Shunta Harada

Thermoelectric properties and crystallographic shear structures in titanium oxides of the Magnèli phases

The thermoelectric properties of Magneli phase titanium oxides TinO2n−1 (n=2,3,…) have been investigated, paying special attention to how the thermoelectric performance can be altered by changing the microstructure. Dense polycrystalline specimens with nominal composition of TiO2−x (x=0.05, 0.10, 0.15, and 0.20) prepared by conventional hot-pressing are all identified to be one of the Magneli phases, in which crystallographic shear planes are regularly introduced according to the oxygen deficiency. Electrical conduction is n-type for all specimens and the carrier concentration increases with the increase in the oxygen deficiency. The values of lattice thermal conductivity, on the other hand, decrease with the increase in the oxygen deficiency, which can be attributed to phonon scattering at the crystallographic shear plane. The largest value of thermoelectric figure of merit Z, 1.6×10−4 K−1 was obtained at 773 K for the hot-pressed specimen of TiO1.90.

High-Efficiency Conversion of Threading Screw Dislocations in 4H-SiC by Solution Growth

The high-efficiency conversion of threading screw dislocations (TSDs) in 4H-SiC by solution growth provides an efficient method of obtaining ultra high-quality SiC crystals. The behavior of TSDs on on-axis and off-axis 4H-SiC0001 seed crystals was investigated by synchrotron X-ray topography. Almost all TSDs in the off-axis Si-face seed crystal were converted to Frank-type stacking faults on the basal planes. The conversion ratio of TSDs was highly influenced by the surface polarity of the seed crystal. The stacking faults laterally propagate toward the outside of the crystal.

Improvement of Grain-Boundary Conductivity of Trivalent Cation-Doped Barium Zirconate Sintered at 1600°C by Co-doping Scandium and Yttrium

Scandium and yttrium co-doped barium zirconate [BaZr 0.85 Sc x Y 0.15-x O 3-δ (x = 0, 0.05, 0.075, 0.10, 0.15)] have been investigated in terms of phase relationship, microstructures, and electrical conductivity. The bulk conductivity of the scandium and yttrium co-doped barium zirconate increased with the dopant ratio of yttria. BaZr 0.855 Sc 0.05 Y 0.10 O 3-δ had the highest grain-boundary conductivity among the scandium and yttrium co-doped barium zirconates in this study. But, BaZr 0.85 Sc 0.15 O 3-δ BaZr 0.85 Sc 0.10 Y 0.05 Ο 3-δ , BaZr 0.85 S 0.75 Y 0.075 O 3-δ , and BaZr 0.85 Sc 0.05 Y 0.10 Ο 3-δ consisted of a single cubic perovskite phase at 1600°C and their densities of grain-boundary were smaller than that of BaZr 0.85 Y 0.15 Ο 3-δ . From the observation of microstructure and results of grain boundary-conductivity measurement, we can say that yttrium is a dopant that increases specific grain-boundary conductivity and bulk conductivity, and scandium is a dopant that increases the grain size. Thus, there is a trade-off relation between the grain size and specific grain-boundary conductivity based on the mixing ratio of scandia to yttria. The total conductivity of BaZr 0.85 Sc 0.05 Y 0.10 O 3-δ at 600°C was estimated to be 1.6 X 10 -2 S cm -1 , which is the highest-class conductivity among reported trivalent cation-doped barium zirconates.

Evolution of threading screw dislocation conversion during solution growth of 4H-SiC

Evolution of threading screw dislocation (TSD) conversion during the solution growth of 4H-SiC on a vicinal crystal of 4H-SiC(0001) was investigated by synchrotron X-ray topography. Selecting appropriate X-ray wavelength and g vector, we can change the penetration of X-ray, and the dislocation behaviors with the different depth were successfully observed. Evidently TSDs parallel to the c-axis having c-component Burgers vector were changed into defects on the (0001) basal planes with the same Burgers vector as the TSDs, propagating to the [112¯0] step-flow direction by advancing macrosteps during the solution growth. The TSD conversions stochastically took place during the growth. The conversion rate was almost uniform and finally almost all TSDs were converted to the basal plane defects. The conversion rate was low at the very early stage of the growth, which implies that the macrosteps were not formed at the initial stage of the solution growth.

Low-dislocation-density 4H-SiC crystal growth utilizing dislocation conversion during solution method

We report a marked reduction in the dislocation density of a 4H-SiC crystal using a high-efficiency dislocation conversion phenomenon. During the solution growth, threading dislocations were efficiently converted to basal plane defects by the step flow of macrosteps. Utilizing this dislocation conversion phenomenon, we achieved the marked reduction of threading dislocation density. Consequently, the threading screw dislocation density was only 30 cm−2, which was two orders of magnitude lower than that of the seed crystal. The 4H-SiC polytype of the seed crystal was replicated in the grown crystal, which was attributed to the spiral growth owing to a few remaining threading screw dislocations upstream of the step flow.

Reduction of Threading Screw Dislocation Utilizing Defect Conversion during Solution Growth of 4H-SiC

Reduction of threading screw dislocation without polytype transformation from 4H-SiC was performed by the combination of step-flow growth and spiral growth. On a vicinal 4H-SiC seed crystal, threading screw dislocations are converted to Frank-type stacking faults by step-flow during solution growth. As the growth proceeds, the defects are excluded to the crystal. Thus utilizing the conversion, high quality SiC crystal growth without threading screw dislocations is expected to achieve. However, at the same time, polytype transformation is caused by the occurrence of 2D nucleation. By using the special shape of seed crystal, we successfully grew high quality 4H-SiC crystal without threading screw dislocation and polytype transformation.

Conversion Mechanism of Threading Screw Dislocation during SiC Solution Growth

Solution growth is considered to be a powerful method for high quality SiC crystals. This work reports that the conversion process from a threading screw dislocation into a few Frank partial dislocations in basal planes was investigated by synchrotron X-ray topography. This process was effectively assisted by step-flow growth on off-oriented (0001) seed crystals. The Frank partials were not extended into the crystal grown toward the [0001] direction perpendicular to the basal plane. Thus, the conclusion of this study suggests the use of off-oriented seed crystal is important to improve crystal quality.

Increase in the Growth Rate by Rotating the Seed Crystal at High Speed during the Solution Growth of SiC

We studied the effect of rotation speed of seed crystal on the growth rate during the solution growth of SiC. The growth rate increased with increasing rotation speed of the seed crystal. The increase in the growth rate was observed in relatively wide range of carbon concentration. According to the numerical simulation, the carbon concentration gradient near the growth interface under 150 rpm condition is larger than 20 rpm (ACRT) condition. This indicates that increase in the growth rate is caused by the increase in the carbon concentration gradient of the diffusion layer.

Surface Morphology and Threading Dislocation Conversion Behavior during Solution Growth of 4H-SiC Using Al-Si Solvent

Surface morphology and threading dislocation conversion behavior during solution growth of 4H-SiC using pure Si and Al-Si solvents was investigated. The growth surfaces on the C face were smoother than the Si face. By the addition of Al to the solvent, the growth surface became smooth on the C face and rough on the Si face. Threading screw dislocation conversion took place only in the grown crystals on the Si face and threading edge dislocation conversion occurs both on the Si face and the C face using the pure Si solvent. On the other hand, in the grown crystal on the C face using the Al-Si solvent, the threading dislocation conversion was hardly observed. These results indicate that the threading dislocation conversion behavior is influenced by the surface morphology.

Improvement of Surface Morphology by Solution Flow Control in Solution Growth of SiC on Off-Axis Seeds

The solution growth of SiC on an off-axis seed is effective on the reduction of threading dislocations. We proposed a novel method to grow a SiC crystal on an off-axis seed by top-seeded solution growth (TSSG). In our previous study, a unidirectional solution flow above a seed crystal is effective to suppress surface roughness in the growth on the off-axis seed. However, it is difficult to apply the unidirectional flow in an axisymmetric TSSG set-up. In this study, the unidirectional flow could be achieved by shifting the rotational axis away from the center of the seed crystal. As a result, the smooth surface was obtained in the wider area where the solution flow direction was opposite to the step-flow direction.