Ellipsometric Measurements Research Articles

Optical Calibration of a Multi-Color Ellipsometric Mapping Tool Fabricated Using Cheap Parts

We developed and applied a new calibration method to make more accurate measurements with our multi-color ellipsometric mapping tool made from cheap parts. Ellipsometry is an optical technique that measures the relative change in the polarization state of the measurement beam induced by reflection from or transmission through a sample. During conventional ellipsometric measurement, the data collection is relatively slow and measures one spot at a time, so mapping needs a long time compared with our new optical mapping equipment made by an ordinary color LED monitor and a polarization-sensitive camera. The angle of incidence and the incident polarization state is varied point by point, so a special optical calibration method is needed. Three SiO2 samples with different thicknesses were used for the point-by-point determination of the angle of incidence and rho (ρ) corrections. After the calibration, another SiO2 sample was measured and analyzed using the calibrated corrections; further, this sample was independently measured using a conventional spectroscopic ellipsometer. The difference between the two measured thickness maps is less than 1 nm. Our optical mapping tool made from cheap parts is faster and covers wider area samples relative to conventional ellipsometers, and these correction enhancements further demonstrate its performance.

Theoretical Model for Determining the Thickness and Optical Constants of Transparent Conducting Oxide Thin Films From the Measured Reflectance Spectra

ABSTRACTThickness and optical constants of transparent conducting oxide (TCO) of metal oxide thin films have been commonly estimated by invoking theoretical models based on measured optical reflectance or transmittance. One of such widely employed models is reflectance‐based Kushev model. However, the relatively higher value of the absorption coefficient at the absorption edge of TCOs of metal oxide thin films results in a large deviation of their thickness values estimated by this model. To address the underlying problems, in the present work, we have improved the Kushev model. Specifically, for minimizing the error in the estimated thickness values from the proposed model, we have employed t test for discarding outliers of a set of thickness values of a given thin film, which were calculated from nearby extrema of a reflectance curve. Then the thickness calculated from model was validated by comparing the value of ZnO thin transparent films extracted from ellipsometric measurement. Our model shows excellent agreement with the thickness values of ZnO films from ellipsometric measurement with only 1.2% deviation. Furthermore, observed values were also compared with the values from already established Cauchy model. The perfectly matching of the simulated reflectance spectra from the present model with the measured reflectance and excellent agreement of thickness values derived from the ellipsometric measurements with the values extracted from the model may infer us that the present model is suitable for fairly accurately estimating film thicknesses of TCO thin films.

Engineering Thermoresponsive Poly(N-isopropylacrylamide)-Based Films with Enhanced Stability and Reusability for Efficient Bone Marrow Mesenchymal Stem Cell Culture and Harvesting.

Poly(N-isopropylacrylamide) (PNIPAM) offers a promising platform for non-invasive and gentle cell detachment. However, conventional PNIPAM-based substrates often suffer from limitations including limited stability and reduced reusability, which hinder their widespread adoption in biomedical applications. In this study, PNIPAM copolymer films were formed on the surfaces of glass slides or silicon wafers using a two-step film-forming method involving coating and grafting. Subsequently, a comprehensive analysis of the films' surface wettability, topography, and thickness was conducted using a variety of techniques, including contact angle analysis, atomic force microscopy (AFM), and ellipsometric measurements. Bone marrow mesenchymal stem cells (BMMSCs) were then seeded onto PNIPAM copolymer films prepared from different copolymer solution concentrations, ranging from 0.2 to 10 mg·mL-1, to select the optimal culture substrate that allowed for good cell growth at 37 °C and effective cell detachment through temperature reduction. Furthermore, the stability and reusability of the optimal copolymer films were assessed. Finally, AFM and X-ray photoelectron spectroscopy (XPS) were employed to examine the surface morphology and elemental composition of the copolymer films after two rounds of BMMSC adhesion and detachment. The findings revealed that the surface properties and overall characteristics of PNIPAM copolymer films varied significantly with the solution concentration. Based on the selection criteria, the copolymer films derived from 1 mg·mL-1 solution were identified as the optimal culture substrates for BMMSCs. After two rounds of cellular adhesion and detachment, some proteins remained on the film surfaces, acting as a foundation for subsequent cellular re-adhesion and growth, thereby implicitly corroborating the practicability and reusability of the copolymer films. This study not only introduces a stable and efficient platform for stem cell culture and harvesting but also represents a significant advance in the fabrication of smart materials tailored for biomedical applications.

Wide spectral range optical characterization of niobium pentoxide ([formula omitted]) films by universal dispersion model

In this work, the optical properties of niobium pentoxide (Nb2O5) films were extensively studied across a wide spectral range using heterogeneous data-processing methods, combining ellipsometric and spectrophotometric measurements for five samples with thicknesses between 20 and 250 nm. This study primarily determined the optical constants of Nb2O5 from the far infrared to the vacuum ultraviolet, presenting these constants as dispersion parameters using the universal dispersion model to describe valence electron excitations in ultraviolet region as well as phonon vibrations in infrared region. These comprehensive and reliable data across such a broad spectral range are unprecedented. Secondly, presented optical characterization proofs that Nb2O5 films can be grown without defects such as surface roughness, porosity, or inhomogeneity. This fact, together with its high refractive index, makes Nb2O5 a promising material for optical applications.

Artificial Intelligence-Enhanced Metasurfaces for Instantaneous Measurements of Dispersive Refractive Index.

Measurements of the refractive index of liquids are in high demand in numerous fields such as agriculture, food and beverages, and medicine. However, conventional ellipsometric refractive index measurements are too expensive and labor-intensive for consumer devices, while Abbe refractometry is limited to the measurement at a single wavelength. Here, a new approach is proposed using machine learning to unlock the potential of colorimetric metasurfaces for the real-time measurement of the dispersive refractive index of liquids over the entire visible spectrum. The platform with a proof-of-concept experiment for measuring the concentration of glucose is further demonstrated, which holds a profound impact in non-invasive medical sensing. High-index-dielectric metasurfaces are designed and fabricated, while their experimentally measured reflectance and reflected colors, through microscopy and a standard smartphone, are used to train deep-learning models to provide measurements of the dispersive background refractive index with a resolution of ≈10-4, which is comparable to the known index as measured with ellipsometry. These results show the potential of enabling the unique optical properties of metasurfaces with machine learning to create a platform for the quick, simple, and high-resolution measurement of the dispersive refractive index of liquids, without the need for highly specialized experts and optical procedures.

Optical Properties of Tlinse2<Au> Single Crystals

This paper presents the results of studying the surface microrelief in 2D and 3D models and analyzing the spectroscopy of a three-junction TlInSe2<Au> crystal. Analysis of the results obtained showed that with a change in the composition of the TlInSe2<Au> crystal, sharp changes occur in the microrelief of its surface. An X-ray optical diffraction analysis of the TlInSe2<Au> crystal was experimentally carried out. Based on ellipsometric data, optical functions were determined - the real and imaginary parts of the dielectric permittivity of crystals, the coefficients of optical absorption and reflection, the dependence of energy losses and electric field power on the effective density, the spectral dependences of the real (σr) and imaginary (σi) parts, optical electrical conductivity were experimentally studied. The fluorescence spectra of the ternary compound TlInSe2<Au> were isolated and analyzed when excited by light with a wavelength of 532 nm. X-ray studies of TlInSe2<Au> showed that this phase crystallizes into tetragonal systems. Ellipsometric measurements showed that the real (ε1) and imaginary (ε2) parts of the dielectric constant are components of the dielectric constant tensor of the uniaxial joints under consideration and do not depend on the angle. Analysis of the dependence of the real and imaginary parts of the refractive index of the TlInSe2<Au> crystal on photon energy showed that the nature of the change in the real and imaginary parts of the dielectric constant does not differ significantly. When analyzing the spectral dependences of the real (σr) and imaginary (σi) parts of the optical electrical conductivity, it was noticed that the real part of the optical electrical conductivity increases exponentially in the energy range 0.894-3.505 eV. In the energy range of 0.654-2.91 eV, the imaginary part of the optical electrical conductivity increases linearly, reaches a maximum value, and decreases at an energy of 2.91 eV. At 3.6 eV, an inversion of the imaginary part of the optical electrical conductivity of the TlInSe2<Au> compound is observed. From the graphs of the effective power density versus electric field energy losses, it is known that the effective power density increases significantly in the energy range of 0.805–3.52 eV. The fluorescence spectrum of the ternary compound TlInSe2<Au> upon excitation with light with a wavelength of 532 nm has been studied and it has been established that this phase has luminescent properties.

Analytic ellipsometric measurement for materials under bulk encapsulation

Accurate measurement of the dielectric functions of emerging optical materials is of great importance for advancements in solid-state physics. However, it is rather challenging since most materials are highly active in ambient conditions, which makes in-situ measurements tough. Here, we report an analytical ellipsometry method (AEM) accessible in ambient conditions for measuring the dielectric functions of chemically reactive materials under bulk encapsulation. Taking the highly pursued low-loss plasmonic materials, such as sodium films, as an example, the effectiveness and measuring errors of the proposed AEM have been systematically demonstrated. This verifies AEM’s superiority in overcoming the limitations of traditional spectroscopic ellipsometry methodologies, which include complex multi-parameter fitting procedures and the issue of potentially unphysical results, especially in newly developed low-loss materials. Our results will provide a generalized and convenient ellipsometric measurement strategy for sensitive materials under bulk encapsulation.

Incoherent partial superposition modeling for single-shot angle-resolved ellipsometry measurement of thin films on transparent substrates

Ellipsometric measurement of transparent samples suffers from substrate backside reflection challenges, including incoherent and partial superposition issues. The recently developed angle-resolved ellipsometry (ARE) can naturally eliminate the backside reflections of substrates with a micro-spot equivalent thickness or thicker; however, for thinner substrates, ARE working with general incoherent backside reflection models shows significant inaccuracy or measurement failure. In this paper, an incoherent partial superposition (IPS) model is proposed to characterize the optical superposition effect between the frontside and uncertain backside reflections from an unknown substrate. IPS introduces a cosine-like correction of the backside reflection, corresponding to the overlapping-area change of backside and frontside reflections along with incident angles. Benefiting from ARE’s wide-angle spectral imaging capability, IPS achieves single-shot measurement of thin film thicknesses on transparent substrates of unknown thickness. An ARE system was built and calibrated regarding the linear relationship between the cosine-corrected angular frequencies and substrate thicknesses. Then, commercial ITO films on glasses of different thicknesses ranging from 200 to 1000 µm were measured. Experimental results show that IPS-ARE results in a root-mean-square accuracy error of ∼1 nm in film thickness measurement and provides a ∼77% error reduction from general incoherent backside reflection models.

RI−Calc: A user friendly software and web server for refractive index calculation

The refractive index of an optical medium is essential for studying a variety of physical phenomena. One useful method for determining the refractive index of scalar materials (i.e., materials which are characterized by a scalar dielectric function) is to employ the Kramers−Kronig (K−K) relations. The K−K method is particularly useful in cases where ellipsometric measurements are unavailable, a situation that frequently occurs in many laboratories. Although some packages can perform this calculation, they usually lack a graphical interface and are complex to implement and use. Those deficiencies inhibit their utilization by a plethora of researchers unfamiliar with programming languages. To address the aforementioned gap, we have developed the Refractive Index Calculator (RI−Calc) program that provides an intuitive and user−friendly interface. The RI−Calc program allows users to input the absorption coefficient spectrum and then easily calculate the complex refractive index and the complex relative permittivity of a broad range of thin films, including of molecules, polymers, blends, and perovskites. The program has been thoroughly tested, taking into account the Lorentz oscillator model and experimental data from a materials' refractive index database, demonstrating consistent outcomes. It is compatible with Windows, Unix, and macOS operating systems. You can download the RI−Calc binaries from our GitHub repository or conveniently access the program through our dedicated web server at nanocalc.org.

Computational and experimental approach for investigation of optical properties of single-crystal MnGa2Se4

In the presented work, we propose a systematic investigation of the electronic and optical properties of MnGa2Se4 by combining the spectroscopic ellipsometry carried out at room temperature in the spectral range of 1–4 eV and density functional theory (DFT) calculations. Ellipsometric measurements were collected at an incidence angle between 60° and 75° with a 5° step. The optical direct gap energies of MnGa2Se4 were estimated from the linear extrapolation of the absorption coefficients that were verified by band structure calculation. DFT calculation of the imaginary part ɛ shows that first critical point occurs at 2.46 eV and represents the direct optical transition at Γ point. The total magnetic moment of compound is 3.38 mB. The major contribution in total magnetic moment is due to 3 d state of Mn atoms.

UV–VIS-NIR optical properties of micrometric-thick polyimide membranes for lightweight devices in space

The optical properties of micrometric-thick polyimide membranes were analyzed between 200 nm and 2500 nm to assess their potential use as substrates for the fabrication of wide optical devices for space applications. These membranes were prepared from fully aromatic polyimides containing trifluoromethyl functional groups which have already been used in space and have shown excellent resistance to the space environment. Various membranes have been synthesized using two different solvents and thermal treatments. Spectrophotometric and ellipsometric measurements were performed to estimate the spectral refractive index (n) and extinction coefficient (k) of various membranes synthesized using a standard solvent, such as N,N-dimethylacetamide (DMAc) and an alternative bio-based solvent, that is dimethyl isosorbide (DMI). The investigation has been extended to consider the morphological properties of the surface and the consequences they have on the diffusion of incident light. This work demonstrates that it is possible to use a green and non-hazardous solvent for the synthesis of thin polyimide membranes, even in the perspective of a large-scale manufacturing process that is greener and more sustainable. Using similar synthesis methods, the surface topography of the polyimide obtained with DMI is found to be smoother. Results suggest that membranes obtained by this bio-based solvent can also be used for optical devices where the scattered component of reflected and transmitted radiation needs to be minimized.

Ellipsometry of ultrathin transparent films. Some aspects of optimum experimental conditions choice

Reflection ellipsometry in visible and infrared range, which enables the analysis of isotropic thin films, is one approach that is extremely sensitive and accurate. There are a few restrictions with this technique when analyzing ultra-thin transparent film (UTTF) structures. In our study, the issue of the independent determination of two parameters of a transparent film that is extremely thin (d < 10 nm) is examined from the standpoint of the optimal conditions choosing for an ellipsometric experiment. An original method to calculation the errors in the main parameters of UTTF depending on the experimental conditions is proposed. One analyzed the influence of the substrate optical parameters and the incidence angle of light on the sensitivity of ellipsometric data. The analysis shows that under certain experimental conditions, the sensitivity of ellipsometric measurements allows to independently determine both the refractive index and the thickness of a transparent film down to 3 nm thick.

The Optical Properties of Thin Film Alloys of ZnO, TiO2 and ZrO2 with Al2O3 Synthesised Using Atomic Layer Deposition

In this work, the results of ellipsometric studies of thin films of broadband oxides (ZnO, TiO2, ZrO2) and broadband oxides doped with Al2O3 (Al2O3–ZnO, Al2O3–TiO2, Al2O3–ZrO2) are presented. All layers have been produced using the atomic layer deposition method. Ellipsometric studies were performed in the wavelength range of 193–1690 nm. Sellmeier and Cauchy models were used to describe the optical properties of the tested layers. Dispersion dependencies of refractive indices were determined for thin layers of broadband oxides on silicon substrates, and then for layers of Al2O3 admixture. The EDX investigations enabled estimation of the composition of the alloys. The Bruggeman effective medium approximation (EMA) model was used to determine the theoretical dependencies of the dispersion refractive indices of the studied alloys. The refractive index values determined using the Bruggeman EMA model are in good agreement with the values determined from the ellipsometric measurements. The doping of thin layers of ZnO, ZrO2 and TiO2 with Al2O3 enables the creation of anti-reflective layers and filters with a specific refractive index.

Study of reactive electron beam deposited tantalum penta oxide thin films with spectroscopic ellipsometry and atomic force microscopy

The optical and surface morphological properties of thin films are profoundly influenced by the deposition technique and the various process parameters employed. In present work, several tantalum penta oxide thin films have been fabricated using reactive electron beam evaporation at varying oxygen (O2) pressures, ranging from 3 × 10−3 Pa to 8 × 10−2 Pa. Refractive index, extinction coefficient, and voids in thin film microstructure were determined through the analysis of spectroscopic ellipsometric measurements. The refractive index, which exhibits a decreasing trend, varies between 1.859 and 2.067, with the exception being the lowest O2 pressure. The variation has been explained in terms of varying oxygen content and voids in the thin films. The extinction coefficient exhibits a decreasing trend due to the increasing oxidation of tantalum, except at O2 pressure of 8 × 10−2 Pa. Increase in the extinction coefficient at this pressure could be due to porous film microstructure, resulting in dominant light scattering. Variation of film density which is similar to that of refractive index, has been attributed to the varying oxidation of tantalum (Ta) and film porosity. Surface root mean square (rms) roughness, correlation length and Hurst parameter, have been derived from suitable analysis of atomic force microscopy measurements. Variations of correlation length and rms roughness have been explained in terms of varying surface shadowing effects and oxidation of Ta with varying O2 pressure.

Tailoring crystallisation of anatase TiO2 ultra-thin films grown by atomic layer deposition using 2D oxides as growth template

TiO2 ultrathin films are required in many material research areas. The anatase phase was found to be more stable in case of atomic layer deposition growth, nevertheless a critical thickness of around 10 nm appears necessary to obtain crystallization on native silicon oxide on Si (001). This work focuses on direct liquid injection Atomic Layer Deposition (ALD) of TiO2 films with thicknesses about some nanometers, using titanium tetra-isopropoxide as precursor and H2O as oxidant. A particular care to the treatment of ellipsometric measurements is employed. Below the threshold value, films remain amorphous unless the growth starts onto a crystalized surface, a thick TiO2 layer in our case. Here we propose and show the efficiency of 2-dimensional (2D) [Ca2Nb3O10]− and [Ti0.865O2]0.54− nanosheets as seeds at the surface for the crystallization and epitaxy of ultra-thin films of TiO2 below the critical thickness. Furthermore, we show that the structure of the 2D nanosheets determines the growth orientation of the epitaxial anatase, namely (010) for [Ti0.865O2]0.54−, and (100) for [Ca2Nb3O10]−. These results, based on the tailoring of crystal anisotropy via adapted 2D seed nanosheets and low temperature ALD, opens the way for opto-electronics applications of titanium oxide layers of a few unit cells.

Study of Structure, Morphology and Optical Properties of Cobalt-Doped and Co/Al-co-Doped ZnO Thin Films Deposited by Electrospray Method

A versatile electrospray method was utilized for deposition of thin ZnO films doped with Co (5%) (CZO) or co-doped with Co (2.5%) and Al (2.5%) (CAZO). Thin polycrystalline films with approximate thickness of 200 nm and high transmittance (more than 80%) were obtained. No additional XRD peaks due to dopant impurities or dopant oxides were observed. The cobalt doping led to decrease in grain size and increase in crystallite size from 22 nm to 29 nm in the (101) direction. Smaller changes were observed for the CAZO films. Surface roughness of the films was measured using a 3D optical profiler. Surface roughness of the doped films was from 5 nm to 9 nm higher than that of the pure films. Refractive index, extinction coefficient and thickness of the films were calculated using ellipsometric measurements and were further used for determination of optical band gap and Urbach energy. Refractive index and optical band gap increased with doping from 1.86 and 3.29 eV for pure ZnO to 2.00 and 3.35 eV for CZO and 1.97 and 3.33 eV for CAZO films. Through calculation of Urbach energy (119 meV for pure ZnO, 236 meV for CZO and 138 meV for CAZO), it was demonstrated that doping leads to an increase in structural disorder, most pronounced in the case of Co doping.

Temperature-dependent optical dispersion spectra for a single CeO2 thin-film layer in UV–vis and its reflection and transmission in the IR region

CeO2 is a rare-earth refractory material with high refractive index, high thermal stability and infrared transparency; this knowledge can be utilized to develop practical thin films for high temperature thermophotovoltaics (TPVs) and thermal barrier coatings (TBCs). Here, we report ellipsometric measurements of a CeO2 thin film in the wavelength range 210 nm–2500 nm from room temperature to 500 ∘C. Because most previous spectroscopic studies of CeO2 evaluated its optical properties only at room temperature, our findings provide insights into the potential impacts of temperature change on the aforementioned applications, induced by the changes in its electronic structure. We also provide the reflectance and transmittance measurements of the same CeO2 thin film at room temperature using Fourier transform infrared (FTIR) spectroscopy. This knowledge can be leveraged to develop TPV and TBC designs. Finally, we used a software package called Stanford Stratified Structure Solver (S4) to simulate the angle-dependent reflectance spectrum. The optical properties of CeO2 films measured by ellipsometry are used as inputs to predict reflectance spectra. Comparison of the experimental FTIR reflectance with simulated reflectance in S4 shows strong agreement at three different angles.

Preparation of Concanavalin A Imprinted Surface Plasmon Resonance Based Biosensors

In this study, Concanavalin A imprinted surface plasmon resonance (SPR) biosensor was prepared for the determination of Concanavalin A. Concanavalin A imprinted SPR biosensors were characterized by atomic force microscopy, ellipsometer and contact angle measurements. The kinetic studies were performed with pH scanning. For this purpose, Concanavalin A solutions were prepared with different pH buffers and were used for detection of Concanavalin A. The kinetic analysis of SPR biosensor was with Concanavalin A solutions ranging from 0.01 to 50 ng/mL. The limit of detection and limit of quantification was calculated 0.0011 ng/mL and 0.0038 ng/mL, respectively. Selectivity studies were conducted using SPR biosensors that were either Concanavalin A imprinted or non-imprinted, as well as competing molecules such as bovine serum albumin and myoglobin, which have a similar molecular structure. In order to determine Concanavalin A in clinical studies, kinetic analyzes were performed with concanavalin a solutions prepared in artificial plasma and jack bean solutions. The recovery of Concanavalin A was calculated using the obtained sensorgram data. Four consecutive analyses were performed on the same day to test the concanavalin A molecule’s reusability. In order to test the reusability of the biosensor at various points in time, kinetic analyses were also performed out at the first, second, fourth, and sixth months.

Chemico-Physical Properties of Some 1,1'-Bis-alkyl-2,2'-hexane-1,6-diyl-bispyridinium Chlorides Hydrogenated and Partially Fluorinated for Gene Delivery.

The development of very efficient and safe non-viral vectors, constituted mainly by cationic lipids bearing multiple charges, is a landmark for in vivo gene-based medicine. To understand the effect of the hydrophobic chain's length, we here report the synthesis, and the chemico-physical and biological characterization, of a new term of the homologous series of hydrogenated gemini bispyridinium surfactants, the 1,1'-bis-dodecyl-2,2'-hexane-1,6-diyl-bispyridinium chloride (GP12_6). Moreover, we have collected and compared the thermodynamic micellization parameters (cmc, changes in enthalpy, free energy, and entropy of micellization) obtained by isothermal titration calorimetry (ITC) experiments for hydrogenated surfactants GP12_6 and GP16_6, and for the partially fluorinated ones, FGPn (where n is the spacer length). The data obtained for GP12_6 by EMSA, MTT, transient transfection assays, and AFM imaging show that in this class of compounds, the gene delivery ability strictly depends on the spacer length but barely on the hydrophobic tail length. CD spectra have been shown to be a useful tool to verify the formation of lipoplexes due to the presence of a "tail" in the 288-320 nm region attributed to a chiroptical feature named ψ-phase. Ellipsometric measurements suggest that FGP6 and FGP8 (showing a very interesting gene delivery activity, when formulated with DOPE) act in a very similar way, and dissimilar from FGP4, exactly as in the case of transfection, and confirm the hypothesis suggested by previously obtained thermodynamic data about the requirement of a proper length of the spacer to allow the molecule to form a sort of molecular tong able to intercalate DNA.

Interband Electron Transitions Energy in Multiple HgCdTe Quantum Wells at Room Temperature

The studies of the interband electron transition energy in multiple Hg1-xCdxTe/Hg1-yCdyTe quantum wells (MQWs) at room temperature were carried out. The MQWs were grown on the (013) GaAs substrate by molecular beam epitaxy, with the layer composition and thickness being measured by the in-situ ellipsometric parameters measurement at the nanometer level. The Hg1-xCdxTe barrier composition and width were x = 0.69 and 30 nm, respectively. The Hg1-yCdyTe well composition was y = 0.06–0.10, and the width varied in the range of 2.7–13 nm. The experimental data of the interband electron transition energy were determined by the absorption spectral analysis. The calculation of the interband electron transition energy was carried out on the basis of the four-band Kane model. A good agreement between the experimental and calculated data was obtained. It was shown that MQWs may be used as a photosensitive material for creating infrared optoelectronic devices operating in different modes in the range of 3–10 μm at room temperature.