Monte Carlo simulation of low-energy electron trajectories and energy loss in ZnS phosphor powders

During electron beam irradiation of ZnS phosphor powders, a non-luminescent ZnO layer is formed on the powder due to electron-beam-stimulated surface reactions. As the thickness of the oxide layer increases, the energy loss in the ZnS bulk decreases with a subsequent degradation in cathodoluminescence. Using the Monte Carlo technique, the trajectories of low-energy electrons were simulated in a ZnS phosphor powder with a ZnO overlayer of varying thickness based on recent models describing the energy loss and scattering angles of low-energy electrons in a solid. A diffusion interface between the ZnO layer and ZnS bulk was simulated by varying the concentration of O and S atoms in the interface. Modelling the interface in this way describes the electron trajectories and energy loss in the interface region, because a sharp interlace between two dissimilar layers very seldom exists. In the energy-loss profiles the transition between ZnO and ZnS corresponds to a sharp increase in energy loss due to the increased rate of energy loss of electrons in ZnS. The diffusion interface has a smoothing effect on this sudden increase. From the electron trajectory data and corresponding energy loss, energy loss profiles were determined indicating the cumulative distribution of all the electron energy losses as a function of the interaction volume depth and thickness of the ZnO layer. When a distribution of incident angles is used, the profile differs from the typical energy-loss profile seen at normal incident angles. As the thickness of the ZnO layer increases, the total energy loss in the solid decreases due to the increase in the backscattering coefficient of electrons in ZnO. Copyright (C) 2001 John Wiley & Sons, Ltd.