**2. Experimental**

The samples used in this study were prepared from a 4*H* and 6*H*-SiC boules grown in Ar by the sublimation sandwich technique [31]. The Ar pressure in the growth chamber was 80 mbar. The growth temperature was 2180◦C, and the growth rate was 0.5 mm × <sup>h</sup><sup>−</sup>1. The crystals were N-doped and had a *n*-type conductivity with a donor concentration 2 × 1018 cm<sup>−</sup>3. They contained B to a concentration (1–2) × <sup>10</sup><sup>17</sup> cm<sup>−</sup>3. Besides, doping of SiC by Al to a concentration of approximately (2–7) × <sup>10</sup><sup>17</sup> cm−<sup>3</sup> occurred because of the Al presence in the polycrystalline SiC source. After polished and treated to eliminate damaged layers on both sides, the sample thicknesses were controlled as about 500 *µ*m.

The foreign polytype inclusions located close to the surface were revealed with the aid of a photoluminescence (PL) microscope in visible as well as in ultraviolet light under the magnifications of 50× – 200×. The polytype was identified by the color of PL [28]. The opening of pores and MPs on molten KOH etched sample surfaces was detected using optical and scanning electron microscopies (SEM), performed with Zeiss universal microscope and JEOL JSM-6330F FESEM, respectively.

Imaging experiments were done at the 7B2 X-ray microscopy beamline of Pohang Light Source (PLS), Korea [2]. The 7B2 bending magnet port of the PLS storage ring provided an effective source size of the order of 60 *µ*m (160 *µ*m) in the vertical (horizontal) direction at the distance 34 m from the sample. Unmonochromatized ('white') radiation with the spectrum ranged from 6 to 40 keV was propagated through a polished beryllium window of 2 mm thick and then through a specimen with no optical elements in between.

X-ray phase contrast in the white beam is formed because of a small angular size and a high spatial coherence of third generation SR source. The white beam spectrum has a curve shape with a maximum at a certain energy in the case at hand. Decreases in the radiation intensity at high and low energies are caused by the SR generation's nature and the absorption in the sample, respectively. Therefore, even 'white' SR is partially coherent, which is quite sufficient for recording phase-contrast images of small objects such as MPs in SiC [19].

2 Physics and Technology of Silicon Carbide Devices

I II III

fixed perpendicular to SR beam.

explain them.

**2. Experimental**

JEOL JSM-6330F FESEM, respectively.

*3*

(a) (b)

*4*

both sides, the sample thicknesses were controlled as about 500 *µ*m.

thick and then through a specimen with no optical elements in between.

*5 6*

*7*





0-2.2

1-3.3 1-3.4 1-3.5


2-4.6

2-2.2

3-2.2 3-2.3 3-2.4 3-2.5

2-3.2 2-3.3 2-3.4 2-3.5

21.3 -31.3

2-2.3

3-4.7

2-2.4 4-4.7

3-1.2 3-1.3 3-1.4

4-2.6

20.-4 20.-3 20.-2

3-1.5 5-2.9

**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)

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

The samples used in this study were prepared from a 4*H* and 6*H*-SiC boules grown in Ar by the sublimation sandwich technique [31]. The Ar pressure in the growth chamber was 80 mbar. The growth temperature was 2180◦C, and the growth rate was 0.5 mm × <sup>h</sup><sup>−</sup>1. The crystals were N-doped and had a *n*-type conductivity with a donor concentration 2 × 1018 cm<sup>−</sup>3. They contained B to a concentration (1–2) × <sup>10</sup><sup>17</sup> cm<sup>−</sup>3. Besides, doping of SiC by Al to a concentration of approximately (2–7) × <sup>10</sup><sup>17</sup> cm−<sup>3</sup> occurred because of the Al presence in the polycrystalline SiC source. After polished and treated to eliminate damaged layers on

The foreign polytype inclusions located close to the surface were revealed with the aid of a photoluminescence (PL) microscope in visible as well as in ultraviolet light under the magnifications of 50× – 200×. The polytype was identified by the color of PL [28]. The opening of pores and MPs on molten KOH etched sample surfaces was detected using optical and scanning electron microscopies (SEM), performed with Zeiss universal microscope and

Imaging experiments were done at the 7B2 X-ray microscopy beamline of Pohang Light Source (PLS), Korea [2]. The 7B2 bending magnet port of the PLS storage ring provided an effective source size of the order of 60 *µ*m (160 *µ*m) in the vertical (horizontal) direction at the distance 34 m from the sample. Unmonochromatized ('white') radiation with the spectrum ranged from 6 to 40 keV was propagated through a polished beryllium window of 2 mm

X-ray phase contrast in the white beam is formed because of a small angular size and a high spatial coherence of third generation SR source. The white beam spectrum has a curve shape with a maximum at a certain energy in the case at hand. Decreases in the radiation intensity at high and low energies are caused by the SR generation's nature and the absorption in the

5-3.9 4-3.7

0-2.3



0-2.4 0-4.7

*8*

*3*

*1*

*2*

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 its orientation, during its growth [1].

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 Laue pattern served for orientation.

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 from 1× to 50×.

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 the wafer II, and so on.
