SiO2 Slab Research Articles

Gas-phase etching mechanism of silicon oxide by a mixture of hydrogen fluoride and ammonium fluoride: A density functional theory study

We report the selective etching mechanism of silicon oxide using a mixture of hydrogen fluoride (HF) and NH4F gases. A damage-free selective removal of native oxide has been used in semiconductor manufacturing by forming and removing the ammonium fluorosilicate [(NH4)2SiF6] salt layer. A downstream plasma of NF3/NH3 or a gas-phase mixture of HF and NH4F was used to form (NH4)2SiF6. We modeled and simulated the fluorination of silicon oxide and the salt formation by density functional theory calculation. First, we simulated the successive fluorination of silicon oxide using SiO2 slab models. The fluorination reactions of SiO2 surfaces by the mixture produced a volatile SiF4 molecule or a surface anion of –OSiF4−* with an NH4+ cation with low activation energies. Unlike HF, NH4F produced surface salt species consisting of a surface anion and an ammonium cation. Next, we simulated the (NH4)2SiF6 formation from the two reaction products on fluorinated SiO2 surfaces. (NH4)2SiF6 can be formed exothermally with low activation energies (0.27 or 0.30 eV). Finally, we compared silicon with SiO2 to demonstrate the inherently selective etching of silicon oxide. The fluorination reactions of silicon by the mixture showed the activation energies significantly higher than the SiO2 cases, 1.22–1.56 eV by HF and 1.94–2.46 eV by NH4F due to the less stable transition state geometries. Therefore, the selective salt formation on silicon oxide, not on silicon, is expected in near-room temperature processing, which enables selective etching of silicon oxide.

Structural Transitions of Anionic, Cationic, and Nonionic Surfactant Solutions Confined between Amorphous SiO2 Slabs: Molecular Dynamics Simulations

Adsorption of surfactants of a different nature on solid surfaces has found remarkable enthusiasm in many industrial processes such as enhanced oil recovery, detergency, solubilization, and flocculation/dispersion. Surfactants can interact with solid slabs and alter their surface charge and hydrophobicity, which are essential in interfacial phenomena. Interestingly, the surface charges of amorphous materials are thoroughly different from crystalline materials due to the disordered distribution of surface atoms. Therefore, we performed molecular dynamics simulations to examine the interactions of surfactant solutions with SiO2 surface in detail. To this aim, aqueous solutions of various surfactants were confined between two amorphous SiO2 walls, which were 10 nm apart. The results indicate that the structure of the amorphous solid surfaces is changed substantially due to their interactions with surfactant solutions. The simulation results indicate the presence of the nonionic surfactant aggregates in the bulk solvent, while ionic surfactants without branching, such as SDS, form aggregates that adsorb on the SiO2 surface. The interactions of the confined surfactants with glassy SiO2 slabs change the structure of the surface layer and consequently alter the surface charge density during the simulation, which leads to the separation of cationic and anionic parts of the confined surfactants in the vicinity of amorphous SiO2 slabs.

Omnidirectional High Reflectors Using Silica/Superconductor Fibonacci Photonic Crystal for Optical Communication Applications

Using the transfer matrix method and Fibonacci structure, an omnidirectional high reflector that covers the fiber optics wavelengths is searched. We aim to make the reflector covers the operating wavelengths of Plastic and Glass fiber optics (650, 850, 1300 and 1550nm). Firstly, an ordinary Fibonacci structure composed of TiO2 and SiO2 slabs materials was studied and for TE and TM polarization modes the reflectance spectra cover only the 850, 1300 and 1550 nm wavelengths and for a limited range of wave-angle of incidence. Secondly, the SiO2 slabs were replaced with the superconductor YBCO slabs which allows the extension of the reflectance bands to cover all fiber optics wavelengths for the entire angle of incidence interval [0-1.5 radian] and for both polarization modes. During that, the structure thickness was optimized by varying the iteration number and the optical thickness of Fibonacci sequence. Finally, the optical behavior of the reflector was tested when changing the ambient temperature.

Metal ion implantation into transparent dielectric slab: an effective route to high-stability localized surface plasmon resonance sensors

Ag/SiO2 and Au/SiO2 samples were prepared by separately implanting 30 keV Ag and Au ions into 0.5-mm-thick SiO2 slabs at a fluence of 6 × 1016 ion·cm−2, and their optical and structural properties were studied in detail by using a fiber spectrometer and a transmission electron microscope, respectively. Our results showed that the two samples featured by their respective nanocomposite surface layers were asymmetrical in structure, and hence, their characteristic signals in the reflectance spectra excited by the lights incident from the rear surfaces were able to exhibit corresponding blueshifts when the overlays on the implanted surfaces were increased in refractive index with respect to air. Our results also showed that each of characteristic signals was strongly dependent on the localized surface plasmon resonance (LSPR) behavior of the involved Ag or Au nanoparticles (NPs), and it could not appear at a wavelength position smaller than or equal to that of the LSPR absorption peak since the involved Ag or Au NPs were quite small in size. These results meant that the two samples could be regarded as the LSPR sensors with a negative refractive index sensitivity (RIS), although their sensing abilities would lose when the overlays were very large in refractive index. Especially, the two samples were demonstrated to be relatively high in stability because the involved Ag and Au NPs were closely hugged and chemically protected by the matrices of SiO2, and consequently, they could have a chance to become prospective sensing devices in some special fields as long as their RISs and linearities could be improved in the future. The above findings substantially confirmed that the metal ion implantation into transparent dielectric slab was an effective route to the high-stability LSPR sensors.

Molecular Dynamics Simulation of Silicon Dioxide Etching by Hydrogen Fluoride Using the Reactive Force Field.

In this study, we develop a reactive force field (ReaxFF) for a Si/O/H/F system to perform etching simulations of SiO2 with an HF etchant. Quantum mechanical (QM) training sets from density functional theory calculations, which contain structures of reactant/product and energies with bond dissociation, valence angle distortions, and reactions between SiO2 clusters and SiO2 slab with HF gases, are used to optimize the ReaxFF parameters. Structures and energies calculated using the ReaxFF match well with the QM training sets. Using the optimized ReaxFF, we conduct molecular dynamics simulations of the etching process of SiO2 substrates with active HF molecules. The etching yield and number of reaction products with different incident energies of the HF etchant are investigated. These simulations show that the developed ReaxFF offers insights into the atomistic surface reaction of the SiO2 etching process.

Omnidirectional high reflector at 650, 850, 1300 and 1550 nm for optical fiber communication by fused silica/YBCO-superconductor Octonacci photonic crystal

The transfer matrix method (TMM) is deployed to determine the reflectance spectra of Octonacci photonic structures. First, the structures are designed using the TiO2 and SiO2 slabs materials. This design allows the obtainment of limited high reflectors for some optical communication wavelengths and for both TE and TM polarizations. After that, the superconductor YBa2Cu3O7 (YBCO) and the silica slabs replace the previous materials in Octonacci structures. This modification permits to extend the omni-directional reflectance bands for both polarizations. The iteration number of the Octonacci sequence has an important effect on these bands. Later the geometric thickness of the photonic structure is optimized by changing the reference wavelength λ0 and omnidirectional high reflectors that cover all optical communication wavelengths (650, 850, 1300 and 1550 nm) are obtained. In addition, for these wavelengths, a tiny effect of the ambient temperature is noticed. These omnidirectional high reflectors can find application in glass (GFO) and plastic (PFO) fiber optics and may be used in sensing applications.

Modulating Near-Field Radiative Heat Transfer through Thin Dirac Semimetal Films

ABSTRACT We propose a thermal modulation structure made of two identical SiO2 slabs coated by Dirac semimetal (DSM) films and separated by a nanoscale vacuum gap. The energy transmission probability reveals that the coupled surface plasmon polaritons (SPPs) between the two DSM films, and the surface phonon polaritons (SPhPs) supported by the SiO2 substrate can vary sensitively with the Fermi level, the degenerate factor of 3D Dirac points and the thickness of the DSM film, thus providing the possibilities for modulating the radiative heat transfer by tuning these parameters. Based on Maxwell’s equations incorporating fluctuational electrodynamics, the effects of these parameters on the heat transfer coefficient and the thermal modulation contrast are numerically analyzed. Under proper parameters, higher modulation contrasts are obtained by continuously tuning the Fermi level from 0.05 eV to 0.3 eV. The obtained results might be helpful in designing a DSM-based thermal modulator with higher modulation contrasts.

Graphene based photonic crystals including anisotropic defect layers with highly tunable optical responses in infrared frequency range

Abstract‒ Optical features of three one dimensional photonic crystals are studied theoretically. First structure which consists of SiO2 slabs separated by graphene sheets is considered as an ideal crystal with tunable response. Adding anisotropic liquid crystal (LC) layers inside the ideal structure, two defective crystals are also proposed to assess their potential of being used in photonics. Dependence of the crystals transmittance to periods of the layers as well as size of the SiO2 and LC slabs are studied at first. Extracting the optimum values for these structural parameters, tunability of the optical response of the proposed crystals by using externally controllable parameters such as: optical axis of the anisotropic layer, light incident angle, crystal temperature and graphene layer chemical potential are evaluated systematically. Results clearly reflect that all suggested crystals might be useful to support the increasing demands of optical integrated circuit technology for adjustable photonic elements.

Graphene based photonic crystal optical filter: Design and exploration of the tunability

Optical characteristics of two new graphene based photonic crystals are studied in detail. A structure containing alternating layers of graphene and SiO2 slabs is considered as the ideal crystal. The dependency of the photonic band gaps (PBGs) to the dielectric layer thickness and the period number is explored at first step. Potential of the proposed crystal to be used as an optical filter is then investigated. Adding a nonlinear electro-optic polymer as a defect layer, the alterations of the optical features are inspected. Results show that the defect layer insertion causes a resonant mode inside the PBGs. However, the location of the defect layer inside the crystal is very effective on both the frequency and width of the resonant mode. Tunability of the optical features is probed by taking into account of the dependencies to the wave incident angle, graphene chemical potential and the applied external voltage to the defect layer.

Broad band temperature independent photonic crystal based optical filter with response in visible wavelength range

A new photonic crystal-based wide band (⩾200 nm) filter/mirror with a response in the visible frequency range is proposed. Alternating layers of Si–Ge and SiO2 slabs are considered as the main building blocks of the crystal. Having inserted stacks of the SiO2/TiO2 unit cells inside the crystal, the alterations of the optical features of the ideal crystal can be explored. The number of defects, crystal temperature, light incident angle and the polarization of the light are considered as the controlling parameters for tuning the crystal optical characteristics. Investigating the dependencies systematically, it is shown that both the ideal and defective crystals have unique temperature independent features. More interestingly, the proposed crystals have an almost similar response for TE and TM polarizations. However, the light incident angle is a very effective factor for controlling not only the photonic band gap interval, but also for tuning the defect layer(s) induced resonant modes. Due to their 100% reflectivity in ~450–650 nm, simplicity in fabrication (limited number of layers and reasonable sizes), temperature independency (in 50 °C–300 °C) and comfortable tunability, the proposed structures might be interesting in photonic technology for manufacturing wide-band filters/mirrors.

Plasmonic Behaviors of Two-Dimensional Ag/SiO2 Nanocomposite Gratings: Roles of Gap Diffraction and Localized Surface Plasmon Resonance Absorption

Two-dimensional Ag/SiO2 nanocomposite gratings of 400 and 600 nm in grating constant are fabricated by etching the SiO2 slabs implanted with Ag ions, and their plasmonic extinction, absorption, and reflection behaviors are investigated. Our results indicate that no scattering light fields can exist near the localized surface plasmon (LSP) resonance wavelength (about 405 nm) of Ag nanoparticles (NPs) due to the intense LSP resonance absorption. Especially, when the gaps between nanocomposite veins have a width close in value to the LSP resonance wavelength of Ag NPs, the local light fields in the grating plane can be slightly enhanced due to an in-phase addition of the incident light fields and the diffractive light fields induced by the gap diffraction, leading to a slight red shift of LSP resonance mode of Ag NPs. Moreover, in the LSP resonance absorption region, although the grating diffraction can still occur, the diffractive light fields are extremely weak, and thus, the local light fields in the grating plane cannot be modified by coherently adding these extremely weak diffractive light fields to the incident light fields. As a result, the LSP resonance mode of Ag NPs will keep its position unchanged even though the grating constant is set to make the first grating order rightly change from evanescent to radiative character.

Excitation of long-wavelength surface optical vibrational modes in films, cubes and film/cube composite system using an atom-sized electron beam.

Using spatially resolved Electron Energy-Loss Spectroscopy, we investigate the excitation of long-wavelength surface optical vibrational modes in elementary types of nanostructures: an amorphous SiO2 slab, an MgO cube, and in the composite cube/slab system. We find rich sets of optical vibrational modes strongly constrained by the nanoscale size and geometry. For slabs, we find two surface resonances resulting from the excitation of surface phonon polariton modes. For cubes, we obtain three main highly localized corner, edge, and face resonances. The response of those surface phonon resonances can be described in terms of eigenmodes of the cube and we show that the corresponding mode pattern is recovered in the spatially resolved EELS maps. For the composite cube/substrate system we find that interactions between the two basic structures are weak, producing minor spectral shifts and intensity variations (transparency behaviour), particularly for the MgO-derived modes.

Flexible broadband plasmonic absorber on moth-eye substrate

High efficiency, broadband and flexible plasmonic absorbers comprised of top Ag nanoparticle layer, SiO2 dielectric slab and bottom Ag reflective layer on moth-eye polydimethylsiloxane substrates were fabricated by sputtering. The moth-eye polydimethylsiloxane substrates were obtained by simply replicating the pattern of the commercial U-shaped anodic aluminum oxide templates. For our designed absorber, the integrated absorption efficiency over the wavelength range from 350 nm to 1000 nm at normal incidence is optimized to 95.8% when the nominal thickness of the top Ag nanoparticle layer is 5.4 nm and the thickness of the SiO2 slab is 96 nm. In addition, our absorber performs angle-insensitively with the integrated absorption efficiency suffering only around 10% degradation when the incidence changes from normal to 60° oblique. Moreover, the absorber is very robust against bending and the integrated absorption efficiency almost maintains its original value even after bending for 10, 000 times. Compared to the absorbers fabricated on a flat substrate, the absorbers made on the structured moth-eye substrates have high tolerance of the thickness of the SiO2 slab. Simulation results indicate that the Ag nanoparticle layer allows light penetrate into the corrugated dielectric slab, forming various photonic resonances at different wavelengths, resulting in efficient light harvesting over a broadband wavelength range.

Plasmonic broadband absorber by stacking multiple metallic nanoparticle layers

High efficiency, broadband plasmonic absorbers are constructed based on a stack of alternating metallic nanoparticle layers (MNLs) and SiO2 slabs on top of a reflective Ag substrate. Experimental results show that the stacks with thick MNLs absorb light better than those with thin MNLs when the number of MNL/SiO2 cells (N) is small (e.g., 1 or 2), but the situation gets reversed when N is greater than 3. When the nominal thickness of MNL is as thin as 5 nm, the acquired Ag nanoparticles are so small that light penetration through all of the stacked MNLs in the proposed design is possible. Thus, an increase in N leads to a growing number of light trapping elements. Our simulation reveals that the Ag nanoparticles at different layers are hybridized to excite rich localized plasmonic resonances, resulting in multiple absorption peaks at optical frequencies and thus a broader absorption band. The broadband absorbers with an integrated absorption efficiency of 96% over the 300–1100 nm wavelength range were achieved by stacking 18 MNL/SiO2 cells. The proposed absorbers can be used for applications in solar energy harvesting and thermal emission tailoring, due to their easy fabrication procedure and excellent optical properties.

Realization of Directional-Coupler-Based Optical Waveguide Devices by Ultraviolet-Imprinting Technique Using Sol-Gel Derived Inorganic GeO$_{2}$ : SiO$_{2}$ Films

An efficient fabrication scheme of directional coupler (DC) optical waveguide devices is demonstrated using a direct ultraviolet imprinting (DUI) technique in sol-gel derived inorganic 0.2GeO2:0.8SiO2 slab waveguide. The slab waveguide has a ~3-mum-thick photosensitive inorganic 0.2GeO2:0.8SiO2 film which is prepared by the sol-gel spin-coating technique on (10 mum) SiO2-Si substrate. This inorganic thick film of slab waveguide has the ability to induce a high refractive index change of ~5times10-3 under ultraviolet irradiations, which is suitable for the fabrication of waveguiding channel of photonic devices by a DUI technique. DC geometries were theoretically evaluated by the beam propagation method for various optical power splitting ratios and experimental performances of DCs 3-dB light splitter and complete power exchange from one waveguide to another were demonstrated and found to be in good agreement with the simulation.

Adsorption of Atomic Oxygen and Nitrogen at β-Cristobalite (100): A Density Functional Theory Study

The adsorption of atomic oxygen and nitrogen on the beta-cristobalite (100) surface is investigated from first principles density functional calculations within the generalized gradient approximation. A periodic SiO2 slab model (6 layers relaxing 4 or 6) ended with a layer of Si or O atoms is employed throughout the study. Several adsorption minima and diffusion transition states have been characterized for the two lowest spin states of both systems. A strong chemisorption is found for either O or N in several sites with both slab endings (e.g., it is found an average adsorption energy of 5.89 eV for O (singlet state) and 4.12 eV for N (doublet state) over the Si face). The approach of O or N on top O gives place to the O2 and NO abstraction reactions without energy barriers. Atomic sticking coefficients and desorption rate constants have been estimated (300-1900 K) by using the standard transition state theory. The high adsorption energies found for O and N over silica point out that the atomic recombination processes (i.e., Eley-Rideal and Langmuir-Hinshelwood mechanisms) will play a more important role in the atomic detachment processes than the thermal desorption processes. Furthermore, the different behavior observed for the O and N thermal desorption processes suggests that the published kinetic models for atomic O and N recombination reactions on SiO2 surfaces, based on low adsorption energies (e.g., 3.5 eV for both O and N), should probably be revised.