Inhomogeneous Films Research Articles

Optical characterization of inhomogeneous thin films containing transition layers using the combined method of spectroscopic ellipsometry and spectroscopic reflectometry based on multiple-beam interference model

This paper presents the results of the optical characterization of inhomogeneous thin films of polymer-like SiOxCyHz and non-stoichiometric silicon nitride SiNx. An efficient method combining variable angle spectroscopic ellipsometry and spectroscopic reflectometry applied at the near-normal incidence based on the multiple-beam interference model is utilized for this optical characterization. The multiple-beam interference model allows us to quickly evaluate the values of ellipsometric parameters and reflectance of the inhomogeneous thin films, which exhibit general profiles of their optical constants. The spectral dependencies of the optical constants of the inhomogeneous SiOxCyHz and SiNx thin films are determined using the Campi–Coriasso dispersion model. The profiles of the optical constants of these films can also be determined. Furthermore, the transition layers at the lower boundaries of the characterized films are also taken into account. Spectral dependencies of the optical constants of these transition layers are also determined using the Campi–Coriasso dispersion model.

Characterization of optically inhomogeneous polymer layers with silver nanoparticles by spectroscopic ellipsometry

In this work it was shown the applicability of the spectroscopic ellipsometry technique for characterization of an optically inhomogeneous polymer film with silver NPs. The histogram of silver NPs distribution based on SEM images shows the largest number of NPs have a size in the range 15-35 nm and the filling factor as large as 6%. By using the possibilities of ellipsometry it was suggested a 3-layer inhomogeneous structure with different sizes of silver NPs covered by thin polyamide-6 film. Also existence of localized plasmon resonance in such NPs was observed by optical absorption measurements.

Non-destructive chemical and phase layer profiling of multicomponent multilayer thin ultrathin films

This work covers a method of non-destructive layer profiling of ultra-thin films on solid. The method is based on solution of the problem of elastic and inelastic photoelectron scattering in multilayer inhomogeneous films. An example of depth profiling of an air-oxidized ultra-thin chromium film on a silicon substrate is given.

Theoretical modeling and experimental validation for the effective thermal conductivity of moist silica aerogel

In this paper, the influence of water content on insulating performance of moist silica aerogel nano-porous composite material is studied theoretically and experimentally. A new fractal-intersecting sphere with inhomogeneous water film structural model is proposed. It considers the microstructures in aerogel and analysis of condensation process in moist environment based on surface physics theory. The effective thermal conductivity of composite aerogel material has been also derived using the equivalent circuit method. The nano-scale heat transfer effect and the influence of micron-scale additives such as opacifiers and reinforced fibers are involved in the present theoretical model. The theoretical model is semi-empirical because some parameters are introduced to better predict the effective thermal conductivity based on partial experimental data at a reference temperature. Experiments were conducted to measure thermal conductivities of nano-porous aerogel composites within temperature range of 15–45 °C and water uptake range of 0–110 mg/g by the transient plane source method. Validation study for both current samples and material in Zhang et al. (2015) was carried out, where the relative errors between experimental thermal conductivity and theoretical calculations are less than 7% and 8%, respectively. The prediction accuracy of present model for this super insulating material is much higher than that of Bjurström’s model which are widely used to calculate effective thermal conductivity for common porous media containing moisture. In addition, the results show that the increase of thermal conductivity with water uptake is linear at the same temperature. Lower porosity would result in a greater sensitivity to moisture content for aerogel material. Comparing to the influence of various additives, the properties of silica aerogel matrix is the most dominant factor in determining the effective thermal conductivity under different water content.

Light induced degradation of the transport length of CH3NH3PbI3 studied by modulated surface photovoltage spectroscopy after Goodman

Abstract Transport length is one of the most important parameters of any absorber material in a solar cell. It affects the design and performance of solar cells given that it determines the photogeneration of charge carriers in the device. In this work, we studied the transport length of CH3NH3PbI3 perovskite layers deposited on sputtered TiO2 under blue light soaking using surface photovoltage (SPV) spectroscopy developed after Goodman. SPV developed after Goodman is a direct, sensitive and accurate method for determining the transport length of thin absorber layers. However, the method has been inadequately utilized. In the study, we used blue light for light soaking due to its short wavelength that can probe the surface properties of the perovskite layers in comparison to red light. We observed light induced degradation of transport length within 12 h of blue light soaking in TiO2/CH3NH3PbI3 layers. This is attributed to trap states at TiO2/CH3NH3PbI3 interface which create defect states that increases the recombination rate. In addition, the decrease in the transport length within 12 h of light soaking can be due to the charging and discharging effects at TiO2/CH3NH3PbI3 interface. With prolonged light soaking, we observe an increase in transport length, which is probably due to generation of compensating defects that neutralizes trap states at the TiO2/CH3NH3PbI3 interface. Above 12 h of light soaking, we observe dependence of transport length with surface photovoltage spectroscopy. This is probably related to change in quasi fermi level splitting in the space charge region which arises due to the in-homogeneity of the perovskite layers. We conclude that the strong variation of Transport length with SPV signals give an indication of the presence of defects and in-homogeneity of the perovskite films.

Approximate methods for the optical characterization of inhomogeneous thin films: Applications to silicon nitride films

Abstract In this paper the overview of the most important approximate methods for the optical characterization of inhomogeneous thin films is presented. The following approximate methods are introduced: Wentzel–Kramers–Brillouin–Jeffreys approximation, method based on substituting inhomogeneous thin films by multilayer systems, method based on modifying recursive approach and method utilizing multiple-beam interference model. Principles and mathematical formulations of these methods are described. A comparison of these methods is carried out from the practical point of view, ie advantages and disadvantages of individual methods are discussed. Examples of the optical characterization of three inhomogeneous thin films consisting of non-stoichiometric silicon nitride are introduced in order to illustrate efficiency and practical meaning of the presented approximate methods.

Plastic deformation of a film-substrate with inhomogeneous inclusions under contact loading

In this paper, a semi-analytic solution is developed to investigate the plastic deformation of a film-substrate with inhomogeneous inclusions subjected to contact loading. In this solution, the surface pressure distribution and contact area can be determined by solving a set of governing equations via a modified conjugate gradient method. The inhomogeneous inclusions and the coating material are modeled as homogeneous inclusions with known initial eigenstrains plus unknown equivalent eigenstrains, according to the Eshelby’s equivalent inclusion method. A plasticity loop and an incremental loading process are used to obtain the accumulative plastic strain iteratively. This model considers not only the interactions among the contact loading body, embedded inhomogeneous inclusions and film materials, but also the plastic deformation of the film-substrate system. This solution is of great significance to understand the plastic deformation mechanism of a film-substrate with inhomogeneous inclusions under contact loading.

Understanding the influences of temperature and microstructure on localized corrosion of subsea pipeline weldment using an integrated multi-electrode array

The localized corrosion behaviour of an X65 subsea pipeline weldment in CO2-saturated formation water was studied using weldment coupon and a novelly designed weldment multi-electrode array (WMEA). The dynamic progression of localized corrosion on different welding areas i.e. base metal (BM), heat affected zone (HAZ) and welding metal (WM) was probed by WMEA. The influences of the solution temperature and the steel microstructure on the localized corrosion of subsea pipeline weldment were analysed. The test results show that HAZ and WM would act as anodes registering a high corrosion rate which was controlled by both micro-cell corrosion and macro-cell corrosion below 50 °C. While, if the temperature increased higher than 50 °C, WM would transfer from major anode to significant cathode due to the preferential deposition of FeCO3 crystal on WM. Part areas of HAZ would act as the main anodic sites and maintain a high local anodic dissolution rate due to the macro-cell current which is induced by the inhomogeneous FeCO3 film covering.

Automated Laser Ablation of Inhomogeneous Metal Oxide Films to Manufacture Uniform Surface Temperature Profile Electrical Heating Elements

This paper presents automated laser ablation strategies to improve the temperature distribution across the surface of inhomogeneous Ni-Fe-Cr-NiO electrical heating elements during joule heating. A number of iterative closed-loop laser control algorithms have been developed and analyzed in order to assess their impact on the efficacy of the heating element, in terms of homogeneous temperature control, and on the implications for automated fabrication of inhomogeneous metal oxide films. Analysis shows that the use of the leading method, i.e., use of a temperature-dependent variable-power approach with memory of previous processes, showed a 68% reduction in the standard deviation of the temperature distribution of the heating element and a greater uniformity of temperature profile as compared to existing manual methods of processing.

Contactless monitoring method both carbon-chain thermoplastic thickness and microstructure inhomogeneity as applicable to the blown type extruders

The polymer films unevenness within the range of 5-10 µm arises from the blown-extrusion method production features that limits the end-product feasibility using in a number of industry areas. Current contactless inspection methods ignore the fact of optical density sensible difference presence to the different wavelength range radiation at the various stretching stages. In the study the pleochroism effect appearing in the extrusion films, produced by polymer melt orientation stretching, is proposed to use as a base for both unevenness and microstructural film inhomogeneity complex monitoring method. The unevenness monitoring effectiveness according to CIE La*b*- colour difference of polymer film areas is shown. The device design layout with the built-in mounting possibility with no necessity of significant extrusion line upgrade is offered.

Incoherent scattering from dielectric metasurfaces under the influence of electromagnetic eigenmodes.

Anomalous optical properties of microscopically inhomogeneous dielectric films placed on a thick metal sublayer are investigated. We study the reflection, scattering, and absorption of the coherent electromagnetic radiation as a function of the incidence angle. Computer simulations show the existence of the incidence angle of the laser beam when the scattering and absorption increase simultaneously for the s-polarization so that almost 60% of the incident light goes in the scattering channel. The critical angle corresponds to the excitation of Fabry-Perot mode. The effect makes it possible to manipulate the reflection from the metafilms.

Charge‐Carrier Dynamics, Mobilities, and Diffusion Lengths of 2D–3D Hybrid Butylammonium–Cesium–Formamidinium Lead Halide Perovskites

AbstractPerovskite solar cells (PSCs) have improved dramatically over the past decade, increasing in efficiency and gradually overcoming hurdles of temperature‐ and humidity‐induced instability. Materials that combine high charge‐carrier lifetimes and mobilities, strong absorption, and good crystallinity of 3D perovskites with the hydrophobic properties of 2D perovskites have become particularly promising candidates for use in solar cells. In order to fully understand the optoelectronic properties of these 2D–3D hybrid systems, the hybrid perovskite BAx(FA0.83Cs0.17)1‐xPb(I0.6Br0.4)3 is investigated across the composition range 0 ≤ x ≤ 0.8. Small amounts of butylammonium (BA) are found that help to improve crystallinity and appear to passivate grain boundaries, thus reducing trap‐mediated charge‐carrier recombination and enhancing charge‐carrier mobilities. Excessive amounts of BA lead to poor crystallinity and inhomogeneous film formation, greatly reducing effective charge‐carrier mobility. For low amounts of BA, the benevolent effects of reduced recombination and enhanced mobilities lead to charge‐carrier diffusion lengths up to 7.7 µm for x = 0.167. These measurements pave the way for highly efficient, highly stable PSCs and other optoelectronic devices based on 2D–3D hybrid materials.

Determination of optical parameters of lithium niobate films by srectrophotometry

Lithium niobate films on silicon substrates were synthesized by high−frequency magnetron sputtering of a target. The resultant film was a layer of polycrystalline lithium niobate. By the method of spectrophotometry we obtained the spectral dependences of the reflectance in the wavelength range 300—700 nm at small angles of incidence. The angular dependence of p− and s− polarized light were measured for a discrete set of wavelengths from 300 to 700 nm increments of wavelength 50 nm and increments for angles of 1°. The values of the refractive indicies, film thickness and extinction coefficients were determined using a numerical method for solving inverse problems. As the film is absorbing we accepted the simulation optical system as an isotropic monolayer absorbing film on a semi-infinite absorbing substrate with a sharp interface. Initial approximation for the solution of inverse problems were defined by the methods based on the estimation of the interference extrema position in the reflection-angular spectra. Values of the refractive indicies of the film differ from the values typical for LiNbO3 single crystals obtained both from the reference literature, and by refractive indices direct goniometric method measurements of a certified standard enterprise sample (SES) made from a lithium niobate single crystal. We additionally studied the specimens with X−ray diffraction and scanning probe microscopy. These deviations are attributed to the film inhomogeneity, the presence of the second phase, and disordering of the structure. Inclusions of the second phase in the form of crystallites with a predominant orientation along the Z axis are observed.

Spin-Wave Resonance Detection of Nanostructured Magnetic Alloy Inhomogeneities, Using the Example of Co–P and Co–Ni Planar Systems

Inhomogeneous layered magnetic thin films of amorphous and nanocrystalline Со–Р and Co–Ni alloys are studied via spin-wave resonance. It is found that the formation of a magnetic potential profile specified over the coating thickness leads to characteristic modifications of the spin-wave resonance spectrum. Another important factor that determines the type of modification is the dominant magnetic parameter (the constant of magnetization or exchange coupling).

High-energy density in Si-based layered nanoceramic/polymer composites based on gradient design of ceramic bandgaps

Abstract It is difficult to simultaneously achieve a high electrical breakdown strength, dielectric permittivity, discharged energy density and charge-discharge efficiency by simply blending electrically conductive nanoparticles with insulating polymers in large-scale preparations. In this work, a family of gradient sandwich-structured Si-based semiconducting nanoparticle/fluoropolymer nanocomposite films with a high breakdown and good energy storage performances was fabricated using a triple solution casting process by fine tuning the type and volume concentration of the Si-based nanofiller used in each layer of the sandwich films. Differing from traditional homogeneous composite films with mono-layered structures, novel inhomogeneous composite films with gradient sandwich structures were fabricated. This fabrication was achieved by implementing a gradient-design strategy used to produce high breakdown layer-by-layer composite dielectrics and the induced polarization tactics used to produce high-permittivity Si-based semiconductor/polymer composite dielectrics. Beta-Si 3 N 4 , alpha-SiC and monocrystalline Si nanoparticles (all ∼100 nm) were introduced into the upper, middle and bottom layers of the fluoropolymer matrices, respectively. The volume concentrations of the Si-based nanofillers in each of the three layers were controlled, and 1 vol% filler in each layer was found to be optimal. In this case, the preferred electric field applied to each layer due to the gradient design could result in the highest breakdown strength (360 MV m −1 ) of the composite. The interface blocking effect present between two adjacent layers might inhibit the electrical tree growth between the two electrodes, contributing to a significantly enhanced breakdown property. As a result, a discharged energy density of 13 J cm −3 and a charge-discharge efficiency of 67% at 350 MV m −1 were obtained. This work might pave the way for the large-scale preparation of high-performance nanocomposite dielectrics based on the interface blocking effect.

Global modeling of wall material migration following boronization in NSTX-U

Abstract Boronization is commonly utilized in tokamaks to suppress intrinsic impurities, most notably oxygen from residual water vapor. However, this is a temporary solution, as oxygen levels typically return to pre-boronization levels following repeated plasma exposure. The global impurity migration model WallDYN has been applied to the post-boronization surface impurity evolution in NSTX-U. A “Thin Film Model” has been incorporated into WallDYN to handle spatially inhomogeneous conditioning films of varying thicknesses, together with an empirical boron conditioning model for the NSTX-U glow discharge boronization process. The model qualitatively reproduces the spatial distribution of boron in the NSTX-U vessel, the spatially-resolved divertor emission pattern, and the increase in oxygen levels following boronization. The simulations suggest that oxygen is primarily sourced from wall locations without heavy plasma flux or significant boron deposition, namely the lower and upper passive plates and the lower private flux zone.

Modeling of Label-Free Optical Waveguide Biosensors with Surfaces Covered Partially by Vertically Homogeneous and Inhomogeneous Films

Optical Waveguide Lightmode Spectroscopy (OWLS) is widely applied to monitor protein adsorption, polymer self-assembly, and living cells on the surface of the sensor in a label-free manner. Typically, to determine the optogeometrical parameters of the analyte layer (adlayer), the homogeneous and isotropic thin adlayer model is used to analyze the recorded OWLS data. However, in most practical situations, the analyte layer is neither homogeneous nor isotropic. Therefore, the measurement with two waveguide modes and the applied model cannot supply enough information about the parameters of the possible adlayer inhomogeneity and anisotropy. Only the so-called quasihomogeneous adlayer refractive index, layer thickness, and surface mass can be determined. In the present work, we construct an inhomogeneous adlayer model. In our model, the adlayer covers the waveguide surface only partially and it has a given refractive index profile perpendicular to the surface of the sensor. Using analytical and numerical model calculations, the step-index and exponential refractive index profiles are investigated with varying surface coverages from 0 to 100%. The relevant equations are summarized and three typically employed waveguide sensor structures are studied in detail. We predict the errors in the calculated optogeometrical parameters of the adlayer by simulating the OWLS measurement on an assumed inhomogeneous adlayer. We found that the surface coverage has negligible influence on the calculated refractive index below film thicknesses of 5 nm; the calculated refractive index is close to the refractive index of the adlayer islands. But the determined quasihomogeneous adlayer refractive index and surface mass are always underrated; the calculated quasihomogeneous thickness is heavily influenced by the surface coverage. Depending on the refractive index profile, waveguide geometry, and surface coverage, the thickness obtained from the homogeneous and isotropic modeling can even take negative and largely overestimated values, too. Therefore, experimentally obtained unrealistic adlayer values, which were dismissed previously, might be important indicators of layer structure.

Approximations of reflection and transmission coefficients of inhomogeneous thin films based on multiple-beam interference model

A multiple-beam interference model is used to derive approximate formulae for the reflection and transmission coefficients of inhomogeneous thin films exhibiting large gradients of refractive index profiles. It is shown that these formulae are constituted by series containing the Wentzel-Kramers-Brillouin-Jeffreys term and correction terms with increasing order corresponding to number of considered internal reflections inside the films. A numerical analysis enabling us to show the influence of a degree of inhomogeneity on spectral dependencies of reflectance and ellipsometric parameters of inhomogeneous films is performed. Advantages and disadvantages of our approach compared with other approximate approaches are discussed. The optical characterization of a selected non-stoichiometric silicon nitride film prepared by reactive magnetron sputtering onto silicon single crystal substrate is performed for illustration of using our formulae in practice.

Strain engineering in perovskite solar cells and its impacts on carrier dynamics

The mixed halide perovskites have emerged as outstanding light absorbers for efficient solar cells. Unfortunately, it reveals inhomogeneity in these polycrystalline films due to composition separation, which leads to local lattice mismatches and emergent residual strains consequently. Thus far, the understanding of these residual strains and their effects on photovoltaic device performance is absent. Herein we study the evolution of residual strain over the films by depth-dependent grazing incident X-ray diffraction measurements. We identify the gradient distribution of in-plane strain component perpendicular to the substrate. Moreover, we reveal its impacts on the carrier dynamics over corresponding solar cells, which is stemmed from the strain induced energy bands bending of the perovskite absorber as indicated by first-principles calculations. Eventually, we modulate the status of residual strains in a controllable manner, which leads to enhanced PCEs up to 20.7% (certified) in devices via rational strain engineering.

Technique for Calculating the Critical Current of Inhomogeneous Superconducting Films

A method for calculating the critical current of an inhomogeneous superconducting film (plate) in different external magnetic fields has been proposed. The superconducting properties are changed by varying the coherence length and London penetration depth over the thickness. The coherence length is maximum at the center of the plate and decreases upon approaching its boundaries and the London depth, on the contrary, is minimum at the center of the plate and increases toward its boundaries. Using the proposed approach, the magnetic field dependences of the critical current on the external magnetic field for the cases of nonuniform and uniform distributions of the superconducting properties over the film thickness have been calculated and compared. It has been established that at the nonuniform distribution of the superconducting properties the critical current of the plate is higher than at the uniform distribution at the same external magnetic fields. This is due to the fact that the order parameter is redistributed over the inhomogeneous plate thickness in such a way that the superconducting state becomes more stable against the current and magnetic field. It has been shown that the vortex-free Meissner state in inhomogeneous films is maintained at higher fields than in homogeneous films. The greater the film nonuniformity, the higher the stability of the Meissner state against an external magnetic field.