Medium-energy backscattered electron diffraction as a probe of elastic strain in epitaxial overlayers

We show that, for certain experimental geometries, the angular dependence of medium-energy backscattered primary electrons from Ni(001) and pseudomorphic Cu on Ni(001) is very similar to that observed in high-energy LMM Auger emission. Both experiments reveal strong forward-scattering induced maxima along low-index directions which are sensitive to the tetragonal distortion accompanying pseudomorphism. The measured displacement of the diffraction feature along [011] in polar-angle scans is the same within experimental error in both measurements. Comparison with theoretical angular distributions obtained by kinematical scattering theory indicates an overlayer lattice constant perpendicular to the interface of 3.75±{}0.02 A{}, in excellent agreement with the value obtained by Auger electron diffraction (3.71±{}0.03 A{}). Significantly, medium-energy backscattered electron diffraction should prove to be a powerful tool for overlayer structural characterization because the technique is not limited to elements which emit a high-energy Auger electron of sufficient intensity to be useful as a diffraction probe.