**1. Introduction**

[23] Karmann, S., Suttrop, W., Schöner, A., Schadt, M., Haberstroh, C., Engelbrecht, F., Helbig, R., Pensl, G., Stein, R., & Leibenzedar, S. (1992). Chemical vapor deposition and characterization of undoped and nitrogen-doped single crystalline 6H-SiC. *J.*

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[25] Iwata, H., Itoh, K. M., & Pensl, G. (2000). Theory of the anisotropy of the electron

[26] %\*+/\$%0\_z^\_z 0+\$\_z^z
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[27] Volm, D., Meyer, B. K., Hofman, D. M., Chen, W. M., Son, N. T., Persson, C., Lindefelt, U.,

nation of the electron effective-mass tensor in 4H SiC. *Phys. Rev. B*, 53, 15409-15412. [28] Kojima, K., Suzuki, T., Kuroda, S., Nishio, J., & Arai, K. (2003). Epitaxial Growth of %#\$w1(%05zGw%z.+\*w!z5z+3w.!//1.!z+0w((z\$!)%(z,+.z!,¥

[29] Rutsch, G., Devaty, R. P., Choyke, W. J., Langer, D. W., & Rowland, L. B. (1998). Measurement of the Hall scattering factor in 4H and 6H SiC epilayers from 40 to 290

[30] Götz, W., Schöner, A., Pensl, G., Suttrop, W., Choyke, W. J., Stein, R., & Leibenzeder, S. (1993). Nitrogen donors in 4H-silicon carbide. *J. Appl. Phys.*, 73, 3332-3338.

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[35] Senzaki, J., Fukuda, K., & Arai, K. (2003). Influences of postimplantation annealing conditions on resistance loweing in high-phosphorus-implanted 4H-SiC. *J. Appl.*

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\*6!\*\_z^zcDLLId^z!0!.)%¥

Hall mobility in n-type 4H- and 6H-SiC. *J. Appl. Phys.*, 88, 1956-1961.

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K and in magnetic fields up to 9 T. *J. Appl. Phys.*, 84, 2062-2064.

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26 Physics and Technology of Silicon Carbide Devices

*Appl. Phys.*, 89, 6228-6234.

Optimization of Silicon Carbide (SiC) bulk growth by physical vapor transport (PVT) has allowed commercial realization of SiC wafers with low defect density. Dislocated micropipes (MPs), known as the most killing defects in SiC devices, have been reduced to the density levels as low as 0.7–0.5 cm<sup>−</sup>2. However applications of SiC as a material for high-power electronics are still hampered. High-performance high-power electric systems require SiC devices capable of handling the current capacity of at least 100 A, which means that the MP density of SiC wafer should be less than 0.5 cm−<sup>2</sup> [16]. Fabrication of such high quality wafers, in particular, in low cost is indispensable for manufacturing of high-current devices.

Various methods such as the enhancement of polytype stability [26, 31], the reduction of screw dislocations [22], the restriction of inclusions [23], *etc.*, help control MP density. Polytype transformation, dislocations, and MPs, and their interaction or conversion are complicated phenomena which are of particular interest to SiC crystal growers and device engineers. Dislocations are largely induced by foreign polytype inclusions (FPIs) [27], but disappear when the inclusions transform back to the initial polytype [26]. The improvement of the polytype stability immediately and dramatically decreases the MP densities [29]. When accumulated at the boundaries of FPIs, MPs may coalesce each other, forming pores [10, 11, 13].

Nowadays, dislocation processes in SiC single crystals are studied by means of Synchrotron Radiation (SR) X-ray topography [24, 25, 33] combined with defect selective etching [30] or computer simulation of Bragg-diffraction images [3, 18]. A very good methodology to study hollow defects in SiC — to map them and to evaluate their sizes — is phase contrast X-ray imaging [19], which is available due to high spatial coherency of third generation SR sources.

Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2013 Argunova et al.; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2013 Argunova, et al., licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

©2012 Je et al., licensee InTech. This is an open access chapter distributed under the terms of the Creative

sample, respectively. Therefore, even 'white' SR is partially coherent, which is quite sufficient

Characterization of Defects Evolution in Bulk SiC by Synchrotron X-Ray Imaging

http://dx.doi.org/10.5772/52058

29

White rather than monochromatic beam gives higher intensity, larger exposed area, the use of various research methods during a single experiment, but has the attendant disadvantage that obtained information is mostly qualitative. The task of obtaining reliable information can be solved by developing numerical simulation of phase-contrast images. Such an approach was recently proposed [1, 19] based on Kirchhoff integral calculation for monochromatic SR harmonics followed by summation over an actual spectrum, taking into account the absorption in sample. In particular, the Fit Microtube Image (FIMTIM) program was developed for automatic determination of the parameters of the MP cross section in two dimensions from the good-fit condition for calculated and experimental profiles of relative intensity. From this it was shown that MP cross section can vary not only in its size, but also

In our studies, MPs are assumed to be screw dislocations with hollow cores [17], as made certain by combining phase contrast imaging and topography. Evolution of pores and MPs during SiC growth presents a substantial challenge, because a SiC boule is not transparent as a whole even for hard synchrotron X-rays. The series of slices were cut perpendicular (on-axis) and parallel (axial-cut) to the growth direction of 6*H* and 4*H*-SiC boules. Between on-axis 6*H*-SiC slices, the shapes and distributions of defects were variable. It was essential to have some special means of finding the same area of interest between the slices. Taking into account the microscopic sizes of defects, this area was controlled as rather small as 1.5 mm × 1.5 mm. X-ray phase-contrast micro-imaging was utilized for the area mapping, and the

Fig. 1(a) shows the experimental set-up combining phase contrast imaging and topography techniques. In a diffraction mode, topographs were recorded on a photographic film. Phase-contrast images were taken with a charge coupled device camera (14-bit gray scale and 1600 × 1200 pixels range). Before recorded X-ray image was converted into visible lights by a 150 *µ*m thick CdWO4 scintillator and magnified by a lens system with a magnification

Six on-axis 6*H*-SiC wafers were numbered as 'wafer I' (adjacent to the seed), 'wafer II' (the next to grow), and so the last wafer near the top of the boule as 'wafer VI'. The area mapping was preceded by the following orientation procedure. Each wafer was mounted with a miscut line, that is, an intersection between the surface and the basal plane (0001), vertical and the surface perpendicular to the beam. Each wafer was aligned to have the same Laue pattern of the orientation. For instance, the wafers I, II, III in the succession of one to another are sketched on Fig. 1(a). Fig. 1(b) shows an indexed Laue pattern for the (0001) orientation obtained from the wafer I at a distance of 9 cm. Similar patterns were then recorded from

Defect evolution from wafer to wafer was investigated on the area of interest from the matched overlap of the wafer images. For example, Fig. 2(a) shows the matched overlap of the wafer I (gray) cut off near the seed and the next wafer II (black). The region 1 is the area of interest, while the region 2 is a reference point to refine the wafer match. Here a group of MPs undeviatingly propagated through all the wafers, as demonstrated in Fig. 2(b). Pores and MPs in the region of interest 1 in the wafers I and II are displayed in Figs. 2(c) and (d),

for recording phase-contrast images of small objects such as MPs in SiC [19].

in its orientation, during its growth [1].

Laue pattern served for orientation.

from 1× to 50×.

the wafer II, and so on.

**3. The evolution of defects during SiC growth**

**Figure 1.** (a) Scheme for obtaining diffraction and phase-contrast images on-line: (*1*) SR beam from storage ring, (*2*) beam defining slits, (*3*) diffracted beams, (*4*) film, (*5*) mirror, (*6*) scintillator, (*7*) objective, (*8*) detector. I, II and III correspond to the succession of 6*H*-SiC axial-cut slices investigated in turn. (b) Laue pattern of the (0001) 6*H*-SiC wafer (direction [1210] horizontal) fixed perpendicular to SR beam.

The purpose of the present paper is to characterize the evolution of pores and MPs during SiC growth using SR white beam phase contrast imaging and Bragg diffraction topography. We document defect reduction mechanisms and suggest theoretical and computer models to explain them.
