Magnetocrystalline anisotropy of zinc-blende CrTe (001) surface: A first-principles study

By using the highly precise all-election full potential linearized augmented plane-wave method based on density functional theory within the generalized gradient approximation, we investigated magnetocrystalline anisotropy and magnetism of zinc-blende CrTe (001) surface. We observed that both of the Cr- and Te-terminated zinc-blende CrTe (001) surfaces maintain the half-metallicity by analyzing the density of states. The magnitudes of the calculated magnetic moments for the Cr(S) and Cr(S-1) atoms are calculated to be 3.92 and 3.16 μ_B for the Cr- and Te-terminated surfaces, respectively. The Te atoms show significantly induced negative spin polarizations of 0.13-0.30 μ_B. The spin orientations at the Te-terminated (Cr-terminated) surfaces were found to be in-plane (out-of-plane) regardless of its thickness. Since a Te-terminated surface is known as to be more stable than a Cr-terminated one, our result is consistent with an experiment which observed in-plane magnetic anisotropy at a CrTe (001) surface.     

High-pressure plastic flow in silicon: A first-principles theoretical study

Summary Microindentation experiments have recently shown that silicon can exhibit plastic flow when subjected to high pressure. Assuming that under these conditions the relevant reference structure is the -Sn high-pressure phase of silicon, we apply the magic-strain concept to explore the space of configurations that could describe the observed behavior. We use first-principles total-energy calculations (including full relaxation of the atomic basis for every structure) to evaluate the relevance of strained configurations. Using this approach, we were able to identify a low-energy path that corresponds to planar flow of the atoms. The atomic configurations along this path provide insight into possible microscopic motions under high pressure that may be relevant to plastic flow in silicon.     

Heterogeneous nucleation and growth of polycrystalline silicon

The heterogeneous nucleation theory of silicon on SiO₂ and Si₃N₄ substrates has been developed using classical theory. It is shown that the experimental observations can be explained on the basis of the bond energies of O-H, N-H and Si-H. A reaction model is proposed for the growth of silicon on silicon from silane, using hydrogen as a carrier gas in the temperature region 600–900°C. The growth rate of silicon is shown to be equal toP _{SiH 4} P _{H 2} when the partial pressure of hydrogen is high, and is independent of the total pressure and the partial pressure of hydrogen in the lower region.     

Growth of noble metal nanostructures on the Bi nanoline surface: A first-principles study

Recent experiments suggest that self-assembled Bi nanolines on the passivated Si(0 0 1) surface can be used as templates to grow one-dimensional arrays of noble-metal (Au, Ag) nanoclusters. In order to ascertain this template effect, the gold atom on the H-terminated Si(0 0 1) surface and that on the Bi line are examined by density-functional methods. The calculation indicates that the gold atom enters a Si dimer on the H-terminated surface and the BiSi backbonds of a Bi dimer on the Bi line. The gold atom is 0.6 eV more stable on the Bi line than on the H-terminated surface. This result suggests the preferential adsorption of gold on the Bi line and confirms its template effect.     

Synthesis, characterization and magnetoresistance properties study of magnetite thin films by electroless plating in aqueous solution

Polycrystalline half-metallic Fe₃O₄ films with 1 μm in thickness were synthesized on glass substrates directly by electroless plating in aqueous solution at 90 °C without heat treatment. The films have single pure spinal phase structure and the well-crystallized columnar grains grow perpendicularly to the substrates, as revealed by XRD, XPS and SEM. At room temperature, the films exhibit negative magnetoresistance (MR) ratio of about −5.1%, which is ascribed to intergranular tunneling of spin polarized electrons of Fe₃O₄. The resistivity R of the films with 1 μm in thickness at room temperature is about 5.2 × 10⁻¹ Ω cm. The cation distribution and the arrangement of the magnetic moments of the plated Fe₃O₄ ferrite thin films are different from that of the bulk materials, which is likely to be one of the reasons for the modification of R and MR properties.     

Two-gap superconductivity in R2Fe3Si5 (R=Lu,Sc) and Sc5Ir4Si10

Abstract

R₂Fe₃Si₅ (R= Sc, Y, Lu) contains nonmagnetic iron and has a relatively high superconducting transition temperature T_c among iron-containing superconductors. An anomalous temperature dependence of specific heat C(T) has been reported for polycrystalline samples down to 1 K. We have grown R₂Fe₃Si₅ single crystals, confirmed the anomalous C(T) dependence, and found a second drop in specific heat below 1 K. In Lu₂Fe₃Si₅, we can reproduce C(T) below T_c, assuming two distinct energy gaps 2Δ ₁/k_BT_c = 4.4 and 2Δ ₂/k_BT_c = 1.1, with nearly equal weights, indicating that Lu₂Fe₃Si₅ is a two-gap superconductor similar to MgB₂. Hall coefficient measurements and band structure calculation also support the multiband contributions to the normal-state properties. The specific heat in the Sc₂Fe₃Si₅ single crystals also shows the two-gap feature. R₅Ir₄Si₁₀ (R = Sc, rare earth) is also a superconductor where competition between superconductivity and the charge-density wave is known for rare earths but not for Sc. We have performed detailed specific heat measurements on Sc₅Ir₄Si₁₀ single crystals and found that C(T) deviates slightly from the behavior expected for weak-coupling superconductors. C(T) for these superconductors can also be reproduced well by assuming two superconducting gaps.     

Two-gap superconductivity in R2Fe3Si5 (R=Lu,Sc) and Sc5Ir4Si10

R₂Fe₃Si₅ (R= Sc, Y, Lu) contains nonmagnetic iron and has a relatively high superconducting transition temperature T_c among iron-containing superconductors. An anomalous temperature dependence of specific heat C(T) has been reported for polycrystalline samples down to 1 K. We have grown R₂Fe₃Si₅ single crystals, confirmed the anomalous C(T) dependence, and found a second drop in specific heat below 1 K. In Lu₂Fe₃Si₅, we can reproduce C(T) below T_c, assuming two distinct energy gaps 2Δ ₁/k_BT_c = 4.4 and 2Δ ₂/k_BT_c = 1.1, with nearly equal weights, indicating that Lu₂Fe₃Si₅ is a two-gap superconductor similar to MgB₂. Hall coefficient measurements and band structure calculation also support the multiband contributions to the normal-state properties. The specific heat in the Sc₂Fe₃Si₅ single crystals also shows the two-gap feature. R₅Ir₄Si₁₀ (R = Sc, rare earth) is also a superconductor where competition between superconductivity and the charge-density wave is known for rare earths but not for Sc. We have performed detailed specific heat measurements on Sc₅Ir₄Si₁₀ single crystals and found that C(T) deviates slightly from the behavior expected for weak-coupling superconductors. C(T) for these superconductors can also be reproduced well by assuming two superconducting gaps.     

Fullerene-like CPx: A first-principles study of the relative stability of precursors and defect energetics during synthetic growth

Inherently nanostructured CP_x compounds were studied by first-principles calculations. Geometry optimizations and cohesive energy comparisons show stability for C₃P, C₂P, C₃P₂, CP, and P₄ (P₂) species in isolated form as well as incorporated in graphene layers. The energy cost for structural defects, arising from the substitution of C for P and intercalation of P atoms in graphene, was also evaluated. We find a larger curvature of the graphene sheets and a higher density of cross-linkage sites in comparison to fullerene-like (FL) CN_x, which is explained by differences in the bonding between P and N. Thus, the computational results extend the scope of fullerene-like thin film materials with FL-CP_x and provide insights for its structural properties.     

Defect generation during silicon oxidation: A Kinetic Monte Carlo study

We present a synthetic review of elementary chemical mechanisms source of the oxidation of pure silicon (100) surfaces. These mechanisms are then discussed from their ability to build a mesoscale model of the Kinetic Monte Carlo type dedicated to the process simulation of silicon thermal oxidation. We show that oxidation is driven by two main processes: (i) charge transfer arising from the formation of SiO bonds in contact to pure silicon at the interface, (ii) destructive oxidation in which SiO building blocks rearrange at the interface to form a hexagonal-based oxide network directly in contact to cubic Si layers. Based on these considerations, simulations at the process scale exhibit epitaxial behavior within the interfacial domain. The resulting oxide layers are analyzed in terms of local to more extended defects. We observe two types of defects: (i) “intra-domain defects” which are related to local distortion of the elementary hexagonal oxide pattern Si₆O₆ (ii) “inter-domain defects”, which are related to global oxide structural transitions from one orientation to another.     

Preparation and characterization of bio-functionalized iron oxide nanoparticles for biomedical application

Amine modified iron oxide (Fe₃O₄) nanoparticles were synthesized by thermal decomposition method and were further used to bio-functionalize by grafting of N-hydroxysuccinimide (NHS) ester of folate and ethylenediaminetetraacetate (EDTA). Fe₃O₄ nanoparticles of ~ 22 nm were confirmed from X-ray diffraction (XRD) and transmission electron microscopy (TEM) studies. FT-IR studies indicated two bands at 1515 cm^− 1and 1646 cm^− 1, which can be attributed to carboxylic group and the amide linkage respectively, revealing the conjugation of folate with Fe₃O₄. The conjugation of the chelating agent showed strong C=O stretch and Fe-O vibrations at 1647 and 588 cm^− 1 respectively. The value of saturation magnetization for Fe₃O₄ nanoparticles was found to be 88 emu/g, which further reduced to 18 and 32% upon functionalization with EDTA and NHS ester folate, respectively. These amine modified Fe₃O₄ nanoparticles can also be functionalized with other bifunctional chelators, such as amino acids based diethylene triamine pentaacetic acid (DTPA), and thus find potential applications in radio-labeling, biosensors and cancer detection, etc.     

Nitridation enhancing effect of ZrO2 on silicon powder

The nitridation behavior of silicon powder with added Zr compounds was studied in order to assess the catalytic effect of zirconium on the formation of reaction bonded silicon nitride, using high purity silicon powder and monoclinic zirconia as starting materials. Thermogravimetric analysis revealed that the addition of ZrO₂ to Si powder reduced the temperature of the main nitridation reaction, and increased the amount of silicon converted to silicon nitride at a given temperature. On the other hand, the nitridation rate at higher temperatures (1380-1400 °C) indicated similar values for both pure Si and Si with ZrO₂ additions.     

UV-visible absorption spectra of silicon CVD intermediates

Transient absorption in 394-435 nm wavelength range following 193 nm photolysis of disilane has been measured by using cavity ring-down spectroscopy (CRDS). A broad and continuum absorption band was observed. Time profiles of the absorption measured at several wavelengths were similar and found to have at least two components. The decaying part of the absorption can be attributed to Si(H₂)Si based on the kinetic consideration and available information from the literature. The absorption was also measured in the hot wire CVD (HW-CVD) of SiH₄. A broad and continuum band was observed.     

One-step pyrolysis route to C/Fe3O4 hybrids from EDTA ferric sodium salt

C/Fe₃O₄ hybrid materials have potential applications in sensors and anode materials for lithium-ion batteries. In this text, a one-step pyrolysis method was used to prepare C/Fe₃O₄ hybrid materials from EDTA ferric sodium salt. The magnetic Fe₃O₄ nanoparticles can be homogeneously incorporated into carbon materials to form C/Fe₃O₄ hybrids during the reaction. The morphology and magnetism of the C/Fe₃O₄ hybrids are strongly affected by pyrolysis temperature. This method supplies an ideal template to facilely synthesize C/metal-oxide hybrid materials from EDTA metallic salts.     

Solid-phase epitaxy of undoped amorphous silicon by in-situ postannealing

The solid phase epitaxy (SPE) of undoped amorphous Si (a-Si) deposited on SiO₂ patterned Si(001) wafers by reduced pressure chemical vapor deposition (RPCVD) using a H₂-Si₂H₆ gas system was investigated. The SPE was performed by applying in-situ postannealing directly after deposition process. By transmission electron microscopy (TEM) and scanning electron microscopy, we studied the lateral SPE (L-SPE) length on sidewall and mask for various postannealing times, temperatures and a-Si thicknesses. We observed an increase in L-SPE growth for longer postannealing times, temperatures and larger Si thicknesses on mask. TEM defect studies revealed that by SPE crystallized epi-Si exhibits a higher defect density on the mask than at the inside of the mask window. By introducing SiO₂-cap on the sample with 180 nm Si thickness following postannealing at 570 °C for 5 h, the crystallization of up to 450 nm epi-Si from a-Si is achieved. We demonstrated the possibility to use this technique for SiGe:C heterojunction bipolar transistor (HBT) base layer stack to crystallize Si-buffer layer to widen the monocrystalline region around the bipolar window and to improve base link resistivity of the HBT.     

Structural and magnetic study of nitrided Co74Fe8B12Si6 and Co74Fe4Mn4B12Si6 ribbons

The nitriding thermochemical treatment (NTT) is commonly used for steels. In this paper, the experimental conditions required for NTT, and the influence of such treatments on the structure and hysteresis loops of Co₇₄Fe₈B₁₂Si₆ and Co₇₄Fe₄Mn₄B₁₂Si₆ ribbons are reported. The results have been compared with those obtained with ribbons treated according to conventional thermal treatment (CTT) as well.     

Effect of solid content on pore structure and mechanical properties of porous silicon nitride ceramics produced by freeze casting

Porous Si₃N₄ ceramics were prepared by freeze casting using liquid N₂ as refrigerant. The pore structure, porosity, α → β-Si₃N₄ transformation and mechanical properties of the obtained materials were strongly affected by the solid contents of the slurries. Increasing the solid content would reduce the porosity, decrease the pore size and change the pore structure from the aligned channels with dendrites to the round pores with decreased pore size. The formation of this round pores impeded the α → β-Si₃N₄ phase transformation, but was beneficial to the mechanical properties of the obtained porous Si₃N₄ due to its unique pore structure.     

Structure of double interfaces system of Si3N4/SiO2/Si irradiated by γ-rays

The interfacial structures of double interfaces system of Si₃N₄/SiO₂/Si were examined using X-ray photoelectron spectroscopy (XPS) before and after ⁶⁰Co radiation. The experimental results demonstrate that there existed two interfaces, one consisted of Si₃N₄ and SiO₂, while another was made of Si and SiO₂, the interface between SiO₂ and Si was extended towards the interface of the Si₃N₄/SiO₂ meanwhile the center of the former interface was removed in the direction of the latter interface by ⁶⁰Co. The concentration of silicon in the Si₃N₄ state (BE 101.8 eV) was decreased with the variation of radiation dosage as well as bias field within the SiO₂-Si interface, remarkably. The mechanism for the experimental results is analyzed.     

Textured silicon nitride: processing and anisotropic properties

Abstract

Textured silicon nitride (Si₃N₄) has been intensively studied over the past 15 years because of its use for achieving its superthermal and mechanical properties. In this review we present the fundamental aspects of the processing and anisotropic properties of textured Si₃N₄, with emphasis on the anisotropic and abnormal grain growth of β-Si₃N₄, texture structure and texture analysis, processing methods and anisotropic properties. On the basis of the texturing mechanisms, the processing methods described in this article have been classified into two types: hot-working (HW) and templated grain growth (TGG). The HW method includes the hot-pressing, hot-forging and sinter-forging techniques, and the TGG method includes the cold-pressing, extrusion, tape-casting and strong magnetic field alignment techniques for β-Si₃N₄ seed crystals. Each processing technique is thoroughly discussed in terms of theoretical models and experimental data, including the texturing mechanisms and the factors affecting texture development. Also, methods of synthesizing the rodlike β-Si₃N₄ single crystals are presented. Various anisotropic properties of textured Si₃ N₄ and their origins are thoroughly described and discussed, such as hardness, elastic modulus, bending strength, fracture toughness, fracture energy, creep behavior, tribological and wear behavior, erosion behavior, contact damage behavior and thermal conductivity. Models are analyzed to determine the thermal anisotropy by considering the intrinsic thermal anisotropy, degree of orientation and various microstructure factors. Textured porous Si₃N₄ with a unique microstructure composed of oriented elongated β-Si₃N₄ and anisotropic pores is also described for the first time, with emphasis on its unique mechanical and thermal-mechanical properties. Moreover, as an important related material, textured α-Sialon is also reviewed, because the presence of elongated α-Sialon grains allows the production of textured α-Sialon using the same methods as those used for textured β-Si₃N₄ and β-Sialon.     

Growth studies and characterization of silicon nitride thin films deposited by alternating exposures to Si2Cl6 and NH3

We deposited silicon nitride films by alternating exposures to Si₂Cl₆ and NH₃ in a cold-wall reactor, and the growth rate and characteristics were studied with varying process temperature and reactant exposures. The physical and electrical properties of the films were also investigated in comparison with other silicon nitride films. The deposition reaction was self-limiting at process temperature of 515 and 557 °C, and the growth rates were 0.24 and 0.28 nm/cycle with Si₂Cl₆ exposure over 2 × 10⁸ L. These growth rates with Si₂Cl₆ are higher than that with SiH₂Cl₂, and are obtained with reactant exposures lower than those of the SiH₂Cl₂ case. At process temperature of 573 °C where the wafer temperature during Si₂Cl₆ pulse is 513 °C, the growth rate increased with Si₂Cl₆ exposure owing to thermal deposition of Si₂Cl₆. The deposited films are nonstoichiometric SiN, and were easily oxidized by air exposure to contain 7-8 at.% of oxygen in the bulk film. The deposition by using Si₂Cl₆ exhibited a higher deposition rate with lower reactant exposures as compared with the deposition by using SiH₂Cl₂, and exhibited good physical and electrical properties that were equivalent or superior to those of the film deposited by using SiH₂Cl₂.     

Surface morphology and growth mechanisms for sputtered amorphous silicon nitride thin films

Evolution of surface of sputter-deposited amorphous Si₃N₄ films growth on Si (100) substrates was investigated using atomic force microscopy (AFM). The scaling behaviors of the AFM topographical profiles were analyzed using the one-dimensional power spectral density. The results of root-mean-square surface height variation showed that there is a power law relationship between the surface roughness and deposition time. It is interesting to note that the growth exponent can be divided into one region and two regions, respectively, when Si₃N₄ films are deposited at different working pressures. A very low growth exponent of β = 0.07 ± 0.01 was found when Si₃N₄ films were deposited at a working pressure of 1.6 × 10^− 1 Pa. However, the growth exponent β can be divided into two regions, which is β₁ = 0.09 ± 0.01, β₂ = 0.24 ± 0.03 and β₁ = 0.09 ± 0.01, β₂ = 0.33 ± 0.04, when the films were deposited at a working pressure of 2.1 × 10^− 1 Pa and 2.7 × 10^− 1 Pa, respectively. The mechanisms of anomalous dynamic scaling exponents of Si₃N₄ films deposited at different working pressures were discussed.