Stable structures and electronic states of Si(100)/SiO
2 interface are investigated using the first-principles molecular dynamics method. Quartz, tridymite, and pseudo beta-cristobalite are employed as the initial structures of the SiO
2 at the interface to find the stable ones by the structural optimization. It is found that the optimized tridymite-type SiO
2 structure on Si is the most stable for thin (about 7Å) SiO
2 layer. For the thicker (about 15Å) layer, however, this structure becomes less stable and the optimized quartz-type SiO
2 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 SiO
2 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 SiO
2 region between 1 and 4Å.
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