Density Of Structural Defects Research Articles

Effect of thallium doping on the thermal conductivity of PbTe single crystals

The thermal conductivity of thallium-doped PbTe single crystals has been measured in the temperature range 80–310 K, and the electronic and lattice (χph) components of their thermal conductivity have been evaluated. The results indicate that thallium doping markedly reduces the lattice thermal conductivity of PbTe single crystals, by up to ∼29%. Heat treatment at ∼473 K reduces the density of structural defects in the crystals and increases their χph. Thallium doping has a significant effect on χph starting at 0.05 at % before annealing and starting at 0.01 at % after annealing. We have calculated the density of structural defects in the unannealed samples and the doping-induced thermal resistance Δ W of the unannealed and annealed samples. ΔW has been shown to increase with thallium content.

Evolution of the microstructure and magnetic properties of Sm(Co0.6Cu0.4)5 alloys prepared with different Sm excess content

The structure of annealed Sm(Co0.6Cu0.4)5 compounds, prepared with different Sm excess content, has been investigated by means of high resolution x-ray diffraction and scanning electron microscopy. The samples were also magnetically characterized by thermomagnetic analysis and M versus H curves at room temperature. Increasing Sm excess improves the compositional order of the 1 : 5 phase. The coercivity (HC) and the Curie temperature (TC) are both changed as a function of Sm excess content. The decrease in the structural defects density, resulting from the compositional order, is responsible for the observed magnetic behaviour.

A novel approach towards selective bulk synthesis of few-layer graphenes in an electric arc

The paper demonstrates the selective bulk synthesis of few-layer graphenes by optimizing an external magnetic field assisted electric arc. An ultra-high purity glassy graphite anode was sublimated in an argon atmosphere, and carbon nanotubes (CNTs), along with graphene sheets, were found inside the deposit formed on the cathode. Both the high purity CNTs and the graphene sheets, with minimal structural defects, were synthesized separately by varying the strength and orientation of the external magnetic field. The as-synthesized carbonaceous samples were characterized with the help of transmission electron microscopy, selected area electron diffraction (SAED), Raman spectroscopy and thermogravimetry with the objective of optimizing the highest selective production of 2D graphene structures. The as-synthesized graphene sheets exhibited a relatively high degree of graphitization and low structural defect density as confirmed by Raman spectroscopy. They were found to exhibit higher oxidation temperature (767 °C) than that of the carbon nanocrystalline particles (690 °C), as inferred from the thermogravimatric analysis. Moreover, they were found to roll up at their edges on account of their surface energy minimization. This was confirmed by the SAED analysis. With this new technique, we could successfully synthesize 2D graphene structures at the rate of a few g h−1.

Surface processing of TlBr single crystals used for radiation detectors

Abstract The processing method for obtaining the high-quality surfaces of TlBr single crystals, providing removal of a mechanically destroyed surface layer by chemical etching, is developed. The crystals grown from the melt of purified materials by the Bridgman–Stockbarger method were used for the experiments. The Vickers microhardness as a structure-sensitive technique was used in a study of the crystal quality and properties of the plastically deformed surface layer created by cutting. It was shown that even under highly accurate conditions of cutting, the depth of the work-hardened surface layer with a high density of dislocations, vacancies and other structural defects exceeds 20 μm. The advantages of offered processing are demonstrated by the spectrometric properties of the TlBr detectors

ZnO/GaN heterostructure for LED applications

White electroluminescence (EL) from ZnO/GaN structures fabricated by pulsed laser deposition of Zn:In onto GaN:Mg/GaN structures MOCVD-grown on Al2O3 substrates has been observed. The white light is produced by superposition of two strongest emission lines, narrow blue and broad yellow, peaked at 440 and 550 nm, respectively. The intensity ratio of different EL lines from ZnO/GaN/Al2O3 structures depends on the ZnO film quality and drive current. The white EL is due to the high density of structural defects at the n-ZnO/p-GaN interface. A band diagram of the n-ZnO/p-GaN/n-GaN is constructed and a qualitative explanation of the EL is given. Conditions of ZnO deposition strongly affects the properties of the recombination emission and predetermines the EL spectrum of the LED structure if it does not have high quantum efficiency (more than 1%) such as in commercial LEDs.

Experimental Observation of Sequential Tunneling Transport in GaN/AlGaN Coupled Quantum Wells Grown on a Free-Standing GaN Substrate

AbstractA GaN/AlGaN multiple-quantum-well structure based on an asymmetric triple-quantum-well repeat unit was grown by molecular beam epitaxy, and its vertical electrical transport characteristics were investigated as a function of temperature. To minimize the density of dislocations and other structural defects providing leakage current paths, homoepitaxial growth on a free-standing GaN substrate was employed. The measured vertical-transport current-voltage characteristics were found to be highly nonlinear, especially at low temperatures, consistent with sequential tunneling through the ground-state subbands of weakly coupled adjacent quantum wells. Furthermore, different turn-on voltages were measured depending on the polarity of the applied bias, in accordance with the asymmetric subband structure of the sample repeat units.

一种新颖改善印刷碳纳米管薄膜的场发射特性

A novel post-treatment method, including hard hairbrush and electrical treatment, is performed intentionally to improve the field emission capability and stability of screen-printed carbon nanotubes (CNTs). Compared with untreated films, the field emission properties of the treated ones are greatly enhanced. Scanning electron microscopy (SEM) and Raman spectrum studies reveal that field emission properties are enhanced by two factors. Firstly, the improved field emission properties of CNT films can be attributed to the more active CNT surface by removing the organic material cover on the CNTs. Secondly, the generation of a high density of structural defects and the lower resistance contact to the topside CNT emitters after treatment are all helpful to improving the field emission properties of the treated CNTs.

In situ electron backscattered diffraction of individual GaAs nanowires

We suggest and demonstrate that electron backscattered diffraction, a scanning electron microscope-based technique, can be used for non-destructive structural and morphological characterization of statistically significant number of nanowires in situ on their growth substrate. We obtain morphological, crystal phase, and crystal orientation information of individual GaAs nanowires in situ on the growth substrate GaAs(1 1 1) B. Our results, verified using transmission electron microscopy and selected area electron diffraction analyses of the same set of wires, indicate that most wires possess a wurtzite structure with a high density of thin structural defects aligned normal to the wire growth axis, while others grow defect-free with a zincblende structure. The demonstrated approach is general, applicable to other material systems, and is expected to provide important insights into the role of substrate structure on nanowire structure on nanowire crystallinity and growth orientation.

Origin of trapping in multicrystalline silicon

Defect sites in silicon, which temporarily capture excess charge carriers (traps), are a promising source of information on defect structures relevant for photovoltaic application of the material. In this work the correlation between traps in p-type silicon, structural crystal defects, and impurities is explored in order to find the origin of these traps in multicrystalline silicon. The trap density is compared to the density of different impurities and structural crystal defects. These comparisons reveal that the trap density is positively correlated with the oxygen density and negatively correlated with the density of the metallic impurities analyzed. In addition we show that structural crystal defects are necessary but not sufficient for the existence of high trap densities. In summary, structural crystal defects that are decorated by oxygen precipitates arise as likely origin of trap centers.

Enhanced field emission from printed CNTs by hard hairbrush and electrolytic treatment in NaCl solution

The effects of hard hairbrush and electrolytic treatment on field emission characteristics of the printed carbon nanotubes (CNTs) cathode films were investigated. Significant improvement in emission current density, emission stability and uniformity were achieved for the hard hairbrush- and electrolytic-treated CNTs films. The improved field emission characteristics of CNTs film can be attributed to the CNT surface more actively by removing organic material cover on the CNTs, which dominates initial characteristics of field emission. Besides, the generation of a high density of structural defects and lower resistance contact CNT to the substrate after treatment are all helpful for improving the field emission characteristics of the treated CNTs. It is suggested that hard hairbrush and electrolytic treatment may be a key technology for the commercialization of CNT field emission display.

Defect-dependent annealing behavior of multi-walled carbon nanotubes

We report Raman spectroscopy and electron microscopy studies on multi-walled carbon nanotubes annealed in hydrogen atmosphere at low temperatures (<1300 °C). The pristine samples have different I D/ I G ratios (ratio of the area of D peak to the area of G peak in the Raman spectra) between 0.19 and 2.6. I D/ I G ratio represents defect density in graphitic structures. We find that as the annealing temperature is increased, I D/ I G ratio first increases and then decreases. The annealing temperature at which the I D/ I G ratio is maximum, decreases with increasing the pristine I D/ I G value. We explain this by invoking a mathematical expression for I D/ I G ratio, consisting of an exponential and a stretched exponential term for production and annealing of defects, respectively. The obtained activation energies and the pre-exponential factors of the two contributions are found to vary systematically with pristine I D/ I G ratio. The contributions of various carbon phases to I D/ I G ratio on annealing have been discussed.

Analysis of S-rich CuIn(S,Se)2 layers for photovoltaic applications: Influence of the sulfurization temperature on the crystalline properties of electrodeposited and sulfurized CuInSe2 precursors

This paper reports the microstructural analysis of S-rich CuIn(S,Se)2 layers produced by electrodeposition of CuInSe2 precursors and annealing under sulfurizing conditions as a function of the temperature of sulfurization. The characterization of the layers by Raman scattering, scanning electron microscopy, Auger electron spectroscopy, and XRD techniques has allowed observation of the strong dependence of the crystalline quality of these layers on the sulfurization temperature: Higher sulfurization temperatures lead to films with improved crystallinity, larger average grain size, and lower density of structural defects. However, it also favors the formation of a thicker MoS2 interphase layer between the CuInS2 absorber layer and the Mo back contact. Decreasing the temperature of sulfurization leads to a significant decrease in the thickness of this intermediate layer and is also accompanied by significant changes in the composition of the interface region between the absorber and the MoS2 layer, which becomes Cu rich. The characterization of devices fabricated with these absorbers corroborates the significant impact of all these features on device parameters as the open circuit voltage and fill factor that determine the efficiency of the solar cells.

Dissolution kinetics of dehydroxylated nickeliferous goethite from limonitic lateritic nickel ore

The dissolution kinetics in 2 M H 2SO 4 of variously dehydroxylated nickeliferous goethites was investigated for five oxide-type lateritic nickel deposits. Goethite was the main constituent with minor amounts of quartz, talc, kaolinite and Mn oxides. Dissolution of Fe from heated materials followed the Kabai equation. There was a 9–34-fold increase in the Kabai dissolution rate constant ( k) for samples heated at 340–400 °C due to both the increased surface area (1.5–2.6 fold) and higher density of structural defects (5–10 fold) in the variously dehydroxylated products. The presence of structural Al and Cr in goethite appears to reduce dissolution rate possibly through the greater M 3+–OH, O bond strength relative to Fe 3+, Ni 2+–OH, O. Nickel showed congruent dissolution with Fe indicating that Ni was uniformly incorporated in the goethite structure. Pre-heating goethite to 600–800 °C for 30 min resulted in incongruent dissolution of Fe and Ni. It is postulated that some Ni is ejected from the neo-formed hematite structure and resides on the crystal surface or in voids. These results may contribute to the development of more efficient procedures for Ni extraction including heap leaching of lateritic nickel ores.

K-feldspar phenocrysts in microgranular magmatic enclaves: A cathodoluminescence and geochemical study of crystal growth as a marker of magma mingling dynamics

The crystallization history of zoned K-feldspar phenocrysts in microgranular magmatic enclaves in the Karkonosze granite (SW Poland) reveals that the crystals grew in stirred coeval magmas of contrasting compositions. The growth mechanism and crystal compositions are investigated using cathodoluminescence and profiling by Electron Microprobe and Laser Ablation ICP-MS. These methods provide insight into the crystallization process and the varying compositions of the host melt. The phenocrysts show two types of growth patterns — with or without resorption interfaces. The trace-element distribution, heterogeneous across different zones and within single phenocryst zones, reflects a dynamic process of crystal growth from compositionally heterogeneous magma. Hybridization is also reflected in the density of structural Al–O −–Al defects — features that relate to coupled Ba–Al incorporation into the crystal structure. Differences in structural-defect densities and crystal composition trace the degree of hybridization in the granitic magma during the growth of the K-feldspar phenocrysts.

Pentacene devices: Molecular structure, charge transport and photo response

AbstractIn this feature article, we will discuss to which extend the peculiar growth properties of pentacene on metallic contacts and on gate dielectrics contribute to the device performance of organic thin film transistors. The early growth state of pentacene monolayers is reviewed, as well as the molecular structure of the so called thin film phase. Then, the relation of structural defect densities to trap densities is discussed. The spatially resolved photo response of a pentacene transistor will be presented in the context of injection barriers and contact homogeneity. (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Magnetotransport properties of manganite based magnetic tunnel junctions

We have studied spin-dependent tunneling in La2/3Ba1/3MnO3−δ based trilayer magnetic tunnel junctions fabricated by pulsed laser deposition. In the heteroepitaxial trilayer structures, a strained SrTiO3 layer of different thickness was used as a tunneling barrier. For the junctions with a SrTiO3 barrier, the measured tunneling characteristics deviated considerably from the ideal characteristics expected for elastic tunneling through an ideal barrier. The data indicate that inelastic multistep tunneling as described by the Glazman-Matveev model is present. Our analysis indicates that the SrTiO3 barrier layer consists of two different parts: The interface region with the manganite electrodes containing a high density of structural defects, and the central barrier layer with a small density of defects. The observed temperature dependence of the junction resistance can be explained by a series connection of the resistance contributions from the interfacial and central barrier part. The magnetic field dependence of the junction resistance is found to depend strongly on the barrier thickness and can show complicated behavior due to both a complex magnetic state in the junction electrode and magnetic interactions between the ferromagnetic electrodes. A very large tunneling magnetoresistance up to ΔR/Rp≃1200% is observed that, however, depends sensitively on the magnetic history.

Influence of ZnO seed crystals and annealing on the optical quality of low-temperature grown ZnO nanorods

The influence of ZnO seed crystals and postgrowth annealing on low-temperature aqueous chemically grown ZnO nanorods is analyzed. At the seed crystal/nanorod interface a high density of structural defects leads to emission at 3.332 eV, attributed to excitons bound to structural defects. This peak is absent for seed crystals, very pronounced for rods of shorter lengths grown on seed crystals, and reduced for longer nanorods. After annealing in oxygen and nitrogen atmosphere, the near-band-edge excitonic transitions sharpen and deep-level emission is strongly reduced. Time-resolved photoluminescence measurements show a striking similarity between donor-bound excitons and excitons bound to structural defects.

Structural imperfections and magnetoresistive properties of the ceramic La0.6Sr0.2Mn1,2−xFexO3±δ

The x-ray structural, resistive, magnetic, and Mn55 NMR methods are used to investigate the ceramic magnetoresistive lanthanum manganite perovskites La0.6Sr0.2Mn1,2−xFexO3 (x=0, 0.02, 0.05, and 0.1) annealed at 1170 and 1500°C. It is found that increasing the Fe content decreases the metal-semiconductor phase transition temperature Tms and the Curie temperature TC, and increases the peak of the magnetoresistive effect near these phase transitions and results in growth of the peak at low temperatures, where the magnetoresistive effect is due to tunneling transitions of carriers between crystallites. The wide asymmetric Mn55 NMR spectrum, whose resonance frequency shifts with increasing x to lower frequencies, confirms the high-frequency electron-hole exchange between the Mn3+ and Mn4+ ions and the high degree of lattice imperfections, including not only vacancies but also clusters. The hysteresis in the field dependences of the magnetization at 4.2K is due to a change in the fraction of low-spin Mn2+ in clusters, whose magnetism appears below 42K. The increase of the activation energy with increasing Fe content is explained by the influence of these ions on the structural defect density, the charge carrier concentration, and the electron-hole exchange between heterovalent manganese ions in B positions.

Effects of microwave plasma treatment on the field emission properties of printed carbon nanotubes/Ag nano-particles films

The effects of Ar microwave plasma treatment on field emission properties of the printed carbon nanotubes (CNTs) cathode films using Ag nano-particles as binder were investigated. The field emission J– E characteristics were measured at varied plasma treatment time. Significant improvement in emission current density, emission stability and uniformity were achieved for the Ar treated CNTs films, even though the plasma treatment increased the turn on electric field slightly. High-resolution transmission electron microscopy (HRTEM) and Raman spectroscopy revealed the microstructural changes of CNTs after the plasma treatment. The improved field emission properties of CNTs film can be attributed to the generation of a high density of structural defects after treatment, which increased greatly the possible emission active sites. Besides, the formation of the sharpened and open-ended CNTs tips is all helpful for improving the field emission properties of the treated CNTs.

Reduced structural defect densities in a‐plane GaN layers on r‐plane sapphire through buffer layer engineering

AbstractNonpolar a‐plane GaN epilayers were grown by metalorganic vapour phase epitaxy on r‐plane sapphire substrates using three different buffer layer structures. The investigated structures consisted of GaN grown on: (A) AlN nucleation layer; (B) AlN plus Al0.25Ga0.75N intermediate layer; (C) AlN, Al0.25Ga0.75N, and GaN/Al0.25Ga0.75N superlattice. The samples were characterized by X‐ray diffraction, transmission electron microscopy, and atomic force microscopy. Structure (B) exhibited much lower overall structural defect densities than structure (A), with narrower X‐ray rocking curves by a factor of 1.5‐2. Compared to structure (B), structure (C) showed a factor of 2 lower density of screw dislocations in TEM although the densities of other types of defects were not significantly reduced. Type (B) and (C) structures also exhibited smoother surface morphologies than Type (A). (© 2007 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)