CHARGING EFFECTS OF MGO UNDER ELECTRON-BOMBARDMENT AND NONOHMIC BEHAVIOR OF THE INDUCED SPECIMEN CURRENT

A MgO single crystal partly coated with a thin layer of Ag was irradiated with incident electrons while the time evolution of the surface potential and specimen current were recorded. A zero surface potential, V(s), is measured when the uncoated part of the specimen is irradiated, while the potential is very negative on the coated part for primary beam energies in the 2-5 keV range. The results invalidate the usual approach which uses a theoretical total yield curve for evaluating the sign of the surface potential as well as the use of an ohm law V(s) = RI(s) for correlating the surface potential V(s) and the specimen current I(s). Explanations of the observed phenomena are suggested and their correlation with the secondary electron emission of insulators is indicated. One surprising result is the possibility of injecting (nonohmic) current densities of 1 mA/mm2 or greater into insulators having electrical resistivities of 10(15)-OMEGA-cm.