Materials Science Forum Vols. 679-680 (2011) pp 370-373
Online available since 2011/Mar/28 at www.scientific.net© (2011) Trans Tech Publications, Switzerland
doi:10.4028/www.scientific.net/MSF.679-680.370

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Abstract. We present an electrically detected electron-spin-resonance (ESR) study on SiO2-SiC
interface regions of -channel lateral 4 -SiC MOSFETs with hydrogen annealing. This
characterization technique can reveal electrically active defects that interact with channel currents of
the MOSFETs. The defects were observed at 20 K, and were labeled “ H0” and “ H1”, one of which( H1) exhibited a 1H hyperfine splitting of 5.3 mT.
Introduction
Interface states in SiO2-SiC gate stacks are crucially important for the performance of SiC-based
metal-oxide-semiconductor field effect transistors (MOSFETs). For controlling such states,
incorporations of hydrogen and nitrogen atoms into the interfaces were found to be effective,
similarly to the case of well-studied SiO2-Si systems. Mechanism of such a control as well as a more
efficient control has been surveyed both experimentally and theoretically. However, microscopic
origins of the interface states are still not sufficiently clear. To identify their origins,electron-spin-resonance (ESR) studies have been performed [1-6] by analogy with the case of thecenters [Si dangling-bond (DB) centers] at SiO2-Si interfaces [7]. Samples studied by ESR wereoxidized 4 -SiC wafers [1], oxidized porous-SiC wafers [2,3], 4 /6 -SiC gate-controlled diodes[4,5], and 4 -SiC MOSFETs [6]. All these studies have been done at room temperature (R.T.).
Following to those works, we performed an ESR study on the SiO2-SiC interfaces at low
temperatures. We used an electrically detected ESR [electrically detected magnetic resonance(EDMR)] technique, which enables us to characterize interface regions inside fully-processed
MOSFETs.
Experimental
We prepared -channel lateral 4 -SiC MOSFETs with different processes. In this study, we would
like to focus on the MOSFETs with post oxidation annealing with H2 at a temperature of 800 ºC. They
were fabricated on epitaxial layers of Cree 8°-off 4 -SiC(0001) Si-face wafers with -type doping.
The gate length and width are 100 and 150 µm, respectively. The initial gate oxide is a 50nm-thick dry
oxide. Source and drain regions were formed by a high-dose phosphorous ion implantation. Poly-Si
gate electrodes and nickel contacts were also made. After preparing the MOSFETs, some of themwere subjected to a γ-ray irradiation (average energy = 1.3 MeV, dose = 2.7 Mrad). This irradiation is
known to damage hydrogen bonds, e.g., Si-H or O-H bonds, creating DBs there [8]. Thus, we
expected that the γ-irradiation can activate hydrogen-passivated defects in the samples.
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Materials Science Forum Vols. 679-680

371

EDMR measurements are similar to ESR ones, except that we monitor a change in the device
current instead of a change in the microwave absorption of a sample. We used amagnetic-field-modulation (1.5 kHz, 0.5 to 1.0-mT width) technique for amplifying a small current
change due to ESR. The key points for our measurements are that (1) we detect EDMR signals using
lateral channel currents of MOSFETs in order to selectively observe defects interacting with thechannel currents (on the contrary, previous EDMR works monitored diode currents flowing between
source/drain and substrate [4-6]) and that (2) we measure EDMR signals at low temperatures. The
latter condition will be necessary for detecting energetically shallow interface states that have a great
influence on the channel currents.

40

20

0

-20
0

(f) 20K
5.4mT
PH1
2.00

Resonant field (mT)60
Current change (arb.units)

Fig. 1. EDMR spectra forchannel currents of 4 -SiC(e) 20K
MOSFETs with post hydrogen
340
treatment at 800 ºC. Channel
18°
PH1 5.4mT
currents and bias conditions are
as follows: (a) 44 nA at ds =
338 P
2.01
H0
320
330
340
350 after γ-irr.
0.3V, gs = 0.5V, (b) 58 nA at ds= 1V, gs = 6.5V, (c) 15 nA at dsgvalue(a) R.T.
8°336= 1V, gs = 7V, (d) 370 nA at ds(b) 50K= 1V, gs = 17V, (e) 51 nA at ds
0 40 80= 2V, gs = 10V, where ds is a
PH0/PH1
Angle(°)
source-drain voltage and gs is a(c) 20K
substrate-gate voltage. (f) shows
angular dependences of H0/ H1
before γ-irr.
9.5GHzsignals where magnetic field was(d) 20K
200mW
rotated from [0001] (0º) to[1-100] (90º). To excite ESR
100 200 300 400 500 600 700 800
transitions, microwave of 9.5
Magnetic field (mT)
GHz and 200 mW was applied.

PH0

-5

10

(a)

channel10
1010devicecurrents

2-9µFE (cm /Vs)

Current (A)

-7

10
10

(b)

forward

-11

5

R.T.

0
0

10

currentchange

-13

-4

-2

0
ds (V)

2

4

6

8

10 12
gs (V)

20
(V)
gs

14

Fig. 2. Channel currents and
EDMR signals (current changes
due to ESR) as a function of ds
and gs measured at 20 K. In (a),gs was set to be 7V. For forward
currents ( ds < 0), EDMR signals
were not detected and one orderweaker than the smallest signal
in the channel currents. In (b),ds was set to be 1V.
Field-effect mobility (µFE) data
at room temperature are also
included in (b). The MOSFETs
used in this study showed amaximum µFE of 9 cm2/V·s.

Results and discussions
Figure 1 summarizes EDMR results for the channel currents of our samples. From R.T. to 50 K, we
could not detect any EDMR signals [Figs. 1(a) and (b)]. On the other hand, at 20 K, EDMR signals
were found after γ-irradiation, as shown in Figs. 1(c) and (d). The signals appeared only in the channel
currents, as shown in Fig. 2.
Figures 1(e) and (f) shows a higher-resolution spectrum of the EDMR signals and their angulardependences, respectively. The EDMR signals consisted of two signals, labeled “ H0” and “ H1”, andone of them ( H1) exhibited a doublet splitting of 5.4 mT. This splitting is most probably due to a
hyperfine interaction of a 1H atom (nuclear spin = 1/2, natural abundance = 99.9%), because the372

Silicon Carbide and Related Materials 2010

splitting was isotropic [see Fig. 1(f)]. The g values of the two signals were tentatively estimated to be|| = 2.004 and ⊥= 2.003, as is seen in Fig. 1(f). Their common g values suggest that they originate
from the same defect but are distinguishable by the presence or absence of a 1H hyperfine splitting(hfs).
In Table 1, ESR parameters of H0/ H1 are examined by comparison with various reported centers.
First, we compare their g values. Most of the centers can be classified into either carbon DB centers[(2), (3), and (8)] or silicon DB centers [(1), (5), (6), and (7)]. It is well known that silicon DB centers
exhibit a large g anisotropy (| || – ⊥| = 0.004-0.010), while carbon DB centers are characterized by avery small g anisotropy (≤ 0.001) and g values of ~ 2.003. Judging from such trends, we propose thatthe H0/ H1 centers are a type of carbon DB centers.
For H1, we observed an isotropic 1H hfs as large as 5.4 mT. This splitting is similar to the case of(6) (1H hfs = 7.5 mT) where a hydrogen atom is located at a back-bond site of a DB. Only for such acase, a strong isotropic 1H hfs can be observed. Therefore, we propose that the origin of H1 is acarbon DB with a hydrogen atom at a back-bond site.
For H0, we cannot conclude whether this center coupled with any hydrogen atoms. One probablemodel is that H0 is a carbon DB after missing a passivated hydrogen atom (probably, such a hydrogen
atom diffused far away). On the other hand, we can also consider a “vacancy + hydrogen” model likethe cases of (7) and (8). If a DB and a hydrogen atom are separated inside a monovacancy space, 1H
hfs should be smaller than 0.3 mT, and no hfs signatures could be resolved. Such a case is applicableto the H0 signal. Therefore, both “carbon-DB alone” model and “vacancy + hydrogen” model are
CenterH0/ H1 centersin -channel 4 -SiC MOSFET
with post hydrogen annealing

ESR parameters

Remarks|| = 2.004,(tentative)

measured for channel currents,
created by γ-ray irradiation,observable at 20K.
isotropic hfs ( H1).

1⊥= 2.003

H hfs = 5.4 mT ( H1)

Ref. andmethods
present
EDMR(1) Si DB-like centerin oxidized 4 /6 -SiC wafers|| = 2.0028,⊥ ≤ 2.0062

observed in -type wafers
observable at R.T.

[1]
ESR

(2) Carbon DB ( C)in oxidized porous-SiC|| = 2.0023,⊥ = 2.0032

located at SiC-SiO2 interface,observable at R.T.

[2][3]ESR(3) Si-vacancy center (carbon
DB center) in 4 /6 -SiC
gate-control diodes= 2.0027

same as the Si− center in bulk
SiC, measured for diode currents,
observable at R.T.

[4][5]EDMR(4) DB center in -channel
4 -SiC MOSFET withdeposited ONO gate stack(5) Si DBs in Si-SiO2( centers)|| = 2.0026,-axial DB center, measured for
diode currents, observable at R.T.

[6]
EDMR

the well-known Si DB centers,
observable at R.T.

[7]
ESR, EDMR

generated by hydrogen plasma
treatments on SiO2, observable at
R.T.

[7]
ESR

Si vacancy containing a hydrogenatom, showing a weak dioplar
intreaction of 1H (anisotropic).

[9]
ESR

Carbon vacancy containing ahydrogen atom, showing a similar
weak dipolar interaction of 1H.

[10]
ESR

(6) E’ center (Si DB in SiO2)+ hydrogen(7) Si DB + hydrogenin a monovacancy of silicon⊥ = 2.0010|| = 2.0014-2.0015,⊥ = 2.0080-2.0087
1

H hfs = 7.5 mT (H in a
back-bond site)1
H hfs = 1.0 mT (H in a
2nd nearest neighbor site)= 2.001-2.011
1(8) Carbon DB + hydrogenin a monovacancy of diadmondH hfs < 0.3 mT= 2.002-2.003

1

H hfs < 0.3 mT

Table 1. Comparison between the H0/ H1 EDMR signals with other related ESR/EDMR signals. “ ”, “ ||”, and “ ⊥”represent the g value and two principal values of an axial g tensor, respectively.

Materials Science Forum Vols. 679-680

373

possible for the origin of H0.
Finally, other important features of the H0/ H1 EDMR signals are mentioned. One is that both the
H0/ H1 signals appeared after γ-irradiation (see Fig. 1). This result will be reasonable, because one
can naturally expect that the DBs were passivated by hydrogen atoms after hydrogen annealing. The
hydrogen passivation of the DBs is known to be much more stable for carbon (~850 ºC) rather than forsilicon (~450 ºC) [3]. Thus, our hydrogen annealing at 800 ºC will be effective for passivating carbon
DBs. As a result, carbon DB centers were observed after γ-irradiation. In this study, we have notobserved Si DB centers. We speculate that the high-temperature hydrogen annealing may remove
such DBs or that such DB centers may be unobservable owing to their deep energy levels (see thefollowing discussion).
Second important feature is that the H0/ H1 signals were detected in the channel currents (see Fig.
2). This means that the H0/ H1 centers should be located at the SiC-SiO2 interface and/or in thechannel region of SiC. In the channel region, all energetically deep levels should be doubly occupied
by electrons, and should be ESR inactive. Therefore, only energetically shallow defects are detectablein our measurements. Accordingly, the H0/ H1 centers should correspond to shallow energy levels
near the conduction band. They interact with carriers (electrons) in the channel region, as evidenced
by the appearance of their EDMR signals. These aspects are also consistent with the fact that the
H0/ H1 signals were detectable at low temperatures such as 20 K.
Summary
We have performed a low-temperature EDMR (electrically detected ESR) study on the channel
currents of fully-processed -channel lateral 4 -SiC MOSFETs. Our EDMR measurements revealed
the presence of hydrogen-passivated defects in the channel regions of the MOSFETs after posthydrogen annealing. Such defects were a type of carbon DB centers and could be converted into the
H0/ H1 EDMR centers after γ-irradiation. They correspond to energetically shallow levels near the
conduction band, and interact with the channel currents.
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Silicon Carbide and Related Materials 2010
doi:10.4028/www.scientific.net/MSF.679-680
Electrically Detected ESR Study of Interface Defects in 4H-SiC Metal-OxideSemiconductor Field Effect Transistor
doi:10.4028/www.scientific.net/MSF.679-680.370