Practical Electron Microscopy and Database

In principle, for CBED the largest obtainable convergence angle is used to observe as much structure as possible within the CBED disks.

To draw an unambiguous conclusion about Burgers vectors, in many cases a combination of different techniques is applied. For instance, LACBED (large angle convergent beam electron diffraction) can provide more information on the properties of Burgers vectors. It had been suggested that when a LACBED Bragg line intersects a dislocation, displacement and splitting of the Bragg line may occur [1], from which the sign and magnitude of b can be obtained. [2 - 4] This method is called Cherns–Preston rules. These rules give the magnitude of b by g·b = m+1, where m is the number of subsidiary maxima in between the main peaks obtained in a dark-field image. For instance, Figure 1998 shows the dark-field contour of D1, taken by g = [0006], has m = 5 corresponding to g·b = 6. After applying the rules for the sign of b, [5] the Burgers vector is given by b = - c = [000-1] as listed in Table 3463. Note that the intensity and position of the subsidiary fringes vary with the dislocation character and depth in the TEM sample.

Figure 1998. (a) Bright field image and (b) Dark-field LACBED pattern
(g = [0006]) taken from dislocation D1 in page3463. Adapted from [6]

[1] D. Cherns and A. R. Preston, Proceedings of the 11th International Congress on Electron Microscopy (Japan Society of Electron Microscopy, Kyoto, 1986), p. 721.
[2] D. Cherns and J. P. Morniroli, Ultramicroscopy 53, 167 (1994).
[3] J. P. Morniroli, F. Strzelczyk, A. Redjaı¨mia, and D. Cherns, Proceedings of the 13th International Congress on Electron Microscopy, Les Ulis, Les Editions de Physique, Paris, 1994!, Vol. 1, p. 901; J. P. Morniroli and D. Cherns, Ultramicroscopy 62, 53 (1996).
[4] P. Cordier, J. P. Morniroli, and D. Cherns, Philos. Mag. A 72, 1421 (1995).
[5] P. B. Hirsch, A. Howie, R. B. Nicholson, D. W. Pashley, and M. J. Whelan, Electron Microscopy of Thin Crystals (Krieger, New York, 1977).
[6] F. A. Ponce, D. Cherns, W. T. Young, and J. W. Steeds, Characterization of dislocations in GaN by transmission electron diffraction and microscopy techniques, Appl. Phys. Lett. 69 (6), (1996) 770.