High-resolution transmission electron microscopy of an atomic structure at a Si(001) oxidation front

Direct observations of the atomic structure at a SiO2/Si interface have revealed that an epitaxial cristotbalite-like SiO2 layer is formed at a Si(001) oxidation front. We found that Si back-bond oxidation occurred in at least two atomic layers at the interface, forming cristobalite-like protrusions beneath the amorphous SiO2 layer. Each protrusion was about 1 nm in lateral width, and covered about half of the interface. We thus conclude that formation of the crystalline SiO2 at the interface is an essential process for layer-by-layer oxidation to occur. This is because further vertical oxidation would be suppressed at the front of the SiO2 protrusions, thus preserving the initial atomic-scale flatness of the interface during the oxidation.