Electron-spin-resonance and electricallydetected-magnetic-resonancecharacterization on PbC center in various 4HSiC(0001)/SiO2 interfaces
Cite as: J. Appl. Phys. 127, 145301 (2020); https://doi.org/10.1063/1.5134648
Submitted: 01 November 2019 . Accepted: 18 March 2020 . Published Online: 09 April 2020
T. Umeda, Y. Nakano, E. Higa

, T. Okuda, T. Kimoto, T. Hosoi, H. Watanabe, M. Sometani,and S. Harada

J. Appl. Phys. 127, 145301 (2020); https://doi.org/10.1063/1.5134648© 2020 Author(s).

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Electron-spin-resonance and electricallydetected-magnetic-resonance characterization on
PbC center in various 4H-SiC(0001)/SiO2 interfaces
Cite as: J. Appl. Phys. 127, 145301 (2020); doi: 10.1063/1.5134648
Submitted: 1 November 2019 · Accepted: 18 March 2020 ·Published Online: 9 April 2020
Y. Nakano,1 E. Higa,1T. Umeda,1,a)4
and S. Harada

T. Okuda,2T. Kimoto,2View Online

T. Hosoi,3Export Citation

CrossMark

H. Watanabe,3 M. Sometani,4

AFFILIATIONS
1
2

Institute of Applied Physics, University of Tsukuba, Tsukuba 305-8573, Japan
Graduate School of Engineering, Kyoto University, Kyoto 615-8510, Japan

3

Graduate School of Engineering, Osaka University, Osaka 565-0871, Japan

4

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

Note: This paper is part of the Special Topic on Defects in Semiconductors 2020.
a)

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

ABSTRACT
We characterized an intrinsic interface defect, called the “PbC center,” formed at 4H-SiC(0001)/SiO2 interfaces by means of electron-spinresonance (ESR) and electrically detected-magnetic-resonance (EDMR) spectroscopies. The formation of the PbC center was observed with aspin density of 3–4 × 1012 cm−2 after standard thermal oxidation. This center could be effectively removed by the NO post-oxidation-anneal(POA) process or ultra-high-temperature oxidation and could be passivated by H atoms via the H2 POA process. There was a clear correlation between the PbC center and field-effect mobility (μFE) of 4H-SiC(0001) metal–oxide–semiconductor field effect transistors (MOSFETs).
The PbC center decreased μFE because this center acts as electron traps, reducing the free-carrier density in the inversion channel of 4H-SiC(0001) MOSFET. We also examined the counter doping effect of NO POA by introducing 15N impurities; however, the counter doping of
15
N donors was not detectable by ESR (much lower than 2 × 1011 cm−2). Highly sensitive EDMR measurements revealed that the PbC centerhas two isotropic hyperfine (HF) interactions at 1.3 and 6.8 mT and suggested that its main 13C HF interaction should be larger than
14 mT. Based on the present experimental data, the origin of the PbC center was ascribed as a carbon-related interface defect that forms a
C–H bond after hydrogen passivation. This feature is similar to that of the porous-PbC centers (carbon dangling-bond centers) found inporous-SiC/SiO2 systems. However, their HF signatures indicated that the PbC center at 4H-SiC(0001)/SiO2 interfaces and the porous-PbC
centers in porous-SiC should be different centers associated with different wave functions.
Published under license by AIP Publishing. https://doi.org/10.1063/1.5134648

I. INTRODUCTION
The 4H-SiC/SiO2 interface is the key part for 4H-SiC metal–
oxide–semiconductor field effect transistors (MOSFETs) and is abenchmark system for every wide-bandgap semiconductor MOS
interface.1 The standard method for fabricating good 4H-SiC/SiO2
interfaces is thermal oxidation by dry O2 gas (dry oxidation) followed by interfacial nitridation such as NO post-oxidation anneal(POA).2 This process can reduce shallow interface states near the
conduction band edge (EC)2,3 and deeper electron traps,4 resulting
in a drastic improvement on the field-effect mobility (μFE) of

J. Appl. Phys. 127, 145301 (2020); doi: 10.1063/1.5134648
Published under license by AIP Publishing.

SiC-MOSFETs. As a result, the improved μFE reaches
30–40 cm2 V−1 s−1 for 4H-SiC(0001) (so-called “Si face”) MOS
interfaces, realizing ultra-low-energy-loss performances of power
SiC-MOSFETs as compared to those of conventional power
Si-MOSFETs.1,5 However, the improved μFE is still orders of magnitude lower than the electron mobility of 1000 cm2 V−1 s−1 in bulk
4H-SiC. Therefore, many challenges have been made for further
improving μFE, such as phosphorous (P) incorporation by POCl3
POA,6 boron incorporation,7 Ba incorporation,8 hydrogen (H)incorporation by “wet-POA,”9 counter doping of shallow donors

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into the channel region,10 and oxidation techniques without any
additive elements.11
Recently, we reported an electron-spin-resonance (ESR) observation on carbon-related paramagnetic defects at the 4H-SiC(0001)/
SiO2 interface,12 which correlate closely to the improvements on μFE.
They were intrinsically formed by the dry oxidation of 4H-SiC(0001)
surfaces and were successfully removed by NO POA or POCl3 POA.
Their typical density was 3–4 × 1012 cm−2, which is high enough for
affecting μFE drastically. We found that their ESR signal has somesimilarities to previous ESR signals called “PbC centers.”13,14 The PbC
centers were originally found in porous-SiC/SiO2 interfaces and havebeen identified as carbon dangling-bond (DB) centers with several
different orientations.13 The name of “PbC” comes from its analogy
to the Pb centers (Si DB centers15) at Si/SiO2 interfaces. Here, we distinguish our PbC center in 4H-SiC(0001) and the PbC centers inporous-SiC; we call the former one as “PbC center” and the latter one
as “porous-PbC centers.” The reason for this distinction originatesfrom spectroscopic differences between two types of PbC centers,
which will be described later.
In this paper, we characterize the PbC center at 4H-SiC(0001)/
SiO2 interfaces in more detail by employing two techniques: one isconventional ESR spectroscopy12 and another is electricallydetected-magnetic-resonance (EDMR) spectroscopy16–19 enabling
electrical detection of ESR signals. ESR can give us spin densities of
the PbC center. Using this technique, we examined a correlation
between the PbC center and μFE for a wide variety of 4H-SiC(0001)/
SiO2 interfaces prepared by a ultra-high-temperature (UHT) oxidation process11 and an 15NO POA process. The former process canachieve several times higher μFE without adding any intentional
impurities. The latter process enables us to monitor 15N impurities
introduced by the counter doping mechanism.10 Furthermore, we
demonstrate that the PbC center can be passivated via the H2 POA
process, likewise the porous-PbC centers14 and the original Pb
centers.15 In the next place, we discussed the origin of the PbC
center on the basis of EDMR data. EDMR achieved an excellent
sensitivity to the PbC center in real 4H-SiC(0001) MOSFETs.

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Taking advantage of higher sensitivity, we focused on spectroscopic
features of the PbC center, in particular, hyperfine (HF) interactions
between its electron spin and surrounding nuclear spins.
II. EXPERIMENTAL
The PbC center was detected by both ESR and EDMR
spectrometers. For ESR measurements, we used a Bruker
E500 continuous-wave X-band spectrometer equipped with asuper-high-Q cavity and an Oxford ESR900 continuous-flow liquid
He cryostat. All spectra were measured using magnetic-field modulation at 100 kHz and microwave excitation at 9.43 GHz. The PbC
center was observed at room temperature because roomtemperature ESR spectra simply consisted of the PbC signal and an
E0 signal from a SiO2 sample rod. On the other hand, the shallow
N donors were measured at either 20 K or 50 K. The photoexcitation (100-W Halogen light) was used in low-temperature
measurements because the shallow donors were partially compensated in our substrates. Magnetic field (B) was usually set to asurface-normal direction (B parallels to the c axis or the [0001]
axis) unless otherwise stated. Magnetic-field values were calibrated
by a built-in nuclear-magnetic-resonance (NMR) tesla meter withan accuracy of 0.01 mT or finer.
Since ESR measures the whole of the substrate, we have tocarefully prepare ultra-high-quality 4H-SiC(0001) substrates toobserve interface defects. We used full-epitaxial-layer substrates
with very low N or Al concentrations (4 × 1014 cm−3). Both front
and back surfaces of the substrate were finished by the chemicalmechanical polish (CMP) process. For most of the substrates, wehave further carried out surface cleaning by dry etching, which wecall a “reset process.”12 The effect of the reset process can be seenin Sec. III A. Using 4H-SiC(0001) faces of such substrates, we characterized an UHT oxidation process and a 15NO POA process.
Table I describes details of the ESR samples and processes. The
UHT oxidation samples were prepared by the same UHT furnaceand the same experimental procedure as used in previous works.11
For the 15NO POA experiment, we prepared a 15NO gas with 15N

TABLE I. List of 4H-SiC(0001) ESR samples. All substrates are 4H-SiC(0001) full-epitaxial layers with both-side CMP finishing. Oxidations and POA processes were done at
atmospheric pressure. Just before oxidation, standard RCA surface cleaning was subjected to substrates. After oxidation, the back-side SiO2 layer (i.e., a C-face oxide layer)
was removed by one-side HF etching.12 Field-effect mobilities (μFE) shown here are maximum values.

4H-SiC(0001) ESR
samplesDry (Ref)aDry (UHT ox.)a
Substrateb
Dry (Normal)b
Dry (Normal) + 15NOb
Dry (Normal) + NOc

Process
Dry oxidation at 1200 °C by 100% O2
Dry oxidation at 1600 °C by 0.3% O2 + 97.7% Ar
Only the same surface cleaning process
Dry oxidation at 1200 °C by 100% O2“Dry (Normal)” after 15NO POA (1.1% 15NO + 98.9%
N2) at 1440 °C for 20 min“Dry (Normal)” after NO POA at 1250 °C for 60 min

SiO2 thickness(nm)

PbC density(×1012 cm−2)

μFE(cm2 V−1 s−1)

30
30
0
50
50

3.2
0.7
…
3.8
0.5

3.0d
9.7d
…
7.2e
20.5e

50

<0.11232e

Epitaxial layer with [N] = 4 × 1014 cm−3 and 100 μm-thick, subjected to the reset process + RCA cleaning before oxidation.
Epitaxial layer with [N] = 4 × 1014 cm−3 and 160 μm-thick, subjected to sacrificed oxidation cleaning + RCA cleaning before oxidation.
cEpitaxial layer with [Al] = 4 × 1014 cm−3 and 130 μm-thick, subjected to the reset process + RCA cleaning before oxidation.12d
MOSFETs were fabricated on 4°-off p-type epitaxial layer ([Al] = 5 × 1015 cm−3).11e
MOSFETs were fabricated on 4°-off p-type epitaxial layer ([Al] = 1 × 1015 cm−3).
ab

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enrichment = 100%. The POA temperature was set to be higher
than the standard condition (1250 °C4,12 → 1440 °C) because weaimed for interfacial nitridation as complete as possible using adiluted 15NO atmospheric mixture (1.1% 15NO + 98.9% Ar).
Together with ESR samples, lateral n-channel 4H-SiC(0001)
MOSFET samples were also prepared for electrical characterizations. Maximum μFE values are listed in the table.
For H2 POA experiments, we used 100% H2 gas and the same
experimental setup used in our previous studies.20,21 Using thissetup, we successfully observed the H-passivation behaviors oflifetime-killer centers in p-type epitaxial layers20 and even those of
intrinsic defects in thick substrates.21 We also carried out aH-dissociation experiment by Ar-gas annealing at 900 °C.
For EDMR measurements, we prepared lateral n-channel
4H-SiC(0001) MOSFETs on an optimum 4°-off p–-type epitaxial
layer ([Al] = 1 × 1015 cm−3). A gate SiO2 layer (30 nm) was formed
by standard dry oxidation as used for the ESR samples. The
MOSFETs with an optimum gate size (gate length/width = 5/2000μm) were characterized by a home-built EDMR spectrometer basedon a Bruker ESP300 X-band spectrometer. The magnetic-field
modulation frequency was set to 1.5 kHz for an optimum
signal-to-noise ratio (S/N). To enhance EDMR signals of the PbC
center, we adopted the bipolar-amplification-effect (BAE) EDMR
technique developed by Aichinger and Lenahan.17 In this regime,
we activated a constant drain current of −10 μA (a negative signmeans a forward-biased current) and monitored a source current
by an EDMR (spin-dependent current) detector. Then, by varying
a gate bias, we searched for the best BAE EDMR signals arisingfrom paramagnetic interface states. The highest S/N figure wasobtained for a source current of ∼200 nA and a gate bias of approximately −8 V in our MOSFETs. All EDMR measurements werecarried out at room temperature and microwave excitation at
9.46 GHz and 200 mW. For EDMR, magnetic-field values were calibrated by an Echo Electronics EFM-2000 NMR tesla meter with anaccuracy of the order of 0.01 mT.

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FIG. 1. ESR measurements on the PbC center at the 4H-SiC(0001)/SiO2 interface and after the 15NO POA process (see the sample list in Table I). ESR
spectra were measured under a microwave excitation of 0.2 mW and a modulation amplitude of 0.25 mT.

III. ESR RESULTS
A. Formation of the PbC center
Here we examine the formation behavior of the PbC center,
which is the following work of our previous paper.12 Figures 1 and 2
show ESR studies on two different types of substrates (see Table I).
The substrates examined in Fig. 1 were prepared without the reset
process.12 An isotropic broad ESR signal appeared at the g factor (g)of 2.003, which is most probably due to residual carbons on thesurface.12 This signal could not be eliminated even by sacrificed oxidation cleaning (surface oxidation + HF etching). However, after standard dry oxidation (50-nm growth), we can observe an appreciable
increase in the ESR signal, which is due to the formation of the PbC
center. By extracting the increment of the signal, the spin density of
the PbC center was estimated to be 3.8 × 1012 cm−2.
On the other hand, a series of ESR spectra shown in Fig. 2
were obtained by taking advantage of the reset process. As shownin (a), the residual-carbon ESR signal completely disappeared after
the reset process, showing a flat baseline except the E0 signal. Afterdry oxidation (30-nm growth), the PbC center with 3.2 × 1012 cm−2
was clearly detected as shown in (b). We always found spin

J. Appl. Phys. 127, 145301 (2020); doi: 10.1063/1.5134648
Published under license by AIP Publishing.

FIG. 2. ESR spectra of the PbC center in various 4H-SiC(0001)/SiO2 interfaces(see the sample list in Table I). (a) After the reset process (surface dry etching).
(b) After standard dry oxidation at 1200 °C. (c) After ultra-high-temperature(UHT) oxidation at 1600 °C. (d) After H2 POA at 900 °C. All ESR spectra were
measured after C-face-oxide etching, and ESR signals arise from the Si-faceside, except a reference E0 signal from a sample rod. All ESR spectra were
measured under a microwave excitation of 0.2 mW and a modulation amplitudeof 0.25 mT.

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densities of PbC in the range between 3 × 1012 and 4 × 1012 cm−2,
independent of oxide thicknesses from 10 to 60 nm.
It is worth noting that the above spin densities measured forthe n–-type substrates were nearly equal to those measured for the
p–-type substrates.12 This suggests that the surface Fermi levelapproached the same position in both types of substrates. Possibly
the PbC center may pin the surface Fermi level close to its energy
level. Since the PbC center is considered as a spin-1/2 paramagnetic
center ( judging from the absence of any fine splittings), its
one-electron energy level can capture an additional electron.
Namely, each PbC center acts as an electron trap. When the inversion channel is going to form in an n-channel 4H-SiC(0001)
MOSFET, the PbC center captures free electrons at the outset. After
filling out all the energy levels of the PbC center, the Fermi levelcan be released from there, moving toward EC for a strong inversion condition. This is a mechanism of how the PbC center affects
electrical characteristics of the 4H-SiC(0001) MOSFET.
B. Removing the PbC center by UHT oxidation and POA
processes
Previously, we reported that the PbC center can be eliminatedby optimum NO POA and POCl3 POA.12 We here confirm
whether the PbC center can also be removed by controlling oxidation conditions alone and without any additive impurities.
Figure 2(c) shows an ESR spectrum measured after an optimum
UHT oxidation at 1600 °C (see Table I), which clearly indicates a
successful reduction in the PbC signal. The PbC density decreased to
0.7 × 1012 cm−2. The UHT oxidation used here was developed tomaximize the removal of residual-carbon atoms by tuning bothoxygen partial pressure and oxidation temperature toward a passiveoxidation–active oxidation boundary.11 The ESR result demonstrates the removal of carbon-related defects (the PbC centers), supporting the above concept from a microscopic viewpoint.
In Fig. 1, we examine the 15NO POA effect on the PbC center.
After the 15NO POA process, the ESR signal almost returned to theoriginal signal (before oxidation), indicating the annihilation of the
PbC center. The remaining density of PbC was estimated to be
0.5 × 1012 cm−2. This value is larger than that of an optimum NO
POA process (≤0.1 × 1012 cm−2),12 implying some incompleteness of
the interfacial nitridation.
C. Influence of the PbC center on μFE
Figure 3 summarizes a relationship between the PbC signals
and the maximum μFE values. Basically, removing the PbC center
increased the top values of μFE. This behavior is reasonable because
as we mentioned in Sec. III A, the PbC center has a role of reducing
the free-carrier density in the inversion channel of the 4H-SiC(0001) MOSFET. Note that μFE depends on both the mobility andthe density of the free carriers.4 The removal of the PbC centers
directly connects to a higher free-carrier density, resulting in ahigher μFE. Since the PbC density of 3–4 × 1012 cm−2 is just comparable with the free-carrier density in the inversion channel,3,4 thePbC center strongly suppresses μFE. In Sec. III E, we will demonstrate using 15N impurities that the counter doping effect due to
NO POA was much weaker than the effect of removing the PbC
centers. Therefore, the improved μFE shown in Fig. 3 originates

J. Appl. Phys. 127, 145301 (2020); doi: 10.1063/1.5134648
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FIG. 3. Spin densities of the PbC center versus maximum μFE values.

primarily from the removal of the PbC centers. In addition, thereduction in the shallow interface states, which were clearly
detected in the range between EC – 0 eV and EC – 0.7 eV after NO
POAs,3,4 will also contribute to the improvement.
D. Hydrogen passivation of the PbC center
Another way for reducing the influence of the PbC center may
be H passivation. Such passivation is well established for the original Pb centers at Si/SiO2 interfaces.15 Moreover, in porous-SiC/
SiO2, H passivation of the porous-PbC centers was observed by aforming-gas POA (H2 5% + N2 95%) at 400 °C, resulting in a
drastic reduction in the porous-PbC signal (<5% of the original
intensity before the POA).14 We, therefore, examined H passivationof the PbC center in 4H-SiC(0001) using the H2 POA process.
Figure 4 shows normalized intensities of the PbC signal, which
represent the passivation rate (%) as a function of H2 POA temperature. Efficient passivation was observable only at around 900 °C.
The largest passivation rate was 10%–40% depending on the H2
POA time. It seems that H passivation of the PbC center is not aseasy as the cases of the Pb center (efficient passivation was available
at 220–550 °C15) and the porous-PbC centers (H passivation started
at 400 °C and maintained up to 800 °C14). This result may be inline with our common knowledge that 4H-SiC(0001)/SiO2 interfaces are generally insensitive to the incorporation of H atoms sothat other additive impurities (N, B, P, Ba, etc.) have been tried so
far.1 Nevertheless, the present result shows the potential ability of
H passivation if the passivation condition is carefully designed.
Actually, the “wet-POA process” (the POA process by H2O
ambient)9 was proposed for obtaining good 4H-SiC(0001)/SiO2
interfaces comparable to those after NO POA. We speculate thatsuch a process may realize an efficient H passivation of the PbC
center.
As seen in Fig. 4, Ar annealing at 900 °C or H2 POA over
900 °C caused the recovery of the PbC signal due to dissociation of127, 145301-4

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doublet HF splitting for 15N. Since the HF interactions of shallow
N donors in 4H-SiC are already well known (see Table II),24
we can accurately predict those of shallow 15N donors via the following procedure. From the known 14N HF tensor (A// and A⊥
principal values), the isotropic and anisotropic HF parameters(Aiso and Aaniso, respectively) are derived using the equations
Aiso = (A// + 2A⊥)/3 and Aaniso = (A// – A⊥)/3.22,23 Ratios of Aiso/A0
and Aaniso/b0 represent 2s- and 2p-orbital fractions on a 14N donor,
where A0 and b0 are HF constants for pure 2s- and 2p-orbitals of
14 22
N. Supposing the same 2s- and 2p-orbital fractions on a 15N
donor (it is a quite reasonable assumption), we then obtain the Aiso
and Aaniso values for a 15N donor and subsequently its A// and A⊥
values, too. The predicted 15N HF splitting is 2.55 mT for the 15N(k) donor and is 0.15 mT for the 15N(h) donor, when B // [0001].
In Fig. 5, we independently examine 15N(k) and 15N(h) donor
signals by comparing the 15NO POA samples before and after the
POA. In every ESR spectrum, the 14N donor signals were dominant, which arise from residual donors in the n–-type substrates.
For the 15N(k) donor, its doublet signal must appear as simulated
in Fig. 5(a). However, we hardly detected such a doublet signal in
the experimental spectrum, and the ESR spectra before and after
15
NO POA involved only the 14N(k) donor signal with a spin
density of 1.8 × 1012 cm−2 or 1.1 × 1014 cm−3. The 15N(k) density
must be smaller than 1 × 1011 cm−2. The situation seems to be
similar for the 15N(h) donor [Fig. 5(b)]. Although small HF splittings of 14N(h) and 15N(h) do not allow to separate these twodonor signals, the ESR spectra before and after 15NO POA can be
accounted for only the 14N(h) donor signal. In fact, a spectral simulation [a dotted curve in Fig. 5(b)] calculating the 14N(h) donor
signal (a blue solid line, a spin density of 3.7 × 1012 cm−2) + a
carbon signal (a gray solid line) perfectly reproduced the experimental spectrum. We, therefore, judged that the contribution of the15
N(h) donors must be smaller than 1 × 1011 cm−2 [see a simulated
15
N donor signal in Fig. 5(b)]. Accordingly, there was no detectablecounter doping of 15N in the 15NO POA experiment. Totally, thecounter doping density must be rather smaller than 2 × 1011 cm−2.
Note that the observed spin density of the 14N donors[14N(k) + 14N(h)] is 3.4 × 1014 cm−3, which is similar to the doping
14

FIG. 4. H passivation of the PbC center by the H2 POA process. Solid circles
were measured for H2 POA for 2 min on 4H-SiC(0001)/SiO2 (50-nm thick).
Open squares and open triangles were taken for H2 POA for 30 min on 4H-SiC(0001)/SiO2 with 44 nm and 10 nm in oxide thicknesses, respectively. Solid anddashed curves are guides for eyes. Right-hand-side symbols show
H-dissociation experiments by Ar anneal for 30 min.

H atoms. Note that this temperature range is consistent with thedissociation temperature of C–H bonds.14,21 The porous-PbC
centers exhibited the same dissociation behavior above 850 °C.14
Accordingly, it is reasonable to consider that the PbC center forms
a C–H bond when H passivation combines a H atom with its
unpaired electron.
E. 15N doping by the 15NO POA process
In the present NO POA experiment using 15NO, we can distinguish residual 14N donors in the substrate and interfacial 15N
donors intentionally doped by NO POA. As tabulated in Table II,
14
N and 15N isotopes have different nuclear spins (I = 1 and 1/2,
respectively), which generate a triplet HF splitting for 14N or a

TABLE II. HF constants (A0 and b022) and HF parameters (A//, A⊥, Aiso, and Aaniso22,23) of shallow 14N and 15N donors. 14N HF parameters are experimental values,24 which
are converted in mT via 1 mT = 28.02 MHz. 15N HF parameters are predicted from 14N experimental values (see details in the text). All HF tensors are c-axial symmetric; i.e.,their A/ principal values correspond to HF splittings when B // [0001].

HF parameters
22

Nuclear spin (I) (natural abundance)

HF constants

14

N (I = 1)(99.63%)A0 = 64.6 mT
b0 = 1.98 mT

15

A0 = 90.6 mTa
b0 = 2.78 mTa

N (I = 1/2)
(0.37%)

k site, N(k)A// = 1.82 mT
A⊥ = 1.82 mT
Aiso = 1.82 mT
Aaniso < 0.001 mT
A// = 2.55 mT
A⊥ = 2.55 mT
Aiso = 2.55 mTb
Aaniso < 0.001 mTb

h site, N(h)A// = 0.11 mT
A⊥ = 0.10 mT
Aiso = 0.10 mT
Aaniso = 0.003 mT
A// = 0.15 mT
A⊥ = 0.14 mT
Aiso = 0.145 mTb
Aaniso = 0.004 mTb

a1.40 times of the HF constants of 14N.22
1.40 times of the measured HF parameters (Aiso and Aaniso) of 14N.

b

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FIG. 6. Observation of the PbC center by means of ESR and EDMR spectroscopy. (a) Typical ESR spectrum measured for a 28-mm2 specimen preparedby a “Dry (Ref )” condition (using a microwave power of 0.2 mW and a modulation amplitude of 0.25 mT). An E0 signal (a SiO2 defect) arises from a sample
quartz rod. (b) EDMR spectrum measured for 4H-SiC(0001) MOSFET with a
0.01-mm2 gate area prepared by a “Dry (Ref )” condition (using a microwave
power of 200 mW and a modulation amplitude of 0.5 mT). Signal-to-noise ratios(S/N) shown here are calculated as follows: (peak-to-peak height of PbC signal)/(root-mean-square noise level).

FIG. 5. ESR studies on 15N doping after 15NO POA on 4H-SiC(0001)/SiO2. (a)N(k) donors measured at 50 K under photo-excitation and a microwave power of
0.2 mW. (b) N(h) donors measured at 20 K under photo-excitation and a microwave power of 0.02 mW. 14N and 15N donor signals were simulated by adopting
HF parameters shown in Table II. The 15N donor signal was simulated byassuming a spin density of 1 × 1011 cm−2. The dotted curve in (b) is a simulated
spectrum by calculating a combination of the 14N donor signal (blue solid line) +carbon signal (gray solid line).

density of our epitaxial layer (4 × 1014 cm−3). This consistency
ensures the validity of the above ESR evaluation.
We finally mention that the above conclusion was also consistent with the previous result.12 Using the p–-type substrate (Table I),
the counter doping was also estimated to be <1 × 1011 cm−2 for an
optimum NO POA condition12 [the same as “Dry (Normal) + NO”
shown in Fig. 3]. It is a striking contrast with the case of POCl3
POA, which caused a strong counter doping of P donors over1 × 1012 cm−2 (Ref. 12). We speculate that such a strong counter
doping brings a maximum μFE of 100 cm2 V−1 s−1.(Ref. 6).
IV. EDMR RESULTS
As was seen in the preceding section, the PbC center is an
important interface defect that is related closely to the improved
μFE. The next major interest is its microscopic origin. To achieve aconclusive identification, we must analyze HF interactions of the

J. Appl. Phys. 127, 145301 (2020); doi: 10.1063/1.5134648
Published under license by AIP Publishing.

PbC center, especially, due to a nuclear spin of 13C (I = 1/2, naturalabundance = 1.1%). However, it was a very hard task for conventional ESR spectroscopy. A typical ESR signal of the PbC center[Fig. 6(a)] shows only S/N ∼ 30 for over 1-day accumulation. To
observe a 13C HF satellite signal (relative intensity = 0.55% of thetotal signal) with S/N = 10, we will need 4240-day accumulation.
Moreover, its ESR signal suffered from a strong microwave saturation behavior at low temperatures because the PbC center is acarbon-related defect that involves only a weak spin-lattice relaxation as compared to any Si-related defects. This feature prevents us
from a low-temperature signal enhancement on the PbC center.
Therefore, we alternatively employed EDMR spectroscopy forthis purpose. By optimizing EDMR measurements on our 4H-SiC(0001) MOSFETs (noted in Sec. II), a very strong EDMR signal of
the PbC center was observable, as shown in Fig. 6(b). The S/Nfigure reached 850 for only 2-hours accumulation. Accordingly, the
EDMR spectrum clearly exhibits the presence of HF satellite
signals. In principle, only 0.4-day accumulation is enough forresolving a 13C HF satellite signal with S/N = 10 using EDMR.
Figure 7(a) focuses on weak HF satellite signals of the PbC
center. Within a 14-mT range examined here, all EDMR lines
were nearly isotropic, which are also plotted in Fig. 7(b). We
could not detect the known 13C HF splitting of the porous-PbC
centers [Aiso = 5.1 mT (48 × 10−4 cm−1) and Aaniso = 1.4 mT(13 × 10−4 cm−1)].13 It is thus clear that the PbC center in 4H-SiC(0001) is different from the porous-PbC centers in porous-SiC. For
the PbC center, we found two isotropic HF splittings of 1.3 and6.8 mT, as shown in Fig. 7(b). These HF splittings are also clearly
different from those of an “interfacial Si-vacancy center” (13C HF
splittings = 1.3 mT and 2.8 mT when B // [0001]) observed in
4H-SiC(0001) MOSFETs.25

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FIG. 7. (a) Magnified EDMR spectra as a function of a magnetic-field angle.
Angles of 0° and 90° correspond to B parallel to [0001] and [1120], respectively. The modulation amplitude was set to be 0.5 mT for focusing on weak HF
satellite signals. (b) Angular map of a PbC EDMR signal. Solid lines are simulated results.

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correlation was ascribable to that the PbC center acts as electron
traps, reducing the free-carrier density in the inversion channel of
4H-SiC(0001) MOSFET. We also demonstrated another way fordeactivating the PbC center using the H2 POA process at 900 °C.
This is so-called the H passivation effect, similar to the cases of the
Pb centers in Si/SiO2 systems and the porous-PbC centers inporous-SiC. The H passivation suggests the formation of a C–H
bond, supporting a recent C DB model for the PbC center.26 On theother hand, we could not detect the counter doping of shallow 15N
donors in the 15NO POA experiment; the doping density should bemuch below 2 × 1011 cm−2, which is smaller than 1/10 of the PbC
density (3–4 × 1012 cm−2). The PbC center was also detectable withan extremely high-S/N using EDMR spectroscopy on 4H-SiC(0001)
MOSFETs. According to the high-S/N EDMR data, (1) the PbC
center has two sets of nearly isotropic HF interactions at 6.8 mT
and 1.3 mT, (2) its main 13C HF interaction is larger than 14 mT,
and (3) the PbC center is different from the porous-PbC centers inporous-SiC13,14 as well as the Si-vacancy center in 4H-SiC
MOSFETs.16,17,25
ACKNOWLEDGMENTS

The largest HF interaction observed at 6.8 mT corresponds toonly 5% of the isotropic HF constant of 13C (134.77 mT22) or 4% ofthat of 29Si (163.93 mT22). Namely, it originates from a minor partof the wave function of the PbC center; in other words, the main HF
interaction should appear outside the 14-mT range. Actually, we
recently found the Aiso value of 18.9 mT for a 13C HF interaction of
the PbC.26 This large Aiso value means a strong s-character for thewave function of the PbC center. Detailed analyses of its 13C and 29Si
HF interactions are described in a recent paper.26 The reason for thestrong s-character can be accounted for using a carbon-adatom DB
model based on first-principles calculations.26
We mention that the strong s-character of the PbC center isconsistent with its nearly isotropic g factor. An angular-dependence
simulation shown in Fig. 7(b) (solid lines) nominally findsg//c = 2.0029 and g⊥c = 2.0032 for the PbC center. This nearly isotropic g factor is reasonable because an anisotropic g factor is causedby the p-character of an unpaired electron.15,22,23 The g factors estimated from EDMR seem to be deviated from those observed by
ESR (g// = 2.0023 and g⊥ = 2.003212). Since an accurate determination of the g factors was difficult for the PbC center because of its
relatively large signal width, we judged that EDMR and ESR
observed the same PbC signal. At least, the observed g factors areclose to the free-electron g factor (2.0023), supporting that the PbC
center is a carbon-related defect. The atomic model of the PbC
center is given in Ref. 26.
V. SUMMARY
We have studied the PbC center intrinsically formed at 4H-SiC(0001)/SiO2 interfaces. This center is a dominant carbon-related
interface defect and played an important role in changing μFE. The
present study demonstrated that the PbC center can be removed by
the NO POA process2–4,12 and the UHT oxidation process,11 resulting in lower PbC densities and simultaneously higher μFE. This

J. Appl. Phys. 127, 145301 (2020); doi: 10.1063/1.5134648
Published under license by AIP Publishing.

This work was supported by the Council for Science,
Technology and Innovation (CSTI), 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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