Spectral interferometry and reflectometry used to measure thin films

A new method for a precise measurement of the oscillatory part of phase change on reflection (interferometric phase) from a thin-film structure is presented. The method, which is based on phase retrieval from the spectral interferograms recorded at the output of a slightly dispersive Michelson interferometer, is combined with reflectometry. The interferometric phase of the thin-film structure is measured precisely using a reference sample of known phase change on reflection. The spectral reflectance of the thin-film structure is also measured in the interferometer. The feasibility of the method is confirmed in processing the experimental data for SiO₂ thin film on a silicon wafer of known optical constants. Four samples of the thin film are used and their thicknesses are determined. We confirm very good agreement between the thicknesses obtained from the interferometric phase and reflectance measurements. PACS  07.60.Ly; 68.55.Jk; 78.20.Bh     

Nano-imprint for near-zero reflected Si solar cells

The periodically patterned Si nanopyramid solar cell shielded with SiN_x-passivation layer was fabricated using nano-imprint method. The fabricated nanopyramid Si solar cell demonstrated the near-zero reflectivity over a wide range of wavelengths. With a SiN_x-coating layer, nanopyramids showed 0.366% reflection. A SiN_x-coated nanopyramid Si is efficient to improve the carrier collection for broad wavelengths due to intrinsically low light-reflection of Si nanopyramid structures. SiN_x-passivation effectively controls carrier collection efficiencies of nanopyramid structures, resulting in significant improves short wavelength lights. We systematically investigated optical and electrical performances of nanoscale solar cells and suggest a route to realize high-efficient nanoscale solar cells. The importance and influence of SiN_x-passivation layer and nanopyramid structure in Si based solar cell were explained in detail with optical and electrical characteristics.     

Ultraviolet to far infrared optical properties of the one-dimensional conductor K2Pt(CN)4Br0·3. 3H2O

An experimental study of the optical properties of single crystals of the quasi-one-dimensional conductor K₂Pt(CN)₄Br_0·3, 3H₂O in the spectral range from 45,000 to 10 cm^?1 is presented.For light polarized perpendicular to the highly conducting direction the material behaves as a transparent dielectric, as evidenced by reflection and transmission spectra. The reflection spectrum measured with polarization parallel to the chain direction (E∥Z) shows, in addition to the previously reported ‘metal-like’ optical reflectance and plasma edge in the visible, deviations from free carrier type reflectance in the i.r. This was expected because the conductivity is thermally activated at low temperatures. The optical constants for E∥Z polarization have been determined by Kramers-Kronig analysis of the reflection spectrum and, at a few discrete wavelengths, from measurements of polarized reflectance under oblique incidence. The results are qualitatively consistent with various models of the electronic structure but do not distinguish between them.     

Aperiodic multilayer mirrors for efficient broadband reflection in the extreme ultraviolet

Recent extreme ultraviolet sources using high-harmonic generation in a rare gas make new optics developments necessary. We report on the study and development of multilayer structures with efficient reflectivity in the 35–75 eV energy range. We have optimized, deposited and characterized two aperiodic broadband mirrors consisting of a Mo, Si and B₄C thin-film stack. We used the needle procedure in order to optimize mirror reflectivity. The magnetron sputter deposited multilayers have been calibrated and characterized using Cu K _α grazing incidence X-ray reflectometry. Reflectivity measured at near-normal incidence on a synchrotron radiation source reaches 12% with a full width at half maximum of nearly 40 eV. Experimental results are compared with theoretical simulation using available optical constants for Mo, Si and B₄C in this spectral range.     

Quantitative evaluation of the electro-optic effect and second-order optical nonlinearity of lithium tantalate crystals of different compositions using Raman and infrared spectroscopy

Nominally pure lithium tantalate (LiTaO₃) crystals with different chemical compositions were characterized by Raman scattering, infrared reflection, and infrared absorption spectroscopy. In these crystals specific phonon bands are correlated to electro-optic and nonlinear optical properties. The electro-optic and nonlinear optical coefficients were determined and the data are compared with literature values obtained by direct measurements. Our measurements are in good accordance with the literature data, which opens up a new possibility for fast and easy estimation of electro-optic and nonlinear optical performance of newly developed LiTaO₃-based materials modified by stoichiometry. PACS 42.70.Mp; 78.20.Jq; 78.30.-j     

A method of online quantitative interpretation of diffuse reflection profiles of biological tissues

We have developed a method of combined interpretation of spectral and spatial characteristics of diffuse reflection of biological tissues, which makes it possible to determine biophysical parameters of the tissue with a high accuracy in real time under conditions of their general variability. Using the Monte Carlo method, we have modeled a statistical ensemble of profiles of diffuse reflection coefficients of skin, which corresponds to a wave variation of its biophysical parameters. On its basis, we have estimated the retrieval accuracy of biophysical parameters using the developed method and investigated the stability of the method to errors of optical measurements. We have showed that it is possible to determine online the concentrations of melanin, hemoglobin, bilirubin, oxygen saturation of blood, and structural parameters of skin from measurements of its diffuse reflection in the spectral range 450–800 nm at three distances between the radiation source and detector.     

Use of surface plasmon excitation for determination of the thickness and optical constants of very thin surface layers

We present a method using attenuated total reflection (ATR) with excitation of surface plasma waves at different wavelengths and angles of incidence, which allows an accurate determination of the thickness and optical constants of absorbing surface layers from optical measurements only. This method is applied to the case of very thin Au surface layers on (111)Ag, and the results are discussed.     

Accuracy of ellipsometric monitoring during growth of semiconductor nanostructures

The determination accuracy of the parameters of multilayer structures during their in situ ellipsometric monitoring is analyzed. A qualitative criterion is proposed that makes it possible to evaluate the validity of replacement of the multilayer structure by a certain medium with effective optical constants. According to this criterion, this replacement is possible if the complex reflection coefficients R _{p, s} of the structure satisfy the relation (R _p /R _s + cos2φ)/(R _s + R _p cos2φ) = 1, where φ is the angle of incidence of light. It is shown that, for structures consisting of layers with a high refractive index, this condition is implemented with a high accuracy. This proves that, upon the successive solution of the ellipsometry problem for layers of this structure, errors are not accumulated. In particular, this inference can be applied to structures based on a cadmium-mercury-tellurium ternary compound.     

Laser nonlinear-optical probing of silicon/SiO2 interfaces: Surface stress formation and relaxation

A nonlinear optical technique based on optical second harmonic generation in reflection is shown to provide information on the surface layer structure of semiconductor crystals, thin films and layered systems. The second harmonic intensity is sensitive to inhomogeneous stress in centrosymmetric materials via spatial selection rules and the appearance of an electric dipole contribution to the second order nonlinear optical susceptibility. The technique is used to monitor mechanical stress relaxation in the SiO₂/Si interface during several annealing procedures.     

CdxHg1-xTe混晶的远红外反射光谱和多振子模型

在温度4.2—300K和波数15—450cm^-1范围内研究了x=0.18到0.45的不同组份的Cd_xHg_1-xTe样品的远红外反射光谱。实验观察到了类CdTe光学声子反射带的精细结构和低组份样品类HgTe反射带的复杂结构,研究了这些结构对组份和温度的关系,并用多振子模型讨论了反射光谱的这种精细结构。对实验反射光谱用经典的振子匹配方法进行了拟合计算,并从这种拟合运算获得了Cd_(x^-)Hg_1-xTe的远红外介电函数谱及其它光学常数和频率的关系。 关键词：     

Spectral interferometric technique to measure the relative phase change on reflection from a thin-film structure

A two-step white-light spectral interferometric technique to measure the relative phase change on reflection from a thin-film structure is presented. The technique is based on recording of the channeled spectra at the output of a Michelson interferometer and their processing by using a windowed Fourier transform to retrieve the phase functions. In the first step, the phase difference between the beams of the interferometer with a thin-film structure is retrieved. In the second step, the structure is replaced by a reference sample of known phase change on reflection and the corresponding phase difference is retrieved. From the two phase differences, the relative phase change on reflection from the thin-film structure is obtained. The feasibility of the simple method is confirmed in processing the experimental data for a SiO₂ thin film on a Si wafer of known optical constants. Four samples of the thin film are used and their thicknesses are determined. The thicknesses obtained are compared with those resulting from reflectometric measurements, and good agreement is confirmed.     

A nanostructured hybrid material based on polymer infiltrated porous silicon layer

In this work, we report on the fabrication of an hybrid material based on a porous silicon (PSi) layer infiltrated by an amino functionalized poly(ε-caprolattone) (PCL-NH₂). The organic–inorganic structure has been characterized by variable angle spectroscopic ellipsometry, optical reflectometry and water contact angle measurements. A polymer coated PSi layer, 3 μm thick, shows improved wettability and chemical stability against long lasting, up to 18 min, exposure to an alkaline environment. Even after basic treatment, the structure still retains a very good quality optical response, so that it may be proposed as a high performance platform for biochemical sensing applications.     

生长在Si(001)衬底上应变合金层GexSi1-x的光学性质

采用经验的紧束缚方法对生长在Si(001)衬底上的应变合金Ge_xSi_1-x的光学常数进行了计算。应变对电子能带结构的影响,通过紧束缚参数随键角方向余弦的变化以及键长按经验的标度定则的变化而进行计算。其中标度指数根据对Ge和Si的畸变势常数的实验值进行拟合而确定。计算介电常数虚部时出现的动量矩阵元,根据对Ge和Si的介电常数虚部的实验曲线拟合而决定。列出了当x=0.2和1时的光学常数——介电常数虚部ε₂、折射率n、吸收系数α和反射率R的计算结     

Magnetorefractive effect in La<Subscript>0.7</Subscript>Ca<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript> in the infrared spectral range

The reflection and magnetic reflection spectra, magnetic resistance, electrical properties, and equatorial Kerr effect in La_0.7Ca_0.3MnO₃ crystals have been complexly investigated. The measurements have been performed in wide temperature and spectral ranges in magnetic fields up to 3.5 kOe. It has been found that magnetic reflection is a high-frequency response in the infrared spectral range to the colossal magnetore-sistance near the Curie temperature. Correlation between the field and temperature dependences of the magnetic reflection and colossal magnetoresistance has been revealed. The previously developed theory of the magnetorefractive effect for metallic systems makes it possible to explain the experimental data at the qualitative level. Both demerits of the theory of the magnetorefractive effect in application to the magnets and possible additional mechanisms responsible for the magnetic reflection are discussed.     

Influence of MgO doping on the structure of monocrystalline LiNbO3

Conditions for the growth of LiNbO₃:MgO single crystals by the Czochralski method were optimized. An upper limit of the molar MgO concentration in the melt for obtaining limpid, optically homogeneous crystals was determined. The Curie temperature was measured on precisely defined samples and so real MgO contents in the single crystal phase, distribution coefficients and longitudinal concentration profiles could be determined using this calibration curve. X-ray diffraction and optical absorption in the near infra-red and in the ultra-violet and visible regions were also studied on LiNbO₃:MgO single crystals. Lattice constants and positions of the OH^– absorption band and of the short-wave absorption edge were determined on the basis of described measurements. Dependences of all the above-mentioned properties on the molar MgO concentration were found out. The simple model of microscopic mechanisms for explaining these experimental data was proposed.     

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.     

Der Einfluß der freien Ladungsträger auf die optischen Konstanten des Bi2Se3 im Wellenlängengebiet von 2 bis 23 μm

The influence of free carriers on the optical constants of Bismuthtriselenide Bi₂Se₃ was investigated in the infrared (2–23 μm). The reflection and transmission ofn-type Bi₂Se₃ single crystals was measured at approximately 30, 80 and 300 °K. Up till now,p-type Bi₂Se₃ is not known. The dependence of the refraction index and the absorption constant on the wavelength was calculated from the results of the measurements. It was found that the optical constants strongly depend on the carrier concentration. The crystals were prepared according the bridgman method. They normally have a carrier concentration of about 10¹⁹ cm^?3, which can be diminished by annealing in Selenium vapour of various pressure up to 10¹⁸ cm^?3. Bismuthtriselenide is a degenerated semiconductor. We obtained for the optical energy band gap by extrapolation the value 0,21 eV. It increases with increasing carrier concentration. This is known as the Burstein-Moss-effect.     

Tunneling of X-ray photons through a thin film under total internal reflection conditions

Tunneling of 0.154-and 0.139-nm x-ray photons through a thin film under total internal reflection conditions has been experimentally demonstrated. The NiSi₂ film 13 nm thick is deposited by magnetron sputtering on a polished Si substrate. A beam with an angular spread of 20″ is directed to the Si/NiSi₂ interface from the inside through the lateral surface of a sample. A peak associated with tunneling of photons from Si to air through the NiSi₂ film is observed at grazing angles of θ₁ > 0.4θ_c, where θ_c is the critical angle of total internal reflection at the Si/NiSi₂ interface. The integral intensity of tunneling peaks that is measured for various θ₁ angles agrees with the calculations.     

A study of Ba x Mg1 − x F2 optical films

Ba _x Me_{1 ? x} F₂ binary fluoride films (“Me” denotes calcium or magnesium fluoride) are studied. A method of processing the reflection and transmission spectra is proposed to determine the optical constants. The dispersion dependences of the refractive indices and absorption coefficients of films in the range of 1.3–12 μm are found. Dispersion in films in the regions of additional absorption bands, which are absent in single crystals, is observed for the first time. It is shown that the films of binary fluorides have a higher packing density, a lower absorption, and better operating characteristics than do films of pure fluorides. The films are promising for application as optical interference coatings in the mid-IR spectral region.