Study of hydrogenated AlN as an anti‐reflective coating and for the effective surface passivation of silicon

Stacks of aluminum oxide and silicon nitride are frequently used in silicon photovoltaics. In this Letter, we demonstrate that hydrogenated aluminum nitride can be an alternative to this dual‐layer stack. Deposited on 1 Ω cm p‐type FZ silicon, very low effective surface recombination velocities of 8 cm/s could be reached after firing at 820 °C. This excellent passivation is traced back to a high density of fixed charges at the interface of approximately –1 × 10¹² cm^–2 and a very low interface defect density below 5 × 10¹⁰ eV^–1 cm^–2. Furthermore, spectral ellipsometry measurements reveal that these aluminum nitride layers have ideal optical properties for use as anti‐reflective coatings. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

First principles study of pressure-induced magnetic transition in CrN

First principles calculation was performed using tight-binding LMTO method with local density approximation (LDA) and atomic sphere approximation (ASA) to understand the electronic properties of chromium nitride. The equilibrium geometries, the magnetic moment, the electronic band structure, the total and partial DOS are obtained under various pressures and are analyzed in comparison with the available experimental data. The most stable structure of CrN is NaCl structure in the FM state. A pressure-induced second order magnetic phase transition from ferromagnetic (FM) to non-magnetic (NM) at very high pressure of 0.5549 Mbar is predicted. Our results indicate that CrN can be used as a hydrogen storage material.     

Stability of Disclinations in Nematic Liquid Crystals

In the light of φ-mapping method and topological current theory, the stability of disclinations around a spherical particle in nematic liquid crystals is studied. We consider two different defect structures around a spherical particle: disclination ring and point defect at the north or south pole of the particle. We calculate the free energy of these different defects in the elastic theory. It is pointed out that the total Frank free energy density can be divided into two parts. One is the distorted energy density of director field around the disclinations. The other is the free energy density of disclinations themselves, which is shown to be concentrated at the defect and to be topologically quantized in the unit of (k-k₂₄)π/2. It is shown that in the presence of saddle-splay elasticity a dipole (radial and hyperbolic hedgehog) configuration that accompanies a particle with strong homeotropic anchoring takes the structure of a small disclination ring, not a point defect.     

六方氮化硼单层中一种(CN)3VB缺陷的第一性原理计算

二维六方氮化硼(hBN)的点缺陷最近被发现可以实现室温下的单光子发射,而成为近年的研究热点.尽管其具有重要的基础和应用研究意义,hBN中发光缺陷的原子结构起源仍然存在争议.本文采用基于密度泛函理论的第一性原理计算,研究hBN单层中一种B空位附近3个N原子被C替代的缺陷(CN)3VB.在hBN的B空位处,3个N原子各自带一个在平面内的悬挂键及相应的未配对电子,而通过C替换可以消除未配对的电子.系统研究了(CN)3VB缺陷的几何结构、电子结构以及光学性质,结果表明,缺陷可以由一个对称的亚稳态经过原子结构弛豫变成1个非对称的、3个C原子连在一起的基态结构.缺陷的形成在hBN中引入了一些由缺陷悬挂σ键及重构的π键贡献的局域缺陷态.这些缺陷态可以导致能量阈值在2.58 eV附近的可见光内部跃迁.本文的工作有助于进一步理解hBN中点缺陷的构成及光学性质,为实验上探讨发光点缺陷的原子结构起源及其性质提供理论依据.     

Twist disclination system in a planar sample of chiral smectic C* liquid crystal

In thick samples of Sm C^* liquid crystals the helicoidal structure in the sample centre is connected to the unwound structure near sample surfaces by a regular system of (?2π) and (+2π)-twist disclinations associated in rows near the upper and lower sample surfaces. The geometry and stability of such a disclination system in a high electric field is investigated and the critical electric field causing the disclination annihilation is estimated. The mutual disclination annihilation is the proposed mechanism for the field induced transition from helicoidal Sm C^* structure to unwound Sm C^* structure with the twist along the sample thickness.     

Microstructure of Mo – 47% Re – 0.4% Zr alloys after rolling at room temperature. I. Anisotropy of microband structure and peculiarities of internal structure of microbands

The results of an electron microscopy investigation of microstructure of a Mo – 47% Re – 0.4% Zr alloy after rolling deformation (ε ≈ 90%) at room temperature are presented. A special focus is made on investigation of anisotropy of microband nanostructured states and high-energy defect substructures with high values of the crystal lattice curvature, dislocation density and local internal stresses. A disclination mechanism of reorientation as a mechanism of fragmentation of the internal microband structure is proposed.     

First-principles calculations of BC4N nanostructures: stability and electronic structure

In this work, we apply first-principles methods to investigate the stability and electronic structure of BC₄N nanostructures which were constructed from hexagonal graphite layers where substitutional nitrogen and boron atoms are placed at specific sites. These layers were rolled up to form zigzag and armchair nanotubes, with diameters varying from 7 to 12 Å, or cut and bent to form nanocones, with 60^° and 120^° disclination angles. The calculation results indicate that the most stable structures are the ones which maximize the number of B–N and C–C bonds. It is found that the zigzag nanotubes are more stable than the armchair ones, where the strain energy decreases with increasing tube diameter D, following a 1/D ² law. The results show that the 60^° disclination nanocones are the most stable ones. Additionally, the calculated electronic properties indicate a semiconducting behavior for all calculated structures, which is intermediate to the typical behaviors found for hexagonal boron nitride and graphene.     

Diffusion processes near triple joints of special grain boundaries

Diffusion processes are studied near triple joints of special grain boundaries, whose dilatation field is modeled by a wedge disclination. An exact analytic solution of the diffusion equation in the stress field produced by the structural defect considered is obtained. The results of the theoretical analysis are used to determine the concentration field of point defects near triple joints of special grain boundaries in a polycrystal. Fiz. Tverd. Tela (St. Petersburg) 41, 64–67 (January 1999)     

Precipitation of γ′ phase and helium bubbles during proton and α-particle irradiation of nickel–silicon

Segregation of silicon was induced by light-ion irradiation at elevated temperatures in Ni–8Si specimens. Its occurrence at external surfaces, helium-induced cavities, dislocation loops, coherent twin boundaries, grain boundaries, and precipitate-matrix interfaces has been investigated by transmission electron microscopy. Layers of ordered γ^′ (Ni₃Si) phase were formed at most of these point defect sinks. The behaviour of grain boundary precipitation was found to be exceptional in various respects. In particular, a high rate of precipitation distinguishes grain boundaries from all other kinds of point defect sinks investigated here. This phenomenon of rapid precipitation was found to be adjoined to precipitation-driven grain boundary migration and is attributed to a radiation-induced “discontinuous” precipitate reaction. Observations of helium bubble distributions created during α-particle irradiations at growing dislocation loops and at migrating grain boundaries are also briefly discussed.     

Zero modes for Dirac fermions on a sphere with fractional vortex

Normalized zero-energy states are shown to emerge for massless Dirac fermions in an external gauge field that gives rise to nonquantized vortices on a sphere. A field-theory model is used to describe electronic states of a fullerene-like molecule. In particular, we predict the existence of exactly one zero-energy mode due to a disclination. For 60° disclination, the normalized electron density at zero energy is found to behave as R ^?5/3, with R being the fullerene radius.     

Topological Aspect and Bifurcation of Disclination Lines in Two—Dimensional Liquid Crystals

Using φ-mapping method and topological current theory,the topological structure and bifurcation of disclination lines in two-dimensional liquid crystals are studied.By introducing the strength density and the topological current of many disclination lines,the total disclination strength is topologically quantized by the Hopf indices and Brouwer degrees at the singularities of the director field when the Jacobian determinant of director field does not vanish.When the Jacobian determinant vanishes,the origin,annihilation and bifurcation processes of disclination lines are studied in the neighborhoods of the limit points and bifurcation points,respectively.The branch solutions at the limit point and the different directions of all branch curves at the bifurcation point are calculated with the conservation law of the topological quantum numbers.It is pointed out that a disclination line with a higher strength is unstable and it will evolve to the lower strength state through the bifurcation process.     

Increasing the B/N ratio in boron nitride nanoribbons,a possible approach to dilute magnetic semiconductors

Density functional theory (DFT) employing the local spin density approximation and including correlation functionals is used to show that increasing the boron content relative to the nitrogen content in boron nitride nanoribbons can significantly reduce the band gap making the ribbons semiconducting. Armchair ribbons, but not zigzag ribbons, having excess borons are predicted to have a more stable optimized triplet structure than the optimized singlet structure. The triplet structure is predicted to have a higher density of states at the top of the valence band near Fermi level for the spin down state indicating it could be a ferromagnetic semiconductor. The results suggest a possible new approach to developing ferromagnetic semiconductors.     

Melting of cubic boron nitride at extreme pressures

Due to its large pressure range of stability and inert nature, cubic boron nitride has been proposed as a potential pressure standard for high pressure experiments. It is extremely refractive upon compression, although its melting temperature is not known beyond 10 GPa. We apply first-principles molecular dynamics to evaluate the thermodynamics of zincblende structured (cubic) and liquid boron nitride at extreme temperatures and pressures, and compute the melting curve up to 1 TPa by integration of the Clapeyron equation. The resulting equations of state reveal that liquid boron nitride becomes denser than the solid phase at pressures of around 0.5 TPa. This is expressed as a turnover in the melting curve, which reaches a maximum at 510 GPa and 6550 ± 700 K. The origin of this density crossover is explained in terms of the underlying liquid structure, which diverges from that of the zincblende structured solid as the phases are compressed.     

Theoretical study of the influence of vacancies on the electronic structure of a hexagonal boron nitride monolayer

The influence of boron and nitrogen vacancies and divacancies on the electronic structure of a hexagonal boron nitride h-BN monolayer is studied. In the presence of vacancies in the structure, the introduced states appear in the forbidden band. The position of an introduced state with respect to the upper occupied level and the lower vacant level depends on deformation. Calculations show that, depending on the defect type and the magnitude of the applied deformation, the introduced state can be both localized and not localized on atoms surrounding the defect. When the state is localized in the system, the inhomogeneous distribution of the spin density is observed, resulting in the appearance of the magnetic moment in the system.     

Defect structure near grain boundaries in deformed polycrystalline Cu–Al alloys

The evolution of defect structure upon the deformation of Cu–Al polycrystalline solid solutions with grain sizes of 10, 100, and 200 μm is studied by means of transmission diffraction electron microscopy. Alloys are deformed by tension at a rate of 10^?2 s^?1 at room temperature. Different parameters that characterize defect structure are measured. Patterns of changes in them are revealed upon moving away from grain boundaries. Analysis of the results testifies to the presence of a reinforced zone near the grain boundaries.     

Energy of grain boundaries between cusp misorientations

The disclination model of high angle grain boundaries proposed before is used to understand the energy-angle relations between cusp misorientations. It is found that the model predicts the correct relationship within experimental error with only one adjustable parameter which resembles the Burgers vector of dislocations at low angles. Experimental data for [100] symmetric tilt boundaries in Cu and Al and for [110] symmetric tilt boundaries in Al are used for illustration.     

Electronic conductivity and chemical diffusion in n-conducting barium titanate ceramics at high temperatures

The electronic conductivity as well as the chemical diffusion coefficient of barium titanate ceramics doped with Y and Mn (donor-doped and acceptor co-doped) have been determined by application of conductivity relaxation experiments. The equilibrium values of the electronic conductivity of n-conducting BaTiO₃ have been analyzed by application of a defect chemical model involving electrons and cation vacancies as the predominant defect species at oxidizing conditions (fairly high oxygen partial pressures). The relaxation curves of the electronic conductivity yield the chemical diffusion coefficient of the bulk by employing a spherical grain model where the appropriate diffusion length is the radius of grains (average grain size). The conductivity relaxation experiments have been performed as a function of temperature ranging from 1100 to 1250 °C at oxygen partial pressures between 0.01 and 1 bar. The kinetics of the oxygen exchange process can be interpreted in terms of extremely fast diffusion of oxygen via oxygen vacancies along the grain boundaries and slow diffusion of Ti (cation)-vacancies from the grain boundaries into the grains. The Ti-vacancy diffusion coefficients were extracted from the chemical diffusion coefficients as a function of temperature. Typical values for the Ti-vacancy diffusivity are around 10^− 15 cm² s^− 1 with an activation energy of 3.9 ± 0.7 eV.     

Junction Line Disclinations: Characterisation and Observations

Experimental observations obtained using high resolution transmission electron microscopy (HREM) of junctions between two or more interfaces are analysed to determine whether they exhibit disclination character. The method of analysis used is circuit mapping, and the advantage of using rotation and mirror symmetry operations in the present context, rather than translation operations as is done conventionally, is demonstrated. This technique is applied to micrographs of junctions selected from published literature, and includes junctions in homophase and heterophase materials. It is concluded that few observations currently provide unequivocal evidence of junction line disclination character. This situation may be due in part to the special crystallographic constraints on the applicability of HREM to studies of junction lines. The examples which have been identified all arise at intersections where favourable interfaces, such as epitaxial and twin boundaries, intersect. Moreover, such juncions occur either in pairs, in the form of disclination dipoles, or in small particles.     

Structural state of second-generation HTSCs obtained by laser ablation

The structure of buffer layers and deposited YBa₂Cu₃O _x (Y123) films with a high critical current density (～10⁶ A/cm²) has been investigated by different methods. These superconducting films and buffer layers are found to have a fine-grained structure, which, along with the high texture of Y123 films of the (001) type, is believed to be responsible for the high critical current density. An unusual texture is revealed in buffer layers, which differs from that of substrates and Y123 films. The superconducting films deposited on buffer CeO₂ layers exhibit a system of orthogonal lines; in the case under consideration, this is a manifestation of a domain structure with Y123 particles at boundaries.