Low-energy displacement damage and the effect on the AEM of Ceramics

Abstract

The observation that the signal from light elements such as carbon, oxygen, and nitrogen decreases in strength with time is to be expected for displacement in ceramic materials at high voltage, not only because the momentum transfer from an electron is more favorable, but also because the lattices are often full of defects. Even though the sensitivities of these materials can be below that of close parked metals at high voltage, during analytical electron microscopy the beam currents are sufficiently intense that the effect can be substantial. The loss rates for the signal are however quite predictable in the early stages and need not interfere with quantitative assessments of composition.An example of the loss of carbon signal from titanium carbide is shown in Figure 1 and a determination of the overall cross-section for carbon loss at 100 keV indicates a value close to 0.3 barns. This is about the cross-section for carbon displacement at 20 eV which is a conservative estimate of the binding energy for carbon in the lattice. Early experiments on vanadium carbide (Venables, 1969) indicate a similar overall cross-section however the threshold energy for the electrons to induce the damage is about only 5 eV. These results are consistent with the transfer of about 5 eV from an incident electron inducing a very small displacement of the carbon atom, of the order of an interatomic distance, with a damage cross-section of about 200 barns. Since the process needs to be repeated several times before the carbon atom is lost from the specimen the measured cross-section is proportionality reduced.