Quantum chemical study on the oxidation of hydrogen-terminated silicon surface by oxygen anions

The oxidation mechanism of hydrogen-terminated Si(111) surface by oxygen anions has been studied using ab initio molecular orbital method. We have found that oxygen anions are stable in a cage of silicon clusters, which is a local model of the hydrogen-terminated Si(111) surface augmented by an extended model of periodic boundary condition. Oxygen anions can attack Si atoms to form a penta-coordinated complex, which will lower the diffusion barrier for oxygen anion transfer into the Si bulk. An oxygen anion is inserted into a Si-Si bond to form a local Si-O-Si structure. The stabilization energy is enhanced if the deformation energy in the local Si-O-Si structure is released. That the oxidized Si has a tendency to accept additional oxygens is concluded based on a series of cluster model calculations assuming that the crystallographic strain is completely relaxed.