8. Indexing diffraction patterns
The automatic indexing of SAED diffraction patterns is activated by menu item Indexing-Spot Pattern (Alt-X + Ctrl-S). The indexing window is shown on figure 8.1. It contains a tool box with tools for printing,, saving,, loading an experimental SAED diffraction pattern,, to start the indexing search,, and to scale the experimental diffraction pattern. The plot area contains a base formed of a green cross inside a circle and 2 arrows, 0, and 1, that must be moved and adjusted on the experimental DP in order to measure the spacing to the origin of 2 spots (reflections) and their angle. The right of the window is occupied by 6 controls panels:
Calibration panel : to calibrate the SAED pattern.
Crystals panel : to load the crystal files that are being used for the auto indexing.
Mask panel : to overlay a mask on the SAED DP.
Microscope panel : to set the experimental camera length, accelerating voltage and experimental measurement errors.
Options panel : to set options.
Spots panel : to set the arrows length and angle (used instead of adjusting the arrows with the mouse or keyboard).
Figure 8.1 (Automatic indexing window)
The first adjustment consists of adjusting the experimental accelerating voltage and camera length. Figure 8.2 shows the "Microscope" panel:
Figure 8.2 (Microscope panel)
As the SAED pattern that will be used here for demonstration was recorded at 200 kV with a camera length of 653 mm, these values are entered. The experimental SAED pattern can now be loaded (). Figure 8.3 shows the indexing window after loading a pattern (gif format). The recording film was scanned with an unknown resolution. What is available is the distance on the film between the origin, transmitted beam and a reflection. In this case the distance was measured between the 2 closest spots to the origin: 12.5 mm.
Figure 8.3 (After loading the experimental SAED pattern)
It is now time to center the base and to adjust the arrows on some spots. To adjust the origin one selects the green cross, and drags it on the transmitted spot. The result is shown on figure 8.4.
Figure 8.4 (Centering the arrows origin)
The arrows are then centered on the spots by dragging the arrows. In order to increase the accuracy of the measurement one can adjust the arrows on higher order spots as shown in figure 8.5.
Figure 8.5 (After adjustment of the arrows)
As arrow 0 and 1 points to second order reflections, the spot order of them has been set on 2. The check box "Show mask" of the "Options" panel has also been checked to overlay a mask .
The last step before searching for a zone axis pattern of GaN, one has to calibrate the pattern. This step is indeed required. It is in this case fairly simple because the length of arrow 0 is known: it was measured on the film (12.5 mm). The calibration slider is adjusted on 12.5 and the unit is "mm". Then the calibrate tool, , is activated. It will allow the use of the "Calibrate" button placed near the calibration slider. Once pushed, a calibration data bar is drawn on the plot (figure 8.6).
Figure 8.6 (Calibrated SAED pattern)
The "Crystals" panel allows to load the crystal files that will be used by the auto indexing procedure to find possible zone axis (figure 8.7).
Figure 8.7 (Crystal files used by the auto indexing procedure)
One can then use the search tool,, to start the indexing procedure. jems starts a new indexing thread for each crystal file (figure 8.8). The stop button stops the thread.
Figure 8.8 Indexing thread using GaN crystal file
A little stereographic projection will show the zone axis found using the lattice parameters of each crystal files (figure 8.9). Moreover the length of the arrows and their angle are shown on the indexing window (see figure 8.7).
Figure 8.9 a,b,c and d (GaN, BeO, NiAs and Ceo2 possible indexing)
Possible zone axis have been found in the 3 hexagonal crystals. Clicking on one of the proposed solutions (for example [2,1,0] in GaN) will open the diffraction window with a plot of the [2,1,0] zone axis (figure 8.10). It is up to the user to qualify or disqualify the proposed solution.
Figure 8.10 (GaN [2,1,0] solution)
The avalanche panel shows all the solutions for a particular crystal. The "Fire" button will plot these solutions in separate windows (figure 8.11).
Figure 8.11 (Solution of the indexation)
When there is no solution, one can first increase the measurement errors adjusted on the "Microscope" panel. Then it is advisable to check carefully the calibration, the accelerating voltage and the camera length.
8.2 Kikuchi patterns
The indexing of Kikuchi patterns follows the same principles. Figure 8.12 shows the calculated Kikuchi pattern that will be used for checking the indexing.The beam has indices close to [7, 9, 24].
Figure 8.12 Kikuchi pattern (simulation)
The Kikuchi indexing window is shown on figure 8.13. The first actions to take are to set the camera length, the accelerating voltage and the scale. The distance between the two red lines gives the scale, here 30 nm-1.
Figure 8.13 The Kikuchi indexing window
One next adjusts the position of the center (yellow cross inside the yellow circle). Then the lines are positioned. They are dragged and rotated using the square, cross and round hot spots. Figure 8.14 shows the adjusted Kikuchi bands.
Figure 8.14 Adjusted Kikuchi bands
An indexing thread is launched for each crystal file. The possible beam directions are shown on a little stereogram (figure 8.15).
Figure 8.15 Indexing of the Kikuchi pattern
Selecting one of the proposed directions opens a diffraction pattern (figure 8.16).
The indexing is given as a direction, here [1,1,4] and the indices of the Laue circle center (4.727, 16.883, -5.403).