Dense Si Research Articles

Mechanism of synthesizing dense Si–SiC matrix C/C composites

The following technique is known to synthesize C/C (carbon fiber-reinforced carbon) composites. The organic matter in the preformed yarn (plastic straw covered yarn including bundles of long carbon fibers, carbon powder, and organic binder) is pyrolyzed at 500 °C and concurrently hot-pressed. Then, the carbon ingredient is graphitized in an atmosphere of nitrogen at 2000 °C. The authors used the above mentioned C/C composites as a starting material and developed a dense Si–SiC matrix C/C composites in which most long carbon fibers remain without reacting with Si which is infiltrated in argon at 1600 °C and 100 Pa. As a result, production of 1 × 2 m large size plates free from warps and cracks was attained in NGK Insulators, Ltd. This mechanism consists of three steps. First, a trunk-shaped Si–SiC matrix is synthesized between yarn and yarn. Then a trunk-shaped Si–SiC matrix extends a yarn by force. Only differential gap is made in a yarn surface. Finally, branch-shaped Si–SiC matrix is synthesized so that a trunk-shaped Si–SiC matrix leads to the yarn inside.

Three-Dimensional High-Density Hierarchical Nanowire Architecture for High-Performance Photoelectrochemical Electrodes

Three-dimensional (3D) nanowire (NW) networks are promising for designing high-performance photoelectrochemical (PEC) electrodes owing to their long optical path for efficient light absorption, high-quality one-dimensional conducting channels for rapid electron-hole separation and charge transportation, as well as high surface areas for fast interfacial charge transfer and electrochemical reactions. By growing titanium dioxide (TiO(2)) nanorods (NRs) uniformly on dense Si NW array backbones, we demonstrated a novel three-dimensional high-density heterogeneous NW architecture that could enhance photoelectrochemical efficiency. A 3D NW architecture consisting of 20 μm long wet-etched Si NWs and dense TiO(2) NRs yielded a photoelectrochemical efficiency of 2.1%, which is three times higher than that of TiO(2) film-Si NWs having a core-shell structure. This result suggests that the 3D NW architecture is superior to straight NW arrays for PEC electrode design. The efficiency could be further improved by optimizing the number of overcoating cycles and the length/density of NW backbones. By implementing these 3D NW networks into electrode design, one may be able to advantageously impact PEC and photovoltaic device performance.

Preparation of Dense and Porous Silicon Oxycarbide Submicrometer‐Sized Spheres Using a Modified Stöber Process

Dense and highly porous silicon oxycarbide (Si–O–C) submicrometer‐sized spheres, with diameters in the range 100–400 nm are synthesized through pyrolysis of sol–gel‐derived hybrid precursors. A modified Stöber process is used for the synthesis of silsesquioxane submicrometer‐sized particles from different organotriethoxysilanes RTES, R=CH3, C5H11, and C6H5. The hybrid particles are transformed into dense inorganic Si–O–C submicrometer‐sized spheres through a pyrolysis process in controlled atmosphere and the spherical morphology is kept provided that the glass transition temperature of the silsesquioxane network is higher than the onset of the polymer‐to‐ceramic transformation. The Si–O–C spheres are stable up to 1400°C. By HF etching the silica nanodomains present in the silicon oxycarbide structure, the dense Si–O–C particles can be further engineered and transformed into highly porous Si–O–C submicrometer‐sized spheres with specific surface area up to 564 m2 g−1 and pore volume up to 0.7 cm3 g−1.

Fabrication and Optical Properties of Silicon Nanowires Arrays by Electroless Ag-catalyzed Etching

Abstract In order to realize ultralow surface reflectance and broadband antireflection effects which common pyramidal textures and antireflection coatings can’t achieve in photovoltaic industry, we used low-cost and easy-made Ag-catalyzed etching techniques to synthesize silicon nanowires (SiNWs) arrays on the substrate of single-crystalline silicon. The dense vertically-aligned Si NWs arrays are fabricated by local oxidation and selective dissolution of Si in etching solution containing Ag catalyst. The Si NWs arrays with 3 μm in depth make reflectance reduce to less than 3% in the range of 400 to 1000 nm while reflectance gradually reached the optimum value with the increasing of etching time. The antireflection of Si NWs arrays are based on index-graded mechanism: Si NWs arrays on a subwavelength scale strongly scatter incident light and have graded refractive index that enhance the incidence of light in usable wavelength range. However, surface recombination of Si NWs arrays are deteriorated due to numerous dangling bonds and residual Ag particles.

Fabrication of Silicon Pillar with 25 nm Half Pitch Using New Multiple Double Patterning Technique

For the higher-density cells of next-generation semiconductor memories, many recent studies have focused on the vertical cell structure technology, which includes various performance merits such as small cell size, high drivability, and suitability for cell-stacked-type arrays. The authors developed a new method to fabricate 25 nm half pitch dense Si pillars that would be applicable to the fabrication of vertical cell devices. Using the proposed multiple double patterning techniques, 23.6 nm diameter, 114 nm height Si cylindrical pillars with a half pitch of 25 nm were fabricated. We confirmed the uniformity in a 300 mm wafer at 30 points, and its 3σ was only 1.7 nm. Moreover, we examined the presence of pillar collapse at arbitrarily selected chip dies for confirmation. Surprisingly, there was no pillar collapse within any of the inspected areas. From these verifications, we conclude that our proposed fabrication technique for slim Si pillars is now available.

Realization of ultra dense arrays of vertical silicon nanowires with defect free surface and perfect anisotropy using a top-down approach

The routine synthesis of ultra dense nanowires arrays appears as an inescapable requirement to implement future generations of nanodevices. In this study, we demonstrate the fabrication of vertical of ultra dense (4×1010cm−2) Si NWs arrays using a top-down fabrication strategy. The developed process also feature nearly perfect anisotropy (98.5%), 100% yield and an excellent surface cleanliness based a self-limiting oxidation mechanism that develops in 1D nanostructure.

Influence of polymer infiltration and pyrolysis process on mechanical strength of polycarbosilane-derived silicon carbide ceramics

In this article, strength evaluation of silicon carbide (Si–C) ceramics fabricated from polycarbosilane (PCS) precursor is described. Si–C ceramics was prepared by firing a green body made of the mixture of Si–C nano-powders and a PCS solution at 1,273 K in N2 gas for an hour. To obtain dense Si–C, the solution was infiltrated into the produced body, and then it was fired again. The polymer infiltration and pyrolysis (PIP) process was conducted up to 12 cycles. Si–C ceramics was diced to be rectangle shape measuring 1.0 mm × 3.0 mm × 0.5 mm, and was subjected to the three-point bending test for measurement of the Young’s modulus and bending strength. Si–C specimens fabricated through PIP processes less than 2 cycles showed non-linear force–displacement curves like a polymer, whereas those through the processes more than 3 cycles showed linear relations and fractured in a brittle manner. The Young’s modulus of 12-cycles-PIPs specimen was found to be 56 GPa on average, which was approximately 22-fold of non-PIP specimen. The bending strength was also increased with an increase in the number of PIP process. The maximum value was found to be 157 MPa. The cause of the influence of PIP process on the mechanical characteristics is discussed using a PCS-derived Si–C model.

Three-dimensional mesoporous structures fabricated by independent stacking ofself-assembled films on suspended membranes

We report an original iterative method for fabricating three-dimensional mesoporous structuresby independently stacking multiple self-assembled block copolymer films supported bySi membranes. A first layer is formed on the substrate by a self-assembled PS-b-PMMA (polystyrene-block-poly(methyl methacrylate)) film. A porous,permeable Si membrane deposited on top of the first block copolymer filmprovides mechanical support, preventing pattern collapse during the wetdeveloping used to selectively remove the PMMA component of the PS-b-PMMA film. A second, dense Si membrane is deposited to seal the porous membrane,resulting in an impermeable coating suspended atop the self-assembled mesoporouspolystyrene structures. The process can then be iterated using the sealed membrane as thenew substrate to support a subsequent self-assembled block copolymer film. This multilayerapproach provides a flexible three-dimensional fabrication technique where, in each layer,pattern morphology, domain orientation and degree of ordering can be designedindependently. Furthermore, the process is compatible with electron-beam directedassembly, used to achieve regular patterns with feature density multiplication at any levelin the stack.

Modification of the Adhesive Properties of XeF2-Etched Aluminum Surfaces by Deposition of Organic Self-Assembled Monolayers

Removal of Si sacrificial layer from Al surfaces using XeF2 gas results in highly adhesive aluminum (oxy)fluoride surfaces. We describe the preparation and characterization of improved, highly nonstick surfaces modified with octadecyltrichlorosilane (OTS). High hydrophilicity of the AlFx surfaces is the key factor for attachment of OTS, where the Al−F surface bond is hydrolyzed to form Al−O−Si link with OTS. Infrared absorption data indicate that a dense Si−O−Si network is formed above the AlFx surfaces upon attachment of OTS. Based on the optical modeling of ν(CH2) absorption bands, an average tilt angle of the molecular layer on fluorinated aluminum is estimated to be 27° ± 2° to the surface normal. Owing to the high packing of the grafted OTS layer, the modified surfaces exhibit an excellent hydrophobicity with a contact angle of 105°. The aging and stability of the interface are determined using X-ray photoelectron spectroscopy.

Defects in SiO2/Si Structures Formed by Dry Thermal Oxidation of RF Hydrogen Plasma Cleaned Si

The present paper reports characterization of the defects in ultrathin (∼10 nm) oxides grown by low-temperature (850°C) thermal oxidation of hydrogen plasma hydrogenated (100)Si and (111)Si substrates. Electrically active defects, studied by analyzing the frequency dispersion of the capacitance-voltage (C-V) and conductance-voltage (G-V) characteristics are dominated by distinct defects, related to interface traps with different localized energy levels in the Si bandgap, border traps and bulk Si traps. Precursors of the trapping centers are defects in a thin, less dense Si surface region containing voids, which is formed during hydrogenation and is incorporated into the growing oxide layer. Oxidation-induced stress level, evaluated from ellipsometric and electroreflectance data analysis, is in the order of 108 N/m2.

Rare-earth element doped Si 3N 4/SiC micro/nano-composites—RT and HT mechanical properties

Dense Si 3N 4/SiC micro/nano-composites with varying grain boundary phase composition were fabricated by hot-pressing under the same conditions. Six different sintering aids (Lu 2O 3, Yb 2O 3, Y 2O 3, Sm 2O 3, Nd 2O 3 and La 2O 3) were used. The formation of SiC nano-inclusions was achieved by in situ carbothermal reduction of SiO 2 by C during the sintering process. Room temperature, fracture toughness, hardness and strength tended to increase when the cation radius of the rare-earth element used in the oxide additive decreased (i.e. from La 3+ to Lu 3+). The composite material with Lu 2O 3 sintering additive showed the highest hardness and had reasonably high fracture toughness and strength. The same micro/nano-composite also possessed the highest creep resistance in the temperature range from 1250 °C to 1400 °C and with loads in the range 50–150 MPa.

Spark plasma sintering: A powerful tool to develop new silicon nitride-based materials

Several key topics on the current assisted sintering of Si 3N 4-based materials are reviewed. First, different proposed mechanisms for spark plasma sintered (SPSed) ceramics are presented, discussing the electric field effect on the liquid phase behaviour and how it may influence the liquid phase sintering of Si 3N 4 ceramics. Next, we show that the SPS is a powerful tool to develop new Si 3N 4-based materials with tailored microstructures, such as functionally graded materials (FGMs) and carbon nanotubes (CNTs) containing Si 3N 4 matrix composites. Si 3N 4 FGMs are fabricated from a sole homogenous Si 3N 4 mixture just modifying the SPS system punches set-up, thus creating a temperature gradient through the specimen. Finally, the capability of SPS to get dense Si 3N 4/CNTs composites overcoming both constraint densification and nanotubes degradation is proved.

Plasma-Mediated Modification of Austenitic Stainless Steel: Application to the Prevention of Yeast Adhesion

The work is focused on defining a stainless steel surface treatment to prevent microbial adhesion. The yeast Saccharomyces cerevisiae was selected as the model for eukaryotic cells. Different surface cleaning procedures, including chemical and plasma-mediated treatments, were studied. Detachment experiments displayed no modification in yeast adhesion, despite significant variations in the physico-chemical properties of the solid surface. Plasma-mediated thin coatings (≈375 nm) were then deposited on stainless steel applying hexamethyldisiloxane/oxygen plasma. FT-IR study revealed a dense SiOSi network, while XPS analysis detected no more iron or chromium at the extreme surface. Detachment experiments demonstrated that these films were successful in preventing cell adhesion, due to a barrier effect.

MECHANISM AND MICROSTRUCTURE TRANSFORMATION OF 3D-Si/LG5 COMPOSITES BY HIGH TEMPERATURE DIFFUSION TREATMENT

A dense and uniform Si p / LG 5 composite were fabricated by squeeze casting technology, and high temperature diffusion treatment was adapted to the composite. Microstructure observation indicated that Si transformed from irregular particles to 3D-structure. Fine dispersive precipitates Si were also observed on Si - Al interface and within Al matrix, smoothing and improving the interface. Based on the microstructure observation, three transformation stages were designated: melting, dissolution and precipitation, solidification. Thermodynamics and kinetics of the transformation can be explained by Gibbs-Thomson effect.

Nanostructural characterizations of hydrogen-permselective Si–Co–O membranes by transmission electron microscopy

Nanostructural characterizations of liquid metal–organic precursors-derived cobalt-doped amorphous silica (Si–Co–O) membranes supported on a mesoporous anodic alumina capillary (MAAC) tube were performed to study their unique high-temperature hydrogen gas permeation properties. Cross-sectional scanning transmission electron microscopy images and selected-area electron diffraction patterns indicated that the metal cobalt and the different oxidation states of cobalt oxides (CoO and Co3O4) nanocrystallites having a size range of 5–20 nm were in situ formed in the mesopore channels of the MAAC tube. In addition, high-resolution transmission electron microscopy micrographs and electron energy loss spectroscopy elemental mapping images indicated that the highly dense Co-doped amorphous Si–O formed within the mesopore channels of the MAAC tube. These nanostructural features could contribute to the hydrogen-selective permeation properties observed for the membranes.

Electroluminescent devices based on amorphous SiN/Si quantum dots/amorphous SiN sandwiched structures

A single layer of dense Si quantum dots with average size of 4 nm sandwiched in amorphous SiN layers was prepared by laser crystallization of ultrathin amorphous Si film followed by subsequently thermal annealing. The electroluminescent diodes were fabricated by evaporating Al electrodes on back sides of p-Si substrates and the top surface of samples. Room temperature electroluminescence can be detected with applying the negative voltage around 10V on the top gate electrode and the luminescent intensity is increased with increasing the applied voltage. It was found that the integrated luminescent intensity is linearly proportional to the injection current which suggested the intensity depends on the concentrations of injected carriers after Fowler-Nordheim tunneling through amorphous SiN barriers. The influence of the amorphous SiN with different band gap on the device performance was also discussed briefly.

Growth process and surface structure of MnSi on Si(1 1 1)

The solid-phase epitaxial growth process and surface structure of MnSi on Si(111) were investigated by coaxial impact-collision ion scattering spectroscopy (CAICISS) and atomic force microscopy (AFM). For the Si(111) sample deposited with 30 monolayers (ML) of Mn at room temperature, the intermixing of Mn and Si gradually started at 100°C and reached equilibrium at approximately 400°C. At this equilibrium state, the Mn atoms were transformed into crystalline MnSi film. Further annealing caused the desorption of Mn atoms. We identified the structure of MnSi as cubic B20 and the crystallographic orientation relationships as Si(111)//MnSi(111) and Si[1¯21¯]//MnSi[1¯10]. The MnSi(111) surface was found to have a dense Si terminating layer on its topmost surface. On the other hand, 3ML of Mn deposited on Si(111) reacted with Si even at room temperature and formed a pseudomorphic structure. This structure was transformed into MnSi after annealing. A filmlike morphology with protrusions was observed for the sample with 30ML of Mn, while island growth occurred for the sample with 3ML of Mn.

In situ preparation of Si 3N 4–TiN composite by pyrolysis of polytitanosilazane (PTSZ)

Si 3N 4–TiN composite powders were obtained by in situ pyrolysis of polytitanosilazane . Dense Si 3N 4–TiN composites were prepared by hot-pressing at 1800 °C under 20 MPa for 2 h without sintering additive. Crystallization of amorphous PTSZ powders occurred between 1400 and 1500 °C with major phases, α-Si 3N 4, β-Si 3N 4, and small amount of phase TiN. Mechanical properties and microstructure of Si 3N 4–TiN composites were characterized. The results showed that the mechanical strength was 620 MPa, the fracture toughness was 7.8 MPa m 1/2 and the Vickers hardness was 8.5 GPa. SEM analysis indicated that Si 3N 4–TiN composite possessed excellent fracture toughness because TiN grains produced by in situ pyrolysis were well dispersed in Si 3N 4 matrix.

Fabrication and characterization of fully dense Si–C–N ceramics from a poly(ureamethylvinyl)silazane precursor

Characterization of the intrinsic physical and mechanical properties of precursor-derived ceramics (PDC) is hitherto hindered by the unavailability of suitable specimens with the representative material structure, homogeneity and of sufficient dimensions amenable to the characterization method. The experimental response is often significantly modified by the included porosity and possible pseudo-microstructures, introduced through the sample fabrication. This paper describes the fabrication of fully dense homogeneous precursor-derived Si–C–N ceramic specimens with material structures covering amorphous to nano-crystalline state using a casting technique, from a liquid polysilazane precursor. The three critical problems involved in the dense PDC processing, viz., (i) bubble formation, (ii) gas evolution induced bloating and cracking and (iii) transformation induced cracking are addressed through controlled cross-linking and thermolysis techniques. Structural characterization of the specimens is carried out using FT-IR, Raman, XRD and HRTEM. The changes in the material structure in the amorphous and phase segregated state are correlated to the material properties, through preliminary physical and mechanical characterization.

Pretreatment and sintering of Si 3N 4 powder synthesized by the high-temperature self-propagation method

Self-propagation high-temperature synthesis (SHS) was applied for the synthesis of low-cost Si 3N 4 powder. The powder was purified and ground until its particle size reached submicron levels and its purity reached 98%. Using this pretreated powder, with α/β = 60/40 content, fully dense Si 3N 4 ceramics, having improved mechanical properties, were obtained by liquid-phase sintering in the presence of (Y, La) 2O 3-AlN. The mechanical properties achieved finally were as follows: strength, 784 MPa; hardness, 15.1 GPa; and fracture toughness, 5.2 MPa m 0.5. The behaviors of the SHS-Si 3N 4 powders before and after the pretreatment were compared. The relation between microstructure and mechanical properties of the sintered specimens and the effect of different β content in the powder on the sintering process of Si 3N 4 were also studied.