Surface Charging of Insulators by Ion Irradiation

The charging of insulating surfaces by irradiation with gaseous ions is discussed from the viewpoint of basic interactions between individual ions and surfaces. Experiments are reported in which mass‐selected beams of low‐energy positive and negative ions are used to irradiate insulating amorphous selenium films. The surface‐charging efficiency (the number of charges retained on the surface per incident ion) is found to be initially equal to 1, independent of the ion species and ion kinetic energy. As charge accumulates on the surface, the charging efficiency decreases, the extent of decrease being a function of species and kinetic energy. The results are shown to be consistent with a model in which the ion injects a hole or electron into the solid and the injected carrier is trapped in a surface state. The deviation of the charging efficiency from unity arises as a result of free carrier generation in the solid by means of a transfer of kinetic and/or potential energy from the neutralized ion. Implicatio...