Monocrystalline SiC Research Articles

Investigation of the elastic-plastic evolution mechanism of 4H-SiC substrates with external thermal-assisted abrasion

Plastic removal is easier to achieve nanometric surfaces of monocrystalline SiC with brittleness in nano-polishing. This paper mainly focuses on the nano-scale elastic-plastic evolution mechanism of 4H-SiC substrates in thermal-assisted polishing using molecular dynamics simulation. The results show that 4H-SiC substrates undergo elastic deformation, elastic-plastic deformation, and plastic deformation in ramp nano-abrasion. The critical depth from elastic to elastic-plastic deformation rises with higher external temperature, and elastic-plastic deformation stage is compressed. Amorphization dominates the elastic recovery in elastic-plastic deformation stage. Furthermore, the step-terrace structure appears in elastic deformation stage, and is observed at a greater depth rising from 0.679 nm to 1.226 nm with increasing external temperature from 298 K to 700 K. High surface quality and subsurface damage-free of SiC substrates are performed with forming the atomic step-terrace structure.

Structural and Mechanical Properties of SiC-Rich By-Products of the Metal Grade Si Process

Mechanical properties of composites produced from the SiC-rich furnace slag using traditional stone and ceramic machining technologies were studied. A non-uniform mixture of coarse monocrystalline SiC grains soaked with Si-metal and glassy oxide phases represented the microstructure of dense monolithic SiC-rich samples. The fracture mechanism of coarse-grained SiC-rich composites was susceptible to the grain size/sample geometry and machining conditions yielding flexural strength in the range of 50-106 MPa and high compression strength of 750 MPa. Despite inhomogeneous macro and microstructure, mechanical and thermal properties are comparable to the traditionally produced siliconized SiSiC ceramics. It opens up the opportunity for the circular economy and value-added recycling of the Si/FeSi industries’ wastes.

Research on analyzing and valuing residual stress for micro wire busbar

The electroplated diamond wire saw made from high-carbon steel wire busbar (busbar), which directly affects the quality of cutting monocrystalline silicon, polysilicon, sapphire and SiC. When the wire diameter is on the micro-scale, there is no equipment to precisely measure the distribution of residual stress at present. The free circle diameter (FC-diameter) is used as the quality index to measure the distribution of axial residual stress of the busbar post-drawing. The existing finite element simulation of residual stress of steel wire post-drawing is mainly aimed at steel wire diameter of millimeter, and most of it is two-dimensional finite element simulation. The effect of actual factors on residual stress of the busbar is not considered in the simulation. In this paper, finite element simulation is carried out for steel wire with diameter in micron scale. The influences of die deflection, die friction coefficient, die sizing band ellipse and steel wire ellipse on axial residual stress is considered. The relationship between residual stress distribution and FC-diameter is established, and the correctness of simulation results is verified by experiments.

High-temperature active oxidation of nanocrystalline silicon-carbide: A reactive force-field molecular dynamics study

Flexible woven SiC ceramics are prone to accelerated fiber embrittlement under high temperature oxidation in dynamic oxygen environments. The nanocrystalline structure of the constituent fibers impacts the reaction kinetics and phase transformations during active oxidation. However, fundamental understanding and quantification of grain boundary effects on oxidation behavior in nanocrystalline SiC remain elusive when temperatures exceed 1500 K. This study deploys large-scale molecular dynamics simulations with a reactive force-field to elucidate the complex roles of atomic oxygen reservoir conditions and grain size on oxidation kinetics and the nature of oxides produced in both monocrystalline and nanocrystalline 3C-SiC between 1100 K and 2000 K. The simulations with dynamically replenished oxygen provide good agreement with oxidation kinetics and activation energies for the monocrystalline Si(100) and C(100) orientations published in the available literature. This study reveals that, by contrast, nanocrystalline SiC samples exhibit two distinct oxidation kinetics with a transition point at 1500 K due to surface melting, which is supported by experimental evidence. The introduction of a grain-boundary network produces a two-fold decrease in oxidation activation energies compared to monocrystalline SiC below 1500 K. Above 1500 K, however, the activation energies rise substantially due to the formation of a liquid Si phase at the SiC/Si oxide interface. It is shown that the stability of the interfacial liquid phase is promoted by incoherent grain boundaries in the crystalline SiC. These findings are important for the deployment of nanocrystalline SiC fibers in advanced thermal protection systems for high-temperature applications.

Strain-rate sensitivity of brittle deformation and removal mechanisms of monocrystalline 3C–SiC induced by nano cutting process

In this work, the dynamic crack propagation mechanism of 3C–SiC at the micro-scale and the influence mechanism of the strain rate effect are studied, and the deformation mechanism under different strain rate is deeply analyzed. These simulations show many new phenomena, such as crack nucleation, deflection and arrest, defect accumulation and evolution near crack, strain rate effect on material deformation and removal. When the cutting velocity is equal to 5 m/s, the deformation mode of 3C–SiC is mainly sliding deformation, and the chip is separated from the matrix in the form of massive slump. When the cutting velocity reaches 500 m/s, the plastic deformation mode of the material in the machining region is the same, but the dislocation density is significantly reduced, and the material removal mode is transformed into a pure fracture mode. The brittle removal mode of monocrystalline SiC is closely related to the strain rate. The results will not only enhance the understanding the strain rate sensitivity of the damage evolution and removal behaviors of monocrystalline 3C–SiC involved at nano- and micro-scales, but also provide a theoretical guidance for optimizing process parameters during the SPDT process of monocrystalline 3C–SiC.

Thermal effects on removal mechanism of monocrystal SiC during micro-laser assisted nanogrinding process

As a hard-brittle material, research into efficient processing methods of monocrystalline SiC with low damage has attracted a lot of attention. In this paper, removal behavior of monocrystalline SiC using micro-laser assisted method is analyzed using molecular dynamics method. This enables an in-depth theoretical analysis of thermal effects on material removal mechanism for 3C–SiC during nanogrinding process. Taking average laser pulse intensity as a variable, the micro deformation mechanism and machinability under the micro-laser assisted method are analyzed in detail. The results show that laser irradiation can directly affect material removal rate and subsurface damage depth of 3C–SiC. The results of this work significantly improve the understanding of the feasibility of micro-laser assisted method and its optimization for 3C–SiC.

Applying molecular dynamics simulation to take the fracture fingerprint of polycrystalline SiC nanosheets

Graphene-like nanosheets are the key elements of advanced materials and systems. The mechanical behavior of the structurally perfect 2D nanostructures is well documented, but that of polycrystalline ones is less understood. Herein, we applied molecular dynamics simulation (MDS) to take the fracture fingerprint of polycrystalline SiC nanosheets (PSiCNS), where monocrystalline SiC nanosheets (MSiCNS) was the reference nanosheet. The mechanical responses of defect-free and defective MSiCNS and PSiCNS having regular cracks and circular-shaped notches were captured as a function of temperature (100–1200 K), such that elevated temperatures were unconditionally deteriorative to the properties. Moreover, larger cracks and notches more severely decreased the strength of PSiCNS, e.g. Young’s modulus dropped to ca. 41% by the crack enlargement. The temperature rise similarly deteriorated the failure stress and Young's modulus of PSiCNS. However, the stress intensity factor increased by the enlargement of the crack length but decreased against temperature. We believe that the findings of the present study can shed some light on designing mechanically stable nanostructures for on-demand working conditions.

Vacancy growth of monocrystalline SiC from Si by the method of self-consistent substitution of atoms

A modified technique for the growth of silicon carbide from silicon by the method of self-consistent substitution of atoms is proposed, which makes it possible to increase the achievable thickness of the formed silicon carbide layer by about an order of magnitude. The technique includes an additional step before the growth process: the surface of the silicon substrate is being saturated with vacancies by annealing in vacuum at T = 1350 ℃ for ~ 30 min. This leads to a change in the mechanism of mass transfer from the interstitial to the vacancy one when silicon atoms are being substituted by carbon atoms. As a result, not only the thickness of the silicon carbide layer increases, but also the separation of silicon carbide from the silicon substrate occurs if the thickness of the SiC layer surpasses 400 nm.

SiC, Si and diamond detectors for comparison of laser-generated plasma in TNSA regime

Monocrystalline SiC, Si and Diamond detectors have been used to monitor the radiations emitted from TNSA laser-generated plasma using a high-intensity fs laser. The comparison of their spectra was performed monitoring in time-of-flight the forward emitted TNSA plasma radiation. Carbon, aluminum, copper and gold targets were irradiated with an intensity of the order of 1019 W/cm2. SiC detector, with 80 microns depth active region, gave the best response to detect fast UV and X-rays, relativistic and cold electrons, and energetic protons and light accelerated ions. The experiments demonstrate the advantages to use semiconductor detectors to characterize the plasma enhancing the importance of the thickness of the target with respect to its electronic density.

Precision Deep Reactive Ion Etching of Monocrystalline 4H-SiCOI for Bulk Acoustic Wave Resonators with Ultra-Low Dissipation

Integrated mechanical resonators with high quality factors (Q) made in high acoustic velocity materials are essential for a wide range of applications, including chemical sensors, timing resonators, and high-performance inertial sensors for navigation in GPS-occluded environments. While silicon is the most popular substrate for the implementation of microelectromechanical systems (MEMS) resonators, SiC exhibits an exceptionally small intrinsic phononic dissipation due to its low Akhiezer damping limit. This paper reports on the latest developments of precision deep reactive ion etching (DRIE) of monocrystalline 4H SiC-on-Insulator (SiCOI) substrates with the aim to fully take advantage of the exquisite mechanical properties of crystalline SiC. To wit, capacitive Lamé mode micromechanical resonators exhibit ƒ·Q products beyond 1 × 1014 Hz independent of crystalline orientation. The contribution of surface roughness to dissipation and practical considerations to etch mirror-polished trenches in SiCOI substrates are discussed, paving the way towards micromechanical monocrystalline SiC resonators with Qs beyond 100 Million.

Investigating Elastic Anisotropy of 4H-SiC Using Ultra-High Q Bulk Acoustic Wave Resonators

Hexagonal 4H-silicon carbide (4H-SiC) is a transversely isotropic substrate garnering interest for precision MEMS devices such as resonant gyroscopes. This paper investigates the elastic anisotropy of 4H-SiC by utilizing capacitive bulk acoustic wave (BAW) resonators with ultra-high mechanical quality factors ( $Q$ ) enabled by phononic crystals. We directly measure the value of $C_{66}$ using Lame mode resonators for the first time and numerically fit the values of $C_{11}$ and $C_{12}$ using BAW elliptical modes in center-supported solid disk resonators. We compare (0 0 0 1) 4H-SiC to (1 1 1) Si, another in-plane isotropic material and validate (0 0 0 1) 4H-SiC’s superior robustness to fabrication and design variations. Measurement of in-plane BAW elliptical modes in multiple disk resonators with as-born frequency splits as low as 3 ppm reveal (0 0 0 1) 4H-SiC’s transverse isotropy across process corners. Lame mode resonators display a temperature coefficient of frequency (TCF) three times lower compared to its Si counterpart. Finally, this paper provides a modified set of elastic constants for 4H-SiC with a view towards monocrystalline SiC MEMS devices. [2020-0254]

Facile integration of MoS2/SiC photodetector by direct chemical vapor deposition

Abstract The MoS2 photodetector on different substrates stacked via van der Waals force has been explored extensively because of its great potential in optoelectronics. Here, we integrate multilayer MoS2 on monocrystalline SiC substrate though direct chemical vapor deposition. The MoS2 film on SiC substrate shows high quality and thermal stability, in which the full width at half-maximum and first-order temperature coefficient for the E 2 g 1 $E_{2g}^1$ Raman mode are 4.6 cm−1 and −0.01382 cm−1/K, respectively. The fabricated photodetector exhibits excellent performance in the UV and visible regions, including an extremely low dark current of ~1 nA at a bias of 20 V and a low noise equivalent of 10−13–10−15 W/Hz1/2. The maximum responsivity of the MoS2/SiC photodetector is 5.7 A/W with the incident light power of 4.35 μW at 365 nm (UV light). Furthermore, the maximum photoconductive gain, noise equivalent power, and normalized detectivity for the fabricated detector under 365 nm illumination are 79.8, 7.08 × 10−15 W/Hz1/2, and 3.07 × 1010 Jonesat, respectively. We thus demonstrate the possibility for integrating high-performance photodetectors array based on MoS2/SiC via direct chemical vapor growth.

Impact of the dangling bond defects and grain boundaries on trapping recombination process in polycrystalline 3C SiC

The bulk polycrystalline (pc) 3C SiC of n- and p-type obtained by thermal decomposition of methyltrichlorosilane vapor have been studied by applying the electron paramagnetic resonance (EPR), direct current (DC) conductivity, photoconductivity (PC) and PC time-resolved decay methods. The energy level of the two donor-like minority carrier traps at ε1 = 77 meV and ε2 = 92.6 meV located at the grain boundaries (GB) of pc-3C SiC has been obtained using the measurements of the temperature dependence of DC conductivity and PC in the range of 80–600 K. The minority carrier traps assigned to carbon and silicon dangling bonds with the carbon back bonds were observed in the EPR spectra of pc-3C SiC of n- and p-type at g = 2.0029(3), g = 20042(3), respectively. The PC time decay after the termination of the photo-excitation was studied in monocrystalline and pc-3C SiC of n- and p-type at 80 K. The persistent relaxation of PC has been described by kinetic equations accounting the trapping, ionization, and recombination processes of non-equilibrium charge carriers bound dynamically to shallow donors and acceptors. We have concluded that the main process responsible for the long-lived relaxation of the PC is trap-assisted electron-hole recombination in n-type pc-3C SiC and ionization of boron acceptors, as well as the hole escape/capture at the boron level in p-type pc-3C SiC. The differences in the relaxation process of the PC in n- and p-type pc-3C SiC were explained by the presence of the potential barrier height of about 8.6 meV at GB for the capture of the majority carriers in p-type pc-3C SiC.

Atomic-scale study of vacancy defects in SiC affecting on removal mechanisms during nano-abrasion process

The mechanical properties of mono-crystalline SiC with vacancy defects in fixed abrasive polishing processes are not well known at the nanometric scale. In the molecular dynamic (MD) simulation, the removal mechanism of mono-crystalline SiC substrates with vacancy defects is investigated. So is the wear mechanism of diamond abrasives explored. The simulation result reveals that the increase of vacancy defects in SiC substrates leads to reduced von Mises Stress, however, to increased temperature of SiC substrates during nano-abrading process. More vacancy defects are found to lead higher removal efficiency and less subsurface damage on SiC substrates. Furthermore, the diamond abrasives are worn out through a combination of thermo-chemical wear, graphitization wear and abrasive wear in the simulation.

Microstructural characterization of reaction products in Cu/Graphene/SiC system

The results of microstructural examinations of reaction products formed at liquid copper/graphene-coated monocrystalline SiC interface are presented. Samples were prepared during a wettability test performed under a vacuum at T = 1100 °C kept constant for 30 min by capillary purification-sessile drop method. Careful analyses of the microstructure and chemical composition were carried out at the interfaces by high resolution scanning electron microscopy combined with local analysis of chemical composition, Raman spectroscopy and computed tomography. The detailed structural investigations showed that in both systems, at the drop/substrate interface, the substrate (SiC) was dissolved and the zone of reaction products was composed of alternately arranged dark and bright layers.

Modeling and verifying of sawing force in ultrasonic vibration assisted diamond wire sawing (UAWS) based on impact load

Ultrasonic vibration assisted diamond wire sawing (UAWS) is an effective sawing process for hard and brittle materials such as monocrystalline SiC and Si. Compared with the conventional diamond wire sawing (CWS), both of sawing force and workpiece surface quality are improved by UAWS greatly, but the principle of improvement is still unclear. In order to reveal the mechanism of sawing force reduction in UAWS, this paper presents a theoretical model for sawing force in UAWS based on the theory of impact load. Firstly, the transverse vibration model of the diamond wire saw in UAWS was established based on the transverse vibration theory of continual system. Secondly, the impact load model of single abrasive was established based on the vibration model. The validity of this impact load model was verified by the finite element simulation. Thirdly, the sawing force caused by multi abrasives was derived based on the distribution of abrasives on the surface of wire saw. Finally, the verification experiments were conducted on the monocrystalline Si workpiece in various groups of processing conditions. The average error between the experimental and theoretical results of sawing force is 7.50%, which verifies the validity of the theoretical model. The measured results also indicate that the workpiece surface roughness of UAWS are 4.3%–29.7% lower than that of CWS.

Experimental investigation on the ultrasonically assisted single-sided lapping of monocrystalline SiC substrate

This paper studied the effect and optimization of ultrasonically assisted single-sided lapping for monocrystalline SiC wafer using grey relational analysis method. 27 experiments based on the Taguchi method of orthogonal arrays were performed to determine the best factor level condition. Besides, the comparative experiments using three lapping plates (cast iron, copper, and aluminum) were also performed under ultrasonic lapping and non-ultrasonic lapping, respectively. Material removal rate (MRR) and surface roughness (Ra, Rmax) were selected as the output indexes. The degree of influence that the controllable process factors exert on the indexes was studied by investigating the correlation between them. By analyzing the signal-to-noise ratio and grey relational grade, the most influential process factor is the interaction item of normal pressure and rotating speed. Ultrasonically assisted vibration has a positive correlation with the MRR, while simultaneously increases the surface roughness, but the higher ultrasonic power is beneficial to inhibit the increase of surface roughness. The lower hardness of lapping plate can obtain the better surface at the cost of decreasing MRR, but much too low hardness may destroy the machined surface.

Molecular dynamics study on femtosecond laser aided machining of monocrystalline silicon carbide

To solve SiC machining processes problems such as low processing efficiency, surface/subsurface damage, and machining tool wear, a femtosecond-laser-aided machining process was studied. In this paper, the diamond-machinability and removal mechanism of SiC-modified layer during femtosecond-laser-aided machining process are evaluated at the nanoscale using molecular dynamics. The results show that micro/nano structures in the modified layer significantly influence the material removal process, and SiC surface structures effectively improve the removal efficiency and reduce subsurface damage depth. The surface micro/nano structures introduced by femtosecond laser scanning improve the diamond-machinability of mono-crystalline SiC.

Molecular dynamics simulation of SiC removal mechanism in a fixed abrasive polishing process

Abstract Precision polishing of mono-crystalline SiC wafers on a fixed abrasive pad is investigated by double-nano-abrasives cutting at micro/nano scale in this report. Prior to this report, a single abrasive approach in molecular dynamics simulation had been employed to illustrate the material removal mechanism in SiC polishing process, which is quite different from the real situation of the fixed abrasive polishing process. Cutting depth and spacing of abrasive particles in a fixed abrasive pads were tested to gain insights on phase transformation, subsurface damage, surface quality, material removal and friction characteristics of polished SiC wafers by molecular dynamics simulation. By following the coordination number and radial distribution function, we clearly see that the number of phase transformation atoms caused by cutting and abrasion increases with the cutting depth of nano-abrasives on the surface of SiC workpiece. Simulation results also suggest that t he phase transformation of the SiC crystal phase increases with the lateral spacing of abrasive particles in pads, while does not change much with the increase of the longitudinal spacing. It is also found that the deeper the abrasive cutting depth, the deeper subsurface damage, resulting more materials’ removal from SiC workpiece. The lateral and longitudinal abrasive spacings lead to little change the depth of subsurface damage on the wafer in MD simulation for a fixed double abrasive polishing. The surface roughness is better with the larger lateral abrasive spacing, but no clear correlation with the longitudinal abrasive spacing.

Material removal mechanisms in chemical-magnetorheological compound finishing

With a view to ultra-precision polishing of SiC wafers, the chemical-magnetorheological compound finishing (CMRF) method was proposed based on the principle of the Fenton reaction. To study material removal characteristics of CMRF, a force model for polishing pads based on magnetorheological (MR) effects was built. Through the theory of solid-phase particles, this study conducted a force analysis of carbonyl iron powders and abrasives and calculated polishing forces of a single polishing pad based on MR effects on a workpiece surface. Based on this, according to the Preston equation, a material removal model was established. By conducting the CMRF test on monocrystalline SiC wafers, it is found that the test results were consistent with theoretical calculations.