Optical properties of AsSe1.5−xTex glassy system

Thin AsSe_1.5−xTe_x films with 0x<1.5 have been prepared by a thermal vacuum evaporation technique onto quartz and glass substrates kept at room temperature (300 K). The optical constants, the refractive index, n, and the absorption index, k, of the films were determined for the investigated compositions of different thickness values (100–300 nm) using spectrophotometric measurements of the transmittance, T, and the reflectance, R, at normal incidence in the spectral range 400–2500 nm. The obtained values of both n and k were found to be independent of the film thickness within the above mentioned thickness range. The estimated indirect and direct optical energy gap decreased as tellurium content increased in the parent sample AsSe_1.5. The values of dispersion energy, E_d, and lattice dielectric constant, _L, of the system have been determined and correlated with the type and amount of chemical bonds and the relative proportion of the constituent elements in the examined compositions.     

Dispersion studies and electronic transitions in nickel phthalocyanine thin films

Thin films of the organic semiconductor nickel phthalocyanine (NiPc) are structurally investigated using X-ray diffraction and infrared light absorption. The optical absorption and dispersion studies of nickel phthalocyanine were investigated using spectrophotometric measurements of transmittance and reflectance at normal incidence in the wavelength range 190–2100 nm. The absorption spectra recorded in the UV-VIS region show two well-defined absorption bands of the phthacyanine molecules, namely, the Soret and the Q-band. The Davydove splitting of the main absorption peak in the metal phthalocyanines correlates with the relative tendencies of the metal to out-of-plane bonding. The refractive index n as well as the absorption index k were calculated and showed an independent of the film thickness in the film thicknesses range 400–770 nm. The refractive index n showed an anomalous dispersion in the absorption region as well as normal dispersion in the transparent region. Some of the important optical absorption such as the molar extinction coefficient, the oscillator strength, the electric dipole strength have been evaluated. The analysis of the spectral behavior of the absorption coefficient α in the absorption region revealed two indirect allowed transitions with corresponding energies 2.77±0.03 and 1.66±0.02 eV. An energy band diagram has been proposed to account for the optical transitions of NiPc thin film. All previous parameters were as well obtained for films annealed at 523 K for 2 h. Discussion of the obtained results and their comparison with the previously published data are also given.     

Optical properties of nanostructured InSe thin films

Thin films of InSe were prepared by thermal evaporation technique. The as-deposited films have nano-scale crystalline nature and the annealing enhanced the degree of crystallinity. The optical properties of nanocrystalline thin films of InSe were studied using spectrophotometric measurements of transmittance, T, and reflectance, R, at normal incidence of light in the wavelength range 200–2500 nm. The optical constants (refractive index, n, and absorption index, k) were calculated using a computer program based on Murmann's exact equations. The calculated optical constants are independent of the film thickness. The optical dispersion parameters have been analysed by single oscillator model. The type of transition in InSe films is indirect allowed with a value of energy gap equals to 1.10 eV, which increased to 1.23 eV upon annealing.     

Cu膜的光学特性尺寸效应及最小连续膜厚研究

采用离子束溅射在K9玻璃基底上沉积了不同厚度的Cu膜,利用Lambda-900分光光度计,测量了波长为310 nm到1300 nm范围内Cu膜的反射率和透射率.选定波长为310、350、400、430、550、632、800、1200 nm时对薄膜的反射率、透射率和吸收率随膜厚变化的关系进行研究.同时,对Cu膜的光学常量也进行了讨论.结果显示,Cu膜的光学特性都有明显的尺寸效应.将波长为550 nm时的反射率和透射率随Cu膜厚度变化关系的交点对应厚度作为特征厚度, 该厚度可认为是金属Cu膜生长从不连续膜进入连续膜的最小连续膜厚.根据这一特征判据,离子束溅射沉积Cu膜样品的最小连续膜厚为33 nm.利用原子力显微镜观测了膜厚在特征厚度附近时Cu膜的表面形貌.     

Novel optical film sensor design based on p-polarized reflectance

A new scheme of optical film sensor is presented. The sensor is based on p-polarized reflectance, consisting of a sensing coated substrate, is easily optimized for maximum sensitivity in different applications. The resolutions of refractive index n_f, extinction coefficient k_f and thickness h_f of the sensitive films are predicted to be 10⁻⁷, 10⁻⁵ and 10⁻³ nm, respectively. Experimentally, we selected the sol–gel derived SnO₂ films as gas-sensitive films and conducted preliminary gas-sensing test. The results indicate that novel optical film sensor scheme has higher sensitivity, and the detection sensitivity is available to 10⁻¹ ppm on the condition of optimum optical parameters and incident angle.     

Optical constants of nanosized films of metal titanium

A new ellipsometric technique for optical measurements of the refraction and extinction indices and thickness of nanosized metal-titanium films in air is proposed and implemented. It is shown that the determination of optical constants of titanium films by measuring the ellipsometric parameters Ψ and Δ for a light beam incident upon the metal/substrate interface through the substrate of fused silica allows one to obtain optical constants of the metal, which correlate well with the results of the most reliable measurements performed in vacuum chambers. In this case, one can additionally determine the thickness and refractive index of the natural oxide film on the titanium surface. The obtained values of the optical constants of titanium, n ₂ = 3.42 ± 0.05 and k ₂ = 3.75 ± 0.05 (λ = 632.8 nm), agree well with the results of the measurements made in vacuum. The proposed technique makes it possible to measure the thicknesses of titanium films within the range 7–30 nm with an accuracy of 0.7 nm. The technique is tested on titanium films deposited onto fused silica substrates obtained by vacuum thermal evaporation. The possible error of determining the thickness due to various additional factors is estimated. The results of ellipsometric measurements of the thickness are compared with the data obtained from parallel measurements of electric resistance of the films.     

Accuracy of Determining the Optical Parameters of Thin Films by the Method of the Reflectance-Spectrum Extrema Envelopes

A technique to evaluate the accuracy with which the optical parameters of thin films are determined by the method of reflectance-spectrum extrema envelopes is presented. The general case of s-polarized light impinging obliquely on a weakly absorbing thin film formed on an absorbing substrate is discussed. Rather simple analytical expressions are derived which can easily be used in program realization on computers for calculating errors. On the basis of the error analysis, a procedure for determining the optical constants and thickness of thin films is proposed, which allows one to reach the maximum accuracy in solving the inverse problem of spectrophotometry. The optical constants and the thicknesses of the films of perylenetetracarboxylic acid and 2-[4-(4-aminophenyl)phenyliminomethyl]phenol formed on silicon substrates are found by the developed technique in the spectral range 550–900 nm.     

Experimental determination of optical constants in the vacuum ultra violet wavelength region between 80 and 140 nm: A reflectance versus thickness method and its application to ZnSe

The 80-120 nm spectral range is a key domain for solar physics. Below 105 nm solids do not transmit light and the reflectance of available mirrors is particularly low which makes optical measurements specifically difficult. Optical constants of the materials may consequently be unavailable or unreliable.We present here a two media reflectance method at normal incidence suited to this VUV range, in which the variable is not the incidence angle but the thickness of the top layer made of the material to be analyzed. The real (n) and imaginary (k) parts of the complex index are directly and graphically determined in the (n, k) plane as the common intersection point of isoreflectance curves corresponding to samples different only in the thickness of the top layer.The method is tested and illustrated with ZnSe films evaporated on Al covered float glass substrates. In the literature, the reflectance magnitudes measured on ZnSe crystals differ strongly from an author to the other, leading to discrepant data for ZnSe in the VUV domain.We obtain precise and reliable values of (n, k), which fit the experimental values determined on freshly cleaved ZnSe crystals by J.L. Freeouf and the theoretical values calculated from the electronic band structure of ZnSe by John P. Walter and Marvin L. Cohen, but strongly differ from the optical constants selected by E.D. Palik in his tables.     

离子束溅射沉积不同厚度铜膜的光学常数研究

利用Lambda-900分光光度计对离子束溅射沉积不同厚度Cu膜测定的反射率和透射率,运用哈德雷方程,并考虑基片后表面的影响,对离子束溅射沉积的Cu膜光学常数进行了计算。结果表明,在同一波长情况下,膜厚小于100 nm的纳米Cu膜光学常数随膜厚变化明显;膜厚大于100 nm后,Cu膜的光学常数趋于一定值。Cu膜不连续时的光学常数与连续膜时的光学常数随波长变化规律不同;不同厚度的连续膜的光学常数随波长变化规律相同,但大小随膜厚变化而变化。     

A study of some optical properties of hafnium dioxide (HfO2) thin films and their applications

2 ), a series of films ranging in thickness from 50 to 10000 nm was prepared by using an electron beam gun inside an evacuated coating chamber of pressure 1×10^-5 mbar. The films were obtained on optical glass substrate by using oxygen with a backfill pressure of 2.4×10^-4 mbar during the deposition processes. The optical constants of the films were computed in the spectral wavelength region (350–2000 nm) from the transmission, reflection and thickness measurements. A computer program was created to determine two optical parameters n and k of the films, and this was achieved by entering the practical results into the computer program, which solved a series of equations for each wavelength. The effects created by changing various evaporation conditions (thickness, substrate temperature and evaporation rate) were studied in the spectral wavelength range, and the optimum values of the various conditions were obtained while achieving the best optical performance. According to the investigations of the HfO₂ material, two applications of the anti-reflection (AR) multi-layer coatings were achieved in two different spectral wavelength ranges. The first application was measured in the visible and near infra-red (VIS/NIR) range from 500 nm to 850 nm deposited on the glass substrate. The second application was measured in the infrared (IR) range from 7500 nm to 11500 nm deposited on germanium substrate. Computer modelling for designing the optical multi-layer system has been presented. The theoretical formulation and experimental results with the same specification were achieved. The correlation between the theoretical and the experimental results reveals a close agreement that offers a convenient method for predicting and controlling the multi-layer coating. By continuous measurement of the optical and mechanical (durability) performances of the coating process, high-quality films were produced in the manufacture of various optical devices. Received: 16 April 1997/Accepted: 5 August 1997     

Radiation Effects on Optical Properties of Ethylene Vinyl Acetate Copolymer Films

The optical properties of ethylene vinyl acetate (EVA) film have been studied. The effects of gamma irradiations on the optical spectrum of EVA films have been investigated using spectrophotometric measurements of reflectance and transmittance in the wavelength range 200–1100 nm. The absorption spectra were recorded in the UV–vis region for the unirradiated and irradiated films (from 0 to 50 kGy). Optical constants such as refractive index (n), extinction coefficient (K), and complex dielectric constant have been determined, as well as the optical dispersion parameters and high frequency dielectric constants. A large dependence of the fundamental optical constants on the irradiation dose was noticed. On irradiation, a higher refractive index was obtained as compared with that for unirradiated film. The dispersion parameters, such as E ₀ (single‐oscillator energy), E _d (dispersive energy), and M _?1 and M _?3 (moments), are discussed in terms of the single‐oscillator Wemple–DiDomenico model.     

Influence of thickness on the optical properties of vacuum-deposited a-Si:H films

The influence of the thickness of the a-Si : H film on its optical properties was studied using spectrophotometric measurements of the film transmittance and reflectance in the wavelength range 200–3000 nm. Both the refractive index and the absorption coefficient were found to increase as the film thickness increased and the absorption edge shifted to lower energies. Both the optical energy gap, E_g, and Urbach parameter values, E₀, decreased with increasing thickness. This decrease in E_g and E₀ was slow for thicknesses above 400 nm.     

Optical properties of stannous oxide thin films

Thin films of stannous oxide (SnO) have been prepared by the electron-beam evaporation method. Film properties, such as refractive indexn, extinction indexk, and absorption coefficient, were studied in the wavelength range between 200 nm and 2 000 nm with different methods. The surface roughness of SnO thin films has been found to depend linearly on the film thickness in the range 400 <d < 2 000 nm. The optical transition in SnO thin films shows direct transition corresponding to an optical gap of 2.85 eV.     

Infrared optical properties of Mn1.56Co0.96Ni0.48O4 thin films prepared by chemical solution deposition

～2 μm thick Mn_1.56Co_0.96Ni_0.48O₄ (MCN) films have been prepared on a Al₂O₃ substrate by the chemical solution deposition method. X-ray diffraction and microstructure analyses show good crystallization and the thickness of the films is 2.12 μm. Mid-infrared optical properties of MCN films have been investigated using transmission spectra and infrared spectroscopic ellipsometry. The optical band gap of the MCN film has been derived to be 0.64 eV by assuming a direct transition between valence and conduction bands. The optical constants and thickness of the thin films have been obtained by fitting the measured ellipsometric parameter data with the classical infrared model. The refractive index n of the MCN films decreases as the wavelength increases, but the extinction coefficient k monotonously increases in the wavelength range of 2–7 μm. The maximal n value is 2.63, and the maximal k value is only 0.024. The above results are instructive for the applications of MCN films in infrared detecting.     

Thickness dependence of temperature coefficient of resistivity of polycrystalline bismuth films

Results for the temperature coefficient of resistivity (TCR) of polycrystalline bismuth films deposited on to glass substrate are reported for the thickness range 30–300 nm. The film TCR is found to be negative for all thicknesses studied and its absolute value exhibits a maximum of 3.70×10^–3 K^–1 near 72.5 nm. The variation of charge carrier density with film thickness has been estimated from the presence of surface states. To include the thickness dependence of charge carrier density, a modified theory has been used to explain the observed behaviour of the TCR. The experimental results for the TCR of Bi films are found to be consistent with the theoretical values. The existence of the extremum is theoretically verified. From the analysis, the specularity parameter p is about 0.44 and the reflection coefficient R is 0.1.     

Optical constants of thin CoSi2 films on silicon

The optical transmission of CoSi₂ films of thickness 2.6–15 nm is measured in the wavelength range 1–20 m. The optical constants are evaluated by taking into account multiple reflections in the film and by fitting a Drude model. The plasma frequency _p=5.4–7.6 eV is equivalent to a carrier density n _eff=3×10²² cm^–3 and one carrier per unit cell. The relaxation frequency of the plasma resonance assumes high values =2 eV near the interface to silicon and decreases into the bulk film over several nanometers. Films grown off-axis from the (111) Si orientation exhibit an enhanced relaxation frequency.     

Determination of the depth of formation of magnetooptical effects in CoNi films

The optical constants of CoNi films with magnetic properties that are nonuniform across their thickness are determined in reflected light by two methods, viz., optical and magnetooptical measurements. The values of the parameters L=λ/4πk and Z ₀=λ/8n, one of which (specifically, the one which has the smaller value at a given value of λ) determines the depth of formation of reflective magnetooptical effects (l _mo) according to the current theories, are calculated on the basis of the values obtained for the optical constants n and k of the films (λ is the wavelength of the light used, and n and k are the refractive index and the absorption coefficient of the magnet). It is established for the CoNi films investigated that l _mo is determined by L and varies from about 200 to 300 ? in the range 0.33 μm⩽λ⩽0.83 μm. In CoNi films, which are inhomogeneous across their thickness and are characterized by significant variation of the magnetic properties over distances ∼l _mo, variation of the form of the magnetization curves determined by measuring the equatorial Kerr effect is observed as λ increases. Zh. Tekh. Fiz. 68, 69–72 (February 1998)     

Optical band gap and optical constants in a-Se80Te20−xPbx thin films

The optical constants (absorption coefficient, refractive index, extention coefficient, real and imaginary part of dielectric constant) have been studied for a-Se₈₀Te_20−xPb_x (where x = 0, 2, 6, 10) thin films as a function of photon energy in the wave length range (500–1000 nm). It has been found that the optical band gap increases while the refractive index and the extinction coefficient (k) decreases on incorporation of lead in Se–Te system. The value of absorption coefficient (α) and the extinction coefficient (k) increases, while the value of refractive index (n) decreases with incident photon energy. The results are interpreted in terms of the change in concentration of localized states due to the shift in fermi level.     

Synthesis and optical characterization of nanocrystalline CdTe thin films

From several years the study of binary compounds has been intensified in order to find new materials for solar photocells. The development of thin film solar cells is an active area of research at this time. Much attention has been paid to the development of low cost, high efficiency thin film solar cells. CdTe is one of the suitable candidates for the production of thin film solar cells due to its ideal band gap, high absorption coefficient. The present work deals with thickness dependent study of CdTe thin films. Nanocrystalline CdTe bulk powder was synthesized by wet chemical route at pH≈11.2 using cadmium chloride and potassium telluride as starting materials. The product sample was characterized by transmission electron microscope, X-ray diffraction and scanning electron microscope. The structural characteristics studied by X-ray diffraction showed that the films are polycrystalline in nature. CdTe thin films with thickness 40, 60, 80 and 100 nm were prepared on glass substrates by using thermal evaporation onto glass substrate under a vacuum of 10⁻⁶ Torr. The optical constants (absorption coefficient, optical band gap, refractive index, extinction coefficient, real and imaginary part of dielectric constant) of CdTe thin films was studied as a function of photon energy in the wavelength region 400–2000 nm. Analysis of the optical absorption data shows that the rule of direct transitions predominates. It has been found that the absorption coefficient, refractive index (n) and extinction coefficient (k) decreases while the values of optical band gap increase with an increase in thickness from 40 to 100 nm, which can be explained qualitatively by a thickness dependence of the grain size through decrease in grain boundary barrier height with grain size.     

Dispersive optical constants of glassy AsSe1−xTex system

The compositional dependence of the optical constants, the refractive index n, and the absorption index k, of the AsSe_1−xTe_x thin films with 0<x<1.0 were determined in the spectral range of 400–2500 nm. The maximum value of the refractive index n, is shifted toward the long wavelength by increasing the Te content in the examined system. The values of the forbidden energy gap of the system have been determined and were correlated with the type and the amount of chemical bonds formed by the increasing Te content in the AsSe_1−xTe_x glassy system. The value of the dispersion energy E_d exhibits low value at the composition containing the same atomic percent of Se and Te.