Bulk and surface components of electric influence on the magnetoplasticity

The influence of weak electric fields on the magnetically induced mobility of individual dislocations in NaCl crystals has been investigated. It is shown that the strong influence of an electric field on the magnetoplasticity, which is caused by the transformation of impurity centers at dislocations from the diamagnetic state (Me ⁺⁺) to the paramagnetic state (Me ⁺), has both bulk and surface components. The surface effect dominates over the bulk one in weak electric fields (E < 0.5 kV/m), which are insufficient for the exciting bulk electromigration of charged defects near dislocations. However, the contribution of the surface effects becomes insignificant in comparison with the bulk processes in stronger fields E. Lacquering crystal faces to block the surface electromigration of defects excludes this stimulation component at any E. It is shown that dislocation motions occurring due to a rapid switching on an electromagnet also disappear in lacquered samples. These effects observed may be related to the same surface electromigration of defects under the vortex electric fields induced by switching on a magnetic field.     

Electric field induced structural modification and second order optical nonlinearity in potassium niobium silicate glass

Second harmonic generation properties have been studied in 23 K₂O · 27Nb₂O₅ · 50 SiO₂ glass subjected to thermal poling. The poling-induced optical nonlinearity, with χ⁽²⁾ = 3.8 pm/V, has been related to structural modifications within a surface layer of a few microns on the anode side, as evidenced by means of confocal micro-Raman mapping along the sample thickness. The data indicate that the structural changes result from a charge transport process that causes network modifications in an alkali depleted layer whose thickness is comparable with that of the non-linear region. The Raman data also indicate that in the alkali depleted layer the network polymerization degree increases as a consequence of ion migration. The origin of the nonlinearity and the mechanisms activated by poling are discussed. The mechanism of non-bridging-oxygen to bridging-oxygen bond switching is proposed to explain ion migration and the subsequent structural changes in the glass.     

Bulk GaN crystals grown by HVPE

We succeeded in preparing very thick c-plane bulk gallium nitride (GaN) crystals grown by hydride vapor phase epitaxy. Growth of the bulk GaN crystals was performed on templates with 3 μm GaN layer grown by metal organic chemical vapor deposition on (0 0 0 1) sapphire substrates. Colorless freestanding bulk GaN crystals were obtained through self-separation processes. The crystal's diameter and thickness were about 52 and 5.8 mm, respectively. No surface pits were observed within an area of 46 mm diameter of the bulk GaN crystal. The dislocation density decreased with growth direction (from N-face side to Ga-face side) and ranged from 5.1×10⁶ cm⁻² near the N-face surface to 1.2×10⁶ cm⁻² near the Ga-face. A major impurity was Si, and other impurities (O, C, Cl, H, Fe, Ni and Cr) were near or below the detection limits by SIMS measurements.     

Coarse crystal layer growth and liquid entrapment study with gradient freeze technology

The coarse crystal layer growth and liquid entrapment processes were investigated with gradient freeze technology in this paper. The research system was hemihydrate phosphoric acid (H₃PO₄·0.5H₂O) crystal‐phosphoric acid aqueous solution. The distribution coefficients of ions (Ca²⁺, Fe³⁺ and Na⁺) in this system were measured. The effect of supercooling degree gradient on layer growth and the effect layer growth rate on ions redistribution were studied. The result indicated that the layer growth rate increased with supercooling degree gradient as an exponential curve. The distribution coefficient tended to increase as an approximate ‘S’ curve when coarse crystal layer growth rate increased. The ‘three region’ theory was applied to explain this phenomenon. Each ion's diffusion parameter was obtained, which contributed to explain the separation differences between different ions. The work in this paper also indicated that layer crystallization was an effective separation technology for electronic grade phosphoric acid preparation.     

Formation of nanoclusters through silver reduction in glasses: The model

The system of equations describing the formation of silver nanoclusters through the reduction of ionic silver in the course of thermal processing of silver containing glasses in hydrogen is formulated and solved numerically. The processes of the clusterization of neutral silver within the glass and at the surface, and the growth of the nanoclusters are modeled with the account of different mobilities and concentrations of the participating species. The influence of the variation of diffusion coefficients of Ag⁰ atoms, Ag⁺ and H⁺ ions as well as concentration of Ag⁺ ions in the glass matrix. It is shown that concentration distribution of neutral hydrogen (H⁰) strongly depends on the relationship between concentration of Ag⁺ ions (C_Ag⁺) and concentration of neutral hydrogen at the glass surface (C₁), as well as between diffusion coefficients of silver and hydrogen ions. When D_Ag⁺ ≪ D_H⁺ and C_Ag⁺ ≫ C₁, the total hydrogen concentration profile (C_Htot = C_H⁰ + C_H⁺) is defined by silver ion diffusion coefficient D_Ag⁺. Concentration of neutral silver, C_Ag⁰, represents a bell-shaped depth distribution, whose maximum moves to the depth linearly with t^1/2 with the rate depending both on the diffusion coefficient of neutral silver, D_Ag⁰, and neutral hydrogen, D_H⁰. It is shown that the depth position of the maximum coincides with the frontier of the layer filled by clusters. The kinetics of the cluster radius growth in the bulk of the glass also depends both on D_Ag⁰ and D_H⁰ and essentially deviated from the kinetic law R² ∼ t predicted earlier in the assumption of time-independent oversaturation. The kinetics of Ag cluster growth on the glass surface turned out independent of the D_Ag0 diffusion coefficient.     

Na2CaSi2O6–P2O5 based bioactive glasses. Part 1: Elasticity and structure

The glass structure and elastic properties of two bioglasses having bulk compositions near Na₂CaSi₂O₆ (45S5.2) and Na₂CaSi₃O₈ (55S4.1) were studied using both Raman and Brillouin scattering techniques. The annealed 45S5.2 glass has more Q² and Q⁰ but less Q³ species than 55S4.1 glass due to lower (Si⁴⁺ + P⁵⁺)/(Na⁺ + Ca²⁺) ratio. Brillouin scattering measurements of the as-annealed glasses indicated that 45S5.2 glass is ca. 2% and 9% higher in Young’s and bulk moduli than 55S4.1 glass due to more modifiers in the 45S5.2 glass. Nearly full crystallization of 45S5.2 glass was observed after treating it at 715 °C for ca. 30 min. Devitrification of the 45S5.2 glass caused an increase in the elastic moduli up to ca. 30% (fully crystallized) but a negligible change in density. This 45S5.2-derived crystalline phase displayed at least 17 Raman bands, and has the average elastic moduli of 72.4 (bulk), 41.6 (shear), and 104.7 (Young’s) GPa. The comparable elastic moduli with hydroxyapatite and the ability for developing a HCA layer in simulated body fluid indicate that the 45S5.2-derived phase may be better for using as a substitute of bone than its parent glass.     

Ablation and compaction of amorphous SiO2 irradiated with ArF excimer laser

The structure of amorphous SiO₂ exposed to ArF excimer laser irradiation was examined. Threshold fluence for causing ablation with a single pulse depended on sample preparation: more specifically, 1 J/cm² for thermally grown SiO₂ films on silicon and 2.5 J/cm² for bulk SiO₂. It was found that the bond angle of Si-O-Si was reduced by irradiation near the interface of thermally grown SiO₂ films. In contrast, evolution of the bond angle by irradiation was absent in both the bulk SiO₂ and SiO₂ film-near the top surface, even though the concentration of puckered four-membered rings deduced from Raman spectra dramatically increased. It is assumed that planar three-membered rings were generated in the SiO₂ thin layer near the interface, and puckered four-membered rings were generated in the bulk SiO₂. The concentration of both the Si³⁺ and Si²⁺ structure was increased at a fluence of 800 mJ/cm² with an increasing number of pulses, although generation of both was absent at higher fluence for a single pulse. The author proposes that the structure of SiO₂ is created by flash heating and quenching by pulse laser irradiation. Structural similarities were found between the irradiated SiO₂ and SiO₂ at high temperatures.     

Structure of porous surface layers of single-crystal GaAs(001) wafers from data of X-ray diffractometry and reflectometry

The surface morphology and structure parameters of the surface layers of the single-crystal GaAs(001) wafers subjected to He⁺ ion implantation (E = 300 keV, D = 10¹⁶ atoms/cm²) and subsequent electrochemical etching in a 0.5 M H₂SO₄ aqueous solution are investigated by X-ray reflectometry and double-crystal X-ray diffractometry. The strain and amorphization profiles over the layer thickness are determined from X-ray diffraction data. The density of surface layers, their thickness, and the changes of the surface relief upon etching are evaluated from the reflectometric data. The experimental parameters of the studied layers are compared with the theoretical distributions of implanted impurities and intrinsic point defects, which are calculated by the Monte Carlo method. A correlation is revealed between the layer thickness and the depth of the maximum defect concentration. It is found that the electrochemical etching predominantly occurs in strongly amorphized regions of the surface layer and does not lead to a change in the total layer thickness. The results of X-ray diffraction investigations are confirmed by scanning electron microscopy.     

Solitons in Polyacetylene Induced and Probed by Positive Mupons

Abstract

Positive muons were injected into both trans- and cis- (CH) as well as (CD)_x. In cis-polyacetylene, the μ⁺ was found to form a radical state with an unpaired electron localized near the μ⁺ In trans-polyacetylene, the longitudinal μ⁺ spin relaxation rate at 293 K showed H^?1/2 dependence on the applied field H. After considering possible mechanisms, we concluded that it is due to the soliton-like one-dimensional motion of induced unpaired electron.     

The validity of the double layer model on the amorphous semiconductor surface including the bulk localized state within the gap

Under the two assumptions that the origin of surface states may be different from that of bulk localized states within the gap and the density of surface states is sufficiently high, the validity of the double layer model on the amorphous semiconductor surface is investigated in comparison with the case of a crystal. It is suggested that the criteria concerning the double layer should be determined by the relative value of the surface states to that of bulk localized states. The existence of the double layer can be confirmed when the bulk localized state density n(E_f) is smaller than 10¹⁹ cm^?3 eV^?1. When n(E_f) is high at about 10²⁰ cm^?3 eV^?1, surface states cannot be distinguished from the localized states within the gap. This double layer model is strongly supported by the results of previous experiments by others who have measured the dependence of the Schottky barrier height on the work function of metal and the dependence of the surface potential on the preparation conditions of a-SiH.     

Underlying reverse current mechanisms in a-Si:H p-i-n solar cell and compact SPICE modeling

The defect states in bulk of i-layer and at p⁺/i interface have been studied by using dark reverse current-voltage (J-V) measurements. The dark reverse current as a function of voltage has been analyzed on the basis of thermal generation of the carriers from mid-gap states in i-layer. Based on its behavior the thermal generation mechanism has been divided into two types. Thermal generation at lower bias (<5 V) shows V^1/2 behavior, whereas at higher bias follows an exponential dependence of voltage (>5 V). This was explained using a thermal generation zone at lower bias, which is a source of reverse currents, and its evolution towards p⁺/i interface with increasing voltage. The analytical result has shown that at lower reverse bias (V < 5 V) the defect states in the bulk of i-layer and at higher bias (V ∼ 25 V) the defect states at p⁺/i interface are contributing to the reverse currents. Reverse bias annealing (RBA) treatment which has been performed on these cells shows that a reduction of defect states more in the i-region near to the p⁺-layer and at p⁺/i interface as compared to the deep regions in bulk of i-layer. The calculated defect state density (DOS) is varying from its intrinsic value of 2.4 × 10¹⁷ cm⁻³ in the bulk of the i-layer up to 2.1 × 10¹⁹ cm⁻³ near and at p⁺/i interface. These values decrease to 7.1 × 10¹⁶ cm⁻³ and 2.7 × 10¹⁷ cm⁻³, respectively, in the samples annealed under reverse bias at 2 V. The bias dependent leakage current behavior has been modeled and implemented in simulation program with integrated circuit emphasis (SPICE) using simple circuit elements based on voltage controlled current sources (VCCS). Simulated and measured reverse leakage current characteristics are in reasonable agreement.     

Structural change of OH-free fused quartz tube by blowing with hydrogen-oxygen flame

The effect of blowing with a hydrogen-oxygen flame on the structure of an OH-free fused quartz tube was studied by microscopic spectroscopy. The number of OH groups increased within 1000 μm from the outside surface (OSS). The peak decomposition of the IR absorption at around 3600 cm⁻¹ showed that most OH structures were ‘free SiOH’ without the hydrogen bond, and H₂O molecules were also distributed throughout the cross section. The distribution of the fictive temperature, T_F, in the cross section was measured from the peak position of infrared absorption at ≈2200 cm⁻¹. T_F before blowing was ≈1420 K, which is near the strain point, except in the region within 200 μm from the OSS, where T_F steeply decreases approaching the OSS. After blowing, the values of T_F became greater than those before blowing, increasing steeply from the OSS and having a maximum at ≈1000 μm. The steep change near the OSS must be caused by the shorter structural relaxation time due to the SiOH introduced by blowing and H₂O vapor in the flame. The distribution of the intensities of Raman bands at 495 (D₁) and 606 cm⁻¹ (D₂) derived from the planar four- and three-member rings was also measured.     

Production of CN(BΣ) state from dissociative excitation reaction of BrCN with microwave discharge flow of Ar

The mechanism of the dissociative excitation reaction of BrCN with the microwave discharge flow of Ar was investigated based on the CN(B²Σ⁺-X²Σ⁺) emission-spectroscopic and the electrostatic-probe measurements. By passing Ar and BrCN through P₂O₅, the contamination of H₂O molecules into the reaction region was reduced to ≈30%, being confirmed by monitoring the reduction of the OH(A²Σ⁺-X²Π) emission intensity. The variation of the CN(B²Σ⁺-X²Σ⁺) emission intensity on the pressure of Ar, P_Ar, of 0.1-0.3 Torr was compared with that of the density of electrons whose kinetic energy was lower than the ionization energy of BrCN, 11.9 eV. The dissociation of BrCN was found to proceed predominantly via the charge transfer from Ar⁺ followed by the BrCN⁺ − e⁻ recombination. When the contamination of H₂O molecules in the reaction region was not reduced, the dissociation via the energy transfer from metastable Ar atoms became significant. The present study shows that the contamination of H₂O molecules in the reaction system makes significant effect onto the gas-phase plasma process.     

Up-conversion enhancement in Er3 +-Ag co-doped zinc tellurite glass: Effect of heat treatment

The melt quenching method was used to synthesize the Ag⁰ nanoparticles and Er^3 + ions co-doped zinc tellurite glass. The glasses were characterized by differential thermal analyzer, UV–VIS-IR absorption, photoluminescence spectroscopy and TEM imaging. Heat treatment at different annealing time intervals above the glass transition temperature was applied to reduce the Ag⁺ ions to Ag⁰ NPs. The influence of heat treatment on structural and optical properties is examined. Intense and broad up-conversion emissions of silver are recorded in the visible region. Up-conversion luminescence spectra revealed three major emission peaks at 520, 550 and 650 nm originating from ²H_11/2, ⁴S_3/2 and ⁴F_9/2 levels, respectively. An efficient enhancement in visible region is observed for samples containing silver NPs. The absorption plasmon peaks are evidenced around 560 and 594 nm. The effect of localized surface plasmon resonance and the energy transfer from the surface of silver NP to trivalent erbium ions are described as the sources of enhancement.     

Solid-Solid Phase Transitions of Long-Chain n-Alkyltrimethylammonium Halides

Transition properties of some compounds of the type, n-C _n H_2n+1N⁺(CH₃)₃X_-, with n = 10, 12, 14, 18, 22, and X = Cl, Br, and I, were investigated. All these compounds show a solid-solid phase transition with a large enthalpy change in the range of temperature 350 ～ 400K. The transition behaviors were investigated by the aid of DSC, TG, X-ray, and optical microscope. The solid-solid phase transition is regarded to be caused by melting of the layer of hydrocarbon chains of the compounds, while the rigid ionic layer retains the regular arrangement. Some relationships between the physical properties and the structure of the compounds are discussed.     

Investigation of the localization of H+ ions in stoichiometric LiNbO3

The possible positions of H⁺ ions in stoichiometric crystals of lithium niobate are analyzed using computer simulation. It is shown that all hypothetical positions of H⁺ ions at the midpoints of the O-O bonds are unstable and must be excluded from further consideration. An analysis of the trajectories of displacement of H⁺ ions in the structure of LiNbO₃ shows that an H⁺ ion is associated with one of the O^2? ions of the upper oxygen face of the NbO₆ octahedron and that the direction of the dipole moment of the OH^? group differs only slightly from the direction of the short O-O bond.     

Micro-photoluminescence study on defects induced by ion microbeam in silica glass

We have evaluated the irradiation effects by ion microbeam on silica glass for various ion species by means of a micro-photoluminescence technique. Defect generation and refractive index change were observed for silica at the area of 10 μm × 50 μm scanned by ion microbeam of H⁺, He⁺, N⁴⁺, C⁴⁺, O⁴⁺, and Si⁵⁺ with energy from 1.7 to 18 MeV. The μ-PL spectroscopy measurements were performed along the side surface perpendicular to the microbeam irradiated surface. Based on the comparison with a result of SRIM (stopping and range of ions in matter) simulation, the defect generation mechanism was discussed in terms of the energy deposition processes due to electronic and nuclear stopping powers. We conclude that the electronic stopping power is responsible for the defect generation at the track of ions. The effect of the nuclear stopping power is also not negligibly small at the end of range.     

Synthesis,Crystal Structure,and Luminescent Properties of Ag(I) Coordination Polymer with Tricarboxylic Acid and Flexible N-donor Ligand

The reaction of AgNO₃ with combinations of 1,3-bis(4-pyridyl)propane (bpp) and 1,3,5-benzenetricarboxylic acid (H₃btc) in aqueous alcohol/ammonia at room temperature produces crystals of {[Ag₆(H₂O)₂(bpp)₆] · (btc)₂ · 25H₂O} _n (Ι). Single crystal X-ray diffraction analysis reveals that the complex Ι consists of 1D infinite cationic chains of [Ag(bpp)] _n ⁿ⁺ and [Ag(H₂O)(bpp)] _n ⁿ⁺ which are further linked into the cation layer of [Ag(bpp)] _n ⁿ⁺ by Ag···π interactions. The noncoordinated btc^3? serves as template driving surrounding water molecules to aggregate into the anionic water layer. The neighboring anionic water layer and cationic layer were further alternately joined into a 3D sandwich-like framework by hydrogen bonding. In addition, the luminescent properties of Ι were investigated.     

Oxygen and silver diffusion into float glass

In the float glass process, molten glass is floated on a molten metallic tin bath, such that tin penetrates the glass surface. Consequently, the glass has distinctly two different faces; the tin-penetrated face (bottom face) and the opposite face (top face). In this paper, the effects of tin on oxygen and silver diffusion into the top and bottom faces of a soda–lime–silica float glass are reported. It was revealed that oxygen diffusion from the atmosphere into the bottom face at temperatures above glass transition temperature was extremely suppressed near the surface region of the glass. This was not observed for the top face. This effect was ascribed to chemical reactions between the diffused oxygen and Sn²⁺ near the surface of the glass. Silver diffusion was also influenced by the tin due to chemical reactions of Ag⁺ ions with Sn²⁺, leading to the precipitation of nanometer-sized Ag crystals. As a result, the coloration due to the surface plasmon resonance of the Ag crystals was significantly different between the top and bottom faces because of differences in the nucleation and growth processes associated with the Ag crystals.     

Cross‐sectional and surface Raman mapping of thick GaN layers

Raman scattering spectroscopy was utilized for investigation of the structural properties of thick GaN layers. These layers with thickness ∼ 40 μm have been grown by HVPE technique on the sapphire substrates. The investigations have been focused on the strain distribution in GaN layer cross‐section as a function of distance from an interface sapphire/GaN and mapping of the surface and of the inner layer, near the sapphire/GaN interface. From the observed phonon shifts in the Raman spectra strain differences lower than 6.4×10^–4 corresponding to stress differences of 240 MPa were estimated across the thick GaN epitaxial layer. The measurements exhibit that strain in the layer causes changes in the Raman spectra and allow determining the relaxation process in the crystal. The obtained results confirmed, that the mode frequencies in the measured Raman spectra in both directions (parallel or perpendicular to the growth direction) for layer thicknesses over 30 µm are comparable with typical values for bulk material and match the low strain in the structure due to relaxation processes. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)