High-purity SiC Research Articles

Densification behavior of high purity SiC by hot pressing

Densification behavior of 99.998% purity SiC by hot pressing without metallic additives was investigated. A theoretical densification of 92% was achieved at 2350°C with 50MPa in 3h. The densification was more affected by the sintering pressure rather than by the sintering temperature and was independent of initial particle size. Data were interpreted according to the power-law creep model. The β-SiC phase was preserved in the high purity, hot-pressed SiC specimen without β→α phase transformation even at 2350°C.

Effect of Beveled SiC Substrate on Light Extraction of Flip-Chip Light-Emitting Diodes

We fabricated GaN-based flip-chip light-emitting diodes (FC-LEDs), which were grown on both conventional and high purity SiC substrates. The influence of beveling, thickness, and absorption of the SiC substrate on the light extraction efficiency (LEE) of the FC-LED was investigated by simulation and experiment. From the simulation results, the LEE of the FC-SLED with a 60° beveling angle is higher than that on a rectangular substrate. The experimental results demonstrate that about 15% enhancement of the LEE was achieved by increasing the thickness of the high purity SiC substrate from 120 to 470 μm. A sample with an X pattern on the top face exhibits the highest light output power. The LEE of the FC-SLED can be efficiently enhanced by suitably beveling the substrate with no degradation of the electrical properties.

Irradiation creep of nano-powder sintered silicon carbide at low neutron fluences

The irradiation creep behavior of nano-powder sintered silicon carbide was investigated using the bend stress relaxation method under neutron irradiation up to 1.9dpa. The creep deformation was observed at all temperatures ranging from 380 to 1180°C mainly from the irradiation creep but with the increasing contributions from the thermal creep at higher temperatures. The apparent stress exponent of the irradiation creep slightly exceeded unity, and instantaneous creep coefficient at 380–790°C was estimated to be ∼1×10−5[MPa−1dpa−1] at ∼0.1dpa and 1×10−7 to 1×10−6[MPa−1dpa−1] at ∼1dpa. The irradiation creep strain appeared greater than that for the high purity SiC. Microstructural observation and data analysis indicated that the grain-boundary sliding associated with the secondary phases contributes to the irradiation creep at 380–790°C to 0.01–0.11dpa.

Impurity Behavior of High Purity SiC Powder during SiC Crystal Growth

Two kinds of SiC powder having a different impurity contents and particle size were prepared by carbothermal reduction under different conditions from traditional process for controlling the purity of product. SiC single crystal was grown in the RF heating PVT machine at the temperature above 2,100 °C. After crystal growth, boule was cut to wafers in 1mm thickness and fine polished using diamond abrasive slurry. The impurity in the powder and wafer was analyzed using glow discharged mass spectroscopy (GDMS). Major impurities in the SiC wafer were aluminum, boron, iron and titanium which were accorded in the SiC powder and these impurities were decreasing in proportional to those in the powder. However, behavior of each elemental impurity was different from each other during the crystal growth. In case of boron was increased after crystal growth while aluminum decreased. In case of titanium and boron were higher in the wafer than in the powder. It can be explained to other impurity source such as graphite crucible and insulation felt.

FABRICATION OF CERAMIC POWDERS SiC AND Si3N4 USING CO2-LASER

The process of obtaining powders of SiC and Si3N4 with CO2-laser has been studied. The technology differs in that the used reagent gases - silane and ammonia (for silicon nitride) or ethylene (for silicon carbide) are passed through the CO2-laser beam. Fine agglomerated powders of high purity SiC and Si3N4 containing spherical particles of the same size (monodispersed) have been obtained.

Plasma Resistance and Etch Mechanism of High Purity SiC under Fluorocarbon Plasma

Etch rates of Si and high purity SiC have been compared for various fluorocarbon plasmas. The relative plasma resistance of SiC, which is defined as the etch rate ratio of Si to SiC, varied between 1.4 and 4.1, showing generally higher plasma resistance of SiC. High resolution X-ray photoelectron analysis revealed that etched SiC has a surface carbon content higher than that of etched Si, resulting in a thicker fluorocarbon polymer layer on the SiC surface. The plasma resistance of SiC was correlated with this thick fluorocarbon polymer layer, which reduced the reaction probability of fluorine-containing species in the plasma with silicon from the SiC substrate. The remnant carbon after the removal of Si as volatile etch products augments the surface carbon, and seems to be the origin of the higher plasma resistance of SiC.

Improving high temperature properties of hot pressed ZrB2–20 vol% SiC ceramic using high purity powders

Fully dense ZrB2–20vol% SiC (ZS) ceramics were fabricated by self-made high purity ZrB2 and SiC powders hot-pressing at 1900°C/30MPa. Their properties were compared with other ZS ceramics with the same nominal composition, but prepared by pressureless sintering, using low purity commercially available powders. The hot-pressed composites show superior hardness (17.82GPa), elastic modulus (504GPa), flexure strength (462MPa at 25°C and 463MPa at 1300°C in air), electrical conductivity (7.68×10−6Ω−1m−1 at 25°C), thermal conductivity (93.7W/(mK) at 25°C, 77.2W/(mK) at 1200°C) and a lower average thermal expansion coefficient (6.66×10−6K−1 from 25 to 2000°C) compared to the pressureless sintered materials. The improvement in these properties could be primarily explained by the decrease of impurities and second phase levels in hot pressed samples.

Point Defects in Chemical-Vapor Deposited Diamond, High-Purity Semi-Insulating SiC, and Epitaxial GaN

The native and impurity induced point defects in undoped chemi-cal-vapor deposited diamond, high-purity semi-insulating SiC and grown by molecular beam epitaxy GaN were investigated by isothermal charge-based deep level transient spectroscopy (Q-DLTS). It was found that deep levels in undoped diamond have a continuous energy spectrum with two maximums at energies 0.71 and 0.54 eV. Several discrete deep levels were found in SiC. Defect level E1 is attributed to atom displacement on the carbon sublattice in SiC. Levels E2, E4 are related to (Vc - Vsi) complex and silicon vacancy Vsi, respectively. Obtained values of active-tion energy and capture cross-section allow expecting that deep level E3 associated with the carbon vacancy Vc, while acceptor level En is vanadium-induced level. Eight deep levels were observed in GaN. Levels E1, E2, E7, and E8 have been observed in low doped n-GaN are characterized for the first time in our studies.

Investigation of the properties of SiC membrane on alumina by using polycarbosilane

We investigated the properties of a SiC membrane on alumina, derived from polycarbosilane. The thickness of the deposited SiC layer was approximately 1μm. The topmost layer, having a thickness of approximately 0.6μm, comprised high-purity SiC. The 2.8μm thick layer present below it appeared to consist of SiO2. The contact angles between the SiC membrane and molten alumina at 973K and 1373K were approximately 67% and 86% of that between non-modified alumina and molten alumina.

911 高純度SiC製造プロセスにおける副生成物の精密研磨用スラリーへの応用 : 粉砕SiC微粒子の分粒方法と研磨特性の検討(CMP加工2)

911 高純度SiC製造プロセスにおける副生成物の精密研磨用スラリーへの応用 : 粉砕SiC微粒子の分粒方法と研磨特性の検討(CMP加工2)

The EI4 EPR centre in 6H SiC

We present the results of our recent electron paramagnetic resonance (EPR) studies of the EI4 EPR centre in electron-irradiated high-purity semi-insulating 6H SiC. Higher signal intensities and better resolution compared with previous studies have enabled a more detailed study of the hyperfine (hf) structure. Based on the observed hf structure due to the interaction with Si and C neighbours, the effective spin S=1, the C1h-symmetry and the annealing behaviour, we suggest a carbon vacancy–carbon antisite complex in the neutral charge state, VCVCCSi0, with the vacancies and the antisite in the basal plane, as a new defect model for the centre.

Purifying Mechanism in the Acheson Process - A Thermodynamic Study

To improve the purity of SiC and the yield of high purity SiC in the conventional Acheson Process, the behaviour of metal oxide impurities (Al2O3, CaO, Fe2O3 and MgO) with/without addition of sodium chloride (NaCl) is studied with the help of FactSageTM from a thermodynamic point of view. Equilibrium phases and compositions during evaporation and gas deposition are examined in the system of SiO2 + 3C. The calculation results are compared with industrial observations and published data. Purifying mechanism is clarified for the first time since the invention of the Acheson Process in 1891.

β-SiC 분말 Seeds가 Sol-gel 공정으로 합성된 고순도 β-SiC 분말 형상 및 크기에 미치는 영향

High purity β-SiC powders were synthesized using sol-gel processing. TEOS and phenol resin were used as the starting material for the silicon source and carbon source, respectively. The process turned out to be capable of producing high purity SiC powder purity degree with 99.98 %. However, it was difficult to control the shape and size of β-SiC powders synthesized by sol-gel process. In this study, β-SiC powder with size of 1~5 ㎛ an 30 ㎚ were used as the seeds for β-SiC to control the β-SiC powder morphology. It was found that β-SiC powder seeds was effective to increase the powder average size of synthesized β-SiC using sol-gel process by acting as the preferred growing sites for β-SiC.

새로운 가이드 튜브를 통한 6H-SiC 단결정의 직경 확장에 관한 연구

A sublimation method using the SiC seed crystal and SiC powder as the source material is commonly adopted to grow SiC bulk single crystal. However, it has proved to be difficult to achieve the high quality crystal and the process reliability because SiC single crystal should be grown at very high temperature in closed system. The present research was focused to improve SiC crystal quality grown by PVT method through using the new inner guide tube. The new inner guide tube was designed to prevent the enlargement of polycrystalline region into single crystalline region and to enlarge the diameter of SiC single crystal. The 6H-SiC crystals were grown by conventional PVT process. The seed adhered on seed holder and the high purity SiC source materials are placed on opposite side in sealed graphite crucible surrounded by graphite insulation. The SiC bulk growth was conducted around 2300 <TEX>$^{\circ}C$</TEX> of growth temperature and 50 mbar in an argon atmosphere of growth pressure. The axial thermal gradient across the SiC crystal during the growth was estimated in the range of 15<TEX>${\sim}$</TEX>20 <TEX>$^{\circ}C$</TEX>/cm.

Compatibility Issues for a High Temperature Dual Coolant Blanket

One proposed U.S. test blanket module (TBM) for ITER uses ferritic-martensitic alloys with both eutectic Pb-Li and He coolants at ∼475°C. In order for this blanket concept to operate at higher temperatures (∼750°C) for a DEMO-type reactor, several Pb-Li compatibility issues need to be addressed. A SiC/SiC composite flow channel insert is proposed to reduce the steel dissolution rate (and the magnetohydrodynamic pressure drop). Prior capsule testing examined dense, high-purity SiC in Pb-Li at 800°-1200°C and found detectable levels of Si in the Pb-Li after 2,000h at 1100°C and 1,000h at 1200°C. Current capsule experiments are examining several different SiC/SiC composite materials at 1000°C. Another issue involves Pb-Li transport between the first wall and heat exchanger. Aluminide coatings on type 316 stainless steel and Al-containing alloys capable of forming an external alumina scale have been studied in capsule experiments at 700° and 800°C for 1,000h. Model aluminide coatings made by chemical vapor deposition reduced the dissolution rate for 316SS at 800°C by a factor of 50.

Silicon Carbide Coating on Graphite and Isotropic C/C Composite by Chemical Vapour Reaction

The application of Carbon and graphite based materials in unprotected environment is limited to a temperature of 450 o C or so because of their susceptibility to oxidation at this temperature and higher. To over come these obstacles a low cost chemical vapour reaction process (CVR) was developed to give crystalline and high purity SiC coating on graphite and isotropic C/C composite. CVR is most effective carbothermal reduction method for conversation of a few micron of carbon layer to SiC. In the CVR method, a sic conversation layer is formed by reaction between carbon and gaseous reagent silicon monoxide at high temperature. Characterization of SiC coating was carried out using SEM. The other properties studied were hardness density and conversion efficiency.

Clustering of vacancy defects in high-purity semi-insulating SiC

Positron lifetime spectroscopy was used to study native vacancy defects in semi-insulating silicon carbide. The material is shown to contain (i) vacancy clusters consisting of four to five missing atoms and (ii) Si-vacancy-related negatively charged defects. The total open volume bound to the clusters anticorrelates with the electrical resistivity in both as-grown and annealed materials. Our results suggest that Si-vacancy-related complexes electrically compensate the as-grown material, but migrate to increase the size of the clusters during annealing, leading to loss of resistivity.

The Influence of SiC Powder Source on 6H-SiC Single Crystals Grown by the Sublimation Method

We examined the influence of thermal treatment of high-purity SiC powder on 6H-SiC crystal growth. The doping concentration was decreased by increasing either thermal treatment temperature or time. It was also found that the defects such as micropipes and planar cavities were generated under relatively long treatment time (13 hours), because SiC powders were significantly graphitized. A 6H-SiC crystal grown by using SiC source treated at 2100oC for 6 hours revealed the best result with relatively low micropipes. For the effects of thermal treated sources on the improvement of crystallinity, it could be explained that both the amount of alpha phase transformed from high-purity beta-SiC powder and the elimination of porous powders in SiC powder had an influence on the removal of silicon droplets, resulting in higher Si vapor pressure at the initial growth stage.

Electrical properties of unintentionally doped semi-insulating and conducting 6H-SiC

Temperature dependent Hall effect (TDH), low temperature photoluminescence (LTPL), secondary ion mass spectrometry (SIMS), optical admittance spectroscopy (OAS), and thermally stimulated current (TSC) measurements have been made on 6H-SiC grown by the physical vapor transport technique without intentional doping. n- and p-type as well semi-insulating samples were studied to explore the compensation mechanism in semi-insulating high purity SiC. Nitrogen and boron were found from TDH and SIMS measurements to be the dominant impurities that must be compensated to produce semi-insulating properties. The electrical activation energy of the semi-insulating sample determined from the dependence of the resistivity was 1.0eV. LTPL lines near 1.00 and 1.34eV, identified with the defects designated as UD-1 and UD-3, were observed in all three samples but the intensity of the UD-1 line was almost a factor of 10 more in the n-type sample than in the the p-type sample with that in the semi-insulating sample being intermediate between those two. OAS and TSC experiments confirmed the high purity of this material. The results suggest that the relative concentrations of a dominant deep level and nitrogen and boron impurities can explain the electrical properties in this material.

Crystal growth of micropipe free 4H–SiC on 4H–SiC [formula omitted] seed and high-purity semi-insulating 6H–SiC

Micropipe free c-plane 4H–SiC wafers were achieved by sublimation growth on the 4H–SiC { 0 3 3 ¯ 8 } seed. 4H–SiC { 0 3 3 ¯ 8 } seeds were obtained by inclining the c-plane to 〈 0 1 1 ¯ 0 〉 at 54.7°. A transmission X-ray topograph of the micropipe free c-plane wafer revealed that there were no macroscopic defects with lattice displacements. Crystal growth of undoped (vanadium-free) semi-insulating 6H–SiC was carried out by our sublimation system. In order to achieve high resistivity, high-purity SiC source and controlled instruments were used for the reduction of nitrogen, boron and metal impurity backgrounds. Hence high-purity and high-resistivity 6H–SiC 2 and 3 in in diameter were developed for high-frequency power transistors.