Electrically detected magnetic resonancestudy on interface defects at nitrided Si-face,a-face, and m-face 4H-SiC/SiO2 interfaces
Cite as: Appl. Phys. Lett. 116, 171602 (2020); https://doi.org/10.1063/5.0002944
Submitted: 30 January 2020 . Accepted: 08 April 2020 . Published Online: 27 April 2020
E. Higa, M. Sometani, H. Hirai, H. Yano, S. Harada, and T. Umeda

COLLECTIONSThis paper was selected as an Editor’s Pick

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Electrically detected magnetic resonance study
on interface defects at nitrided Si-face, a-face,and m-face 4H-SiC/SiO2 interfaces
Cite as: Appl. Phys. Lett. 116, 171602 (2020); doi: 10.1063/5.0002944
Submitted: 30 January 2020 . Accepted: 8 April 2020 .
Published Online: 27 April 2020
E. Higa,1

M. Sometani,2

H. Hirai,2

H. Yano,1

S. Harada,2and T. Umeda1,a)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 investigated interface defects formed on a-face and m-face 4H-SiC/SiO2 interfaces after interface nitridation by nitric oxide (NO)post-oxidation annealing (POA). Using electrically detected magnetic-resonance spectroscopy, we observed interface defects on these faces.
The a- and m-face interface defects were found to be similar to a carbon-related interface defect (the PbC center) observed on the standard
Si-face, but their amounts were signiﬁcantly lower than those of the Si-face after the same NO POA. Such a reduction was correlated with adrastic increase in the ﬁeld-effect mobility (80–90 cm2 V1 s1) of the a- and m-face metal-oxide-semiconductor ﬁeld-effect transistors after
NO POA. We also found that over-nitridation caused the formation of two types of nitrogen-related defects on the Si-face. These nitrogenrelated defects resemble the K center (Si dangling-bond center) observed in Si3N4.
Published under license by AIP Publishing. https://doi.org/10.1063/5.0002944

Silicon carbide metal-oxide-semiconductor ﬁeld-effect transistors(4H–SiC MOSFETs) have many advantages such as lower energy loss,
higher power, higher frequency, and higher temperature operation for
power electronics compared to conventional silicon (Si)-based powerelectronics.1,2 However, their performances are severely limited due totheir ﬁeld-effect mobility (lFE) being much lower than the ideal electron mobility (1000 cm2 V1 s1).3 Improvement in lFE can beachieved using two basic techniques: introducing a high density ofnitrogen (N) atoms into the interface4–7 and using either a4H-SiC(112 0) surface (“a-face”) or a 4H-SiC(11 00) surface (“m-face”)
for the MOSFET channel.2,8,9 By using interface nitridation by postoxidation annealing (POA) with nitric oxide (NO),4–7 the maximum
lFE increases from 1–7 cm2 V1 s1 for standard 4H–SiC(0001) (theso-called “Si-face”) MOSFETs10 to 25–40 cm2 V1 s1 for nitrided
Si-face MOSFETs. Furthermore, the use of the a-face or m-facetogether with interface nitridation increases the maximum lFE to
100 cm2 V1 s1.8,9 However, the microscopic mechanism of such an
improvement is not fully clear. To reveal microscopic information oninterface defects, electrically detected magnetic-resonance (EDMR)
spectroscopy has been applied to fully processed 4H–SiC
MOSFETs.11–16 Previous EDMR studies revealed interface defects
such as the “interface Si-vacancy center,”11–13 “PbC center” [interface

Appl. Phys. Lett. 116, 171602 (2020); doi: 10.1063/5.0002944
Published under license by AIP Publishingcarbon dangling-bond (C DB) center],14,15 and “dual-PbC center.”16
However, all of them were found on the Si-face, and no EDMR data
have been reported for the a- and m-faces thus far.
In this Letter, we present EDMR observations on the a- andm-face interfaces after NO POA. Both interfaces revealed EDMR signals of C-related interface defects similar to the PbC center observedon the Si-face.14,15 However, the amounts of C-related defects were
found to be two orders of magnitude lower on the nitrided a- andm-faces than on the Si-face. We, therefore, suggest that the drastic
improvement in the maximum lFE (79 cm2 V1 s1 for the a-face and89 cm2 V1 s1 for the m-face) is mainly caused by eliminating
C-related defects. However, we found that interface nitridation is lesseffective for the Si-face and over-nitridation even created additional
interface defects related to N atoms. We speculate that these additional
defects may be similar to the K center (Si DB center in Si3N4)17,18 andrelated to the negative effects of over-nitridation.19
We prepared n-channel lateral 4H–SiC MOSFETs on Si-, a-, andm-face wafers, as shown in Table I. The Si-face MOSFETs were
fabricated on 4 -off p epitaxial layers (Al concentration ¼ 4–5
 1015 cm3) on 4H-SiC(0001) wafers. The a- and m-face wafers wereprepared by cutting 4H–SiC crystal ingots each with an off angle ofabout 10 . On these substrates, p epitaxial layers were grown with Al

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ARTICLETABLE I. 4H–SiC MOSFET samples studied by EDMR.

Label

NO POA time (min)lFE (cm2 V1 s1)

0
10
60
120
60
60

6.8
28
32
31
79
89

Si-face dry14,15
Si-face NO10
Si-face NO60
Si-face NO120
a-face NO60
m-face NO60

concentrations of 1.3  1016 cm3 for the a-face and 1.8  1016 cm3for the m-face. For all the MOSFETs, gate oxides (50 nm) were grownby standard dry oxidation. After dry oxidation, NO POA was carriedout at 1250  C for each duration shown in Table I. Before the NO
POA, a- and m-face MOSFETs hardly activated the channel currents.
We call the MOSFETs prepared by the NO POA times of 0, 10, 60,
and 120 min “dry,”14,15 “NO10,” “NO60,” and “NO120,” respectively.
The gate electrodes were fabricated by poly-Si deposition. The gate
length (L) and width (W) were 5 lm and 2000 lm, respectively, except
for the m-face MOSFET (L ¼ 5 lm and W ¼ 200 lm). A top view ofthe MOSFET is shown in Fig. 1(a). The maximum lFE values for each
MOSFET were evaluated and are listed in Table I.
EDMR measurements were carried out at room temperature byusing an X-band EDMR spectrometer we constructed and used in ourprevious studies.14,15 We adopted bipolar-ampliﬁcation-effect (BAE)

FIG. 1. (a) Photograph of a 4H–SiC MOSFET examined here and its schematic crosssection. We adopted the BAE EDMR setup shown here. (b) EDMR spectra of Si-face, aface, and m-face MOSFETs. The traces are vertically offset for clarity. Each spectrum
was measured under optimized bias conditions such that Vg ¼ 7.5 V and IEDMR¼ 200 nA for Si-face dry,14,15 Vg ¼ 5 V and IEDMR ¼ 500 nA for Si-face NO60,
Vg ¼ 5 V and IEDMR ¼ 50 nA for a-face NO60, and Vg ¼ 5.5 V and IEDMR ¼ 6 lA for
m-face NO60. (c) Correlation between EDMR signal intensities and maximum ﬁeld-effectmobilities. These intensities express the peak-to-peak intensities of EDMR signals.

Appl. Phys. Lett. 116, 171602 (2020); doi: 10.1063/5.0002944
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EDMR measurements that were optimized to detect interface signals
with a high signal-to-noise ratio (S/N).20,21 In this regime, we activateda constant current (Id) from the drain to the well and monitored electron-spin-resonance (ESR)-induced current changes in the drainsource current (IEDMR), as shown in Fig. 1(a). The gate voltage (Vg)
was optimized to maximize the S/N. We used lock-in detection synchronized with magnetic-ﬁeld modulation at 1.56 kHz. ESR transitions
were excited by microwave at 9.462 GHz and 200 mW.
Figure 1(b) shows the EDMR spectra for the Si-, a-, and m-face“NO60” MOSFETs together with the “Si-face dry” MOSFET14,15 whenthe external magnetic ﬁeld (B) was parallel to the [0001] axis (c axis). It
is immediately clear that the NO POA signiﬁcantly reduced an EDMR
signal of the PbC center, which was dominantly observed on the Si-facedry sample.14,15 As plotted in Fig. 1(c), the signal intensity on the Si-facedecreased to 1/40 (2.5%) after NO POA for 60 min. A consistent result
was previously obtained using conventional ESR spectroscopy where aninterface signal with a spin density of 3–4  1012 cm2 was reduced
below the detection limit (<2  1011 cm2) after optimum NO POA or
POCl3 POA.22 In the present study, thanks to the higher S/N of EDMR
spectroscopy,20,21 we could still observe the defect signal even after optimum NO POA. We could also conﬁrm that the a- and m-faces after the
NO60 process displayed a reduced interface EDMR signal, compared tothe untreated Si-face,14,15 to 1/110 (0.91%) and 1/380 (0.26%), respectively [see Fig. 1(c)]. These EDMR observations suggest that NO POA
can remove the PbC center and its formation can be much more suppressed on the nitrided a- and m-faces than on the nitrided Si-face. This
provides microscopic information on why the nitrided a- and m-faces
show excellent lFE. In fact, as shown in Fig. 1(c), maximum lFE values
are correlated with the EDMR signal intensities.
To examine the origins of the interface defects observed on
the a- and m-faces, normalized EDMR spectra measured for B parallel to or perpendicular to the c axis are shown in Fig. 2. Similar to
the PbC center observed on Si-face dry, which shows slightly anisotropic g factors of g// ¼ 2.0029 and g? ¼ 2.0032,14,15 the a- andm-face NO60 interfaces exhibited similar EDMR signals in thesame range of g factors (2.0025–2.0031). These g factors were close
to the free-electron g factor (2.0023), and their anisotropy was
found to be very weak. These features are characteristic of
C-related ESR centers.22 Therefore, we conclude that C-related
defects similar to the PbC center are formed on the nitrided a- andm-faces although their structural details could not be resolved due
to too small signal intensities. On the Si-face, the microscopic origin of the PbC center has been identiﬁed as a C DB on a C-adatom
structure.15 We deduced that the surface structures of the a- andm-faces prevent the formation of C adatoms.
We further investigated the effect of N incorporation using the
Si-face. The Si-face NO60 interface (Fig. 2) revealed additional shoulders
on both sides of the central signal. These shoulders are asymmetric andshould differ from a pair of symmetric shoulders of the PbC center, whicharise from its hyperﬁne (HF) interaction.14 Thus, we judged that the
Si-face NO60 interface involves a different signal(s). Figure 3(a) shows
the EDMR spectra of the nitrided Si-face MOSFETs with different POA
times. EDMR signal intensities increased with the increasing POA time.
The NO10 spectrum was identical to the PbC spectrum with a reduced
intensity (1.2% of the PbC intensity in the Si-face dry sample14,15). In thissample, the maximum lFE suddenly increased to 28 cm2 V1 s1 (seeTable I), supporting our idea that C-related defects strongly affect lFE.

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We found, however, that the NO60 and NO120 spectra cannot
be ﬁtted by the PbC spectrum; alternatively, they could be well ﬁtted by
a combination of one broad central signal (dotted line) and anothertriplet signal (three colored lines). The latter signal is necessary forreproducing the asymmetric shoulders. In addition to the spectra

shown in Fig. 3(a), other EDMR spectra for different magnetic-ﬁeld
angles (30 , 60 , and 90 , where 0 and 90 correspond to B//[0001]
and B//[112 0], respectively) could also be reasonably ﬁtted by the sumof the two signals. The former central signal shows an isotropic g factor of 2.003 and a signal width broader than the PbC signal. The latter
triplet signal represents a 14N HF splitting due to a 14N nuclear spin
(spin number I ¼ 1; natural abundance ¼ 99.6%). Its g factor and 14N
HF splitting were found to be isotropic at g ¼ 2.0032 and 0.57 mT,
respectively.
As shown in Fig. 3(b), these two signals increased with theincreasing POA time or the amount of interfacial N atoms shown on
the right-side axis. The amount was measured by x-ray photoelectron
spectroscopy (XPS) on the same series of nitrided samples.23
Accordingly, we argue that the central and 14N HF signals are related
to incorporated N atoms. The g factor of 2.003 coincides with that of
the K center in Si3N4 (Si DB center, N3Si•, where “•” represents an
unpaired electron).17,18 Thus, it is reasonable to consider that the central signal may arise from the K center formed inside N-incorporated
regions, as schematically drawn in Fig. 4.
We also argue that the 14N HF signal also corresponds to a familyof K centers because its 14N HF splitting of 0.57 mT is similar to 14N
HF splitting due to back-bonded N atoms of the K center (0.46 mT).17
Since the 14N HF signal revealed a single 14N HF splitting, we imagine
that its origin may be ascribed as an incomplete type of K center, suchas C2N1Si•. This type of defect may be formed at the boundary
between N-incorporated and non-incorporated regions, as shown on
the right-hand side of Fig. 4. Since this incomplete K center has only asingle back-bonded N atom, we could resolve its 14N HF splitting. If
we consider other types of N-related defects such that their unpaired
electron mainly distributes on a N atom, much larger 14N HF splitting
should be observed. For instance, larger isotropic 14N HF splitting was
detected, e.g., 3 mT for a bridging N center (Si2¼N• in Si3N4),25
3.3 mT for a substitutional N atom surrounded by C atoms (C4N• in

FIG. 3. EDMR study on N-related interface defects on the nitrided Si-face. (a)EDMR spectra for B//[0001] and ﬁtting results. The traces are vertically offset forclarity. NO10 spectrum could be simply ﬁtted by the PbC signal (observed in the
Si-face dry sample14,15). For NO60 and NO120, ﬁtted spectra were calculated by
the sum of the “central signal” and the “14N HF signal.” EDMR spectra were measured by Vg ¼ 5 V and IEDMR ¼ 10 lA for NO10, Vg ¼ 5 V and IEDMR ¼ 500 nA
for NO60, and Vg ¼ 5.5 V and IEDMR ¼ 500 nA for NO120. The modulation amplitude was set to 0.5 mT. (b) Relative EDMR intensities of the central signal and triplet 14N HF signal vs NO POA time. These intensities were normalized by integrated
EDMR intensity of the PbC signal in the Si-face dry sample shown in Fig. 1(b).

FIG. 4. Schematic image of the nitrided Si-face (NO120) and formation of two typesof N-related defects, in addition to the PbC center (rightmost edge).15 On this surface, the N-atom density was set to 4  1014 cm2 (approximately 35% of surface
C atoms were replaced with N atoms).24 N-incorporated honeycomb units are represented as red regions. EDMR detected two types of N-related defects. We argue
that one type is similar to the K center (N3Si•) formed inside red regions, andanother type is formed at the boundary between red and non-red regions. We also
argue that the latter is an incomplete K center (e.g., C2N1Si•). Symbols“”represent the unpaired electrons. Fractions of former and latter defects were 3
defects/7128 Si atoms (0.44  1012 cm2) and 1 defect/7128 Si atoms(0.17  1012 cm2), respectively. ESR signatures of three EDMR centers are also
summarized. See the details in the text.

FIG. 2. Normalized EDMR spectra of Si-face, a-face, and m-face MOS interfaces
when B//[0001] or B ? [0001]. The traces are vertically offset for clarity. Bias conditions were the same as those in Fig. 1(b). The modulation amplitude was set to
0.5 mT except for the m-face (0.25 mT).

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diamond),26 and 1.8 mT for a substitutional N atom surrounded by Si
atoms (Si4N• in 4H-SiC).27 Therefore, we consider the model shownin Fig. 4 to be plausible.
It is known that the K center is an amphoteric charge trap withnegative-U behavior.18 Thus, this type of defects may also reduce thefree-electron density in the inversion channel layer by capturing electrons, similar to the case of the PbC center.14,15 The valence-band-side
energy levels of PbC largely decrease the free-electron density(7  1012 cm2 for Vg ¼ 15 V),7 resulting in the drastic reduction inlFE.14,15 In contrast, the K-center-related defects appeared to have an
weaker impact on lFE, as shown in Fig. 1(c). It is probably because thedensity of the K-center-related defects shown in Fig. 4 is one order ofmagnitude smaller than that of the PbC center. Judging from relative
EDMR intensities of the central and 14N HF signals, we roughly estimated the spin densities of the K center and the incomplete K center
to be 0.4  1012 cm2 and 0.2  1012 cm2, respectively, for theNO120 interface, supposing that the PbC EDMR intensity in the
Si-face dry sample corresponds to 3.5  1012 cm2.14,15,22
At least, the K-center-related defects may be related to thethreshold-voltage (Vth) instability observed for the over-nitridation of
the Si-face19 because Vth is highly sensitive to the presence of thecharge traps. For the NO120 interface, the total estimated density(0.6  1012 cm2) of the K-center-related defects corresponds to a Vthshift of 1.4 V for the oxide capacitance of 6.9  108 Fcm2 (the oxide
thickness of 50 nm), which may be practically detectable.
In summary, we investigated interface defects formed in nitrided
Si-face, a-face, and m-face 4H-SiC MOSFETs using EDMR. For thenitrided a- and m-faces, the EDMR signal of the PbC-related centerdrastically decreased compared to the Si-face dry interface, resulting ina signiﬁcant improvement in lFE from 7 to 80–90 cm2 V1 s1. For
the nitrided Si-face, the PbC signal rapidly decreased to 1/80 after
10 min NO POA. By increasing the NO POA time up to 60–120 min,the formation of two types of N-related interface defects was detectedby EDMR. We argued that both N-related defects may be similar to
the K center (N3Si•) observed in Si3N4. These K-center-related
defects may cause reliability issues of Si-face 4H-SiC MOSFETs after
over-nitridation.
We would like to thank C. Shinei for useful discussions. This
work was supported by the Council for Science, Technology andInnovation (CSTI), Cross-ministerial Strategic Innovation
Promotion Program (SIP), and “Next-generation powerelectronics” (funding agency: NEDO). This work was also partly
supported by Grants-in-Aid (Grant Nos. 17H02781 and 20H00340)
from the Ministry of Education, Culture, Sports, Science andTechnology of Japan.

Appl. Phys. Lett. 116, 171602 (2020); doi: 10.1063/5.0002944
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