Deep levels in GaN studied by deep level transient spectroscopy and Laplace transform deep-level spectroscopy

In this study we present the results of investigations on Schottky Au-GaN diodes by means of conventional DLTS and Laplace DLTS methods within the temperature range of 77–350 K. Si-doped GaN layers were grown by Molecular Beam Epitaxy technique (MBE) on sapphire substrates. DLTS signal spectra revealed the presence of four majority traps: two hightemperature and two low-temperature peaks. Using LDLTS method and Arrhenius plots the activation energy and capture cross sections were obtained. For two high-temperature majority traps they are equal to E1 = 0.65 eV, σ₁ = 8.2 × 10^?16cm² and E2 = 0.58 eV, σ₂ = 2.6 × 10^?15 cm² whereas for the two low-temperature majority traps E3 = 0.18 eV, σ₃ = 9.72 × 10^?18 cm² and E4 = 0.13 eV, σ₄ = 9.17 × 10^?18 cm². It was also found that the traps are related to point defects. Possible origin of the traps was discussed and the results were compared with the data found elsewhere [1–5].     

Enhancement on the high-temperature joint reliability and corrosion resistance of Sn–0.3Ag–0.7Cu low-Ag solder contributed by Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Nanoparticles (0.12 wt%)

In this study, the evolution of interfacial microstructures and mechanical properties of the joints soldered with Sn–0.3Ag–0.7Cu (SAC0307) and SAC0307-0.12Al₂O₃ nanoparticles (NPs) aged at 150 °C for different hours (72–840 h) were investigated. It was found the joint soldered with SAC0307-0.12Al₂O₃ displayed a significantly enhanced high-temperature joint reliability, reflected in a higher shear force than that of the original. This enhancement in shear force primarily benefited from the refinement in solder microstructure contributed by Al₂O₃ NPs. As aging time reached 840 h, a controlled growth of interfaical IMC layer resulted from the pinning effect of Al₂O₃ NPs contributed to the increase in shear force. Theoretical analysis showed 0.12 wt% Al₂O₃ NPs effectively lowered the growth constant of total interfacial IMCs (D_T) from 3.19?×?10^?10 to 1.02?×?10^?10 cm² s^?1. Moreover, comparative studies on the corrosion resistances of SAC0307 and SAC0307-0.12Al₂O₃ were also conducted by electrochemical test and analyzed by electrochemical impedance spectroscopy (EIS). The results revealed SAC0307-0.12 Al₂O₃ solder displayed a stronger corrosion resistance (R_t; ~?3.1 kΩ cm² vs?~?7.1 kΩ cm²). This is also related with the tailored microstructure, which provides more grain boundaries for the initial nucleation of passive film.     

Self-organization of three-dimensional lead telluride nanoislands under conditions close to thermodynamic equilibrium

Three-dimensional lead telluride (PbTe) nanoislands were grown on (111)BaF₂ substrates by hotwall epitaxy (HWE) from vapor phase under conditions close to thermodynamic equilibrium and their surface morphology was studied by atomic force microscopy in various growth stages, including the initial stage of nucleation and the subsequent evolution of the size and shape of nanoislands. The distributions of island dimensions in the samples grown under various thermodynamic conditions were statistically analyzed. It is shown that the proposed HWE method ensures the formation of dense (∼8 × 10¹⁰ cm⁻²) self-organized arrays of PbTe quantum dots with parameters comparable with those of the quantum dots of the same material grown by molecular beam epitaxy according to the Volmer-Weber mechanism.     

Simultaneous functionalization and reduction of graphene oxide with diatom silica

The simultaneous coupling and reduction of graphene oxide (GO) with diatom silica (Amphora sp., Navicula ramossisira and Skeletonema sp.) were demonstrated in this work. Binding of GO with diatom silica via direct esterification reaction at 100 °C was observed as well as the reduction of GO. The Raman spectra of GO-diatom silica revealed the typical peaks for reduced graphene oxide at 1350 cm^?1 (D band) and 1585 cm^?1 (G band). Infrared spectroscopy also showed the presence of a unique peak at 1260–1300 cm^?1 indicative of Si–O–C=O bond formation. This confirms the successful functionalization of GO with silica. Scanning electron microscopy showed the presence of GO on the diatom. For the pennate diatoms, Amphora sp. and N. ramossisira, their pores were closed demonstrating that GO was able to cover the surface of the diatom via the Si–O–C bond formation. For the centric diatom, Skeletonema sp., GO was found to be on its rib cage-like body structure and on its centric top. Electrochemical measurements by cyclic voltammetry using a redox probe, K₃[Fe(CN)₆], showed that GO-Amphora and GO-Navicula had more surface negative charge compared with bare GO or bare diatom silica. Furthermore, they demonstrated similar surface charge characteristics as the chemically reduced GO (by hydrazine hydrate). This implies that the composite (reduced GO-diatom) can possibly replace chemically reduced GO (by exposure to hydrazine vapor) and it could probably function as an electrode in sensing cationic biomolecules.     

Modifying the morphology and structure of graphene oxide provides high-performance LiFePO4/C/rGO composite cathode materials

In this study we synthesized LiFePO₄/carbon/reduced graphene oxide (LFP/C/rGO) composite cathode materials using a method involving sol–gel processing, spray-drying, and calcination. To improve the electrochemical performance of LFP/C, we tested graphene oxides (GOs) of various morphologies as conductive additives, including pristine GO, three-dimensional GO, and hydrothermal porous GO (HTGO). Among our samples, the cathode material prepared through spray-drying with the addition of 1 wt% of HTGO (denoted SP-LFP/C/1%rHTGO) displayed the best electrochemical performance; its discharge capacities at 0.1C, 1C, 5C, and 10C were 160.5, 151.8, 138.8, and 130.3 mA h g⁻¹, respectively. From measurements of its long-term cycling performance, the discharge capacity in the first cycle and the capacity retention after 30 cycles at 0.1C were 160.2 mA h g⁻¹ and 99.6%, respectively; at 10C, these values were 132.2 mA h g⁻¹ and 91.8%, respectively. The electronic conductivity of SP-LFP/C/1%rHTGO (6.58 × 10⁻⁵ S cm⁻¹) was higher than that of the pristine LFP/C (9.24 × 10⁻⁶ S cm⁻¹). The Li⁺ ion diffusivities (D_Li⁺) of the SP-LFP/C/1%HTGO cathode, measured using AC impedance (3.91 × 10⁻¹³ cm² s⁻¹) and cyclic voltammetry (6.66 × 10⁻¹⁰ cm² s⁻¹ for discharge), were superior to those of the LFP/C cathode (9.31 × 10⁻¹⁵ cm² s⁻¹ and 1.79 × 10⁻¹⁰ cm² s⁻¹ for discharge, respectively). Galvanostatic intermittent titration revealed that the value of D_Li⁺ was located in a reasonable range from 1 × 10⁻¹⁰ to 1 × 10⁻¹⁷ cm² s⁻¹; its value dropped to its lowest point when the state of charge was close to 50%. Thus, the use of spray-drying and the addition of conductive HTGO (having a 3D wrinkled morphology and interconnected pore structure) can enhance the electronic conductivity and Li⁺ ion diffusivity of LFP/C cathode materials, thereby improving the electrochemical performance significantly.     

Influence of ion energies on the structure,composition, and properties of multilayer Ti–Al–Si–N ion-plasma-deposited coatings

It is established that the energy of deposited particles influences the structure, composition, and properties of multilayer nitride coatings consisting of alternating layers of nanocrystalline TiN and amorphous Si₃N₄ phases with inclusions of nanocrystalline hexagonal AlN formed at energies of titanium, aluminum, and silicon ions exceeding ~317 × 10^–19, 267 × 10^–19, and 230 × 10^–19 J, respectively. As the energy of titanium ions bombarding the substrate increases above ~512 × 10^–19 J, the phase transition from disordered TiN _x to Ti₃N₂ and the appearance of 2- to 3-nm-thick sublayers in 15-nm-thick nanocrystalline TiN _x layers take place in the coating. The maximum hardness of such coatings reaches a level of ~54 GPa.

     

Measurement of the diffusion coefficient of rare earth species in PbO-B2O3 flux used for liquid phase epitaxial growth of magnetic garnet films

The solute diffusion coefficient in a PbO-B₂O₃ flux system used for the growth of magnetic garnet films has been measured by two different techniques and found to be (5±1.5)×10^?7 cm²/sec in the growth temperature range of 840°C–887°C. In the first technique, for a nearly stagnant melt, the film thickness is proportional to the square root of the growth time after subtraction of the residual effects of convection. In the second technique the melt is stirred by rotating the substrate. The growth rate is constant after an initial transient, and is proportional to the square root of the rotation rate.     

Electrical properties of post-oxidized In2O3:Sn films

In₂O₃:Sn (ITO) films were prepared on soda lime glass by evaporating suboxides and oxidizing these in air. The specific resistivity of the films showed a broad minimum of about 2×10^-3ω cm for a doping level range of 10–30 wt.% SnO₂ and they had decreased mobility (10 cm² V^-1 s^-1) and carrier density (2×10²⁰ cm^-3) compared with high performance ITO films obtained by reactive evaporation onto a high temperature substrate. Unlike the behaviour of films treated in vacuum or in forming gases, the change in specific resistivity with oxidation for our films exhibited three characteristic stages: a sharp decrease accompanied by increasing transparency in the first stage, a steep increase in the second and a very slow increase in the third. The lowest resistivity was attained at the end of the first stage with a slight absorption. This change in resistivity was explained qualitatively on the basis of the diffusion model. It was also found that the resistivity could be further reduced if the residual gas pressure during the deposition was maintained as low as possible.     

Mechanically Robust Gel Polymer Electrolyte for an Ultrastable Sodium Metal Battery

Sodium dendrite growth is responsible for short circuiting and fire hazard of metal batteries, which limits the potential application of sodium metal anode. Sodium dendrite can be effectively suppressed by applying mechanically robust electrolyte in battery systems. Herein, a composite gel polymer electrolyte (GPE) is designed and fabricated, mainly consisting of graphene oxide (GO) and polyvinylidene fluoride‐hexafluoropropylene (PVDF‐HFP). With the addition of an appropriate amount of GO content, the compressive Young's modulus of 2 wt% GO+PVDF‐HFP (2‐GPH) composite GPE is greatly enhanced by a factor of 10, reaching 2.5 GPa, which is crucial in the suppression of sodium dendrite growth. As a result, uniform sodium deposition and ultralong reversible sodium plating/stripping (over 400 h) at high current density (5 mA cm^?2) are achieved. Furthermore, as evidenced by molecular dynamics simulation, the GO content facilitates the sodium ion transportation, giving a high ionic conductivity of 2.3 × 10^?3 S cm^?1. When coupled with Na₃V₂(PO₄)₃ cathode in a full sodium metal battery, a high initial capacity of 107 mA h g^?1 at 1 C (1 C = 117 mA g^?1) is recorded, with an excellent capacity retention rate of 93.5% and high coulombic efficiency of 99.8% after 1100 cycles.     

Influence of precursor molar concentration on the electrical and magnetic properties of synthesized MnS nanocrystals

MnS Nanocrystals have been synthesized with 1:1, 1:2 and 2:1 molar ratio of precursors using wet chemical method. The electrical and magnetic properties of as-synthesized MnS nanocrystals have been investigated using the impedance spectroscopy and vibrating sample magnetometer respectively. An increase in dielectric constant from 68.5 to 87.9 with increasing Mn content and decrease to 54.1 with increase in the sulfur content was observed. Both the dielectric constant and the loss factor increased with temperature due to the Maxwell–Wagner type interfacial space-charge polarization. The Cole–Cole plot confirmed that the conduction in as-synthesized samples are through the grain and grain boundaries. The resistance and capacitance of grain and grain boundaries have been calculated. The grain resistance varies from 248 to 199 Ω whereas the grain boundary resistance varies from 16 to 6.7 KΩ over the temperature ranges of 323–473 K for 1:1 sample. It endorsed the NTCR type behavior in all the samples. The electric modulus representation revealed the well-defined relaxation peaks. The relaxation time versus temperature behavior revealed that the relaxation time decreases with increase in temperature. AC-conductivity (σ_ac) increases with increasing frequency and temperature. σ_ac increases from 1.58 × 10^?5 to 1.51 × 10^?3 S cm^?1 with the increase of Mn content and then decreases to 1.22 × 10^?8 S cm^?1 with increase of sulfur content. The activation energy is found to be 0.35 eV (1:1), 0.64 eV (1:2) and 0.28 eV (2:1) at 3 kHz. The chemically modified MnS nanocrystals exhibit paramagnetic behavior. A typical saturation magnetization of 0.56, 0.38, and 0.57 emu/g and coercivity of 2.31, 8.42, and 5.57 Oe at room temperature for 1:1, 1:2 and 2:1 sample respectively.     

Preparation of superconducting LiTi2O4 thin films

Superconducting LiTi₂O₄ thin films were prepared for the first time by the r.f. magnetron sputtering method and subsequent heat treatment. Their superconducting transition temperatures are 10.9–11.5 K, close to the transition temperatures of samples prepared by sintering or metling. Uniform films shows metallic conduction and have a resistivity of $\text{(4.3?8.8) × 10}{}^{-4}\text{Ω}\text{cm at}\text{15}\text{K}$ and a superconducting critical current density of 1.2 × 10⁴ A cm^-2 at 4.2 K. It was also found that the films are stable in an argon gas atmosphere.     

Synthesis of zirconium silicide in Zr thin film on Si and study of its surface morphology

Zirconium silicide was synthesized on Si (100)/zirconium interface by means of swiftly moving 150 MeV Au ion beam. Thin films of zirconium (~60 nm) were deposited on Si (100) substrates in ultra high vacuum conditions using the electron-beam evaporation technique. The system was exposed to different ion fluencies ranging from 3 × 10¹³ to 1 × 10¹⁴ ions/cm² at room temperature. Synthesized zirconium silicide thin film reasonably affects the resistivity of the irradiated system and for highest fluence of 1 × 10¹⁴ ions/cm² resistivity value reduces from 84.3 to 36 μΩ cm. A low resistivity silicide phase, C-49 ZrSi₂ was confirmed by X-ray analysis. Schottky barrier height was calculated from I–V measurements and the values drops down to 0.58 eV after irradiation at 1 × 10¹⁴ ions/cm². The surface and interface morphologies of zirconium silicide were examined by atomic force microscopy (AFM) and scanning electron microscopy (SEM). AFM shows a considerable change in the surface structure and SEM shows the ZrSi₂ agglomeration and formation of Si-rich silicide islands.     

Growth of polycrystalline Cd3As2 films on room temperature substrates by a pulsed-laser evaporation technique

Cd₃As₂ films were prepared by a pulsed-laser evaporation technique. The deposition onto fused quartz substrates was carried out in a vacuum chamber under a background pressure of about 0.6 × 10^?4 Pa. Energy-dispersive X-ray analysis, scanning electron microscopy, transmission electron microscopy and Hall measurements were used to characterize the films. The properties of the film as a function of their thickness and of the deposition parameters are discussed. It is shown that polycrystalline layers of Cd₃As₂ grow on substrates held at temperatures as low as 295 K. The room temperature electron concentrations and mobilities for such layers were (3.5–5.3) × 10¹⁸ cm^?3 and (0.6–1.06) × 10³ cm² V^?1 s^?1 respectively.     

Self-absorption of Hβ line in pellet ablation clouds in devices with magnetic confinement of high-temperature plasma

We have determined the optical absorption depth for H_β line in clouds of secondary plasma near pellets ablating in high-temperature plasma of devices with magnetic confinement. It is established that, for all values of the electron concentration (10¹⁶–3 × 10¹⁷ cm^?3) and electron temperature (2–5 eV) measured in typical impurity pellet clouds, the absorption is negligibly small. At the same time, it can be significant in clouds of higher density (10¹⁷–5 × 10¹⁷ cm^?3) with a temperature of 1–2 eV, which are typical of cryogenic fuel pellets.     

Using Graphene Oxide to Enhance the Barrier Properties of Poly(lactic acid) Film

The ultra‐thin (polyethyleneimine/graphene oxide)_n [(PEI/GO)_n]multilayer films on poly(lactic acid) (PLA) were constructed via the layer‐by‐layer assembly. Here, the electrostatic interactions between PEI and GO were used to obtain the nanoscale composite membrane of (PEI/GO)_n on the surface of PLA film. With the number of assembling layers increased, the oxygen permeability (PO₂) of PLA film decreased substantially. As a 0.06 wt% GO solution was used with only four layers, the PO₂ decreased from 53.8 to 0.377 × 10^?4 cm³/m²/d/Pa, only 0.7% of the original PLA film. At the same time, the coated PLA film also presented a good transparency and better mechanical properties. It is a novel way to use GO on biodegradable packaging materials. Copyright © 2014 John Wiley & Sons, Ltd.     

Multifilamentary superconducting (NbTa)-Sn Composite wire by solid-liquid reaction for possible application above 20 tesla

Nb alloyed with Ta was employed in fabricating multifilamentary composite wires of (NbTa)-Sn using the liquid-infiltration process. The superconducting A15 phase was formed with subsequent heat treatments at 800–950°C by the solid-liquid reaction. High inductive T_c's of 18.2K with sharp transition width (<0.3K) and high overall J_c's of ～1.6 × 10⁴ A/cm² at 20T and 4.2K were obtained. It was found that 2 wt.% Ta in the Nb was sufficient in the enhancement of the overall J_c at the high fields and in increasing the H_c2 (4.2K) to 25T.     

Growth and some properties of barium-cadmium formate single crystals: (I) Crystal growth and morphology

The growth of barium-cadmium formate BaCd(HCO₂)₄·2H₂O single crystals by slow cooling method and their characterization by selective etching are reported. It was found that BaCd(HCO₂)₄·2H₂O crystallizes from aqueous solution in 2/m class of the monoclinic system. Crystals grown during a period of 1 month have dimensions of about 2 × 1.5 × 10 cm³. The typical twinning for these crystals has been observed and investigated by the selective etching. The dislocation density has been estimated to be 3·10² – 2·10³ cm^?2.     

Infrared spectroscopic characterization of Na5Lu9F32 single crystals doped with various Er3+ concentrations

This work reports on optical spectra of Na₅Lu₉F₃₂ single crystals doped with various Er³⁺ concentrations from 0.5 to 5 mol%. In our improved Bridgman method, the X-ray powder diffractions were investigated and optical parameters were also calculated by the Judd–Ofelt theory. Results showed that Er³⁺ ions entered the Lu³⁺ sites successfully without causing any obvious peak changes, and the doping concentration of Er³⁺ had important influence on the Er³⁺ local structure in Na₅Lu₉F₃₂ crystals. The maximum emission intensities of ~1.5 and ~2.7 μm were obtained in present research when the doping concentration of Er³⁺ were 4 and 5 mol%, respectively, under the excitation of 980 nm LD. In these doping concentration, the maximum emission cross-sections were calculated to be 1.37 × 10^?20 cm² (~1.5 μm) and 2.1 × 10^?20 cm² (~2.7 μm). The gain cross-section at 2.7 μm was also estimated according to the absorption and emission cross section spectra. All these spectroscopic characterizations suggested that this fluoride crystal would possess promising applications in infrared lasers.     

Removal of barium impurities from selenium by vacuum distillation

We have studied the behavior of barium impurities in the form of BaF₂ and BaO during vacuum distillation of selenium. The effective Ba partition coefficient is shown to depend on the impurity concentration and evaporation rate. At initial barium contents in the range 3 × 10⁻³ to 2 × 10⁻⁵ wt %, the effective Ba partition coefficient is 600 and 1.5 at distillation rates of 4.7 × 10⁻⁶ and 1.5 × 10⁻³ cm³/(cm² s), respectively. The chemical form of barium has no significant effect on its partition coefficient. Using a Rayleigh distillation equation, we have determined the equilibrium partition coefficient and the thickness of the diffusion boundary layer in the system.     

Defect Structure of GaAs Layers Implanted with Nitrogen Ions

Structural defects formed in epitaxial GaAs layers as a result of 250-keV N⁺ ion implantation to doses within 5 × 10¹⁴–5 × 10¹⁶ cm^–2 have been studied by the X-ray diffraction (XRD) and transmission electron microscopy techniques. No amorphization of the ion-implanted layer took place in the entire dose range studied. The implantation to doses of 5 × 10¹⁴ and 5 × 10¹⁵ cm^–2 led to the appearance of an additional peak on XRD curves, which was related to the formation of a stressed GaAs layer with positive deformation arising due to the formation of point-defect clusters. The implantation to a dose of 5 × 10¹⁶ cm^–2 led to the formation of a dense structure of extended defects in the implanted layer, which was accompanied by the relaxation of macrostresses to the initial state.