Charging effects under electron beam injection on sapphire implanted with zirconium ions

During bombardment of a dielectric material with a high energy electron beam (few 10 kV), the electrons trapped in the material induce a high negative electric field which increases the surface potential and the secondary electron emission. So, in order to understand the space charge formation in insulators, it is necessary to characterize these charging effects which can modify the injection conditions (primary energy and spot size) and the trapping ability. Focused electron injection on sapphire implanted with zirconium ions was achieved in a SEM with an energy of 30 kV. The trapping of charges was followed during the electron injection by recording the absorbed current. After injection, the quantity of trapped charges was measured by observing the mirror "image" created at low potential and compared to the quantity of "image" charges. The trapping rate decreases rapidly when the quantity of electrons trapped in the material increases. This decreasing is related to the electric field due to the electrons already trapped in the material.