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.     

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.     

Determining crystallization kinetic parameters of Li2O–Al2O3–SiO2 glass from derivative differential thermal analysis curves

The kinetic parameters such as crystallization activation energy, E, and the frequency factor, ν, of Li₂O–Al₂O₃–SiO₂ glass were determined by a new non-isothermal method. The method is described by the equation

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, where β is the heating rate and T_f is the inflection-point temperature of differential thermal analysis (DTA). The value of T_f is determined as the maximum peak temperature on derivative differential thermal analysis (DDTA) curves. Values of E and ν of Li₂O–Al₂O₃–SiO₂ glass were also determined by two existing non-isothermal methods, namely the Kissinger plot and the Ozawa plot, and compared with those determined by isothermal method. Values of E and ν determined by the proposed equation were 332 kJ/mol and 1.4×10¹³ s⁻¹, respectively. They are excellent agreement with the isothermal analysis results, 336 kJ/mol and 1.8×10¹³ s⁻¹, respectively. In contrast, both the Kissinger equation and the Ozawa equation give much higher values of E and ν.     

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.     

Dynamic fracture toughness of X70 pipeline steel and its relationship with arrest toughness and CVN

The dynamic fracture toughness K_1d and J_1d, arrest toughness K_1a and Charpy V-notched impact toughness (CVN) of a pipeline steel, X70, were studied at different temperatures. It was found that fracture toughness was strongly affected by temperature and loading rate. The fracture toughness decreases with decreasing temperature from 213 to193 K and increasing loading rate from to . At constant temperatures, only increasing loading rate can induce the transition from ductile to brittle. There exists a fracture transition caused by loading rate. Through thermal activation analysis, a quantitative relationship has been derived:

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. It can describe the fracture process at different temperatures and loading rates. At a loading rate of , the relationship can predict arrest toughness well. It provides the possibility of measuring arrest toughness with small size specimen. An empirical equation has been derived: CVN=4.84×10⁶T^−2.8K_1d(K_1a), which correlates K_1d and K_1a with CVN in one equation. This means that we can calculate K_1d and K_1a when we get CVN.     

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.     

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.     

The surface of carbon fibres continuously treated by cold plasma

The surface of carbon fibres prepared from polyacrylonitrile (PAN) precursor have been continuously treated by means of a cold plasma. The interlaminar shear strength of carbon fibre reinforced epoxy composites containing fibres so treated was increased from about 60 MPa to 100 MPa. There are four possible mechanisms for this increase:

1. (1) Higher reactivity between fibre surface and matrix as a result of an increase of

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groups on the fibre surface.

2. (2) The surface constitution was changed by the plasma treatment so as to improve the wetting properties of fibre surface. The contact angle θ between water and the carbon fibre was decreased from 75° to 61°.

3. (3) Electron micrographs of the surface of carbon fibre show that the surface striations and surface roughness were changed increasingly on fibre surfaces after plasma etching. This increases the interfacial adhesion and the effect of mechanical interlocking.

4. (4) The strength of the carbon fibre was decreased very little (about 1·6%) by this surface treatment method.

Analysis of fracture morphology by scanning electron microscopy (SEM) indicates that debonding and fibre pull-out between fibre and matrix do not occur in the treated carbon fibre reinforced epoxy composite. These results all show that adhesion between fibre and matrix is very strong. In addition, the plasma treatment technology is very simple and the cost is low. This treatment process produces no environmental pollution and has promising future for engineering applications.     

Synthesis of AlN by reactive sputtering

We present a systematic study of the sub-band gap optical absorption coefficients (hν) in the range 1.2–6 eV vs. deposition-temperature (T_s from 27 to 450°C) films deposited on silica by 13.6 MHz magnetron sputtering of an Al target with 53 and 72% N₂ in the reactive mixture. X-ray diffraction, infrared absorption and Raman diffusion are also presented, mainly on films deposited on Si in the same run to help in the characterisation of the films. All signals are specific of AlN polycrystalline films, which are of better quality when deposited with 72% N₂. The lowest sub-band gap optical absorption around 5×10² cm⁻¹ is obtained for deposition on silica at T_s=300°C with 72% N₂ and is close to that of heteroepitaxial films deposited on sapphire.     

The temperature dependence of luminescence from a long-lasting phosphor exposed to ionizing radiation

The temperature dependence of luminescence from a long-lasting phosphor (LLP), SrAl₂O₄ : Eu²⁺,Dy³⁺, exposed to ionizing radiation has been measured to understand the LLP luminescence mechanism. Evaluation of the decay constants of the LLP exposed to -, β- or γ-rays at temperatures from 200 to 390 K showed that the decay constant is divided into four components ranging from 10⁻⁴ to 10⁻¹ s⁻¹ with activation energies of 0.02–0.35 eV.

Total luminous intensity from the LLP with changing irradiation temperature has its maximum value around the room temperature. Irradiation at elevated temperature (390 K) has the total luminescence pattern with monotonous decrease as temperature rises. As a result of evaluating the temperature dependence of luminescence, the luminescence mechanism is considered as follows:

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Observation of ferroelectric domains in bismuth-layer-structured ferroelectrics using Raman spectroscopy

Raman spectroscopy has been used to investigate the domain structures in bismuth-layered-structured ferroelectrics (BLSFs). In Bi₄Ti₃O₁₂ single crystal, the lowest frequency mode (soft mode) at 30 cm⁻¹ appears exclusively for the xx polarization configuration (xpolar axis). We found that the polarization dependence of the Raman signal exhibits spatial symmetries that reflect the presence of different domain variants present in Bi₄Ti₃O₁₂. This highly anisotropic character of the soft mode shares with other BLSFs such as Bi_3.25La_0.75Ti₃O₁₂ and SrBi₂Ta₂O₉, which demonstrate the usefulness of the soft-mode spectroscopy for the study of ferroelectric domain structures in BLSFs. We also applied Raman spectroscopy to in situ observation of domain structures in Bi₄Ti₃O₁₂ under applied electric field.     

Preparation of Aluminum Nitride Thin Films by CVD

AlN films were prepared by CVD using aluminum halide (AlCl₃) and aluminum alkyl ((CH₃)3Al) precursors. The appropriate deposition conditions to obtain polycrystalline AlN films using A1C1₃ precursor were found at T_dep = 1173 K, P_tot = 66.6 Pa and N₂/NH₃ = 0.75. It was found that AlN films of different crystallinity can be obtained from (CH₃)₃Al precursor at T_de = 973-1023 K, P_tot = 1.99 kPa, only under a H₂ atmosphere. AlN films can be grown highly oriented in the (210) direction on amorphous quartz substrates depending on their thicknesses. The 0.1 -0.2 μm thick AlN films were transparent and their refractive indexes were about 1.4-1.6.     

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%:

.     

Lattice vibrations of materials for lithium rechargeable batteries. VI: Ordered spinels

Raman scattering spectra have been investigated to evaluate the local structure of lithiated oxides used as electrode materials for lithium-ion batteries. We report the analysis of the vibrational spectra of ordered spinel phases including the partially delithiated λ-Li_0.5Mn₂O₄ ( SG), the partial charge-ordered LiMn₂O₄ orthorhombic form (Fddd SG) and the LiNi_0.5Mn_1.5O₄ substituted oxide (P4₁32 SG). Analysis of spectroscopic data is performed using the classical factor group theory and the vibration features are compared with those of the ordered lithium ferrite -LiFe₅O₈ and the normal spinels LiMn₂O₄ and LiNi_0.5Mn_1.5O₄ (Fd3m SG), and the inverse spinel LiNiVO₄.     

Molecular alignment structure of dimeric liquid crystals

While an antiferroelectric liquid crystal (AFLC) with dimeric molecules which take a bent molecular structure may show the phase transition from the isotropic liquid to the chiral smectic-C AFLC () phase via the nematic (N) phase, it has not been obvious yet why the and N phases may coexist in the phase sequence of the dimeric AFLC medium. In this study, the molecular alignment structures of the N and the SmC_A phases of an achiral dimeric LC were researched in detail with a polarized Fourier transform infrared spectroscopy (FT-IR). The domain structure with two orientational directions is formed both in the SmC_A and N phase, and the occupation ratio of two types of domain does not change in the phase transition. Since their alignment structures are basically same except for the existence of layer structure, their phases may coexist in the phase sequence of the dimeric AFLC.     

Inductively coupled plasma — plasma enhanced chemical vapor deposition silicon nitride for passivation of InP based high electron mobility transistors (HEMTs)

Silicon nitride thin films have been deposited on InP-based structures at both room and high temperatures in an RF-inductively coupled plasma enhanced chemical vapor deposition (ICP-PECVD) equipment. Metal insulating semiconductor (MIS) diodes have been widely investigated using either SiH₄+NH₃ or SiH₄+N₂ gas phase. I–V measurements conducted on these diodes reveal high resistivity and breakdown electric field even at low deposition temperature (50°C). Double channel (DC) High electron mobility transistors (HEMTs) have been passivated by SiN_x films deposited at room temperature using SiH₄+NH₃ precursors. Passivated devices exhibit a very low drift over a 45 h period of stress under high gate-drain electric field.     

Deposition chemistry in the Cat-CVD processes of the SiH4/NH3 system

Laser spectroscopic as well as mass-spectrometric techniques were employed to examine the deposition chemistry in the catalytic chemical vapor deposition processes of the SiH₄/NH₃ system. The absolute densities of NH, NH₂ and SiH₃ radicals were measured under various conditions. The densities of the stable products, H₂ and N₂, as well as those of the reactants, NH₃ and SiH₄, were also measured. The NH₂ density is always higher than that of NH and both densities decrease by the addition of SiH₄. The SiH₃ density increases nonlinearly with the increase in the SiH₄ pressure. The SiH₃ density was found to be much higher than that of NH₂ under near practical deposition conditions to fabricate Si₃N₄ films (an NH₃ to SiH₄ flow-rate ratio of 50:1, a total pressure of 20 Pa and a catalyzer temperature of 2300 K). No aminosilane molecules were identified, suggesting that the contribution of aminosilyl radicals to the film deposition is minor. Thus, NH₂ and SiH₃ must be the major deposition species to form Si₃N₄.     

A method for the measurement of thin film optical constants with a spectral photometer from 230 nm to 850 nm and its application to plasma silicon (oxy) nitride

A technique to measure the complex index of refraction of thin solid films in the visible and near-UV range with a spectral photometer using polarized light is introduced. The complex index of refraction of amorphous silicon (oxy)nitride films deposited in a glow discharge with different gas flow ratios of NH₃:SiH₄ and N₂O:SiH₄ at 270°C was measured as a function of wavelength. The real part of the index of refraction at 632.8 nm of the films investigated varies from 1.5 to 2.5. The results could be confirmed by scanning ellipsometry.