Silicon Carbide Crystals Research Articles

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.

Formation of Nanocomposites on the Surface of Silicon Carbide Crystals under Impact of Iron Fluxes

The formation of graphene–iron composite structures on the surface of cleavages of silicon carbide single crystals using vacuum thermal destruction with simultaneous deposition of iron atoms has been investigated. The effect of the iron deposition rate on the formation of single- and multilayer graphene structures is established. It is shown by X-ray photoelectron and Raman spectroscopy that iron partially dissolves in the graphene structures without formation of nanocrystallites, clusters, etc.

Oxygen-incorporated single-photon sources observed at the surface of silicon carbide crystals

The formation of high-brightness single-photon sources (SPSs) that emit single photons at room temperature was recently confirmed in oxygen-annealed SiC semiconductors (surface SPSs.) However, the defect structure of surface SPSs remains unclear, which makes device fabrication and property control difficult. To verify the incorporation of oxygen in surface SPSs, we fabricated SPSs using stable 18O isotopes as oxidants. By comparing this to the case of natural oxygen annealing, we found that the SP emission spectra for the 18O sample tended to have shorter peak wavelengths, slightly narrower peak widths, and higher intensities. Thus, it appeared that, in the case of the 18O sample, the phonon sideband was located closer to the zero-phonon line and that oxygen was incorporated into the defects attributed to the surface SPS.

Morphology and Chemical Composition of Silicon Carbide Surfaces Interacting with Iron, Cobalt, and Nickel in Microscratching

The paper dwells upon the specifics of worn spots being formed on a silicon-carbide crystal in microscratching of iron, cobalt, and nickel. Analysis was done using a Versa 3D dual-beam electron microscope. The chemical composition of worn spots was studied by local X-ray microanalysis. It was found out that the amount of metal transferred to the silicon-carbide worn spot was associated with the electron structure of metal atoms.

Influences of pre-forming on preparation of SiC by microwave heating

Silicon carbide (SiC) crystals were synthesized by microwave sintering using coal and tetraethoxysilane (TEOS) as raw materials. A sol-gel method was carried out to coat coal mineral particles with silicon dioxide (SiO2). The mixed raw powders were pre-formed by uniaxial pressing into cylindrical pellets in dimension of ~ 30 × 3 mm2. The pre-forming pressure was selected at 0 MPa, 1 MPa, 2 MPa, 3 MPa, 4 MPa and 5 MPa respectively, which led to different apparent density of the green pellets. The influence of apparent density of green pellets on microwave heating behavior was investigated. Different microwave thermal effects were analyzed. Techniques of XRD、SEM were carried out to characterize samples. It was found that pre-forming pressure showed crucial influences on microwave thermal effects and electric field (E-field) intensification. No SiC crystal could be formed without pre-forming pressure. Pre-forming pressure might be the prerequisite for synthesis of SiC by microwave heating. Five consecutive and indispensable heating stages including accumulation of residual air, microwave plasma generation, complex chemical reactions, nucleation and grain growth of SiC crystallites could be distinguished for samples under pre-forming pressure. Different pre-forming pressure leads to changes in heating behavior as well as morphologies of SiC crystals. ~ 4 MPa might be the optimized pre-forming pressure for both microwave plasma effects and E-field intensification.

Hardness and mechanical anisotropy of hexagonal SiC single crystal polytypes

The mechanical response of single crystal silicon carbide (SiC) of two hexagonal polytypes (six layer, 6H- and four layer, 4H) was investigated using nanoindentation. Indentations were performed on two specific crystallographic orientations of single crystals i.e., normal to the basal, (0001) and prismatic, (101¯0) planes, in the load range between 25 mN and 500 mN. A significant anisotropy in the hardness is observed with the basal orientations showing a higher hardness compared to prismatic orientations. In both orientations, the 6H-SiC polytype exhibits higher hardness than the 4H-SiC polytype. It is also observed that the hardness decreases with increasing indentation load, suggesting that SiC crystals exhibit indentation size effect. However, unlike hardness, elastic modulus is independent of indentation load and the elastic anisotropy is insignificant. Severe cracking, particularly at higher indentation loads is noticed near the edges of the indentation imprints. The indentation fracture toughness, KICi computed from the imprints shows slightly higher values for 6H-SiC compared to the 4H-SiC. However, for both the polytypes, a slightly higher KICi is observed for basal indentations compared to the prismatic ones.

In-Situ Synchrotron X-Ray Topography Study on the Stress Relaxation Process in 4H-SiC Homoepitaxial Layers

Recent advances made towards the improvement of the quality of 4H silicon carbide (4H-SiC) have enabled many successful applications in the power device industry. While defect densities in homoepitaxial active layers have been lowered significantly, there is still a strong interest in understanding the formation mechanism of crystal defects such as dislocations preferably through direct observation. During 4H-SiC homoepitaxy and post-growth annealing, the process of stress relaxation has previously been reported to lead to the formation of misfit dislocations or interfacial dislocations (IDs). [1] The driving force for such a relaxation process is believed to be the lattice misfit induced by the difference in nitrogen doping concentrations across the substrate/epilayer interface. According to van der Merwe [2], at initial stage of epitaxy growth, misfit is accommodated by elastic strain in epilayers; as the growth continues, misfit strain builds up until a critical amount when crystals start to deform plastically with misfit dislocations introduced at the substrate/epilayer interface, relaxing additional elastic strain and minimizing the free energy of the epi-substrate system. Matthews and Blakeslee, [3] from a mechanics point of view, predict the onset of stress relaxation process to be the moment when the mismatch stress applied on a single threading glide dislocation in the epilayer (basal plane dislocation in the case of 4H-SiC homoepitaxy) exceeds the sum of the line tension and the Peierls-Nabarro stress as the epilayer grows thicker. Nevertheless, the direct observation of the stress relaxation process in silicon carbide crystals has not yet been reported. In this paper, we present an in-situ study in which a sample cut from a 150mm commercial n-/n+ 4H-SiC homoepitaxial wafer was subject to high-temperature heat treatment while sequential synchrotron white beam X-ray topographs were recorded simultaneously. The heat treatment was carried out in a halogen lamp furnace and the local temperature was elevated to as high as 1600°C. Figure 1 shows a series of topographs recorded in this manner and the time interval between consecutive topographs is 10 seconds. As time advances, a basal plane dislocation (BPD) in the epitaxial layer is observed to glide from left to right, depositing a straight segment of misfit dislocation right at the substrate/epilayer interface. Figure 2 schematically shows the entire process. Such dynamic observations of relaxation process suggest that the lattice misfit once again exceeds the critical value during high temperature heat treatment. Since the epilayer thickness does not change, it can be inferred that the misfit strain simply increases as temperature is raised. The reason may be that the thermal expansion coefficient of 4H-SiC crystal is a function of doping concentration. Sasaki et. al [4] also confirmed this by measuring the lattice constants of both n+ substrate and n- epilayer at elevated temperatures. [1] M. Dudley, H. Wang, J. Guo, Y. Yang, B. Raghothamachar, J. Zhang, B. Thomas, G.Y. Chung, E. Sanchez, D.M. Hansen, S.G. Mueller, MRS Advances, 1(2), 91-102 (2016). [2] J.H. van der Merwe, J. Appl. Phys. 34, 117 (1963). [3] J.W. Matthews, and A.E. Blakeslee, Journal of Crystal Growth, 27, 118–125 (1974). [4] S. Sasaki, J. Suda, and T. Kimoto, Materials Science Forum, 717-720, 481-484 (2012) Figure 1

Numerical Simulation of Temperature Fields in a Three-Dimensional SiC Crystal Growth Furnace with Axisymmetric and Spiral Coils

Three-dimensional numerical simulation platform for silicon carbide crystal growth furnace was established using C programing language, where a physical model of the furnace was built based on cylindrical coordinates; governing equations for electromagnetic and temperature fields were discretized by finite volume method; radiation characteristics were studied with the help of S2S model (surface to surface radiation model); and the least distance method was proposed to check radiation faces visibility efficiently. LU decomposition algorithm based on graphic processing unit (GPU) technology was developed to accelerate the solving process of surface to surface radiation. Then the radiation heat transfer in silicon carbide crystal (SiC) growth chamber and temperature field of silicon carbide growth furnace were studied quantificationally at I = 1250 A and F = 16 kHz. The effects of coil structures (axisymmetric and spiral) on temperature field and its gradient distributions were investigated by standard deviation method. The simulation results demonstrate that spiral electromagnetic coil generates non-axisymmetric temperature field easily; the radiation heat flux is 102~103 times more than conduction heat flux, radiation heat transfer is helpful to increase temperature evenness; the spiral temperature field on the SiC crystal cross-section reduces the poor homogeneity of temperature gradient, which will cause crystal to generate large defects.

Effects of the Temperature Gradient Near the Crystal‐Melt Interface in Top Seeded Solution Growth of SiC Crystal

The top seeded solution growth (TSSG) method is a promising technique for fabricating high‐quality silicon carbide (SiC) single crystals. The carbon required to grow SiC is provided by dissolving the graphite crucible in the silicon melt, and the carbon distribution in the silicon melt is governed by various factors. In this study, two hot zone structures are evaluated using the finite element analysis (FEA) simulation, especially for the temperature distribution, the velocity field, and the carbon concentration in the silicon melt. The results of the simulations revealed significant differences between the two hot zone structures in terms of temperature and carbon concentrations, especially near the interface between the crystal and the melt. SiC crystals are experimentally grown to verify the simulation results with the two hot zone structures. The grown crystals are evaluated to study their surface morphology, crystal quality, polytype stability, and dislocation density by using optical microscopy, high‐resolution X‐ray diffraction, micro‐Raman spectroscopy, and chemical etching, respectively. The simulations and experiments suggests that the hot zone structure with a small temperature gradient especially near the interface between the crystal and the melt promotes stable conditions for growing SiC crystals via the TSSG method.

Group III Acceptors with Shallow and Deep Levels in Silicon Carbide: ESR and ENDOR Studies

Results of investigations of Group III acceptors (B, Al, and Ga) in crystals of silicon carbide using the most informative electron spin resonance and electron nuclear double resonance methods are presented. Structural models of the acceptors with shallow and deep levels are considered. In addition to the data obtained earlier, studies using high-frequency magnetic resonance were obtained, which allowed revealing orthorhombic deviations from the axial symmetry for the deep acceptors; theoretical analysis explains experimentally found shifts of g factors for the deep acceptors arising due to the orthorhombic deviations, which appear probably due to the Jahn–Teller effect.

The Changes in Microstructure and Thermal Constant in Conversion of Carbonized Wood to Silicon Carbide Composite

Carbonized wood which possesses microstructure with random orientation graphitic crystallites and with pores between the graphitic crystallites, is potentially developed into new material of silicon carbide (SiC) composite, a high performance material for engineering purposes. This paper investigates the development of the microstructure in the turbostratic carbon phase and the formation of SiC crystal from the reaction of carbon and SiO2. Results show the turbostratic microstructure in carbonized wood lead to the possible formation of SiC compound in the manufacturing of SiC/SiO2/Ccomposite. The heat treatment at 1800ºC on the mixture of SO2 and carbonized wood creates the formation of SiC compound, which improves the degree of microstructure ordering. The improvement of microstructure turbostratic carbon and the growth of graphitic crystallites in turbostratic carbon improves the thermal conductivity of SiC/SiO2/C composite comparingwith those of carbonized wood composite.

Magnetic properties of the natural and isotope-modified diamond and silicon carbide

The magnetic properties of single crystals of synthetic diamond and crystals of silicon carbide were studied. High-purity samples of diamonds synthesized with HPHT and CVD technologies were used. The crystals of silicon carbide were grown by sublimation and industrial technology. Along with samples with a natural isotopic composition, monoisotopic crystals of diamond (99.96% 12C and 99.96% 13C) and silicon carbide (99.993% of 28Si) were studied. On the basis of the data obtained, the diamagnetic susceptibility was determined and the concentration of paramagnetic centers and the content of the ferromagnetic component were evaluated. The results are discussed.

Акцепторы III группы с мелкими и глубокими уровнями в карбиде кремния: исследования методами ЭПР и ДЭЯР

AbstractResults of investigations of Group III acceptors (B, Al, and Ga) in crystals of silicon carbide using the most informative electron spin resonance and electron nuclear double resonance methods are presented. Structural models of the acceptors with shallow and deep levels are considered. In addition to the data obtained earlier, studies using high-frequency magnetic resonance were obtained, which allowed revealing orthorhombic deviations from the axial symmetry for the deep acceptors; theoretical analysis explains experimentally found shifts of g factors for the deep acceptors arising due to the orthorhombic deviations, which appear probably due to the Jahn–Teller effect.

Determination of stress components in 4H-SiC power devices via Raman spectroscopy

The stress dependencies of the phonon modes in a 4H silicon carbide (SiC) crystal were investigated. The deformation potentials of the A1(TO), E2, and E1(TO) modes were determined on the basis of uniaxial stress tests. The A1(TO) mode was sensitive to stress along the c axis, whereas the E1(TO) and E2 modes exhibited larger dependencies on the stress perpendicular to the c axis than the A1(TO) mode as expected from their atomic displacements. The coefficient of the frequency shift in the E2 mode for an isotropic biaxial stress in the c plane was determined to be −323 MPa/cm−1. We applied cross-sectional Raman measurements to SiC metal-oxide-semiconductor field-effect transistors. The horizontal and vertical stress components, σ11 and σ33, were derived from the frequency shifts of the E2 and A1(TO) modes using the deformation-potential constants. A compressive horizontal stress was observed near the interface between the SiC chip and the metals. The observed compressive stress increased as the temperature decreased. The main cause for the compressive horizontal stress at low temperature is considered to be the difference in the coefficients of thermal expansion of the SiC chip and the metals. The results suggest that the temperature of the fabrication process is one of the key factors for achieving a reduction in the thermal stress in the SiC power devices.

Preparation of silicon carbide coating by chemical vapor deposition by using hexamethyldisilylamine precursor

Silicon carbide (SiC) coating is deposited on C/C composite substrate for the first time by chemical vapor deposition (CVD) with hexamethyldisilylamine (HMDS, C6H19NSi2) as precursor and N2 as carrier gas in an intermediate deposition temperature range. The effects of deposition temperature on phase constitution, surface morphology and deposition rate of the coating are investigated by using X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The coating exhibits a β-SiC (3C) phase at 1010°C, 1060°C and 1100°C, coexistence of both α-SiC (2H) and β-SiC (3C) phases at 1130°C and 1180°C, as well as a single β-SiC (3C) phase again at 1220°C. The morphology and microstructure of coating change significantly with deposition temperature. The change of phase constitution and surface morphologies are closely linked to nucleation, growth and defects formation in SiC crystals during CVD process.

Quantum mechanical theory of epitaxial transformation of silicon to silicon carbide

The paper focuses on the study of transformation of silicon crystal into silicon carbide crystal via substitution reaction with carbon monoxide gas. As an example, the Si(1 0 0) surface is considered. The cross section of the potential energy surface of the first stage of transformation along the reaction pathway is calculated by the method of nudged elastic bands. It is found that in addition to intermediate states associated with adsorption of CO and SiO molecules on the surface, there is also an intermediate state in which all the atoms are strongly bonded to each other. This intermediate state significantly reduces the activation barrier of transformation down to 2.6 eV. The single imaginary frequencies corresponding to the two transition states of this transformation are calculated, one of which is reactant-like, whereas the other is product-like. By methods of quantum chemistry of solids, the second stage of this transformation is described, namely, the transformation of precarbide silicon into silicon carbide. Energy reduction per one cell is calculated for this ‘collapse’ process, and bond breaking energy is also found. Hence, it is concluded that the smallest size of the collapsing islet is 30 nm. It is shown that the chemical bonds of the initial silicon crystal are coordinately replaced by the bonds between Si and C in silicon carbide, which leads to a high quality of epitaxy and a low concentration of misfit dislocations.

Effect of SiBCN content on the dielectric and EMW absorbing properties of SiBCN-Si3N4 composite ceramics

Siliconboron carbonitride ceramics (SiBCN) were introduced into porous Si3N4 substrates via low pressure chemical vapor deposition and infiltration from SiCl3CH3-NH3-BCl3-H2-Ar system. To improve the electromagnetic wave(EMW) absorbing properties, the molar ratio, nCH3SiCl3/(nNH3+nBCl3), was increased based on thermodynamics analysis. The results show that nanosized silicon carbide crystals and free carbon dispersed uniformly in the amorphous SiBCN phase, resulting in suitable dielectric properties and improved absorption capabilities of SiBCN-Si3N4 ceramics. Additionally, with increasing SiBCN ceramics loading, the amount of nanocrystals and interface between nanocrystals and amorphous SiBCN phase increased, leading to enhanced polarization and dielectric loss of the composite ceramics. When SiBCN content was up to 3.64wt%, the electromagnetic reflection coefficient (RC) of SiBCN-Si3N4 composite ceramics reached −40dB (>99.97% absorbing) with the effective electromagnetic absorbing bandwidth of 3.64GHz in the X-band. This study makes it possible to fabricate SiBCN-based composite materials with excellent EMW absorbing properties at a low temperature.

Интенсивность контактного взаимодействия и перенос материалов при шлифовании и микроцарапании тугоплавких металлов

Peculiarities in the contact surfaces formation and material transfer at micro-scratching and grinding of refractory metals are investigated. There is shown a connection of metal quantity transferred to the area of crystal wear, a degree of ground surface charging with silicon carbide crystals and wear of an abrasive tool with the electron structure of atoms in refractory metals. It is defined, that the intensity of metal transfer in a crystal surface layer decreases with the increase of a principle quantum number of metal valence electrons. According to the intensity of the interaction silicon carbide during grinding and micro-cutting the refractory metals are classified into adhesion-active metals of IVB, VB sub-groups and inert metals of VIB subgroup of the Periodic Table.

Li-intercalated graphene on SiC(0001): An STM study

We present a systematical study via scanning tunneling microscopy (STM) and low-energy electron diffraction (LEED) on the effect of the exposure of Lithium (Li) on graphene on silicon carbide (SiC). We have investigated Li deposition both on epitaxial monolayer graphene and on buffer layer surfaces on the Si-face of SiC. At room temperature, Li immediately intercalates at the interface between the SiC substrate and the buffer layer and transforms the buffer layer into a quasi-free-standing graphene. This conclusion is substantiated by LEED and STM evidence. We show that intercalation occurs through the SiC step sites or graphene defects. We obtain a good quantitative agreement between the number of Li atoms deposited and the number of available Si bonds at the surface of the SiC crystal. Through STM analysis, we are able to determine the interlayer distance induced by Li-intercalation at the interface between the SiC substrate and the buffer layer.

4H-SiC wafer slicing by using femtosecond laser double-pulses

Silicon carbide (SiC) is promising as a key material for power electronics devices owing to its wide bandgap property. Meanwhile, by the convention wire-saw technique, it is difficult to slice off a thin wafer from bulk SiC crystal without reserving space for cutting. In this study, we have achieved exfoliation of 4H-SiC single crystal by the femtosecond laser induced slicing method. By using this technique, the exfoliated surface with the root-mean-square roughness of 5 μm and the cutting-loss thickness smaller than 24 μm was successfully achieved. We have also observed the nanostructure on the exfoliated surface in SiC crystal.