Anomalous carbon clusters in 4H-SiC/SiO2
interfaces
Cite as: J. Appl. Phys. 125, 065302 (2019); https://doi.org/10.1063/1.5066356
Submitted: 15 October 2018 . Accepted: 21 January 2019 . Published Online: 11 February 2019
Y. Kagoyama, M. Okamoto, T. Yamasaki, N. Tajima, J. Nara, T. Ohno, H. Yano, S. Harada, and T. Umeda

J. Appl. Phys. 125, 065302 (2019); https://doi.org/10.1063/1.5066356© 2019 Author(s).

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Anomalous carbon clusters in 4H-SiC/SiO2
interfaces
Cite as: J. Appl. Phys. 125, 065302 (2019); doi: 10.1063/1.5066356
Submitted: 15 October 2018 · Accepted: 21 January 2019 ·Published Online: 11 February 2019
Y. Kagoyama,1 M. Okamoto,2View Online

Export Citation

CrossMarkT. Yamasaki,3 N. Tajima,3 J. Nara,3 T. Ohno,3 H. Yano,1 S. Harada,2 and T. Umeda1

AFFILIATIONS
1
2

Institute of Applied Physics, University of Tsukuba, Tsukuba 305-8573, Japan
Advanced Power Electronics Research Center, National Institute of Advanced Industrial Science and Technology (AIST),

Tsukuba 305-8568, Japan
3

Computational Materials Science Unit, National Institute for Materials Science (NIMS), Tsukuba 305-0044, Japan

ABSTRACT
We investigated a metal-oxide-semiconductor interface of dry-oxidized (0001) 4H-SiC, which was known as the most electricallydeteriorated SiC MOSFET, by electrically detected magnetic resonance (EDMR) and observed a signal with an isotropic g factor(2.0024) and magnetic-ﬁeld angular dependent signal widths. Judging from the g factor, the signal comes from sp2-bonded
carbon clusters. In addition, we found that the angular dependence of EDMR signal widths was caused by two-dimensional
dipolar broadening with exchange interaction between electron spins. However, the density of electron spins or carbon clusters
was 5.4 × 1013 cm−2, which was not high enough for exchange interaction. Therefore, we propose inhomogeneous distribution ofcarbon clusters in the interface. At the interface, π* peaks from sp2-bonded carbon atoms were detected by electron energy loss
spectroscopy. Scanning the electron beams along the interface revealed uneven existence of the π* peaks, which also provedthat the sp2-bonded carbon atoms were distributed inhomogeneously in the interface. In addition, we found the formation of
sp2-bonded carbon clusters at 4H-SiC(0001)/SiO2 interfaces and interaction between π-conjugate electron spins on the carbon
clusters by ﬁrst principles calculation. Such carbon clusters generated electrically active states widely in the energy gap of
4H-SiC. The states result in the Fermi level pinning of the MOSFET.
Published under license by AIP Publishing. https://doi.org/10.1063/1.5066356

I. INTRODUCTION
Silicon carbide (4H-SiC) and its metal-oxide-semiconductor(MOS) interfaces have attracted much attention as the heartof next-generation power transistors such as SiC MOS ﬁeldeffect transistors (MOSFETs).1 This system also provides ascientiﬁcally interesting issue: what types of interface defects
are formed using typical covalent-semiconductor elements.
The SiC-MOS interfaces are abrupt and simply consist of Si,
C, and O atoms.2 Nevertheless, they exhibit unpredictably
wide variations; for instance, the ﬁeld-effect mobility (μFE) of
the channel of SiC-MOSFETs shows a wide range of degradation from 0 to 100 cm2 V−1 s−1, depending on surface orientations and oxidation processes.3–5 The identiﬁcation of interface
defects in this system is a timely issue. Electron spin resonance(ESR) and electrically detected magnetic resonance (EDMR)
spectroscopy have so far revealed various interface defects
such as carbon clusters (C clusters),6,7 PbC centers (interfacial

J. Appl. Phys. 125, 065302 (2019); doi: 10.1063/1.5066356
Published under license by AIP Publishing.

carbon dangling bonds),8,9 Si-vacancy centers,10 C-facedefects,11 and carbon-related defects.12
In this paper, we identify another key defect in
the SiC-MOS system, which was found in dry-oxidized
4H-SiC(0001)/SiO2 interfaces that are known as the mostdeteriorated SiC-MOS interfaces. In such interfaces, we
identiﬁed a new EDMR center of sp2-bonded C clusters.
This center is essentially different from the C cluster reported
previously.6,7 The previous ESR centers [g factor (g) ≈ 2.003]
of the C cluster originated from carbon dangling bonds in theclusters, which are characterized by a spatially localized wave
function at a single C atom. On the other hand, the present
EDMR center of g ≈ 2.003 originates from π-like electrons inthe sp2-bonded C clusters, which are spatially delocalizing
over the cluster. As a consequence, our C-cluster signal
revealed two-dimensional (2D) dipolar broadening13,14 as well
as 2D exchange interaction between electron spins.15–17
Normally, the exchange interaction of 2D electron spins was

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generally observed when the spin density exceeded
4 × 1014 cm−2.15–17 In contrast, the present center exhibited
2D exchange interaction with one order of magnitude lower
spin density. This unusual exchange interaction is closely
related to the nature of sp2-bonded C clusters. The presenceof such C clusters was also evidenced from transmissionelectron-microscope (TEM) observation. Furthermore, ﬁrstprinciples molecular dynamics (FPMD) calculations predicted
the formation of sp2-bonded C clusters, which is consistent
with various experimental results.

II. METHODS
A. EDMR sample fabrication
Fully processed n-channel lateral C-face 4H-SiC MOSFETs
were prepared on a p-epitaxial layer of 4°-off 4H-SiC(0001)
surface. Gate oxides were formed by either dry oxidation at
1200 °C (60 nm in thickness) or wet oxidation at 1100 °C (50 nm
in thickness). We named the dry-oxidized sample “Dry” andthe wet-oxidized one “Wet.” The Dry and Wet samples were
subjected to a post oxidation anneal with Ar gas at 1100 °C
and H2 gas at 1100 °C, respectively. The post oxidation anneal
with Ar gas for the dry-oxide MOSFET was for the same
thermal history with the wet-oxide MOSFET. The gate lengthand the width are 5 and 150 μm, respectively. The source anddrain regions were formed by phosphorous ion implantation.
The same type of SiC-MOSFETs was studied in our previous
papers, and other detailed information on the MOSFETs canbe found in Refs. 4 and 11. The two C-face MOSFETs exhibited
strikingly different current-voltage characteristics, e.g., μFE
was quite high (67 cm2 V−1 s−1) for the Wet sample but was
10−8 cm2 V−1 s−1 for the Dry sample.

B. EDMR setup
We measured EDMR spectra to investigate what type of
defects exist at the MOS interfaces of Dry and Wet samples.
We adopted the bipolar-ampliﬁcation-effect (BAE) technique.
This technique was developed by Aichinger and Lenahan,18
enabling us to amplify EDMR signals via a current ampliﬁcation function of MOSFET itself. The BAE-EDMR setup is
shown in Fig. 1. A current source and an EDMR detector areconnected to the drain and source, respectively. Using aforward-biased current source (Id), electrons are injected into
the MOS interface. We can control the density of electrons
that recombine at the interface defects by changing a gatebias (Vg), while most of Id ﬂows into the well region as a wellcurrent (Iw, see Fig. 1) which is unrelated to the interface
defects. We then measured ESR-induced current changes (ΔI)
due to the MOS interface defects using the drain-source currents (I). For maximizing the signal-to-noise ratio of EDMR
signals in the Dry MOSFETs, Id was set to be 100 μA, and the I
was 750–780 nA, in the Vg range between −30 and +15 V.
For the case of the wet MOSFETs, the details of the optimizing
EDMR setup were already reported in Ref. 11. All measurements
were performed at room temperature and with a microwave of
9.462 GHz and 200 mW. EDMR signals were detected using aJ. Appl. Phys. 125, 065302 (2019); doi: 10.1063/1.5066356
Published under license by AIP Publishing.

FIG. 1. BAE-EDMR setup used for maximizing EDMR signals of MOS interface
defects.

lock-in detection synchronized to a magnetic-ﬁeld modulation
at 1.56 kHz. The magnetic-ﬁeld modulation amplitudes were
set to be 0.5 mT for the Dry sample and 0.25 mT or less for the
Wet sample.

C. TEM/EELS setup
Using electron-energy-loss spectroscopy (EELS), we
can distinguish sp2- and sp3-bonded C atoms, which aredetectable as a π* peak at 285 eV and σ* peak at 290 eV in the
C-K energy-loss edge, respectively.19 The TEM sample (40 nm
in thickness) was prepared from the EDMR sample by focusedion-beam machining. High-resolution TEM/EELS measurements conventionally used an acceleration voltage of over
100 kV (typically 200 kV)20 and a focused electron beam with a
sub-nm diameter. However, such conditions strongly damaged
SiC/SiO2 interface regions, as evidenced in a scanning TEM(STEM) image of Fig. 2(a). As is shown in the ﬁgure, a 200-kV
EELS observation along the interface (indicated by a dashedline) can generate amorphized regions at the interface over thescanning area. Note that such damages were not observed at
Si/SiO2 interfaces with similar observation conditions. The
amorphized regions showed a strong π* carbon-EELS signal
due to the formation of amorphous carbons. Therefore, tominimize such artiﬁcial damages, we have optimized the acceleration voltage, beam diameter, scanning steps, and measurement times at each point. STEM images were obtained using a
JEOL HEM-ARM 200F electron microscope operated with a
low acceleration voltage of 60 kV. The electron beam was defocused into 1 nm in diameter and scanned just along the interface every 2 nm over a 150-nm length. When scanning thebeam, the beam position was accurately calibrated relative tothe standard position deﬁned inside 4H-SiC lattice image.
We also examined EELS intensity proﬁles across the Dryinterface as similarly to the previous work such as Ref. 20.
The result is summarized in Fig. 2(b). In this measurement,a higher-resolution condition with a focused electron
beam was used: acceleration voltage of 80 kV, beam spot
diameter = 0.2 nm, and beam step = 0.2 nm. At every scanningpoint, we measured intensities of Si-L edge (95—105 eV), C-K
edge (285—295 eV), and O-K edge (535—545 eV) in each EELS

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FIG. 2. (a) Annular-dark-ﬁeld STEM image of a 4H-SiC(0001)/SiO2 interface (the Dry interface) after a conventional 200-kV EELS scanning analysis along the interface(indicated by a dashed line). Amorphized damages were induced over the EELS scanning area. (b) EELS intensity proﬁles across the Dry interface measured by a lowdamage (80 kV) EELS scanning analysis. (c) Compositional proﬁles along the Dry interface obtained by EDX spectrometry (60 kV). The vertical intensities were normalized
by the standardless correction. The horizontal range (0—150 nm) was the same as that shown in Fig. 5(d).

spectrum. The obtained proﬁles of Si, C, and O indicate
an interfacial region of 1-nm width (dashed lines). This interfacial region was much narrower than that shown in theprevious work (3—4 nm),20 suggesting that our TEM/EELS
analysis has achieved a lower damage analysis. From thispreliminary EELS analysis, we decided that a beam spot was
expanded to 1-nm diameter to fully cover the interfacial
region and to minimize the damages. If the carbon EELS
signal was lost, it should be judged that the beam spot strays
off the interfacial region.
In EELS scanning analyses along the interface, we also
simultaneously measured energy-dispersive X-ray spectrometry (EDX) to check compositional fractions of Si, C, and O
atoms at each scanning point [Fig. 2(c)]. As is seen in theﬁgure, C atoms were always detected, ensuring that the beam
spot traced the interface. About 10% ﬂuctuation was present
in the compositional fraction of C atoms, although that of Si
atoms was almost constant. These proﬁles may suggest thepresence of non-stoichiometric C atoms at the interfacial
region. Their density is nominally estimated to be 1014 cm−2
(10% of the interfacial atomic density). However, we must be
careful in quantitative arguments, and hence we tried to
examine non-stoichiometric C atoms by observing the carbon
π* EELS signals, which will be described in Sec. III C.

D. FPMD simulations
We performed O2 oxidation simulations of 4H-SiC(0001)
using the FPMD program PHASE/0,21 which is based on thedensity functional theory22 with pseudopotentials and plane
wave basis. Pseudopotentials for Si and H atoms were normconserving, and those for C and O atoms were ultra-soft
types. Exchange and correlation effects were taken within thegeneralized gradient approximation (PBE96). Cut off energiesof plane wave basis for wave functions and charge density
were 16 and 144 Ry, and the Γ point was sampled in the

J. Appl. Phys. 125, 065302 (2019); doi: 10.1063/1.5066356
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Brillouin zone. We set the temperature as 2000 K aiming toenhance oxidation reactions at the interface. We executed 14
series of simulations, in each of which a total of 22 O2 molecules were introduced one by one at regular intervals (1 ps forthe ﬁrst 11 O2 and 2 ps for the latter 11 O2) into the interface
region. The unit cell of an initial structure model contains asubstrate of six SiC bilayers, 6.5-Å thick amorphous-like SiO2
layer smoothly connected to the substrate with no danglingbond generation, and a 10-Å thick vacuum region, where O
atoms at the SiO2 surface and Si atoms at the SiC backsurface are terminated with H atoms.

III. RESULTS AND DISCUSSIONS
A. EDMR experiments
The EDMR results of Dry and Wet samples are shown in
Fig. 3(a). In the Dry sample, a single broad Lorentzian signal
appeared at an isotropic g factor of 2.0024. In contrast, theWet sample revealed sharper anisotropic EDMR signals, whichwere named “C-face defects,”11 and they showed a c-axial gtensor of g// = 2.0016 and g⊥ = 2.0023.11 We tentatively attributethe broadened signal of the Dry sample to high-density 2Delectron spins because a similar phenomenon was determined
for the Pb centers at Si(111)/SiO2 interfaces, if their density is≳1013 cm−2.13,14 Such high-density 2D electron spins reveal a
broadened ESR signal due to 2D dipolar broadening.13,14
Figure 3(b) illustrates a gate-bias (Vg) dependence of the
EDMR signal in the Dry sample. The EDMR intensity
increased linearly with increasing Vg over a very wide range
from −30 V to +15 V. This nearly linear response is one of thecharacteristic features of the EDMR signal in the Dry sample,
because EDMR signals of interface defects generally exhibited
peak-shaped Vg dependences, as observed in the Wet sample[Fig. 2(c)].11 Basically, a peak-shaped Vg dependence is associated with a particular energy level of the interface defect;e.g., in Ref. 11, the defect level of the Wet sample was assumed

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assumed a peak-shaped Vg dependence in a strong negative Vg
region (Vg < −30 V), the zero EDMR signal could be observed atsmaller negative Vg bias, and the defect density could besmaller than the above value. However, we should consider
that the defect density in the Dry sample is larger than thatin the Wet sample (1.2 × 1013 cm−2),11 judging from the muchbroader 2D dipolar broadening in the Dry sample.
Figure 4(a) shows the angular dependence of thepeak-to-peak width (ΔBpp) of the EDMR signals in the Dry andWet samples. Both signals revealed angular-dependent signal
widths. Such an angular dependence is actually known for
2D dipolar broadening with high-density electron spins(≳1013 cm−2).13,14 In fact, the dashed lines in the ﬁgure were
simulated assuming the 2D dipolar broadening [numericallycalculating Eq. (5) in Ref. 14], showing good agreement withthe case of the Wet signal. In this simulation, we assumed
that the spin density at the Wet interface is 1.2 × 1013 cm−2,
which was experimentally found in Ref. 11. On the contrary,
for the Dry signal, the 2D dipolar broadening cannot accountfor the experimental result when we assumed the spin
density of ∼5 × 1013 cm−2. Even varying the spin density, no
good ﬁtting results were obtained.
Instead of simple 2D dipolar broadening, we considered
a combination of 2D dipolar broadening and 2D exchangeinteractions between electron spins. In the case of“ultra-high-density” spins (>1014 cm−2), the angular dependence of ΔBpp is modiﬁed by the 2D exchange interactions asfollows:
FIG. 3. (a) EDMR spectra of “Dry” and “Wet” C-face 4H-SiC MOSFETs formagnetic ﬁeld (B) parallel to [0001]. ESR transitions were excited by a 200-mW
microwave at 9.46 GHz. Magnetic-ﬁeld modulation at 1.56 kHz with an amplitudeof 0.25–1.0 mT was used for signal ampliﬁcation. (b) EDMR signal intensity(ΔI/I) as a function of Vg in the Dry sample, where ΔI is the peak-to-peak amplitude of current change. (c) ΔI/I versus Vg in the Wet sample with the defectdensity of 1.2×1013 cm−2. This graph is the same as Fig. 3(a) in Ref. 11. The
signal intensity was largely enhanced due to a different measurement techniquewhich was optimized for the much high-μFE sample.11

to be at EV (valence band edge) + 0.5 eV. Contrary to such acase, we speculate a much broader distribution of the defectlevels in the Dry sample. In fact, our theoretical calculations
on the C clusters (Sec. III C) predict a broad distribution of
their defect levels over the whole range of the energy gap.
At least, by changing Vg from −30 V to +15 V, electrons of
1.6 × 1013 cm−2 are supplied to the interface, where we calculated CoxΔV/e = 1.6 × 1013 cm−2 with the oxide capacitance (Cox)of 5.6 × 10−8 F/cm2, ΔV = 45 V, and e = 1.602 × 10−19 C. Theseelectrons were trapped into the defect levels, forming singly
occupied ESR-active states. As a result, the EDMR signal intensity increased. From the observed linear relationship, weroughly estimated a zero EDMR signal at Vg = −140 V. Thismeans that the EDMR centers, which act as electron traps andare ESR-active after capturing electrons, may exist by a densityof 5.4 × 1013 cm−2, where ΔV is 155 V ( = 140 + 15 V). This spin
density is high enough to cause 2D dipolar broadening. If weJ. Appl. Phys. 125, 065302 (2019); doi: 10.1063/1.5066356
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ΔB pp ¼ A þ B(3 cos2 θ  1)2 ,where A and B are constants and θ is an angle between themagnetic ﬁeld and the normal of the 2D plane.15 In fact, theangular dependence of the Dry signal can be well ﬁtted by
the above formula, as shown with a solid line in Fig. 4(a) with

FIG. 4. (a) ESR signal width (peak-to-peak widths of ﬁrst-derivative signals,
ΔBpp) as a function of magnetic-ﬁeld angle θ. Dashed curves were simulated
for pure 2D dipolar broadening.13,14 In addition to 2D dipolar broadening, 2D
exchange interaction15 was taken into account for calculating a solid curve. (b)Relationship between signal widths and 2D spin densities. The left dashed oval
indicates the class of pure 2D dipolar broadening observed in high-density Pb
centers at Si(111)/SiO2 interfaces.13 The upper-right dashed oval indicates theclass of ultra-high-density spin systems involving 2D exchange interaction,
which were observed in 2D magnetic systems.15–17

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A = 0.99 and B = 0.66 mT. Therefore, we conclude that the Drysignal should involve the 2D exchange interactions, indicatinga very short distance between electron spins at the Dryinterface.
Interestingly, the present Dry interface is quite unusualcompared to the known examples of ultra-high-density 2D
spin systems.15–17 Figure 4(b) shows a relationship between
ΔBpp and 2D spin densities for the high-density Pb systems14
and the ultra-high-density 2D spin systems.15–17 The present
experimental data are also plotted in the ﬁgure. The Wet signal
belongs to the class of the high-density 2D spin systems, exhibiting pure 2D dipolar broadening. In contrast, for the Drysignal, its ΔBpp and spin density approached those of the highdensity 2D spin systems; however, the angular dependence ofΔBpp is similar to that of the ultra-high-density 2D spin systems(>4 × 1014 cm−2).
The observed isotropic g factor of 2.0024 in the Drysample is close to those of the C clusters6,7 and Si-vacancy
center.10 In addition, similar g factors are commonly observedfor other carbon-related interface defects8,9,11,12 as well as
electron spins in various carbon materials.19 We therefore
argue that the present EDMR center is a sort of carbon
electron spins. The observed g-isotropy suggests a randomorientation of electron spins at the Dry interface. For theorigin of such carbon electron spins, we propose sp2-bonded
C clusters that are inhomogeneously formed at the Dry interface. This model naturally accounts for the random orientation of carbon electron spins. Carbon electron spin(s) may be
conﬁned in a C cluster and delocalize over the size of thecluster via a sp2 π-conjugate electronic system. Such features
may promote the 2D exchange interaction even with a low
spin density. In addition, an inhomogeneous formation of
the C clusters may assist in a spatial condensation of carbon
electron spins.

B. EELS experiments
By optimizing the damage-less TEM/EELS observations
described in Sec. II C, we obtained no appreciable structuralchanges at the interface, which was conﬁrmed by the second
TEM observation as shown in Fig. 5(a). In the inset of Fig. 5(a),a lattice image of 4°-off 4H-SiC and an amorphous structureof SiO2 are resolved. At every scanning point [the yellow dots
in Fig. 5(a)], we measured C-K edge, O-K edge, and Si-L edgein the EELS spectrum, conﬁrming the presence of all C, O,
and Si atoms. It ensures that the electron beam is just locatedat the SiC/SiO2 interface. We also performed a double checkof the co-presence of C, O, and Si atoms in every scanning
spot by means of EDX [Fig. 2(c)]. Figure 5(b) shows the C-K
edge of the EELS spectra measured at positions A and B in
Fig. 5(a). Spectrum A consists of the σ* peak only, while spectrum B includes the π* peak. By scanning 150 points along theinterface, scattered π* peaks appeared. This result is reasonably ascribed to the presence of sp2-bonded C clusters at the
Dry interface.
The observed C-K edge EELS spectra can be ﬁtted by a
combination of σ* and π* peaks. An example is shown at theJ. Appl. Phys. 125, 065302 (2019); doi: 10.1063/1.5066356
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FIG. 5. (a) Cross-sectional annular-bright-ﬁeld STEM image of the Dry interface. The image shows the (1100) plane. Yellow dots along the interface
express spots of the 1-nm-diameter electron beam. (b) EELS spectra of C-Kedge measured at the spots A and B in (a). (c) and (d) x-direction proﬁles of σ*(red) and π* (green) intensities measured for a 4H-SiC bulk region and an interface region of the TEM sample, respectively. Intensities were normalized by theaverage of the σ* peak heights (dashed lines).

bottom of Fig. 5(b). The reference σ* peak was obtained fromthe average of 76 C-K edge EELS spectra measured for a
4H-SiC bulk region of the same TEM sample. The π* peak was
assumed to be always located at 285 eV with a constant width.
All C-K edge EELS spectra could be well deconvoluted into
these two peaks. We then estimated the distribution proﬁlesof sp3- and sp2-bonded C atoms along the x direction, which
are shown with red and green histograms, respectively, inFigs. 5(c) and 5(d). Figure 5(c) was obtained for a 4H-SiC bulk
region. Obviously, this region consists of purely sp3-bonded C
atoms. On the contrary, in Fig. 5(d), an interface region unambiguously shows the scattered appearance of the sp2-bonded
C atoms. This observation supports the inhomogeneous formation of sp2-bonded C clusters at the Dry interface.
Figure 6(a) illustrates such an inhomogeneous formationof the sp2-bonded C clusters. The dots indicate carbon electron spins that are separated by less than 0.3 nm (equivalent
to an average spin density of 1 × 1015 cm−2) to satisfy theobserved 2D exchange interactions. The gray areas surrounding the dots express the aggregation of sp2-bonded C atoms,
each of which corresponds to C clusters. The density of dots
(carbon electron spins) was set to 5 × 1013 cm−2. As is clear in
the ﬁgure, the inhomogeneous distribution of the C clusters
is indispensable for ensuring 2D exchange interaction.
Moreover, we speculate the delocalization of carbon electron
spins over the size of each cluster (i.e., each gray areas),

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C. FPMD results

FIG. 6. (a) A top view of the sp2-bonded C-cluster model at the Dry interface.
The details are given in the text. (b) A top view of the spin distribution in the
Wet interface. This distribution was used for calculating the 2D dipolar distribution shown in Fig. 4(a).

further assisting in 2D exchange interaction. On the other
hand, in Fig. 6(b), a spin distribution for the Wet interface (thespin density = 1.2 × 1013 cm−2) is simulated, which was used for
calculating the 2D dipolar broadening shown in Fig. 4(a).
Obviously, this type of distribution cannot generate the 2D
exchange interactions.
To further check the EELS results, we also performed
the same EELS analysis on a 4H-SiC(0001)/SiO2 interface(the so-called “Si-face” interface). The Si-face interface
do not show the EDMR/ESR signals of the C clusters.4,9,10
Furthermore, surface-enhanced Raman spectroscopy strongly
suggested that the dry oxidation on the C face generated
much stronger graphitic signal (G peak) than on the Si
face.23,24 Therefore, we expected much less appearance of theC sp2 component at the Si-face interface. Figure 7 summarizes
our EELS analysis on a dry-oxidized Si-face interface (oxide
thickness = 50 nm) inside a Si-face 4H-SiC MOSFET. The π*
peak on the Si-face interface was clearly much smaller thanthe case of Fig. 5(d), supporting our above conclusion and theprevious results.4,9,10,23,24

FIG. 7. Carbon EELS σ* (red) and π* (green) intensity proﬁles measured for adry-oxidized 4H-SiC(0001)/SiO2 interface of a Si-face 4H-SiC MOSFET. We
employed the completely same condition and analysis as used for Figs. 5(c)and 5(d).

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In the FPMD simulation described in Sec. II C, most of
the dissociated O atoms from the O2 molecules reacted with
the Si atoms and distributed them from the interface to thedeeper region of SiO2, and a part of the O atoms generated
CO and CO2 molecules with interfacial C atoms. The C atoms
left behind at the interface condensed into C clusters. We
examined the sizes and conﬁgurations of C clusters at theend of the intervals in the period from 15 to 22 O2 molecule
introductions. The sizes of C clusters were counted at theends of intervals in a period of 15 to 22 O2 molecules for the14series of the FPMD simulations. The distribution of the sizesof C clusters is shown in Fig. 8(a). Here, we deﬁne the size ofa C cluster as the number of C atoms contained in the C
cluster. The average size of C cluster was 5.4. We also analyzed electronic structures at the ends of intervals andcounted gap states that appeared in the SiC energy bandgap.
Figure 8(b) shows the correlation between the number of gap
states (ngs) and a sum of the number of electrically active C
clusters (nC-clusters) and the number of defects (ndefects) like —Si=O2 and —CSi3: the vertical and horizontal axes represent
ng and nc-cluster + ndefects, respectively. Approximately, eachactive C cluster generates only one gap state.
As an example, Fig. 9(a) shows one of the interface
structures obtained after the introduction of 16 O2 molecules
among the 14 series. A defect of —Si=O2 and four C clusters(two C2 clusters, C5 and C15 clusters) occurred in the interface
region. In the energy bandgap of 4H-SiC, three states weregenerated: EV + 0.1 eV, EC − 0.4 eV, and EC (to be accurate, this is
distributed between EC − 0.1 and EC + 0.1 eV), as shown in
Fig. 9(b). The state at EV + 0.1 eV originated from the —Si=O2
defect and was spatially localized, and the state at EC – 0.4 eVoriginated from the C15 cluster and was not localized compared to the former one. The state around EC originated from
C15, C5, and C2 clusters and was extended, as indicated by thepartial charge density shown in Fig. 9(a). The other C2 cluster[C20 cluster, the second from the left cluster in Fig. 9(c)] gaveno bandgap states (we deﬁne this kind of C cluster as

FIG. 8. (a) A distribution of the C cluster size observed at the ends of intervaltimes in a period of 15 to 22 O2 molecules for 14 series simulations. (b)Numbers of gap states appeared in the 4H-SiC bandgap versus summed
numbers of active C-clusters and other defects observed for the 14 series ofsimulations.

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The large-scale classical MD simulation (which uses theinteratomic potentials developed with our FPMD simulation)
predicted the formation of various-sized and various conﬁgurations of C clusters at a 4H-SiC(0001)/SiO2 interface
much more dominantly than at a 4H-SiC(0001)/SiO2 interface.26 In contrast with dry oxidation, wet oxidation seems
to prevent the formation of C clusters, alternatively forming
point-defect-like interface defects, as shown in Fig. 6(b).
Although our FPMD simulation did not sufﬁciently examine
wet oxidation (introducing H2O instead of O2), a preliminary
result suggests that the formation of C—H bonds may
disturb the aggregation of C atoms. Clarifying atomistic
mechanisms in wet oxidation will be the next challenge.

IV. SUMMARY

FIG. 9. (a) SiO2/SiC interface structure. Si and O atoms are denoted with blue
and red balls, respectively. C atoms in C clusters are orange, and other C
atoms are yellow. The superposed yellow isosurface is the charge density integrated from 2.2 eV to 2.4 eV. (b) Calculated density of states in the interface
region [the dashed square in (a)]. EV and EC are the valence band maximum
and the conduction band minimum of 4H-SiC, respectively. (c) Four types of C
clusters formed in the interfacial region shown in (a).

“inactive”). All C atoms of the C5 cluster and nine C atoms of
the C15 cluster were sp2-bonded. The majority of the C atoms
in the C clusters observed among the 14 series were sp2- or
sp-bonded.
Through the analysis of gap states for the 14 series, wefound several interesting features of the sp2/sp-bonded C
clusters as follows. (1) Gap states are not carbon danglingbond states but are delocalized states possibly due to a
hybridization of sp2 and sp π orbitals. This is consistent with
our defect model. (2) The number of gap states correlates
to that of electrically active C clusters and defects such as—Si=O2 and —CSi3. (3) There is no correlation between the
C-cluster size and the number of gap states. Each active C
cluster generates approximately only one gap state. Therefore,
the theoretical calculation suggests that 2D exchange interactions may be mainly caused between close C clusters, such as
among C2, C5, and C15 shown in Fig. 9(a). (4) Electrically
active C clusters generate gap states over a wide range ofthe 4H-SiC bandgap, independent of the C-cluster size andtheir structures. These gap states can capture electrons,
resulting in a Fermi-level-pinning phenomenon, as observedin the Dry interface.

D. Comparison between the Dry and the Wet samples
The reason why the C clusters are so much formed at the
Dry interface is supposedly related to the fast oxidation rateof the C face.25 Faster oxidation generates more excess C
atoms at the interface,25 enhancing the formation of C clusters.

J. Appl. Phys. 125, 065302 (2019); doi: 10.1063/1.5066356
Published under license by AIP Publishing.

In summary, we observed sp2-bonded C clusters at a dryoxidized 4H-SiC(0001)/SiO2 interface by using EDMR andTEM/EELS. They were dominant interface defects at theworst-quality SiC-MOS interfaces. We found the characteristic nature of the sp2-bonded C clusters, e.g., 2D exchangeinteraction even with a low spin density. The formation of the
sp2-bonded C clusters and their characteristic features wererevealed from FPMD simulation, which supports the experimental ﬁndings.

ACKNOWLEDGMENTS
We thank T. Aoki, K. Ogata, and S. Hashimoto ( JFETechno-Research Corp.) for conducting TEM/EELS measurements shown in Figs. 2, 5, and 7 and also thank H. Sako (Toray
Research Center, Inc.) for his TEM/EELS measurements
shown in Fig. 2. This work was supported by the Council
for Science, Technology and Innovation (CSTI), the Crossministerial Strategic Innovation Promotion Program (SIP), andthe “Next-generation power electronics” (funding agency:
NEDO). This work was also partly supported by a Grantin-Aid (Grant Nos. 17H02781 and 16H03830) and the priority
issue 6 of FLAGSHIP2020 from the Ministry of Education,
Culture, Sports, Science and Technology of Japan. The calculations were done on the K computer (Project ID: hp160226,
hp170253, and hp180187) and NIMS supercomputer.

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