Figure 2. Identification and confirmation of 3C-SiC polytype for as-grown crystals. a) Raman spectra of 3C-SiC measured on 20 points on the 2-inch crystal. The inset shows the distribution of all measured points. b) Raman spectra of seed 4H-SiC, TZ (transition zone) and as-grown 3C-SiC. c) PL spectrum of 3C-SiC measured at 300 K. d) Plan-view high-angle annular dark field scanning TEM (HAADF-STEM) image of 3C-SiC. Si and C atoms are superimposed. Inset is SAED measured along\(\left[1\overset{\overline{}}{1}0\right]\) Z.A. (zone axis).
The crystal grows by stacking of (111) crystallographic planes as only diffraction peaks (111) and (222) are present in the θ -2θscan on the surface of the grown boule, see Figure 3a. To assess the crystallinity of the wafer, we perform the X-ray rocking curve (XRC) measurements. The full width at half maximum (FWHM) for as-grown (111) surface (Figure 3b) ranges from 28.8 to 32.4 arcsec with an average value of 30.0 arcsec (Table 1). The FWHM is very homogeneous across the whole wafer, indicating the high uniformity of 3C-SiC. To our best knowledge, the values stand for the best results on wafers larger than 2-inch obtained so far (Table S3). Defects are characterized on the wafer after being etched at 500 ℃ for 10 min in KOH melt. Linear ridges, triangle pits and rounded-triangle pits are clearly seen on Si-terminated surface under an optical microscope (OM) and a scanning electron microscope (SEM) (Figures 3c-f). The ridges from dozens to more than one hundred microns in length are due to the stacking fault (SF) (Figures 3c, f, Figure S8), a common defect in 3C-SiC.[9, 36-38] The thickness of typical SFs revealed by the bright-field and dark-field TEM images (Figure 3g, h) is 3 layers of (111) planes. Its density, defined by the total length of all SFs divided by the observed area, is averaged to be 92.2 /cm (Table 1, Figure S9), much less than what’s previously reported (Table S4). Our results are in good agreement with the reported results that N-doping can substantially increase the SFs length.[39] In addition, the SFs, seen from a slice of 3C-SiC, are delimited by two triangle pits or two rounded-triangle pits (Figure S10). The typical triangle pits are ~5 μm in size, probably originating from thread screw dislocations (TSDs) (Figures 3c, d). The rounded-triangle pits, a little bit smaller in size, are from thread edge dislocations (TEDs) (Figures 3c, e, Figure S9).[37, 38, 40-42] They are about 4.3\(\times\)104 /cm2 and 13.9\(\times\)104 /cm2 in density, respectively (Figure S9). No double-positioning boundaries (DPBs), which are quite common in 3C-SiC,[23, 26, 43-45] are observed in our 3C-SiC wafers.
The electrical characterizations are conducted on a slab crystal cut from the grown boules. Electrical resistivity, carrier density and mobility are measured by the standard six-wire method (Figure S11). Figure S12a shows the variations of electrical resistivity with temperature from 5 to 300 K. We can see that the samples grown under\(p_{N_{2}}\) of 15 and 20 kPa exhibit a metallic character. The resistivity decreases with lowering temperature, suggesting the 3C-SiC should become a semi-metal with a room-temperature resistivity of 0.58 mΩ·cm (Table 1, Tables S5-6), much lower than 4H-SiC’s (15~28 mΩ·cm) (Table S8).[46] We note that the crystal grown with\(p_{N_{2}}\) of 10 kPa behaves like a semiconductor below about 100 K (Figure S12a). The carrier density for the 20 kPa sample is calculated to be 1.89×1020 /cm3 (Figure S12b and Table S5), in good agreement with the doping concentration of N (1.99×1020 /cm3) measured by secondary ion mass spectroscopy (SIMS) (Figure S13). It demonstrates that almost all of the doped electrons are activated to the conduction band at room temperature. The calculated mobility ranges from 56.95 to 62.66 cm2/V·s (Table S5). The mobility is enhanced to be 66.24 cm2/V·s when the carrier density is lowered (Figure S12c, Tables S5-7), meanwhile the resistivity mounts up to 5.77 mΩ·cm, about one fourth of 4H-SiC’s (15~28 mΩ·cm) at room temperature,[46] which is much lower than the reported results (Table S8). In this case, the PL at about 523 nm due to the band-edges transition is observed, as state above, see Figure 2c.