Theoretical analysis of optical properties of dielectric coatings dependence on substrate subsurface defects

A model for refractive index of stratified dielectric substrate was put forward according to theories of inhomogeneous coatings. The substrate was divided into surface layer, subsurface layer and bulk layer along the normal direction of its surface. Both the surface layer (separated into N₁ sublayers of uniform thickness) and subsurface layer (separated into N₂ sublayers of uniform thickness), whose refractive indices have different statistical distributions, are equivalent to inhomogeneous coatings, respectively. And theoretical deduction was carried out by employing characteristic matrix method of optical coatings. An example of mathematical calculation for optical properties of dielectric coatings had been presented. The computing results indicate that substrate subsurface defects can bring about additional bulk scattering and change propagation characteristic in thin film and substrate. Therefore, reflectance, reflective phase shift and phase difference of an assembly of coatings and substrate deviate from ideal conditions. The model will provide some beneficial theory directions for improving optical properties of dielectric coatings via substrate surface modification.     

基底微缺陷对介质薄膜光学性能影响的理论研究

基于非均匀膜理论提出一种存在微缺陷的介质基底的折射率分层模型，将基底依次分为表面层、亚表面层和体材料层，其中表面层和亚面层分别等效为折射率服从统计分布的非均匀膜，将它们分别再次细分为N₁和N₂个子层，每一子层均视为均匀介质 膜.应用光学薄膜特征矩阵法对其进行理论分析，并对单层介质膜的光学性能进行数值计算. 研究结果表明：基底的表面和亚表面微缺陷改变了薄膜和基底的等效折射率，导致了准Brew ster角和组合反射率与理想情形的偏离.同时这些微缺陷也改变了光在薄膜和基底中的传播 特性，因此反射相移和相位差均偏离理想情形.在研究基底的微缺陷对多层介质膜光学性能 影响的分析和计算时，该模型同样适用. 关键词： 微缺陷 介质薄膜 非均匀膜 光学性能     

Optical and electrical properties of Y2O3 thin films prepared by ion beam assisted deposition

Y₂O₃ thin films were deposited by ion beam assisted deposition (IBAD) and the effects of fabrication parameters such as substrate temperature and ion energy on the structure, optical and electrical properties of the films were investigated. The results show that the deposited Y₂O₃ films had less optical absorption, larger refractive index, and better film crystallinity with the increase of substrate temperature or ion energy. The as-deposited Y₂O₃ films without ion-beam bombardment had larger relative dielectric constant (?_r) and the ?_r decreased with time even over by 40%, while the ?_r of films prepared with high ion energy had less changes, only less than 3%. Also, with the increase of ion energy, the electrical breakdown strength and the figure of merit increased.     

Broadband dielectric film transflector for variable transmission optical filters

A broadband multilayer dielectric film transflector for applications in variable transmission optical filters is proposed. The dielectric film is composed of alternating TiO₂ and SiO₂ layers deposited on a substrate. The transmittance of the dielectric film transflector can be varied from 0 to 100% by adjusting the individual dielectric layer thickness and the number of layers. The conjugate gradient method is used to optimize the design of the transflectors. The obtained transflectors have good transmittance and reflectance within the whole visible range and the incident angle range 0-20°. The manufacturing tolerances on film thickness and refractive index are reasonably large.     

Optical characterization of band gap graded ZnMgO films

We investigated the optical properties of compositionally graded Zn_1?xMg_xO (g-ZnMgO) films using spectroscopic ellipsometry. The g-ZnMgO and ZnO films were grown on Pt/Ti/SiO₂/Si substrates by ultrasonic spray pyrolysis. We simulated a uniformly graded optical band gap layer on the Pt substrate to reproduce the experimental result. The band gap of the bottommost layer of the g-ZnMgO film was estimated to be ～3.22 eV, the same as the undoped ZnO film. Then we considered a linearly increasing band gap with the film composition, and obtained a band gap of ～3.56 eV for the topmost layer of the film. In addition, the exciton peak showed a strong increase for the topmost layer of the film suggesting an important role of doping.     

Structural,optical and dispersion characteristics of nanocrystalline GaN films prepared by MOVPE

In this work, nanocrystalline GaN film was grown on a c-plane sapphire substrate by metal-organic vapor phase epitaxy (MOVPE). The structural and optical properties of the nanocrystalline GaN thin film were studied. The morphological and structural properties of GaN film were studied by scanning electron microscopy (SEM) and X-ray diffraction (XRD), respectively. According to the X-ray diffraction spectrum, a GaN film was formed with a wurtzite structure, which is the stable phase. The optical parameters were determined using spectrophotometric measurements of transmittance and reflectance in the wavelength range 200–2500 nm. The analysis of the spectral behavior of the absorption coefficient in the intrinsic absorption region reveals a direct allowed transition with a band gap of 3.34 eV. The dispersion of the refractive index is discussed in terms of the single oscillator Wemple–Didomenico (WD) model. The single oscillator energy (E_o), the dispersion energy (E_d), the high frequency dielectric constant (ε_∞), the lattice dielectric constant (ε_L) and the free charge carrier concentration (N) were estimated. From the optical dielectric analysis, the optical conductivity, volume and surface energy loss functions were calculated. Moreover, the third-order nonlinear optical susceptibility χ⁽³⁾ was also considered.     

Electron density profile at the interface of SiO2/Si(0 0 1)

In this report we present grazing incidence X-ray reflectivity (GIXR) study of SiO₂/Si(0 0 1) system. We have analysed the X-ray reflectivity data using recursive formalism based on matrix method and distorted wave Born approximation (DWBA). From the analysis of the reflectivity data we could obtain the electron density profile (EDP) at the interface of the dielectric SiO₂ film and the Si(0 0 1) substrate. The EDP obtained from the matrix method follows the DWBA scheme only when two transition layers are considered at the interface of SiO₂/Si. The layer which is in proximity with the Si substrate has a higher electron density value than the Si and SiO₂ values and it appears as a maximum in the EDP. The layer which is in proximity with the dielectric SiO₂ layer has an electron density value lower than the SiO₂ value and it appears as a minimum in the EDP. When the thickness of the SiO₂ layer is increased the lower density layer diminishes and the higher density layer persists.     

A facile high‐performance SERS substrate based on broadband near‐perfect optical absorption

Plasmonic systems based on metal nanoparticles on a metal film with high optical absorption have generated great interests for surface‐enhanced Raman scattering (SERS). In this study, we prepare a broadband‐visible light absorber consisting Au nanotriangles on the surface of a continuous optically opaque gold film separated with a dielectric SiO₂ layer, which is a typical metal‐insulator‐metal (MIM) system, and demonstrate it as an efficient SERS substrate. The MIM nanostructure, prepared using nanosphere lithography with a very large area, shows a broadband with absorption exceeding 90% in the wavelength regime of 630–920 nm. We observe an average SERS enhancement factor (EF) as large as 4.9 × 10⁶ with a 22‐fold increase compared to a single layer of Au nanotriangles directly on a quartz substrate. A maximum SERS EF can be achieved by optimizing the thicknesses of the dielectric layer to control the optical absorption. Owing to the simple, productive, and inexpensive fabrication technique, our MIM nanostructure could be a potential candidate for SERS applications. Copyright © 2015 John Wiley & Sons, Ltd.     

In situ ellipsometric characterization and process control for amorphous thin film deposition

Characterization of the growth of hydrogenated amorphous silicon (a-Si:H) and carbon (a-C:H) thin films by in situ ellipsometric analysis at 3.4 eV and 3.2 eV is reported. For a-Si:H, prepared on metal substrates from an rf discharge of SiH₄, in situ ellipsometry data are strongly influenced by the SiSi bond packing density in the growing film. Deviations in the data from model calculations assuming a thickness independent a-Si:H dielectric function, when analyzed using an effective medium approximation, reveal the geometry and scale of the initial nucleation process. Effects of the deposition conditions and substrate microstructure on the coalescence of initial nuclei are understood on the basis of new measurements. For a-C:H, prepared on c-Si substrates from CH₄ by direct ion beam deposition, ellipsometry measurements in the initial stages of growth provide monolayer sensitivity to the formation of an absorbing SiC_x layer at the substrate interface. Fits to the data in the later stages of growth establish the real and imaginary parts of the bulk dielectric function at 3.2 eV, allowing real time categorization of the nature of the bonding in such films.     

Effects of interface on the dielectric properties of Ba0.6Sr0.4TiO3 thin film capacitors

Ba_0.6Sr_0.4TiO₃ thin films were deposited on Pt/SiO₂/Si substrate by radio frequency magnetron sputtering. High-resolution transmission electron microscopy (HRTEM) observation shows that there is a transition layer at BST/Pt interface, and the layer is about 7-8 nm thickness. It is found that the transition layer was diminished to about 2-3 nm thickness by reducing the initial RF sputtering power. X-ray photoelectron spectroscopy (XPS) depth profiles show that high Ti atomic concentration results in a thick interfacial transition layer. Moreover, the symmetry ν of ?_r-V curve of BST thin film is enhanced from 52.37 to 95.98%. Meanwhile, the tunability, difference of negative and positive remanent polarization (P_r), and that of coercive field (E_C) are remarkably improved.     

Application of spectroscopic ellipsometry to the investigation of the optical properties of cobalt-nanostructured silica thin layers

Spectroscopic ellipsometry is used to investigate optical properties of cobalt-implanted silica thin films. The films under investigation are 250 nm thick thermal SiO₂ layers on Si substrates implanted with Co⁺ ions at energy of 160 keV and at fluences of 10¹⁷ ions/cm² for different temperatures of substrate during implantation (77 and 295 K). Changes due to Co⁺ implantation are clearly observed in the optical response of the films. Optical behaviours are furthermore different for the three implantation temperatures. To understand the optical responses of these layers, the ellipsometric experimental data are compared to different models including interference effects and metal inclusions effects into the dielectric layer. The simulated ellipsometric data are obtained by calculating the interferences of an inhomogeneous layer on a Si substrate. The material within this layer is considered as an effective medium which dielectric function is calculated using the Maxwell-Garnett effective medium approximation. We show that although the structures of these layers are very complicated because of ion-implantation mechanisms, quite simple models can provide relatively good agreement. The possibilities of ellipsometry for the study of the optical properties of such clusters-embedded films are discussed. We especially provide the evidence that ellipsometry can give interesting information about the optical properties of nanostructured layers. This is of special interest in the field of nanostructured layered systems where ellipsometry appears to be a suitable optical characterization technique.     

Optical parameters of epitaxial GaN thin film on Si substrate from the reflection spectrum

A method has been proposed for determining the optical properties of a thin film layer on absorbing substrates. The film optical parameters such as thickness, refractive index, absorption coefficient, extinction coefficient and the optical energy gap of an absorbing film are retrieved from the interference fringes of the reflection spectrum at normal incidence. The envelopes of the maxima of the spectrum E_M and of the minima E_m are introduced in analytical forms to find the reflectance amplitudes at the interfaces and approximate values of the thin film refractive index. Then, the interference orders and film thickness are calculated to get accurate values of the needed optical parameters. There are no complex fitting procedures or assumed theoretical refractive index dispersion relations. The method is applied to calculate the optical properties of an epitaxial gallium nitride thin film on a silicon (1 1 1) substrate. Good agreement between our results and the published data are obtained.     

Optical switch with low-phase transition temperature based on thin nanocrystalline VOx film

An optical switch is fabricated by using micromachining technology, which is based on thin nanocrystalline vanadium oxide (VO_x) film, and it consists of four layers: a silicon (Si) substrate layer, a VO_x layer, a Si₃N₄ buffer layer, and an aurum (Au) electrode layer. By applying a switching power supply to a pair of the Au electrodes, the optical switch is controlled to exhibit from an “on” state with semi-conducting phase to an “off” state with metallic phase. The optical switch performance is investigated, and testing results show that its extinction ratio is about 14 dB, its switching response time can achieve about 1.5 ms, and the power dissipation required for stimulating switching to work can be below about 15 mW at least, which is lower than the power dissipation of conventional optical switches based on microstructure thin vanadium dioxide (VO₂) films. This kind of optical switch is potential to be applied as optical switch for optical communication.     

Surface plasmons in small metal particles

The dielectric response of a small metal particle to a perturbing potentialv _L =r ^L Y _LM e ^iωt is considered within the random phase approximation (RPA). The static dielectric polarizability is found and the size dependence of the surface plasmon frequencies are then determined from sum rule calculations. When the particle radiusa is large compared to the Thomas-Fermi screening lengthr ₀ the RPA equation is transformed into a form appropriate for an analytical solution. The dynamic electric polarizability, the position and the width of the surface plasma resonance are found in the limita/r ₀?1.     

Numerisch genaue Lösungen des nichtlokalen Schalenmodells

The nonlocal Schrödinger-equation has been solved in the shell model case with a kernel-function, which is well known from optical model calculations. Now in the eigenvalue region the energies, as well as the wave-functions, of the exact and the approximate nonlocal calculations (E andE ₀) differ slightly on the average. The exact solutions show in general larger energy-level distances. Although these differences are small, fitting of experimental neutron binding-energies requires nevertheless a relatively big change of the set of physical parameters; the exact nonlocal calculations tend to larger potential depthV and smaller radius constantr ₀ than the approximate nonlocal ones. For the wave-functions (Φ andΦ ₀) it can be shown that an application of the Perey-effect leads to a wrong result in the shell model case. Here the difference function shows two inflection points, whereas there is only one in the optical model. The form of nuclear matter distribution thus becomes more potential-like. The eigenfunctions of the exact nonlocal calculation are orthogonal, whereas the local equivalent ones are not.     

Specific features of magnetooptical spectra of Co/SiO2 hybrid multilayers

Magnetooptical spectra of Co(x)/SiO₂(y) bilayers and [Co(x)/SiO₂(y)]_n hybrid multilayers (x and y are the layer thicknesses) are studied in the range 1.5–4.0 eV. In these layers, the ultrathin ferromagnetic layers are not continuous but consist of ellipsoidal Co nanoparticles with a concentration near the percolation threshold. It is found that (a) the sign, magnitude, and spectral shape of the magnetooptical signal depend strongly on the Co particle size; (b) the signal for the [Co(1.6 nm)/SiO₂(3.0 nm)]₆ multilayers at 3.0 eV has a record-high magnitude for Co-based structures, 3×10^?2, which exceeds that for bulk Co about fivefold and about 50-fold that for a uniform Co film of an equivalent thickness; (c) the magnetooptical signal of multilayers depends non-linearly on the number of periods and the thickness of the dielectric layer y; and (d) the dependence of the signal of bilayers and multilayers with x=1.3 and 1.6 nm on the wavelength is nonmonotonic and exhibits clearly pronounced extrema. The data obtained for bilayers are attributed to the strong influence of percolation on the optical and magnetooptical parameters of the structure and interpreted in the framework of the effective medium approximation and macroscopic Fresnel magnetooptics.     

Growth and electric properties of (100)-oriented Mn-doped (Bi0.5Na0.5)TiO3-BaTiO3 thin film by pulsed laser deposition

Mn-doped (Bi_0.5Na_0.5)TiO₃-BaTiO₃ (BNBMT) thin film with the composition near the morphotropic phase boundary was grown on (La_0.6Sr_0.4)CoO₃-electroded SrTiO₃ single-crystal substrate by using pulsed laser deposition method. Ascribed to the crystal structure and lattice similarity, (100)-oriented BNBMT film with pure single-phase perovskite structure was obtained through carefully controlling the growth conditions. Enhanced ferroelectric and dielectric properties were obtained with large remanent polarization P _r of ～21 μC/cm², coercive field E _c of ～7.4 kV/mm and dielectric constant $\varepsilon_{33}^{T}/\varepsilon_{0}$ of ～750 at 1 kHz. The excellent global electrical properties of the BNBMT film are promising for environmentally friendly ferroelectric devices.     

Optical band-edge absorption of oxide compound SnO2

Tin oxide (SnO₂) is an important oxide for efficient dielectrics, catalysis, sensor devices, electrodes and transparent conducting coating oxide technologies. SnO₂ thin film is widely used in glass applications due to its low infra-red heat emissivity. In this work, the SnO₂ electronic band-edge structure and optical properties are studied employing a first-principle and fully relativistic full-potential linearized augmented plane wave (FPLAPW) method within the local density approximation (LDA). The optical band-edge absorption α(ω) of intrinsic SnO₂ is investigated experimentally by transmission spectroscopy measurements and their roughness in the light of the atomic force microscopy (AFM) measurements. The sample films were prepared by spray pyrolysis deposition method onto glass substrate considering different thickness layers. We found for SnO₂ qualitatively good agreement of the calculated optical band-gap energy as well as the optical absorption with the experimental results.     

Obtaining three surface-film parameters from two ellipsometric measurements

A powerful approximation method for obtaining the thickness, d, and optical constants, n₂ and k₂, of a thin absorbing film on a known opaque substrate is described. The advantages of the method for certain ellipsometer systems are discussed. Instead of requiring a third measurement, for example the reflectance, |r|, as is often done, the effective reflectance, |r_eff|, is calculated in the two-phase approximation from the measured ellipsometric parameters. This provides the third quantity needed to obtain n₂, k₂ and d. Examples worked out for the systems Ag₂S/Ag and CoO/Co illustrate the accuracy and utility of the method. The technique is shown to be applicable to a wide range of films, roughly $\text{n}{}_2\text{? 2.0}$, k₂ ? 1.0 and $\text{d ? 40}\text{A}\text{?}$ on semiconducting and highly reflecting metal substrates and 2.0 ? n₂ ? 2.7, k₂ ≈ 0.5, and $\text{d ? 10}\text{A}\text{?}$ on less highly reflecting metal substrates. The important systems GeO₂/Ge, SiO₂/Si and Al₂O₃/Al are well within these limits. When the method is used in these ranges, the errors resulting from the approximation are less than those that would be introduced by a measurement of reflectance change on most ellipsometer systems.