Charge dynamics of MgO single crystals subjected to KeV electron irradiation

Abstract

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 σ with a high precision (a few percent), as a function of energy, electron current density, and dose. The intrinsic secondary electron emission yield σ0 of uncharged MgO single crystals annealed at 1000 °C, 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) σ0 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 E2, for which the intrinsic SEE yield σ0=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 J⩽105 pA/cm2, the variation of σ with the injected dose makes possible the observation of a self-regulated regime characterized by a steady value of the SEE yield σst=1. At low energies 1.1 and 5 keV, there is no current density effects in MgO, but at high energies ≈30 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 °C. This phenomenon is the result of a disruptive process similar to flashover, which takes place at the surface of the material.

Determination of the electron beam irradiated area by using a charging effect in oxide surfaces

“…According to recent works [14] MgO is classified as an oxide in which the trapped charges remain localized. Then, the proposed experimental procedure is performed on a MgO 3 lm thick layer deposited on glass [17].…”

“…As a result, the achieved SEE yield r st is equal to one. At larger current density (10 6 6 J 6 10 10 pA/cm 2 ), the steady-state has a SEE yield r st = 1 ± e(J), in which e(J) is a few percent [14]. It is important to note that a sign inversion of the net trapped charge can occur, before the achievement of the steady-state, in the case of larger current density.…”

“…Indeed a larger surface potential can be the cause of the non-complementary of the recorded currents I r and I ab with regard to the incident beam current I p (I p 5 I r + I ab ), which limits the accuracy of the r measurement [14]. Such surface potentials are considered a source of spurious effect that hamper the performance of most spectroscopic methods such as electron microscopy, electron microprobe analysis and secondary ion mass spectrometry.…”

“…It is important to note that a sign inversion of the net trapped charge can occur, before the achievement of the steady-state, in the case of larger current density. The lack of control of the current density, J, in the majority of the experiments reported in the literature is the cause of the scatter in the data of the charging kinetics of insulating materials [14].…”

Thermodynamic investigation of an insulator irradiated by a low-energy electron beam

“…Below the first threshold value E c1 the primary particle energy is too low, thus the number of secondary electrons per incident electron reemitted by the surface is smaller than one, so the surface charges negatively, more or less to the beam energy. In between E c1 and E c2 , the second threshold energy, the yield of secondary electrons is greater than one and the surface charges positively [5,6]. Above E c2 , the surface charges negatively.…”