Influence of heat treatments and ionizing irradiations on the charge distribution and the number of surface states in the Si-SiO2system

The number of surface states present at an oxidized silicon surface is generally much larger when oxidation or further heat treatment has been carried out in a dry than in a wet environment. Especially at low temperatures (400-600°C) water very much tends to cause their disappearance. As hydrogen acts in a similar way, the effect seems to be due to a reaction of centers (probably unsaturated silicon bonds) with hydrogen, so that they are no longer able to capture holes or electrons. The hydrogen may also be evolved during the reaction of an Al-electrode on top of the oxide with hydroxyl or water. Irradiation of oxidized silicon by X-rays or ultraviolet light can cause charge redistributions, which may be accompanied by the formation of new centers. The latter effect is most pronounced when hydroxyl groups can be supposed to occur in the oxide. In these cases the irradiation may cause a disappearance of n-type inversion on p-type silicon, especially when X-ray irradiation is followed by ultraviolet (u. v.) illumination. Irradiation experiments have been used to detect differences in Si-SiO2systems in cases where direct electrical measurements failed.