Photooxidation of hydrogen-terminated Si(111) surfaces studied by optical second harmonic generation

Photooxidation of hydrogen-terminated Si(111) surfaces in air has been studied by using optical second harmonic generation (SHG) in reflection from the silicon surface. The mechanism of photooxidation induced by a mercury pen lamp is shown to involve oxygen atoms produced by 185-nm photodissociation of O-2 in the gas phase above the Si(111)-H surface. Several fundamental wavelengths were used for SHG, including two-photon resonant excitations that are characteristic of H-terminated and oxidized Si(I 11) surfaces. By focusing on the anisotropic part of the SHG response, the specific response of intra-bilayer Si-Si bonds was isolated. A pronounced nonmonotonic variation of the SHG signal was observed during photooxidation and also during oxidation of Si(111)-H surfaces in air in the dark. A simple model is described that relates modulations in the SHG signal with progressive oxidation of distinct layers of Si-Si bonds near the surface. An analysis suggests that inter- as well as intra-bilayer bonds are attacked by oxygen atoms at an early stage during photooxidation.