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

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),

**Figure 2.** (a) The outline of the wafers I (gray) and II (black) imaged after the Laue pattern adjustment is completed. The area 1 is the area of interest. The wafer images are lapped over in such a way that the positions of propagating MPs encircled by the area 2 overlie in SR phase contrast images of (b). Comparison of SR phase contrast images of pores in the wafers I (c) and II (d).

200 µm

MPs

MPs

MPs

1

**Figure 4.** SR phase contrast image of the pores and MPs in the wafer I. The arrows point to the group of MPs (*1*) and the pore

<sup>100</sup>µ<sup>m</sup> <sup>100</sup>µ<sup>m</sup> (a) (b)

**Figure 5.** Pores and MPs in the wafers III, IV. The pores are counted III-1 and III-2 (a). MPs in the wafer IV (b).

(*2*). The group of MPs is magnified in the inset: (*3, 4*) — MPs, (*5*) — pore.

MPs

III-1 III-2

100 µm

4 3

5

2

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

**Figure 3.** (a) SR phase-contrast image of the pores in the wafer I. (b) PL micrographs of 4*H*-SiC inclusions in 6*H*-SiC matrix; the white in (b) and the black in (c) arrows point to the boundaries of the inclusions where the pores are located in (a).

<sup>30</sup> Physics and Technology of Silicon Carbide Devices Characterization of Defects Evolution in Bulk SiC by Synchrotron X-Ray Imaging 5 Characterization of Defects Evolution in Bulk SiC by Synchrotron X-Ray Imaging http://dx.doi.org/10.5772/52058 31

4 Physics and Technology of Silicon Carbide Devices

**1**

**2**

I-1

I-2

I-3 I-4 I-5

> I-6 I-7

I-8

I-10

(a) (b)

I-11 I-12

II-1

**Figure 2.** (a) The outline of the wafers I (gray) and II (black) imaged after the Laue pattern adjustment is completed. The area 1 is the area of interest. The wafer images are lapped over in such a way that the positions of propagating MPs encircled by the area 2 overlie in SR phase contrast images of (b). Comparison of SR phase contrast images of pores in the wafers I (c) and II (d).

<sup>200</sup>µ<sup>m</sup> (a)

<sup>200</sup>µ<sup>m</sup> (b) <sup>200</sup>µ<sup>m</sup> (c)

**Figure 3.** (a) SR phase-contrast image of the pores in the wafer I. (b) PL micrographs of 4*H*-SiC inclusions in 6*H*-SiC matrix; the

white in (b) and the black in (c) arrows point to the boundaries of the inclusions where the pores are located in (a).

II-2

II-3

II-4

II-6

2 mm 100 µm

II-7

200 µm 200 µm

II-5

II-8 II-9

II-10

MPs

MPs

(c) (d)

I-13 I-14

I-9

**Figure 4.** SR phase contrast image of the pores and MPs in the wafer I. The arrows point to the group of MPs (*1*) and the pore (*2*). The group of MPs is magnified in the inset: (*3, 4*) — MPs, (*5*) — pore.

**Figure 5.** Pores and MPs in the wafers III, IV. The pores are counted III-1 and III-2 (a). MPs in the wafer IV (b).

intensity inside the MP starts oscillating. The black contrast appears as a result of average, because in the white radiation fringes are not resolved. In such a way, the image simulation confirms that MPs can move parallel to the growth front towards one another and other

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

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A detailed investigation of MPs shows that the MP #3 and, especially, the MP #4 sharply inclined toward the slit-shaped pores nearby (for instance, to the pore #5 in the inset to Fig. 4). The attraction of the MPs to the pores is confirmed by their contrast behavior and verified by observing the density change of MPs during the SiC growth. The MP density in the group 1 reduced in the wafer II compared to I. The attraction of the MPs to the pore was suggested and explained by their absorption to pores [11, 13]. We believe that, at the initial stage of growth, the pores are generated by the attraction of MPs by FPIs, followed by MP

The number of pores reduced during the growth, as shown in Fig. 2(c) and (d). Fourteen pores, labeled in the sequence from I-1 to I-14, are identified from the wafer I as shown in Fig. 2(c). They reduce to ten pores on the same location of the wafer II, as shown in Fig. 2(d). We note that pores migrate and noticeably change their configuration. With further growth, the number of pores much more reduces from ten pores in the wafer II to two pores [III-1 and III-2 in Fig. 5(a)] in the wafer III. It is worth noting that new MPs appear in the wafer III. As the growth proceeds, pores are overgrown: no pores detected in the wafer I are seen in the place of interest of wafer IV [Fig. 5(b)]. However, one sees more MPs which obviously group together. They are bent and irregular in shape, size and propagation direction. In the last wafer VI, MPs density remains very inhomogeneous: it is high in some local places, as highlighted in the inset to Fig. 6 by black squares, but low on average. Phase-contrast image of the figure shows the distribution of MPs in a representative location #1. One can see short projection lengths of the MPs running in various directions inside the sample fixed perpendicular to the beam. The imaged area in Fig. 6 is 1.2 mm × 1.2 mm. Taking into account that the area displayed in Fig. 5(b) is slightly smaller (1.2 mm × 0.8 mm), and the number of MPs is noticeably bigger, one can conclude that the density of MPs decreases with

The observations of MPs in this 6*H*-SiC boule and many other similar 6*H* and 4*H*-SiC boules provided strong evidences for the reactions of MPs [7–9]. Reaction between MPs is always a

Ramification of MPs is accompanied by the partitioning of their Burgers vectors. According to the Frank rule [6], the equilibrium MP radius is proportional to the squared magnitude of its Burgers vector. As a result, ramification of MPs results in a decrease of their radii. The sketch of Fig. 7(a) represents a dislocated MP with the radius *r*<sup>0</sup> and the Burgers vector *b*<sup>0</sup> splitted into two smaller ones with the radii and the Burgers vectors *r*1, *r*<sup>2</sup> and *b*1, *b*2, respectively. A typical phase-contrast image of a ramified dislocation with a hollow core is

positive process in view of their elimination during the crystal growth.

**4. Reduction of micropipes density via self-reactions**

**4.1. Radii reduction and gradual healing**

*4.1.1. Micropipe ramification at the front of a growing crystal*

defects.

growth.

shown in Fig. 7(b).

coalescence [10, 11].

**Figure 6.** SR phase-contrast image of MPs in the latest-to-grow wafer VI. Schematic of the wafer with black squares highlighting the locations of MPs is shown in the inset. MPs are located on the square #1.

respectively. One can see that the wafer I has higher density of pores. In addition, Figs. 3(a), (b) and (c) taken in SR X-rays and PL, respectively, show pores and FPIs in the wafer I located just outside the scope of the image in Fig. 2(c). The high resolution of Fig. 3(a) demonstrates that the groups of pores (marked by black and white arrows) consist of short tube-shaped or slit-like segments. The morphology of such pores was investigated and attributed to the elastic interaction between MPs and boundaries of FPIs, resulting in coalescence of MPs into larger pores elongated along the boundaries [11, 13]. FPIs were indeed observed on the same location, as revealed by the yellow PL images [28] of *n*-type 4*H*-SiC containing N and B in Fig. 3(b) and (c), recorded at 77 K. The comparison with the pore images in the optical micrographs taken prior to the excitation of PL (data not shown), indicates that the pores trace the FPI boundaries.

A group of pores in Fig. 3(a) is partly displayed in the phase contrast image of Fig. 4. One pore from the group is marked by the arrow 2. Notice that the arrow 1 points to the group of MPs located at the place of interest in Fig. 2(c). The magnified image of the group is shown in the inset to Fig. 4. MPs appear as line segments, of which lengths are dependent on the wafer thickness, the miscut angle, and the sample tilting relative to the beam. The MPs run in different directions instead of lying parallel to the growth direction, similar to the observation described in an earlier paper [32]. In fact, the majority of MPs deviated from the growth direction and inclined toward one another or other defects in all six wafers studied.

Interestingly, one can see that the sign of contrast changes per every MP as well as along its axis. For instance, a typical MP image with black edges and a white inside shows a reversal to light edges and a black inside, as is the case in the upper right corner of the inset of Fig. 4. The MP #3 from the group of three MPs and the MP #4 nearby demonstrate the features of black contrast. The effect was well explained by the simulation of MP images measured in a white SR beam [20]. At high angles between the MP axis and the beam, the section size of MP along the beam is small. At low angles, the section size increases, and the wave field intensity inside the MP starts oscillating. The black contrast appears as a result of average, because in the white radiation fringes are not resolved. In such a way, the image simulation confirms that MPs can move parallel to the growth front towards one another and other defects.

A detailed investigation of MPs shows that the MP #3 and, especially, the MP #4 sharply inclined toward the slit-shaped pores nearby (for instance, to the pore #5 in the inset to Fig. 4). The attraction of the MPs to the pores is confirmed by their contrast behavior and verified by observing the density change of MPs during the SiC growth. The MP density in the group 1 reduced in the wafer II compared to I. The attraction of the MPs to the pore was suggested and explained by their absorption to pores [11, 13]. We believe that, at the initial stage of growth, the pores are generated by the attraction of MPs by FPIs, followed by MP coalescence [10, 11].

The number of pores reduced during the growth, as shown in Fig. 2(c) and (d). Fourteen pores, labeled in the sequence from I-1 to I-14, are identified from the wafer I as shown in Fig. 2(c). They reduce to ten pores on the same location of the wafer II, as shown in Fig. 2(d). We note that pores migrate and noticeably change their configuration. With further growth, the number of pores much more reduces from ten pores in the wafer II to two pores [III-1 and III-2 in Fig. 5(a)] in the wafer III. It is worth noting that new MPs appear in the wafer III. As the growth proceeds, pores are overgrown: no pores detected in the wafer I are seen in the place of interest of wafer IV [Fig. 5(b)]. However, one sees more MPs which obviously group together. They are bent and irregular in shape, size and propagation direction. In the last wafer VI, MPs density remains very inhomogeneous: it is high in some local places, as highlighted in the inset to Fig. 6 by black squares, but low on average. Phase-contrast image of the figure shows the distribution of MPs in a representative location #1. One can see short projection lengths of the MPs running in various directions inside the sample fixed perpendicular to the beam. The imaged area in Fig. 6 is 1.2 mm × 1.2 mm. Taking into account that the area displayed in Fig. 5(b) is slightly smaller (1.2 mm × 0.8 mm), and the number of MPs is noticeably bigger, one can conclude that the density of MPs decreases with growth.

The observations of MPs in this 6*H*-SiC boule and many other similar 6*H* and 4*H*-SiC boules provided strong evidences for the reactions of MPs [7–9]. Reaction between MPs is always a positive process in view of their elimination during the crystal growth.
