SiO2 Thin Films Research Articles

Controlling surface functional composition and improving the gas-sensing properties of metal oxide sensors by electron beam processing

It has been established that the electron beam processing of the ZnO–SiO2 and SnO2–SiO2 thinfilm gas sensors synthesized using the sol-gel method at an electron energy of 900 keV and an absorbed dose of 50 to 200 kGy leads to a significant increase in their sensitivity towards acetone and isopropanol vapors. It is found that the observed effect, which is enhanced with an increase in the absorbed dose, is correlated to an increase in the concentration of the Bronsted acid sites which are formed under the influence of accelerated electrons and capable of reacting to the molecules of the detected substances.

Selectivity in Thermal Atomic Layer Etching Using Sequential, Self-Limiting Fluorination and Ligand-Exchange Reactions

Atomic layer etching (ALE) can result from sequential, self-limiting thermal reactions. The reactions during thermal ALE are defined by fluorination followed by ligand exchange using metal precursors. The metal precursors introduce various ligands that may transfer during ligand exchange. If the transferred ligands produce stable and volatile metal products, then the metal products may leave the surface and produce etching. In this work, selectivity in thermal ALE was examined by exploring tin(II) acetylacetonate (Sn(acac)2), trimethylaluminum (TMA), dimethylaluminum chloride (DMAC), and SiCl4 as the metal precursors. These metal precursors provide acac, methyl, and chloride ligands for ligand exchange. HF-pyridine was employed as the fluorination reagent. Spectroscopic ellipsometry was used to measure the etch rates of Al2O3, HfO2, ZrO2, SiO2, Si3N4, and TiN thin films on silicon wafers. The spectroscopic ellipsometry measurements revealed that HfO2 was etched by all of the metal precursors. Al2O3 was et...

Comparative study of SiO2, Si3N4 and TiO2 thin films as passivation layers for quantum cascade lasers.

The aim of this article is to determine the best dielectric between SiO2, Si3N4 and TiO2 for quantum cascade laser (QCL) passivation layers depending on the operation wavelength. It relies on both Mueller ellipsometry measurement to accurately determine the optical constants (the refractive index n and the extinction coefficient k) of the three dielectrics, and optical simulations to determine the mode overlap with the dielectric and furthermore the modal losses in the passivation layer. The impact of dielectric thermal conductivities are taken into account and shown to be not critical on the laser performances.

Ultra-Low Switching Voltage Induced by Inserting SiO2Layer in Indium–Tin–Oxide-Based Resistance Random Access Memory

A lower switching voltage of indium–tin–oxide (ITO)-based resistance random access memory (RRAM) with an inserted SiO2 thin film was presented. The amplitude of switching voltage of device was below 0.2 V whether measured by direct current or alternating current sweep operation. Notably, the observed reset voltage increased with temperature. To clarify the switching mechanism, conduction current fitting and switching voltage statistics were applied to explore the regular voltage variation dependent on temperature. In addition, a reaction model was proposed to explain the oxygen concentration gradient induced between the inserted SiO2 and ITO electrode on the ITO-based RRAM device.

Effect of NiO crystallinity on forming characteristics in Pt/NiO/Pt cells as resistive switching memories

Resistive switching (RS) in metal/oxide/metal stack structures plays a key role in resistive RAM. The formation and rupture of conductive filaments have been widely accepted as an origin of RS mechanism especially in binary transition metal oxides. Forming exhibits some analogies with a dielectric breakdown of SiO2 thin films. In this study, Time-Dependent Forming (TDF) characteristics of Pt/NiO/Pt stack structures have been investigated. The results revealed that the formation of conductive filaments at the forming process by applying constant voltage followed a weakest-link theory and that the weakest spots were almost randomly distributed in NiO thin films according to the Poisson statistics. Furthermore, the distribution of TDF characteristics depends on NiO crystallinity. A small variation of initial resistance tends to result in a large variation of time to forming and vice versa.

Control of Asymmetric Scattering Behavior of Plasmonic Nanoparticle Ensembles

A transparent or translucent film shows the same color for both front and back sides in general. However, different colors can be shown by the aid of strong plasmonic scattering of metal nanoparticles. Here we propose a design strategy of films exhibiting asymmetric scattering behavior. Typically two different types of nanoparticles are embedded in a SiO2 thin film that is coated on a TiO2 thin film. One of the particle types is selectively excited for the front incidence, and the other type for the back incidence, by tuning the electric field distribution inside the SiO2 film. We experimentally prepared two different samples that scatter light of different colors from the front and the back sides by using Au nanorods and Ag nanospheres.

Gravimetric biosensor based on a 1.3 GHz AlN shear-mode solidly mounted resonator

We investigate the performance of solidly mounted resonators based on Ir/tilted-AlN/Ir piezoelectric stacks as biosensors. These films are deposited by varying the pressure, the cathode power and the temperature of a two-step process based on depositing (00·2)-tilted AlN active layers over an (10·3)-oriented AlN seed layer. To minimize the influence of the temperature coefficient of frequency on the stability of the biosensor, we use insulating acoustic mirrors made of layers of SiO2 and amorphous TaOx with non-λ/4 thicknesses, which enables to reduce the TCF to −14ppm/°C. The mass loading of the resonators with SiO2 thin films results in a sensitivity of 1800kHz/pg·cm2. Surface functionalization consists on the binding of silane groups on plasma oxidized SiO2 surfaces. After a glutaraldehyde link, streptavidin is bonded to the surface to receive biotinylated receptors for several species. We test thrombin-binding aptamer (TBA29 against thrombin, and IgG antibody against immunoglobulin). The sensors response to species of different molecular weight like TBA-29 (9.75kDa) or IgG antibody (150kDa) is monitored. Finally, we assess the response of the biosensors to different thrombin concentrations (ranging from 4nM to 270nM) on surfaces functionalized with the TBA29 aptamer.

Shear-horizontal surface acoustic wave characteristics of a (110) ZnO/SiO2/Si multilayer structure

Shear-horizontal (SH) surface acoustic wave (SAW) propagation characteristics in IDT/(110)ZnO/SiO2/Si multilayered structure were theoretically investigated using 3-dimensions (3D) finite element analysis (FEA). X-ray diffraction result shows that the prepared ZnO films have preferred (110) orientation with its c-axis parallel to the substrate. 3D FEA results and the out of plane vibration experiment confirmed that SH-SAW was successfully excited in the multilayered structure. The phase velocities Vp, electromechanical coupling coefficients K2, and temperature coefficient of frequency (TCF) dispersion properties of mode 0 were studied considering various thicknesses of ZnO and SiO2 thin films, which were verified by experiments. SiO2 film with a positive TCF not only compensate the negative TCF of the (110) ZnO but also enhance the K2. A large K2 of 3.37 and nearly zero TCF of −0.1 ppm/°C was achieved in SH-SAW device based on IDT/(110)ZnO(2 μm)/SiO2(1 μm)/Si multilayer structure, which is promising for high sensitive and temperature stable bio-sensing application.

Hollow-cathode activated PECVD for the high-rate deposition of permeation barrier films

This paper shows a study on the variation of the flow ratio between oxygen and Hexamethyldisiloxane (HMDSO) in a hollow cathode arc PECVD process. Dynamic deposition rates were measured between 300 and 500nm∗m/min. The deposited plasma polymer films were analyzed using visible light spectroscopy, X-ray photo-electron spectroscopy (XPS), X-ray reflectivity (XRR), Fourier-transformed infrared spectroscopy (FTIR), Scanning electron microscopy (SEM) and water vapor transmission rate (WVTR) measurements. The deposited samples were compared to a reactively sputtered SiO2 thin film. By increasing the oxygen to HMDSO flow ratio, the chemical, density and optical properties of the coating approached the sputtered SiO2 film. However the permeation barrier of the SiO2 film only shows a slight improvement over that of the bare PET substrate. The organic coating with high power and low oxygen flow, which was deposited with a deposition rate of 450nm∗m/min, approached the barrier of the sputtered SiO2 thin film with a WVTR of 0.16g/(m²day).

Plasma Damage Effects on SiO2 Thin Films Determined by Nanoindenter Under Various Etching Conditions Using ACP Source

Plasma Damage Effects on SiO2 Thin Films Determined by Nanoindenter Under Various Etching Conditions Using ACP Source

Effect of SiO2 addition on photocatalytic activity, water contact angle and mechanical stability of visible light activated TiO2 thin films applied on stainless steel by a sol gel method

Nanostructured N doped TiO2/20%SiO2 thin films were developed on steel surface via sol gel method using a painting airbrush. Thin films then were calcined at various temperatures in a range of 400–600°C. The effect of SiO2 addition on phase composition and microstructural evolution of N doped TiO2 films were studied using XRD and FESEM. Optical properties, visible light photocatalytic activity, hydrophilic behavior, and mechanical behavior of the films were also investigated by DRS, methylene blue degradation, water contact angle measurements, and nanoscratch testing. Results indicated that the band gap energy of N doped TiO2/SiO2 was increased from 2.93 to 3.09eV. Crack formation during calcination was also significantly promoted in the composite films. All composite films demonstrated weaker visible light photocatalytic activities and lower mechanical stability in comparison with N doped TiO2 films. Moreover, the N doped TiO2/SiO2 film calcined at 600°C showed undesirable hydrophilic behavior with a water contact angle of 57° after 31h of visible light irradiation. Outcomes of the present study reveal some different results to previous reports on TiO2/SiO2 films. In general, we believe the differences in substrate material as well as application in visible light are the main reasons for the above mentioned contradiction.

Mesoporous materials from SiO2 and NiTiO3

ABSTRACTIn this work, we synthesized and characterized mesoporous thin films of SiO2 and NiTiO3 structured by a surfactant called Brij58. These films were fabricated by the method of dip coating and the best conditions for well-structured thin films were investigated as a function of surfactant concentration and different types of substrates. These films have been characterized by X-ray reflectivity which was calculated using the matrix formalism. We demonstrated that the silicon substrate had a great effect on the structure and porosity of the fabricated films for both SiO2 and NiTiO3. Furthermore, we found that mesoporosity has been increased as a function of the surfactant concentration in the solution. This experimental procedure allows also to produce NiTiO3 powders which have been characterized by X-ray diffraction. The XRD coupled to the crystallographic software “Maud”shows that the samples are constituted by 98, 2% NiTiO3 powders.

Preparation and characterization of SiO2 thin film and SiC nanofibers to improve of graphite oxidation resistance

In this study, a multiphase coating was prepared on a graphite substrate by a two-step technique to improve oxidation resistance of graphite. The first step was to prepare a functionally graded SiC layer by a pack cementation process and the second step was to obtain a nano SiC and SiO2 thin film coating by a slurry painting technique. The microstructure and the surface chemistry of the coating were characterized by X-ray diffraction, scanning electron microscopy and X-ray photoelectron spectroscopy. The spectral deconvolution was conducted using CasaXPS software based on Gaussian-Lorentzian function. The isothermal oxidation test of the coated samples was performed at 1773K for 10h. A 500μm thick graded C–SiC layer was observed at the graphite-coating interface. An amorphous SiO2 thin film and SiC nanofibers with the diameter in the range of 30–75nm were formed on the coatings. The SiC nanofibers can efficiently improve oxidation resistance of graphite.

Full-atomistic nanoscale modeling of the ion beam sputtering deposition of SiO2 thin films

The previously developed high-performance parallel method of the atomistic simulation of the ion beam sputtering deposition process is applied to the SiO2 thin films. Structural properties of deposited films such as density, concentration of point defects, ring statistics, as well as effects arising from the interaction of high energy sputtered Si atoms with the growing film are discussed.

NBI Optical Filters in Minimally Invasive Medical Devices

The integration of the narrow band imaging (NBI) technique in highly miniaturized minimally invasive medical devices is presented. NBI provides a more reliable sensing performance for in vivo studies on tissues as compared to approaches based on white light illumination. NBI uses a selective filtered light with peak transmission at 415 (blue) and 540 nm (green). The blue light improves the visualization of the superficial mucosal layer, while the deeper penetration of the green light highlights the vascular patterns of the subepithelial vessels. The optical filters are based on a multilayer thin-film stack, using the Fabry–Perot configuration with titanium dioxide (TiO2) and silicon dioxide (SiO2). The blue light-emitting diode (LED) combined with the blue filter results in a maximum central wavelength at 414 nm, full-width half-maximum (FWHM) of 19 nm and maximum relative transmittance of 21%. The green LED combined with the green filter yields maximum peak intensity at 536 nm, FWHM of 30 nm, and maximum relative transmittance of 35%. RF-sputtering was used for the deposition of NBI optical filters. The refractive index and extinction coefficient of the TiO2 and SiO2 thin films were characterized and the green and blue filter designs were experimentally validated.

Characterization of Thin Film Dissolution in Water with in Situ Monitoring of Film Thickness Using Reflectometry.

Reflectometry was implemented as an in situ thickness measurement technique for rapid characterization of the dissolution dynamics of thin film protective barriers in elevated water temperatures above 100 °C. Using this technique, multiple types of coatings were simultaneously evaluated in days rather than years. This technique enabled the uninterrupted characterization of dissolution rates for different coating deposition temperatures, postdeposition annealing conditions, and locations on the coating surfaces. Atomic layer deposition (ALD) SiO2 and wet thermally grown SiO2 (wtg-SiO2) thin films were demonstrated to be dissolution-predictable barriers for the protection of metals such as copper. A ∼49% reduction in dissolution rate was achieved for ALD SiO2 films by increasing the deposition temperatures from 150 to 300 °C. ALD SiO2 deposited at 300 °C and followed by annealing in an inert N2 environment at 1065 °C resulted in a further ∼51% reduction in dissolution rate compared with the nonannealed sample. ALD SiO2 dissolution rates were thus lowered to values of wtg-SiO2 in water by the combination of increasing the deposition temperature and postdeposition annealing. Thin metal films, such as copper, without a SiO2 barrier corroded at an expected ∼1-2 nm/day rate when immersed in room temperature water. This measurement technique can be applied to any optically transparent coating.

Substrate and atmosphere influence on oxygen p-doped graphene

The mechanisms responsible for p-type doping of substrate supported monolayer graphene (Gr) by thermal treatments in oxygen ambient have been investigated by micro-Raman spectroscopy, atomic force microscopy (AFM) and X-ray Photoelectron Spectroscopy (XPS), considering commonly employed dielectric substrates, such as SiO2 and Al2O3 thin films grown on Si. While a high p-type doping (∼1013 cm−2) is observed for Gr on SiO2, no significant doping is found for Gr samples on the Al2O3 substrate, suggesting a key role of the Gr/SiO2 interface states in the trapping of oxygen responsible for the Gr p-type doping. Furthermore, we investigated the doping stability of Gr on SiO2 during subsequent thermal treatments in nitrogen (N2), carbon dioxide (CO2), water (H2O) or in vacuum controlled atmospheres. These processes induce only minor effects on the doping of Gr but for H2O and principally affect its defectiveness, suggesting that the literature reported air influence on the doping depends on water present in the atmosphere.

Selective synthesis of carbon-nanotubes/graphite or carbon-nanotubes/multi-graphene composites on 3-D nickel foam prepared with different nickel catalyst and pre-treatment

The research is to deposit carbon on porous 3-D nickel foam (NiF) as a gas diffusion electrode (GDE) by chemical vapor deposition (CVD). Effects of three different preparations of the nickel (Ni) catalyst and two pre-treatments of oxidation and reduction for carbon deposition were investigated. In the first method, the carbon was deposited on NiF with and without sputtering a Ni seed layer before the CVD. In the second method, the NiF was oxidized, then reduced by hydrogen into Ni particle as a catalyst and the carbon also was deposited by CVD. In the third method, a thin film of SiO2 was deposited on the NiF as a separation layer, then the Ni catalyst was deposited and finally, carbon was also deposited by CVD. The results exhibited large grain sizes of Ni catalyst cannot grow carbon nanotube (CNTs) during CVD. Pre-treatment of oxidation and the reduction reduce the grain size of the Ni catalyst, and an oxidation temperature equal or higher than the reduction temperature always makes it easy to grow CNTs. A smooth SiO2 thin film was used as a separation layer to deposit the Ni catalyst to grow the SWCNTs and a low area ratio of ID/IG for a non-crystallized sp3 graphite microstructure always creates a graphene microstructure. A suitable hydrogen gas could help the amount of growth of CNTs. Anyway, the specified Ni catalysis could promise strategy the selective synthesis of CNTs/graphite or CNTs/multi-graphene composites on NiF.

Frequency-dependence of the switching voltage in electronic switching of Pt-dispersed SiO2 thin films

The switching time-voltage dependence of electronic resistive switching was studied for understanding the switching dynamics in Pt-dispersed SiO2 thin film devices. Trapezoidal voltage pulses with opposite polarities were consecutively introduced and thereby transient on-switching and offswitching were examined. A prior on-switching voltage determines the off-switching voltage regardless of the sweeping rate of the pulse for the prior on-switching. However, the off-switching voltage was sensitive to the sweeping rate of the subsequent pulses for off-switching. The frequencydependent impedance of both the device and the surrounding circuit element are thought to result in the variation of the off-switching voltage; otherwise, the switching voltage is independent of time.

Study of temperature-dependent charge conduction in silicon-nanocrystal/SiO2 multilayers

Silicon-nanocrystals (Si-NCs) realized by SiOx<2/SiO2 multilayer (ML) approach have shown promise for realizing tightly-controlled dimensions, thus efficiently exploiting the size-dependent quantum effects for device applications. Unfortunately, the confining insulating barriers (SiO2 sublayers), instrumental for realizing quantum size effects in Si-NC MLs, can also hinder the charge conduction which is crucial for device applications including Si-NC based tandem solar cells and multi-exciton solar cells. Owing to this, a comprehensive study of conduction mechanisms has been carried out using a thorough analysis of temperature-dependent dark I-V measurements of SiO2 thin film and Si-NC multilayer samples fabricated by Inductively Coupled Plasma CVD (ICPCVD). As the ML samples consisted of interleaved SiO2 sublayers, current in SiO2 thin film has initially been studied to understand the conduction properties of bulk ICPCVD SiO2. For 21nm thick SiO2 film, conduction is observed to be dominated by Fowler–Nordheim (FN) tunneling for higher electric fields (>8MV/cm; independent of temperature), while for lower electric fields (5–8MV/cm) at higher temperatures, the trap-related Generalized Poole–Frenkel (GPF) is dominant. This signified the role of traps in modifying the conduction in bulk ICPCVD SiO2 films. We then present the conduction in ML samples. For multilayer samples with SiO2 sublayer thickness of 1.5nm and 2.5nm, Direct Tunneling (DT) is observed to be dominant, while for SiO2 sublayer thickness of 3.5nm, Space Charge Limited Conduction (SCLC) with exponential trap distribution is found to be the dominant conduction mechanism. This signifies the role of traps in modifying the conduction in Si-NC multilayer samples and SiO2 sublayer thickness dependence.