Slightly negative surface potential and charging model of insulator in the negative-ion implantation

In the negative-ion implantation into insulators, a slightly negative surface potential was obtained from the energy analysis of emitted electrons. Since emitted electrons offer an information of the surface potential, the surface potential was evaluated as an energy shift of the peak in the electron-energy distribution from the peak energy for a SiO2 (40 nm)/Si without charging effect. From the obtained results of surface potentials of many kinds of insulators during C- implantation as a function of the ion energy from 5 keV to 35 keV, it was found that the surface potential had a slightly negative value in a range from -2 to -12 V, and that the negative surface potential had a linear dependence on the ion velocity. This negative surface potential of insulators could not be explained by the charging model adopted for isolated electrodes. Therefore, taking account of the true electron-emission yield more than unity and of a quite low mobility in insulators, we have proposed a charging model of an electric double layer formed as a negative-charge layer at the surface and a positive-charge layer below the surface. The relationship between the surface potential and the true electron-emission yield, or the surface barrier are discussed.