ECS Transactions, 50 (4) 305-311 (2012)©The Electrochemical Society

SiC MOS Interface States: Similarity and Dissimilarity from Silicon
T. Umedaa, R. Kosugib, Y. Sakumab, Y. Satoha, M. Okamotob, S. Haradab, and
T. OhshimacabInstitute of Applied Physics, University of Tsukuba, Tsukuba, 305-8573, Japan
National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba
305-8568, Japanc
Japan Atomic Energy Agency (JAEA), Takasaki 370-1292, Japan

We here discuss SiC MOS interface states based mainly on
microscopic information given by electron spin resonance (ESR)
spectroscopy. Despite structural similarities between Si-SiO2 and
SiC-SiO2, SiC MOS interfaces exhibit some dissimilarities from Si
MOS ones. One example is the presence of carbon-related defects
at there. Reactions to hydrogen and nitrogen atoms are also quite
different between the two interfaces. We present recent ourfindings on the behaviors of these atoms at SiC MOS interfaces.

SiC MOS interfaces
In the last two decades, silicon carbide (SiC) is intensively studied as a promising wideband-gap semiconductor for power electronics. This material is unique compared to otherwide-band-gap semiconductors [1], because it has a high compatibility to the silicon
technology. For instance, ion implantation techniques are available for both n- and pdoping, making it possible to design a variety of device structures with both small andlarge sizes. Moreover, we can use thermally grown silicon dioxide (SiO2) for
dielectric/insulating layers in SiC devices. These features realize normally-off metaloxide-semiconductor field effect transistors (MOSFETs) of SiC as similarly to Si
MOSFETs. Currently, SiC devices including SiC MOSFETs are going to becommercialized into many electric apparatus. Since SiC-SiO2 interfaces have structuralsimilarities to Si-SiO2 ones (abrupt interfaces [2] consisted of Si-O bonds [3]), the Si
MOS system will set a good model for SiC MOS systems.
On the other hand, it is also known that there are some dissimilarities between Si and
SiC MOS systems. Such dissimilarities seem to link with unresolved issues in SiC MOS
technology. The primary issue is that microscopic origins of SiC MOS interface states are
not yet established. In silicon, main interface states originate from the Pb centers (Sidangling bonds (DBs) at the interface) [4]. Since the Pb centers can interact withhydrogen (H) atoms, H passivation is quite useful for the Si MOS technology. On thecontrary, this technique was often ineffective for SiC MOS interfaces. This is a big
difference between Si and SiC, and is related to a different nature of SiC MOS interface
states. So far, these states have been studied by many groups [5-13] including Afanas’evStesmans [5] and Lenahan [10] who are also great specialists of Si MOS interfaces.
Briefly looking at the whole results, carbon-related defects are believed to play animportant role in this interface. The microscopic information about such defects was
mainly revealed by electron-spin-resonance (ESR) spectroscopy, as similarly to the caseof Si-SiO2 interface states. In this paper, we discuss SiC MOS interface states based on
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ESR data. Here our discussions are limited to 4H-SiC MOS interfaces of c-facet wafers,
because the state-of-the-art SiC devices were fabricated on such wafers. The 4H-SiC
crystal (band gap = 3.26 eV) exhibits excellent physical properties and the highest wafer
quality.
Si face vs. C face. In 4H-SiC, it is notable to _distinguish two different surfaces andinterfaces, called SiC(0001) (Si face) and SiC(0001) (C face). These surfaces have quite
different chemical properties. For instance, an oxidation rate is much lower for Si face (afew % of the case of Si(001)) than for C face (ibid ~80%) at 1200 °C. A method for
improving their MOS interfaces is also quite different between Si and C faces [13]. For C
face, hydrogen incorporation into the interface is often effective to reduce the interfacestate density (Dit) and increase the channel mobility. On the contrary, for Si face, nitrogenor phosphorous incorporation into the interface gains the highest improvement. Currently,
Si face is mostly studied and used for device fabrication. Since most of SiC MOSFETs
are operated using n-channel, interface states near the conduction band edge (EC) are of
the highest importance. The Dit value was successfully reduced to ~1011 cm-2/eV at EC –0.2 eV after some improving processes. Owing to this reduction, the field-effect mobility(μFE) was increased from 1 ~ 5 cm/V·s to 20 ~ 100 cm/V·s. However, these improved
values are still much lower than μFE in a low doped bulk region of 4H-SiC (~1000
cm/V·s), which is one of the most serious problem in SiC MOS technology.
ESR analysis on SiC MOS interfaces
Current status and issues
ESR spectroscopy is a powerful tool for identifying microscopic origins of singlyoccupied energy levels. In particular, ESR has a fruitful history in Si MOS technology [4].
On the contrary, for SiC MOS interfaces, ESR may not be so successful in spite of theirhigher Dit. Possible reasons for this are considered as follows. One essential reason is
supposed that SiC-SiO2 interface states may not be singly-occupied defect levels (either
empty or doubly-occupied levels) unlike the case of Si. This situation is more likely forshallower interface states near the band edge. To observe such shallow states, it may benecessary to perform low-temperature ESR studies under charge pumping (photoexcitation or current injection). However, previous ESR studies on SiC MOS samples
were mostly carried out at room temperature (R.T.).
Another technical reason is related to the quality of SiC substrates. In general, SiC
substrates include residual defects (mainly vacancy-related defects) and impurities (e.g.,
unintentionally doped nitrogen). A typical density is > 1014 cm-3 or > 1012 cm-2 for a 0.01cm-thick specimen. Their ESR signals are overlapping over interface signals, especiallyat low temperatures. To avoid such a disturbance, a few methods have been attempted.
One is to enhance interface signals using porous-SiC samples [9]. The other is to apply
electrically detected ESR technique (EDMR: Electrically Detected Magnetic resonance)[10] which can focus on an interface region by means of electrical currents. We tried thelatter approach.
Table I summarizes ESR signals in SiC MOS structures. In the early stages, carboncluster signals (amorphous carbon DB signals) were reported for oxidized SiC wafers[5,6]. The Afanas’ev-Stesmans’s group proposed that carbon clusters with various sizes
are the origin of a continuous distribution of Dit near the band edge [5]. This model may
be valid for the valence band side of 3C-SiC MOS interfaces, because the carbon-cluster
signals were strong in p-type 3C-SiC. In 4H-SiC and 6H-SiC, however, such signals were306

ECS Transactions, 50 (4) 305-311 (2012)©The Electrochemical Society

usually absent. In addition, residual carbon clusters in the interface region has not been
observed by other spectroscopic analyses [2,3]. Judging from these facts, the carbon
clusters are probably negligible in Dit near EC at the-state-of-the-art 4H-SiC MOS
interfaces. The 2nd signal in Table I (assigned to a sort of Si DBs [7]) may be also
negligible nowadays, because of its absence in the present MOS samples.
Later, the PbC center (interfacial carbon DB) was found in porous SiC-SiO2 samples[8]. This name means “the Pb center on a carbon atom”, and it was really similar to theoriginal Pb center, except a difference between Si and C. In fact, the PbC center was
passivated by a H atom in the temperature range of 400~800 °C [9]. However, this centerhas not been observed in single-crystal samples. Its areal density as well as its location(either Si face or C face) is also unclear. Therefore, it is also unclear whether this defectrelates to the major part of Dit.
More recently, the Lenahan’s group have found a strong Si-vacancy signal [10] (the–VSi signal of negatively charged Si vacancy at k (quasi-cubic) site [14]) by taking
advantage of EDMR technique. This signal was observed in 4H/6H-SiC MOSFETs (Siface) with dry oxidation. The location of the Si vacancies seems to be separated from theinterface, because they preserved the same high symmetry (Td) and high spin state(electron spin = 3/2) as they are in bulk. In 4H-SiC, a Si vacancy in low symmetricpositions should appear as a different ESR signal such as the TV2a signal (negatively
charged Si vacancy at h (hexagonal) site, C3v symmetry [14]). With lowering thesymmetry, a fine splitting should become detectable in ESR.
In nitrided SiC MOSFETs, the Si-vacancy signal vanished [10], suggesting that theSi-vacancy levels (close to the midgap [14]) were eliminated by the nitridation. The
Lenahan’s group also reported another interesting signal (the 5th signal in Table I) which
was observed in the other type of 4H-SiC MOSFET [10]. This signal was assigned to asort of c-axial interface DBs. For these centers, further experiments are desired to clarify
their roles in SiC MOS interfaces.

1
2
3
4
5
6
7

TABLE I. ESR signals of SiC MOS interfaces. g means gyromagnetic value ( g value) of each signal.
MethodCenterSample & Remarks
ESR signatures
Ref.
(Temp.)
C cluster (carbon DB
ESR
oxidized 3C-SiC (p-type),g = 2.003 (isotropic)[5]in amorphous-C)(R.T.)
some re-oxidized 4H/6H-SiC[6]
DB center (Si DB?)
ESR
oxidized 4H/6H-SiC (p-type)g = 2.0028~2.0062[7](R.T.)PbC (carbon DB
ESR
oxidized porous-SiC,
g = 2.0023~2.0032[8]
at interface)(R.T.)[9]reacting with H at 400~800°C
VSi- (Si vacancy
EDMR
4H/6H-SiC MOSFET (Si face), g = 2.0027 (isotropic) [10]near interface)(R.T.)
in sub. current
new DB center (cEDMR
4H-SiC MOSFET (Si face, ONO
g = 2.0026~2.0010[10]
axial DB at interface)(R.T.)gate), in sub. current
PH0/PH1 (carbon DBs
EDMR
4H-SiC MOSFET (Si face, H2
g = 2.004~2.003[11]interacting with H)(20K)
annealed), in channel current& 1H hfs (isotropic)
Nh (channel N donor)
EDMR
4H-SiC MOSFET (Si face, NO
g = 2.0047~2.0008
[12]
(4K)
annealed), in channel current

Our low-temperature ESR and EDMR studies
We have also investigated SiC MOS interfaces using ESR and EDMR. Our
measurements were carried out mainly at low temperatures for exploring shallow

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interface states. In fact, we have found different signals and results from the previous
studies, as is seen in the following two subsections.
Hydrogen and SiC MOS interfaces. So far the relationship between H and interfaces
has been revealed in two cases. The first one is the PbC center [9] as noted in the above.
Probably, the PbC center associates with relatively deep interface states, because its signal
was observable at R.T. The second case is our EDMR study on 4H-SiC MOSFETs (Siface) with a typical hydrogen treatment (H2 annealing at 600°C) [11]. The observed
signals were named “the PH0/PH1 centers” (see the inset of Fig. 1) which were observable
at around 20 K in the channel current of the MOSFETs. Therefore, they surely correlatedwith the conduction-band-side interface states. These centers were formed after γ-ray
irradiation (energy ~ 1.4MeV, dose = 6 Mrad) to the MOSFETs, suggesting that they
were initially passivated by H atoms. In addition, the PH1 signal revealed an isotropic
hyperfine splitting (hfs) due to 1H nuclear spin (5.4 mT). This signature indicates thepresence of a H atom in a nearest-neighbor site of the PH1 center. A small anisotropy of
their g values suggests that they are carbon-related defects.
Accordingly, we proposed a model of the PH0/PH1 centers as shown in Fig. 1. At theinterface, there are DB-like sites due to carbon-related defects. They reacted with H
atom(s) during H2 annealing. After γ-ray irradiation, H passivation is broken, and thedefect site shows ESR signals. The dissociated H atoms may diffuse far away from thedefect sites, which create the PH0 centers at there. On the other hand, when the
dissociation is partial or the dissociated H atom is captured again in a neighboring site ofa defect, the PH1 center is generated. In 4H-SiC MOSFETs with dry oxidation, an EDMR
signal similar to PH0 was observed at around 20 K [12].
The PH0/PH1 centers may be consistent with carbon-dimer defects predicted by thetheoretical calculation on SiC-SiO2 interfaces [13]. Such defects hold DB-like orbitals on
their threefold coordinated C atoms, which can interact with H atoms. They were
proposed as the origin of the major interface states [13]. However, we suggest that thePH0/PH1 centers are minor defects at the interface. The reason for this is based on theirsmall signal intensities in both H-incorporated and dry-oxidation MOSFETs (Si face).
Our idea is consistent with the small response of Si face MOS interfaces against H
treatments. On the contrary, C face MOS interfaces show a higher reactivity to H
treatments, so that we have to also examine C face.

PH1
SiO2
20K

Γ-irradiation
C-related defects(C dimer?)H

Si faceSi

C

4H-SiCPH1 PH0
H removalelectron spin at PH0/PH1 centers

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PH0
5.4mT

ECS Transactions, 50 (4) 305-311 (2012)©The Electrochemical Society

Figure 1. Model of hydrogen and the PH0/PH1 centers in 4H-SiC MOS interface (Si face).
The graph shows a typical EDMR spectrum of PH0/PH1 at 20 K in the channel current of
4H-SiC MOSFET [11].

N density (1014 cm-2)

XPS intensity (a.u.)

Nitrogen and SiC MOS interfaces. In nitrided 4H-SiC MOSFETs (NO/N2O annealing
at 1250°C for 6-nm dry SiO2), we have not detected the PH0/PH1 signals, indicating that
nitrogen (N) incorporation eliminates shallow interface states due to carbon-related
defects [12]. Even after γ-ray irradiation, no defects were detectable at 20 K. N atoms at
the interface may form stable bonding. Alternatively, at lower temperatures such as 4 K,
a strong EDMR signal was observed, which we name “Nh”. The signal is shown in theinset of Fig. 2. From its ESR signature, the Nh center has been identified as channel N
shallow donors (at h site) [12]. The Nh signal resembled close to the N shallow donor
signal, NC(h), in N-doped bulk region. However, there were found quantitative
differences between Nh and NC(h), which may reflect on different environments of ashallow donor in channel and bulk regions.
In addition to the channel N donors, we have also found “fixed N atoms” at theinterface [15]. After the nitridation of the interface, chemically unremovable N atoms
were formed at there, and their density increased over 1014 cm-2 or up to 0.4 monolayer
coverage. They were evidenced by x-ray photoemission spectroscopy (XPS), as shown in
the inset of Fig. 2. This XPS spectrum was taken after removal of a top SiO2 layer byhydrofluoric (HF) treatments (both diluted and concentrated conditions were subjected).
The density of the fixed N atoms was linearly increased with nitridation temperature (Fig.
2). The capacitance-voltage analyses revealed that the fixed N atoms eliminate shallow
interface states by a fraction of 1 interface state to ~1000 fixed N atoms. The origin of
these N atoms may be analogue to a silicon oxynitride epitaxial layer observed for
nitrided 6H-SiC surfaces [16].

Fixed N atoms

2
1

N 1s

800
1200
Nitridation temp.
(°C)

410

405
400
Binding energy (eV)Si face SiO2

Si
Cchannel N
donor

4H-SiCN

395

silicon oxynitride epilayer?

Fixed N atoms at interface
Removal of C-related defects

channel N donor

Figure 2. Model of fixed N atoms and channel N donors in 4H-SiC MOS interface (Siface) after nitridation. The left graphs show a typical XPS spectrum of fixed N atoms and309

ECS Transactions, 50 (4) 305-311 (2012)©The Electrochemical Society

their areal density as a function of nitridation temperature [15]. The right graph shows atypical EDMR spectrum of channel N donors [12].
Those two behaviors of N (the formation of the fixed N atoms and N doping to thechannel region) are characteristic of Si face SiC MOS interfaces. Using ESR, the amountof the N doping was estimated to be ~1012 cm-2 (we will report separately in theforthcoming paper), which is comparable to a carrier density in the inversion layer of thechannel. Thus, the N doping should have a large impact on electrical properties of the
MOSFET channel. We have to optimize the nitridation processes in terms of the
reduction in Dit as well as the N doping effect.
Summary
4H-SiC MOS interface states were discussed based on ESR and EDMR results ofprevious pioneering works and by our group. Several ESR/EDMR signals have beenreported in this interface system as summarized in Table I. They were connected mainly
to carbon-related defects. We pointed out that there are notable dissimilarities between Si
and SiC MOS interfaces. One is a small reactivity of H atoms at SiC MOS interfaces. We
observed minor reactive sites (the PH0/PH1 centers) on 4H-SiC MOS interfaces (Si face).
The other notable dissimilarities were seen in nitrided 4H-SiC MOS interfaces (Si face),
where we have found the formation of the fixed N atoms and the N doping to the channel
region. These behaviors are the key to utilize the nitridation processes. For SiC MOS
interfaces, it is still necessary_ to accumulate more complete data not only on Si face but
also on C face, a-facets ((1120) planes), and off-angled interfaces (note that standard 4HSiC wafers are prepared with a 8°-off angle), and so on.
Acknowledgments
We thank K. Esaki and Prof. J. Isoya for their experimental assistances.

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Invited presentation on PRiME 2012 (ECS 222nd) @ Honolulu, Oct. 7-12.
http://www.electrochem.org/meetings/biannual/222/222.htm

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