AES of bulk insulators - control and characterisation of the surface charge

Basic rules are described to avoid the charging problem for studying bulk i nsulators in AES, for use by analysts. Charging is observed as both a near- instantaneous effect, governed by secondary electron emission, and also as a time-dependent effect, governed by the implanted charge and the dynamical ly changing secondary electron emission. The possible methods of charge red uction are outlined and the main rules are set out with illustrative exampl es of pure single crystals and of commercial sintered material of NaCl, MgO , SiO2, Al2O3 or Si3N4. The main rules are to (i) work quickly and avoid a build-up of charge, (ii) defocus the electron beam, (iii) increase the samp le tilt angle and (iv) reduce the electron beam energy. In order of increas ing charging effect were the materials: MgO, Al2O3, Si3N4, NaCl and SiO2. T his order, however, will depend on the precise form and purity of the relev ant materials. For low doses, excessive charging was not observed for energ ies, E-C (keV), below and angles of incidence, theta, above values given by E-C(0.6) cos theta equal to a constant, the value of which ranged from abo ve 4.0 for MgO to 1.85 for SiO2. In a plot in E-C, theta space for low dose studies, there is thus a zone of low charging and a zone of excessively hi gh charging caused by the electron beam incident on insulators. For high do se studies, the zone of excessively high charging expands with the constant value above reducing to below 0.88 of the low dose value for SiO2 and belo w 0.55 of the low dose value for Si3N4. Crown copyright (C) 2000 Published by Elsevier Science B.V. All rights reserved.