Si Compound Research Articles

First-Principles Forecast of Gapless Half-Metallic and Spin-Gapless Semiconducting Materials: Case Study of Inverse Ti2CoSi-Based Compounds

First-principles calculations were used to investigate several inverse Ti2CoSi-based compounds. Our results indicate that Ti2CoSi could transform from a spin-gapless semiconductor to a half metal if a quarter of the Co atoms are replaced by Ti. Ti2.25Co0.75Si would keep stable half-metallic properties in a large range of lattice parameter under the effect of hydrostatic strain, and would become a gapless half metal under the effect of tetragonal distortion. Furthermore, we substituted B, Al, Ga, P, As, and Sb for Si in the Ti2.25Co0.75Si compound. Our results demonstrate that Ti2.25Co0.75Si0.5B0.5, Ti2.25Co0.75Si0.5Al0.5, and Ti2.25Co0.75Si0.5Ga0.5 are half-metallic ferromagnetic materials, and Ti2.25Co0.75Si0.5P0.5, Ti2.25Co0.75Si0.5As0.5, and Ti2.25Co0.75Si0.5Sb0.5 are spin-gapless semiconducting materials. The introduced impurity atoms may adjust the valence electron configuration, change the charge concentration, and shift the location of the Fermi level.

Effect of rubidium incorporation on the optical properties and intermixing in Mo/Si multilayer mirrors for EUV lithography applications

High reflectivity and long-term stability in multilayer mirrors (MLMs) are crucial for extreme ultraviolet (EUV) photolithography. The conventional base stack to reflect 13.5 nm light is a Mo/Si multilayer, which offers a maximum theoretical reflectivity of 75%. In practice, however, the efficiency of the mirror is strongly affected by intermixing between Mo and Si. Diffusion barriers have therefore been adopted, which nevertheless do not provide a perfect solution. In this work, we propose to suppress intermixing in Mo/Si MLMs by substituting the pure Si with a Si compound that can ensure higher thermodynamic stability, while simultaneously providing comparably high EUV theoretical reflectivity, with the net effect of increasing both reflectivity and lifetime. Our theoretical calculations show that rubidium silicide is the most promising material for this purpose. We estimate the optical and thermodynamic properties for each phase of rubidium silicide, and we show that Mo/Rb12Si17 provides the highest theoretical reflectivity, while Mo/RbSi is the most thermodynamically stable. The suppression of intermixing in Mo/RbSi MLMs should lead to a maximum reflectivity at least 2% higher than the best Mo/Si MLMs, integrated with diffusion barriers. The proposed Mo/RbSi MLM solution has the potential to increase the total EUV lithography throughput by ~50%.

Off-Eutectic Au–Ge Die-Attach—Microstructure, Mechanical Strength, and Electrical Resistivity

In this paper, off-eutectic Au–Ge joints were formed between Si substrates to investigate their high-temperature compatibility. High-quality joints made with small bond pressure, 53 kPa, were fabricated. The joints comprised three different types of morphologies: 1) a layered structure of Au/Au–Ge/Au; 2) a layered structure of Au/Au–Ge/Au where some sections of the central Au–Ge band have been replaced by an Au section that extended across the entire section; and 3) a roughly homogenous Au layer comprising the primary $\alpha $ phase. The average Ge concentration was 10 ± 2 at%. Joints formed with a higher bond line pressure, 7.6 MPa, were of a reduced quality with voids and cracks at the original bond line. Annealing at 400 °C for 1000 h transformed the microstructure into an Au–Ge–Si compound with Au precipitates. The shear strength of the fabricated joints was found to be at least 50 MPa, and the fracture mode was an adhesive fracture between the adhesion layer and the die or substrate. Heated dies detached from the substrates at 460 °C, i.e., more than 100 °C above the eutectic melting point of the binary Au–Ge system. Electrical resistivity measurements confirmed a melting process at the eutectic melting point by an abrupt increase in resistivity.

Microstructure and micro-hardness behavior of Ti–Y2O3 –Al–Si composite coatings prepared in electron-plasma alloying

The paper investigates micro-hardness behavior, element and phase composition of Al–11%Si compound subjected to electron-plasma alloying (EPA). Essentially, EPA is an electrical explosion of a Ti conductor and a weighted Y2O3 powder sample with further high-speed heating created by an intense pulse electron beam. The work has established a considerable increase in micro-hardness in surface layers of the material. A penetration depth of alloying elements, being a thickness characteristic of the modified layer, is reported to be ˜ 170 μm. A maximal micro-hardness (155 ± 15.5 HV) is detected in a layer to be as close as possible (5 μm) to the surface of modification. Micro-hardness drops gradually at a depth of 110 μm, reaching its initial value 85.9 ± 8 HV at a depth of 170 μm. The research data collected by the methods of X-ray microanalysis and transmission electron diffraction microscopy demonstrate that the structure of EPA-treated composite 5 μm below the surface comprises nano-dimensional (1–100 nm) crystallites composed mainly of aluminum. The study has revealed that in some crystallites and their joining points there are inclusions formed by particles of Ti and Y aluminides (Al3Ti and Y3Al2) and Ti silicides (TiSi2). The structure of alloy ≈70 μm below the treated surface is made of high-speed crystallization cells ranging 0.5 μm–0.6 μm. The crystallization cells are shown to be formed by a solid Al-based solution and surrounded by second phase layers containing Si and Cu2.7Fe6.3Si; cross dimensions of which vary 50–70 nm. A comparison of micro-hardness behavior and concentration of alloying elements in the surface layer of the composite has highlighted a boost of micro-hardness caused by Ti and Y in the modified layer, and multiphase sub-micro-and nano-dimensional structure made of crystallites with dimensions within a range of several units to hundreds of nanometers.

Cavitation Corrosion of Cast Al-Alloy in an Ethylene Glycol-Water Solution

As a heat media, an ethylene glycol-water solution is widely used as a key component in a cooling liquid for the cooling systems of motors and electric equipment or devices. Therefore, cast Al alloy pumps are often used to circulate the ethylene glycol-water solution in the cooling systems. Thus, cast Al alloy are often subjected to cavitation corrosion. In this study, the cavitation behaviors of cast Al alloy in an ethylene glycol-water solution were studied using the ultrasonic cavitation corrosion apparatus which was made in accordance with ASTM G32. Then, the electrochemical kinetic of cavitation corrosion and the chemical structures of the corroded surface were investigated with EIS in conjunction with XPS analysis. The results showed that the synergistic effect between electrochemical corrosion and cavitation resulted in the increment of the cumulative weight loss of cast Al-alloy in an ethylene glycol-water solution with cavitation time. The calculation of the impacting efficient for cathodic and anodic processes demonstrated that the cavitation corrosion of cast Al-alloy was controlled by anodic process. In this case, the polarization resistance increased with cavitation time. In a stagnant ethylene glycol-water solution, EIS spectra of cast Al-alloy consisted of a deformed capacitance loop, but it had two time constants based on the Bode plot. Under cavitation, its EIS spectra consisted of a capacitance loop at high frequency zone and an inductive loop at a low frequency zone. It was indicated that the cavitation corrosion mechanism of cast Al alloy in an ethylene glycol-water solution under cavitation was changed obviously. Under cavitation, corroded morphology became rougher with cavitation time and many of pits and cracks existed after cavitation. At the same time, the volume of Al and Si compounds decreased, and that Si compound decreased much more compared with Al(OH)3. At the same time, the volume of OH- increased with cavitation time. This was shown that cavitation accelerated Si compound to peel off from the substrate, resulting in the changes of both the morphology and the chemical structures of surface film under cavitation. Finally, the cavitation corrosion model was proposed based on the above studies.

Effect of application of opposite polarity voltage on interface separation of anodically bonded kovar alloy–borosilicate glass joints

Anodic bonding is a process for bonding metal to ion-conductive glass at a temperature lower than the glass softening point, where a DC voltage is applied to the metal and glass with an anode and a cathode, respectively. In this study, when opposite polarity voltage was applied to anodically bonded Kovar alloy–borosilicate glass joints, the bond interface separated. Because Na accumulated at the bond interface by application of opposite polarity voltage, a brittle Na and Si compound oxide layer removed from the vitrification region formed adjacent to the bond interface. It is considered that destruction of the brittle oxide layer is caused by the thermal stress owing to a slight difference in the linear expansion coefficient and the elastic stress caused by bonding between bond surfaces with imperfect flatness, resulting in separation of the bond interface. The interface separation speed decreased with higher bonding temperature and lower temperature during application of opposite polarity voltage. Because movement of alkali ions becomes easy when the bonding temperature is high, the width of the alkali ion depletion layer formed adjacent to the bond interface was large in joints with high bonding temperature. When opposite polarity voltage was applied, the potential gradient of the alkali depletion layer became large and the potential gradient of the glass bulk became gradual. As a result, movement of Na ions toward the bonding interface was suppressed and it took a long time to reach the Na concentration required to separate the interface.

Influence of Magnesium on Hardness and Microstructure of ADC 12 Alloy Produced by Gravity Casting Method

Magnesium addition produce modification level of silicon’s eutectic for Al-Si-Mg. An increase in magnesium content will increase mechanical properties value of ADC 12 alloy. Highest hardness value reached when Mg content is over 4%. Properties of material’s increased because Mg that added to the alloy will bind with Si and form Mg2Si compound. Where the Mg2Si compound will close the empty space in crystal structure of Al-Si-Mg and the distance between the grain is getting closer so that ADC 12 alloy possess improved strength and hardness. ADC 12 alloy with the addition of 8% magnesium variation will have the highest hardness value, which is 143.44 BHN

Effect of Chromium Addition on the Cyclic Oxidation Resistance of Pseudo-Binary (Mo,Cr)3 Si Silicide Alloy

AbstractThis paper describes the performance under cyclic oxidation of (Mo,Cr)3 Si compound with different Cr additions for possible coating application. Cyclic oxidation was carried out at 1,000 °C at different intervals during 250 h. Oxidized surface samples were analysed by scanning electron microscope where epitaxial oxide scales were observed mainly in samples with higher Cr content which may provide protection against surface oxidation. X-ray diffraction studies have shown the Cr2O3 and SiO2 formation as the main oxide scale; after analyses, it was found that these oxides are responsible for the best oxidation protection, with 36 at.% Cr being the optimal chromium concentration. At lower chromium concentrations, pest reaction occurred in the oxidized samples at times less than 25 h as a result of the formation of the unstable molybdenum oxide.

Electronic properties of the two-dimensional (Tl, Rb)/Si(1 1 1) compound having a honeycomb-like structure

Heavy metal layers having a honeycomb structure on the Si(1 1 1) surface were theoretically predicted to show prospects for possessing properties of the quantum spin Hall (QSH) insulators. The (Tl, Rb)/Si(1 1 1) atomic-layer compound synthesized in the present work is the first real system of such type, where atoms of heavy metal Tl are arranged into the honeycomb structure stabilized by Rb atoms occupying the centers of the honeycomb units. Electronic properties of the (Tl, Rb)/Si(1 1 1) compound has been fully characterized experimentally and theoretically and compared with those of the hypothetical (Tl, H)/Si(1 1 1) prototype system. It is concluded that the QSH-insulator properties of the Tl-honeycomb layers on Si(1 1 1) surface are dictated by the stable adsorption sites occupied by Tl atoms which, in turn, are controlled by the atom species centering the Tl honeycombs. As a result, the real (Tl, Rb)/Si(1 1 1) compound where Tl atoms occupy the T4 sites does not possess QSH-insulator properties in contrast to the hypothetical (Tl, H)/Si(1 1 1) system where Tl atoms reside in the T1 (on-top) sites and it shows up as a QSH material.

Ferromagnet/semiconductor/ferromagnet hybrid trilayers grown using solid-phase epitaxy

The direct growth of semiconductors (SC) over metals by molecular beam epitaxy is a difficult task due to the large differences in crystallization energy between these types of materials. This aspect is problematic in the context of spintronics, where coherent spin injection must proceed via ballistic transport through sharp interfacial Schottky barriers. We report the realization of single-crystalline ferromagnet/SC/ferromagnet hybrid trilayers using solid-phase epitaxy, with combinations of Fe3Si, Co2FeSi, and Ge. The slow annealing of amorphous Ge over Fe3Si results in a crystalline film identified as FeGe2. When the annealing is performed over Co2FeSi, reflected high-energy electron diffraction and x-ray diffraction indicate the creation of a different crystalline Ge(Co,Fe,Si) compound, which also preserves growth orientation. It was possible to observe independent magnetization switching of the ferromagnetic layers in a Fe3Si/FeGe2/Co2FeSi sample, thanks to the different coercive fields of the two metals and to the quality of the interfaces. This result is a step towards the implementation of vertical spin-selective transistor-like devices.

Wafer-Scale Ultrathin, Single-Crystal Si and GaAs Photocathodes for Photoelectrochemical Hydrogen Production.

Crystalline Si and III-V compound semiconductors with appropriate band edge positions for the reduction of water have been widely utilized in photoelectrochemical (PEC) cells for the hydrogen evolution reaction (HER). However, the high cost of manufacturing those PEC cell photoabsorbers makes it difficult to achieve cost-effective hydrogen production. To overcome this issue, a new approach to fabricate a photoabsorber with low cost yet high performance for the HER is highly necessary. Here, we present a controlled fracture method, the so-called spalling process, to fabricate a cost-effective thin semiconductor applicable to the PEC HER. Using this method, a wafer-scale thin Si, whose thickness can be controlled from a few micrometers to sub-50 μm, was fabricated from a thick Si mother substrate without material loss. Pt nanoparticle-decorated 16 μm thick spalled Si with an np+ rear junction exhibited an HER onset potential of 332 mV (vs reversible hydrogen electrode (RHE)) and a photocurrent density of 20.1 mA cm-2 at 0 V (vs RHE), which are the best performances among previously reported planar-type thin Si-based photocathodes. Finally, we demonstrated that 20 μm thick GaAs could also be successfully fabricated by the spalling process, while exhibiting a PEC HER performance comparable to 350 μm thick bulk GaAs.

Preparation and application of energy materials from biomass

Silicon-based anode materials in recent years has gained tremendous interest due to high theoretical specific capacity for next generation lithium ion battery. Some biomass, such as rice husks (RHs), has contained a lot of inorganic silicon, which are abundant in all the countryside and farmland. Considering that RHs are mainly composed of organic lignin, cellulose, hemicellulose, and inorganic Si compound, they could be used to prepare low-cost electrode materials, such as carbonaceous and silicon-based anode materials. In this work, we will present the synthesis of various anode materials from RHs with prominent performance for lithium ion battery application, such as porous C/SiO[Formula: see text] composites and Al-doped porous carbonized RHs husks composites.

Effect of N and Zr on as-cast microstructure and properties after annealing of a high-speed steel

Effects of N and Zr on the as-cast microstructure and properties after annealing of high-speed steel (HSS) were investigated by using electronic probe micro-analysis, Rockwell hardness test, X-ray diffractometry and differential scanning calorimetry with combination of microstructure analysis. The results indicate that the addition of N and Zr will refine the eutectic structures and enhance the stability of carbides which are mainly MC, M2C and M7C3. The coarse dendritic structures decrease significantly and most of the carbides are distributed in the microstructure uniformly. Moreover, a kind of Zr–Si compound which only exists in VC is discovered, and this new phase is speculated to be related with the spheroidization of VC. The annealing process is set up to 6 different time periods which are 1, 3, 6, 10, 15 and 20 h, respectively. In different annealing processes at 750 °C which is lower than austenitizing temperature, the addition of N and Zr makes the decrease of hardness more obvious and restrains the precipitation of secondary carbides with the extension of time. Moreover, when the annealing time reaches 20 h, some clusters appear in the matrix of the two samples, and the density of clusters in HSS1 is lower, but the matrix of HSS1 contains more C and alloying elements which indicate more carbides precipitate.

Poole-Frenkel emission and defect density in a-Si:H/nc-Si:H multilayer films for “all silicon” third generation photovoltaics

A layered structure of nanocrystalline Silicon (nc-Si) and Si compound dielectric (SiO2, Si3N4, SiC) is becoming popular for the third generation photovoltaics because of its tunable optical bandgap. However, this structure makes poor current conduction, as the barrier potential between Si and Si dielectric is very high. An excellent current conduction can be obtained by replacing the Si dielectric with amorphous Si (a-Si:H), as the barrier potential between a-Si and nc-Si is very low compared to nc-Si/Si dielectric structure. This paper presents the electrical properties of an a-Si:H/nc-Si:H multilayer film fabricated using hot-wire chemical vapour deposition. Hydrogen (H2) dilution in the nc-Si:H layers was considered as a variable parameter in different multilayer films. Formation of a nc-Si:H layer sandwiched between two a-Si:H layers was confirmed by Raman spectroscopy and X-ray diffraction. The current-voltage measurements performed in low temperature reveal different vertical charge transport mechanisms, such as Poole-Frenkel (PF) emission, phonon assisted tunnelling and direct tunnelling. Below room temperature, the PF emission rate increases the current with increase in H2 dilution in the multilayer films. The capacitance-voltage measurement performed near the room temperature explains the increased defects in the multilayer films with an increase in H2 dilution.

(Tl, Au)/Si(1 1 1) 2D compound: an ordered array of identical Au clusters embedded in Tl matrix

Formation of the highly-ordered -periodicity 2D compound has been detected in the (Tl, Au)/Si(1 1 1) system as a result of Au deposition onto the Tl/Si(1 1 1) surface, its composition, structure and electronic properties have been characterized using scanning tunneling microscopy, angle-resolved photoelectron spectroscopy and density-functional-theory calculations. On the basis of these data, the structural model of the Tl–Au compound has been proposed, which adopts 12 Tl atoms and 10 Au atoms (in total, 22 atoms) per unit cell, i.e. ∼1.71 ML of Tl and ∼1.43 ML of Au (in total, ∼3.14 ML). Qualitatively, the model can be visualized as consisting of truncated-pyramid-like Au clusters with a Tl atom on top, while the other Tl atoms form a double layer around the Au clusters. The (Tl, Au)/Si(1 1 1) compound has been found to exhibit pronounced metallic properties at least down to temperatures as low as ∼25 K, which makes it a promising object for studying electrical transport phenomena in the 2D metallic systems.

Preparation of alginate flame retardant containing P and Si and its flame retardancy in epoxy resin

ABSTRACTIn the present paper, a novel biomass flame retardant based on alginic acid was synthesized through chemical combination with a reactive P–Si compound. Compared with alginates, the modified alginate showed obviously increased thermal stability and water resisting property, as well as better compatibility with epoxy resin, which can satisfy the requirements of a flame‐retardant additive in the polymer. The flame‐retardant properties were evaluated by vertical burning tests, limiting oxygen index, and microscale combustion calorimetry. Due to the self‐charring capacity of alginate combined with the charring catalyst from P and the charring reinforcer from Si, the modified alginate exhibited much better flame retardancy, taking advantage of the formation of a more continuous, denser, and strengthened char layer than either individual alginate or P–Si flame retardant. The corresponding flame‐retardant mechanisms were investigated and discussed. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017, 134, 45552.

From Si(II) to Si(IV) and Back: Reversible Intramolecular Carbon-Carbon Bond Activation by an Acyclic Iminosilylene.

Reversibility is fundamental for transition metal catalysis, but equally for main group chemistry and especially low-valent silicon compounds, the interplay between oxidative addition and reductive elimination is key for a potential catalytic cycle. Herein, we report a highly reactive acyclic iminosilylsilylene 1, which readily performs an intramolecular insertion into a C═C bond of its aromatic ligand framework to give silacycloheptatriene (silepin) 2. UV-vis studies of this Si(IV) compound indicated a facile transformation back to Si(II) at elevated temperatures, further supported by density functional theory calculations and experimentally demonstrated by isolation of a silylene-borane adduct 3 following addition of B(C6F5)3. This tendency to undergo reductive elimination was exploited in the investigation of silepin 2 as a synthetic equivalent of silylene in the activation of small molecules. In fact, the first monomeric, four-coordinate silicon carbonate complex 4 was isolated and fully characterized in the reaction with carbon dioxide under mild conditions. Additionally, the exposure of 2 to ethylene or molecular hydrogen gave silirane 5 and Si(IV) dihydride 6, respectively.

Brazing SiC matrix composites using Co-Ni-Nb-V alloy

Co-13.5Ni-16.2Nb-10 V (wt.%) filler alloy was designed for joining silicon carbide (SiC) matrix composites, including Cf/SiC and SiCf/SiC composites. The wettability of the Co-Ni-Nb-V alloy on two kinds of composites was studied by the sessile drop method. After heating at 1573 K for 20 min, the new filler alloy exhibited a low contact angle of 18° and 2° on the Cf/SiC and SiCf/SiC composite, respectively. The interfacial microstructure was investigated by SEM equipped with EDS. At the interface between the Cf/SiC composite and the brazing alloy, sandwiched reaction band VC/NbC/VC was formed. In the case of SiCf/SiC, only VC band was formed at the interface, signifying that the activity of Nb was lower than V. The elements of Co and Ni mainly existed in the form of Co-Si or (Co,Ni)-Si compound. The Co-Ni-Nb-V alloy was fabricated into the filler foils with thickness of 100 μm by rapid solidification technique. The Cf/SiC-Cf/SiC joints and SiCf/SiC-SiCf/SiC joints were produced using the filler foil at 1553 K for 10 min. The bend strength of the Cf/SiC-Cf/SiC joints brazed with double-layer filler foil is 21.3 MPa tested at room temperature, whereas the maximum strength of the SiCf/SiC-SiCf/SiC joints brazed with single-layer filler foil reaches 30.8 MPa.

Editors' Choice—Behaviors of Si, B, Al, and Na during Electrochemical Reduction of Borosilicate Glass in Molten CaCl2

The electrochemical reduction of borosilicate glass in molten CaCl2 at 1123 K was investigated. The behaviors of the constituent elements, i.e., Si, B, Al, Na, and K, were estimated using potential–pO2− diagrams constructed from the thermodynamic data for the species. The diagrams suggested that the first cathodic wave in the cyclic voltammogram results from the reduction of the B2O3 component. The dissolution of the Na2O and K2O components, which was predicted from the diagrams, was confirmed by energy dispersive X-ray analysis of a borosilicate glass plate after immersion into molten CaCl2 without electrolysis. The scanning electron microscopy/wavelength dispersive X-ray mappings and Raman spectrum for borosilicate glass reduced at 0.9 V vs. Ca2+/Ca indicated that SiO2 and B2O3 are reduced to Si and B–Si compound. The formation of Ca–Al–O compounds owing to the increase of O2− ions is suggested. The pO2− range during electrolysis at 0.9 V was indicated to be 2.95–3.46.

First principle study of ternary combined-state and electronic structure in amorphous silica

In this paper, for the ITO-SiOx (In, Sn)/n-Si photovoltaic device, the molecular coacervate of In–O–Si bonding and two kinds of quantum states for indium-grafted in amorphous silicon oxide a-SiOx (In, Sn) layers are predicted by molecular dynamics simulation and density function theory calculation, respectively. The results show that the SiOx layers are the result of the inter-diffusion of the In, Sn, O, Si element. Moreover, In–O–Si and Sn–O–Si bonding hybird structures existing in the SiOx layers are found. From the result of formation energy calculations, we show that the formation energies of such an In–O–Si configuration are 5.38 eV for Si-rich condition and 4.27 eV for In-rich condition respectively, which are both lower than the energy (10 eV) provided in our experiment environment. It means that In–O–Si configuration is energetically favorable. By the energy band calculations, In and Sn doping induced gap states (Ev+4.60 eV for In, Ev+4.0 eV for Sn) within a-SiO2 band gap are found, which are different from the results of doping of B, Al, Ga or other group-Ⅲ and V elements. The most interesting phenomena are that there is either a transition level at Ev+0.3 eV for p-type conductive conversion or an extra level at Ev+4.60 eV induced by In doping within the dielectric amorphous oxide (a-SiOx) model. These gap states (GSⅡ and GSIS) could lower the tunneling barrier height and increase the probability of tunneling, facilitate the transport of photo-generated holes, strengthen the short circuit current, and/or create negatively charged defects to repel electrons, thereby suppressing carrier recombination at the p-type inversion layer and promoting the establishment of the effective built-in-potential, increasing the open-circuit voltage and fill factor. Therefore, the multi-functions such as good passivation, built-in field, inversion layer and carriers tunneling are integrated into the a-SiOx (In, Sn) materials, which may be a good candidate for the selective contact of silicon-based high efficient heterojunction solar cells in the future. This work can help us to promote the explanations of the electronic structure and hole tunneling transport in ITO-SiOx/n-Si photovoltaic device and predict that In–O–Si compound could be as an excellent passivation tunneling selective material.