Papers by Author: Yasuo Kito

Abstract: We investigated a way of reducing the stacking fault (SF) density on a highly nitrogen (N) doped 4H-SiC crystal. SFs were generated at highly N doped crystal exceeding 4 x 1019 cm-3 and the density was increased with increasing N concentration. We found that Al co-doping had the potential to suppress this SF generation and was effective up to an N concentration of about 1 x 1021cm-3. This effect depended strongly on the Al concentration. We discussed the reason for the SF suppression effect of Al co-doping.

Abstract: The main electronic characteristics of silicon carbide (SiC) are its wide energy gap, high thermal conductivity, and high break down electric field which make of it of one of the most appropriate materials for power electronic devices. Previously we reported on a new electrical conductivity evaluation method for nano-scale complex systems based on our original tight-binding quantum chemical molecular dynamics method. In this work, we report on the application of our methodology to various SiC polytypes. The electrical conductivity obtained for perfect crystal models of 3C-, 6H- and 4H-SiC, were equal to 10-20-10-25 S/cm. For the defect including model an extremely large electrical conductivity (of the order of 102 S/cm) was obtained. Consequently these results lead to the conclusion that the 3C-, 6H-, and 4H-SiC polytypes with perfect crystals have insulator properties while the electrical conductivity of the crystal with defect, increases significantly. This result infers that crystals containing defects easily undergo electric breakdown.

Abstract: The mechanism of layer growth as well as defect formation in the SiC crystal is fundamentally important to derive its appropriate performance. The purpose of the present study is to investigate competitive adsorption properties of growth species on the various 4H-SiC polytype surfaces. Adsorption structure and binding energy of growth species in the experimentally condition on various SiC surfaces were investigated by density functional theory. For the SiC(000-1) and SiC(0001) surfaces, the adsorption energy by DFT follows the orders C > H > Si > SiC2 > Si2C > C2H2. Furthermore, based on the DFT results, amount of adsorption of each species in the experimental pressure condition were evaluated by grand canonical Monte Carlo method. H and Si are main adsorbed species on SiC(0001) and SiC(000-1) surfaces, respectively. The ratio of amount of adsorption of Si to H was depending on the surface structure that might explain different growth rate of the surfaces.

Abstract: A simulation study for high temperature chemical vapor deposition (HTCVD) of silicon carbide (SiC) is presented. Thermodynamic properties of the species were derived from the first-principles calculations in order to evaluate the activation energy (Ea) in the gas phase reaction. Pathways producing SiC2 and Si2C from SiCl4-C3H8-H2 system were proposed to investigate the effect of chlorinated species on HTCVD. A thermo-fluid analysis was carried out to estimate the partial pressures of the species. It was found that the main sublimed species of Si, SiC2, Si2C decreased in the SiCl4-C3H8-H2 system compared to the SiH4-C3H8-H2 system. This suggests that the growth rate would decrease in the atmosphere of chlorinated species at around 2500°C.

Abstract: High temperature chemical vapor deposition (HTCVD) simulations of silicon carbide (SiC) were demonstrated with experimental results. A vertical cylindrical reactor was used in an RF inductive heating furnace and the temperature was more than 2200
. SiH4 and C3H8 were used as source gases and H2 as carrier gas. A gas phase reaction model from the literature was used on the condition that the gas phase reaction is a quasi-equilibrium state. It was found that the major species were Si, Si2C, SiC2 and C2H2 in the gas phase reaction model as well as in the thermodynamic equilibrium calculation. Sublimation etching was considered in the surface reaction rates by modifying partial pressures of species with equilibrium vapor pressures. CFD-ACE+ and MALT2 software packages were used in the present calculation. The sticking coefficients were determined by fitting the calculated growth rates to the experimental ones. The simulated growth rate in a different reactor is in good agreement with the experimental value, using the same sticking coefficients. The present simulation could be useful to design a new reactor and to find optimum conditions.