Papers by Author: Toru Ujihara

Abstract: In order to achieve a high-quality SiC crystal in solution growth, one of the most difficult issues is to grow a thick layer on Si face avoiding polytype transformation. In this case, two-dimensional nucleation, which leads to the polytype transformation, is frequently induced because a density of threading screw dislocations acting as a source of spiral step decreases and wide terraces form by step bunching as growth proceeds. Therefore, it is very difficult to stabilize the polytype of crystals grown with extremely low density of threading screw dislocations. In this study, we tried to overcome these problems by using specially designed seed crystal and optimizing growth temperature and temperature distribution. We successfully grew thick low-threading-dislocation density SiC crystal without polytype transformation under the condition of high growth temperature and homogeneous temperature distribution.

Abstract: The relationship between surface morphology and spatial distribution of basal plane dislocations in 4H-SiC crystal grown by top-seeded solution growth on the C face was investigated by the differential interference microscopy as well as X-ray topography. Basal plane dislocations were generated at the boundaries of the domains with the different macrosteps advance directions. On the other hand, at the position where macrosteps advance to the same direction, BPDs were hardly observed. This results suggest that BPD density can be decreased by the suppression of the collision of macrosteps during the solution growth on the C face controlling the surface morphology.

Abstract: In order to design a solvent for high-purity SiC solution growth, the impurity incorporation and the carbon solubility of various solvent materials have been investigated. Among the transition metal elements, the impurity elements of Cr, Ti, V and Hf are more readily incorporate during the solution growth than the other transition metal elements. The thermodynamic calculation revealed that the Y-Si solvent has relatively large carbon solubility, which is comparable to the Cr-Si and Ti-Si solvents often used in the solution growth of bulk SiC crystals. From these results, the Y-Si solvent is expected to be a suitable solvent for the high-purity SiC solution growth. Furthermore, we have demonstrated that the Y-Si solvent can achieve lower incorporation of metal impurity in the grown crystal than the Cr-Si solvent maintaining the growth rate.

Abstract: We achieved a high growth rate in solution growth of AlN single crystal by suppressing unintentional precipitations near the surface of solvent and by increasing the equilibrium nitrogen concentration in the solvent. In order to suppress unintentional precipitations, we made the solvent supersaturated just above the surface of the substrate by optimizing the composition of the solvent and the temperature distribution based on thermodynamic numerical analysis. In particular, we focused on interactions between nitrogen or aluminum and solvent elements, leading to the increase of the equilibrium nitrogen concentration. We selected chromium and cobalt due to their high affinity with nitrogen or aluminum. Consequently, we successfuly achieved growth rate as high as 200 μm/h in maximum.

Abstract: We investigated the spatial distribution of carrier concentration in n-type 4H-SiC grown by the solution method from the peak frequency of the longitudinal optical phonon-plasmon coupled (LOPC) mode of the Raman spectra on the surface. The carrier concentration at the position of the smooth terrace was higher than the carrier concentration at the position where the macrosteps were formed. This indicates the nitrogen incorporation efficiently occurs on the smooth surface where the density of macrosteps is relatively low. The different incorporation of nitrogen depending on the surface morphology can be understood from the view point of the adsorption time of impurity on the terrace. The present result implies that the uniform surface morphology is necessary to achieve uniform doping concentration in SiC crystal.

Abstract: The influences of solution flow and lateral temperature distribution on the surface morphology of the 4H-SiC single crystal grown from solution was investigated. A flat surface region was enlarged by the seed-rotation rate. The solution flow simulation indicated that the higher rotation rate made the outward solution flow ordered beneath the solution surface. Such a solution flow was thought to be effective to enlarge the flat region of growth front. Furthermore, a full-flat surface was obtained with a hollow-type graphite rod at a seed-rotation rate of 60 min^-1. The simulated results of temperature distribution showed the hollow-type graphite rod reduced the lateral temperature gradient at the SiC-solution interface. The ordered solution flow and the small temperature gradient at the growth front were found to be effective to make the growth front flat in the solution-growth method.

Abstract: In this paper, we will discuss how to cope with the smoother growth front and higher growth rate by the forced convection. When the rotation rate of the upper part of the solution is different from that of the lower part in the crucible, the centrifugal force of the upper part is different from that of the lower part. As a result, a forced convection occurs in the solution. This kind of convection was achieved with accelerating/decelerating rotation of crucible and a plate fixed on the bottom of the crucible. By optimizing conditions of rotation program patterns and the crucible design for the forced convection, the growth rate could almost be doubled while maintaining smooth morphology.

Abstract: In solution growth of 4H-SiC, we have investigated changes in macrostep height with addition of the Group III (B, Al, Ga), Group IV (Ge, Sn), Group V (N) elements, and transition metals (Ti, V, Cr, Ni) to Si solvents, in order to find additives improving severe step bunching which often occurs during growth. The addition of Al, B, Sn, N, and V decreased the average macrostep height compared with the crystal grown with Si solvents. The addition of Al, B, Sn, N, and V suppressed the generation of trench-shaped surface defects in long-term growth of 10 hours. This result demonstrated that the addition of Al, B, Sn, N, and V has an advantage to achieve high quality bulk crystal growth from solution.

Abstract: We realized the growth of 3C-SiC crystal on sapphire by solution growth method. The carbon deposition on a sapphire substrate before growth is the key point for this technology. This carbon layer plays a role to protect the dissolution of sapphire by Si solvent. Single crystal of 3C-SiC was grown on the whole surface of the sapphire substrate. Surprisingly, the 3C-SiC layer did not directly grown on the sapphire substrate. The single crystal 3C-SiC layer formed by the reaction between the deposited carbon and the Si wafer that is a solvent material below the melting point of silicon during heating process before the growth. The 3C-SiC grew on the 3C-SiC layer. In this process, the deposited carbon play another important role.

Abstract: We achieved the convex growth interface shape in top-seeded solution growth of SiC applying non-axisymmetric solution convection induced by non-axisymmetric temperature distribution. The detailed solution flow, temperature distribution and carbon concentration distribution were calculated by 3-dimensional numerical analysis. In the present case, the solution flow below the crystal was unidirectional and the supersaturation was increased along the solution flow direction. By the rotation of the crystal in the unidirectional flow and the temperature distribution, we successfully obtained the crystal with the convex growth interface shape.