44: Probing the critical oxide thickness for electron-mediated O2 adsorption on n-type silicon using second harmonic generation

Abstract: Second harmonic generation (SHG) was used to explore the role played by conduction band electrons in n-type silicon in promoting molecular O2 physisorption on samples covered with native and thermal oxide layers of varying thickness. In prior work, SHG intensity from n-type silicon was found to be enhanced by approximately 40–45% under O2 relative to N2 at ambient temperatures, while p-type and undoped silicon showed no such dependence. This behavior was attributed to conduction band electron backbonding into the π* orbitals of O2, enabling weak physisorption. When the oxide thickness was increased to approximately 600 nm, the SHG response of n-type silicon became independent of atmospheric composition, indicating that electron-mediated adsorption is suppressed when the oxide layer grows too thick. In the current work, we extend these findings by systematically investigating molecular adsorption on n-type silicon as a function of thermal oxide thickness. Using SHG as a surface-sensitive probe, we examine samples with oxide layers ranging from native thickness (~1–3 nm) up to several hundred nanometers. By comparing SHG responses under O2 and N2 environments, we identify how the Si surface becomes increasingly insulating as oxide thickness increases until, at last, the surface of n-type silicon behaves like that of p-type and undoped silicon. The results seek to establish the thickness-dependent limit of conduction band electron influence across insulating oxide layers and provide deeper insight into electron-mediated physisorption at semiconductor interfaces.