Mechanism of charge decay in UV-irradiated SiO2 electret films

The charge stability in thermally grown silicon dioxide layers during irradiation with monochromatic ultraviolet light was investigated. The samples were prepared by thermal oxidation of the Si wafer at 1000°C and charged by a constant-voltage corona method. A xenon arc lamp in combination with a monochromator was used to generate monoenergetic photons. The surface potential was measured in dependence on the photon energy, the irradiation intensity, and the charge polarity. During an irradiation period of several hours, the surface potential of negatively charged samples remained constant. In contrast, positively charged samples showed a charge decay which was accelerated by a reduction of the light wavelength as well as an increase of the irradiation intensity. A theoretical model for the interpretation of the experimental data is presented in the second part of this contribution