Location of positive charge trapped near the Si-SiO2 interface at low temperature.

A new technique has been developed to obtain the depth profile of trapped positive charge which has been introduced into the SiO2 layer in metal-oxide-silicon structures at low temperature. We use photon-assisted tunneling at a fixed photon energy to inject electrons from the Si into the SiO2. Trapped positive charge located closer to the interface than the point at which the electrons emerge into the oxide conduction band is not affected, while that farther away can capture electrons. From the shifts of the capacitance versus voltage curves at various biases we can profile the location of the trapped positive charge. We demonstrate the use of this technique with a comparison of the positive charge introduced by x irradiation and Fowler–Nordheim tunneling (high-field stress). For the x-ray irradiated samples, the positive charge is located considerably farther (20–35 Å) from the Si-SiO2 interface than for high-field stressed samples (<20 Å). [ABSTRACT FROM AUTHOR]