Electrically detected-magnetic-resonanceidentifications of defects at 4H-SiC(000 )/SiO2 interfaces with wet oxidation
Cite as: Appl. Phys. Lett. 115, 151602 (2019); https://doi.org/10.1063/1.5116170
Submitted: 22 June 2019 . Accepted: 20 September 2019 . Published Online: 10 October 2019
T. Umeda, Y. Kagoyama, K. Tomita, Y. Abe, M. Sometani, M. Okamoto, S. Harada, and T.

Hatakeyama

Appl. Phys. Lett. 115, 151602 (2019); https://doi.org/10.1063/1.5116170© 2019 Author(s).

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Electrically detected-magnetic-resonanceidentifications of defects at 4H-SiC(0001)/SiO2
interfaces with wet oxidation
Cite as: Appl. Phys. Lett. 115, 151602 (2019); doi: 10.1063/1.5116170
Submitted: 22 June 2019 . Accepted: 20 September 2019 .
Published Online: 10 October 2019
T. Umeda,1,a)Y. Kagoyama,1 K. Tomita,1 Y. Abe,1
and T. Hatakeyama2

M. Sometani,2

M. Okamoto,2S. Harada,2

AFFILIATIONS
1Institute of Applied Physics, University of Tsukuba, Tsukuba 305-8573, Japan

2

National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba 305-8569, Japana)

Author to whom correspondence should be addressed: umeda@bk.tsukuba.ac.jp

ABSTRACT
We present electrically detected-magnetic-resonance (EDMR) identiﬁcation of major and minor interface defects at wet-oxidized 4HSiC(0001 )/SiO2 interfaces for C-face 4H-SiC metal-oxide-semiconductor ﬁeld-effect transistors. The major interface defects are identiﬁed asc-axial types of carbon-antisite-carbon-vacancy (CSiVC) defects. Their positive (þ1) charge state generates a spin-1/2 EDMR center named“C-face defects” and behaves as an interfacial hole trap. This center is responsible for the effective hydrogen passivation of the C face. We
also identify a minor type of interface defect at this interface called “P8 centers,” which appear as spin-1 centers. Judging from their similarity
to the P7 centers (divacancies, VSiVC) in SiC, they were assigned to be a sort of basal-type interfacial VSiVC defect. Since both the CSiVC andVSiVC defects are known as promising single photon sources (SPSs) in SiC, the wet oxidation of the C face will have good potential for developing SPSs embedded at SiC surfaces.
Published under license by AIP Publishing. https://doi.org/10.1063/1.5116170
4H-SiC metal-oxide-semiconductor ﬁeld-effect transistors(MOSFETs) are promising high-power and ultra-low-energy-loss
switching transistors for next-generation power electronics. The
standard 4H-SiC MOSFETs are fabricated on the front side of
4H-SiC(0001) wafers (the “Si face”) in association with the thermaloxidation of the Si face in dry O2 ambience.1 On the other hand, theback side of the same wafer, i.e., 4H-SiC(0001 ) surface or “C face,”
exhibits quite different behaviors. For the C face, wet oxidation in
H2O ambience is extremely useful for fabricating high-quality MOS
interfaces.2,3 For instance, the ﬁeld-effect mobility (lFE) easily exceeds
70 cm2 V1 s1 in wet-oxidized C-face MOSFETs, which is muchhigher than the standard values (40 cm2 V1 s1) in Si-face
MOSFETs.4,5 While the reason behind such a drastic improvement isstill a mystery, it is commonly held that interface defects associatedwith the wet oxidation are closely related to this behavior.6 The wet
oxidation is also important for other useful SiC surfaces such as
4H-SiC(1 100) (“m face”) and 4H-SiC(112 0) (“a face”), as these surfaces exhibit a similar sensitivity to the wet oxidation.7 To reveal interface defects related to the wet oxidation, we have so far applied
electrically detected magnetic resonance (EDMR) spectroscopy to thewet-oxidized C face and found some characteristic interface defects

Appl. Phys. Lett. 115, 151602 (2019); doi: 10.1063/1.5116170
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there,6,8 including one major type called “C-face defect.” However,their microscopic origins still remain unclear.
In this Letter, we identify the origin of the C-face defects on thebasis of their hyperﬁne (HF) interaction due to a 13C nuclear spin. The
resolved 13C HF interaction clearly indicates that the C-face defects
belong to the family of positively charged carbon-antisite-carbonvacancy complexes, CSiVC(þ). This type of defect is known as a fundamental defect in p-type 4H-SiC. In the bulk, four types of bulk
CSiVC(þ) have been identiﬁed,9 as summarized in Figs. 1(a) and 1(b).
In contrast, the wet oxidation of the C face selectively generates c-axial
types of CSiVC at the interface [Figs. 1(a) and 1(c)], which is believed
to correlate with atomistic features of wet oxidation and the C face.
Moreover, we here show the presence of other characteristic interface
defects at the wet-oxidized C face, labeled “P8 centers.” They are spin1 ESR centers belonging to the family of divacancy defects (VSiVC, theP6/P7 centers10). It is worth noting that both the CSiVC and VSiVC
defects are known as promising single photon sources (SPSs) in
SiC.11,12 Thus, we can expect that the wet oxidation of the C face has
potential for exploring SPSs embedded at SiC surfaces.
For EDMR measurements, n-channel lateral C-face 4H-SiC
MOSFETs (gate length/width ¼ 2/200 lm) were prepared using a 4 -off

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FIG. 1. Atomic models for interface defects at the wet-oxidized 4H-SiC(0001)/SiO2
interface and for bulk CSiVC(þ) centers9 and bulk VSiVC centers10 in 4H-SiC. (a) In
the upper part, bulk CSiVC and VSiVC defects (hh, kk, hk, and kh types) are displayed. In the lower part, interface defects at the C face are shown. Blue bonds
express unpaired-electron orbitals in CSiVC and VSiVC, which are located on a CSi
atom and on three C atoms of the VSi side, respectively. (b) Bulk c-axial CSiVC(þ)
centers (HEI9a and HEI9b centers).9 (c) Interface c-axial CSiVC(þ) centers (indistinguishable hh and kk types). Blue dotted lines represent the periodicity of their
2nd nearest neighbors.

p-epitaxial layer (net acceptor concentration  5  1015 cm3).6,13
Wet oxidation at 1000  C (oxide thickness ¼ 60 nm) and H2 postoxidation anneal (POA) at 1100  C were applied to the MOSFETs. We
have already reported that EDMR observations on the C-face defects
were successful only in the wet oxidation of the C face.6,13 In fact,
the C-face defects were not observed in dry-oxidized C-face
MOSFETs with the same epitaxial layer, the same implanted source/
drain/well regions, and the same thermal history.8 We also conﬁrmed that H2 POA after the dry oxidation is not useful to create
the C-face defects and the P8 centers. It is also sure that these centers are not implantation damages because they were not observablein SiC-MOSFETs with an implanted (1020 cm3) channel region.13
Their EDMR signals increased after c-ray irradiation due to
breaking of the hydrogen (H) passivation of the C-face defects.6 In thiswork, we further tried to maximize their EDMR signals by applying
moderate postoxidation annealing at 900  C in N2 ambience, whichremarkably damages the H passivation of C-face MOSFETs.4 After the
900  C anneal treatment, the maximum lFE of the MOSFETs
decreased from 80 to 28 cm2 V1 s1. We then performed roomtemperature EDMR measurements on the annealed C-face MOSFETs
by utilizing either the gate-controlled-diode (GCD) EDMR modeor bipolar-ampliﬁcation-effect (BAE) EDMR mode.14 We foundthat the C-face defects were detectable in both the modes, and theGCD EDMR mode gives us a better signal-to-noise ratio for them(instead, the BAE mode gave us larger current changes). In this mode,
a forward current of 50–60 nA was driven from the drain/source
region to the well region, under applying a gate bias (VG) of 10 V.
The negative VG was indispensable for increasing the EDMR signals of
the C-face defects because they can be converted from ESR-inactive
states (neutral states) into ESR-active states (þ1 charge states) by

Appl. Phys. Lett. 115, 151602 (2019); doi: 10.1063/1.5116170
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capturing holes.6 The EDMR signals (current changes induced by ESR
transitions) were excited by a microwave of 9.462 GHz and 200 mW
and were ampliﬁed by a lock-in ampliﬁer synchronized to a magneticﬁeld modulation at 1.56 kHz.
Figure 2(a) shows a typical EDMR spectrum of the C-face defects
in the annealed C-face 4H-SiC MOSFET. The spectrum clearly showsa doublet HF splitting of 1.1 mT, which is a characteristic signature of
the C-face defects.6,8 Figure 2(c) examines angular dependences of the
EDMR signals, which can be well ﬁtted by three central solid lines.
These lines were simulated by adopting an electron spin (S) of 1/2, a
C3v (c-axial) symmetry, the g tensor shown in Table I, and with/without an isotropic doublet HF splitting of 1.1 mT. All the above signatures are consistent with those of the C-face defects.6
Looking carefully at both tails of the EDMR signals [Fig. 2(b)],we ﬁnd a weak doublet HF splitting due to a 13C isotope (I ¼ 1/2,
natural abundance ¼ 1.1%). Blue solid lines express simulated 13C HF
doublets, which were made from two replicas of the experimental
spectrum shown in Fig. 1(a). The relative intensity of the replicas was
set to be 0.55% in the whole spectrum. By assuming appropriatesmooth baselines [dashed blue lines in Fig. 2(b)], the simulated 13C
HF doublets can excellently reproduce the tails of the EDMR signals.
In Fig. 2(c), we analyzed the angular dependences of the 13C HF splitting (solid symbols). In the 13C HF doublets, we could resolve one sideof additional HF doublets (open symbols) due to the 1.1-mT splitting.
As shown in the ﬁgure, both the 13C HF and 1.1-mT doublets are wellsimulated by solid lines, which were calculated using a 13C HF tensor
shown in Table I.
As shown in Table I, the determined 13C HF tensor is similar to
those of the known CSiVC(þ) centers (the HEI9a and HEI9b centers)

FIG. 2. (a) EDMR spectrum of C-face defects in annealed C-face 4H-SiC
MOSFETs with wet oxidation (SiO2 thickness ¼ 60 nm). The magnetic ﬁeld (B)was modulated by an amplitude of 0.25 mT and the spectrum was measured for B//[0001]. (b) A doublet 13C HF structure of C-face defects. Blue lines indicate simulation of 13C HF doublets (see details in the text). (c) Angular maps of C-face defects.
Solid and open circles represent their resonant ﬁelds for without and with an isotropic 1.1-mT HF splitting, respectively. The magnetic ﬁeld was rotated from 30 to120 where B//[0001] for 0 and B//½1 120 for 90 . Solid lines are simulated byusing the g and 13C HF tensors listed in Table I. There are other minor defects
which revealed isotropic signals (see grayed symbols). They are assigned toannealing damages because they were absent before annealing.

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TABLE I. Spin-Hamiltonian parameters of C-face defects at the 4H-SiC(0001)/SiO2 interface and bulk CSiVC(þ) centers in p-type 4H-SiC. The spin-Hamiltonian in this system
is deﬁned as H ¼ lBS  g  B þ S  A  I  gnlnI  B,15 where lB is the Bohr magneton, S is an electron spin operator, g is a gyromagnetic-factor tensor, B is an external
magnetic-ﬁeld vector, A is a 13C HF tensor, and the third term represents a nuclear-spin Zeeman energy for a 13C nucleus. Both the g and A tensors are well described byaxially symmetric tensors (e.g., Azz Ajj and Axx ¼ Ayy A?) with respect to the c axis.

A(13C) (mT)gCenterOrigin (charge)

Spin (symmetry)Wet-oxidized C-face 4H-SiC MOSFETs, present and Ref. 6
C-faceInterfacial
1/2
defectCSiVC(þ)(C3v)
p-type 4H-SiC bulk, Ref. 9
HEI9a
CSiVC(þ) kk site
HEI9b
CSiVC(þ) hh site

1/2
(C3v)

in p-type 4H-SiC;9 speciﬁcally, we found good agreement between thepresent center and the HEI9a center. In these bulk CSiVC(þ), a main
13
C HF interaction is generated from a CSi atom of CSiVC where anelectron spin is localized on a strongly p-like orbital [blue bonds in
Figs. 1(a) and 1(b)]. Moreover, comparing the C-face defects with the
HEI9a/b centers, their electron spin (1/2), their symmetry (C3v), andtheir charge state (þ1) are also consistent with each other. Therefore,
we conclude that the C-face defects belong to the family of HEI9a/b
centers, i.e., they are c-axial types of interface CSiVC(þ) centers at the
wet-oxidized C face, as illustrated in Figs. 1(a) and 1(c).
However, a few fundamental questions on the above model
remain. The ﬁrst is an apparent inconsistency in the g tensors between
the C-face defects and the HEI9a/b centers (Table I). We attribute thisinconsistency to a perturbation of the interface. In fact, we alreadyreported such a perturbation on the g tensor for the cases of typical
dopants (nitrogen donors16 and phosphorous donors17) when they areincorporated into the close vicinity of the interface.
The second is why only one type of EDMR center was observable
at the interface despite four types of ESR centers distinguishable inbulk 4H-SiC [Fig. 1(a)]. In the bulk, two c-axial types [kk and hh types,
see Fig. 1(b)] are distinguishable as the HEI9a and HEI9b centers,
respectively.9 A distinguishable point between them is the periodicityof their 2nd nearest neighbors (NNs), which are indicated by the blue
dotted lines in Fig. 1(b). For the HEI9a and HEI9b centers, each 2nd
NN atom connects to either the same column or a next column,
respectively. Such a difference results in the distinguishable ESR centers. In contrast, at the interface shown in Fig. 1(c), each 2nd NN atomshould connect to the amorphous SiO2 network. This constraint supposedly makes the hh and kk types indistinguishable at the interface.
The interface CSiVC defects reasonably account for the effective
H passivation of the C face.2–4 Our previous ESR study conﬁrmed thatthe HEI9/10 centers (CSiVC) are one of the most reactive ESR centersto the H passivation in p-type 4H-SiC.18 An unpaired electron on the
CSi atom can be passivated in the form of C-H bonds, and such passivation can be removed at above 800  C.18 This temperature range isconsistent with the degradation temperature observed for wetoxidized C-face MOSFETs.4
In addition to the CSiVC defects, there is a minor but interesting
ESR center at the wet-oxidized C face, which we call the P8 centers.
Figure 3(a) shows EDMR signals of two P8 centers (P8a and P8b) that

Appl. Phys. Lett. 115, 151602 (2019); doi: 10.1063/1.5116170
Published under license by AIP Publishing

g//

g?

A//

A?

2.00182.00278.0

2.1

2.002272.00195

2.00408
2.00379

8.25
9.95

2.27
3.71

were only detectable in wet-oxidized C-face MOSFETs. These P8 centers exhibit a large zero-ﬁeld splitting (ﬁne splitting) due to a close pairof two electron spins (S ¼ 1). From the angular-map analyses shownin Fig. 3(b), their ﬁne-splitting constants (D and E10) are estimated to
be D ¼ 50 mT and D ¼ 43 mT for P8a and P8b, respectively, andE  1 mT for both the centers. These parameters are quite similar to

FIG. 3. (a) EDMR spectrum of P8 centers at the wet-oxidized 4H-SiC(0001)/SiO2
interface. This spectrum was measured for a forward current of 85 nA under VG¼ 27 V in wet-oxidized (nonannealed) C-face MOSFETs. A strong central line
arises from C-face defects (see Fig. 2). At least two spin-1 ESR centers (P8a andP8b) are resolved in the ﬁgure. (b) Angular maps of P8 centers. Magnetic-ﬁeld rotation was set to be the same manner as used in Fig. 2. Solid lines are simulated
angular patterns (see the text). (c) Gate-bias dependence of P8 centers. (d)Energy-level calculations on spin-1 systems of P8a and P8b (red and green lines,
respectively) for a magnetic-ﬁeld angle of 55 .

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Applied Physics Lettersthose of the P7 centers in bulk 4H-SiC (D ¼ 43–48 mT andE < 0.7 mT).10,19 The P7 centers are basal-type divacancies (VSiVC)
conﬁgured by 70 -off from the c direction, as is drawn on the righthand side of Fig. 1(a). Judging from the similarity between the P8 andP7 centers, we assign the P8 centers to be a sort of interfacial VSiVC
defect in a basal conﬁguration. The P8 centers became detectable
under a strong negative gate bias [Fig. 3(c)]. This is due to the gatebias control of P8’s charge state, similar to the case of the C-facedefects.6 One notable difference between the P8 and P7 centers is seenin their symmetrical axes where each ﬁne splitting has a maximum.
For the P8 center, the symmetrical axis is tilted by about 55 from thec axis [Fig. 3(b)], which is deviated from 70 for the P7 centers10 [seeFig. 1(a)]. The tilt angle of 55 implies that the P8 centers are slightly
rotated from the original VSiVC centers, like shown in the right-handbottom side of Fig. 1(a), possibly due to an inﬂuence of the interface.
Looking over the above results, one ﬁnds an interesting contrast:
basal types of CSiVC (kh and hk types) are missing, while c-axial typesof VSiVC (kk and hh types) are absent at the interface [see the bottomof Fig. 1(a)]. Unfortunately, we do not have a clear answer to this question; however, we believe that this is strongly correlated with the wetoxidation mechanism of the C face. Some tentative reasons are given
with the help of Fig. 1(a). As seen in the ﬁgure, the basal types of interface CSiVC place VC as well as surrounding Si atoms in the interface
side, while the c-axial types place them in the bulk side. As a result, thebasal types are expectedly much more reactive to O atoms than thec-axial types, which may promote the selective annihilation of the basal
types. With the same analogy, we reasonably expect that the amountof P8 centers (the basal-types VSiVC) should be much less than that of
C-face defects (the c-axial types of CSiVC). On the other hand,“missing” c-axial types of VSiVC [bottom of Fig. 1(a)] possess
unpaired-electron orbitals in the interface side, which are directly
exposed to the oxidants. As a result, the c-axial types of interface
VSiVC may be difﬁcult to survive.
Another interesting feature is that both interface defects are“vacancy types.” Also for the Si face, the formation of interfacial Si
vacancies (VSi) was reported via EDMR analyses.20 It is worth mentioning that the bistability of CSiVC and VSi exists in SiC.9,21 In p-type
4H-SiC (note that the present channel region is p-type), CSiVC should
be more stable than VSi. We imagine that the oxidation of the C face
may initiate the extraction of a Si atom from SiC, forming a VSi-likestructure because the oxidation requires a reaction between Si and O.
Such a precursor may be subsequently stabilized into a CSiVC form.
The details of such atomistic mechanisms will be revealed by theoretical calculations in the future. Here, we should emphasize that the formation of CSiVC and VSiVC can only be successful in cooperation with
H atoms. When the C face is oxidized by dry O2, its interface generates
sp2-bonded C cluster defects,8 instead of the CSiVC and VSiVC defects.
We observed the isotropic HF splitting of 1.1 mT for CSiVC, suggesting
the presence of a 1H HF interaction.6 Such a H atom possibly attaches
to the VC side of CSiVC. Since this HF interaction was alive even after
the 900  C anneal treatment, the survival of a Si-H bond in the VC side
may occur. Another possible assignment for the 1.1-mT HF splitting
may be 29Si HF interactions of three Si atoms of VC. Both the models
will be inspected in the future.
The identiﬁcation of interface defects in this study rehighlights
the importance of wet oxidation of the C-face, as both CSiVC andVSiVC are known as promising SPSs in SiC.11,12 In fact, the ﬁrst step inAppl. Phys. Lett. 115, 151602 (2019); doi: 10.1063/1.5116170
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creating SPSs in SiC-MOSFETs has been recently achieved using wetoxidized C-face MOSFETs.22,23 In Fig. 3(d), we found zero-ﬁeld splittings of 1.40 GHz and 1.21 GHz for the P8a and P8b centers. Note that
the P8 centers may be more suitable to SPSs rather than bulk VSiVC,
because the P8 centers are easily observable at room temperature.
In summary, we have clariﬁed the microscopic origins of major
and minor interface defects at wet-oxidized 4H-SiC(0001 )/SiO2 interfaces in C-face 4H-SiC MOSFETs. The major defects, called C-facedefects,6,8 have been identiﬁed as c-axial types of interface CSiVC
defects. We also found a minor type of interface defect (the P8 centers), exhibiting spin-1 ESR centers with a zero-ﬁeld splitting of
1.2–1.4 GHz. They were ascribed to a sort of basal-type interfacial
VSiVC defects. The selective formation of c-axial CSiVC and basal
VSiVC at the interface is believed to strongly correlate with atomistic
mechanisms of wet oxidation of the C-face, which were brieﬂy discussed in conjunction with the interface structure.
This work was supported by the Council for Science,
Technology and Innovation (CSTI), the Cross-ministerial Strategic
Innovation Promotion Program (SIP), and “Next-generation powerelectronics” (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 ofJapan.
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19

The D and E parameters of P8 are also close to those of the VSiNC centers (divacancy þ N atom) reported in H. J. von Bardeleben, J. L. Cantin, A. Csore, A.
Gali, E. Rauls, and U. Gerstmann, Phys. Rev. B 94, 121202(R) (2016). However,since the P8 centers were absent in N-doped implanted channel13 and basal
types of VSiNC centers have not been found so far, we assigned P8 to a familyof basal-type divacancies.
20
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