Effect of electron beam parameters on CBED patterns from interfaces

Convergent beam electron diffraction (CBED) at vertical grain boundaries (p arallel to the electron beam) can be used to determine the symmetry of bicr ystals. It can also be used to investigate the structure of the boundary re gion 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-co nvergence angle, and the microscope mode (micro- or nanoprobe). Theoretical ly expected CBED patterns for an electron probe on a vertical grain boundar y are observed up to a certain maximum defocus \D-c\, which is shown to inc rease linearly with probe size and specimen thickness. The experimental unc ertainty in positioning the probe in the specimen midplane is independent o f the probe size but increases linearly with specimen thickness. A comparis on of this uncertainty in \D\ with \D-c\ shows that for decreasing probe si zes it is increasingly difficult to obtain the theoretically expected CBED pattern from a vertical boundary. The effect of a defocused probe (\D\ > \D -c\ up to \D\ >> \D-c\) upon the CBED pattern is discussed and experimental ly illustrated for two electron beam directions parallel to a vertical twin boundary in Si. (C) 1998 Published by Elsevier Science B.V. All rights res erved.