Initial stages of growth of GaN over (0001) Al2O3 substrate using MBE: a crystallographic analysis of the defects

An AlN buffer layer grown on (0001) sapphire substrate by molecular beam epitaxy has been studied. It is found to be made of small grains having a common [0001] axis parallel to that of the substrate. Some grains are rotated around this axis and the angle rotation can reach 20° leading to a new epitaxial relationship (0001)_sap//(0001)_AlN and [11

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0]_sap//[21

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0]_AlN. A model for the atomic structure of one of these grain boundaries is proposed using high resolution electron microscopy and extensive image simulation.     

GaN thin films deposited by pulsed laser ablation in nitrogen and ammonia reactive atmospheres

Well-oriented, crystalline GaN films were grown on (11

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0) sapphire substrates in reactive atmospheres of N₂ and NH₃ by pulsed laser deposition. GaN targets were ablated at 2.8 J cm⁻² and the substrate temperature was varied from 500 to 700°C. The background gas pressure was varied from 0.04 to 0.3 mbar. All the films had a wurtzite structure. The crystal quality and preferential orientation depended on the substrate temperature, laser fluence and the presence of the nitriding atmosphere. For both N₂ and NH₃, the most resistive films were preferentially orientated in the [000l] direction. For 700°C the film resistivity was found to increase from 10⁻³ Ω cm when deposited in NH₃ to 10² Ω cm when deposited in N₂. The band-gap, obtained from optical transmission measurements shifted from 3.1 to 3.4 eV. Violet photoluminescence was found in all samples and was centered at 3.2 eV with a full width at half maximum of 0.2 eV. A broad peak in the yellow, centered at 2.1 eV, was detected for films grown in vacuum and ammonia.     

Comparison of luminescence and physical morphologies of GaN epilayers

In this paper we examine a series of four GaN epilayers grown by MOVPE on sapphire substrates with different AlN buffer layer thicknesses. We examine the effect of the buffer layer thickness on the physical and optical properties of the samples via optical microscopy, cathodoluminescence imaging, photoluminescence, and cathodoluminescence spectroscopy. While the morphological and optical properties of all the films (excepting that with the thinnest buffer layer of 30 nm) are good, i.e. the films are smooth and the luminescence is dominated by excitonic luminescence, a number of circular island like features are observed in all the films whose density decrease with increasing buffer layer thickness. A large circular island present on the sample with the thinnest buffer layer and surrounded by cracks in the 11

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0 directions, displays some interesting acceptor related luminescence.     

Characterisation of dislocations, nanopipes and inversion domains in GaN by transmission electron microscopy

Transmission electron microscopy is used to analyse a range of defects observed in hexagonal GaN films grown on sapphire and GaN substrates by metalorganic chemical vapour deposition. Large angle convergent beam electron diffraction is used to analyse the Burgers vectors of dislocations and to show that hollow tubes, or nanopipes, are associated with screw dislocations having Burgers vectors±c. Weak-beam electron microscopy shows that dislocations are dissociated into partials in the (0001) basal plane, but that threading segments are generally undissociated. The presence of high densities of inversion domains in GaN/sapphire films is confirmed using convergent beam electron diffraction and the atomic structure of the {

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} inversion domain boundary is determined by an analysis of displacement fringes seen in inclined domains.     

Spectroscopic studies of Nd-doped alkali fluoroborophosphate glasses

A new family of three optical glasses of the following chemical composition with 1 mol% of Nd³⁺ were prepared to examine the effects of alkali fluorides in unmixed form:

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, where RF=LiF, NaF and KF. On the basis of the measured values of densities and refractive indices, the dielectric constant, reflection losses, molar refraction, Nd³⁺ ion concentration in glasses and several other physical properties were determined. Absorption spectra of these glasses were recorded and Judd–Ofelt intensity parameters (Ω_λ) have been calculated. Radiative lifetimes (τ_R) and branching ratios (β_R) for the fluorescent levels have been determined. To understand the laser efficiency of these materials, the values of the spectroscopic quality factor (Ω₄/Ω₆) has been evaluated and it is found that glass C could be suggested as a suitable lasing material.     

Classification of 2 × 2 non-strictly hyperbolic systems for plane waves in isotropic elastic solids

The governing equations for plane waves in generally nonlinear isotropic elastic solids are a system of 6 × 6 hyperbolic conversation laws. For the half-space Riemann problem in which the initial conditions at t = 0, x > 0 and the boundary conditions at x = 0, t > 0 are constant, the system is equivalent to 3 × 3 system in the full-space Riemann problem. It is further reduced to a 2 × 2 system due to the fact that one of the characteristic wave speeds is linearly degenerate. For hyperelastic materials for which there exists a potential W whose gradients provide the strains, the wave curves near an isolated umbilic point are represented by the potential of the form

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which contains two parameters k and m. The classification of the geometry of wave curves depends on the values of k and m and can be classified into five cases. The potential function considered here is equivalent to considered by Schaeffer and Shearer where a and b are parameters. The classification presented here seems to provide simpler algebraic expressions. It also renders more refined classification as shown in the paper.     

Simple formulas for the energy loss of ultrarelativistic muons by direct pair production

Simple formulas for direct pair production are derived from the general equation for deeply inelastic lepton scattering. Applications to energy loss by ultrarelativistic muons are discussed. For muons above the critical energy, E_c^μ 200 GeV, where the radiative effects of direct pair production and bremsstrahlung are dominant, the expressions for energy losses are considerably simplified when quoted in terms of the fractional energy loss per radiation length. The differential probability for direct pair production in a thickness x of material of radiation length X₀ for an incident lepton of energy E, mass M, can be expressed as

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where ν is the energy of the produced pair, υ is the fractional energy loss, υ = ν/E, m_e is the electron mass and the variable z is defined by

For indicent muons, this simple expression agrees very well with the exact calculation to within 30% over the entire range of υ, for E 1 TeV. At higher energies complete screening occurs, and the agreement is further improved, to better than 15% (except for the range 0.005 ≤ υ ≤ 0.01 where it is 25%). The integral of this expression gives the energy loss due to direct pair production by muons (complete screening) which is accurate to 10%:

.     

Near-tip fields of growing cracks and resistance curves

This paper gives the formulation and solution of near-tip fields of mode-I cracks growing quasi-statically in compressible elastic perfectly plastic materials. As Poisson's ratio v tends to , the solution approaches the solution of crack growth in incompressible elastic perfectly plastic materials. The rate of crack opening is determined as

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, where β = 5.454 for V = o.3. The evaluation of fracturing based on the criterion of the near-tip opening is discussed.     

Magnetic and electrical characterization of heavily boron-doped diamond

For a heavily boron-doped diamond (BDD) film, temperature variations of the electrical conductivity σ and magnetic susceptibility χ are reported. The room temperature σ 143 (Ω-cm)⁻¹ corresponds to a carrier concentration 10³ ppm, and its temperature variation yields an activation energy E_a 28 meV from 140 to 300 K and E_a0.88 meV from 40 to 80 K. It is argued that larger boron doping leads to lower magnitudes of E_a. The χ vs. T data (1.8–350 K) fits the Curie–Weiss law, with the concentration of paramagnetic species 120 ppm and a diamagnetic susceptibility −0.4×10⁻⁶ emu/g Oe. The results obtained from the measurements of σ and χ are discussed and compared.     

Structural and optical properties of (100 − x)(Li2B4O7) − x(SrO–Bi2O3–0.7Nb2O5–0.3V2O5) glasses and glass nanocrystal composites

Glasses of various compositions in the system (100 − x)(Li₂B₄O₇) − x(SrO–Bi₂O₃–0.7Nb₂O₅–0.3V₂O₅) (10  x  60, in molar ratio) were prepared by splat quenching technique. The glassy nature of the as-quenched samples was established by differential thermal analyses (DTA). The amorphous nature of the as-quenched glasses and crystallinity of glass nanocrystal composites were confirmed by X-ray powder diffraction studies. Glass composites comprising strontium bismuth niobate doped with vanadium (SrBi₂(Nb_0.7V_0.3)₂O_9−δ (SBVN)) nanocrystallites were obtained by controlled heat-treatment of the as-quenched glasses at 783 K for 6 h. High resolution transmission electron microscopy (HRTEM) of the glass nanocrystal composites (heat-treated at 783 K/6 h) confirm the presence of rod shaped crystallites of SBVN embedded in Li₂B₄O₇ glass matrix. The optical transmission spectra of these glasses and glass nanocrystal composites of various compositions were recorded in the wavelength range 190–900 nm. Various optical parameters such as optical band gap (E_opt), Urbach energy (ΔE), refractive index (n), optical dielectric constant and ratio of carrier concentration to the effective mass (N/m^*) were determined. The effects of composition of the glasses and glass nanocrystal composites on these parameters were studied.     

Decrease of dielectric loss in giant dielectric constant CaCu3 Ti4 O12 ceramics by adding CaTiO3

CaCu₃ Ti₄ O₁₂–x CaTiO₃ ceramics (x=0,0.1,0.2,0.3,0.4 and 0.5) was studied by X-ray diffraction, scanning electron microscope and dielectric measurements. It was indicated that some CaTiO₃ entered the boundaries of CaCu₃ Ti₄ O₁₂ grains and/or subgrains. Dielectric measurement showed that the addition of CaTiO₃ lowered the dielectric loss remarkably, especially at low frequencies, while the giant dielectric constant still remained. At room temperature, the dissipation factor of the x=0.5 sample was decreased to 0.02 over the frequency range from 50 to 2000 Hz, and the dielectric constant was kept to be 4000. We explain this phenomenon in terms of internal barrier layer capacitance model by using the impedance spectroscopy analysis.     

Ultrasonic studies of the binary mixtures of ethyl acetate and cresols—application of Kosower and Dimroth treatments

The ultrasonic velocity (ν) studies were carried out at a frequency of 2 MHz (transducer of x-cut quartz crystal) using ultrasonic pulse echo system (model UX4400-M) on cresols in ethyl acetate at constant temperature of 311 K. The values of internal pressure ( π_i) and molar free volume (V_f) were calculated from measured values of ultrasonic velocity (ν), viscosity (η) and density (ρ). An attempt is made to rationalize the ultrasonic velocity (ν), internal pressure ( π_i) and free volume (V_f) of binary mixtures using Kosower solvent parameter (Z), Dimroth solvent parameter (E_T) and Dielectric constant (). It is found that there is linear correlation between ultrasonic velocity and acidity constant pk⁻¹a, indicating the dependence of acidity. Correlation of Ksower and Dimroth parameters with ultrasonic velocity confirms that solvent polarity is an important factor in the variation of ultrasonic velocity in the present investigation.     

Effect of Al2O3 particles on mechanical properties of directionally solidified intermetallic Ti–46Al–2W–0.5Si alloy

The effect of Al₂O₃ particles on microhardness and room-temperature compression properties of directionally solidified (DS) intermetallic Ti–46Al–2W–0.5Si (at.%) alloy was studied. The ingots with various volume fractions of Al₂O₃ particles and mean ₂–₂ interlamellar spacings were prepared by directional solidification at constant growth rates ranging from 2.78×10⁻⁶ to 1.18×10⁻⁴ ms⁻¹ in alumina moulds. The ingots with constant volume fraction of Al₂O₃ particles and various mean interlamellar spacings were prepared by directional solidification at a growth rate of 1.18×10⁻⁴ ms⁻¹ and subsequent solution annealing followed by cooling at constant rates varying between 0.078 and 1.889 K s⁻¹. The mean ₂–₂ interlamellar spacing λ for both DS and heat-treated (HT) ingots decreased with increasing cooling rate according to the relationship λ∝ ^−0.46. In DS ingots, microhardness, ultimate compression strength, yield strength and plastic deformation to fracture increased with increasing cooling rate. In HT ingots, microhardness and yield strength increased and ultimate compression strength and plastic deformation to fracture decreased with increasing cooling rate. The yield stress increased with decreasing interlamellar spacing and increasing volume fraction of Al₂O₃ particles. A linear relationship between the Vickers microhardness and yield stress was found for both DS and HT ingots. A simple model including the effect of interlamellar spacing and increasing volume fraction of Al₂O₃ particles was proposed for the prediction of the yield stress.     

区熔法制备金属硫化物Ag2S及其热电性能研究

金属硫化物Ag2S具有优异的物理化学性能,在催化、传感及光电子等领域具有广阔的应用空间.本工作利用一种区熔技术制备了尺寸为?18 mm×50 mm的Ag2S并对其潜在热电性能进行了研究.Ag2S在450 K以下具有标准的 α-Ag2S单斜P21/c结构,450 K以上发生相变成为立方 β-Ag2S相.Ag2S在300～...     

An electron diffraction study of the ZnIn2Se4 crystal structure: A novel phase

A novel layered-structure ZnIn₂Se₄ phase has been obtained. Texture electron diffraction patterns aid in the identification of a crystal structure with lattice parameters a = 4.045 Å and c = 52.29 Å, space group R m, and z = 4.5. Crystal electron diffraction patterns displayed superstructural reflection, thus indicating a √3-fold increase in the a parameter. The similirity of reflection locations and intensities both on the crystal rotation electron diffraction pattern and on texture electron diffraction patterns showed that no phase transition occurred on specimen pounding. Electrophysical and optical parameters (E_g = 1.68 eV; N = 8 × 10²² m^-3; = 0.1Ωm) are studied at 300 K. The Hall coefficient is constant (R_H = 7.2 × 10^-5m³C^-1, mobility μ = 8 × 10^-3m²V^-1s^-1 at 200–300 K.     

Dissociation process of CH4/H2 gas mixture during EACVD

The dissociation process of CH₄/H₂ gas mixture during EACVD has been investigated using Monte Carlo simulation for the first time. The electron velocity distribution and H₂ dissociation were obtained over a wide range: 100<E/N<2000 Td. The variation of CH₄ dissociation with CH₄ concentration in the filling gas has been simulated. The electron velocity profile is asymmetric for the component parallel to the field. Most electrons possess non-zero velocity parallel to the substrate. The number of atomic H is a function of E/N. There are two peaks at E/N=177 Td and 460 Td. The appropriate E/N is suggested to be 500–800 Td for low temperature deposition. The main diamond growth precursor is proposed to be CH₃ and CH₃⁺.     

Controllable formation of Er–Yb codoped Al2O3 films by the non-aqueous sol–gel method

1 mol%Er³⁺–10 mol%Yb³⁺ codoped Al₂O₃ thin films have been prepared on thermally oxidized SiO₂/Si(110) substrates by a dip-coating process in the non-aqueous sol–gel method from the hydrolysis of aluminum isopropoxide [Al(OC₃H₇)₃] under isopropanol environment. Addition of N,N-dimethylformamide (DMF) as a drying control chemical additive (DCCA) into the sol suppresses formation of the cracks in the Er³⁺–Yb³⁺ codoped Al₂O₃ thin films when the rare-earth ion is doped with a high doping concentration. Homogeneous, smooth and crack-free Er³⁺–Yb³⁺ codoped Al₂O₃ thin films form at the conditions by a molar ratio of 1:1 for DMF:Al(OC₃H₇)₃. A strong photoluminescence spectrum with a broadband extending from 1.400 to 1.700 µm centered at 1.533 µm is obtained for the Er³⁺–Yb³⁺ codoped Al₂O₃ thin films, which is unrelated to the addition of DMF. Controllable formation of the Er³⁺–Yb³⁺ codoped Al₂O₃ thin films may be explained by the fact that the DMF assisted the deprotonation process of Al–OH at the surfaces of gel particles, resulting in enhancement of the degree of polymerization of sols and improvement of the mechanical properties of gel thin films.     

Photoinduced nonlinear optics in Sb2Se3–BaCl2–PbCl2 glasses

Photoinduced structural transformations in amorphous Sb₂Se₃–BaCl₂–PbCl₂ glasses were studied using a differential IR spectroscopy Fourier technique in the spectral region between 100 and 300 cm⁻¹. A stage of the reversible photodarkening is realized in the Sb₂Se₃ fragments after the first cycle of photoexposure and thermoannealing. The whole scheme of the photo- and thermoinduced transformations in the amorphous system may be explained as a coordination of formation and annihilation of defects. The vibrational density of states calculated using quantum chemical solid state methods confirms our experimental results and their interpretation. Photoinduced photodarkening changes using a CO₂ pulse laser (λ=10.6 μm) in new synthesized Sb₂Se₃–BaCl₂–PbCl₂ glasses were investigated. At the same time we have studied photoinduced second harmonic generation (SHG) and two-photon absorption (TPA). The possibility of using this glass as perspective materials for IR optoelectronics and nonlinear optics was shown.     

Microwave dielectric characteristics of Ba(Mg1/3Ta2/3)O3 ceramics sintered at low-temperatures

The microwave dielectric properties and microstructures of Ba(Mg_1/3Ta_2/3)O₃ (BMT) ceramics sintered at low temperatures with 2–3 wt.% NaF additives were investigated. BMT ceramics sintered at 1340 °C for 3–12 h showed dielectric constants (_r) of 25.5–25.7, Qf values of 41 500–50 400 GHz and temperature coefficients of the resonator frequency (τ_f) of 10.9–21.4 ppm °C⁻¹. The variation of sintering time almost had no effect on the dielectric constant. The Qf value increased and the τ_f decreased with increasing sintering time. The ordering degree of Mg²⁺ and Ta⁵⁺ at B-sites increased with increasing sintering time.     

Optical transitions near the band edge in bulk CuInxGa1−xSe2 from ellipsometric measurements

From the analysis of the variation of optical absorption coefficient with incident photon energy between 0.8 and 2.6 eV, obtained from ellipsometric data, the energy E_G of the fundamental absorption edge and E_G′ of the forbidden direct transition for CuIn_xGa_1−xSe₂ alloys are estimated. The change in E_G and the spin-orbit splitting Δ_SO=E_G′−E_G with the composition x can be represented by parabolic expression of the form E_G(x)=E_G(0)+ax+bx² and Δ_SO(x)=Δ_SO(0)+a′x+b′x², respectively. b and b′ are called “bowing parameters”. Theoretical fit gives a=0.875 eV, b=0.198 eV, a′=0.341 eV and b′=−0.431 eV. The positive sign of b and negative sign of b′ are in agreement with the theoretical prediction of Wei and Zunger [Phys. Rev. B 39 (1989) 6279].