Geometric and electronic structures of SiO₂/Si(001)interfaces

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 (˜7 Å), the lowest-energy structure is the tridymite, followed by the quartz phase. As the SiO₂ layer becomes thicker (˜15 Å), 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 SiO₂side, resulting in an effective decrease of the oxide thickness by 2–5 Å.