Charge dynamics of MgO single crystals subjected to KeV electron irradiation

A scanning electron microscope has been equipped to study the fundamental aspects of charge trapping in insulating materials, by measuring the secondary electron emission (SEE) yield sigma with a high precision (a few percent), as a function of energy, electron current density, and dose. The intrinsic secondary electron emission yield sigma(0) of uncharged MgO single crystals annealed at 1000 degreesC, 2 h, has been studied at four energies 1.1, 5, 15, and 30 keV on three different crystal orientations (100), (110), and (111). At low energies (1.1 and 5 keV) sigma(0) depends on the crystalline orientation wheras at high energies (30 keV) no differentiation occurs. It is shown that the value of the second crossover energy E-2, for which the intrinsic SEE yield sigma(0)=1, is extremely delicate to measure with precision. It is about 15 keV+/-500 eV for the (100) orientation, 13.5 keV+/-500 eV for the (110), and 18.5 keV+/-500 eV for the (111) one. At low current density Jless than or equal to10(5) pA/cm(2), the variation of sigma with the injected dose makes possible the observation of a self-regulated regime characterized by a steady value of the SEE yield sigma(st)=1. At low energies 1.1 and 5 keV, there is no current density effects in MgO, but at high energies approximate to30 keV, apparent current density effects come from a bad collect of secondary electrons, due to very high negative surface potential. At 30 keV energy, an intense erratic electron exoemission was observed on the MgO (110) orientation annealed at 1500 degreesC. This phenomenon is the result of a disruptive process similar to flashover, which takes place at the surface of the material. (C) 2004 American Institute of Physics.