Jun Hasegawa

Computational Evaluation of Electrical Conductivity on SiC and the Influence of Crystal Defects

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.

Simulation Study for HTCVD of SiC Using First-Principles Calculation and Thermo-Fluid Analysis

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.

Density Functional Calculation on the Structure of Cordierite

Density functional theory calculations were performed to analyze atomic and electronic structures of cordierite. The most stable configuration of Al and Si atoms in the six-membered ring of a unit cell was decided by geometry optimizations for sixteen different initial structures. A correlation was observed between the number of Al-O-Al bonds in a unit cell and the lattice energy. We concluded that the number of Al-O-Al bonds is a determining factor of the lattice stability. The structural parameters of the most stable crystal structure obtained by geometry optimization agreed well with the experimental observation. According to the distribution of atomic charges and analysis of bond order, the followings were clarified: (i) the covalent bond in a T1 tetrahedron was stronger than that in a T2 tetrahedron, (ii) the covalent bond of a cation in six-membered ring with O atoms in a T1 tetrahedron was the weakest, and (iii) Mg in the cordierite is of ionic nature.

Multi-level Simulation Study of Crystal Growth and Defect Formation Processes in SiC

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.