Electron diffraction from epitaxial crystals—a convergent-beam electron diffraction of the interface structure for NiSi₂/Si and Al/GaAs

Abstract

Electron diffraction from a plan-view specimen of a coherent epitaxial bicrystal is studied both theoretically (in a Bloch wave approach using a coincident site lattice) and experimentally (using convergent-beam electron diffraction (CBED)). CBED pattern symmetries can be used to find the symmetry of the bicrystal, thus limiting the possible displacements of the two crystals. Numerical calculations are made to show that these effects should be highly sensitive to different shifts of the two crystals. Theory also predicts that features in the higher-order Laue zone (HOLZ) rings can then be used to determine this rigid-body shift. Experiments on the NiSi2/Si(001) system show both the symmetry and the HOLZ features predicted for the previously determined ‘tetrahedrally coordinated’ interface structure. The technique is then applied to molecular-beam-epitaxy-grown Al/GaAs(001). The symmetry of the patterns indicates a relative displacement of high-symmetry form; examination of the HOLZ, however, indicates a lower-symmetry structure. The apparent contradiction can be explained in terms of a high-symmetry structure (with a 1/4[100] (GaAs) shift between the two crystals) with distortions due to misfit dislocations. The structure deduced is consistent with the arrays of misfit dislocations observed, which relate symmetry-equivalent sites for this interface structure.