Light harvesting architectures for electron beam evaporated solid phase crystallized Si thin film solar cells: Statistical and periodic approaches

The fabrication of light trapping architectures for electron beam (e-beam) evaporated polycrystalline Si thin film solar cells is investigated based on tailored self-organized light scattering silica nanospheres and 2 dimensional periodic nanoimprinted structures on glass. A microscopic analysis reveals a unique correlation between the microstructure of high-rate e-beam evaporated Si and the substrate topography. These features provide the basis for the design of nanostructured Si that complies with its distinctive growth characteristics. A layer of self-organized nanospheres embedded in a sol–gel matrix and an anti-reflection coating is found to be an e-beam compatible light trapping approach for poly-Si solar cells, contributing to an increase of 50% in current collection. We developed a preparation process for arrays of equidistant free-standing Si crystals with remarkable optical absorption characteristics based on a nanoimprinted glass substrate by selectively etching e-beam evaporated Si. This periodic approach opens design possibilities for effective three-dimensional architectures for advanced photon management.     

Lattice vibration and photoinduced light scattering in Co-, Cr- and Fe-doped lithium niobate

Raman scattering and photoinduced light scattering in Co-, Cr- and Fe-doped lithium niobate were examined. The A₁(TO) modes appear in E symmetry spectrum of the doped lithium niobate. Their intensities vary with different dopings. In the spectrum of z(yy)x geometry, the properties of the lowest-frequency E(TO) mode of the Cr-doped lithium niobate are different from those of pure, Co- and Fe-doped lithium niobate. The intensity of the A₁(TO) mode at 637cm^-1 I is decreased in doped lithium niobate compared with the pure crystal. We attribute these properties to both the photorefractive effect which is enhanced by dopants and to the different occupation of the doping ions. A light climbing effect was observed in Co- and Cr-doped lithium niobate for the first time. A higher photodamage threshold and quicker light climbing speed were found in Co- and Cr-doped lithium niobate in comparison with the light climbing effect in the Fe-doped lithium niobate. The results from the photoinduced light scattering experiments were compared with those from a Raman spectroscopic study.     

Spherical and lamellar octadecylsilane hybrid silicas

Silicas bearing different contents of octadecylsilane groups were synthesized by the sol–gel method and characterized by solid-state magic angle spin ¹³C nuclear magnetic resonance spectroscopy, Raman spectroscopy, attenuated total reflectance infrared spectroscopy, small angle X-ray scattering, laser light scattering and atomic force microscopy. A structural model for such hybrid materials is proposed in which spherical or lamellar morphology, fern-like or Porod’s structure, are proposed depending on the tetraethyl orthosilicate (TEOS)/octadecylsilane (ODS) molar ratio. The effect of the ODS addition time on the chain conformation and on the particle morphology and texture was also investigated. The degree of organization lowered as the amount of ODS increased. The nanostructured xerogel obtained in the case of pure TEOS evolutes from spherical to lamellar patterns, as the amount of octadecylsilane is increased.     

Orientation of spheroid-shaped silver nanoparticles in mesostructured silica films studied by polarized absorption and low-frequency Raman spectroscopy

Orientation of spheroid-shaped silver (Ag) nanoparticles (NPs) embedded in mesostructured silica sol gel films is studied by polarized absorption measurements and low-frequency Raman scattering, two complementary techniques well-adapted to characterize nanostructures in amorphous matrix. Comparison is made with analysis of mesostructured silica sol gel films containing nanospheres. Polarization-dependent absorption spectra clearly evidence spheroid-shaped NP orientation and their mean aspect ratio is estimated from corresponding polar plots. Mean diameter of spherical NPs is deduced from low-frequency Raman scattering spectra as well as size of elongated spheroid NPs.     

Electrical resistivity of amorphous Fe90-xCoxZr10 alloys: Weak localization and spin fluctuation effects

The results of electrical resistivity () measurements performed on amorphous Fe_90-xCo_xZr₁₀ alloys in the temperature range 77 K to 350 K are presented and discussed in the light of existing theoretical models. Besides unambiguously identifying weak localization, electron-phonon scattering, electron-magnon scattering and scattering of conduction electrons from spin fluctuations as main mechanisms governing the temperature dependence of within the temperature range covered in this work, the present results demonstrate that inelastic scattering of electrons from longitudinal phonons provides the most effective dephasing mechanism. Weak localization effects basically account for the negative temperature coefficient of observed for temperatures below the minimum and the inelastic scattering time or dephasing time increases rapidly with Co concentration. Spin fluctuation effects show up clearly for temperatures in the vicinity of the Curie temperature through a contribution to (T) which varies with temperature as . That spin fluctuations get suppressed by Co substitution is strongly indicated by a continuous decline of the coefficient of the term with increasing Co concentration.     

Study of the gel films of Acetobacter Xylinum cellulose and its modified samples by 1H NMR cryoporometry and small-angle X-ray scattering

Gel films of Acetobacter Xylinum cellulose and its modified samples have been investigated by 1H nuclear magnetic resonance (NMR) cryoporometry and small-angle X-ray scattering. The joint use of these two methods made it possible to characterize the sizes of aqueous pores in gel films and estimate the sizes of structural inhomogeneities before and after the sorption of polyvinylpyrrolidone and Se₀ nanoparticles (stabilized by polyvinylpyrrolidone) into the films. According to small-angle X-ray scattering data, the sizes of inhomogeneities in a gel film change only slightly upon the sorption of polyvinylpyrrolidone and nanoparticles. The impregnated material is sorbed into water-filled cavities that are present in the gel film. ¹H NMR cryoporometry allowed us to reveal the details of changes in the sizes of small aqueous pores during modifications.     

Correlation between Light Microscopical and Electron Microscopical Images of Crystal Defects in GaP

Ultramicroscopy and dark-field-illumination basing on the scattering of photons are easily available, they are nondestructive light microscopical methods for real structure investigations of semiconducting materials. The observation of dislocations is possible because of photon scattering at precipitations adherent to the dislocation line. This work gives a direct correlation between light microscopical and electron microscopical images of dislocations and in this way some hints on the nature of scattering centres. The light scattering takes place at precipitations caused due to gettering of impurity atoms by the dislocation line. Besides elongated dislocations prismatic loops were observed. The loop density was found to be in the order of magnitude of about 10¹² cm⁻³ which corresponds to a concentration of about 10¹⁷ atoms contained in the loops in good agreement with the concentration of impurities in the crystals.     

A SAXS study of V2O5·nH2O sols and gels

Small-angle X-ray scattering (SAXS) measurements on V₂O₅·nH₂O sols and gels, prepared by dissolving V₂O₅ glass into water at room temperature, show that there are V₂O₅ polymeric fibers entangled like random coils in the sol of n 5000, while the deviation from the random coil behavior occurs in the dilute sol of n 6000.

A Bragg peak appears at the scattering vector h 0.02 Å⁻¹ to be superimposed on an asymptotic h⁻²-course in the SAXS curve of the concentrated sol of n 680. This means that the spatial correlation between V₂O₅ polymeric fibers takes place even in the fluid state.

V₂O₅·nH₂O sols completely lose fluidity at n 250 to transfer to the gel state, where V₂O₅ polymeric fibers begin to pile up in the parallel with a substrate surface. Such a layer structure is preserved up to the gel of n 4. However, V₂O₅ polymeric fibers are randomly oriented within each layer.     

Methods for measuring light scattering in germanium and paratellurite crystals

Methods for studying the scattered light in germanium and paratellurite (α-TeO₂) crystals are considered. Investigations of the light scattering in Ge crystals were performed in the infrared wavelength range by the photometric-sphere method (in the range 2–3 μm) and by measuring the line-scattering functions (at 10.6 μm). In the visible range, the paratellurite single crystals were investigated by recording and analyzing images of laser beams transmitted through the samples. It is shown that small-angle Mie scattering is characteristic of both materials. Some conclusions about the sizes and the physical nature of scattering inhomogeneities are drawn. The effect of high-temperature annealing on the scattering intensity is studied.     

GdAlO3∶Tb3+-Al2O3有序共晶生长及其微结构研究

两相有序共晶材料由于具有不同折射率的两晶相呈现有序排列,可降低荧光在共晶材料内部的散射而实现导光功能,可被应用于高分辨探测成像器件中。本工作根据定向凝固原理,用微下降法生长技术制备得到了直径为3 mm的GdAlO₃∶Tb³⁺-Al₂O₃两相有序共晶。通过SEM和元素分析,探究了GdAlO₃∶Tb³⁺-Al₂O₃共晶内部的微结构,结果显示,所得共晶中GdAlO₃∶Tb³⁺晶相均匀有序地分布于基质Al₂O₃晶相中,GdAlO₃∶Tb³⁺晶相直径的大小受生长速率的影响,速率越快,直径越小。所制备得到的GdAlO₃∶Tb³⁺-Al₂O₃有序共晶在X射线辐照下发射出明亮的绿色荧光,并在GdAlO₃∶Tb³⁺晶相中定向传播,有望被用作X射线探测成像材料,提高探测器的空间分辨率。     

Neutron measurements of magnetic correlations in metallic glasses

Neutron diffraction experiments have been made on FeB and PdSi transition metal alloy glasses prepared by melt-spinning. The magnetic neutron scattering has been analysed to provide information about the magnetic order in the specimens. An Fe₈₃B₁₇ metallic glass has magnetic correlations at room temperature with a range commensurate with the variations in atomic structure. These correlations appear to have a structure based on fcc gamma-iron, and the magnetic disorder is small - given that the specimen was in a field of 1 kG. Magnetic scattering was observed from Pd₇₀Si₂₀Co₁₀ and Pd₆₇Si₂₀Fe₁₃ at room temperature, which indicated a large magnetic moment value for the transition metal atoms. Cooling the PdSiCo sample to 80 K induced an antiparallel moment correlation, while further cooling to 7 K produced little obvious change in the scattering. These latter results are discussed in the light of the considerable disagreement which exists between different investigations of these alloys.     

Infrared light scattering and absorption microscopy investigations at dislocations in GaAs crystals

In recent years a number of experiments have been published dealing with the light scattering at small particles. All of these experiments have been made in the visible spectral range. In this paper we report an experimental arrangement for infrared light scattering by dark-field illumination. Preliminary results of the observations of dislocations in GaAs crystals are given. The scattering images are correlated with results obtained by means of absorption microscopy with infrared light. Helix-like fine structures of dislocations are found mainly arranged in the 〈110〉 directions.     

Origin of Rayleigh scattering and anomaly of elastic properties in vitreous and molten GeO2

Vitreous (v) and molten (m) GeO₂ were studied by Rayleigh and Mandel’shtam–Brillouin scattering spectroscopy and high-temperature acoustics. Original measurement apparatus and procedure were used that included Bayseian deconvolution of light scattering spectra of vGeO₂ and a specially designed high-temperature (up to 1500 °C) acoustic interferometer to measure temperature and frequency dependence of ultrasonic (US) velocity and attenuation in mGeO₂. Landau–Placzek ratios for vGeO₂ were found optically (from the light scattering spectrum) and acoustically (through the Schroeder’s formalism). Dispersion of optical and other physical parameters of vGeO₂ found by many authors is explained by the existence of small amount of GeO in the samples. It means that properties of vGeO₂ are under the influence of redox synthesis conditions controlling the GeO₂ ↔ GeO and coordination [GeO₄] ↔ [GeO₆] equilibrium in vGeO₂. Measurements of temperature dependencies of longitudinal ultrasonic velocities in mGeO₂ and in the PbO–GeO₂ glass melts as a function of PbO concentration shows existence of ‘water-like anomaly’ in mGeO₂ and in liquid germanates with the rich content of GeO₂ where equilibrium sound velocity increases with the temperature.     

Defects in calcium tungstate crystals

The role of defects formed owing to the deviations from stoichiometry is analyzed for CaWO₄ single crystals grown by the Czochralski method. It is shown that the defects leading to the light scattering in CaWO₄ crystals affect the spectra of X-ray luminescence and the luminescence kinetics only slightly, but give rise to an additional peak in the thermoluminescence spectrum. Addition of WO₃ to the melt eliminates the light scattering and the additional thermoluminescence peak.     

Slow dynamics of liquid Se studied by Infrared Photon Correlation Spectroscopy

We report on the study of the slow dynamics of supercooled and molten Selenium by means of dynamic light scattering. To achieve this in the present case, where the opaqueness of Se prevents visible light scattering measurements, we have developed a novel experimental setup using infrared radiation as the light source. By measuring the scattered intensity autocorrelation function we have been able to extract the average size of the Se clusters in the liquid state and determine the temperature dependence of the related diffusion coefficient which was found to exhibit Arrhenius behavior.     

Sol–gel simulation—I: Scattering response

Scattering of sol–gel structures is investigated computationally. Sol-gels are recreated through an aggregation algorithm incorporating Brownian motion and chemical reactions. Using the fractal character of sol–gels, the concept of recursion is introduced as a tool to perform multi scale computation of the response of sol–gels through the different scales from the molecular level to the macro scale. The concept is illustrated with the prediction of scattering intensity. The relationship between scattering intensity and functionality is investigated, noting that the latter is a function of the Brownian motion and chemical reactivity. Computational simulation tools are developed to predict scattering intensity as a function of density and reactivity, the former represented by the number of particles, or clusters, in the simulation box. Then, the results are correlated to an analytical model that reveals the critical wave number, or critical scale, at which percolation occurs.     

Bestimmung von Diffusionskoeffizienten aus der Röntgen-Kleinwinkelstreuung und Lichtstreuung von Gläsern

It is shown how to conclude from small angle X-ray scattering and light scattering of phase separated glasses what kind of phase separation process is occurring in the glass. In the cases of diffusion controlled particle growth and of spinodal decomposition, methods were presented permitting to find out from diffraction experiments the diffusion coefficients underlying these decomposition processes. The methods were demonstrated with three different glasses.     

Scattering of visible light by glasses undergoing phase separation and homogenization

A kinematic theory of light scattering and modern concepts of phase-separation theory were used to examine the most typical experimental results obtained from the study of light scattering by glasses. The examination was made by taking into account the positions of figurative points in the immiscibility diagram. It has been shown that the anomalous phenomena (the predominant light scattering into the backward semi-sphere and the spectral dependence of intensity of type λ^?p, where p > 4) arise in four cases: at the stage of decomposition when the figurative point lies in both the binodal and spinodal regions; after the completion of the decomposition stage when interparticle interference is essential; and in the one-phase region when the concentration fluctuations tend to a higher equilibrium level of development. In each case these phenomena are due to the sign-changed behavior of the dielectric constant and to the existence of a range of negative values in the correlation function. The processes of complete or partial homogenization of the glass are characterized by either the gradual disappearance of anomalous phenomena or the appearance of normal scattering due to the increase in space extension of the inhomogeneity regions at their dissolution.     

Light Scattering and Contrast in Thermally Addressed Liquid Crystal Displays

The effects of structural order on the light scattering characteristics of liquid crystal display cells containing smetic p-n-octyl-p′-cyanobiphenyl have been investigated. The scattering characteristics of different structures in the cell due to surface treatments are qualitatively analyzed for their effects on the contrast ratio when used in the thermally addressed liquid crystal display (TALC). Of the four structures investigated, i.e., homeotropic, fan-like, uniaxial rods and spherulites, the spherulitic structure has been found to exhibit the scattering profile best suited for high contrast because of the near-zero scattering near the incident beam. Based on the light scattering theory for spherulites, optimization of structural parameters such as size and order of spherulites, and device design such as collecting angle of projection lens for maximum display contrast has been predicted. Methods to improve contrast have also been discussed.     

Compact Permanent Magnetic Circuit with Periodic Magnetic Field Designed for Studies of Liquid Crystals

Abstract

We have developed a high-performance compact magnetic circuit to investigate orientational phase behaviors of biological and synthetic macromolecular systems by using spectroscopic methods such as X-ray scattering and light scattering. This magnetic circuit can apply a periodic magnetic field and gradient to samples. The maximum values of the magnetic field strength and gradient are varied from 1.5 tesla to 0.55 tesla and from 2.4 × 10² tesla/m to 7.3 × 10¹ tesla/m. Owing to the high periodicity of the magnetic field strength and gradient macromolecules are subjected to both rotational and translational forces from the magnetic field. We show some prominent features of this magnetic circuit and its first application to the synchrotron radiation small-angle X-ray scattering (SR-SAXS) studies of supermacromolecular liquid crystals.