LACDIF, a new electron diffraction technique obtained with the LACBED configuration and a Cs corrector: Comparison with electron precession
Introduction
The electron precession technique which was proposed in 1994 by Vincent and Midgley [1] develops rapidly mainly due to the recent availability of commercial devices which can be implanted on most modern transmission electron microscopes. The precession patterns obtained with these equipments are spot patterns and they look similar to the spot patterns obtained with the conventional selected-area electron diffraction (SAED) or microdiffraction techniques. Nevertheless, these precession patterns have two main advantages with respect to the conventional ones:
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The intensity of the diffracted beams is the integrated intensity [1], [2], [3], [4], i.e. the summation of the diffracted intensity over a large range of deviation parameter s. As a result, a [u v w] zone axis pattern looks very well aligned even if the crystal zone axis is not exactly aligned with respect to the incident beam. This also means that the diffracted intensities can be trusted and used for ab-initio structure determinations [5], [6], [7], [8], [9], [10]. In fact, the precession method was proposed by Vincent and Midgley for this application. The pattern symmetries can also be surely inferred from these patterns [11] as well as the Debye–Waller factor [12].
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The precession patterns display more reflections in the zero-order Laue zone (ZOLZ) but also in the high-order Laue zones (HOLZ) than the conventional diffraction patterns and this number increases with the precession angle α. This property has proved to be very useful for the determination of the possible space groups of a crystal from microdiffraction patterns which involves the observation of the shifts and the periodicity differences between the reflections located in the ZOLZ with respect to the ones located in the HOLZs [11].
The patterns are also less dynamical and the two-beam conditions are mostly verified for every diffracted beam [13]. Provided the precession angle is very large (about 3°), the kinematical forbidden reflections can be identified [11].
The diffraction patterns are spot patterns and they look like conventional SAED or microdiffraction patterns. Therefore, their qualitative aspects (position of the spots on the patterns) can be interpreted by means of the classical methods. The interpretation of the integrated intensities requires dedicated software simulations [14], [15].
These interesting properties prove that the observation of the integrated intensity on spot patterns connected with a large number of reflections is particularly useful for many applications. With the development and the availability of spherical aberration correctors on recent transmission electron microscopes [16], another experimental method which fulfils these requirements becomes available. It is based on the large-angle convergent-beam electron diffraction (LACBED) configuration and it is described in the present paper.
Section snippets
Experimental methods to obtain spot patterns displaying the integrated intensity and a large number of reflections
This section describes the various experimental methods available to obtain spot patterns displaying simultaneously the integrated intensities and a large number of reflections.
Experimental conditions
Most of the experiments were performed on the SACTEM-Toulouse, a Tecnai F20 microscope (FEI) operating at 200 kV, fitted with an objective lens aberration corrector (CEOS) and an imaging filter (Gatan Tridiem). The nanoprobe mode was used in order to have a very small spot size in the range 1–10 nm and a large beam convergence semi-angle in the range 0.3–1.55°. This beam convergence was selected by means of the C2 condensor aperture. Some experiments were also performed on a Philips CM30
Experimental results
Fig. 6b shows a diffraction pattern observed in the object and image planes with a silicon specimen set along the [0 0 1] direction and illuminated by a large defocused incident beam having a convergence semi-angle of about 3°. Due to spherical aberrations connected with the large beam convergence each spot is actually surrounded by a large disk of confusion as schematised in Fig. 6a and the resulting pattern quality is very poor.
Two solutions are available to decrease or to suppress the disks of
Applications
Two applications of the LACDIF patterns are given in this section.
Comparison of the LACDIF technique with the precession method
Both LACDIF and precession patterns display a similar general aspect. They contain a large number of diffracted spots and this number increases with the beam convergence or with the precession angle. The intensity of the reflections is well equilibrated with respect to the transmitted spot.
The main difference is about the specimen illumination. In the precession method the incident beam is a hollow cone (Fig. 14b) while this cone is filled cone in the LACDIF technique (Fig. 14a). Therefore,
Conclusions
This study proves that it is possible to obtain high-quality spot patterns in image mode by using a defocused incident beam. This type of patterns becomes very useful when the incident beam convergence is large because many reflections are simultaneously visible whose intensity is integrated over a large range of deviation parameter. With large beam convergence, the quality of the LACDIF patterns is very poor on a conventional electron microscope due to the spherical aberrations. On a
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