Geometric and electronic structures of SiO2/Si(001) interfaces -: art. no. 115314

Interface structures of SiO2/Si(001) are studied by using the first-principles molecular-dynamics method. Three crystalline phases of the SiO2, cristobalite, quartz, and tridymite, are stacked on the Si(001) substrate and are fully relaxed. When the SiO2 layer is very thin (similar to7 Angstrom), the lowest-energy structure is the tridymite, followed by the quartz phase. As the SiO2 layer becomes thicker (similar to 15 Angstrom), the quartz phase has lower energy than the tridymite phase. The cristobalite phase on Si(001) is unstable due to large lattice mismatch, and transforms into a different crystal structure. No defects appear at the interface after the successive bond breaking and rebonding, but the energy of the resulting structure is the highest irrespective of the thickness. Calculations of the local density of states show that the band-gap change occurs on the SiO2 side, resulting in an effective decrease of the oxide thickness by 2-5 Angstrom.