Interface Defects in C-face 4H-SiC MOSFETs: An Electrically-Detected-MagneticResonance Study
T. Umedaa, M. Okamotob, H. Yoshiokab, G.-W. Kima, S. Maa, R. Araia,
T. Makinoc, T. Ohshimac, and S. HaradababInstitute of Applied Physics, University of Tsukuba, Tsukuba 305-8573, Japan
National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba
305-8568, Japanc
National Institute for Quantum and Radiological Science and Technology (QST),
Takasaki 370-1292, Japan

Using electrically-detected-magnetic-resonance spectroscopy and a
device simulation, we studied dominant interface defects, named“C-face defects,” in C-face 4H-SiC metal-oxide-semiconductor
field-effect transistors (MOSFETs). The C-face defects act as holetraps via their donor levels, when they are not passivated byhydrogen atoms. The densities of unpassivated C-face defects
were estimated to be from 4×1012 cm-2 to 13×1012 cm-2 in various
C-face MOSFETs, which correlated with negative thresholdvoltage (Vth) shifts. We explained influences of the C-face defects
on the Vth instability and the channel mobility of C-face MOSFETs.

C-face 4H-SiC MOSFETs
Silicon carbide metal-oxide-semiconductor field-effect transistors (SiC-MOSFETs) arepromising for low-energy-loss and high-power-density power transistors. SiC-MOSFETs
are usually fabricated using a 4H-SiC(0001) (“Si face”) surface to_ ensure a stability intheir threshold voltages (Vth) (1,2). In contrast, a 4H-SiC(0001 ) (“C face”) surface
exhibits a wider range of the Vth instability (2,3), although C-face MOSFETs can achievea much higher field-effect mobility (µFE = 60–100 cm2V-1s-1) than the standard Si-face
MOSFETs (µFE = 20–30 cm2V-1s-1) (1-4). Figure 1 shows a typical example of the Vthinstability observed in C-face MOSFETs. In MOSFET (a) which we call “bad type,” thedrain-current (Id) versus gate-voltage (Vg) curve is horizontally shifted toward thenegative direction after applying a negative Vg stress (-30 V) for 1800 sec. On thecontrary, in MOSFET (b) or “good type,” the negative Vth shift is drastically reduced
even against a much longer stress time (20480 sec). However, the microscopic origin ofsuch Vth instability as well as microscopic differences between C-face and Si-face MOS
interfaces is still unclear.
In this paper, we present an electrically-detected-magnetic-resonance (EDMR) anddevice-simulation study on C-face MOSFETs. EDMR spectroscopy detected a large
number of hole traps at the C-face MOS interface, which we have named the “C-facedefects” (2,5). They were only observed on oxidized C face, and deeply connect with thecharacteristic properties of C-face MOSFETs. For instance, a primary difference in thesamples shown in Figs. 1(a) and (b) was found in the densities of the C-face defects. We
also found that their densities correlated with the negative Vth shifts in C-face MOSFETs.
Accordingly, we propose a microscopic model for the Vth instability that the C-facedefects are incorporated into the SiO2 layer during the oxidation, generating the positive
fixed charges in the gate oxide.
(c)
Id

Vg

Gate

Vd

0V

n

n
p
~
A

0V

EDMR detector @1.5kHz

Figure 1. (a) and (b) Id-Vg characteristics for C-face 4H-SiC MOSFETs (before γ-ray
irradiation). Solid lines represent initial (un-stressed) curves, and dashed lines indicate
the curves after a negative Vg stress. (c) Id-Vg and EDMR setups. The drain voltage (Vd)was set to 0.1 V and -2.2 V for Id-Vg and EDMR measurements, respectively. The source
and well voltages were set to 0 V.
Experimental
C-face MOSFETs. Lateral n-channel C-face 4H-SiC MOSFETs (gate length = 100µm, gate width = 150_ µm) were prepared on a p-type epitaxial layer ([Al] ≈ 1×1016 cm-3)
of 4º-off 4H-SiC(0001) wafers. A 50-nm-thick gate oxide was grown by wet oxidation at
1000ºC and was subjected to H2 post oxidation anneal (POA) at 1100ºC for 30 min. The
gate electrode was deposited on the gate oxide at room temperature for an Al electrodefor “bad type” [Fig. 1(a)] and at 570ºC for a poly-Si electrode for “good type” [Fig. 1(b)].
This difference may relate to the difference shown in Fig. 1, however, we also suspect
other factors, because we have seen a variation of both Al-gate and poly-Si-gate C-face
MOSFETs. After the oxidation and POA, the process temperatures were kept to be lower
than 570ºC, because high-temperature annealing (≥ 800ºC) is known to degrade the Cface MOSFETs (6).
Some of the MOSFETs were subjected to γ-ray irradiation causing the dissociation ofhydrogen atoms from hydrogen-passivated defects. This effect is well known for the caseof the Pb center at Si/SiO2 interfaces (7). In a previous study (5), we found that thehydrogen-depassivation by γ-irradiation increased negative Vth shifts in C-face MOSFETs.
Thus, the γ-irradiation enabled us to prepare a variety of C-face MOSFETs with a wide
range of the Vth instability. We used γ-ray generated from 60Co (average energy ≈ 1.4
MeV) and the dose was set to 0.5–40 Mrad. Our dose range was similar to that used for
Si-MOS samples (7).
EDMR measurements. We carried out EDMR measurements at room temperatureusing a home-built EDMR spectrometer (2,5). In principle, EDMR can detect singlyoccupied energy levels interacting with carriers. In such energy levels, the Pauli’s
principle permits spin-dependent carrier-capture processes, which lead to electron-spinresonance (ESR)-induced current changes. Figure 1(c) shows our electrical setup. In the

EDMR mode, we used Vd = -2.2 V and monitored a drain-to-well current via the interface
by an EDMR detector. Figure 2(a) shows actual current changes induced by the C-facedefects. The current change is negative; ESR enhanced carrier-capture processes at thedefects and reduced the current. EDMR spectroscopy amplified such a current change(generally, in ppm) under sweeping an external magnetic field, B, by using a lock-in
amplifier synchronized to a magnetic-field modulation (we chose a modulation frequencyof 1.5 kHz). ESR transitions were excited by microwave of 200 mW at 9.462 GHz.
EDMR observation on C-face defects (interfacial hole traps)

Signal position (mT)

Normalized EDMR signal (a.u.)

EDMR signal (ppm)

PreAmp. output (V)The C-face defects have c-axial g values of g// = 2.0016 and g⊥ = 2.0023, which isrevealed by an angular map shown in Fig. 2(b). This ESR signature is clearly different
from the case of the Si-vacancy defect (g = 2.003, isotropic) observed in Si-face
MOSFETs (8). Thus, we have to consider that different types of interface defects areformed at C-face and Si-face MOS interfaces, which should relate to opposite electricalproperties between the two types of MOSFETs.
Figure 2(c) shows typical EDMR spectra of the two C-face MOSFETs examined inFig. 1. As is seen in the figure, we could observe the same EDMR signals, which wehave named “C-face defects” (2,5). The EDMR spectra include an isotropic doublet
hyperfine (hf) splitting of 1.0 mT which is most probably due to a 1H nuclear spin

Figure 2. EDMR measurements on interfacial hole traps (named “C-face defects”) in Cface MOSFETs. (a) Example of ESR-induced current changes due to the C-face defects.
Vertical axis is in proportion to a_ monitoring
current. (b) Angular map of the C-face_ _defect signal. B parallels to [0001] and [1120] for 0º and 90º, respectively. (c) EDMR
spectra measured using drain-to-well currents of -15 nA under Vg = -27 V and -10 V for
the “bad-type” and “good-type” samples, respectively. (d) Vg dependence of EDMR
spectra of C-face defects.

EDMR signal intensity (ppm)

Figure 3. Gate-voltage dependences of EDMR signal intensities of the C-face defects.
The signal intensities (peak-to-peak intensities) were measured for the same conditions as
used for Fig. 2. MOSFETs were subjected to 4-Mrad γ-ray irradiation to enhance the
EDMR signals by breaking hydrogen passivation of the C-face defects. The inset in (b)draws spin and charge states of a donor level of a C-face defect in association with thestages (i) to (iii).
(nuclear spin I = 1/2, natural abundance = 99.9%). This is not surprising because our Cface MOSFETs were subjected to wet oxidation and H2 POA. Although the ratio of the1
H hf signal and the central signal (without hf splitting) was found to be different
between “bad type” and “good type,” the largest difference between the two samples wastheir signal intensities, as is seen in the vertical axes of Fig. 2(c). The ratio of the hf andcentral signals was also changed with the Vg bias, as shown in Fig. 2(d).
The Vg dependences of peak-to-peak intensities of the C-face defects are examined in
Figs. 3(a) and (b). In every sample, the C-face defects were observable only under anegative Vg, in other words, by supplying holes to the interface. The signal intensity
increases with increasing a negative Vg, and reaches a peak at a certain Vg (we define it as
Vpeak) and turns into a decrease with a further negative Vg. These behaviors can bereasonably explained by a donor level of the C-face defect. As is shown in the inset of
Fig. 3(b), the donor level exhibits three different charge and spin states. In the stage (i)[when Vg ≥ 0 V, see Fig 3(a)], the donor level maintains a doubly-occupied neutral state,
which is ESR-inactive. Thus, no EDMR signals appeared. In the stage (ii) (when Vg < 0V), the donor level captured a hole, forming a singly-occupied +1 charge state, which isan ESR-active state. We therefore observed the EDMR signals. In the stage (iii), byfurther increasing negative Vg (or supplying excess holes), the formation of empty +2
charge states (ESR-inactive) has started. Then the EDMR signal intensity started todecrease. When returning Vg to 0 V, the EDMR signals immediately disappeared due tothe emission of trapped holes. Obviously, the C-face defects behaved as interfacial holetraps.
As reported in our previous paper (5), the γ-ray irradiation drastically increased the
EDMR signal of the C-face defects. This increase is caused by breaking the hydrogenpassivation of the C-face defects. Similar hydrogen-depassivation phenomena wereobserved for interfacial dangling-bond centers such as the Pb centers (Si dangling bonds)
at Si/SiO2 interfaces (7) and the PbC center (carbon dangling bonds) found in porousSiC/SiO2 interfaces (9). We, however, emphasize that each of the dangling bonds has anacceptor level whose neutral state should be singly-occupied (ESR-active) (7,9,10). Suchdangling bonds must become ESR-inactive when they are positively charged by holetrapping (10). Therefore, we should consider that the C-face defects are not the same
type as the known dangling-bond centers. It is also notable that the doubly-occupied
neutral donor levels of the C-face defects never capture electrons and never generate the
Coulomb scattering of free carriers. Therefore, they have no influences on the channel
mobility. This is consistent with the high channel mobility of C-face MOSFETs, in spiteof the presence of the C-face defects.
Device simulation on donor levels of C-face defects

12-2

NT (x10 cm )Density of donor levels

EF – EV (eV)In the next place, we estimate the density of the active C-face defects, which are notpassivated by hydrogen atoms, under the assumption that all the active C-face defects areconverted into the singly-occupied +1 charge states at Vg = Vpeak (a starting point for theformation of +2 charge states). We performed a device simulation using a commercial
device simulator (Sentaurus Device, Synopsys) (4), where the MOSFET structure is thesame as that used in the experiments. Donor-type single-level interface states are
introduced at ET: neutral state for ET < EF and +1 charge state for EF < ET. Because wecannot pinpoint ET, it is assumed to be EV + 0.5 eV (below EF), which does not affect theestimation of the defect density. Under the same bias conditions as used in Fig. 3, we
calculated EF at the interface as a function of Vg with/without the donor levels [Fig. 4(a)].
At Vg = 0 V, EF was calculated to be about EV + 1.0 eV. Then, EF moved toward EV as Vg
was negatively biased. For the case of the donor-level density (NT) of 6×1012 cm-2, EF is
pinned at the donor levels until they are fully positively charged at Vg ≈ -16 V.
Obviously, the negative Vg bias required for full charge of the donor levels (we define it
Vcharge) depends on NT. We calculated a relationship between Vcharge and NT as shown in

Figure. 4. Device simulations of a C-face 4H-SiC MOSFET including donor levels of the
C-face defects. (a) Fermi level (EF) at the drain edge of the C-face MOS interface as afunction of Vg. The density of the donor levels (NT) was assumed to be 6×1012 cm-2 at EV+ 0.5 eV. Until fully charging the donor levels with holes, EF – EV is pinned at thedonor-level position. (b) Simulation of a relationship between Vcharge and NT. Solidcircles are obtained by the simulation.

Fig. 4(b). From this relation and an assumption that Vcharge ≈ Vpeak, we estimated NT to be
11×1012 cm-2 for Vpeak = -27 V [Fig. 3(a)] and 3.9×1012 cm-2 for Vpeak = -10 V [Fig. 3(b)].
Correlation between C-face defects and positive fixed charges

12-2

Density of positive fixedcharges (x10 cm )Vth shift (V)We prepared various C-face MOSFETs with different γ-ray irradiation, which
revealed a wide range of the negative Vth shifts (5). Figure 5(a) summarizes arelationship between NT (estimated from the procedure described in the preceding
section) and the Vth shifts. As is seen in the figure, NT seems to correlate with thenegative Vth shifts which are caused by positive fixed charges in the gate oxide. To
simply explain such a correlation, we suggest that the C-face defects are formed not onlyat the interface but also in the SiO2 layer. When the oxidation is proceeding and a
SiC/SiO2 interface is going to the inner substrate, the C-face defects at the interface arepossibly incorporated into the oxide, and partly remain as the source of positive fixedcharges at there. This idea gives us a reason why C-face MOSFETs show large negative
Vth shifts.

SiO2

C-face defects
at interface→ hole traps and
EDMR signal

C faceSiO2growthSiO2

C faceC-face defects
incorporated
into SiO2 layer→ positive fixedcharges andnegative Vth shift

Figure 5. (a) Correlation between NT and the negative Vth shifts in various C-face
MOSFETs. The Vth shifts are saturated values measured after a long stress time; e.g., >1500 sec and > 20000 sec for the samples used in Figs. 3(a) and (b), respectively. A solid
curve is a guide for eyes. Horizontal error bars reflect uncertainties in Vpeak. The righthand-side axis represents the density of effective positive fixed charges calculated fromthe Vth shifts and the oxide capacitance. (b) A model of the C-face defects formed at theinterface as well as in the oxide layer.
Summary
We have examined the dominant interface defects (the C-face defects) in C-face 4HSiC MOSFETs. They acted as interfacial hole traps and closely related to the Vthinstability of C-face MOSFETs, when they were not passivated by hydrogen atoms.
Their densities were estimated to be from 4×1012 cm-2 to 13×1012 cm-2 in various C-face
MOSFETs with a wide range of the negative Vth shifts. The whole EDMR results were
understandable based on a deep donor level of the C-face defect. This level causes holetrapping under Vg < 0, while it does not affect the channel mobility under Vg ≥ 0. Wefound that the densities of the C-face defects are correlated with the negative Vth shifts or
the densities of positive fixed charges in the oxide layer. Accordingly, we proposed that
the C-face defects are partly incorporated into the oxide layer during the oxidation andbecome the source of the Vth instability. Although the microscopic origin of the C-facedefects is still unidentified, their ESR signatures (axially-symmetric g values, 1H hf
splitting, spin-1/2 with a single positive charge state, etc.) will provide important hints fortheir identification.
Acknowledgments
This work was party supported by the JSPS KAKENHI grant No. 25286054 and
17H02781.
References
1. G. Y. Chung, C.C. Tin, J. R. Williams, K. McDonald, R.K. Chanana, R. A.
Weller, M. Di Ventra, S. T. Pantelides, L. C. Feldman, O. W. Holland, M. K. Das
and J. W. Palmour, IEEE Elect. Dev. Lett., 22, 176 (2001).
2. T. Umeda, M. Okamoto, R. Kosugi, R. Arai, Y. Sato, S. Harada, T. Makino and T.
Ohshima, ECS Trans., 58(7), 55 (2013).
3. M. Okamoto, Y. Makifuchi, M. Iijima, Y. Sakai, N. Iwamuro, H. Kimura, K.
Fukuda and H. Okumura, Appl. Phys. Express, 5, 041302 (2012).
4. H. Yoshioka, J. Senzaki, A. Shimozato, Y. Tanaka and H. Okumura, AIP
Advances, 5, 017109 (2015).
5. G. W. Kim, R. Arai, S. J. Ma, M. Okamoto, H. Yoshioka, S. Harada, T. Makino,
T. Ohshima and T. Umeda, Mater. Sci. Forum, 858, 591 (2016).
6. S. Harada, M. Kato, T. Yatsuo, K. Fukuda and K. Arai, Mater. Sci. Forum, 600603, 675 (2009).
7. P. M. Lenahan and P. V. Dressendorfer, J. Appl. Phys., 54, 1457 (1983).
8. C. J. Cochrane, P. M. Lenahan and A. J. Lelis, J. Appl. Phys., 109, 014506 (2011).
9. J. L. Cantin, H. J. von Bardeleben, Y. Shishkin, Y. Ke, R. P. Devaty and W. J.
Choyke, Phys. Rev. Lett., 92, 015502 (2004).
10. P. M. Lenahan and J. F. Conley, Jr., J. Vac. Sci. Technol., B 16, 2134 (1998).