Silicon Carbide Crystals Research Articles

Applicability of a Flat-Bed Birefringence Setup for the Determination of Threading Dislocations of Silicon Carbide Wafers

Screw-type dislocations like micropipes (MP) and threading screw dislocations (TSD) are prohibiting the function or at least diminishing the efficiency of electronic devices based on silicon carbide (SiC). Therefore, it is essential to characterize wafers in an efficient and fast manner. Molten potassium hydroxide (KOH) etching or white-beam X-ray topography (SWXRT) are either destructive or not economically viable for an in-depth characterization of every wafer of one SiC crystal. Birefringence microscopy is being utilized as a fast and non-destructive characterization method. Instead of microscopic setups, commercially available flat-bed scanners equipped with crossed polarizer foils can be used for fast large-area scans. This work investigates the feasibility of such a setup regarding the detection rate of MPs and TSDs. The results of a full-wafer mapping are compared with birefringence microscopy and KOH etching. In the investigated sample clusters of MPs caused by a polytype switch in the beginning of the growth could be identified by both birefringence microscopy and the flat-bed scanner setup, as well as small angle grain boundaries and TED arrays. However, the resolution of the scanner was not sufficient to identify TSDs. Nevertheless the setup proves to be an easy-to-setup and cheap characterization method, able to quickly identify defect clusters in 4H-SiC wafers.

Space charge and trap energy level characteristics of SiC wide bandgap semiconductor

Charge carrier transport and accumulation in silicon carbide (SiC) wide bandgap semiconductors caused by the defect and impurity are likely to lead to serious performance degradation and failure of the semiconductor materials, and the high temperature effect makes the charge behaviors more complex. In this paper, charge carrier transport and accumulation in semi-insulating vanadium doped 4H–SiC crystal materials and the correlated temperature effect were investigated. Attempts were made to address the effect of deep trap levels on carrier transport. A combination of pulsed electro-acoustic direct space charge probing, an electrical conduction·current experiment, and x-ray diffraction measurement was employed. Space charge quantities including trap depth and trap density were extracted. The results show hetero-charge accumulation at adjacent electrode interfaces under a moderate electrical stress region (5–10 kV/mm). The charge carrier transports along the SiC bulk and is captured by the deep traps near the electrode interfaces. The deep trap energy levels originating from the vanadium dopant in SiC crystals are critical to carrier transport, providing carrier trapping sites for charges. This paper could promote the understandings of the carrier transport dynamic and trap energy level characteristic of SiC crystal materials.

Near‐Field Phase‐Contrast Imaging of Micropores in Silicon Carbide Crystals with Synchrotron Radiation

The results of in‐line phase‐contrast imaging of micropores in crystals with synchrotron radiation (SR) using an example of silicon carbide (SiC) single crystal are reported. Computer simulations of micropore images have provided a thorough interpretation of specific features of experimental results. Herein, micropores of sizes in the interval from a few micrometers to several tens of micrometers are analyzed. The near‐field condition is realized only on a short distance behind the sample for such dimensions. In addition, the size of experimental images is below the resolution limit of the best possible charge‐coupled device detectors. Therefore, it is impossible to determine the actual transverse dimensions directly under the near‐field experiment. A new experimental setup is proposed that employs a secondary SR source created by a compound refractive lens at the focus. Two conditions have to be satisfied. First, the focus must be located in front of the sample at a short distance. Second, the beam size at the focus has to be as small as possible. The implementation of the new scheme requires a high‐intensity synchrotron or X‐ray laser beam provided by modern X‐ray radiation sources. Computer simulations of a virtual experiment using this new method are performed.

Problems with Evaluation of Micro-Pore Size in Silicon Carbide Using Synchrotron X-ray Phase Contrast Imaging.

We report near- and far-field computer simulations of synchrotron X-ray phase-contrast images using a micropipe in a SiC crystal as a model system. Experimental images illustrate the theoretical results. The properties of nearly perfect single crystals of silicon carbide are strongly affected by μm-sized pores even if their distribution in a crystal bulk is sparse. A non-destructive technique to reveal the pores is in-line phase-contrast imaging with synchrotron radiation. A quantitative approach to evaluating pore sizes is the use of computer simulations of phase-contrast images. It was found that near-field phase-contrast images are formed at very short distances behind a sample. We estimated these distances for tiny pores. The Fresnel zones did not provide any information on the pore size in the far-field, but a contrast value within the first Fresnel zone could be used for simulations. Finally, general problems in evaluating a micro-pore size via image analysis are discussed.

Femtosecond laser nanostructuring on a 4H-SiC surface by tailoring the induced self-assembled nanogratings.

Ultrafast laser micromachining of crystalline silicon carbide (SiC) has great perspectives in aerospace industry and integrated circuit technique. In this report, we present a study of femtosecond laser nanostructuring on the surface of an n-type 4H-SiC single crystal. Except for uniform nanogratings, new types of large-area periodic structures including nanoparticle array and nanoparticle-nanograting hybrid structures were induced on the surface of 4H-SiC by scanning irradiation. The effects of pulse energy, scan speed, and the polarization direction on the morphology and periodicity of nanogratings were systematically explored. The proper parameter window for nanograting formation in pulse energy-scan speed landscape is depicted. Both the uniformity and the periodicity of the induced nanogratings are polarization dependent. A planar light attenuator for linear polarized light was demonstrated by aligning the nanogratings. The transition between different large-area periodic structures is achieved by simultaneous control of pulse energy and scan interval using a cross scan strategy. These results are expected to open up an avenue to create and manipulate periodic nanostructures on SiC crystals for photonic applications.

High-Temperature Piezoresistance of Silicon Carbide and Gallium Nitride Materials

We examine the temperature dependence of the piezoresistive coefficients of silicon carbide (SiC) and gallium nitride (GaN) crystals, which are prospective materials for high-temperature applications owing to their wide-bandgap properties. The temperature-dependent piezoresistive coefficients of these materials were obtained by modeling experimental resistance changes using thermomechanical numerical simulations. This work reports the piezoresistive coefficients of 4H-SiC and GaN at the high-temperature environments, which are still not well researched. The results revealed that the temperature dependences of piezoresistive coefficients were strongly related to the ionization energy, and a high ionization energy stabilized the values of the piezoresistive coefficients at high temperatures. Our proposed temperature modeling method helps in predicting the temperature dependence of the piezoresistive coefficient using the value at the room temperature and the ionization energy of the material, which is useful for evaluating the piezoresistive effect at different temperatures during device simulations.

Band gap controlling of doped bulk silicon carbide structure under the influence of tensile stress: DFT

We investigated the effect of tensile stress on a bulk pristine silicon carbide (SiC) and Titanium-doped silicon carbide in two different substitutional sites employing first-principle calculations. The changes in electronic properties have been monitored after the substitution of a silicon atom or a carbon atom in SiC crystal with titanium atom. The simulation results show a clear band gap closing in all structures with symmetric behavior observed in the case of titaniumin in the silicon substitutional site and asymmetric behavior in the case of titanium in the carbon substitutional site. Furthermore, the changes in the Fermi energy, valence bands, and conduction bands have been studied. Calculations reveal a systematic up-shift in the Fermi energy as a consequence of the applied stress, and a systematic down-shift in the Fermi energy in response to the applied tensile strain.

Grinding of Titanium Alloys

X-ray spectroscopic microanalysis was used to determine the concentration of titanium on the surface of the wear spot of a silicon carbide crystal during micro-scratching and the concentration of silicon on the treated surface after grinding by a wheel of silicon carbide. The wear resistance and grinding coefficient of titanium alloys under micro-scratching with single crystals and grinding with wheels made of corundum, silicon carbide, cubic boron nitride and diamond were determined. The morphology of the treated surface and the regularities of changes in the indicators of the grinding process of titanium alloys with circles of various characteristics were studied. The features of the process of grinding titanium alloys using various lubricants and coolants are shown. The influence of the characteristics of the abrasive tool and grinding modes on low-cycle fatigue on the operation of flat and round grinding of samples made of titanium alloys VT9 and VT22 was studied. The influence of abrasive material, grinding speed, feed and sparkout during finishing on low-cycle fatigue was analyzed. Based on the research, recommendations were given for choosing a rational characteristic of the abrasive tool and processing modes.

Solid solubility and site preference of Ti in 3C-SiC

Ti is one of characteristic background impurities in silicon carbide (SiC) crystals, and it has a significant effect on the electronic properties of SiC. Herein, the calculated C-Si-Ti phase diagrams reveal that TiC, Ti3SiC2, and TiSi2 are the most stable phases depending on the chemical potential of Si, and the solid solubilities of Ti in 3C-SiC were determined at the SiC-TiSi2 thermodynamic equilibrium. The calculated hole transition energy between L1 and M3 for different possible defects explains the observed peak shift for the experimental Ti-Lℓ soft X-ray emissions of the Ti-saturated 3C-SiC, which experimentally confirmed that the Si-site substitution by Ti is the preferred type of defects for Ti in narrower-bandgap 3C-SiC. Furthermore, theoretical calculations reveal the different effects of Ti on the Si-L2,3 and C-K emission spectra.

Study of Metal, Silicon Carbide Crystals and Ceramic Bond Transfer to the Surface of Titanium Alloy during Grinding

The high adhesive activity of titanium alloys in interaction with abrasive materials is the main cause of poor grinding treatment. The most common abrasive material for grinding titanium alloys is silicon carbide. Silicon carbide wheels operate primarily in self-sharpening mode. Wear of the abrasive tool in the self-sharpening mode occurs as a result of brittle destruction of the fret. The purpose of the study was to determine experimentally the crystalline wear products of an abrasive tool, made of silicon carbide, on the treated surface during grinding of a titanium alloy. Samples of VT9 titanium alloy were processed by flat mortise grinding by a wheel of silicon carbide with the use of VOLTES coolant and the characteristic of the abrasive tool - 64CF80L7V. The treated surface was examined on the electron microscope Versa 3D Dual Beam. The condition of the treated surface testifies to the intensive adhesive interaction of the titanium alloy with the abrasive tool. The thickness of the metal deposits reaches 3 microns. As a result of morphological analysis, objects are identified on the treated surface, the appearance of which allows us to attribute them to crystals. The chemical composition of the selected objects was determined by a microprobe analysis in a microscope camera. On the basis of the conducted researches, a presence on the grinded surface of silicon carbide crystals of various sizes and a ceramic ligament is established.

Machine-Learning-Based Atomistic Model Analysis on High-Temperature Compressive Creep Properties of Amorphous Silicon Carbide.

Ceramic matrix composites (CMCs) based on silicon carbide (SiC) are used for high-temperature applications such as the hot section in turbines. For such applications, the mechanical properties at a high temperature are essential for lifetime prediction and reliability design of SiC-based CMC components. We developed an interatomic potential function based on the artificial neural network (ANN) model for silicon-carbon systems aiming at investigation of high-temperature mechanical properties of SiC materials. We confirmed that the developed ANN potential function reproduces typical material properties of the single crystals of SiC, Si, and C consistent with first-principles calculations. We also validated applicability of the developed ANN potential to a simulation of an amorphous SiC through the analysis of the radial distribution function. The developed ANN potential was applied to a series of creep test for an amorphous SiC model, focusing on the amorphous phase, which is expected to be formed in the SiC-based composites. As a result, we observed two types of creep behavior due to different atomistic mechanisms depending on the strain rate. The evaluated activation energies are lower than the experimental values in literature. This result indicates that an amorphous region can play an important role in the creep process in SiC composites.

CONDITION OF THE TREATED SURFACE OF NIOBIUM-BASED ALLOY WHEN GRINDING FROM SILICON CARBIDE

The study of the surface of silicon carbide after grinding niobium was carried out on a Versa 3D electronic microscope. It is shown that as a result of the grinding, traces of the grinding wheel material are formed on the treated surface, and the ground material itself is prone to adhesion to the grinding wheel. As a result of cohesive interaction, the metal adhering to the grinding wheel is transferred, and vice versa. And, as a result of adhesion-fatigue processes, mechanical action, there is a hanging wear of the grinding wheel and the indentation of individual crystals of silicon carbide into the surface of the metal being processed. This article examines such features of the behavior of niobium, when grinding it with silicon wheels and is considered from the standpoint of the electronic structure of the metal atom.

Wet oxidation of 3C-SiC on Si for MEMS processing and use in harsh environments: Effects of the film thicknesses, crystalline orientations, and growth temperatures

An in-depth understanding of the formation of silicon dioxide (SiO2) on silicon carbide (SiC) in thermal oxidation is imperative for micro/nano fabrication processes, integration of electronic components, and evaluation of SiC device performance under extreme conditions. Herein, we report a comprehensive study on the effects of crystalline orientations, thicknesses, and growth temperatures of cubic SiC films on their wet oxidation properties. The oxidation rate and surface morphology were characterized using atomic force microscopy (AFM) and light reflectance measurement systems. Our experimental results revealed the role of defects in the SiC crystal on the oxidation that relates to SiC thickness, deposition conditions, crystal orientation and temperature of wet oxidation. Critically, the electrical properties of SiC films oxidized at 900 °C remained the same as the unoxidized film as confirmed by room-temperature current-voltage measurements, indicating a long-term service temperature of SiC. These findings are expected to provide crucial information on the effects of defects on the formation of SiO2 on SiC films at different oxidation temperatures, which is highly essential for establishing a basic platform for the fabrication of high-performance SiC-based electronic devices.

Molecular dynamics simulation of the material removal in the scratching of 4H-SiC and 6H-SiC substrates

Single crystal silicon carbide (SiC) is widely used for optoelectronics applications. Due to the anisotropic characteristics of single crystal materials, the C face and Si face of single crystal SiC have different physical properties, which may fit for particular application purposes. This paper presents an investigation of the material removal and associated subsurface defects in a set of scratching tests on the C face and Si face of 4H-SiC and 6H-SiC materials using molecular dynamics simulations. The investigation reveals that the sample material deformation consists of plastic, amorphous transformations and dislocation slips that may be prone to brittle split. The results showed that the material removal at the C face is more effective with less amorphous deformation than that at the Si face. Such a phenomenon in scratching relates to the dislocations on the basal plane (0001) of the SiC crystal. Subsurface defects were reduced by applying scratching cut depths equal to integer multiples of a half molecular lattice thickness, which formed a foundation for selecting machining control parameters for the best surface quality.

Formation of Nanoscale Cracks and Fractoluminescence upon Destruction of Carbon Ceramics

We obtained a Raman spectrum for a surface layer of porous carbon ceramic (porosity ≈5%) of ≈80 nm thick. The analysis of the spectrum showed that the ceramics contain crystals of silicon carbide 6H‑SiC and silicon. The destruction of ceramics by diamond microcrystals resulted in fractoluminescence (FL). Its spectrum contained two bands at 1.6 and 1.9 eV. The first is emerged at the destruction of silicon crystals, and the second is emerged at 6H-SiC crystals. We obtained the time dependence of the intensity of the fractoluminescence signals with a time resolution of 2 ns. Three types of signals were observed: one is formed when the 6H-SiC crystals are destroyed; the second is formed when silicon crystals are destroyed; the third is formed when these crystals are simultaneously destroyed. The appearance of signals is associated with the formation of cracks arising from the breakthrough of barriers formed at the intersection of slip planes of dislocations in silicon carbide and silicon crystals. The size of cracks in 6H-SiC has been estimated: the smallest is 5.5 nm, and the largest is ≈18 nm.

Whiskers as perspective input material for production of high temperature resistant ceramic building materials

Production of electric power by nuclear power plants (NPP) in the world and in Russia accounts for about 17%. Much attention is currently paid to safe NPP operation and to enhancement of their reliability. Present-day nuclear power plants with high-temperature gas-cooled rectors require development and application of new composite materials capable to withstand high temperatures and mechanical stresses. Ceramic composite materials where whiskers of heat-resistant chemical compounds are used as reinforcement fibers look promising for those purposes. The paper investigates and analyses the opportunity of applying whiskers as reinforcement fibers for heat-resistant composite ceramic building materials. Studies revealed that whiskers of silicon carbide and aluminium nitride are resistant towards high temperature heating without notable change in their chemical composition. It was found out that mono crystals of silicon carbide have higher mechanical strength owing to lack of defects in its structure. Linear heat expansion factors of silicon carbide that are needed for thermo-chemical calculations of composite materials on its base were determined.

Similarity analysis of stacking sequences in a SiC nanowire pair grown from the same catalyst nanoparticle using Levenshtein distance.

Stacking faults are easily formed in silicon carbide (SiC) crystals, and this is also the case for SiC nanowires. The stacking faults exercise influences on SiC's properties, therefore it is important to understand their formation mechanism and to control their formation for applications of SiC and its nanowires. In this study, we propose a method for investigating stacking faults' formation mechanism in nanowires and provide its proof of concept. Stacking sequences in a pair of SiC nanowires that were grown from the same metal catalyst nanoparticle were quantified as a pair of binary sequences, and Levenshtein distances between partial sequences extracted from the two sequences were measured to detect similarity between them, and the result was compared with that obtained using a surrogate data of one sequence. The similarity analysis using Levenshtein distances works as a probe for investigating possible influences of some phenomena in the catalyst nanoparticle on the formation of stacking faults. The analysis did not detect a correlation between the two sequences. Although a possibility that the formation of stacking faults in the nanowires were owing to some phenomena in the catalyst nanoparticle cannot be denied, the extrinsic cause in the catalyst nanoparticle was not detected through our analysis in this case.

Numerical Study of Three-Dimensional Melt Flows during the TSSG Process of SiC Crystal for the Influence of Input Parameters of RF-Coils and an External Rotating Magnetic Field

Three-dimensional numerical simulations were conducted for the Top-Seeded Solution Growth (TSSG) process of silicon carbide (SiC) crystals. We investigated the influence of coils frequency and peak current, and an applied rotating magnetic field (RMF) on the melt flow developing in this system. Numerical simulation results show that the Marangoni flow in the melt becomes stronger at higher coils frequencies due to the decreasing coils-induced electromagnetic field strength. Results also show that the use of external RMF may improve supersaturation uniformity along the seed if it is properly adjusted with respect to the coils-induced electromagnetic field strength. Furthermore, it is predicted that the application of RMF and seed rotation in the same direction may enhance supersaturation below the seed.

Thermal evolution of silicon carbide electronic bands

Direct observation of temperature dependence of individual bands of semiconductors for a wide temperature region is not straightforward, in particular. However, this fundamental property is a prerequisite in understanding the electron-phonon coupling of semiconductors. Here we apply \emph{ab initio} many body perturbation theory to the electron-phonon coupling on hexagonal silicon carbide (SiC) crystals and determine the temperature dependence of the bands. We find a significant electron-phonon renormalization of the band gap at 0~K. Both the conduction and valence bands shift at elevated temperatures exhibiting a different behavior. We compare our theoretical results with the observed thermal evolution of SiC band edges, and discuss our findings in the light of high temperature SiC electronics and defect qubits operation.

Образование нанометровых трещин и фрактолюминесценция при разрушении углеродной керамики

Raman spectrum of the surface layer with a thickness of approximately 80 nm of a porous carbon ceramic was obtained (the porosity was used equal to 5%). Spectrum analysis showed that the ceramics contains crystals of silicon carbide - 6H-SiC and silicon. The destruction of ceramics by diamond microcrystals resulted in fractoluminescence (FL). Spectrum of FL contains two bands at 1.6 and 1.9 eV. The first band is formed by breaking down the silicon crystals, and the second band is formed by breaking down the 6H-SiC crystals. The time dependence of the intensity of FL signals with a time resolution of 2 ns is obtained. Three types of signals were observed: one type of signal is formed when the 6H-SiC crystals break down, the second type of signal is formed when the silicon crystals break down, and the third type of signal is formed when these crystals break down simultaneously. The appearance of signals is associated with the formation of cracks arising from the breaking down of the barriers formed at the intersection of slip planes of dislocations in silicon carbide and silicon crystals. The size of cracks in 6H-SiC is estimated: the smallest ones is 5.5 nm, and the largest is approximately 18 nm.