The Role of Water in the Radiation Response of Wet and Dry Oxides

We report results of first-principles calculations that explain why radiation-induced interface-trap density increases at a slower rate in wet versus dry oxides in the initial stages of interface-trap buildup. Similar effects are observed in MOS devices after exposure to elevated humidity and temperature or after non-hermetic aging. We explore the possible reactions responsible for passivation of dangling bonds near the Si/SiO₂ interface by water and elucidate a new energetically favorable path of the reaction. Prior to irradiation, the passivation of the dangling bonds happens either with formation of Si-H bonds and release of OH complexes, or with formation of Si-OH bonds and the release of H atoms. Both reactions have activation energies of ~ 0.9 eV and occur rarely at room temperatures. The OH complex is quite mobile, and can migrate along the interface and passivate dangling bonds via the formation of Si-OH bonds. After irradiation and release of protons, the Si-OH and SiH bonds may be formed with lower activation energy through passivation of a positively charged dangling bond by a water molecule. The presence of a hole at the dangling bond causes the resulting passivated SiOH bond and proton to have a lower energy than the initial water and dangling bond, and favors passivation of the dangling bonds. We discuss the relevance of the reactions for modeling the radiation response of wet and dry oxides, aging of MOS devices, and pre-irradiation elevated temperature stress.