Electronic structures of SiO2/Si(001) interfaces

SiO2/Si(001) interfacial structures are studied by using a first principles molecular dynamics method. Three kinds of SiO2 crystalline phases, namely, quartz, tridymite and pseudo-cristobalite are stacked on the Si(001) substrate and are fully relaxed. After structural optimization they formed geometrically abrupt interfaces, where no dangling bonds appear. The tridymite model is the most stable when the SiO2 layer is very thin (similar to 0.7 nm). But as the SiO2 layer grows thick, it becomes unstable and the quartz model becomes stable. The pseudo-cristobalite phase of SiO2 on the Si(001) is very unstable because of large lattice mismatch and transforms into a quite new crystalline phase. Analysis of the local density of states shows the bandgap width varies in the SiO2 side of 0.5nm thick, resulting in an effective decrease of the oxide thickness of 0.2 similar to 0.5nm. Effective electron masses in the SiO2 transition region are estimated to be 0.6 similar to 0.7m(c).