Reactions of water molecules at the Si/SiO2 interface

Water molecules can be present to a varying degree at Si/SiO2 interfaces, either introduced by the oxidation process or through absorption from the atmosphere. Such water molecules may affect critically the electrical properties of metal-oxide-semiconductor (MOS) structures and devices. Here we present results of first-principles density-functional calculations of several pathways of water reactions near the Si/SiO2 interface. We found that (i) the presence of water molecules is energetically favored in the a-silica rings near the interface with an energy gain of $\sim $ 0.3 eV relative molecule in vacuum; (ii) a water molecule causes depassivation of Si-H bonds with formation of a Si dangling bond, H$_{2}$ molecule and loose OH complex; (iii) a water molecule may passivate a Si dangling bond and form a loose OH complex; (iv) a water molecule may oxidize the Si-Si bonds and form a H$_{2}$ molecule. The interplay of reactions results in changes in interface-trap density. We compare the results with radiation-response measurements of nMOSFETS right after production and after 18 years of aging and explain the observed growth of interface trap density (0.7x10$^{11}$cm$^{-2})$ just after X-ray irradiation. This work was supported in part by the AFOSR and the US Navy.