Electric fields at the surface and interface of SiO 2 films on silicon

Abstract

At solid surfaces and interfaces the potential energy of electrons has a sharp discontinuity with respect to the adjacent bulk phases. This change of potential give rise to parallel opposing layers of charged defects or ions within insulating or semiconducting solids. The concentration of charge and the electric field depend not only on the interfacial potential drop, but also on the potential and concentration of electron donor or acceptor sites in the interfacial region. At the surface of thin silicon oxide films grown on silicon, oxide ions provide a layer of negative charge; an equal positive charge (usually sodium ions) is concentrated in the oxide within about 200 Å of the surface. A similar double layer develops at the interface with silicon, where electrons donated to the silicon provide a negative charge having an adjacent positive charge layer in the oxide. Both positive charge layers exhibit a Poisson-Boltzmann distribution, with a potential drop at the surface always about three times greater than at the interface. Sodium oxide impurities are the dominant electron-donors; these supply the electrons for the negative surface and interfacial layers (as well as for any competing electron-acceptor sites). A high density of acceptor sites can be introduced into silicon dioxide by conditions tending to produce oxygen vacancies, and when the electrons from sodium oxide donors are trapped by such acceptor sites the sodium ions are localized near the trapped electrons. Under these conditions few electrons and sodium ions reach the surface or interface. Since a high surface density of oxide ions requires a high concentration of sodium ions, it is not surprising that the rate of oxidation of silicon increases with sodium content.