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

Basic rules are described to avoid the charging problem for studying bulk insulators 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 dynamically changing secondary electron emission. The possible methods of charge reduction are outlined and the main rules are set out with illustrative examples 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 sample tilt angle and (iv) reduce the electron beam energy. In order of increasing charging effect were the materials: MgO, Al2O3, Si3N4, NaCl and SiO2. This order, however, will depend on the precise form and purity of the relevant materials. For low doses, excessive charging was not observed for energies, 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 above 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 high charging caused by the electron beam incident on insulators. For high dose 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 below 0.55 of the low dose value for Si3N4. Crown copyright (C) 2000 Published by Elsevier Science B.V. All rights reserved.