Materials Science Forum Vols. 717-720

Abstract: Large-diameter SiC single crystals are grown at II-VI by the sublimation technique. 100mm substrates of semi-insulating 6H SiC and n-type 4H SiC are produced as commercial products; in development, diameter expansion to 125mm has been achieved. Over the last two years, significant improvements have been made in crystal quality. The values of FWHM of x-ray rocking curves are typically 20-40 arc-seconds for 6H SI wafers and 12-30 arc-seconds for 4H n+ wafers. Micropipe density is less than 3 cm-2, and less than 0.1 cm-2 in best substrates. Electrical resistivity of SI substrates is, typically, of 10¹¹ Ω•cm or above. For 4H n+ substrates, the typical dislocation density is about 9×10³ cm^-2 and the typical BPD density is less than 1×10³ cm^-2.

Abstract: A reduction in threading screw dislocation (TSD) density in 4H-SiC (silicon carbide) crystal is required for SiC power devices. In this study, TSD’s transformation by the RAF (repeated a-face) growth method [1] is observed by transmission X-ray topography (g=0004) of the cross-section of the crystal. Increasing the number of repetitions of a-face growth and offsetting c-face growth to an angle of several degrees reduce TSDs. TSD density is reduced to 1.3 TSD/cm2. The RAF growth method is very effective towards growing high quality SiC crystals.

Abstract: Large diameter 4H-SiC single crystal wafers with higher quality are required to improve the yields of devices fabricated onto the SiC wafers. For crystal growths with higher quality, it is important to prepare seed crystals with lower defect densities. In particular, the edge part of the seed has to be prepared with considerable care because the crystallinity of the enlarged part of grown crystals depends much upon the surface condition of the seed crystal during radial expansion growth. We found that growth with fewer defect and micropipe densities, specifically at the periphery of the crystal, is possible by utilizing in-situ etching process for the seed crystal surface. We have also performed intense numerical calculations of the temperature distribution around the seed surface, and discussed growth conditions which cause the in-situ etching effective to improvement of the crystallinity in enlarged crystals.

Abstract: The present research was focused to investigate various process parameters influenced on the large 4H-SiC crystal growth on a 6H-SiC seed by PVT method. The crucible diameter along horizontal axial direction and inserted graphite ring was modified to change the growth parameter like the temperature gradient. In the initial stage of growth, foreign polytypes such as 6H/4H were observed on 6H-SiC seed, indicating the growth temperature to be unstable on crystal surface. However, from the middle of growth step, 4H-SiC was successfully formed in the ingot with the modification of growth pressure and a SIMS analysis confirmed the high doping concentration in grown 4H-SiC crystal.

Abstract: Sublimation-recrystallization processes occurring during PVT are investigated from the viewpoint of quasi-equilibrium phase transitions of SiC. In addition to the elemental reaction processes of PVT, other phenomena such as silicon droplet formation and in-situ etching are also discussed based upon the Si-C binary phase diagram, and possible mechanisms are proposed.

Abstract: Crystal growth velocity of SiC in a process of physical vapor transport was studied on the basis of numerical calculation including compressible effect, convection and buoyancy effects, flow coupling between argon gas and species, and the Stefan effect. Calculation in 2D configuration was performed to clarify the effect of pressure on growth velocity. The results revealed that the origin of diffusion resistance reported so far was the effect of convection of argon gas and chemical species.

Abstract: In this work results of nitrogen doping in the amount of 0 vol.%, 3 vol.% and 10 vol.% on the growth of the 4H polytype on the 6H-SiC seed are presented. SiC crystals grown by PVT method on the (000-1) C-face of 6H seeds using the open seed backside design have been investigated. Structural and electrical properties of the crystals were studied by different experimental methods.

Abstract: Lateral expansion of small mixed polytype 4H/6H-SiC and 6H-SiC slivers were realized by hot wall chemical vapor deposition (HWCVD). Small slivers cut from m-oriented (11 ̅00) SiC boule slices containing regions of 4H and 6H-SiC or just single polytype 6H-SiC were exposed to HWCVD conditions using standard silane/propane chemistry for a period of up to eight hours. The slivers exhibited approximately 1500 μm (1.5 mm) of total lateral expansion. Initial analysis by synchrotron white beam x-ray topography (SWBXT) confirms, that the lateral growth was homoepitaxial, matching the polytype of the respective underlying region of the seed sliver.

Abstract: Silicon carbide powders were prepared in a vacuum induction melting furnace (VIM). Silica and silicon were used as sources of silicon, and graphite powder was used a source of carbon. Pressures of 0.1 and 0.01 atm were selected as the operation conditions, and different silicon carbide powders were prepared. Free carbon and remnant silica were removed by high-temperature baking in air and acid leaching. Low-pressure powders show better crystallinity; moreover, free carbon and silica were rarely found in the product after baking and leaching. The low-pressure grains were prismatic whereas the high-pressure grains were porous. This shows that pressure is a critical parameter in silicon carbide formation, and low-pressure makes the low-temperature synthesis of silicon carbide feasible. Glow discharge mass spectra were used to analyze the impurity content in silicon carbide powders. After baking and leaching, the purity is increased from 3N5 (99.95 wt.%) to 4N5 (99.995 wt.%). Further purification procedures will be combined to meet the quality requirements for crystal growth.