Effect of electron beam parameters on CBED patterns from interfaces

Convergent beam electron diffraction (CBED) at vertical grain boundaries (parallel to the electron beam) can be used to determine the symmetry of bicrystals. It can also be used to investigate the structure of the boundary region itself when subnanometer probe sizes are employed. In this paper, the influence of the electron-beam geometry on the CBED pattern is discussed in terms of the defocus distance between the probe position and the specimen midplane, D (set by the second condenser lens), the probe size, the beam-convergence angle, and the microscope mode (micro- or nanoprobe). Theoretically expected CBED patterns for an electron probe on a vertical grain boundary are observed up to a certain maximum defocus |Dc|, which is shown to increase linearly with probe size and specimen thickness. The experimental uncertainty in positioning the probe in the specimen midplane is independent of the probe size but increases linearly with specimen thickness. A comparison of this uncertainty in |D| with |Dc| shows that for decreasing probe sizes it is increasingly difficult to obtain the theoretically expected CBED pattern from a vertical boundary. The effect of a defocused probe (|D|>|Dc| up to |D|≫|Dc|) upon the CBED pattern is discussed and experimentally illustrated for two electron beam directions parallel to a vertical twin boundary in Si.