AbstractsChemistry

We report first-principles electronic-structure calculations that clarify the floating nature of electron states in covalent semiconductors. It is found that wave functions of several conduction- and valence-band states, including the conduction-band minima, do not distribute near atomic sites, as was taken for granted, but float in interstitial channels in most semiconductors. The floating states have a nearly-free-electron(NFE)-like character, and extend in the channels broadly without atomic-orbital characters. The electrostatic potential at the channels and the directions and shapes of the interstitial channels depend on the crystal symmetry so that mysterious variation of the energy gaps in silicon carbide (SiC) polytypes is naturally explained by considering the floating nature. In addition, we have found that the floating states are closely related to the anisotropy in effective masses in SiC. It has been found that most conduction-band minima have floating nature. The existence of the floating state comes from the internal space in the crystal structures, and non-spherical charge distribution. The substantial band-gap variation in SiC has been analyzed by an empirical parameter "hexagonality" for a half century. Yet, we have clarified that the parameter "hexagonality" is a misleading parameter. Instead, we have found that a new parameter "channel length", which represents the spatial extension of the floating state, is essential in describing the band-gap variation in SiC. In addition, we have performed the linear-combination-of-atomic-orbitals (LCAO) calculations and compared the results with those calculated by the plane-wave-basis set. It is found that the floating characters in the electron states are difficult to be pursued in the LCAO calculations. We have also examined the floating states in pressurized sp3-bonded materials. We have also found that the energy bands with floating character manifest different behavior from other bands with atomic-orbital character under the pressurized circumstances. 報告番号: ; 学位授与日: 2013-03-25 ; 学位の種別: 博士 ; 学位の種類: 博士(工学) ; 学位記番号: ; 研究科・専攻: 工学系研究科物理工学専攻