Papers by Author: Tetsuo Takahashi

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 have investigated the solution growth under various Al-N co-doping conditions. Both p-type and n-type 4H-SiC were successfully grown under Al-N co-doping conditions, while using the effect of Al-addition to stabilize both growth surface and polytype. The doping and electrical properties were investigated systematically. Interaction between Al and N in the incorporation process and electrical property under heavily co-doped conditions were discussed.

Abstract: 4H-SiC crystallization from Si-C solution in electric current-controlled liquid phase epitaxy was investigated. The dependence of growth speed on a DC current shows that dissolution/growth is controlled by the electric current without altering temperature gradient in the furnace. Application of an electric current leads to reduction of growth speed with negative polarity and enhancement of growth speed with positive polarity. The variation of the growth speed with a DC current density has been explained by the combination of the effects of electromigration of C solute and Joule heating.

Abstract: Void formation in 4H-SiC crystals grown from solution has been investigated by secondary ion mass spectrometry and Raman scattering. It becomes clear that ambient Ar gas is filled in voids and the solvent (Si) partially remains. The result indicates that Ar dissolved in the solvent vaporizes and forms bubbles. The trapped bubbles at the crystal growth front are considered to be incorporated in the growing crystal as voids. We also have developed following methods for suppression of the void formation; (1) dipping seed crystals so that the growth front faces upward, (2) growth under He atmosphere, and (3) the high temperature treatment of the solvent before crystal growth.

Abstract: In this study, we investigated the cluster effect on the occurrence of giant step bunching. We generated carbon clusters on 4H-SiC (0001) surfaces by thermal decomposition of SiC in an Ar atmosphere and controlled the surface concentrations of the clusters by adding H2 gas. We found the boundaries between surfaces with and without giant steps to show Arrhenius-type behavior. This behavior agreed with our predictions deduced from a chemical reaction model that takes the cluster effect into account, suggesting that giant step bunching is attributable to the formation of clusters on SiC.

Abstract: To elucidate the origin of giant step bunching on 4˚ off-axis 4H-SiC (0001) faces, we carried out hydrogen etching and epitaxial growth under various conditions. We found that giant step bunching occurs during hydrogen etching and epitaxial growth at extremely low or high C/Si ratios, i.e., with an excessive supply of SiH4 or C3H8. From these results, we have proposed that the origins of giant step bunching are asymmetry in the step kinetics in etching and Si or C cluster generation on terraces during growth.

Abstract: We have developed a new chemical vapor deposition (CVD) system that is capable of a high growth rate of over 100 µm/h with good uniformities of thickness and carrier concentration. In this CVD system, the process gases contribute efficiently to epitaxial growth. In a demonstration of the abilities of the CVD system, we achieved an average growth rate of 140 µm/h, a thickness uniformity of 3.9%, and a carrier concentration uniformity of 8.9% in a 2-inch wafer, without degradation of the crystallinity.

Abstract: We have proposed a novel model of hydrogen etching of SiC based on thermal equilibrium and have confirmed the validity of our model through the analysis of H2 etching experiments. The experimental results obtained showed that the etching rate is expressed by a linear equation with the H2 flow rate, by an exponential function with the reciprocal of the temperature and by a power law with the pressure. These results agree well with the theoretical behaviors derived from our model.