Slow Electrons Impinging on Dielectric Systems: II. Implantation Profiles, Mobility and Recombination Processes

When an insulator is subject to electron irradiation, a fraction of electrons is absorbed while the other one is backscattered. It is easily proved that injected electrons cannot be definitely trapped but they must instead recombine with positive charges left near the irradiated surface when secondary electrons are emitted; this is justified on the basis that dielectric breakdown is not observed during specific experiments of electron irradiation of insulators. The dynamics of the absorbed electrons depend on a number of parameters: the number of trapped electrons, the charge-space distribution, the mobility, and the number of secondary electrons emitted from the region near the surface of the dielectric. The time evolution of the surface electric has been studied by integration of the continuity equation for the relevant transport processes of the injected charge by adopting, as the charge source term, the distribution of the absorbed electrons as obtained by a Monte Carlo simulation. The image charge has been also introduced in the calculation in order to take into account the change in the dielectric constant when passing from the material to the vacuum. Selected computational results are reported to illustrate the role of the relevant parameters which control the charging effects in electron-irradiated insulators.