Papers by Author: Koichi Kakimoto

Abstract: In order to clarify the mechanical properties of single-crystal silicon carbide (SiC), nanoindentation was performed on a 4H-SiC wafer. The change of hardness with the angle between the wafer orientation flat and the indenter edge, the maximum load and the loading rate were investigated. The hardness reached maximum at an indentation load of 12 mN in the range of 3-50 mN. Hardness decreased under two conditions: when the edge of the indenter tip is parallel to the [11-20] direction, and when a very low loading rate was used. Transmission electron microscopy was used to observe dislocations and cracks under the indents. It was demonstrated that the deformation process of SiC involved three steps with respect to the increase of the indentation load. These results provide information for improving ductile machining process of single crystal SiC.

Abstract: We have proposed single seed cast Si growth and developed a furnace for 50 cm square ingots. By optimizing growth parameters, improving gas condition, coating, the quality of mono Si ingot has improved. Namely, dislocation density, the concentrations of substitutional carbon and interstitial oxygen have been significantly reduced. The conversion efficiency of cast Si solar cells has become comparable with those of CZ Si wafers.

Abstract: In an attempt to understand how and where dislocations are introduced into Si ingots by temperature gradients, bulk dislocation-free FZ crystals are exposed to temperature gradients similar to those in Bridgman Si crystal growth. This heat treatment introduces dislocations, which were analyzed using X-ray topography (XRT) and Scanning InfraRed Polariscopy (SIRP). Hereby, the orientation dependency is taken into account and ingots in (001) and (111) growth orientation are evaluated in this work. It can be found that the dislocation generation takes place at similar regions of the crystal and is independent of orientation, however, their propagation and multiplication differs. This leads to an overall different shape of the dislocation network. Especially intriguing are the long slip lines in the (111)-crystal, which cannot be found in the (001)-crystal. This suggests a different magnitude of slip propagation depending on the sample orientation. This effect should be explained by a different activation of slip systems and is discussed in the paper.

Abstract: To get the optimized condition and ideal furnace structure, we have performed seed cast growth of mono-crystalline Si by using unidirectional solidification furnace. More than 20 ingots of 10 cm diameter and 10 cm height were grown under different growth conditions. The quality of ingots was characterized by using Fourier transform infrared spectroscopy (FTIR), infrared microscopy, scanning infrared polariscope (SIRP), X-ray topography, etc. We have realized reduction of carbon, residual strain and extended defects, which may contribute the increase of solar cell efficiency.

Abstract: The residual strain distribution in cast-grown mono-like Si ingots is analyzed. The effect of the crucible during solidification and the influence of different cooling rates is described. To clarify in which process steps residual strain accumulates, several Si ingots were grown in a laboratory scale furnace (100mm) using different cooling conditions after completion of the solidification. For the cooling, two different cooling rates were distinguished: fast cooling (12deg/min) and slow cooling (5deg/min). It was found that changes in cooling gradients greatly influence the amount of residual strain. The results show that slow cooling in any temperature range leads to strain reduction. The greatest reduction could be found when the temperature gradient was changed to slow cooling in the high temperature region.

Abstract: We evaluated the properties of crystalline defects in silicon substrate, and clarified the origin of small-angle grain boundaries. In order to eliminate the effects of grain boundaries, the ingot was fabricated by unidirectional solidification technique with seed crystal. In single-crystalline region, Σ3 twin boundaries and SiC precipitates were observed near the seed crystal. No obvious relationship between twin boundaries and precipitates was observed. These defect decreased once and the precipitations appeared again. The density of precipitates increased through the crystal growth procedure. These precipitates were consisted of Si, C, and N. After the precipitation density increased, the small-angle grain boundaries appeared and some precipitates were observed at the boundaries. We considered the precipitations consisted of light element impurities such as C and N were one of the major origins of the small-angle grain boundary generation.

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: The content and uniformity of impurities and precipitates have an important role in the efficiency of solar cells made of multicrystalline silicon. We developed a transient global model of heat and mass transfer for directional solidification for multicrystalline silicon and a dynamic model of SiC particles and silicon nitride precipitation in molten silicon based phase diagrams. Computations were carried out to clarify the distributions of carbon, nitrogen and oxygen based on segregation and the particle formation in molten silicon during a directional solidification process. It was shown that the content of SiC precipitated in solidified ingots increases as a function of the fraction solidified. It was also clarified from the results that Si2N2O was first formed near the melt-crystal interface, since oxygen concentration in the melt decreases and nitrogen concentration in the melt increases with solidification of the molten silicon. Si3N4 was formed after Si2N2O had been formed.