Evaluation of coated and uncoated CaF2 optics by variable angle spectroscopic ellipsometry

CaF₂ is one of the dominating optical materials used for ArF excimer laser optics. Surface quality of optically finished CaF₂ plays an important role in the components' lifetime. A variable angle spectroscopic ellipsometry was employed to evaluate surface quality of optically finished CaF₂ in terms of top surface and subsurface damage. The subsurface damage was revealed by removing the top surface. Combining plasma ion assisted deposition and ellipsometric measurement, a dense smooth F-SiO₂ film was developed to prevent fluorine loss of CaF₂ optics under ArF excimer laser irradiation, leading to an extended lifetime. In addition, an integrated protective coating approach was established on fluoride multilayer coated CaF₂ optics, resulting in environmentally stable optical performance.     

Growth of regular-shaped β-Ga2O3 nanorods by Ni2+-ion-catalyzed chemical vapor deposition

Regular-shaped monoclinic β-Ga₂O₃ nanorods with square cross-sections were successfully synthesized and characterized by Ni²⁺-ion-catalyzed chemical vapor deposition method using CaF₂ as a dispersant. The composition, crystal structure, morphology, and optical property were characterized in detail. X-ray diffraction data indicate that the product was a single monoclinic β-Ga₂O₃ phase with high purity. The regular-shaped nanorods had square cross-sections and an approximate size of 250 nm in diameter and 500–1,000 nm in length. A broad and strong emission band that ranged from 300 to 650 nm was observed with three bands centered at approximately 405 (blue), 467 (dark blue), and 520 nm (green). The growth mechanism of β-Ga₂O₃ nanorods was consistent with the vapor–solid growth mechanism.     

Density Measurement of Molten CaF2 by an Electrostatic Levitator

   High-quality single crystalline calcium fluoride (CaF₂) has been of considerable practical concern as an optical lens to an excimer laser stepper in the deep-ultraviolet region due to its excellent transparency. Highly accurate thermophysical properties of molten CaF₂ are essential as input data for a numerical simulation of the crystal growth process. The density of molten CaF₂ has been successfully determined in the stable and undercooled liquid states (1600–1820 K) with an electrostatic levitator. The temperature dependence of the density of molten CaF₂ is given by

Low-temperature vapor synthesis of 1D β-Ga2O3 nanostructures on Si substrate by inert salt-assisted route

The attractive feature of one-dimensional inorganic nanomaterials grown on substrate at low temperature is the good adhesion between the deposited material and the substrate. In this work inert salt-assisted route has been extended to prepare Ga₂O₃ product on bare Si substrate. By using CaF₂ powder as a dispersant, the vapor pressure of metallic Ga is increased greatly as compared to its non-dispersed state. This allows for the β-Ga₂O₃ nanostructures to be formed at a relatively low temperature of 650 °C as a result of direct oxidation of the well-mixed metallic Ga and CaF₂ powder. This temperature is much lower than the synthetic temperature required in the cases of direct oxidation of metallic Ga as reported by others. The as-prepared Ga₂O₃ nanowires and nanobelts are characterized by XRD, SEM, EDX, TEM and PL. The vapor–solid growth process was also discussed for the as-prepared Ga₂O₃ product on Si substrate. The interesting results indicate the wide applications of inert salt-assisted route to vapor growth of other nanomaterials at relative low temperature.     

Influences of Er3+ content on structure and upconversion emission of oxyfluoride glass ceramics containing CaF2 nanocrystals

Transparent 45SiO₂–25Al₂O₃–5CaO–10NaF–15CaF₂ glass ceramics doped with different content of erbium ion (Er³⁺) were prepared. X-ray diffraction (XRD) and transmission electron microscope (TEM) analyses evidenced the spherical CaF₂ nanocrystals homogeneously embedded among the glassy matrix. With increasing of Er³⁺ content, the size of CaF₂ nanocrystals decreased while the number density increased. The crystallization kinetics studies revealed that CaF₂ crystallization was a diffusion-controlled growth process from small dimensions with decreasing nucleation rate. Er³⁺ could act as nucleating agent to lower down crystallization temperature, while some of them may stay at the crystal surfaces to retard the growth of crystal. Intense red and weak green upconversion emissions were recorded for glass ceramics and their intensities increased with the increasing of Er³⁺ content under 980 nm excitation. However, the concentration quenching effect appeared when Er³⁺ doping reached 2 mol%. These results could be attributed to the change of ligand field of Er³⁺ ions due to the incorporation of Er³⁺ ions into precipitated fluoride nanocrystals.     

Influence of the purity of the starting materials on the recording of dynamic holograms in Bi12TiO20 crystals

The parameters of Bi₁₂TiO₂₀ crystals grown with bismuth oxide of different degrees of purity were measured by a holographic recording technique in an external ac electric field and were compared with a crystal specially doped with chromium. Various crystal parameters such as the diffusion length of the photoexcited carriers and the Debye screening length were determined. It was found that the absorption spectrum of the material and the holographic recording efficiency were strongly influenced by light chromium doping and by insufficient purity of the starting mixture. Pis’ma Zh. Tekh. Fiz. 23, 27–32 (March 12, 1997)     

Single crystal growth of actinide compounds

During recent years, the importance of solid state actinide research has been increasingly recognized. Further progress in actinide solid state physics depends on the availability of pure and perfect single crystals. Actinide compounds have large magnetic anisotropy with anisotropy fields of 8 × 10⁷ A·m^?1 or higher. Investigation of the mechanism responsible for such unique behaviour requires large single crystals of high purity for magnetization, neutron diffraction, angular and energy dependent photoemission measurements. Materials of interest for actinide solid state research are the metals and compounds with simple crystal structures like dioxides (CaF₂ structure), monopnictides and monochalcogenides (NaCl structure), and intermetallic compounds (Laves phases).This article gives an overview of actinide single crystal growth facilities in Karlsruhe and Geel. The actinide compounds are prepared by direct synthesis from the purest elements available using non-contaminating techniques. The reaction occurs in vacuum sealed quartz tubes where the actinide metal reacts with the vapour of the other element, or by levitation melting in a Hukin crucible. Different techniques have been developed to grow single crystals of actinide metals and compounds. High temperature solution growth from molten salts is used to prepare actinide dioxide single crystals. Oxides, pnictides and chalcogenides are grown by chemical vapour transport. Large single crystals of the monopnictides and monochalcogenides are obtained with the recrystallization (or mineralization) technique in sealed tungsten crucibles. Single crystals of congruently melting intermetallic compounds are pulled from levitated or semilevitated melts by the Czochralski method. Selected single crystals are characterized, orientated and encapsulated for safe handling during measurements.     

Bridgman growth of bismuth tellurite crystals

The photorefractive crystal, Bi₂TeO₅, was grown by the modified Bridgman method for the first time. High purity Bi₂O₃ and TeO₂ were used as starting materials and were mixed thoroughly with molar ratio of Bi₂O₃/TeO₂ = 1 ∶ 1. Platinum crucible was fabricated with a seed well of 10 mm in diameter and several folds were pressed so that the spontaneous nuclei could be eliminated through competition. The crucible was sealed during the growth so that the evaporation of TeO₂ was controlled effectively. By optimizing growth parameters, transparent and crack-free Bi₂TeO₅ crystal up to 25 mm in diameter and 40 mm in length was grown successfully.     

Synthesis and growth features of copper hydroxide iodide nanoneedles

Single-crystalline Cu₂(OH)₃I nanoneedles were synthesized by the reaction between Cu(CH₃COO)₂ and KI in aqueous solutions with or without poly(ethylene glycol) (PEG) in situ, and techniques of XRD, EDX, SEM and TEM were used to characterize their crystal structure, chemical composition and growth feature. The as-prepared nanoneedles had a high purity in composition and belonged to the botallackite-type Cu₂(OH)₃I. Nanoneedles, together with botallackite-type Cu₂(OH)₃I flakes, were produced in the absence of PEG, and their growth features were consistent with the theoretical predictions; however, the addition of PEG to the solution led to the exclusive formation of nanoneedles, in which the adsorption of PEG on the related crystal surfaces controlled a crystal growth different from theoretical expectations.     

Characterization of vertical Bridgman grown Cd0.9Zn0.1Te0.97Se0.03 single crystal for room-temperature radiation detection

We report a modified vertical Bridgman method to grow Cd_0.9Zn_0.1Te_0.97Se_0.03 (CZTS) single crystals using in-house zone-refined 7 N (99.99999%) purity elemental precursors for room-temperature radiation detection. CZTS is an economic yet high performance alternative to expensive CdZnTe (CZT) detectors for room-temperature gamma-ray detection. Radiation detector in planar geometry has been fabricated on an 11.0?×?11.0?×?3.0 mm³ CZTS single crystal. A bulk resistivity of 10¹⁰ Ω.cm has been achieved without using any compensating dopant. The elemental composition of the grown crystal has been examined using energy-dispersive X-ray (EDX) analysis. Powder X-ray diffraction (XRD) showed formation of zincblende phase with a lattice constant of 6.447 Å, and sharp peaks confirmed the formation of highly crystalline single-phase CZTS crystals. A modified Vegard’s law has been applied to calculate the atomic percentage of Se in the grown crystals from the XRD patterns and compared with the intended and the measured stoichiometry. The electron mobility-lifetime (μτ) product and the drift mobility have been calculated to be 1.5?×?10^–3 cm²/V and 710 cm²/V.s, respectively, using alpha spectroscopy. The presented vertical Bridgman growth method uses a single pass through the controlled heating zone in contrast to the previously reported multiple pass growth techniques, thus, reducing the growth duration by two third which would help to further reduce the cost of production of CZTS-based room-temperature detectors.

     

Structure and superconducting properties of pure and variously doped bulk MgB2 obtained by uniaxial and isostatic hot pressing

The preparation of dense MgB₂ bodies, undoped and doped with different atomic species (Na, Ag, Y), was performed via reactive sintering by uniaxial and isostatic hot pressing, starting from the pure elements, and compared with undoped samples obtained by commercial MgB₂ powder. The superconducting characteristics of the obtained materials, namely critical temperature (T_c) and current (J_c), were obtained through ac susceptibility measurements and compared to their structural features, like phase purity and secondary phases formation and distribution in the MgB₂ matrix. Both the adopted hot pressing techniques gave rise to undoped MgB₂ pieces exhibiting phase purity in the range 85–95% and relative density above 80%; although in most cases the doped samples underwent higher extents of phase decomposition and lesser densification, they all exhibited higher critical temperature and current compared to the corresponding undoped material, indicating a net influence of the doping on the superconducting behaviour of MgB₂, particularly Ag and Y. An opportune quality factor was adopted, to obtain a more reliable comparison between the different MgB₂ samples and evaluation of the samples goodness, in relation to their superconducting characteristics. It was put in evidence that low amounts of doping can improve the superconducting behaviour of MgB₂ and that this influence can be addressed in terms of pinning centres, as there was no experimental evidence of an actual atomic substitutions in the MgB₂ crystal.     

Bio-conjugation of CaF2:Eu/chitosan nanoparticles with BSA and photoluminescent properties

CaF₂:Eu nanoparticles capped with chitosan and bio-conjugation of CaF₂:Eu/chitosan nanoparticles with bovine serum albumin (BSA) were synthesized via microemulsion method. As a surfactant, chitosan was employed to cap the nanoparticles during the synthesis process and provided functional groups, such as amino group for further bio-conjunction with BSA. CaF₂:Eu and CaF₂:Eu/chitosan–BSA nanoparticles were characterized by means of X-ray powder diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), ultraviolet spectrophotometer (UV), infrared spectrophotometer (IR) and photoluminescence spectrophotometer (PL). The XRD results indicated that the CaF₂:Eu and CaF₂:Eu/chitosan-BSA nanoparticles have crystallized well and the average sizes were about 16 and 19 nm, respectively. Images of FE-SEM showed that the average grain sizes of the CaF₂:Eu and bio-conjunction of CaF₂:Eu/chitosan nanoparticles with BSA were about 19 and 20 nm. The patterns of UV spectra and IR spectra showed that BSA was linked to CaF₂:Eu/chitosan nanoparticles. In the emission spectrum of the CaF₂:Eu/chitosan–BSA nanoparticles, characteristic emission peaks of Eu³⁺ within the wavelength ranging from 500 to 700 nm were observed, corresponding to the transitions from the excited ⁵D₀ levels to ⁷F_J levels. This confirmed that the Eu³⁺dopant ion is located in a Ca²⁺ crystal site with Td symmetry.     

The Effect of Laser Annealing on Laser-induced Damage Threshold

Abstract

A significant enhancement of the single-shot laser-induced damage threshold of CaF₂ and fused silica and a moderate enhancement for GaAs and Al has been observed as the result of laser annealing using an excimer laser operating at 248 nm. This phenomenon is primarily attributed to a reduction of the residual surface roughness of the samples.     

Effect of the purity of CdTe starting material on the impurity profile in CdTe/CdS solar cell structures

We have investigated CdTe/CdS/In₂O₃:F/glass solar cell structures using quantitative SIMS for profiling the impurity distribution from the CdTe free surface through to the glass substrate. Ion implanted CdTe standards were used. The effect of the purity of the CdTe starting material was determined by studying two structures grown from 7N and 5N source materials. Particular emphasis was placed on the potentially electrically active impurities that may originate from the CdTe starting material, and are likely to affect the CdTe/CdS solar cell performance. It was shown that Cu, Zn, Sn, Sb and Pb profiles had the same level and shape in the CdTe layer regardless of the purity of the starting material used, and were therefore not originating from the starting material. Cl, O, Na and Si showed higher levels for structures grown using 5N purity CdTe compared to those from 7N, and may, at least in part, be due to the CdTe starting material used. It was also postulated that at least some impurities (in addition to Cl) may partially come from the CdCl₂ treatment, and/or from the TCO (In) and glass (for Si and Na). Te and S interdiffusion at the CdTe/CdS interface was also shown to be enhanced when 5N CdTe source material is used as compared to 7N.     

Molar volumes in fluoride crystals and glasses

In CaF₂·rRF₃ (R = rare earth or Y), r is a solid solution parameter; the CaF₂ fluorite structure is maintained at 0 < r < 0.7. The process allows a maximum of two volume constants. It is shown from the linear dependence of the crystal volume on r that the process constants are the partial molal volumes of CaF₂ and RF₃. In CaF₂·r_cRF₃ compounds, r_c is a stoichiometry parameter for melt-congruency behavior; the RF₃ tysonite structure (hexagonal) is maintained. In the range 2 < r_c < 4, r_c depends uniquely on R. Only one volume constant - the molar volume of RF₃ in the compound - is allowed by the process. In fluoride-glass formation, the process constant is the gram-anion volume.     

Electrical conductivity of a CaF2-BaF2 nanocomposite

A CaF₂-BaF₂ nanocomposite material has been prepared via 70CaF₂ · 30BaF₂ (mol %) melt solidification. The material has a lamellar structure due to the eutectoid decomposition of the solidified melt. The 500°C ionic conductivity of the composite is 25 and 330 times higher than those of the parent BaF₂ and CaF₂, respectively. The enhanced conductivity of the composite can be understood in terms of the electrical properties of its interfaces. Original Russian Text ? N.I. Sorokin, I.I. Buchinskaya, P.P. Fedorov, B.P. Sobolev, 2008, published in Neorganicheskie Materialy, 2008, Vol. 44, No. 2, pp. 234–237.     

Production of SnO2 nano-particles by hydrogel thermal decomposition method

SnO₂ is an important functional material having a wide range of applications in gas sensors and optoelectronic devices. There is a great interest for finding new cost-effective and straight-forward methods for production of these particles. In this research, hydrogel thermal decomposition method (HTDM) is used for production of high purity SnO₂ nano-particles. Cost effective reactants and green routs of production are the advantages of polysaccharide based hydrogel as starting material for this method. Visual observations indicated that there is very little tendency for agglomeration in the SnO₂ nano-particles produced by this method which can be considered as an advantage for this method over other methods for production of SnO₂ nano-particles. SnO₂ nano-particles are also characterized by X-ray diffraction (XRD) in terms of purity and the sizes. It is found that high purity SnO₂ nano-particles in the size range of 25-36 nm can be produced by HTDM.     

Iron impurities in Si3N4 processing

The atomic environment of iron impurities is investigated during the processing cycle of reaction-bonding silicon nitride (RBSN). Several analysis techniques are utilized, including X-ray photoelectron spectroscopy (XPS), extended X-ray absorption fine structure (EXAFS), and electron spin resonance (ESR), to examine iron impurities in the starting silicon powder, in sintered silicon compacts, and in RBSN materials. Results indicate that iron impurities in as-received metallurgical grade silicon powder are incorporated in the silicon bulk as a highly distorted FeSi₂ compound. No surface iron or iron-based particulate is observed in the starting material. Upon sintering, the iron environment becomes an ordered FeSi₂ structure. In the RBNS material, the FeSi₂ structure is again distorted, as observed by both EXAFS and ESR.     

Eu-doped CaF2 nanocrystals in sol-gel derived glass-ceramics

Glass-ceramics containing Eu³⁺-doped CaF₂ nanocrystals of about 16 nm size have been made using the controlled crystallization at higher temperatures of the Eu³⁺-doped CaF₂-SiO₂ xerogels. In the glass-ceramic material the Eu³⁺ ions are embedded in both silica network and in the non-centrosymmetric sites of the CaF₂ nanocrystals structure. Lower phonon energy of CaF₂ and the dehydratation processes reduce the probability of non-radiative de-excitations and as a result a luminescence enhancement and an increase of the PL lifetime is observed. Eu-doping modifies the traps distribution within the glass-ceramics and as a result an additional broad thermoluminescence peak at about 380 °C was observed. The peak was assigned to the recombination of the electrons thermal released from the Eu³⁺-related traps in the glass matrix and in the CaF₂ nanocrystals.     

Epitaxial growth of conducting CdF2:Er layers

Heterostructures of the CdF₂:Er/CaF₂/CaF₂(111) type were grown by the method of molecular beam epitaxy. Doping with erbium was performed, for the first time in materials of this type, by subliming the metal from an effusion cell immediately during the cadmium fluoride layer growth. A special procedure of CaF₂ substrate preparation for epitaxy was developed. Measurements of the lateral conductivity of the heterostructures by a two-point-probe technique showed that, depending on the dopant (erbium) concentration, the resistivity ranges from 2.5×10⁵ to 50 Ω cm.