Papers by Author: Wataru Ohashi

Abstract: 4H-SiC epitaxial growth on 2˚ off-axis substrates using trichlorosilane (TCS) is presented. Good surface morphology was obtained for epilayers with C/Si ratios of 0.6 and 0.8 at a growth temperature of 1600°C. The triangle defect density was reduced to a level below 5 cm^-2 at 1600°C and below 1 cm^-2 at 1625°C for a C/Si ratio of 0.8. Photoluminescence (PL) measurements were carried out with band-pass filters of 420 nm, 460 nm, and 480 nm to detect stacking faults. A stacking fault density of below 5 cm^-2 was achieved at 1600°C and 1625°C with a C/Si ratio of 0.8. The optimal conditions for TCS growth were a C/Si ratio of 0.8 and a growth temperature of 1600°C. The evaluation of stacking faults and etch pit density indicated that the use of 2˚ off-axis substrates and TCS is effective for reducing basal plane dislocations. Comparing these results to those using silane (SiH₄) with HCl added, it was demonstrated that TCS is much more suitable for obtaining high-quality epilayers on 2º off-axis substrates.

Abstract: Defect formation during the early stages of physical vapor transport (PVT) growth of 4H-SiC was investigated using high resolution x-ray diffraction (HRXRD). Characteristic lattice bending behaviors were revealed in the nearby seed crystal regions of grown crystals. The lattice bending was localized in close proximity to the seed/grown crystal interface, and the (0001) basal planes bended convexly toward the growth direction, indicative of the insertion of extra-half planes pointing toward the growth direction during the initial stages of crystal growth. This paper discusses the possible mechanisms of the observed lattice bending and sheds light on the defect formation processes during PVT-growth of 4H-SiC single crystals.

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: 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: Structures and propagating behaviors of threading dislocations (TDs) in PVT-grown 4H-SiC single crystals were both investigated using Synchrotron monochromatic X-ray topography. Comparative studies by examining images obtained for the crystals with different diffraction geometries of (0004) and (11-20) of 4H-SiC revealed that a large amount of TDs are likely to be mixed in character, i.e., dislocations with Burgers vector components of both <0004> and <11-20>. Closer observations of topography images has revealed that, although TDs lie largely along the c-axis direction, some of the TDs show quite a complex propagating behavior: not extending in a straight line but meandering along the growth direction.

Abstract: Time-dependent evolutions of single and quadruple Shockley stacking faults (sSSF and 4SSF) in 4° off 4H-SiC epitaxial layers have been investigated. UV illuminations using an Hg-Xe lamp light source generate dissociations of basal plane dislocations (BPDs) into sSSFs whereas for 4SSFs no significant changes in shape occur. Detailed analyses of Photo-luminescence (PL) signals suggest that Si- and C-core partials have different PL spectrum distributions in the wavelength range larger than 750 nm, giving rise to images with different contrasts in PL mappings.

Abstract: Dislocations in highly nitrogen-doped (N > 1×1019 cm-3) low-resistivity ( < 10 mcm) 4H-SiC substrates were investigated by photoluminescence imaging, synchrotron X-ray topography, and defect selective etching using molten KOH. The behavior of dislocations is discussed particularly in terms of their glide motion in the presence of a high concentration of nitrogen. The results indicate that nitrogen impurities up to mid 1019 cm-3 concentration do not show any discernible influence on the glide behavior of basal plane dislocations (BPDs) in 4H-SiC crystals grown by physical vapor transport (PVT) method.

Abstract: The epitaxial growth process was optimized in order to obtain good surface morphology for epilayers grown on 4˚ off-axis substrates. The optimization was carried out from growth temperatures and gas chemistry including C/Si ratio. Step-bunching was significantly suppressed by the optimized process and a surface roughness Ra of 0.2 nm was achieved. Etch pit density evaluation by KOH etching indicated that the basal plane dislocations were reduced to less than 50 cm-2 by the use of 4˚ off-axis substrates. Photoluminescence evaluation showed that the epilayer grown by the optimized process had a better crystalline quality than that grown by a standard process. Schottky diodes fabricated on the epilayer by the optimized process represented the ideality factor n of 1.01 and the barrier height of 1.67eV. These results demonstrate that high quality epilayers with smooth surfaces comparable to those on 8˚off-axis substrates were obtained on 4˚off-axis substrates.

Abstract: In-grown type stacking faults (SFs) like structures were observed in 100mm diameter 4H-SiC crystals by Photoluminescence (PL) mappings, and structural analyses using HRTEM clarified that the SF-like structures were comprised of 6H (3, 3) stacking sequences. The stacking sequences of the SF-like structures observed are different from the SFs formed in the a-face grown crystals, suggesting that it is due to 6H nucleation on {0001} plane terraces.