ECS Transactions, 58 (7) 55-60 (2013)10.1149/05807.0055ecst ©The Electrochemical Society

SiC MOS Interface States: Difference Between Si Face and C Face
T. Umedaa, M. Okamotob, R. Kosugib, R. Araia, Y. Satoa, S. Haradab, T. Makinoc,
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
This paper discusses the differences between Si-face and C-face
MOS interfaces in 4H-SiC MOSFETs. The two interfaces exhibit
unique electrical characteristics, which will be linked with thedifferences in interface defects. We carried out an electricallydetected magnetic resonance (EDMR) study on Si-face and C-face
4H-SiC MOSFETs, and found that there are different types anddifferent amounts of interface defects on each interface. We
discussed the EDMR results in comparison with the electricalcharacteristics of Si-face and C-face MOSFETs.

SiC MOS Interfaces: Si face vs. C face
Silicon carbide is a promising wide-band-gap semiconductor for high-performance (highvoltage and high-temperature operational, low-energy-loss, and high-energy-density)
power electronics. The main devices of SiC will be Schottky barrier diodes (SBD) andmetal-oxide-semiconductor field effect transistors (MOSFETs) on 4H-SiC wafers. Boththe devices expectedly have wide applications in power electronics. Currently, the SBD
devices have been practically commercialized, but the MOSFETs still have some barriers
for a mass commercialization. The major barriers are caused by the quality of SiC MOS
interfaces. From a structural view point, SiC-SiO2 interfaces are similar to the wellknown Si-SiO2 interface, as discussed in our earlier paper [1]. However, from anelectrical view point, SiC MOS interfaces are much inferior to those of silicon. The
important issues in 4H-SiC MOSFETs are (1) a serious degradation of the channel
mobility (field-effect mobility, µFE), (2) a lower stability of the threshold voltage (Vth),and (3) a lower reliability of gate dielectric layers, as compared to the case of silicon.
Such properties are strongly dependent not only on MOS processes [oxidation and postoxidation anneal (POA), etc.] but also on the type of initial _surfaces. In 4H-SiC wafers,
typical surfaces are 4H-SiC(0001) (Si face) and 4H-SiC(0001) (C face), as shown in Fig.
1(a). The two ideal surfaces are dominated by either Si or C atoms. However, in practicalcases, an intentional off-angle (4° to 7°) of wafers does not allow such ideal surface
structures, and the growth of an oxide layer also destructs an initial surface [see Fig. 1(b)].
Accordingly, SiC MOS interfaces might form a mixed structure consisting of both Si andC atoms.
Nevertheless, the two MOS interfaces exhibit strikingly different behaviors, which
are evidenced by the electrical measurements shown in Fig. 2. In this figure, four types ofn-channel SiC MOSFETs fabricated on either Si face or C face are examined. Their
specifications are summarized in Table I.
55

ECS Transactions, 58 (7) 55-60 (2013)10.1149/05807.0055ecst ©The Electrochemical Society(a)(0001) Si face(b)4 off

SiO2(0001) Si‐face interface

k
k
k
h
C

k
k

Si

_
(0001) C‐face interfacekh_(0001) C face

SiO2_

Figure 1. Schematic views of (a) 4H-SiC(0001) Si face and 4H-SiC(0001) C face and (b)their MOS interfaces. A unit cell and k and h sites are shown in (a). In (b), the standard 4
off-angle is drawn for a 4H-SiC(0001) wafer.
TABLE I. Lateral n-channel 4H-SiC MOSFETs examined in our experiments. Gate length is 100 µm
and gate width is 150 µm.
Label
Substrate
Source/Drain
Gate
mobility
P-implantation
dry SiO2 (50 nm)
Si-face2 cm2/Vs
7-off n-type 4H-SiC(0001)[P] ≈ 1×1020 cm-3
(Dry)poly-Si@20V
optimum n epi-layer
15-3
Ni contact[N] ≈ 5×10 cm
Al-implantation[Al] ≈ 1×1020 cm-3
19 cm2/Vs
Si-facesame as above
same as above
dry SiO2 (60 nm)(Nitrided)poly-Si@20V
NO&N2O POA
1250°C_
C-face0.0 cm2/Vs
P-implantation
dry SiO2 (50 nm)
4-off n-type 4H-SiC(0001)
20
-3
(Dry)
Al
(transistor[P] ≈ 3×10 cm
optimum p epi-layer
did not
Al contact15-3
NAND ≈ 5×10 cm
work.)_
97
cm2/Vs
same
as
above
wet
SiO
C-face(52
nm)
2
4-off p-type 4H-SiC(0001)(Wet)Al
optimum p epi-layer
H2 POA
15-3
NAND ≈ 5×10 cm
1100°CFor C-face MOSFETs [Fig. 2(a)], dry oxidation was never successful and hence theC-face(Dry) MOSFETs did not work. Contrary to dry oxidation, wet oxidation
dramatically improved C-face MOSFETs. The channel mobility increased up to ~100
cm2/Vs [Fig. 2(c)], which is much higher than the standard value of Si-face MOSFETs.
The H2 POA at high temperatures ( 800 C) was also useful for improving µFE [2].
Namely, C-face MOS interfaces are sensitive to the incorporation of hydrogen atoms [2],as similarly to the Si-SiO2 interfaces. However, C-face(Wet) MOSFETs frequently
revealed a large negative shift in Vth after a strong negative gate-bias stress, as is shown56

ECS Transactions, 58 (7) 55-60 (2013)10.1149/05807.0055ecst ©The Electrochemical Society

in Fig. 2(a). The same trend may be also present in the C-face(Dry) MOSFETs [Fig. 2(a)].
This kind of instability seems to be a unique characteristic of C-face MOS interfaces,
because Si-face MOSFETs (both “Dry” and “Nitrided”) did not exhibit such a large shift[Fig. 2(b)]. Furthermore, it should be mentioned that the Vth shift was accompanied with areduction in the channel current [Fig. 2(a)], indicating that a degradation of µFE occurred
simultaneously.
(b) Si face(a) C face

10
10
10
10

Si‐face(Nitrided)

C‐face(Wet)80µFE up‐7µFE drastic
increase‐9‐30V ‐15V
Vthnegative shift‐11

Si‐face(Dry)60

2

Id‐s (A)10

C‐face(Wet)‐5µFE (cm /Vs)

10(c) mobility

100

Ioff up40
Si‐face(Nitrided)

20

‐13

‐15V

‐15

C‐face(Dry)

Si‐face(Dry)
0

‐5

0

5

10

15

0

5

Vg (V)

10

15

0

5

10
Vg (V)

15

Figure 2. Different behaviors of Si-face and C-face 4H-SiC MOSFETs. Id-s is a sourcedrain current, Vg is a gate bias, and µFE is the channel mobility. (a) Id-s-Vg curves of two
typical C-face MOSFETs (“Dry” and “Wet”). Solid and dashed curves were measured
before and after a negative gate-bias stress (-15V or -30V, duration = 400 s), respectively,
where a source-drain bias was set to be 0.1V and a well region was fixed at 0V. The Cface MOS interface is characterized by a drastic increase in µFE and a large negative shift
in Vth. (b) Id-s-Vg curves of two typical Si-face MOSFETs (“Dry” and “Nitrided”)
measured with the same conditions as the case of C face. This face is characterized by alow µFE, a stable Vth, and an increase in off current (Ioff). (c) Channel mobility of C-faceand Si-face MOSFETs. These curves were derived from Id-s-Vg characteristics and MOS
capacitance values. All measurements were carried out at room temperature.
Specifications of these MOSFETs are summarized in Table I.
Contrary to C-face MOSFETs, Si-face MOSFETs showed lower µFE of 1−20 cm2/Vs[Figs. 2(b) and (c)]. The incorporation of hydrogen atoms is usually ineffective for thisface, and nitrogen or phosphorus atoms provide the best way to improve Si-faceinterfaces [3]. In fact, after a nitridation POA, the channel mobility was improved by 10times higher level [Fig. 2(c)]. However, the interfacial nitridation usually reduces amargin of normally-off threshold voltage [4]. The off current (Ioff) is also increased by thenitridation, as is seen in Fig. 2(b). These are additional problems of the nitridation
technique. On the other hand, Vth of Si-face MOSFETs was stable against a negativegate-bias stress, which is demonstrated in Fig. 2(b).
Such a striking contrast between Si-face and C-face MOSFETs are believed tooriginate from the differences in their MOS interface states. Up to date, there are only
partial information on the microscopic origins of SiC-SiO2 interface states; in Si-face
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ECS Transactions, 58 (7) 55-60 (2013)10.1149/05807.0055ecst ©The Electrochemical Society

MOS interfaces, some carbon-related defects are present, such as the VSi centers(generating carbon dangling bonds) [5,6] and the PH0/PH1 centers [7,8]. However, it isdoubtful whether these defects are responsible for the unique behaviors of the twointerfaces, because Si-face MOSFETs still exhibited lower µFE even if these defects had
been fully eliminated [5,6,8].
Understanding the characteristics of Si-face MOS interfaces
In the first place, we discuss how to understand the characteristics of Si-face MOS
interfaces, because they are more intensively studied than C-face interfaces. In fact, wehave studied shallow interface states in Si-face MOSFETs using low-temperatureelectrically detected magnetic resonance (EDMR) spectroscopy [8]. In addition to EDMR,
interfacial nitrogen atoms have been characterized by X-ray photoelectron spectroscopy(XPS) [4]. Combining the EDMR and XPS data, we concluded that the nitrogen
incorporation into the interface caused larger-scaled changes, as compared with a smallchange in the interface-state density (Dit). The primary change is the formation of a large
number of “fixed nitrogen atoms” in the interface region [4]. They were chemically very
stable and survived not only a strong hydrofluoric etching but also an exposure to air.
Their amounts exceeded over 1×1014 cm-2, which is three orders of magnitude larger than
the reduction in Dit (1010−1011 cm-2) [4]. The secondary change is the doping of shallow
nitrogen donors into the channel region [8]. The amount of the doped nitrogen donors
was estimated to be ~1012 cm-2 [9], which is comparable to the concentration of free
carriers in the channel region [8]. Therefore, the nitrogen doping effect has a large impact
on the electrical characteristics of MOSFETs. Basically, the nitrogen doping increases thenumber of carriers in the channel region. They will partially occupy interface states andinactivate them. The residual carriers will contribute to the channel current. With thesefactors, we expect an apparent increase in µFE by the nitridation. Since the increment ofcarriers ( 1012 cm-2) was one or more orders of magnitude larger than the reduction in
Dit (≤ 1011 cm-2), the change in µFE will be dominated by the nitrogen doping effect. Ourmodel can also explain the reduction in Vth and the increase in Ioff associated with thenitridation, because the concentration of donors in the channel region is directly linked
with Vth and Ioff.
From a qualitatively view point, our model gives reasonable explanations on the
behaviors of nitrided Si-face MOEFETs. In the next step, we will build a quantitativemodel accounting for the electrical characteristics of nitrided MOSFETs with respect tonitridation conditions. For example, the nitridation technique is known to be sensitive tonitridation temperatures and nitridation species. We deduce that such conditions will vary
the amounts of both the doped nitrogen donors and the fixed nitrogen atoms. Actually, we
found that the amount of the fixed nitrogen atoms increased as a function of thenitridation temperature, from 0.7×1014 cm-2 to 2.5×1014 cm-2 [4]. Likewise, we expect thatthe amounts of the nitrogen doping will depend strongly on the nitridation conditions.
Differences between Si-face and C-face MOS interfaces:
A perspective from EDMR study
As is seen in Fig. 2, C-face MOS interfaces are entirely different from Si-face ones.
To answer why the two interfaces are so different, we tried EDMR studies to find out thedifferences in their interface defects. The EDMR experiments on C-face MOSFETs are
58

ECS Transactions, 58 (7) 55-60 (2013)10.1149/05807.0055ecst ©The Electrochemical Society

just progressing, and we here present a preliminary result. This will give us a new aspect
for understanding the differences between Si face and C face.
When we carried out EDMR measurements focusing on relatively deep interface
states of MOSFETs, typical EDMR spectra are obtained as shown in Fig. 3. These
spectra were measured at room temperature and using a negative gate bias that lowers the
Fermi level of the MOS interface. In this experiment, the detectable signals should
correspond to relatively deep interface states (located much lower than the conduction
band edge of 4H-SiC), because EDMR spectroscopy detects singly-occupied energy
levels near the Fermi level.
In Fig. 3, we found a striking contrast between Si-face and C-face MOSFETs; muchstronger EDMR signals were observed in C-face MOSFETs, and a different type of asmall signal was detected in Si-face samples. At present, their origins are not yet
unraveled. We here suggest that the strong signals in C-face MOSFETs are closely
related to the instability of Vth, because the deep interface states will capture carriers andbehave like “fixed charges”. The presence of a large number of deep interface states was
also suggested by the previous electrical measurements on C-face MOSFETs [3].
Furthermore, we speculate that the relevant defects may be also responsible for the
drastic changes in µFE as well as the reaction between hydrogen atoms and C-faceinterfaces. Judging from the EDMR spectra, there are at least two types of defects at the
C-face interfaces, as indicated by “signal 1” and “signal 2” in Fig. 3.
For Si-face MOS interfaces, we observed only a much smaller signal. This result will
be consistent with the fact that the Vth shift was much smaller in Si-face MOSFETs thanin C-face samples. In nitrided Si-face MOSFETs, the EDMR signal intensity was similar
to that shown in Fig. 3, however, the type of signal seemed to change. Thus, we imagine
that the interfacial nitrogen atoms reacted with the interface defects, creating another typeof defects at the interface.

10

EDMR signal (ppm)

400

signal 1

Si‐face(Dry)0200

C‐face(Wet)

‐10
330

340

0
signal 2
‐200

330

332

334

336

338

340

342

344

Magnetic field (mT)

Figure 3. EDMR spectra of C-face(Wet) and Si-face(Dry) MOSFETs measured at room
temperature and with 1.5-kHz magnetic-field modulation under microwave of 9.4 GHz
and 200 mW. The currents for monitoring EDMR signals were 15 nA for C-face sample
and 640 nA for Si-face sample, which were optimized to obtain the best signal-to-noise
ratio. These signals correspond to relatively deep interface states in each interface.
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ECS Transactions, 58 (7) 55-60 (2013)10.1149/05807.0055ecst ©The Electrochemical Society

Summary
In this paper, we focused on the striking contrast between Si-face and C-face MOS
interfaces. Using EDMR spectroscopy, we revealed that the types of defects associatedwith deep interface states are clearly different between the two interfaces. Also theamounts of these defects presented a contrast; they were much larger in C face than in Si
face. Such differences are believed to be linked with the differences in the electricalcharacteristics of Si-face and C-face MOSFETs. For nitrided Si-face MOSFETs, wehighlighted the importance of larger-scaled changes ( 1012 cm-2) in the interface region,
as compared with the smaller change in Dit (1010−1011 cm-2). The interfacial nitridation
caused the nitrogen doping into the channel region. Taking this effect into account, we
could explain how the nitridation improves the electrical characteristics of Si-face
MOSFETs.

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