Interface carbon defects at 4H-SiC(0001)/SiO2 interfaces studied by electron-spinresonance spectroscopy
T. Umeda, G.-W. Kim, T. Okuda, M. Sometani, T. Kimoto, and S. Harada

Citation: Appl. Phys. Lett. 113, 061605 (2018); doi: 10.1063/1.5041059
View online: https://doi.org/10.1063/1.5041059
View Table of Contents: http://aip.scitation.org/toc/apl/113/6
Published by the American Institute of Physics

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APPLIED PHYSICS LETTERS 113, 061605 (2018)

Interface carbon defects at 4H-SiC(0001)/SiO2 interfaces studied
by electron-spin-resonance spectroscopy
T. Umeda,1,a) G.-W. Kim,1 T. Okuda,2 M. Sometani,3 T. Kimoto,2 and S. Harada3
1

Institute of Applied Physics, University of Tsukuba, Tsukuba 305-8573, Japan
Graduate School of Engineering, Kyoto University, Kyoto 615-8510, Japan
3
National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba 305-8569, Japan
2(Received 23 May 2018; accepted 26 July 2018; published online 9 August 2018)
We study an electron-spin-resonance (ESR) signal of carbon dangling-bond defects at 4HSiC(0001)/SiO2 interfaces, which we call an “interface carbon defect.” The ESR signal is close to ac-axial type of the PbC centers (interfacial carbon dangling bonds) that have originally been found inporous-SiC/SiO2 interfaces. The interface carbon defects were always formed with an areal densityof 3–4  1012 cm2 after the standard dry oxidation of 4H-SiC(0001) surfaces. They act as electron
traps and decrease the amount of free electrons in the channel region, consequently reducing thefield-effect mobility of Si-face 4H-SiC MOSFETs. They were eliminated by optimum post-oxidation
anneals (POAs) in either NO or POCl3 environment. Furthermore, POCl3 POAs at 1000  C introduced a high density (1.7  1012 cm2) of phosphorus donors into the channel region, increasing thefree-carrier density as compared with the case of NO POAs. Published by AIP Publishing.
https://doi.org/10.1063/1.5041059

The 4H-SiC/SiO2 interfaces are the key components for
SiC power devices such as 4H-SiC metal-oxide-semiconductor field-effect transistors (MOSFETs). The current standard
technique for fabricating a good 4H-SiC/SiO2 interface is ahigh-temperature (1200–1300  C) thermal oxidation of a 4HSiC(0001) surface (the so-called “Si face”) by dry O2 gas
followed by a post-oxidation anneal (POA) in a NO environment. The POA process can make the field-effect mobility(lFE) of 4H-SiC MOSFETs ten times larger than that before
the POA.1,2 The mechanism for this drastic lFE-improvement
has recently been clarified by combining Hall and split
capacitance-voltage measurements.3 According to the recent
findings, the NO POA process effectively increases theamount of free electrons by removing electron traps, while itdoes not increase the electron mobility itself much. The
amount of the removed traps was found to be 2–3  1012 cm2for the standard NO POA recipe.3 However, the origin of suchelectron traps associated with the lFE-degradation is still
unclear. So far, electron-spin-resonance (ESR) spectroscopy,
which is a powerful tool for identifying microscopic origins ofdefect levels, has been applied to SiC-MOS systems.4–6 Up tonow, however, ESR centers with 2–3  1012 cm2 or more
have not been detected at the 4H-SiC(0001)/SiO2 interfaces.
Alternatively, electrically detected magnetic resonance(EDMR) was applied to Si-face 4H-SiC MOSFETs to visualize interface defects in their MOS interfaces.7–10 As a result,an interfacial Si-vacancy center8 and different types of interface defects7,9,10 have been detected under optimum bias conditions for each EDMR signal. However, the densities of these
EDMR centers are unclear yet because of a lack of quantitativity of EDMR signals.
In this Letter, we present quantitative ESR measurements on interface defects or electron traps at 4H-SiC(0001)/a)

Author to whom correspondence should be addressed: umeda@
bk.tsukuba.ac.jp0003-6951/2018/113(6)/061605/5/$30.00

SiO2 interfaces formed by dry oxidation with/without thestandard NO POA process. Furthermore, we also characterized another effective POA process, i.e., POCl3 POA,11,12
which is known to achieve a much higher lFE (100 cm2/V s)than the standard NO POA (20–40 cm2/V s).11 By applying
ESR to special free-standing 4H-SiC epitaxial layers, weobserved an anisotropic ESR signal just after the dry oxidation. This signal is assigned to be a sort of a carbon danglingbond center at the 4H-SiC(0001)/SiO2 interfaces, since it
resembles c-axial type PbC centers (carbon dangling bonds)
that have originally been found in porous-SiC/SiO2 interfaces.6 We here call it an “interface carbon defect.” Our ESR
measurements demonstrated that both optimum NO and
POCl3 POAs can remove it. Their spin density was estimatedto be 3–4  1012 cm2, which is in agreement with the amountof electron traps changed by NO POAs.3 Thus, we conclude
that the interface carbon defects are major electron traps that
decrease the lFE of dry-oxidized Si-face MOSFETs. In addition, we also found phosphorus (P) doping over 1  1012 cm2
after an optimum POCl3 POA process, which contributes toan increase in the free-carrier density in the channel region. In
contrast, we could not detect nitrogen (N) doping after NO
POAs, and hence evaluated the N-doping density to be lower
than 1  1011 cm2.
We carried out ESR measurements on the interface carbon defects at room temperature and on the P and N donors
at 4–20 K by means of a Bruker E500 X-band spectrometerequipped with a super-high-Q cavity and an Oxford ESR900 liquid-He cryostat. Since the carbon interface defectshowed a strong microwave saturation behavior due to a
weak spin-orbit interaction of its electron spin on a carbon
atom, its ESR signal was most clearly observable at room
temperature. We utilized the standard magnetic-field modulation technique at 100 kHz with an amplitude of 0.1–0.3 mT
for the interface carbon defect. ESR transitions were excitedby a microwave of 9.428 GHz and 0.2 mW, unless otherwise

113, 061605-1

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stated. The spin densities were calculated by comparing theirabsorption ESR intensity per cm2 with that of a measurement
standard (a CuSO45H2O specimen with 12.53 mg and3.02  1019 spins).
The ESR substrates were prepared using free-standing 4HSiC epitaxial layers (0.35  0.70  0.013 cm3 or 0.35  0.70
 0.016 cm3) with either Al doping of 4  1014 cm3 or N doping of 4  1014 cm3, respectively. In these epitaxially grown
crystals, the amount of deep levels is controlled to be much
lower than the above doping levels, and hence an obstruction
due to bulk ESR signals should be suppressed to be 1011 cm2(¼1013 cm3  0.01 cm) or lower. This is the key for observing interface ESR signals with a density of 1012 cm2.
Both sides of the substrates were finished by chemicalmechanical polishing (CMP); however, the substrates showeda broad amorphous-carbon signal [g value (g) ¼ 2.003,
Fig. 1(a)], in addition to a reference signal of the E’ center (aparamagnetic silicon center in SiO2,13 g ¼ 2.0008) in a sample
rod. The amorphous-carbon signal is consistent with those ofcarbon dangling bonds in various carbon materials.14–16 The
signal is isotropic, as indicated by solid and dotted lines inFig. 1(a) measured for different magnetic-field angles. The
isotropy was also confirmed by a more detailed angular

FIG. 1. Room-temperature ESR spectra of free-standing 4H-SiC(0001) epitaxial layers before/after oxidation and post-oxidation anneals (POAs),
where an E0 reference signal comes from a sample rod. Solid and dottedlines were measured for magnetic fields (B) parallel and normal to the[0001] axis, respectively (cf., Fig. 2). (a) Initial substrate. (b) After the
reset process. (c), (e), and (g) After the standard dry oxidation on the Si faceof the reset substrate. An anisotropic ESR signal of “interface carbon defect”
appears. (d), (f), and (h) After POAs with either NO or POCl3.

Appl. Phys. Lett. 113, 061605 (2018)

dependence shown in Fig. 2(a). The peak-to-peak signal width(0.55 mT) is consistent with those of sp3-hybridizied carbons.15 We attribute the signal’s origin to be residual carbons
on the polished surfaces, because its ESR intensities strongly
depended on CMP processes as well as on the surface etchingdetailed below. Note that this signal could not have been
removed by either RCA chemical cleaning or sacrificed oxidation (50 nm) þ SiO2 wet etching; so, we have to use a different surface treatment. To eliminate the amorphous-carbon
signal, we carried out a precise dry-etch process by a hightemperature HCl anneal (Table I).17 We hereafter call this the“reset process.” After the reset process, the amorphouscarbon signal was completely eliminated as shown in Fig.
1(b). The reset process was also very useful for preparing a
fresh surface of the substrate by removing its on-top SiO2
layer as well as the surface layer of the substrate.
By taking advantage of the reset process, we could
repeatedly characterize dry oxidation and various POA processes (see Table I) on the fresh surfaces, as demonstrated in
Figs. 1(c)–1(h). It is notable that the oxidation and POA processes we studied here were electrically characterized byprevious works on MOS or MOSFET structures.2,3,9,12 In thefirst step, we repeatedly tried dry oxidation. For ESR specimens after the dry oxidation, their Si face was protected byan acid-proof coat, and then C-face oxide was chemicallyremoved by hydrofluoric-acid etching. This process ensuresthat we measured only ESR-active centers on the side of the
Si face, and the C-face side will be separately studied elsewhere. We always observed the appearance of a spin-1/2
ESR signal, which we call an “interface carbon defect,” asseen in Figs. 1(c), 1(e), and 1(g). As is shown by solid anddotted lines in Fig. 1(c), this signal is clearly anisotropic,

FIG. 2. Angular dependence of ESR signals observed in this study.
Magnetic-field angles of 0 and 90 correspond to magnetic fields paralleland normal to the [0001] axis, respectively. (a) Amorphous-carbon signal ininitial substrates. (b) Interface carbon defect formed at dry-oxidized 4HSiC(0001)/SiO2 interfaces. (c) Phosphorus donors (measured at 4 K) doped
after POCl3 POA at 1000  C. (d) Nitrogen donors (measured at 20 K) originating from residual impurities in bulk.

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Appl. Phys. Lett. 113, 061605 (2018)

TABLE I. Oxidation and optimum POA processes on 4H-SiC(0001) (Si face) examined in this study. The reset process as well as ESR results are also
summarized.
ProcessReset
Dry oxidation
NO POA
POCl3 POA

Condition

ESR result

HCl anneal at 1300  C for 2 min. (etching rate for SiC ¼ 14 nm/2 min.)O2 anneal at 1300  C for 30 min. (44-nm-thick SiO2 growth on Si face)
NO anneal at 1250  C for 70 min. followed by N2 anneal at 1200  C for 30 min.
POCl3 anneal at 1000  C for 10 min. followed by N2 anneal at 1000  C for 30 min.

Remove surface amorphous-carbon signal
Generate interface carbon defects with 3–4  1012 cm2
Remove interface carbon defects
Remove interface carbon defects þ P-doping

unlike the amorphous-carbon signal noted above as well as
the interfacial Si-vacancy signal studied by Cochrane et al.8
By examining its angular dependence in detail [Fig. 2(b)],we found that this signal shows c-axial symmetry withg// ¼ 2.0024 and g? ¼ 2.0032. These principal g values
belong to those of typical carbon dangling bonds.14 It isespecially worth noting that the principal g values show
excellent agreement with those of the c-axial PbC center(g// ¼ 2.0023 and g? ¼ 2.0032), which has been identified asc-axial carbon dangling bonds in porous-SiC/SiO2 interfaces.6 Therefore, the interface carbon defect most probably
originates from a sort of carbon dangling bonds at the 4HSiC(0001)/SiO2 interfaces. However, the final identificationof this defect should wait until its hyperfine interactions witha 13C nuclear spin have been fully analyzed.
In the second step, we examined the changes in the interface carbon defects by subjecting the dry-oxidized substrates
to either NO or POCl3 POA processes. As Fig. 1(d) shows,the interface carbon defects were eliminated by an optimum
NO POA (Table I). Their amount was reduced below ourdetection limit. Also, an optimum POCl3 POA process (TableI) achieved the same result, as shown in Fig. 1(h). On theother hand, for POCl3 POA at a slightly lower temperature(900  C), the interface carbon defects could not be fully
removed [Fig. 1(f)]. This observation is consistent with theprevious experimental finding that the carrier traps wereeffectively annihilated by POCl3 anneal at over 950  C.12
For each process step, the spin densities of the carbon
defects (amorphous carbon or interface carbon defect) were
estimated by deconvoluting a carbon-defect signal from each
spectrum (blue lines in Fig. 1). The quantitative values are
summarized in Fig. 3. As shown in Fig. 3(a), we found theamorphous-carbon signal with 4.9  1012 cm2/one-side (weassumed this signal from both faces of a substrate). In othersubstrates with different CMP processes, its spin density
decreased below 1  1012 cm2. After the reset process, no
substrates showed the amorphous-carbon signal, e.g., shownin Fig. 3(b).
After the dry oxidation processes, we always found theinterface carbon defects with 3–4  1012 cm2 [Figs. 3(c),3(e), and 3(g)]. They could be fully eliminated by the optimum NO and POCl3 POA processes as shown in Figs. 3(d)and 3(h), respectively. In the previous work,3 a NO POA
process (1250  C for 60 min. applied to a 50-nm-thick dry
oxide) reduced the amount of electron traps at the interface
by 2–3  1012 cm2, resulting in significantly improving lFE.
Such a reduction is mostly consistent with the present resultthat the interface carbon defects with 3–4  1012 cm2 are
eliminated by a NO POA process. Furthermore, the interface

carbon defect expectedly works as an electron trap, because
its positive and neutral dangling-bond levels can capture electron(s), the same as in the case of famous Pb centers (silicon
dangling bonds) at Si/SiO2 interfaces.13 We therefore proposethat the observed defects are responsible for causing the electron traps associated with the lFE-variation before/after NO
POAs.
Interestingly, the POCl3 anneal can further increase lFE
up to 100 cm2/V s.11 The present result revealed that both the
optimum POCl3 and NO POAs fully remove the interface carbon defects. The additional increment in lFE after the POCl3
anneal is possibly due to the P-doping with the channel
region. Figure 4 shows the P-doping after the optimum POCl3
anneal. In the figure, ESR spectra measured at 4 K by a rapidpassage mode,18 which exhibit absorption ESR signals, were
examined for (a) the reset surface, (b) and (c) POCl3 POAs at
different temperatures, and (d) NO POA. In Fig. 4(c), a new
broad resonance appears, which we attribute to the P-donor
signal. A doublet hyperfine (HF) splitting of a 31P nucleus(nuclear spin ¼ 1/2, natural abundance ¼ 100%) seems to be
obscured by a broad signal width which is comparable to the31
P HF splitting (0.29 mT) of bulk P donors,19 as shown in
Fig. 4(c). Moreover, we speculate that the 31P HF splitting
itself may be shrinked rather than that of the bulk donors. In
fact, the g principal values of the present P donors are determined to be g// ¼ 2.0079 and g? ¼ 1.9992 [see Fig. 2(c)],which deviate from those of the bulk P donors in 4H-SiC

FIG. 3. Spin densities of carbon defects measured for ESR spectra (a) to (h)in Fig. 1. (a) Amorphous-carbon defect of an initial substrate. We assume
that this signal arises from both Si- and C-faces of the substrate. (b) Afterthe reset process. The reset process was also used to initialize the substrate
after (d), (f), and (h). (c), (e), and (g) Interface carbon defect formed at dryoxidized 4H-SiC(0001)/SiO2 interfaces. (d) After an optimum NO POA following to (c). (f) and (h) After POCl3 POAs at different temperatures.

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Umeda et al.

FIG. 4. Phosphorus doping after POCl3 POAs. ESR spectra were measured
using a rapid-passage technique (out-of-phase and second-harmonic detections with 2-mW microwave excitation) at 4 K under photo illumination(100-W halogen lamp), which show absorption ESR signals of P and N
donors. (a) Initial substrate. (b) and (c) After POCl3 POAs. An arrow indicates a doublet 31P hyperfine splitting (0.29 mT) of the bulk P donor.19 (d)After NO POA.

(g// ¼ 2.0065 and g? ¼ 2.0006).19 This deviation may reflect
the fact that the present P donors are located close to the interface, possibly perturbing their donor states. Actually, aninterface-induced perturbation was theoretically predicted for
conduction-band-edge states.20
The signal intensity of the P donors is in proportion totheir spin density; however, a rapid-passage signal intensity
cannot be straightforwardly compared with the signal intensity of CuSO45H2O. Therefore, for a comparison, we
employed a rapid-passage signal of the EI6 center in a different sample (bulk carbon vacancies,21 8.3  1012 cm2)
observed at 4 K with the same microwave power (2 mW).
Since this center resembles the present P donor in terms of
their microwave-saturation behaviors, we could directlycompare their rapid-passage signals. Using such a standard,
we estimated the P-doping density to be 1.7  1012 cm2. If
we assume a uniform doping thickness of 5 nm, the P-doping
density becomes 3  1018 cm3, which is close to theobserved carrier density of 4  1018 cm3 at a POCl3annealed 4H-SiC(0001)/SiO2 interface measured by a scanning capacitance microscope (SCM).22 Note that the
P-doping density is comparable to the density of the interface
carbon defects. The P-doping is therefore expected toincrease the free-carrier density in the channel region ascomparable as the case of the interface carbon defects. On
the other hand, the ionized P donors simultaneously increasecarrier scattering, consequently deteriorating the carrier
mobility. Even including such deterioration, the increase in
the free-carrier density due to P-doping may realize thehigher lFE after POCl3 POAs, because the lack of mobile
carriers strongly limited lFE even after POAs.3 It is also confirmed that the P donors were completely removed after theAppl. Phys. Lett. 113, 061605 (2018)

reset process, indicating that they are introduced into a very
shallow region within 14 nm from the interface.
On the contrary, the P-doping was not observed at
900  C [Fig. 4(b)], and the elimination of the interface carbon defects was not perfect [Fig. 3(f)]. The reason why
P-doping was observable at 1000  C is ascribed to strong reoxidation during the POAs at 1000  C.12 It is reasonable toconsider that such a strong re-oxidation enables the mixingof P atoms and the SiC lattice, enhancing the substitutional
doping of P atoms into SiC. According to the carrier-lifetime
measurement,12 the POCl3 POAs effectively increased thecarrier lifetimes in both n- and p-type 4H-SiC epi-layers.
The present result also accounts for such behaviors becausea neutral dangling-bond center can capture both an electron
and a hole, which is known in the case of the Pb center.13
In contrast, for the NO POAs [Fig. 4(d)], we could not
find any increment in the N-donor signal, although the bulk
N donors were clearly observable at 20 K and their angulardependence could be traced as shown in Fig. 2(d).
Accordingly, we judged that the N-doping density should be
at least one order of magnitude lower (<1  1011 cm2 or<2  1017 cm3 when we assume a uniform N-doping layerof 5 nm in thickness) than the P-doping density observed inFig. 4(c). At least, for the NO POAs, the elimination of the
carbon interface defects should have a much larger impact
on lFE than the N-doping effect. The present result seems to
be consistent with those in the previous SCM study thatreported the carrier density of 5  1017 cm3 at a NOannealed interface.22
In summary, we have presented quantitative ESR analyses on “interface carbon defects” with 3–4  1012 cm2 thatwere assigned to a sort of carbon dangling bonds at 4HSiC(0001)/SiO2 interfaces, closely resembling c-axial type
PbC centers.6 They could be fully eliminated by optimum
POAs by NO or POCl3. Based on the similarities between our
ESR data and previous electrical characterization,2,3,9,12 wesuggest that the interface carbon defects are the major electron traps at the dry-oxidized 4H-SiC(0001)/SiO2 interfaces,
and hence both the POAs drastically improve lFE of Si-face
4H-SiC MOSFETs. In addition, we found that the POCl3
POAs at 1000  C formed a high density (>1  1012 cm2) ofthe P donors in the channel region, which is possibly related
to further enhancement on lFE. In contrast, the N-doping
effect has a much smaller impact on lFE than the eliminationof the interface carbon defects.
This work was supported by the Council for Science,
Technology and Innovation (CSTI), Cross-ministerial
Strategic Innovation Promotion Program (SIP), and “Nextgeneration power electronics” (funding agency: NEDO).
This work was also partly supported by a Grant-in-Aid(Grant No. 17H02781) from the Ministry of Education,
Culture, Sports, Science and Technology of Japan.
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