. In addition, the current–voltage curves and devices’ endurance properties were measured by an impedance analyzer. Finally, the crystalline style, the preferred phase, the grain size, and surface microstructure of the thin films were analyzed and observed from X-ray diffraction and field emission scanning electron microscope measurements.

High thermal performance hybrid GaInAsP/SOI ridge waveguide lasers with enhanced heat dissipation structure

Hybrid GaInAsP/SOI ridge waveguide Fabry–Pérot lasers with reduced thermal resistance were fabricated and measured. The lasers were formed by room-temperature surface-activated bonding of InP and silicon-on-insulator wafers. Thin SiO2 film was introduced as ridge-sidewall insulation to increase heat flow to the p-electrode side-metal. By incorporating a thermal shunt structure and Au electroplating, a single-facet output power of 20 mW and lasing operation up to 110 °C were achieved for a cavity length of 2.0 mm under continuous-wave conditions. The proposed structure showed low thermal resistance of 14.3 K W−1 and a threshold current density of approximately 0.7 kA cm−2.

Non-destructive processing of silver containing glass ceramic antibacterial coating on polymeric surgical mesh surfaces.

Surgical meshes composed of bioinert polymers such as polypropylene are widely used in millions of hernia repair procedures to prevent the recurrence of organ protrusion from the damaged abdominal wall. However, post-operative mesh infection remains a significant complication, elevating hernia recurrence risks from 3.6% to 10%, depending on the procedure type. While attempts have been made to mitigate these infection-related complications by using antibiotic coatings, the rise in antibiotic-resistant bacterial strains threatens their effectiveness. Bioactive glass-ceramics featuring noble metals, notably silver nanoparticles (AgNPs), have recently gained traction for their wide antibacterial properties and biocompatibility. Yet, conventional methods of synthesizing and coating of such materials often require high temperatures, thus making them impractical to be implemented on temperature-sensitive polymeric substrates. To circumvent this challenge, a unique approach has been explored to deposit these functional compounds onto temperature-sensitive polypropylene mesh (PP-M) surfaces. This approach is based on the recent advancements in cold atmospheric plasma (CAP) assisted deposition of SiO2 thin films and laser surface treatment (LST), enabling the selective heating and formation of functional glass-ceramic compounds under atmospheric conditions. A systematic study was conducted to identify optimal LST conditions that resulted in the effective formation of a bioactive glass-ceramic structure without significantly altering the chemical and mechanical properties of the underlying PP-M (less than 1% change compared to the original properties). The developed coating with optimized processing conditions demonstrated high biocompatibility and persistent antibacterial properties (>7 days) against both Gram-positive and Gram-negative bacteria. The developed process is expected to provide a new stepping stone towards depositing a wide range of functional bioceramic coatings onto different implant surfaces, thereby decreasing their risk of infection and associated complications.