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.