3.3 Synchrotron White beam X-ray diraction
Figure 3.6: Schematic illustration of the dierent geometries that are commonly used in XRT: a) transmission geometry, b) back-re ection geometry, c)re ection geometry, grazing incidence geometry.
provided the sample has a good crystal quality, a transmitted image can be recorded through several millimeters in the material. The information that can be obtained by the XDT include: the distribution of dislocations (most common defect), the distribution of lattice strains, sub-grain structure (misorientation angles), the presence of secondary phases and twins. No information can be obtained about the 0D defects (point defects). When a white (polychromatic) X-ray beam is impinging the surface of a crystal, many diracted spots appear on the X-ray lm. The use of a white beam is particularly simple, in fact it's not necessary to adjust the sample position. Many re ections will ful ll the re ection condition at the same time. The position of the spots follows the Bragg equation:
λ = 2dsinθB (3.13)
where λ is the incident wavelength and θB is the Bragg angle. This 2D diraction spot constitutes the X-ray topography image. As it was mentioned before, the understanding of the topographic image formation arise from the contrast. A detailed analysis of the contrast can be found in many available resources, Tanner (1975), Bowen and Tanner (1998) or Kato (1996). Two cat-egories of contrast are distinguished:
• Orientation contrast. Orientation contrast originates when parts of the sample have dierent orientation, so that they don't satisfy the Bragg law and therefore they are not diracted. The zero intensity area correspond to the geometrically misoriented part of the sample. For a white
radi-ation source however dierent wavelengths are diracted in dierently oriented regions leading to a displacement of their diracted image in a dierent part of the lm. The orientation contrast arises with two main mechanisms: the misorientation exceeds the divergence of the beam, re-sulting in a lack of diraction, or the diracted beam from two parts of the crystal takes dierent directions in the space. In gure 3.7 the two dierent mechanisms for a monochromatic and a continuous radiation are shown. For a monochromatic beam, as explained in example a) the misorientation exceeds the divergence of the incident X-ray beam, giv-ing a zero intensity zone. In the second case, b), is shown a diagram in the case of a continuous radiation, in this case there is a gain or loss of intensity in the boundary of the misoriented region.
Figure 3.7: The scheme proposed by Tanner [28] for the explanation of the orienta-tion contrast. In a) is shown the behavior of a monochromatic beam, in b) the case of a white beam
• Extinction contrast. This contrast origins from a dierence in scattering power near the defect with respect of that of the perfect crystal. Extinc-tion contrast is described by means of kinematical or dynamical theories of X-ray diraction. There are three types of contrast, that are schemat-ically represented in gure 3.8 by Tanner. The image is divided in two cases: a large strain gradient ( gure a)) or a small strain gradient ( gure b)). For a speci c wavelength the re ecting range of a perfect crystal is very small, the peak re ectivity is close to 100% and the scattering power is proportional to the structure amplitude of the Bragg re ection
|F |. For an imperfect part of the crystal the re ecting range is broader, the re ectivity is less than 100% but the integrated re ectivity is pro-portional to |F |2. There are three kind of extinction contrast, in gure 3.8 a) it's shown the mechanism of their formation. The direct image, shown in the picture as 1, is the result of the diraction of x-rays that don't satisfy the Bragg condition in the perfect crystal. The formation of
Figure 3.8: Extinction contrast, image by Tanner [28]. Two cases are studied: a) when the strain eld gradient is high, and b) when the strain eld gra-dient is low
a direct image is the most common mechanism [28] and it's characterized by an enhanced intensity that appear darker with respect to the perfect crystal. The dynamical image, indicated as 2, is formed by the change of intensity in the Bloch wave elds propagating through the perfect crystal, it is most commonly seen in high absorption conditions. The intermedi-ary image, number 3, is formed by the interference by those wave elds and the new wave elds originating at the defect, it is seen in conditions of moderate absorption and small inclination of the defects. It provides informations on the structure factor.
3.3.1 CZT White Beam X-ray Diraction Topography
CZT ingots are commonly aected by the presence of defects. Some of them have a severe in uence on the detector performances, and thus it's necessary to nd a way to remove them. Other defects don't aect the properties and detector response in such a heavy way and thus can be tolerated, provided that the concentration is low. The most common defects in CZT ingots can be seen by the white beam X-ray diraction topography (WXDT) analyzing the orientation and extinction contrast of the images. WXDT is an extremely useful tool for the characterization of the devices. The material is cut in slices or slabs and then the samples are obtained by cutting the wafer into parts of the suited dimensions. The selection of a good or bad part on a wafer is crucial for the device properties. Some defects can be seen by naked eye and then avoided during the cutting process, like twins and grains with dierent orientation. Other defects are instead impossible to be detected without a topography image. A WXDT image can give information on
Figure 3.9: Grain boundaries in a CZT sample
• 1 dimensional defects: such as dislocations (that are also the most com-mon defect)
• 2 dimensional defects: grain boundaries, twin boundaries. It's important to underline that this kind of defect can be easily detected by watching the surface only if the misorientation is high. For low angles the topog-raphy image is necessary
• 3 dimensional defects: inclusions and precipitates. Point defects are not visible in WXDT, but when they cluster to form a precipitate or inclusion they give a characteristic contrast image. They can appear dark or bright depending on the absorption condition of the measurement and on the position of the precipitates (close or far from the entrance surface). Those defects are also very common in CZT ingots
No information is obtained on point defects (0 dimensional defects). From the WXDT a lot of information can be extracted, to determine the crystalline quality of a CZT ingot. Grain boundaries and twins are well known defects aecting the growth of CZT, despite all the eorts performed for limiting the presence of such defects, so far it is still impossible to obtain an entire ingot completely free of defects. In gure 3.9 grain boundaries are present.
As expected from the orientation contrast theory, the region with a dierent orientation appears white in the image, being the result of a lack of diraction.
A clear eect of the extinction contrast in a topography image is present in
gure 3.10 The dark lines are a network of dislocations.
3.3.2 White Beam X-ray Diraction Topography set up
Figure 3.10: Dislocation walls, WXDT on a CZT sample
Figure 3.11: Side picture of the WXDT beamline hutch, the letter A indicates the sample holder, the letter B indicates the position of the X-ray lm. The beam is impinging from the right side of the image on the right
The WXDT measurement were performed in the Brookhaven National Lab-oratory (BNL), Upton, NY. The National Synchrotron Light Source (NSLS) is a research facility in BNL, the accelerator is producing synchrotron light from the UV range up to the X-ray range (100KeV). The beamline equipped for the WXDT has a spectral range up to 35 KeV, the beam is white, in order to avoid the sample alignment necessary when the beam is monochromatic. The source size is small: 300µm x 100µm.
The hutch is quite large, so it's possible to have a distance of 25 m between the source and the specimen. All the measurement were done in re ection geometry. The beam is impinging the surface of the sample and then is re ected on a X-ray sensitive lm. The sample size can vary between few mm and 4 cm.
The sample size is basically limited only by a lead slit (it is present in gure 3.11) that is positioned before the sample, the purpose of the slit is to avoid uncontrolled re ections by the objects present in the hutch. In gure 3.11 are shown two pictures of the WXDT set up. The rst picture is a side image:
the beam is impinging from the left side. In the picture are shown the sample holder and the X-ray lm holder. The second image shows the direction of the beam, the sample position and the lm holder. The beam needs to be shaped in order to have almost the sample size, this is necessary to obtain a better contrast in the spots.