Structure Of Silicon Carbide Research Articles

Charge properties and stability of 2D silicon carbide

In this paper, low-dimensional silicon carbide (SiC) obtained on the basis of SiC monolayers with a stoichiometric composition of 1:1 and a number of layers from 1 to 3 has been studied based on the quantum-chemical analysis of the structure and charge properties of this material. The redistribution of the electron density registered in a stack of graphene-like SiC layers allowed us to propose a method of identification of low-dimensional silicon carbide structures. The analysis of geometric and energy parameters made it possible to suggest the sustainable existence of two structural types of 2D-SiC, which differ in the way they are positioned within the layers. As a result of the analysis of the effective charge magnitude, the correlation between this parameter and the geometry of the optimized structure was established. It is proved that taking into account the charge properties of low-dimensional silicon carbide makes it possible to trace the structural changes in the system, identify a specific allotrope, and establish the order of the monolayers.

Electronic structure, mechanical and optical properties of ternary semiconductors Si1-xGexC (X = 0, 0.25, 0.50, 0.75, 1)

ABSTRACTThe electronic structure of silicon carbide with increasing germanium content have been examined using first principles calculations based on density functional theory. The structural stability is analysed between two different phases, namely, cubic zinc blende and hexagonal phases. The zinc blende structure is found to be the stable one for all the Si1-xGexC semiconducting carbides at normal pressure. Effect of substitution of Ge for Si in SiC on electronic and mechanical properties is studied. It is observed that cubic SiC is a semiconductor with the band gap value 1.243 eV. The band gap value of SiC is increased due to the substitution of Ge and the band gap values of Si 0.75 Ge 0.25 C, Si 0.50 Ge 0.50 C, Si 0.25 Ge 0.75 C and GeC are 1.322 eV, 1.413 eV, 1.574 eV and 1.657 eV respectively. As the pressure is increased, it is found that the energy gap gets decreased for Si1-x GexC (X = 0, 0.25, 0.50, 0.75, 1). The elastic constants satisfy the Born – Huang elastic stability criteria. The bulk modulus, shear modulus, Young’s modulus and Poisson’s ratio are also calculated and compared with the other available results.

Highly-porous hierarchical SiC structures obtained by filament printing and partial sintering

Three-dimensional (3D) porous silicon carbide (SiC) structures with total porosity in the range of 64–85% are achieved from nanosized SiC powders by filament printing and partial sintering in a spark plasma sintering (SPS) furnace. The effects of the SPS temperature (1500 and 1700 °C) and the addition of oxide sintering aids (7 wt. % Y2O3+Al2O3) on the porosity of the scaffolds are quantitatively compared. More specifically, hierarchical porosity consisting of meso-pores (< 50 nm) in the struts and open macro-pores (in the range of 500–700 μm) defined by the patterned structure is verified for the 1500 °C SPS treatment. The strength of the structures as a function of the density as well as their cooling profiles, once subjected to rapid heating under a direct flame, are analyzed. By following this easy route, an adaptable family of robust light 3D SiC structures with hierarchical porosity is accomplished.

The role of chemical disorder and structural freedom in radiation-induced amorphization of silicon carbide deduced from electron spectroscopy and ab initio simulations

Chemical disorder has previously been proposed as an explanation for the anomalously facile amorphization of silicon carbide (SiC), on the basis of topological connectivity arguments alone. In this exploratory study, “amorphous” (formally, aperiodic) SiC structures produced in ab initio molecular dynamics simulations were assessed for their connectivity topology and used to compute synthetic electron energy-loss spectra (EELS) using the ab initio real-space multiple scattering code FEFF. The synthesized spectra were compared to experimental EELS spectra collected from an ion-amorphized SiC specimen. A threshold level of chemical disorder χ (expressed as the ratio of the number of carbon-carbon bonds to the number of carbon-silicon bonds) was found to be χ ≈ 0.38, above which structural relaxation resulted in formally aperiodic structures. Different disordering methodologies resulted in identifiably different aperiodic structures, as assessed by local-cluster analysis and confirmed by collecting near-edge electron energy-loss spectra (ELNES). Such structural differences are predicted to arise for SiC crystals amorphized by irradiations involving different damage mechanisms—and therefore differing disordering mechanisms—for example, when contrasting the respective amorphized products of ion irradiation, neutron irradiation, and high-energy electron irradiation. Evidence for sp2-hybridized carbon bonding is observed, both experimentally in the irradiated sample and in simulations, and related to connectivity topology-based models for the amorphization of silicon carbide. New information about the probable intermediate-range structures present in amorphized silicon carbide is deduced from enumeration of primitive rings and evolution of local cluster configurations during the ab initio-modelled amorphization sequences.

Double layered radar absorbing structures of Silicon Carbide fibers/polyimide composites

High performance radar absorbing structures were fabricated using polyimide as matrix, Silicon Carbide fibers (SiCf) as reinforcement, and the effect of carbon-based absorbent (carbon nanotubes (CNTs) and graphene nanoplatelets (GNP)) content on the mechanical and electromagnetic absorbing properties was studied and analyzed. The results indicated that the complex permittivity increased with increasing the filler content due to the high polarization ability, while the flexural strength of the composites decreased owing to the poor dispersion properties. The absorbing properties of carbon filler-SiCf/polyimide composites can be adjusted by the kinds of absorbent, the content and the thickness. In order to enhance the absorbing bandwidths, double layered structures were designed and simulated, and the minimum reflection value can reach to −29.8dB. This material can be using as a kind of high performance radar absorbing structures.

Ballistic tracks in graphene nanoribbons

High quality graphene nanoribbons epitaxially grown on the sidewalls of silicon carbide (SiC) mesa structures stand as key building blocks for graphene-based nanoelectronics. Such ribbons display 1D single-channel ballistic transport at room temperature with exceptionally long mean free paths. Here, using spatially-resolved two-point probe (2PP) measurements, we selectively access and directly image a range of individual transport modes in sidewall ribbons. The signature of the independently contacted channels is a sequence of quantised conductance plateaus for different probe positions. These result from an interplay between edge magnetism and asymmetric terminations at opposite ribbon edges due to the underlying SiC structure morphology. Our findings demonstrate a precise control of transport through multiple, independent, ballistic tracks in graphene-based devices, opening intriguing pathways for quantum information device concepts.

3D Printed and Additively Manufactured RoboSiC™ for Space, Cryogenic, Laser and Nuclear Environments

Abstract Goodman Technologies has been directly responsive to, and focused on, 3D printing and additive manufacturing techniques, and what it takes to manufacture in zero-gravity. During a NASA Phase I SBIR project, using a small multi-printhead machine, we showed that it was possible to formulate and 3D print silicon carbide into shapes appropriate for lightweight mirrors and structures at the production rate of 1.2 square-meter/day. Gradient lattice coupons with feature sizes on the order of 0.8mm were printed and were easily machined to very fine tolerances, ten-thousandths of an inch by Coastline Optics in Camarillo, CA. To further elaborate on the list of achievements, in Phase I, Team GT demonstrated three different ceramization techniques for 3D printing low areal cost, ultra-lightweight Silicon Carbide (SiC) mirrors and structures, radiation shielding, and electronics, several of which could be employed in microgravity The Goodman Technologies briefing presented at 2017 Mirror Technology Days “3D Printed Silicon Carbide Scalable to Meter-Class Segments for Far-Infrared Surveyor: NASA Contract NNX17CM29P along with sample coupons resulted in extreme interest from both Government and the Contractor communities. Our materials, which we call RoboSiC™, is suited for many other applications including heat sinks and radiation shielding for space electronics, and we have already started to make the first parts for these applications. The successful Phase I project suggests that we will meet or exceed all NASA requirements for the primary mirror of a Far-IR Surveyor such as the Origins Space Telescope (OST) and have a high probability solution for the LUVOIR Surveyor in time for the 2020 Decadal Survey. Results indicate that printing on the ground will achieve an areal density of 7.75 kg/square-meter (~39% of a James Webb Space Telescope (JWST) beryllium segment), a cost to print of $60K/segment, and an optical surface that has nanometer-scale tolerances. Printing in the microgravity environment of space we have the potential to achieve an areal density of 1.0–2.0 kg/square meter (&amp;lt;10% of a JWST beryllium segment), with a cost to print of ~$10K/segment. The areal density is 2–15 times better than the NASA goal of 15 kg/square meter, and the costs are substantially better than the NASA goal of $100K/square meter. The encapsulated gradient lattice construction provides a uniform CTE throughout the part for dimensional stability, incredible specific stiffness, and the added benefit of cryo-damping. For the extreme wavefront control required by the Large UV/Optical/IR Surveyor (LUVOIR) the regularly spaced lattice construction should also provide deterministic mapping of any optical distortions directly to the regular actuator spacing of a deformable mirror (DM). Some of our processes will also allow for direct embedding of electronics for active structures and segments. Encapsulation of the lattice structures will allow for actively cooling with helium for unprecedented low emissivity and thermal control. Several decades of experience and testing with SiC have shown that our materials will survive, nay thrive in, the most extreme Space, Cryogenic, Laser and Nuclear Environments.

Synthesis and thermal shock evaluation of porous SiC ceramic foams for solar thermal applications

AbstractReticulated porous ceramics with structural features spanning across multiple length scales are emerging as the primary media in a variety of demanding mass and heat transfer applications, most notably solar‐assisted synthetic fuel processing. In this study, we focus on engineering of the open pore silicon carbide (SiC)‐based foams in such catalytic applications. We evaluate the mechanical integrity and thermal stability of these porous structures. X‐ray tomography analyses of the 3D structures reveal the presence of dual pore size distribution different by up to an order of magnitude in length scale. We further study the effect of thermal shock—induced via water quenching—on the SiC structures and we conclude that the mechanical properties of the ceramic foams are significantly reduced after thermal stress. Comparison of SEM micrographs—before and after thermal shock—reveals that needle‐like features appear inside the foam matrix. These elongated defects may be responsible for structural and mechanical weakening.

Mechanism of Formation of Carbon–Vacancy Structures in Silicon Carbide during Its Growth by Atomic Substitution

The mechanism of formation of carbon-vacancy structures in silicon carbide SiC from silicon vacancies that inevitably form during synthesizing SiC from Si by atomic substitution has been studied. It is shown that it is useful for one of four nearest carbon C atoms to be displaced to the place of a silicon vacancy with the decrease in the total energy by 1.5 eV in the case of polytype 3C and by 0.9–1.4 eV in the case of polytype 4H. In this case, the C atom must overcome the activation barrier of 3.1 eV in the case of polytype 3C and by 2.9–3.2 eV in the case of polytype 4H. This transition is realized during synthesizing SiC due to thermal fluctuations, since the synthesis temperature is T ≈ 1200–1300°C. Thus, the carbon–vacancy structure is a nearly flat cluster of four C atoms and related carbon vacancy with a characteristic diameter of ∼4 A at a distance of 2.4 A from it. All the characteristics of this transformation, namely, the energy profile, the transformation pathway, the transient state, its frequency spectrum, the eigenvector corresponding to the only negative eigenfrequency are calculated by the elastic band method. The infrared spectrum (IR) and the permittivity of SiC containing carbon–vacancy structures are considered. The new line at 960 cm–1 detected recently in the IR spectrum of grown by the atomic substitution is unambiguously identified with vibrations of C atoms in carbon–vacancy structures. It is concluded that carbon–vacancy structures stabilize cubic polytype SiC–3C.

Band structure of monolayer of graphene, silicene and silicon-carbide including a lattice of empty or filled holes

We have developed a -orbital tight-binding Hamiltonian model taking into account the nearest neighbors to study the effect of antidot lattices (two dimensional honeycomb lattice of atoms including holes) on the band structure of silicene and silicon carbide (SiC) sheets. We obtained that the band structure of the silicene antidot superlattice strongly depends on the size of embedded holes, and the band gap of the silicene antidot lattice increases by increasing of holes diameter. The band gap of SiC antidot lattice, except for the lattice of the small unit cell, is independent of the holes diameter and also depends on the distance between holes. We obtained that, the band gap of the SiC antidot lattice is the same as the band gap of the corresponding sheet without hole. Also, the electronic properties of the SiC antidot superlattice occupied either by carbon or by silicon atoms are investigated, numerically. Furthermore, we study the effect of occupation of graphene antidot by Si atoms and vice versa. Also, we have calculated the band structure of graphene and silicene antidot lattice filled by Si + C atoms. Finally, we compute the band structure of the SiC antidot lattice including the holes which are filled by C or by Si atoms. Really, in this paper we have generalized the method of paper[] about graphene antidot with empty holes to the cases of filled holes by different atoms and also to the case of silicene and silicon carbide antidot lattices.

Raman spectroscopy of neutron irradiated silicon carbide: Correlation among Raman spectra, swelling, and irradiation temperature

AbstractThe effects of neutron irradiation on microstructural evolution and the resultant changes in physical and mechanical properties are of critical importance for the development of silicon carbide (SiC) materials for nuclear applications. This study neutron‐irradiated β‐SiC under a wide range of conditions at temperatures between 235 and 750°C and neutron doses of 0.01–11.8 displacements per atom, and then evaluated the effects on the SiC structure using Raman spectroscopy. The SiC optical phonon lines were shifted to lower wavenumbers by irradiation. Correlations were found among the wavenumber of the longitudinal optical phonon line, irradiation‐induced swelling, and irradiation temperature. The peak shift also correlated indirectly with decreasing thermal conductivity of irradiated SiC. The irradiation‐induced peak shift is explained by combinations of lattice strain, reduction of the elastic modulus, and other factors including decreasing coherent domain size. These findings bridge irradiation‐induced microstructural changes and property changes and illustrate how Raman spectroscopy is a useful tool for nondestructively assessing irradiated SiC materials for nuclear applications.

Mechanisms of epitaxial growth of SiC films by the method of atom substitution on the surfaces (100) and (111) of Si single crystals and on surfaces of Si films grown on single crystals Al2O3

The structure and morphology of silicon carbide (SiC) films formed by a new method of atom substitution on the surfaces (100) and (111) of silicon substrates (Si) were investigated. It is shown that the singular faces (100) of Si in the process of substitution of silicon atoms for carbon atoms are transformed into a face of SiC, consisting of an ensemble of facets resembling saw-tooth structures, the side surfaces of which are covered with planes (111) and (110) and (210). Theoretically, a fundamentally different mechanism of atom substitution on the faces (111) and faces (100) is described and experimentally confirmed. It was found that on the vicinal surface of Si, deviated 4° or more from the singular face (100), during the synthesis of SiC, an ordered phase of SiC is formed, the surface morphology of which appears as of facets (scales) consisting of faces (111). It is shown that the method of topochemical substitution of atoms can grow epitaxial SiC layers not only on a silicon substrate, but also on substrates of other materials, in particular, on single-crystal sapphire substrates.

Two-dimensional silicon carbide structure under uniaxial strains, electronic and bonding analysis

Based on Density Functional Theory (DFT) calculations, the narrowed band gap of two-dimensional (2D) SiC planar structure was investigated under uniaxial strains applied along the zigzag and armchair directions. 2D SiC structure exhibits an indirect band gap in its pristine state which undergoes towards direct band gap at −10.35% compressive zigzag strain and vanishes below −16.87% leading to its metallization. The unidirectional strain, applied along the zigzag direction, induces the closure of SiCSi bond angle bending from 120° to 99° promoting the increase of the ionic character of the disynaptic (SiC) atomic basin while reducing the covalent character observed in the pristine structure as revealed by electron localization function (ELF). Additionally, the strain imposed along the zigzag direction, promotes the appearance of weak interactions in regions where there is no absence of strain by means of non-covalent interactions (NCI) index. These non-covalent interactions promote the stability of 2D SiC planar structure under strain regime as recently observed in its three dimensional counterpart (3C SiC). Our structural-electronic-bonding analysis shows that the band gap engineering can be monitored by unidirectional strain regime optimizing the narrowed band gap opening potential routes on the band gap engineering of 2D semiconductor materials with applications in the new generation of electronic devices.

Periodically Structured Silicon Carbide Nanowires Obtained by Annealing the Mechanically-Alloyed Amorphous 2SiB3CN Powder

The mechanically-alloyed amorphous 2SiB3CN powder was used as the precursor for the preparation of SiC nanowires in the current work. Annealed at 1700 °C in argon for 2 hrs, the composite powder was covered by a large amount of grey-green SiC nanowires. SEM, TEM and XRD results reveal that the nanowires are 200 to 1000 nanometers in diameter and hundreds of micrometers in length. Bamboo-shaped and nodular-like β-SiC nanowire accounts for the major part of the products. The bamboo-shaped nanowires have perfect periodicity and periodically distributed stacking faults. Further research indicates that traces of iron in raw powders acts as catalyst, promoting the V-L-S process of the nanowire growth. Current route provides a new method for the large-scale preparation of the periodically structured SiC nanowires, which may find applications in nano sensors, optoelectronic devices, etc.

Effect of Activator on Properties of SiC Porous Ceramics

In this study, porous silicon carbide ceramics were prepared by using silicon carbide as the main material and sodium carboxymethyl cellulose (CMC) as the pore-forming agent. SiO2-Y2O3-Al2O3 and SiO2-kaolin were used as the builders respectively. This study is about the effect of SiO2-Y2O3-Al2O3 and SiO2-kaolin content on the structure and mechanical properties of porous silicon carbide. SiO2-Y2O3-Al2O3 and SiO2-kaolin affect the porosity, hardness and flexural strength of silicon carbide porous ceramics. In this experiment, the porosity of porous ceramics was measured by Archimedes method. The hardness was measured by Rockwell hardness tester. The flexural strength was measured by universal testing machine. The results show that the properties of silicon carbide porous ceramics prepared by SiO2-Y2O3-Al2O3 are better than that of SiO2-kaolinite when the SiO2 -Y2O3-Al2O3 content is 20%. With SiO2-Y2O3-Al2O3 as the combustion agent, porous ceramic with better performance can be prepared at 1500 ℃. When the SiO2-Y2O3-Al2O3 content is 20%, the prepared silicon carbide porous ceramics has a large porosity and excellent mechanical properties, the opening porosity of 23.73%, hardness and flexural strength of 62 and 15.47MPa, from the fracture can be seen more porous and evenly distributed.

Atomic structures of graphene-like nanomaterials including SiC and BP

Honeycomb sheet, graphene like, structure has been interested and the planar structures of silicon carbide (h-SiC), boron phosphide (h-BP) and mixed of both materials h-(SiC)1-x(BP)x were investigated with x values of 0.00, 0.25, 0.50, 0.75 and 1.00. The 2 atoms per unit cell of hexagonal and 4, 8 and 16 atoms per unit cell of orthorhombic configurations were used in atomic structure calculation with 1.5 nm space distance between layers of graphene like structure. The calculation is set for 500 eV energy cutoff, using local density approximation (LDA) exchange-correlation functional, 200 Ry mesh cutoff. The lattice constant was varied to evaluate the stable atomic structures together with the average bond length. Then, the band structures including energy band gap, used to determine electronic property, was calculated to guide the utilization of novel electronic device for new millennium.

Effect of dose and size on defect engineering in carbon cluster implanted silicon wafers

Carbon-cluster-ion-implanted defects were investigated by high-resolution cross-sectional transmission electron microscopy toward achieving high-performance CMOS image sensors. We revealed that implantation damage formation in the silicon wafer bulk significantly differs between carbon-cluster and monomer ions after implantation. After epitaxial growth, small and large defects were observed in the implanted region of carbon clusters. The electron diffraction pattern of both small and large defects exhibits that from bulk crystalline silicon in the implanted region. On the one hand, we assumed that the silicon carbide structure was not formed in the implanted region, and small defects formed because of the complex of carbon and interstitial silicon. On the other hand, large defects were hypothesized to originate from the recrystallization of the amorphous layer formed by high-dose carbon-cluster implantation. These defects are considered to contribute to the powerful gettering capability required for high-performance CMOS image sensors.

IR spectra of carbon-vacancy clusters in the topochemical transformation of silicon into silicon carbide

Using infrared (IR) spectroscopy and spectral ellipsometry, we experimentally confirmed the previously predicted mechanochemical effect of the stoichiometric composition disorder leading to the formation of carbon-vacancy structures in silicon carbide (SiC) films grown on silicon substrates by the atom substitution method. It was found that a band at 960 cm–1 in the IR spectra of SiC films on silicon, corresponding to “carbon-vacancy clusters” is always present in SiC films grown under pure carbon monoxide (CO) or in a mixture of CO with silane (SiH4) on Si substrates of different orientation and doping level and type. There is no absorption band in the region of 960 cm–1 in the IR spectra of SiC films synthesized at the optimum ratio of the CO and trichlorosilane (SiHCl3) gas pressures. The previously predicted mechanism of the chemical reaction of substitution of Si atoms for carbon by the interaction of gases CO and SiHCl3 on the surface of the silicon substrate, which leads to the formation of epitaxial layers of single-crystal SiC, is experimentally confirmed.

Alkali (Li, K and Na) and alkali-earth (Be, Ca and Mg) adatoms on SiC single layer

First-principles calculations within the density functional theory (DFT) have been addressed to study the energetic stability, and electronic properties of alkali and alkali-earth atoms adsorbed on a silicon carbide (SiC) single layer. We observe that all atoms are most stable (higher binding energy) on the top of a Si atom, which moves out of the plane (in the opposite direction to the adsorbed atom). Alkali atoms adsorbed give raise to two spin unpaired electronic levels inside the band gap leading the SiC single layer to exhibit n-type semiconductor properties. For alkaline atoms adsorbed there is a deep occupied spin paired electronic level inside the band gap. These finding suggest that the adsorption of alkaline and alkali-earth atoms on SiC layer is a powerful feature to functionalize two dimensional SiC structures, which can be used to produce new electronic, magnetic and optical devices as well for hydrogen and oxygen evolution reaction (HER and OER, respectively). Furthermore, we observe that the adsorption of H2 is ruled by dispersive forces (van der Waals interactions) while the O2 molecule is strongly adsorbed on the functionalized system.

Investigation of enhancement in planar fast neutron detector efficiency with stacked structure using Geant4

Geant4 based Monte Carlo study has been carried out to assess the improvement in efficiency of the planar structure of Silicon Carbide (SiC)-based semiconductor fast neutron detector with the stacked structure. A proton recoil detector was simulated, which consists of hydrogenous converter, i.e., high-density polyethylene (HDPE) for generating recoil protons by means of neutron elastic scattering (n, p) reaction and semiconductor material SiC, for generating a detectable electrical signal upon transport of recoil protons through it. SiC is considered in order to overcome the various factors associated with conventional Si-based devices such as operability in a harsh radiation environment, as often encountered in nuclear facilities. Converter layer thickness is optimized by considering 109 neutron events of different monoenergetic neutron sources as well as 241Am-Be neutron spectrum. It is found that the optimized thickness for neutron energy range of 1–10 MeV is ~ 400 μm. However, the efficiency of fast neutron detection is estimated to be only 0.112%, which is considered very low for meaningful and reliable detection of neutrons. To overcome this problem, a stacked juxtaposition of converter layer between SiC layers has been analyzed in order to achieve high efficiency. It is noted that a tenfold efficiency improvement has been obtained—1.04% for 10 layers stacked configuration vis-a-vis 0.112% of single converter layer detector. Further simulation of the stacked detector with respect to variable converter thickness has been performed to achieve the efficiency as high as ~ 3.85% with up to 50 stacks.