SiC Polytypes Research Articles

Effect of van der Waals interactions on the stability of SiC polytypes

Density functional theory calculations with a correction of the long-range dispersion force, namely, the van der Waals (vdW) force, are performed for SiC polytypes. The lattice parameters are in good agreement with those obtained from the experiments. Furthermore, the stability of the polytypes in the experiments, which show 3C-SiC as the most stable, is reproduced by the present calculations. The effects of the vdW force on the electronic structure and the stability of polytypes are discussed. We observe that the vdW interaction is more sensitive to the cubic site than the hexagonal site. Thus, the influence of the vdW force increases with decreasing the hexagonality of the polytype, which results in the confirmation that the most stable polytype is 3C-SiC.

Prominent local transport in silicon carbide composites containing in-situ synthesized three-dimensional graphene networks

In-situ grown graphene/SiC composites developed by spark plasma sintering have emerged as a very interesting family of materials with expected high performance for advanced applications. In this work, the local functional properties of graphene/SiC ceramics are elucidated for distinct α- and β- SiC polytypes combining scanning probe microscopies. We unambiguously identify all composite constituents and demonstrate the formation of a three-dimensional graphene conductive network inside the composite. The investigated composites exhibit grains with different doping level depending on growth rate during sintering so that conduction paths associated to graphene and matrix networks may compete. The relevance of nanoscale characterization on functional graphene/semiconductor materials is proved as it evidences the type of doping and carrier concentration of the semiconductor and the critical role played by the graphene constituent in the formation of ohmic contacts. Both issues are of crucial importance for understanding the macroscale behavior of these materials and determine their applications.

Thermal, electrical, and mechanical properties of pressureless sintered silicon carbide ceramics with yttria-scandia-aluminum nitride

The effects of the polytype of SiC starting powders on the thermal, electrical, and mechanical properties of pressureless sintered SiC ceramics with a new additive system (6.5vol% Y2O3-Sc2O3-AlN) were investigated. Powder mixtures prepared from α- or β-SiC powders were sintered at 1950°C for 6h in a nitrogen atmosphere without an applied pressure. We found that both specimens could be sintered to >96% of the theoretical density without an applied pressure. The SiC ceramic fabricated from β-SiC powders showed lower electrical resistivity, higher thermal conductivity, and better mechanical properties than that from α-SiC powders. The flexural strength, fracture toughness, hardness, electrical resistivity and thermal conductivity values of the SiC ceramics fabricated from β-SiC powders were 520MPa, 5.1MPam1/2, 25.0GPa, 6.7×10−1Ωcm and 110 Wm−1K−1 at room temperature, respectively. The new additive system achieved the highest thermal conductivity in pressureless liquid-phase sintered SiC ceramics.

Total pressure‐controlled PVT SiC growth for polytype stability during using 2D nucleation theory

A total pressure‐controlled physical vapor transport growth method that stabilizes SiC polytype is proposed. The supersaturation of carbon during SiC growth changed as a function of the growth time due to changes in the temperature difference between the surfaces of the source and the grown crystal. Supersaturation also varied as a function of the pressure inside the furnace. Therefore, modification of the pressure as a function of growth time allowed for constant supersaturation during growth. The supersaturation was calculated based on classical thermodynamic nucleation theory using data for heat and species of Si2C and SiC2 transfer in a furnace obtained from a global model. Based on this analysis, a method for polytype‐stabilized SiC growth was proposed that involves decreasing the pressure as a function of growth time. The 4H‐SiC prepared using this pressure‐controlled method was more stable than that of 4H‐SiC formed using the conventional constant‐pressure method.

Characterization of a n+3C/n−4H SiC heterojunction diode

We report on the fabrication of n + 3C/n-4H SiC heterojunction diodes (HJDs) potentially promising the ultimate thermal stability of the junction. The diodes were systematically analyzed by TEM, X-ray diffraction, AFM, and secondary ion mass spectroscopy, indicating the formation of epitaxial 3C-SiC crystal on top of 4H-SiC substrate with continuous interface, low surface roughness, and up to ∼7 × 1017 cm−3 dopant impurity concentration. The conduction band off-set is about 1 V as extracted from CV measurements, while the valence bands of both SiC polytypes are aligned. The HJDs feature opening voltage of 1.65 V, consistent with the barrier height of about 1.5 eV extracted from CV measurement. We finally compare the electrical results of the n + 3C/n-4H SiC heterojunction diodes with those featuring Si and Ge doped anodes in order to evaluate current challenges involved in the fabrication of such devices.

Dimensional isotropy of 6H and 3C SiC under neutron irradiation

This investigation experimentally determines the as-irradiated crystal axes dimensional change of the common polytypes of SiC considered for nuclear application. Single crystal α-SiC (6H), β-SiC (3C), CVD β-SiC, and single crystal Si have been neutron irradiated near 60 °C from 2 × 1023 to 2 × 1026 n/m2 (E > 0.1 MeV), or about 0.02–20 dpa, in order to study the effect of irradiation on bulk swelling and strain along independent crystalline axes. Single crystal, powder diffractometry and density measurement have been carried out. For all neutron doses where the samples remained crystalline all SiC materials demonstrated equivalent swelling behavior. Moreover the 6H–SiC expanded isotropically. The magnitude of the swelling followed a ∼0.77 power law against dose consistent with a microstructure evolution driven by single interstitial (carbon) mobility. Extraordinarily large ∼7.8% volume expansion in SiC was observed prior to amorphization. Above ∼0.9 × 1025 n/m2 (E > 0.1 MeV) all SiC materials became amorphous with an identical swelling: a 11.7% volume expansion, lowering the density to 2.84 g/cm3. The as-amorphized density was the same at the 2 × 1025 and 2 × 1026 n/m2 (E > 0.1 MeV) dose levels.

Effect of Al addition to Si–Ni flux on pulsed laser deposition of SiC thin films

The effects of Al addition to Si–Ni flux on the flux-mediated pulsed laser deposition of SiC thin films were investigated. A small amount of 3 at.% Al addition was found to cause the dominant growth of a polytype of 6H–SiC (0001) at an early stage of the deposition on a 4H–SiC (0001) on-axis substrate even at a low growth temperature of 1160°C, at which a single polytype of 3C–SiC (111) film is thermodynamically favored. The direct observation of the interface between SiC and Si–Ni based flux with our originally developed high-vacuum laser microscope revealed that the dissolution behavior of SiC into Si–Ni–Al flux becomes milder than that before Al addition to Si–Ni flux. This feature is not inconsistent with the commonly accepted surface flattening effect of Al addition to the Si–Cr solvent in the flux growth of SiC bulk single crystals, and might be partially responsible for the initial preference of the 6H–SiC (0001) polytype growth.

Optically Addressable Silicon Vacancy-Related Spin Centers in Rhombic Silicon Carbide with High Breakdown Characteristics and ENDOR Evidence of Their Structure.

We discovered a family of uniaxially oriented silicon vacancy-related centers with S=3/2 in a rhombic 15R-SiC crystalline matrix. We demonstrate that these centers exhibit unique characteristics such as optical spin alignment up to the temperatures of 250°C. Thus, the range of robust optically addressable vacancy-related spin centers is extended to the wide class of rhombic SiC polytypes. To use these centers for quantum applications it is essential to know their structure. Using high frequency electron nuclear double resonance, we show that the centers are formed by negatively charged silicon vacancies V_{Si}^{-} in the paramagnetic state with S=3/2 that is noncovalently bonded to the neutral carbon vacancy V_{C}^{0} in the nonparamagnetic state, located on the adjacent site along the SiC symmetry c axis.

Fabrication of quasi-superlattices at the interface between 3C-SiC epitaxial layer and substrates of hexagonal SiC polytypes by sublimation epitaxy in vacuum

Transmission electron microscopy has been used to study the structure of a transition layer between a hexagonal substrate (6H-SiC and 4H-SiC) and a cubic silicon carbide layer grown by sublimation epitaxy in vacuum. It is shown by microdiffraction analysis that the transition layer with a thickness of 210 nm is constituted by alternating layers of cubic (3C) and hexagonal (6H) silicon carbide. It is demonstrated that 6H-SiC/3C-SiC and 4H-SiC/3C-SiC quasi-superlattices can be produced by this method.

Tunable magnetic interaction in hydrogenated epitaxial graphene modulated by the SiC substrate

We show that the ${d}^{0}$ ferromagnetism with high Curie temperature $({T}_{c})$ can be achieved in the electron-doped hydrogenated epitaxial graphene on certain SiC substrates through first-principles calculations. The pristine systems are found to be Mott insulators regardless of SiC polytype $(2H,4H$, or $6H)$ which, however, plays a significant role in the modulation of magnetic interaction. Carrier doping enhances the ferromagnetic coupling due to the double-exchange mechanism and thus realizes the phase transition from antiferromagnetism to ferromagnetism. A ${T}_{c}$ of around 400 K is predicted for graphene on the $2H$-SiC substrate. We employ a nondegenerate Hubbard model to demonstrate how the SiC affects the interfacial magnetism in intra-atomic Coulomb repulsion and intersite hopping interactions.

Nanotubes, nanobelts, nanowires, and nanorods of silicon carbide from the wheat husks

Nanotubes, nanowires, nanobelts, and nanorods of SiC were synthesized from the thermal treatment of wheat husks at temperatures in excess of 1450 °C. From the analysis based on x-ray diffraction, Raman spectroscopy, scanning electron microscopy, and transmission electron microscopy, it has been found that the processed samples of wheat husk consisted of 2H and 3C polytypes of SiC exhibiting the nanostructure shapes. These nanostructures of silicon carbide formed from wheat husks are of technological importance for designing advance composites, applications in biotechnology, and electro-optics. The thermodynamics of the formation of SiC is discussed in terms of the rapid solid state reaction between hydrocarbons and silica on the molecular scale, which is inherently present in the wheat husks.

Chemical vapor deposition of Si/SiC nano-multilayer thin films

Stoichiometric SiC films were deposited with the commercially available single source precursor Et3SiH by classical thermal chemical vapor deposition (CVD) as well as plasma-enhanced CVD at low temperatures in the absence of any other reactive gases. Temperature-variable deposition studies revealed that polycrystalline films containing different SiC polytypes with a Si to carbon ratio of close to 1:1 are formed at 1000°C in thermal CVD process and below 100°C in the plasma-enhanced CVD process. The plasma enhanced CVD process enables the reduction of residual stress in the deposited films and offers the deposition on temperature sensitive substrates in the future. In both deposition processes the film thickness can be controlled by variation of the process parameters such as the substrate temperature and the deposition time. The resulting material films were characterized with respect to their chemical composition and their crystallinity using scanning electron microscope, energy dispersive X-ray spectroscopy (XRD), atomic force microscopy, X-ray diffraction, grazing incidence X-ray diffraction, secondary ion mass spectrometry and Raman spectroscopy. Finally, Si/SiC multilayers of up to 10 individual layers of equal thickness (about 450nm) were deposited at 1000°C using Et3SiH and SiH4. The resulting multilayers features amorphous SiC films alternating with Si films, which feature larger crystals up to 300nm size as measured by transmission electron microscopy as well as by XRD. XRD features three distinct peaks for Si(111), Si(220) and Si(311).

Optical properties and Zeeman spectroscopy of niobium in silicon carbide

The optical signature of niobium in the low-temperature photoluminescence spectra of three common polytypes of SiC (4H, 6H, and 15R) is observed and confirms the previously suggested concept that Nb occupies preferably the Si-C divacancy with both Si and C at hexagonal sites. Using this concept we propose a model considering a Nb-bound exciton, the recombination of which is responsible for the observed luminescence. The exciton energy is estimated using first-principles calculation and the result is in very good agreement with the experimentally observed photon energy in 4H SiC at low temperature. The appearance of six Nb-related lines in the spectra of the hexagonal 4H and 6H polytypes at higher temperatures is tentatively explained on the grounds of the proposed model and the concept that the Nb center can exist in both ${\mathrm{C}}_{1h}$ and ${\mathrm{C}}_{3v}$ symmetries. The Zeeman splitting of the photoluminescence lines is also recorded in two different experimental geometries and the results are compared with theory based on phenomenological Hamiltonians. Our results show that Nb occupying the divacancy at the hexagonal site in the studied SiC polytypes behaves like a deep acceptor.

Electron and Hole Confinement in Hetero-Crystalline SiC Superlattice

We report first-principles electronic structure calculations that clarify the electronic structures of mismatch-free SiC hetero-crystalline superlattices consisting of 3m bilayers of the 3C polytype and 2n bilayers of the 2H polytype, (3C)m(2H)n. We find that electrons in the conduction-band bottom are confined in the 3C region and are distributed near one of the two inequivalent interfaces. This is due to the floating nature of the conduction-band bottom of SiC and electric polarization caused by the difference in electron affinity between Si and C. This polarization affects the conduction-band states and causes the band-gap narrowing of up to 1.2 eV, depending on the thickness of each layer. Our results for the wave-vector-resolved local density of states and the Kohn–Sham orbitals have unequivocally clarified that the valence-band offset is absent. This reflects the fact that the local tetrahedral structures are common in all the SiC polytypes. The amount of the polarization changes with the thickness ...

Notes on the plasma resonance peak employed to determine doping in SiC

The doping level of a semiconductor material can be determined using the plasma resonance frequency to obtain the carrier concentration associated with doping. This paper provides an overview of the procedure for the three most common polytypes of SiC. Results for 3C-SiC are presented and discussed. In phosphorus doped samples analysed, it is submitted that the 2nd plasma resonance cannot be detected due to high values of the free carrier damping constant γ.

Density functional theory calculation of ideal strength of SiC and GaN: Effect of multi-axial stress

First-principles density functional theory calculations are performed to quantitatively evaluate the ideal strength of SiC and GaN crystals. The effect of transverse normal stress on the tensile strength and that of hydrostatic stress on the shear strength are clarified for various crystal structures: 3C, 2H and 4H for SiC, zincblende (Z) and wurtzite (W) for GaN. The effect of polytype SiC structures on the shear strength of some shear systems appears to be significant, while that on the tensile strength is marginal. While the ideal tensile strength generally decreases by transverse (perpendicular) tension, large transverse compression reduces the strength in some structures and orientations ([110] tension in 3C–SiC and Z–GaN; [21¯1¯0] tension in 2H–SiC, 4H–SiC and W–GaN) due to structural phase transition. Although SiC and GaN are basically brittle materials, our investigation suggests that not only cleavage by tension but also sliding due to shear stress take place depending on the multi-axial stress condition.

Defects as qubits in3C−and4H−SiC

We employ hybrid density functional calculations to search for defects in different polytypes of SiC that may serve as qubits for quantum computing. We explore the divacancy in $4H\text{\ensuremath{-}}$ and $3C\text{\ensuremath{-}}\mathrm{SiC}$, consisting of a carbon vacancy adjacent to a silicon vacancy, and the nitrogen-vacancy (NV) center in $3C\text{\ensuremath{-}}\mathrm{SiC}$, in which the substitutional ${\mathrm{N}}_{\mathrm{C}}$ sits next to a Si vacancy (${\mathrm{N}}_{\mathrm{C}}\ensuremath{-}{V}_{\mathrm{Si}}$). The calculated excitation and emission energies of the divacancy in $4H\text{\ensuremath{-}}\mathrm{SiC}$ are in excellent agreement with experimental data, and aid in identifying the four unique configurations of the divacancy with the four distinct zero-phonon lines observed experimentally. For $3C\text{\ensuremath{-}}\mathrm{SiC}$, we identify the paramagnetic defect that was recently shown to maintain a coherent quantum state up to room temperature as the spin-1 neutral divacancy. Finally, we show that the $({\mathrm{N}}_{\mathrm{C}}\ensuremath{-}{V}_{\mathrm{Si}}{)}^{\ensuremath{-}}$ center in $3C\text{\ensuremath{-}}\mathrm{SiC}$ is highly promising for quantum information science, and we provide guidance for identifying this defect.

(Invited) Quality Improvement and Mapping Analysis of Single Crystal 4H SiC Grown with Purified Beta-SiC Powder Source

The most important consideration when growing single crystal silicon carbide by the physical vapor transport method is to minimize defects. To minimize defects caused by temperature gradient, we used β phase SiC powder, which has a low sublimation temperature, and purified the β phase SiC powder to improve the purity of single crystal SiC. Furthermore, we performed thermodynamic computational simulations based on compositions of purified and non-purified β-SiC powders to study the impact of metallic impurities within SiC powder on the composition of single crystal SiC. We grew SiC at temperatures about 200 °C lower than the previous growth temperature using purified β-SiC powder and mapped the phase change behavior of SiC according to different growth temperatures. Moreover, we compared and analyzed the characteristics of SiC polytype formation and crystallinity according to growth temperature. We compared the distribution of defects and dislocations of single crystal 4H SiC grown from purified and non-purified β-SiC powder to study the impact of source purification on defect generation. We also investigated the effect of metallic impurities on the formation of defects and dislocations through content analysis of metallic impurities.

Crystallographic orientation dependence of SEM contrast revealed by SiC polytypes

In low energy scanning electron microscope (SEM) with primary electron energy less than 1.0 keV, the dependence of SEM contrast on crystallographic orientation within a range of 1.0 nm in depth has been investigated by utilizing 4H-SiC (0001) as a standard sample having a definitive electron penetration depth marker layer at hexagonal sites. Reflecting the difference of the direction of topmost two Si-C bilayers stacking sequence (0.50 nm in depth), clear bright and dark SEM contrast has been observed by adjusting the sample tilting and rotation angles by a conventional Everhart–Thornley type in-chamber detector. It is revealed that the brighter signal emission arises when the incident primary electron beam direction is almost parallel to the topmost stacking sequence direction. This angular coincidence was verified separately by correlating low energy SEM contrast from 3C-SiC (111) of no hexagonal sites with its electron back scattered diffraction pattern for identifying stacking sequence direction. The obtained results suggest a potential of low energy electron to characterize the crystallographic orientation just beneath the surface without using any special detector.

Chemical Vapor Deposition of Boron Nitride Thin Films on SiC

We give here an overview of our recent work on growth of rhombohedral boron nitride (r-BN) thin films on SiC substrates by chemical vapor deposition (CVD). We demonstrate the growth of twinned r-BN on various SiC polytypes at 1500 °C, using H2 as carrier gas and triethyl boron and ammonia as precursors with an N/B ratio of ~ 640. The epitaxial relation with various substrates is determined from XRD and TEM. Adding Si to the gas phase stabilizes the r-BN phase but does not alter the electric properties of the material which remains electrically insulating.