dislocation analysis by means of the weak-beam dark-field

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Dislocation Analysis by means of the Weak-Beam Dark-Field (WBDF) Method 1. Introduction and Motivation: When analysing dislocations in a crystal, it can be most important to determine the type of each dislocation visible in a TEM image. For this purpose it is necessary to determine the Burgers vector of a dislocation in order to decide if the dislocation is a pure screw, pure edge or a mixed dislocation. The imaging of dislocations in a TEM is predicated on the diffraction contrast by exploiting the DF method under special conditions. By using the WBDF method it is possible to image the dislocation l ines and to determine the Burgers vector. In the following the WBDF technique is described. 2. Basics of the WBDF Method: In a WBDF image dislocations can be visualized as sharp bright lines, whereas the perfect parts of the specimen are not visible and appear dark. Thereto the sample and the incident beam must satisfy specific tilt conditions. The general exposure technique is shown in figure 1. In principle, the WBDF technique is an on-axis dark field imaging method by using a diffracted beam with large excitation error for the defect-free sample area. Thus, the defect- free sample area appears dark because of the weak diffraction intensity. However, close to the dislocation core the hkl plane is bended back into the Bragg condition, which gives rise to a bright intensity peak (the dislocation line). The main challenge is to adjust the til t conditions in the way that the excitation error of the g reflection used is close to zero only near the Figure 1: Path of rays in the TEM for the WBDF method using an edge dislocation as an example. For the defect-free area the incident beam is far away from the exact Bragg condition (large excitation error). In the area near the dislocation core (black mark in the specimen) the Bragg condition is exactly satisfied. This is caused by the bending of the hkl plane near the dislocation core. In the WBDF image the dislocation line appears as a bright line to dark background. Objective Ap ert ur e

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