Selected area diffraction (SAD) technique in TEM is performed with a large parallel incident beam. In this case, an aperture (called selected area aperture) is introduced into the image plane of the objective lens to select an area so that we can limit the region which generates the diffraction pattern. And then, by switching to diffraction mode, the focus of the projection lenses can be changed in order to focus on the back focal plane of the objective lens.
Defocus CBED using high-index reflections provides a more accurate determination method of displacement vectors (R) of stacking faults than two-beam method with exciting low-index reflections, selected area electron diffraction method with low index reflections, and HRTEM technique.
On the other hand, unlike selected-area diffraction (SAD) patterns, CBED ZAPs contain three-dimensional (3D) information, due to the enhanced HOLZ signatures.
Figure 4492 illustrates the comparison of the main contrasts in both CTEM and STEM modes.
Figure 4492. Comparison of the main contrasts in both CTEM and STEM modes.
Note that, in general, greater precision in SAD can be obtained if the accelerating voltage of the electron beam is higher.
Table 4492. Examples of SAD applications.
||Page on the EM book
|Determination of space groups