Stable structures and electronic states of Si(100)/SiO2 interface are investigated using the first-principles molecular dynamics method. Quartz, tridymite, and pseudo beta-cristobalite are employed as the initial structures of the SiO2 at the interface to find the stable ones by the structural optimization. It is found that the optimized tridymite-type SiO2 structure on Si is the most stable for thin (about 7Å) SiO2 layer. For the thicker (about 15Å) layer, however, this structure becomes less stable and the optimized quartz-type SiO2 structure is the most stable. The band gap variation along the direction perpendicular to the interface is also investigated for the optimized structures. In the SiO2 region within 1Å from the structural interface, the band gap remains as narrow as that of silicon. The drastic change of the band gap takes place in the SiO2 region between 1 and 4Å.
