Quantification of AES may be impaired by 3 effects of specimen crystallinity which result in peak intensities varying by a factor of 2 or more when examining single crystals, fracture surfaces or polycrystalline materials of grain size larger than the beam diameter. The first of these effects is anisotropy of Auger electron emission and the second and third the diffraction and channelling of the incident and emergent beam, respectively. The aim of the project was to isolate and examine these effects separately for a wide range of materials in order to determine their underlying mechanisms and relative contributions to peak intensity attenuation. With this knowledge procedures were developed to minimize the effects of crystallinity when performing quantitative AES as well as looking to see how the effects could be used to characterize materials.
The project demonstrated that the effects responsible for the crystalline contrast are too complex to be incorporated into generally applicable correction procedures being dependent both the specimen and the type of spectrometer used. Thus Auger electron diffraction is nearly always negligible in a cylindrical mirror analyzer but may be of comparable magnitude to channelling in concentric hemispherical analyzer. As a result the existence of crystallographic effects must be recognised in individual cases and strategies evolved to account for them. This is particularly true when, as in the measurement of the stoichiometry of a crystalline substrate, the relative intensities of all Auger peaks will be affected by diffraction. In some limited cases amorphization of the sample by ion bombardment was shown to be effective in removing the effects of crystallinity but care has to be taken to avoid preferential sputtering which itself may be orientation dependent. On the other hand, by using low beam energies and normalizing the signals from surface layers (e.g. segregant layers) to the N(E) background at a high energy, reliable surface coverage data can be obtained.
In order to understand the interaction of the various effects of crystallinity theoretical calculations were also undertaken. These confirmed the complex nature of the effects but gave insight into how they could both be controlled and used to characterize materials.
Funding SchemeCSC - Cost-sharing contracts
OX11 0RA Didcot