Distribution Of Structural Defects Research Articles

Spectroscopy of crystals with a random distribution of structural defects for the case Debye and Koiwa-Hasiguti relaxations

Spectroscopy of crystals with a random distribution of structural defects for the case Debye and Koiwa-Hasiguti relaxations

Rutile Crystallites Isolated from Degussa (Evonik) P25 TiO2: Highly Efficient Photocatalyst for Chemoselective Hydrogenation of Nitroaromatics

We report that the rutile crystallites, isolated from Degussa (Evonik) P25 TiO2 by a hydrofluoric acid treatment, behave as a highly efficient photocatalyst for hydrogenation of nitroaromatics. Photoirradiation (λ >300 nm) of the isolated rutile particles with alcohol as a hydrogen source successfully promotes chemoselective hydrogenation of nitroaromatics to anilines, with an activity higher than that of commercially available rutile TiO2. The high activity of the isolated rutile particles is due to the specific distribution of structural defects (oxygen vacancy sites) on the particles. These particles contain a relatively small number of inner defects behaving as recombination centers for photoformed electron (e–) and positive hole (h+) pairs, and a relatively large number of surface defects behaving as reduction sites for nitroaromatics. Photoexcitation of the isolated particles therefore promotes efficient charge separation between e– and h+, and facilitates rapid reduction of nitroaromatics adsorbed ...

Fabrication of improved-quality seed crystals for growth of bulk silicon carbide

Sublimation epitaxy in a vacuum has been used to grow silicon carbide layers with a low defect concentration and with SiC wafers cut from single-crystal ingots produced by the modified Lely method serving as substrates. It is concluded on the basis of a comparative study of the distribution of structural defects in substrates and epitaxial layers that an isomorphic pair composed of an epitaxial layer and a single-crystal SiC substrate, which is a composite of materials produced by the same method (sublimation synthesis), but in different technological conditions, is promising for fabrication of improved-quality silicon carbide seed crystals.

Structural defects in germanium single crystals

The origin and distribution of structural defects (dislocations, small angle grain boundaries, slip lines, polycrystalline inclusions) is studied in large single crystals of germanium applied in manufacturing optical elements.

Comparative study on l-alaninium maleate single crystal grown by Sankaranarayanan–Ramasamy (SR) method and conventional slow evaporation solution technique

Single crystals of l-alaninium maleate (LAM) were successfully grown by Sankaranarayanan–Ramasamy (SR) method and conventional slow evaporation solution technique, which have the sizes of 32 mm in length, 18 mm in diameter and 18×13×2 mm 3. The grown LAM crystals were subjected to single crystal X-ray diffraction, high resolution X-ray diffraction (HRXRD), chemical etching, dielectric permittivity, dielectric loss, thermo-gravimetric–differential thermal analysis, UV–vis NIR and Vickers microhardness analysis and the results were compared. Single crystal X-ray diffraction confirmed the lattice parameters of the grown LAM crystals. The HRXRD analysis indicated that the crystalline perfection of the SR method grown LAM crystals is quite good without having any internal structural grain boundaries. Chemical etching studies represent the distribution of structural defects in LAM crystal and etch pit density of conventional and SR method grown LAM crystal was calculated. The dielectric permittivity and loss measurement were made as function of temperature in the range 40–140 °C. From thermo-gravimetric–differential thermal analysis of the grown crystal, the thermal stability was found to be up to 158 °C. The range and percentage of optical transmission are represented by recording UV–vis analysis. Mechanical strength of the grown LAM crystals was analyzed by Vickers microhardness test and the hardness of SR method grown LAM crystal is higher than the hardness of the crystal grown by conventional method.

Raman Spectroscopy Evaluation of Oxygen Vacancy Migration by Electrical Field in Multilayer Ceramic Capacitors

Raman spectroscopy is used to evaluate migration behavior of oxygen vacancies in BaTiO3-based multilayer ceramic capacitors with Ni internal electrodes (Ni-MLCCs) before and after a highly accelerated life test (HALT). The B1 phonon mode of BaTiO3 that is associated with oxygen vibration was hardened and broadened near the Ni electrodes before HALT. However, the hardening and broadening of the B1 mode were observed near the cathodes after HALT. It is considered that the oxygen vacancies were localized near the Ni electrodes before HALT and they electromigrated toward the cathode because they have the positive charges. These results show that oxygen vacancies migrated and accumulated near the cathode under a dc bias and caused insulation resistance degradation in MLCCs. In addition, Raman spectroscopy is a powerful tool for investigation of structural defect distribution including oxygen vacancies with a relatively high spatial resolution.

New data of typomorphic features of diamonds from placers in North Timan

The morphological and spectral-optical properties of diamond crystals from placers in North Timan rivers have been studied with IR-spectroscopy and cathode luminescence methods. As a result, correlation between external characteristics of diamonds (size and degree of mechanical damage) and number of optically active centers has been revealed. The habit and the type of distribution of structural defects in diamond crystals studied are comparable with those in diamonds from the Archangelsk Region and North Timan’s placers. Based on the obtained data, origin issues and possible primary sources of diamonds from North Timan’s placers are discussed.

Gettering Effect in Low and High Density Structural Defect Regions of the Cast Multi-Crystalline-Silicon Wafer

AbstractTwo cast multi-crystalline silicon (mc-Si) sister wafers before and after solar cell processing were investigated to explore the gettering effect in low and high density structural defect regions. For the processed wafer, the minority carrier recombination lifetime was correlated to the structural defect distribution. For the low density region of as-grown wafer, Cr impurities were 3.28x1013cm-3 and they were reduced to 1.74×1012cm−3 for the processed wafer. The isolated Cr impurities dissolved from the precipitates at 900°C which is the typical gettering temperature in the solar cell processing are getterd effectively in the low density regions. Our current understanding for the gettering effect in the high density regions is that the segregated Cr impurities at defect sites are not released to the silicon matrix at 900°C resulting in the poor gettering effect.

Stability of the photoresponse of Cd1−x ZnxTe crystals

Variety and distribution of structural defects, as well as composition fluctuations determine individual properties and instability of photoresponse I of the Cd1−xZnxTe-based detectors and spectrometers under severe operational conditions. It is shown that these phenomena are most completely represented by differential spectral characteristics I(hv) and related diagrams dI/dν=f(I).

Excess Carrier Lifetime Measurement of Bulk SiC Wafers and Its Relationship with Structural Defect Distribution

Excess carrier lifetime in bulk 2-in. SiC wafers was measured by microwave photoconductivity decay (µ-PCD). The mapping technique was used to obtain the lifetime distribution in the entire wafer. We observed the birefringence image and X-ray topograph of the wafers in order to determine the structural defect distribution, and the net donor concentration distribution was also observed by capacitance–voltage measurements. By comparison of lifetime maps with the structural defect distribution, it was found that relatively long lifetime regions correspond to regions with high-density structural defects. The net donor concentration did not show a clear influence on the carrier lifetimes. We confirmed that surface recombination has a negligible effect on the carrier lifetimes, and therefore the lifetimes obtained from mapping measurements are mainly dominated by carrier recombination behavior in the bulk of the wafers.

Seeded growth of bulk AlN crystals and grain evolution in polycrystalline AlN boules

Large AlN crystals were grown by powder sublimation in a nitrogen atmosphere at low supersaturation and growth rates of 0.1–0.3 mm/h. The starting deposition surface was a sintered TaC disc. An appropriate adjustment of the system pressure and source-seed temperature gradient during the early stages of growth allowed epitaxial re-growth on AlN seeds that had been exposed to air. Single-crystalline AlN grains of 1 cm in size were achieved through multiple sublimation growth runs conducted at P = 500 Torr and growth temperatures of 2050–2150 °C. Elemental analysis of impurities in the grown AlN boules confirmed low oxygen contamination levels of ∼10 19/cm 3. No discontinuities were introduced in the structural defect distribution in the individual single-crystalline grains by the multiple re-growth steps. Absence of preferred growth directions of grains suggest the epitaxial re-growth process is suitable for seeded single-crystal growth in any orientation.

Excess Carrier Lifetime Mapping for Bulk SiC Wafers by Microwave Photoconductivity Decay Method and Its Relationship with Structural Defect Distribution

Excess Carrier Lifetime Mapping for Bulk SiC Wafers by Microwave Photoconductivity Decay Method and Its Relationship with Structural Defect Distribution

A solid-state 51V NMR characterization of vanadium sites in LiCo xNi 1− xVO 4

LiCo x Ni 1− x VO 4 compounds ( x=0, 0.2, 0.5, 0.8, 1.0) have been prepared for lithium ion batteries by different preparation methods, namely, wet-chemistry (WC) and solid-state (SS) routes. 51V nuclear magnetic resonance (NMR) has been employed to accurately assess the vanadium environments in these materials and results show that vanadium atoms are distributed in octahedral and tetrahedral sites. The fractions of vanadium atoms in tetrahedral/octahedral sites depends on composition and sample preparation conditions. The tetrahedral site, which is particularly dominant in samples produced via the WC route, is more affected by paramagnetic interactions and its 51V NMR spectrum is essentially Gaussian in shape. The octahedral site, on the other hand, exhibits first-order quadrupolar broadened satellites. Even though tetrahedral sites for a given x are comparable for WC and SS materials, there are some differences amongst octahedral environments, namely in the distributions of quadrupolar broadened satellites. The larger satellite distribution observed for WC materials is indicative of a larger distribution of structural defects for these as compared to SS materials.

A solid-state 51V NMR characterization of vanadium sites in LiCoxNi1$minus;xVO4

LiCoxNi1−xVO4 compounds (x=0, 0.2, 0.5, 0.8, 1.0) have been prepared for lithium ion batteries by different preparation methods, namely, wet-chemistry (WC) and solid-state (SS) routes. 51V nuclear magnetic resonance (NMR) has been employed to accurately assess the vanadium environments in these materials and results show that vanadium atoms are distributed in octahedral and tetrahedral sites. The fractions of vanadium atoms in tetrahedral/octahedral sites depends on composition and sample preparation conditions. The tetrahedral site, which is particularly dominant in samples produced via the WC route, is more affected by paramagnetic interactions and its 51V NMR spectrum is essentially Gaussian in shape. The octahedral site, on the other hand, exhibits first-order quadrupolar broadened satellites. Even though tetrahedral sites for a given x are comparable for WC and SS materials, there are some differences amongst octahedral environments, namely in the distributions of quadrupolar broadened satellites. The larger satellite distribution observed for WC materials is indicative of a larger distribution of structural defects for these as compared to SS materials.

Extended defects and polarity of hydride vapor phase epitaxy GaN

Hydride vapor phase epitaxy (HVPE) GaN layers on sapphire substrates and so-called free-standing platelets (layers removed from the sapphire) were studied by different transmission electron microscopy (TEM) techniques. Polarity, determined by convergent beam electron diffraction (CBED), and distribution of structural defects, determined by conventional TEM, are discussed. The HVPE layers were found to grow primarily with Ga-polarity. A few inversion domains (areas with N-polarity) were observed on the substrate side of one of the free-standing layers. The dominant structural defects in HVPE GaN layers are threading dislocations. A systematic reduction of their density with an increase in layer thickness was observed for all of the samples. The experimental results indicate that the density of dislocations is inversely proportional to the distance from the substrate, which agrees with the theoretical model.

Effects of structural defects on the activation of sulfur donors in GaN xAs 1− x formed by N implantation

The effects of structural defects on the electrical activity of S-doped GaN x As 1− x layers formed by S and N co-implantation in GaAs are reported. S and N ions were implanted to the depth of about 0.4 μm. Electrochemical capacitance voltage measurements on samples annealed at 945°C for 10 s show that in a thin (<0.1 μm) surface layer the concentration of active shallow donors is almost an order of magnitude larger in S and N co-implanted samples than in samples implanted with S alone. The activation efficiency of S donors also shows a broad minimum at a depth of about 0.2 μm below the surface. The results of these electrical measurements are correlated with the distribution of structural defects revealed by transmission electron microscopy (TEM). The TEM micrographs show that in addition to a band of dislocation loops commonly found in ion implanted GaAs, an additional band of small voids is observed in samples co-implanted with S and N. The location of this band correlates well with the region of reduced electrical activation of S donors, suggesting that the formation of the voids through N accumulation results in a lower concentration of “active”, substitutional N atoms.

Synthesis of Superlattices of Intercalated Transition Metal Dichalcogenides

Thin films of oriented Fe and Cr intercalated (NbSe2)x(TiSe2)y superlattices have been synthesized by controlled annealing of elementally modulated reactants. Analytic techniques have been developed to extract the average c-axis structure of the unit cell, the distribution of c-axis crystal sizes, and the distribution of structural defects from X-ray diffraction data. High-resolution synchrotron source X-ray diffraction data were collected from a [(Fe0.33NbSe2)8(Fe0.33TiSe2)8]12 sample (i.e., 12 superlattice unit cells in the film, each cell containing 8 layers of Fe0.33NbSe2 followed by 8 layers of Fe0.33TiSe2) at three anomalously scattering wavelengths. Analysis determined structural parameters within 2% of published values for the parent compounds, typical crystal sizes of 200−500 A, and typical coherence losses over a unit cell of ±1%. Theoretical diffraction profiles calculated directly from the model structure are shown to agree satisfactorily with experiment. The flexibility offered by our growth ...

An equilibrium study on the distribution of structural defects between the lamellar and amorphous portions of poly(vinylidene fluoride) and (vinylidene fluoride-tetra fluoro ethylene) copolymer crystals

An equilibrium study on the distribution of structural defects between the lamellar and amorphous portions of poly(vinylidene fluoride) and (vinylidene fluoride-tetra fluoro ethylene) copolymer crystals

An equilibrium study on the distribution of structural defects between the lamellar and amorphous portions of poly(vinylidene fluoride) and (vinylidene fluoride-tetra fluoro ethylene) copolymer crystals

Samples of poly(vinylidene fluoride) (PVF2) and (vinylidene fluoride-tetra fluoroethylene) (VF2-VF4) copolymer were etched with a chromium-based etching reagent. The etching rate was lower for the VF2-VF4 copolymer samples than for the PVF2 samples. The melting point and enthalpy of fusion increased with increased etching time of the etched specimen. This was also true for the melt-quenched (etched) samples, whose values were always lower than those obtained from the direct run of the etched samples. The scanning electron micrographs of specimens etched for 24 h indicated that only the amorphous portion was etched without affecting the crystalline lamella. The sequence distribution of the PVF2 and VF2-VF4 copolymer crystals were determined by 19F NMR measurements of the samples and their etched species. The observed probabilities (Pobs), calculated from the integrated area of the NMR peaks, indicated that the crystalline lamella had a more oriented chain structure than that of the amorphous overlayer portion. The head-to-head defects calculated from the aforementioned sequence analysis indicated a greater propensity in the amorphous portion than in the crystalline lamella. The equilibrium constant (K) for the distribution of defects between the lamella and amorphous portion of the crystal varied from 0.7 to 0.9. It was higher at a higher quenching rate of the crystallization, and in the isothermal crystallization, it also had a substantially high value, indicating the equilibrium inclusion of defects in the crystal. The distribution constant increased with an increase in the defect content in the chain and decreased with an increase in the defect size. The sequence distribution data, analyzed through a suitable melting-point depression equation, indicated a defect energy of 2.25 kcal/mol for the α-phase PVF2 crystals and 0.68 kcal/mol for the β-phase VF2-VF4 copolymer. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 297–308, 2000

Study of the local defect structure of CdTe-Ge crystals by microindentation

This paper demonstrates the effectiveness of the microindentation method as a way to monitor the quality of cadmium telluride crystals. Measurements of microhardness and the length of dislocation rays from the rosette around the indentation pit can clearly identify nonuniform distributions of structural defects, both along the direction of growth and along a diameter of a CdTe boule. The mobility of α dislocations is more sensitive to nonuniformities in the structure than the mobility of β dislocations and the microhardness. A qualitative correspondence is established between the microplasticity characteristics of CdTe:Ge and its germanium content: the hardness of the crystal is insensitive to changes in the Ge content in the range from 3×1016 to 1×1017 at/cm3, but increases sharply beginning at a concentration of ∼1×1017 at/cm3. The correlation between the value of the microhardness and the length of the dislocation rays from the rosette around the imprint of the indentor are analyzed. An estimate is obtained for the hardness of pure CdTe.