Values Of Optical Constants Research Articles

Influence of the surface roughness and oxide surface layer onto Si optical constants measured by the ellipsometry technique

Si crystal surface after chemical etching was studied using ellipsometry, atomic force microscopy and scanning tunneling microscopy. The ellipsometric parameters as functions of light incidence angles at two light wavelengths 546.1 and 296.7 nm were measured. The calculations based on equations for the plane surface have shown that the refractive index and absorption coefficient values are different from those determined earlier. Two models for surface layers were developed. After etching, the upper layer contains chemical compounds and the lower layer characterizes the sample roughness. By applying Airy's formula to ellipsometric data, optical constants and thicknesses of the layers were obtained. The calculated values of bulk Si optical constants wholly correspond to the data from literature. The calculated thickness of the lower layer is similar to that obtained through scanning tunneling microscopy measurements. Calculations based on Maxwell-Garnett and Bruggeman equations were performed to determine the content of silicon particles within the lower rough layer.

Nanometer thick tunable AlHfN coating for solar thermal applications: Transition from absorber to antireflection coating

A novel AlHfN(H)/AlHfN(L) coating with tunable optical properties was designed and developed for high temperature solar thermal applications. The transition from absorber to antireflection behavior was achieved by varying the chemical composition of AlHfN coatings. The aluminum and hafnium target power densities and nitrogen flow rates were varied in order to attain the different chemical compositions of AlHfN layers, which resulted in different optical constant values. The ellipsometric measurements indicated that bottom AlHfN(H)) is the main absorber layer and the top AlHfN(L) acts as an antireflection coating. The AlHfN(H)/AlHfN(L) coating deposited on stainless steel (SS) substrates exhibited absorptance of 0.93 with an emittance of 0.13. In order to decrease the emittance, approximately 1μm thick tungsten coating was deposited on SS substrates. The tungsten coating acts as an infrared reflector due to which the emittance decreased drastically from 0.13 to 0.06, without affecting the absorptance. The optimized W/AlHfN(H)/AlHfN(L) coating deposited on SS substrates was thermally stable in air at 425°C for 2h. These coatings also exhibited high thermal stability in vacuum at 580°C for 260h.

Optical properties of zirconium carbide in 60-200 Å wavelength region using x-ray reflectivity technique.

Optical constants of zirconium carbide (ZrC) have been determined in the soft x-ray region of 60-200 Å wavelength using angle-dependent x-ray reflectivity measurements. Reflectivity measurements are carried out at the reflectivity beamline of the Indus-1 synchrotron radiation source. Derived optical constants (δ and β) are compared with the tabulated values [At. Data Nucl. Data Tables 54, 181 (1993)]. The optical constants are 15%-35% lower than bulk values in the 60-200 Å wavelength region. Near the Zr M4 edge 187 eV (66.3 Å), the δ values were close to bulk values with a deviation of 5%-10%. A large deviation of ∼20% was found in beta values especially near the Zr M4 edge region, whereas it was in close agreement in the wavelength range away from the edge. To the best of our knowledge, this paper gives the first reported experimental values of optical constants for zirconium carbide in the 60-200 Å wavelength region.

Optical and structural properties of amorphous Se x Te100-x aligned nanorods

In the present work, we report studies on optical and structural phenomenon in as-deposited thin films composed of aligned nanorods of amorphous SexTe100-x (x = 3, 6, 9, and 12). In structural studies, field emission scanning electron microscopic (FESEM) images suggest that these thin films contain high yield of aligned nanorods. These nanorods show a completely amorphous nature, which is verified by X-ray diffraction patterns of these thin films. Optical studies include the measurement of spectral dependence of absorption, reflection, and transmission of these thin films, respectively. On the basis of optical absorption data, a direct optical band gap is observed. This observation of a direct optical band gap in these nanorods is interesting as chalcogenides normally show an indirect band gap, and due to this reason, these materials could not become very popular for semiconducting devices. Therefore, this is an important report and will open up new directions for the application of these materials in semiconducting devices. The value of this optical band gap is found to decrease with the increase in selenium (Se) concentration. The reflection and absorption data are employed to estimate the values of optical constants (extinction coefficient (k) and refractive index (n)). From the spectral dependence of these optical constants, it is found that the values of refractive index (n) increase, whereas the values of extinction coefficient (k) decrease with the increase in photon energy. The real and imaginary parts of dielectric constants calculated with the values of extinction coefficient (k) and refractive index (n), are found to vary with photon energy and dopant concentration.

Effects of dispersion and deformation histories on rheology of semidilute and concentrated suspensions of multiwalled carbon nanotubes

A new sonication method involving in situ measurement of electrical conductivity along with the characterization of dynamic properties was developed to better control the dispersion of multiwalled carbon nanotubes (CNTs) in polymeric liquids. Using this new method, CNTs were incorporated into monoacrylate-terminated poly(ethylene glycol) at CNT volume fractions, ϕ, in the semidilute (3.7 × 10−4 and 5.5 × 10−3) and concentrated (1.1 × 10−3, 1.6 × 10−3 and 2.7 × 10−3) regimes. This dispersion methodology enabled the preparation of stable CNT suspensions, with reproducible electrical conductivity and dynamic properties, to serve as specimens for subsequent steady and oscillatory shear, and compressive squeeze flows that were carried out in combination with optical imaging and continuous monitoring of their electrical conductivity. During small-amplitude oscillatory shear, the networks established by well-dispersed CNTs remained stable, as evidenced by optical imaging and constant values of electrical conductivity and dynamic properties. The demixing of CNTs and formation of flocculates upon stacking of the CNTs into helical bands in the vorticity direction occurred over a wide range of CNT concentrations, apparent shear rates, and rheometer gaps during steady torsional and cone-and-plate flows and gave rise to precipitous decreases in electrical conductivity, dynamic properties, shear viscosity, and first normal stress difference (which reached negative values). On the other hand, electrical conductivity and dynamic properties were observed to increase modestly during squeeze flow, which occurred free of the strong demixing effects observed in simple shear flows. Regardless of the type of deformation, quiescent relaxation following deformation gave rise to the increase of electrical conductivity and dynamic properties to asymptotic values. Overall, these findings shed additional light into the percolating network formation and associated viscoelastic behavior and electrical conductivity of CNT suspensions and demonstrate the significant sensitivities of their ultimate properties to their preparation and subsequent deformation histories.

In situ growth and ab initio optical characterizations of amorphous Ga3Se4 thin film: A new chalcogenide compound semiconductor thin film

This paper reports the first systematic study of the structural and optical characterization of Ga3Se4 thin film. Powder X-ray diffractometery analysis revealed that the films are amorphous. Optical reflection and transmittance measurements have been done by UV–visible–near-infrared spectrophotometry, and the values of various optical constants, such as the lower cut-off wavelength (550 nm), optical band gap (2.14 eV), dispersive energy (6.75 eV), oscillator energy (2.86 eV), static refractive index (1.83) and static dielectric constant (3.35), have been estimated.

Fine structures in refractive index of sapphire at the L(II,III) absorption edge of aluminum determined by soft x-ray resonant reflectivity.

The optical constants of sapphire crystal (α-Al(2)O(3)) and amorphous Al(2)O(3) in the soft x-ray region (67-85 eV) around the aluminum LII,III absorption edge (73.1 eV) are determined by angle-dependent x-ray reflectivity. The differences between the optical constant values of both the samples are discussed. The fine structures obtained in the absorption of crystalline sapphire are explained. An absorption feature at 70.2 eV is observed for the first time for crystalline alumina. Both datasets are compared to the tabulated values of Henke et al. [At. Data Nucl. Data Tables 54, 181 (1993)], Weaver et al. [Physik Daten, Physics Data: Optical Properties of Metals (Fach-information zentrum, 1981), Vols. 18-1 and 18-2], and [Handbook of Optical Constants of Solids II (Academic, 1991)].

Nonlinear optical constants of ionic conductors: a study based on the Sheik‐Bahae equation

AbstractGuided by the prediction of the bond fluctuation model of superionic conductors, the relation between the nonlinear optical constants and the ion transport properties in ionic conductors has been studied. Since the measured values of nonlinear optical constants in ionic conductors are very limited, they have been evaluated through the Sheik‐Bahae equation. Using such values, it is shown that the activation energy of ion transport and the superionic transition temperature decrease with the increase of the nonlinear refractive index. It is also pointed out that the band gap energy and the linear refractive index in superionic conductors are relatively weakly correlated when compared with non‐superionic materials. The development of a new field of study that could be called photoionics is suggested (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Surface topography and optical properties of Ge-Sb(As)-S-Te thin films by atomic-force microscopy and variable angle spectroscopic ellipsometry

Thin films were evaporated from powdered GexSb(As)40−xS50Te10 (x = 10, 20 and 27) glassy materials and their optical properties and surface morphology were studied by spectroscopic ellipsometry, performed in the UV-VIS-NIR range, and AFM imaging. For both kinds of quaternary systems, the optical constants (n and k) values decreased with increasing the Ge content in the films, their values being smaller for the GexAs40−xS50Te10 compositions. All films were transparent (k = 0) above 800 nm. The optical band gap energy increased with the Ge content; its value was larger for the GexAs40−xS50Te10 compositions. The AMF images revealed surfaces which are cracks-free and fully covered with uniformly distributed grains having particular structure depending on the film composition. For all the evaporated films, the average mean square (RMS) roughness did not exceed 5 nm, giving evidence for sufficiently high smoothness.

Correlation between the nonlinear optical constants and the structural relaxation parameters in chalcogenide glasses

It is well known that chalcogenide glasses exhibit high values of nonlinear optical constants in addition to the widely studied photostructural changes. However, the relations between these properties are not sufficiently understood. On the other hand, according to the bond strength-coordination number fluctuation (BSCNF) model proposed by one of the authors, the viscous behavior is described in terms of the average bond strength, coordination number, and their fluctuations of the structural units that form the system. In the present study, the interrelation between the nonlinear optical constants and the structural relaxation parameters defined from the BSCNF model has been investigated for the case of chalcogenide systems. It is found that the third order nonlinear optical susceptibility increases with the fluctuations of the structural relaxation parameters. The correlation found is discussed based on the BSCNF model and the average electronegativity.

Optical constants and band gap expansion of size controlled silicon nanocrystals embedded in SiO 2 matrix

Silicon nanocrystals (Si-NCs) with different sizes embedded in SiO 2 matrix were synthesized by phase separation and thermal crystallization of SiO x/SiO 2 supperlattice approach. The optical constants and band gap expansion of Si-NCs have been investigated by spectroscopic ellipsometry, based on the Maxwell–Garnett effective medium approximation and the Forouhi–Bloomer optical dispersion model. Similar spectra shapes but smaller values of Si-NCs optical constants with respect to bulk crystalline Si is observed. With the size of Si-NCs decreasing from 6 nm to 2 nm, the band gap increases from 1.64 eV to 2.56 eV. The band gap expansion, as compared to bulk crystalline Si, which agrees with the prediction of first-principles calculations based on quantum confinement effect, is presented in this paper.

Study of the optical response of Si-rich a-SiNx : H thin film near Si L2,3-edge using soft x-ray reflectivity

Angle-dependent soft x-ray reflectivity measurements in the photon energy range 82.67–248 eV near the Si L2,3 absorption edge have been performed on Si-rich a-SiNx : H (SRSN) thin film deposited by the Hg-sensitized photo-chemical vapour deposition technique. It is found that experimentally obtained δ (dispersion) values of the SRSN film lie in between that of pure silicon (Si) and silicon nitride (Si3N4). X-ray photoelectron spectroscopy measurements suggest that excess silicon is responsible for reducing optical constants values and hence by controlling the silicon amount one can tune the optical constants to desired values. We further compare the soft x-ray optical spectrum of the SRSN film with those of other silicon compounds such as silicon carbide (SiC) and silicon monoxide (SiO) near the Si L2,3 edge region to shows its possibility as a prospective material for the next generation 13.5 nm (91.85 eV) node extreme ultraviolet (EUV) lithography technology.

Detection of the Nanostructures in ZnO Thin Films by Using Optical Methods

The ZnO films deposited by reactive magnetron sputtering are investigated. SEM provides information on the composition of the films and enables us to observe the nanostructures. AFM provides information on the roughness and the texture of the surfaces. Fourier transform, photoluminescence and Raman scattering are used for confirmation of appropriate results. The spectral dependences of ellipsometric parameters Δ and Ψ are determined by ellipsometry and the effective values of optical constants n and k are calculated. As it can be indicated, the nanostructures in ZnO film are formed when reducing base pressure down to 2.5•10-7 mbar and elevating the temperature of the substrate up to 5500C. The amorphous structure is formed in most of other films (crystalline structure is formed at rather special conditions). Films containing considerable portions of impurities (generally carbon impurities) are formed at reduced base pressure. The values of optical constants of some of the films fit the appropriate Dumont and Jellison tabulations satisfactorily.

Characterization of As–Se–Tl films near infrared region

ِAmorphous chalcogenide of As–Se–Tl glassy alloys was prepared by melt quenching technique. Films were deposited on a transparent glass substrate using thermal evaporation technique. The optical characterization of thin As–Se–Tl films was studied by the analysis of transmission spectra, measured at normal incidence, in the spectral range 200–3000. The envelope method, proposed by Swanepoel, is a commonly used method for studying optical properties of films with uniform and non uniform thickness. Thickness values as well as optical constants, refractive index, Dispersion energy, Urbach and static refractive index or infinite wave length dielectric constant, are calculated using Wemple–DiDomenico model near infrared region using the envelopes of the transmittance spectrum of the films. The values of optical constants: absorption coefficient, Urbach tail, refractive index etc. have been determined. Urbach tail is estimated using Tauc's extrapolation and is found to increase from 0.69 to 0.91 eV with the Tl addition. This behavior of Urbach tail is interpreted in terms of Tauc's Model. Other optical constants are interpreted in terms of Wemple–DiDomenico model, chemical bond approach, electro-negativity difference of the atoms involved and cohesive energy of the system.

Optical properties of ZnTe and ZnS nanocrystals by critical-points and Tauc–Lorentz models

The optical properties of ZnTe and ZnS nanocrystals (ZnTe-NC and ZnS-NC) were determined by Spectroscopic Ellipsometry. The nanocrystals were embedded in a SiO2 matrix by ion implantation technique. Their sizes were characterized by transmission electron microscopy. The ZnTe-NC and ZnS-NC were modelled using Critical Points (CPs) dispersion formulas developed by Adachi. Besides the CPs model, the Tauc–Lorentz model was found to be another choice to get a good spectral fitting. Here we demonstrated that these models yield reasonable values of optical constants of II–VI nanocrystals. The best agreement was found with the experimental data over the entire range of 0.6 to 6.5eV.

Chiroptical Study of α-Aliphatic Amino Acid Films in the Vacuum Ultraviolet Region

A series of natural circular dichroism (CD) and absorption spectra for films of α-aliphatic amino acids--such as alanine, aminobutyric acid, norvaline, norleucine, valine, leucine, and isoleucine--in the vacuum ultraviolet (VUV) region were observed with the absolute values of optical constants at the undulator-based CD beamline TERAS BL5. Preliminary predictions of some CD spectra were also performed, based on quantum-chemical calculations using the crystal structure. Although the absorption spectra show similar features to each other, significant differences between the CD spectra were found, especially in the 7-8 eV region. The CD spectra of aliphatic amino acids with branched alkyl groups in the side-chain--such as valine, leucine, and isoleucine--exhibit strong negative CD peaks in this energy region. In contrast, the corresponding CD peaks were weak or absent in the spectra of amino acids with straight alkyl groups. Our simple calculation, and the absorption spectra of alkanes, suggest that this difference partly originates from the contribution of the alkyl group. Clear discrepancies between the CD spectra of these amino acids in solutions and those in the solid state were also observed; this is probably caused by the different molecular structures in each state. Our results clearly indicated that CD spectra in the VUV region were very sensitive to the conformations of chiral molecules.

Determination of optical properties and thickness of nanolayers from the angular dependences of reflectance

The possibility of multiparametric determination of properties of structures from the data on the dependence of reflectances Rp and Rs for polarized radiation and ratio Rp/Rs on the angle of incidence θ and from the angular dependence 1/Rp(ΔR/Δθ) is investigated. The results of natural and computer experiments revealed a high sensitivity of the angular dependence Rp(θ) of the reflectance to the values of optical constants and thickness of layered structures. Quantitative results of multiparametric measurements are verified by the independent method of spectral ellipsometry. The possibility of multiparametric determination of the properties and thickness of nanometer dielectric, metallic, and semiconducting layers on various substrates and the properties of substrates in such structures is established.

Optical properties of indium phosphide in the 50–200Å wavelength region using a reflectivity technique

The optical constants of indium phosphide (InP) in the soft x-ray region of 50-200 Å are determined from angle-dependent reflectivity measurements. The measurements are carried out using the reflectivity beam line at the Indus-1 synchrotron source. The derived optical constants are compared with tabulated values of Henke et al. [At. Data Nucl. Data Tables 54, 181 (1993)]. Experimental values of δ and β are in close agreement with the tabulated values in the lower wavelength region of 50-120 Å. The experimental value indicates an edge shift of 0.4 Å toward the lower wavelength side from the phosphorous L-edge value of 92 Å. However, above the 120 Å region, where the indium N(2) edge falls at 160.7 Å, there is a huge difference between experimental and tabulated values. Both delta and beta values are significantly higher. In contrast to tabulated values of the β/δ ratio, which is more than 1 above the 140 Å region, the experimental measured ratio is found to be less than 1. This study presents the first reported experimental values of optical constants for InP in this wavelength range, to the best of our knowledge.

Formation of a plane layer of plasma under irradiation by a soft X-ray source

We investigate theoretically the formation of a plasma in a plane layer of polymer foam (density ρ = 0.002 g/cm3 and thickness 800 μm) under the action of an external source of soft X-ray radiation under the conditions of PHELIX experiments. The incident flux is assumed to have a Planck’s distribution over the spectrum with Trad = 20–40 eV. In numerical calculations, the flux of incident X-ray radiation and the spectral constants of the target substance are varied. The action of an external X-ray radiation source on a low-density foam substance with a density of 2 mg/cm3 causes a plasma to be formed with relatively homogeneous profiles of density and temperature T = 15–35 eV. Absorption of externalradiation energy is distributed in the volume. The plasma temperature increases with increase in the external energy, and the energy passed through the plasma also increases. The results prove to be sensitive to the values of optical constants used in numeral simulation. The spectral flux of external radiation passed through the plasma is chosen as a criterion of correctness of the optical constants used in the calculations. In future experiments using the PHELIX facility, we plan to investigate the slowing-down of an ion beam in a plasma formed as a result of indirect heating of low-density polymer triacetate cellulose (TAC) foam with densities ρ = 0.001–0.01 g/cm3 under the action of a pulse of X-ray radiation, into which the laser radiation is preliminarily transformed.

Nonlinear Optical Constants of Superionic Conductors: A Study from the Bond Orbital Theory

According to the bond fluctuation model of superionic conductors, the polarizability of a material that exhibit high ionic conductivity must be large. That is, the model predicts that ion conducting materials must have large values of optical constants. This prediction has been confirmed recently, through a comparative study of the nonlinear optical constants of ion conducting and non-conducting glasses. In the present report, the values of the nonlinear optical constants χ (3) of R 2 O–B 2 O 3 (R = Rb, Cs, Ag) glasses have been estimated by using the bond orbital theory. For the Rb 2 O–B 2 O 3 and Cs 2 O–B 2 O 3 glasses, a good agreement between the calculated and the measured values has been found. On the other hand, for the Ag 2 O–B 2 O 3 system, a deviation was observed. Such a disagreement has been explained by taking into account the contribution of d-electrons of the Ag ions, which plays also a fundamental role in the ion transport mechanism.