STUDY OF THE SPACE-CHARGE INDUCED BY AN ELECTRON-BEAM IN AN INSULATING TARGET .1. MONTE-CARLO SIMULATION-MODEL

A. model for the building up of space charge in an insulating target u nder electron bombardment is proposed. Electron-hole pairs are created during inelastic collisions with the valence band and with the inner shells. The elastic effects are due to interactions with the ionic cor e potentials. Quasielastic contributions come from the excitation of o ptical and acoustic phonon modes. When its energy becomes sufficiently low, a charge carrier can polarize its surroundings. The formation of a polaron leads to the self-localization of the carrier (electron or hole). The polarons can diffuse in the target under the influence of t he internal field due to the space charge. Along their path, they can encounter polaronic traps. In these sites, the polaron can either be s tabilized or recombine. All these effects are taken into account in a Monte Carlo simulation model. The internal field is deduced self-consi stently from the space charge by solving the corresponding Poisson equ ation.