Silicon Carbide Polytypes Research Articles

Manufacturing of Complex Silicon-Carbon Structures: Exploring SixCy Materials.

This paper reports on the manufacturing of complex three-dimensional Si/C structures via a chemical vapor deposition method. The structure and properties of the grown materials were characterized using various techniques including scanning electron microscopy, aberration-corrected transmission electron microscopy, confocal Raman spectroscopy, and X-ray photoelectron spectroscopy. The spectroscopy results revealed that the grown materials were composed of micro/nanostructures with various compositions and dimensions. These included two-dimensional silicon carbide (SiC), cubic silicon, and various SiC polytypes. The coexistence of these phases at the nano-level and their interfaces can benefit several Si/C-based applications ranging from ceramics and structural applications to power electronics, aerospace, and high-temperature applications. With an average density of 7 mg/cm3, the grown materials can be considered ultralightweight, as they are three orders of magnitude lighter than bulk Si/C materials. This study aims to impact how ceramic materials are manufactured, which may lead to the design of new carbide materials or Si/C-based lightweight structures with additional functionalities and desired properties.

Direct writing of divacancy centers in silicon carbide by femtosecond laser irradiation and subsequent thermal annealing

Divacancy (VSiVC) centers in silicon carbide (SiC) have potential applications in quantum communication and sensing due to their attractive optical and spin properties. To realize many of these divacancy-based quantum applications, it is vital that they are created in prescribed locations with high accuracy. Here, we describe the production of arrays of divacancy centers in 4H polytype SiC (4H-SiC) by femtosecond laser irradiation and subsequent thermal annealing. We optically characterized these divacancy centers by photoluminescence (PL) confocal mapping using a custom-built confocal microscope. The created divacancy centers show a bright stable emission that depends on the pulse energy of the femtosecond laser. PL spectra of the divacancy centers were collected using micro-Raman spectroscopy at the low temperature of 4.2 K and room temperature. The effect of thermal annealing was studied at various temperatures from 500 °C to 1000 °C and showed that the maximum divacancy center PL intensity was achieved at 800 °C. These and the aforementioned measurements show that the femtosecond laser writing method enables divacancy centers to be accurately positioned in 4H-SiC.

3C-SiC-on-Si MOSFETs: Overcoming Material Technology Limitations

The cubic polytype (3C-) of silicon carbide (SiC) is an emerging semiconductor technology for power devices. The featured isotropic material properties along with the wide band gap characteristics make it an excellent choice for power metal oxide semiconductor field effect transistors (<small>mosfet</small>s). It can be grown on silicon (Si) substrates which is itself advantageous. However, the allowable annealing temperature is limited by the melting temperature of Si. Hence, devices making use of 3C-SiC on Si substrate technology suffer from poor or even almost negligible activation of the p-type dopants after ion implantation due to the relatively low allowable annealing temperature. In this article, a novel process flow for a vertical 3C-SiC-on-Si <small>mosfet</small> is presented to overcome the difficulties that currently exist in obtaining a p-body region through implantation. The proposed design has been accurately simulated with technology computer-aided design process and device software. To ensure reliable prediction, a previously validated set of material models has been used. Further, a channel mobility physics model was developed and validated against experimental data. The output characteristics of the proposed device demonstrated promising performance, what is potentially the solution needed and a huge step toward the realization of 3C-SiC-on-Si <small>mosfet</small>s with commercially grated characteristics.

Dipolar spin relaxation of divacancy qubits in silicon carbide

Divacancy spins implement qubits with outstanding characteristics and capabilities in an industrial semiconductor host. On the other hand, there are still numerous open questions about the physics of these important defects, for instance, spin relaxation has not been thoroughly studied yet. Here, we carry out a theoretical study on environmental spin-induced spin relaxation processes of divacancy qubits in the 4H polytype of silicon carbide (4H-SiC). We reveal all the relevant magnetic field values where the longitudinal spin relaxation time T1 drops resonantly due to the coupling to either nuclear spins or electron spins. We quantitatively analyze the dependence of the T1 time on the concentration of point defect spins and the applied magnetic field and provide an analytical expression. We demonstrate that dipolar spin relaxation plays a significant role both in as-grown and ion-implanted samples and it often limits the coherence time of divacancy qubits in 4H-SiC.

Modeling and Simulations of 4H-SiC/6H-SiC/4H-SiC Single Quantum-Well Light Emitting Diode Using Diffusion Bonding Technique.

In the last decade, silicon carbide (SiC) has emerged as a potential material for high-frequency electronics and optoelectronics applications that may require elevated temperature processing. SiC exists in more than 200 different crystallographic forms, referred to as polytypes. Based on their remarkable physical and electrical characteristics, such as better thermal and electrical conductivities, 3C-SiC, 4H-SiC, and 6H-SiC are considered as the most distinguished polytypes of SiC. In this article, physical device simulation of a light-emitting diode (LED) based on the unique structural configuration of 4H-SiC and 6H-SiC layers has been performed which corresponds to a novel material joining technique, called diffusion welding/bonding. The proposed single quantum well (SQW) edge-emitting SiC-based LED has been simulated using a commercially available semiconductor device simulator, SILVACO TCAD. Moreover, by varying different design parameters, the current-voltage characteristics, luminous power, and power spectral density have been calculated. Our proposed LED device exhibited promising results in terms of luminous power efficiency and external quantum efficiency (EQE). The device numerically achieved a luminous efficiency of 25% and EQE of 16.43%, which is at par performance for a SQW LED. The resultant LED structure can be customized by choosing appropriate materials of varying bandgaps to extract the light emission spectrum in the desired wavelength range. It is anticipated that the physical fabrication of our proposed LED by direct bonding of SiC-SiC wafers will pave the way for the future development of efficient and cost-effective SiC-based LEDs.

Structural phase transition and amorphization in hexagonal SiC subjected to dynamic loading

Understanding the dynamic behavior of silicon carbide (SiC) at extreme conditions is critical for applications such as coatings and armor. Revealing the plasticity and phase transition in SiC under intensive dynamic loading is among the key concerns. However, the effects of polytypes in SiC as well as the various loading conditions on its dynamic material responses are still not well understood. Here we carry out a series of large-scale molecular dynamics (MD) simulations to explore the inelastic responses of hexagonal SiC subjected to shock and ramp compression along the [0001] direction. The shock responses present three regimes sequentially as the shock particle velocity is increased from 0.5 km/s to 6 km/s: a single elastic wave, a multi-wave structure consisting of an elastic-plastic wave followed by two consecutive phase transition waves, and finally, an overdriven structural phase transformation wave. The original wurtzite structure is observed to transform into a 6-coordinated rock salt structure through a 5-coordinated intermediate structure. Furthermore, slip along the <112‾0> direction on the (0001) plane causes multiple stacking faults and amorphization when subjected to intense shock compression. The effects of temperature and strain rate on the material responses of hexagonal SiC are decoupled by performing ramp loading simulations. The results reveal that a lower temperature rise impedes atomic motion while a lower strain rate promotes it, highlighting a competitive effect. Besides, due to the limited availability of slip planes, dynamic compression can lead to the formation of nanograins in hexagonal SiC, and nanograins refinement with increasing shock intensity is observed. These results provide important insights into the deformation of SiC polytypes under extreme conditions and promote the understanding of the dynamic properties of hexagonal structure covalent materials.

Electron nuclear interactions in spin-3/2 color centers in silicon carbide: A high-field pulse EPR and ENDOR study

High-frequency pulsed electron paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR) were used to determine electron nuclear interactions on remote ligand shells of silicon and carbon in spin-3/2 color centers with an optically induced high-temperature spin alignment in hexagonal $4H$-, $6H$-, and rhombic $15R$-silicon carbide (SiC) polytypes. The EPR and ENDOR experimental data relate unambiguously to spin-3/2 centers in which the optically induced alignment of the spin-level populations occurs. The identification is based on resolved ligand hyperfine interactions with carbon and silicon nearest, next-nearest, and the more distant neighbors and on the determination of the unpaired electron spin densities. The hyperfine interactions with $^{29}\mathrm{Si}$ and $^{13}\mathrm{C}$ nuclei are unambiguously separated due to the selective population of the fine-structure levels with certain values of ${\mathrm{M}}_{\mathrm{S}}$. The signs of these interactions and, as a result, the signs of oscillating spin density at $^{29}\mathrm{Si}$ and $^{13}\mathrm{C}$ nuclei, are determined. On the basis of the EPR and optically induced ENDOR measurements, signs of the fine-structure splitting for all the centers were demonstrated, which made it possible to establish the character of optically induced spin alignment, including the inverse populations of the spin levels for these centers. The values of hyperfine interaction with $^{29}\mathrm{Si}$ and $^{13}\mathrm{C}$ nuclei, including those remote from the localization of the spin-3/2 center, are tabulated, which can be used by a number of algorithms in quantum information processing as long-term memory.

Towards identification of silicon vacancy-related electron paramagnetic resonance centers in 4H -SiC

The negatively charged silicon vacancy [V$_\text{Si}(-)$] in silicon carbide (SiC) is a paramagnetic and optically active defect in hexagonal SiC. V$_\text{Si}(-)$ defect possesses $S = 3/2$ spin with long spin coherence time and can be optically manipulated even at room temperature. Recently, electron spin resonance signals have been observed besides the signals associated with the V$_\text{Si}(-)$ defects in the 4H polytype of SiC. The corresponding centers share akin properties to those of the V$_\text{Si}(-)$ defects and thus they may be promising candidates for quantum technology applications. However, the exact origin of the new signals is unknown. In this paper we report V$_\text{Si}(-)$-related pair defect models as possible candidates for the unknown centers. We determine the corresponding electronic structures and magneto-optical properties as obtained by density functional theory (DFT) calculations. We propose models for the recently observed electron paramagnetic resonance centers with predicting their optical signals for identification in future experiments.

Accurate values of 3C, 2H, 4H, and 6H SiC elastic constants using DFT calculations and heuristic errors corrections

ABSTRACT Silicon carbide is an important material with applications in numerous domains, but for which our knowledge of basic properties like elastic constants is surprisingly limited. Although several density functional theory calculations have been reported, those are usually not accurate enough, with a sizeable dispersion of published values. A heuristic method is proposed here, allowing for largely reducing the uncertainty. It is based on the use of a weighted average of errors for two reference materials, Si and C. It is demonstrated that this method is effective for physical properties like the lattice parameter and the bulk modulus. It is then used for determining the full sets of elastic constants for the 3C, 2H, 4H, and 6H silicon carbide polytypes.

Nonlinear optical properties of 6H-SiC and 4H-SiC in an extensive spectral range

Silicon carbide (SiC), which is the leading representative of the third-generation of semiconductors, possesses many excellent physical properties. However, its advantages also incur difficulties in processing, which calls for special processing techniques, such as femtosecond laser machining. In addition, SiC has shown unprecedented potential for optoelectronic applications. Knowledge of the nonlinear absorption coefficient and the nonlinear refractive index coefficient of SiC is required in both of the fields. In this work, the nonlinear absorption coefficient and the nonlinear refractive index coefficient of semi-insulating (SI) 6H-SiC and SI 4H-SiC, the most pervasive SiC polytypes, are measured in an extensive spectral range from 400 nm to 1000 nm with the Z-scan technique. Besides, the spectral dependence of the nonlinear optical properties is analyzed, facilitated by linear absorption spectrum. Especially, two-photon absorption (2PA) and three-photon absorption (3PA) coefficients of SI 6H-SiC and SI 4H-SiC are characterized in the respective spectral ranges. From the characterization of SiC, we can observe self-focusing phenomenon for nonlinear refraction. In the end, we unravel the potential of SiC for ultrafast all-optical switching based on the measured nonlinear optical properties.

Theoretical study of the band structure of 2H-SiC and 4H-SiC of silicon carbide polytypes

We have studied the electronic band properties of 2H-SiC and 4H-SiC silicon carbide polytypes. The structures of the electronic bands and density of state (DOS) using ab initio Density Functional Theory (DFT) were calculated for the first Brillouin zone both in the generalized gradient approximation and taking into account quasiparticle effects according to the GW scheme. The calculated bandgaps obtained using the GW approximation Eg2H-SiC = 3.17 eV and Eg4H-SiC = 3.26 eV agree well with experimental values. The shape and values of total DOS are within agreement with calculations performed by other authors. The calculated total energy values for 2H-SiC and 4H-SiC were close, but they satisfy the condition E2H &gt; E4H, which implies that the 4H-SiC structure is more stable than 2H-SiC. Our calculations of the band structure and DOS of 2H-SiC and 4H-SiC by the DFT method showed that the application of the GW approximation is an optimum approach to the study of the electronic structure of 2H-SiC and 4H-SiC polytypes.

Syntaxy and defect distribution during the bulk growth of 4H-SiC Single crystal

In the present study, the inclusion of 6H foreign polytype was seen during the initial growth of 4H polytype silicon carbide single crystal by PVT technique. The foreign polytype inclusion was gradually diminished with growth time; the mechanism is described in the present paper. In order to study the defects, the SiC specimen was wet etched using KOH solution at higher temperature and it was found that the foreign polytype inclusions i.e. 6H polytype introduced high density of defects at the polytype boundary. The defect distribution in different regions viz. 6H polytype foreign inclusion, 4H polytype region and polytype boundary were found to be different. It is interesting to note that the threading screw dislocation was not seen in 6H-polytype inclusion region.

(Invited) Epitaxy of Silicon Carbide for Power Devices

We observe an evolution how electric power is generated, converted, distributed and used. The world making an attempt to move away from fossil fuels towards a more renewable, energy efficient and electrified energy ecosystems, and therefor efficiency in power conversion processes becomes critically important. As the most prominent example, various wide band gap semiconductor materials are entering the power electronics in electric and hybrid vehicles. Among them is a Silicon Carbide (SiC). A major benefit of its characteristics, comparing to Silicon (Si), in terms of switching speed, heat loss and size of power electronic devices, is at least 50 % reduction in energy loss in the form of heat. This saving translates into more efficient power electronics and more energy for an electric motor and therefore for longer mileage with the same battery. Therefore, SiC offers an opportunity to move power electronics beyond the boundaries set by the material limits of Si. From production point of view, SiC is especially attractive because of its compatibility with Si manufacturing technologies.Modern semiconductor power electronic devices are fabricated out of epitaxial thin films of semiconductors, creating a desired device’s layers stack. Therefore, the epitaxy is absolutely essential for creation of such devices.SiC is a fascinating material with excellent physical properties. It is a wide band gap semiconductor with high thermal conductivity. It is resistant to chemical etchants and radiation. It has high mechanical strength and biocompatible. SiC is the only Group IV compound semiconductor. And finally one of its polytypes, cubic silicon carbide (3C-SiC), can be grown on Si. There are over 200 polytpes of SiC. All of them are formed from 50% Si and C and consist of hexagonal SiC bilayers. Different stacking sequences of C-Si bilayers produces different crystal structures. Different polytypes have different properties and challenges. Due to this epitaxy is very challenging. Among all known polytypes only 3C-, 4H- and 6H- have been very well researched. Hexagonal polytypes of SiC such as 4H- and 6H-SiC already have commercial applications in power electronics and also used as substrates for gallium nitride (GaN). SiC substrates can be formed from these two polytypes, which can then be used for homoepitaxial growth at high temperatures for device applications. These substrates are extremely expensive, limited to 150 mm diameters and epitaxial growth requires very high temperatures (~1700-1800 °C), which are only achievable in specially designed, hot-walled chemical vapor deposition (CVD) reactors. 3C-SiC is the only polytype of SiC which possesses a zinc-blende crystal structure and can be grown on a Si substrate, below the Si melting point (1410 °C). Epitaxy state of the art and challenges for these polytypes and their application will be reviewed.

Comparative Ellipsometric Analysis of Silicon Carbide Polytypes 4H, 15R, and 6H Produced by a Modified Lely Method in the Same Growth Process

A model is suggested for a quantitative analysis of the dependence of the dielectric function of hexagonal silicon carbide polytypes on photon energy in the range of 0.7–6.5 eV. The model, which is the sum of two Tauc–Lorentz oscillators (main and minor) with a common energy gap, is used to describe three hexagonal silicon carbide polytypes (4H, 15R, 6H) obtained in the same growth process. Both C- and Si-faces of each polytype are analyzed. A number of conclusions are drawn about how the oscillator parameters depend on the degree of hexagonality of a polytype and on the type of a surface face. The strongest dependence is that the amplitude of the minor oscillator grows with increasing degree of hexagonality of a polytype. The increase in the energy gap on passing from the C-face (000 $$\bar {1}$$ ) to the Si-face (0001) is also worthy of note.

Photoluminescence and Raman Spectroscopy Study on Color Centers of Helium Ion-Implanted 4H–SiC

Color centers in silicon carbide (SiC) are promising candidates for quantum technologies. However, the richness of the polytype and defect configuration of SiC makes the accurate control of the types and position of defects in SiC still challenging. In this study, helium ion-implanted 4H–SiC was characterized by atomic force microscopy (AFM), confocal photoluminescence (PL), and confocal Raman spectroscopy at room temperature. PL signals of silicon vacancy were found and analyzed using 638-nm and 785-nm laser excitation by means of depth profiling and SWIFT mapping. Lattice defects (C–C bond) were detected by continuous laser excitation at 532 nm and 638 nm, respectively. PL/Raman depth profiling was helpful in revealing the three-dimensional distribution of produced defects. Differences in the depth profiling results and SRIM simulation results were explained by considering the depth resolution of the confocal measurement setup, helium bubbles, as well as swelling.

Magnetic Resonance Study of p‐Type 3C SiC Microparticles

The silicon carbide (SiC) 3C polytype microparticles with p‐type conductivity are investigated by X‐band electron paramagnetic resonance (EPR) technique in a wide temperature interval. Morphological and Raman studies show that the mean particle diameter is ≈8 μm, and the SiC polytype is purely 3C. At T = 10 K, the EPR spectrum of the shallow boron acceptor substituting cubic site (BK), with C3V symmetry and anisotropic g‐ and A‐tensors resulting from static Jahn–Teller effect, is observed. At T &gt; 30 K, a continuous transition from static to dynamic Jahn–Teller effect for BK is detected. The temperature‐dependent BK principal g‐values are explained by the transitions between the ground and excited vibronic states of BK. At T = 130–150 K, the BK center is characterized by Td symmetry and isotropic g‐ and A‐tensors. At T = 60–190 K, an additional single EPR Lorentzian line with short spin relaxation times and isotropic g‐value close to BK average g‐value is observed. It was explained by averaging the anisotropic spectrum caused by rapid transitions between the boron ground and excited vibronic states split by Jahn–Teller potential. In addition, the EPR signal from carbon vacancy with characteristic superhyperfine structure is observed at T = 10–298 K.

Wurtzite SiC Formation in Plastic Deformed 3C and 6H

Single side clamped 3C and 6H single crystal silicon carbide beams were elastic deformed using a special designed deformation stage in an electron microscope and subjected to high temperatures. The structural transitions occurring during the plastic relaxation process were recorded in situ in the electron microscope using reflection high energy electron diffraction in {110} azimuthal direction. For both polytypes, a polytype phase transition into the wurtzite silicon carbide polytype was observed independent on the surface polarity. The critical initial elastic deformation of the polytype phase transition into the wurtzite phase for the cubic silicon carbide polytype is larger compared to the 6H-SiC. This is due to the higher partial dislocation densities needed to transform the cubic modification into the wurtzite phase.

Nano- and Micro-Scale Simulations of Ge/3C-SiC and Ge/4H-SiC NN-Heterojunction Diodes

During the last decade, silicon carbide (SiC) and its heterostructures with other semiconductors have gained a significant importance for wide range of electronics applications. These structures are highly suitable for high frequency and high power applications in extremely high temperature environments. SiC exists in more than 200 different polycrystalline forms, called polytypes. Among these 200 types, the most prominent polytypes with exceptional physical and electrical attributes are 3C-SiC, 4H-SiC and 6H-SiC. Heterostructures of these SiC polytypes with other conventional semiconductors (like Si, Ge) can give rise to interesting electronic characteristics. In this article, Germanium (Ge) has been used to make heterostructures with 3C-SiC and 4H-SiC using a novel technique called diffusion welding. Microscale and nanoscale simulations of nn-heterojunction of Ge/3C-SiC and Ge/4H-SiC have been done. Microscale devices have been simulated with a commercially available semiconductor device simulator tool called Silvaco TCAD. Whereas nanoscale devices have been simulated with QuantumWise Atomistix Toolkit (ATK) software package. Current-voltage (IV) curves of all simulated devices have been calculated and compared. In nanoscale device, the effects of defects on IV-characteristics due to non-ideal bonding (lattice misplacement) at heterojunction interface have been analyzed. Our simulation results reveal that the proposed heterostructure devices with diffusion welding of wafers are theoretically possible. These simulations are the preparations of our near future physical experiments targeted to fabricate SiC based heterostructure devices using diffusion bonding technique.

Dependence of Photoluminescence Emission on Excitation Power and Temperature in Highly Doped 6H- SiC

The excitation power and temperature dependences of photoluminescence (PL) emission in 6H polytype of silicon carbide are investigated extensively. A sublinear power-function dependence of the donor-acceptor pair emission intensity on the excitation power is explained by a rate-equation model. A very low power exponent $(k=0.3)$ in a power-law dependence of PL intensity on the excitation power of $\mathrm{B}$-$\mathrm{N}$ codoped 6H-$\mathrm{Si}\mathrm{C}$ at room temperature indicates that three-particle Auger recombination is the main carrier recombination pathway. Auger recombination in an $\mathrm{Al}$-$\mathrm{N}$ codoped $\mathrm{Si}\mathrm{C}$ does not manifest as strongly as that of the $\mathrm{B}$-$\mathrm{N}$ codoped sample due to high density of ionized impurities which capture the nonequilibrium carriers faster. The ionized impurities form localized potential wells due to their random distribution, which is revealed by a blue shift of the PL band with increasing excitation intensity (approximately 16 meV per decade) and with decreasing temperature. Low-temperature Hall-effect measurement of the $\mathrm{Al}$-$\mathrm{N}$ codoped sample confirms the high density of ionized impurities and significantly reduced ionization energy of dopant states by approximately 53 meV due to the high dopant densities. These results could lead to realization of tunable $\mathrm{Si}\mathrm{C}$-based light sources in severe operating conditions.

Room-temperature coherent control of implanted defect spins in silicon carbide

Recently, vacancy-related spin defects in silicon carbide (SiC) have been demonstrated to be potentially suitable for versatile quantum interface building and scalable quantum network construction. Significant efforts have been undertaken to identify spin systems in SiC and to extend their quantum capabilities using large-scale growth and advanced nanofabrication methods. Here we demonstrated a type of spin defect in the 4H polytype of SiC generated via hydrogen ion implantation with high-temperature post-annealing, which is different from any known defects. These spin defects can be optically addressed and coherently controlled even at room temperature, and their fluorescence spectrum and optically detected magnetic resonance spectra are different from those of any previously discovered defects. Moreover, the generation of these defects can be well controlled by optimizing the annealing temperature after implantation. These defects demonstrate high thermal stability with coherently controlled electron spins, facilitating their application in quantum sensing and masers under harsh conditions.