Papers by Author: Toshiharu Ohnuma

Abstract: We perform dynamical simulations of dry oxidation and NO annealing of the SiO2/4H-SiC C-face interface at 1500K using first-principles molecular dynamics based on plane waves, supercells, and the projector-augmented wave method. The slab model is used for the simulation. In the dry oxidation simulation, O atoms oxidize not only the C atoms at the SiC interface but also second-atomic-layer Si atoms in the SiC layer. Bilayer oxidation occurs in the oxidation process. The formation of C clusters that grow in the c-axis direction is observed. In the simulation of NO annealing, N atoms passivate interface C atoms. The density of N atoms saturates, then N atoms desorb as N2 molecules. CN molecules are formed by the abstraction of C atoms by the N atoms, and the CN molecules readily react at the interface. The formation of a Si3N structure is also observed.

Abstract: We perform a dynamical simulation of the SiO2/4H-SiC C-face interface oxidation process at 2500K using first-principles molecular dynamics based on plane waves, supercells, and the projector-augmented wave method. The slab model is used for the simulation. Oxygen molecules are dissociated in the SiO2 layers or by Si atoms at the SiO2 interface. The O atoms of the O2 molecule oxidize the C atoms at the SiC interface and form Si-C-O or CO2-C complexes. COx (x=1 or 2) molecules are desorbed from these complexes by thermal motion. COx molecules diffuse in the SiO2 layers when they do not react with dangling bonds. COx molecule formed during C-face oxidation more easily diffuse than those formed during Si-face oxidation in the interface region.

Abstract: To clarify the origin of a characteristic fine grain structure formed under the high burn-up of the nuclear fuel, the comprehensive first-principles calculations for UO2 containing various types of point defect have been performed by the PAW-GGA+U with lattice relaxation for supercells containing 1, 2 and 8 unit cells of UO2. The electronic structure, the atomic displacement and the defect formation energies of defective systems are obtained, and the effects of supercell size on these properties are discussed. Based on this relatively high precise self-consistent formation energies dataset, thermodynamic properties of various types of point defects in UO2 are further investigated in the framework of the point defects model.

Abstract: We performed the dynamical simulation of the SiO2/4H-SiC(0001) interface oxidation process using first-principles molecular dynamics based on plane waves, supercells, and the projector augmented wave method. The slab model has been used for the simulation. The heat-and-cool method is used to prepare the initial interface structure. In this initial interface structure, there is no transition oxide layer or dangling bond at the SiO2/SiC interface. As the trigger of the oxidation process, the carbon vacancy is introduced in the SiC layer near the interface. The oxygen molecules are added one by one to the empty sphere in the SiO2 layer near the interface in the simulation of the oxidation process. The molecular dynamics simulation is carried out at 2500 K. The oxygen molecule is dissociated and forms bonds with the Si atom in the SiO2 layer. The atoms of Si in the SiC layer at the SiO2/4H-SiC(0001) interface are oxidized to form the SiO2 layer. Carbon clusters, which are considered one of the candidate structures of the interface traps, are formed in the interface layer. Oxygen molecules react with the carbon clusters and formed CO molecules.

Abstract: The performance of SiC MOSFET devices to date is below theoretically expected performance levels. This is widely considered to be attributed to defect at the SiO2/SiC interface that degrade the electrical performance of the device. To analyze the relationship between defect structures near the interface and electrical performances, advanced computer simulations were performed. A slab model using 444 atoms for an amorphous oxide layer on a 4H-SiC (0001) substrate was made by using first-principles molecular dynamic simulation code optimized for the Earth-Simulator. Simulated heating and rapid quenching was performed for the slab model in order to obtain a more realistic structure and electronic geometry of a-SiO2/4H-SiC interface. The heating temperature, the heating time and the speed of rapid quenching were 4000 K, 3.0 ps and -1000 K/ps, respectively. The interatomic distance and the bond angles of SiO2 layers after the calculation are agree well with the most probable values of bulk a-SiO2 layers, and no coordination defects were observed in the neighborhood of SiC substrate.

Abstract: First-principles calculations for the abrupt SiO2/4H-SiC interfaces accounting for Si-Si bonding and Nitrogen atom termination have been performed. Interface states due to Si-Si bonds appear at the valence band edge. Interface states at the midgap vanish when N atom terminates the Si dangling bond, but the interface states arising from the Si-N bonds appear at the valence band edge at the same time.