PHYSICAL REVIEW B 75, 245202 共2007兲

Identification of positively charged carbon antisite-vacancy pairs in 4H-SiC
T. Umeda and J. Ishoya
Graduate School of Library, Information, and Media Studies, University of Tsukuba, Tsukuba 305-8550, Japan

T. Ohshima, N. Morishita, and H. Itoh
Japan Atomic Energy Agency, Takasaki 370-1292, Japan

A. Gali
Department of Atomic Physics, Budapest University of Technology and Economics, Budafoki út 8, H-1111 Budapest, Hungary
共Received 24 January 2007; revised manuscript received 18 April 2007; published 7 June 2007兲
An antisite-vacancy pair and a monovacancy are a set of fundamental stable and/or metastable defects incompound semiconductors. Theory predicted that carbon antisite-vacancy pairs would be much more stable inp-type SiC than silicon vacancies and that they would be a common defect. However, no experimental
evidence has yet supported this prediction. We reexamine electron-irradiated p-type 4H-SiC and identify the
positively charged carbon antisite-vacancy pairs 共CSiVC+兲 by means of electron paramagnetic resonance and abinitio calculations. We compare them with other coexisting defects such as carbon vacancy 共VC兲 and divacancy
共VSiVC兲 and show that CSiVC and VC are very similar in terms of thermal stability and electronic levels.
DOI: 10.1103/PhysRevB.75.245202

PACS number共s兲: 61.72.Ji, 71.55.Ht, 71.15.Mb, 76.30.Mi

I. INTRODUCTION

In the past decade, silicon carbide 共SiC兲 single crystals
have been intensively studied for the development of a novel
wide-gap semiconductor electronics. As-grown, irradiated,and heat-treated crystals show a variety of lattice defects
related to silicon and carbon atoms. In 4H-SiC, which is themost studied SiC single crystal, combined studies of electron
paramagnetic resonance 共EPR兲 spectroscopy and firstprinciples calculation have identified four types of fundamental defects. These have been observed in many samples.
The first type of defect is a silicon vacancy 共VSi兲, which canbe thermally stable in the n-type region.1,2 This vacancy canbe observed in the form of the −1 charge state in n-type3 andsemi-insulating 共SI兲 SiC.4 There coexist two distinguishable
types of such vacancies: the VSi− 共I兲/共II兲 center 关nearly undistorted VSi− at k / h sites, S 共electron spin兲 =3 / 2 共Refs. 5 and6兲兴 and the TV2a center 关VSi− with c-axial distortion, S = 3 / 2
共Ref. 3兲兴. The second type of defect is a carbon vacancy 共VC兲that exhibits the opposite behavior; namely, it becomes increasingly stable in the p-type region.1,2 Therefore, the +1
charge state 共VC+兲 is often observed in p-type7 and SI
materials4,8 共EI5 and EI6 center at k and h sites,7 respectively, S = 1 / 2兲, while in some n-type9 or SI SiC 共Ref. 4兲 the−1 charge state 共VC−兲 is also detectable 共the HEI1 or SI6
center,9,10 S = 1 / 2兲. The third type of defect is a
divacancy11,12 共VSiVC兲 that can be observed in the form ofneutral states 共the P6 and P7 centers, on- and off-axis pairsof VSiVC0, respectively, S = 1兲.12 The divacancy signal appeared strongly in SI 共Refs. 12 and 13兲 or n-type SiC,12,14
which is consistent with the theory11 that it is energeticallyfavored in the n-type region. The fourth type of defect is acarbon antisite-vacancy 共AV兲 pair.14 This pair is a stable
and/or metastable counterpart of VSi.2,15 Recently, this typewas identified in n-type and SI SiC 共the SI5 center, CSiVC−,S = 1 / 2兲.14
Theoretically, all four types can be highly thermally stablein SiC.1,2,11,15,16 However, theory predicted that, alone among
1098-0121/2007/75共24兲/245202共6兲

them, the carbon AV pair is much more stable in the positive
charge state in the p-type region.2,15 Therefore, positively
charged carbon AV pairs should be easily detected in p-type
or SI SiC. Although many studies have been done on thesetypes of SiC, no experimental evidence has yet supported thepresence of the AV defects. Their single positive charge state
共CSiVC+兲 can have an electron spin of 1 / 2 and should be
detectable by EPR.
If the CSiVC+ pairs were as stable as predicted by theory, it
could be important in p-type and SI SiC and could be correlated with important defects. For example, deep-level transient spectroscopy revealed famous thermally stable defects
of Z1/2 共EV + 2.6 eV兲 and EH6/7 共EV + 1.7 eV兲 in 4H-SiC.17 A
recent study18 suggested that both of these types of defect are
related to either a VC-related defect or a single VC. In thismodel, the Z1/2 and EH6/7 levels were correlated with negative and positive charge states of the defects, respectively. If
the negative and positive charge states of CSiVC were both
thermally stable, they might also be candidates for such important defects.
We carefully reexamined electron-irradiated p-type
4H-SiC and identified the missing CSiVC+ pairs by means of
EPR measurements and ab initio supercell calculations. Our
results clearly indicate that the CSiVC defect can be an abundant and important defect in p-type SiC, supporting the predictions of earlier theoretical studies.2,15 We also estimatedthermal stability and electronic levels of CSiVC by comparing
them with other coexisting defects such as VC and VSiVC. We
found that the CSiVC and VC defects are quite similar; both
defects exhibit a similar thermal stability and similar positive
and negative electronic levels.
II. EXPERIMENT+

To create CSiVC defects, we carried out 3 MeV electron
irradiation of commercial boron-doped p-type 4H-SiC substrates 共room-temperature carrier concentration= 1015 cm−3兲

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PHYSICAL REVIEW B 75, 245202 共2007兲

UMEDA et al.

0.05 mT amplitude. Most of the measurements were carriedout at room temperature 共295 K兲, while an Oxford ESR900
liquid-He cryostat system was used for low-temperature
measurements.
There is a simple reason why the CSiVC+ EPR centers
have not yet been observed. Figure 1 shows a series of 150
and 295 K EPR spectra measured at different microwave
powers. In both temperatures, when the microwave power
was reduced, instead of the well-known EI5 / 6 EPR signal
from positively charged carbon vacancies 共VC+兲,7 other EPR
signals became clear. We named them HEI9a / b and
HEI10a / b.19 As the temperature decreased from
295 to 150 K, the microwave saturation effect worked more
dramatically for the HEI9 / 10 signals. Eventually, these signals became negligibly small at the lower temperatures 共for
example, 4 – 80 K兲 used in previous studies.4,7–9,12–14 Therefore, we studied the HEI9 / 10 EPR centers at room temperature and at the lowest microwave power 共0.2 ␮W兲. Because
these conditions greatly reduced EPR signal intensities, we
had to use the signal-enhancement techniques noted above.
In Fig. 2, the four EPR signals were measured without thesaturation effect. Each signal originates from an electron spinof 1 / 2. Their angular dependences, which are plotted in Fig.
3共a兲, reveal a C3v symmetry for the two HEI9 centers andC1h symmetry for the two HEI10 centers. Their g tensors
show similar principal values but their symmetric axes 共g储axes兲, which are summarized in Table I, are different. By
analogy with the P6 / P7 EPR centers in 4H-SiC 共neutral
divacancies, VSiVC0兲,12 we assume that the HEI9 / 10 centersbelong to the same defect but have different orientations of
CSiVC. Namely, the C3v centers 共HEI9兲 may correspond to
on-axis CSiVC pairs 共parallel to the c axis兲, while the C1hcenters 共HEI10兲 may be off-axis pairs 共parallel to a basal
Si-C bond兲. In addition to this variation, the 4H-SiC crystal
has two inequivalent lattice sites, called the cubic 共k兲 andhexagonal 共h兲 sites. In total, there are four different configurations and/or EPR signals for CSiVC.
The identification of the CSiVC pairs is based on 13C hyperfine 共HF兲 interactions caused by their CSi atoms. The
magnified 共⫻10 and ⫻20兲 spectra in Fig. 2 reveal very weak
HF doublets created by 13C 共nuclear spin I = 1 / 2, naturalabundance= 1.1%兲. The signal intensity ratios of each HF

Normalized EPR signal intensity (arb.units)

EI5/6
1 mW
0.1 mW

0.001 mW
0.01 mW

150K

HEI9b
HEI9a/10a

HEI10b1 mW

295K

0.1 mW
0.01 mW
0.001 mW
0.0002 mW
335

336

337

338

339

Magnetic field (mT)FIG. 1. 共Color online兲 Microwave saturation effect of EPR centers in electron-irradiated p-type 4H-SiC measured at 150 and
295 K. EPR spectra were normalized by a factor of
共microwave power兲1/2. In these spectra, well-known EI5 / 6 signals
共overlapped兲 and four EPR signals 共HEI9a / 9b / 10a / 10b兲 were detected. Latter signals exhibit a much stronger saturation effect thanformer signals.

at temperatures from 800 to 850 ° C. With the same irradiation of n-type 4H-SiC, we obtained a high concentration of
CSiVC defects 共CSiVC−兲.14 Therefore, we expected a similar
situation in the p-type samples. To obtain a sufficient signalto-noise ratio, we used 1.5-mm-thick substrates and enhanced the defect concentration by high electron doses of 2
⫻ 1018, 4 ⫻ 1018, and 10⫻ 1018 e / cm2. After the irradiation,
the samples became virtually semi-insulating. This had theadvantage of minimizing a dielectric loss of microwaves due
to free carriers in a substrate.
The EPR spectra were measured in the dark 共thermal
equilibrium兲 by a Bruker Bio-Spin E500 X-band spectrometer with a high-Q-factor 共⬎10 000兲 microwave cavity. Microwave frequency was set to 9.452 GHz, and EPR signals
were detected by a 100 kHz magnetic-field modulation with

EPR signal (arb.units)

10b
295K
dark
B // c
13

C

9b

29

29

Si

13

13

x10

333

334

10b 10a

335

13

C
x10

9b

x20

332

Si
13

C

C

9a

9a/10a

336

337

339

13

C

9b

9a

10b 10a

338

C

x20

340

341

342

Magnetic field (mT)FIG. 2. 共Color online兲 EPR spectra of p-type 4H-SiC after 1 ⫻ 1019 e / cm2 irradiation measured at 295 K using a microwave power of
0.2 ␮W. At this power, HEI9 / 10 EPR signals 共depicted by 9a, 9b, 10a, and 10b, respectively兲 were observed without microwave saturation
effect.
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PHYSICAL REVIEW B 75, 245202 共2007兲

IDENTIFICATION OF POSITIVELY CHARGED CARBON…

(a) main lines(b) 13C hyperfine lines
9b

342

338.5

10a

9a

341

10b

340

338.0

29

Magnetic field (mT)

339
9b

337.5

Si

10a
338
337

337.0
10b

9a

336
335

336.5
334
9a

10b

333

336.0

9b
0

10a

90
0
Angle (degrees)

90

FIG. 3. 共Color online兲 Angular maps for HEI9 / 10 centers. Magnetic field was rotated from 关0001兴 共0°兲 to 关1̄100兴 共90°兲. Solid lines
are simulated results using spin-Hamiltonian parameters shown in
Table I.

doublet to each main line are 5.3/ 480 共1.1%兲 for HEI9a,
2.5/ 158 共1.6%兲 for HEI9b, 8.9/ 1020 共0.9%兲 for HEI10a,
and 7.4/ 781 共0.9%兲 for HEI10b, where we separated theoverlapped main lines of HEI9a and HEI10a using a different spectrum, which was measured for a 90° angle. These

ratios indicate that every HEI9 / 10 center is localized mainly
on a single carbon atom. The next largest HF interactions arecaused by 29Si 共I = 1 / 2, natural abundance= 4.7%兲, which
generates a number of much stronger HF satellites 共see Fig.
2兲 with splitting widths between 1.7 and 2.9 mT 关see Fig.
3共b兲兴. Unfortunately, due to the complicated overlapping, it
was difficult to accurately analyze them. Therefore, we focused on the 13C HF interactions.
The angular dependences of the observed 13C HF interactions could be excellently fitted, as shown in Fig. 3共b兲, using
the spin-Hamiltonian parameters 共the g and HF tensors兲 inTable I. The 13C HF tensors are axially symmetric, so they
can be well described by contributions of carbon 2s and 2p
orbitals. By comparing the standard HF constants for 13C
共A2s = 134.77 mT, A2p = 3.83 mT兲,20 we estimated the carbon
2s and 2p orbital fractions in each defect wave function to be
3% and 52% for HEI9a, 4% and 54% for HEI9b, 4% and
53% for HEI10a, and 3% and 53% for HEI10b. The wave
functions are quite similar in each signal, just forming carbon dangling bonds 共DBs兲. The DB directions 共the ␪ anglesof the 13C HF tensors兲 are parallel to the c axis for HEI9a / b
共␪ = 0 ° 兲 and parallel to a basal bond for HEI10a / b 共␪
= 109° 兲. These carbon DBs can be reasonably attributed to
the CSi atoms of on-axis/off-axis carbon AV pairs shown in
Figs. 4共a兲 and 4共b兲, respectively. Since the samples were of p
type and the observed electron spin was 1 / 2, it is reasonable
to assign a single positive charge state 共CSiVC+兲 to the relevant centers. More complete identification of CSiVC+ isgiven by means of ab inito supercell calculations described
below, where we can distinguish the four configurations of
CSiVC 关共hh兲, 共kk兲, 共hk兲, and 共kh兲; see Fig. 4兴 from the four
EPR centers.
III. THEORETICAL IDENTIFICATION

Ab initio calculations based on density-functional theory
in the local-density approximation21 with the Ceperley-Alder

TABLE I. Spin-Hamiltonian parameters of four HEI9 / 10 centers and four CSiVC+ pairs in 4H-SiC. Spin Hamiltonian is expressed as
H = ␮BS · g · B + ⌺i S · A共i兲 · I共i兲 − gN␮NI共i兲 · B as defined in Ref. 9, where g is a g tensor and A共i兲 is a hyperfine 共HF兲 tensor for a nuclear spini. ␪ expresses angle between main 共储兲 principal axis and c axis. Measured and calculated HF constants are expressed in mT. Associated Si
atoms found by theory are defined in Fig. 4. For 共kh兲 and 共hk兲 configurations, three additional detectable 29Si HF interaction lines were foundnear CSi, which is not labeled here or in Fig. 4.
A 共 13CSi兲

gCenterSymmetry

EPR measured at 295 K
HEI9a
C 3v
HEI9b
C 3v
HEI10aC1h
HEI10bC1h

A 共 29Si兲

gxx

gyy

gzz 共g储兲

␪

Axx

Ayy

Azz 共A储兲␪

Axx,yy,zz

2.00408
2.00379
2.003482.003992.00408
2.00379
2.00258
2.00345

2.002272.00195
2.00226
2.00263

0°
0°
145°
116°

2.27
3.71
2.60
2.31

2.27
3.71
2.65
2.45

8.25
9.95
8.75
8.44

0°
0°
109°
109°

1.7–2.9
1.7–2.9
1.7–2.9
1.7–2.9

1.96
3.12
1.69
1.65

1.96
3.12
1.70
1.67

7.01
8.59
6.14
6.00

0°
0°
110°
108°

2.1–2.9 共Si4–6兲
2.1–2.9 共Si4–6兲
2.5–3.4 共Si1兲
2.1–2.7 共Si2,3兲

Theory for CSiVC+ in 4H-SiC
共hh兲
C 3v
共kk兲
C 3v
共kh兲
C1h
共hk兲
C1h

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PHYSICAL REVIEW B 75, 245202 共2007兲

UMEDA et al.
g||, A||
0°

k h
k
k

(hh) h
9a h

k
k

h

(kh)
10a

hk
Si1

k
k
Si4-6

(kk)
9b

A||
109°

h
h

g||
145°

h k

c axis
kh

Si2,3
g||
115°

kh

(hk)
10b

h k

Si4-6

(a) on axis(b) off axis

FIG. 4. 共Color online兲 Atomic models for four CSiVC pairs in
4H-SiC. This figure shows a lattice structure in 共112̄0兲 plane of
crystal. Four HEI9 / 10 centers correspond to four configurations of
CSiVC+. Angles between c axis and main 共储兲 principal axes of g and
HF tensors are noted. Carbon 共silicon兲 atoms are drawn as small
共large兲 balls, and their dangling bonds are drawn as small ovals.
Open balls are carbon vacancies.

exchange correlation22 were carried out in the framework of
the SIESTA 共Ref. 23兲 and CPPAW codes.24 A 256-atom hexagonal supercell of 4H-SiC was used with 23 Monkhorst-Pack
K-point set25 to model the defects. First, the geometry was
optimized by SIESTA code 共see Ref. 26 for details兲, then thegeometry was reoptimized, and, finally, the HF tensor was
calculated by the all-electron CPPAW code 共see Ref. 7 fordetails兲. All four possible configurations were considered inour study. The on-axis 共hh兲 and 共kk兲 configurations have C3v
symmetry, while the 共hk兲 and 共kh兲 configurations have C1hsymmetry. In the optimized structures, the on-axis configurations will produce one a1 defect level at about EV 共valenceband edge兲 +1.3 eV and a double-degenerate e level in theupper part of the band gap, while the off-axis configurations
with lower symmetry have one a⬘ level at about EV+ 1.2 eV and two close levels 共a⬘ and a⬙兲 in the upper part ofthe band gap. As was explained for the on-axis configurations in our previous study,14 the deep a1 defect level ismostly localized on the carbon antisite 共CSi兲 while the e levelis localized on the carbon vacancy part of the complex. In
CSiVC+, the a1 level of 共hh兲 / 共kk兲 configurations 关and thelower a⬘ level of 共hk兲 / 共kh兲 configurations兴 is occupied by
one electron, while the e level 共or a⬘ and a⬙ levels兲 are
empty.
The on-axis configurations have an a1 character, conserving the C3v symmetry after geometry optimization, which isconsistent with the experimental results. In the relaxed structures, there is a subtle difference between the geometry of
the 共hh兲 and 共kk兲 configurations: in the 共hh兲 configuration,
the CSi atom is relaxed more strongly backward than in the
共kk兲 configuration. This results in weaker 13C HF interaction
in 共hh兲 than in 共kk兲 configuration, which can account for theobserved difference between HEI9a / b. In fact, the calculated
HF tensors are in good agreement with those of HEI9a / btaking the possible error in the local-density approximation

共LDA兲 functional and the supercell size into account 共Table
I兲. Therefore, we identify HEI9a as the 共hh兲 configurationand HEI9b as the 共kk兲 one, as shown in Fig. 4共a兲. The localization of the spin density in the carbon vacancy is negligible
in this case. Because of the strong localization of the spin
density on the CSi atom, the localization of the spin density isdetectable on the second-neighbor Si atoms of the CSi atomalong the c axis, depicted as Si4–6 in Fig. 4共a兲.
For the off-axis configurations, 13C HF interactions were
only slightly different between the HEI10a / b centers or the
calculated 共kh兲 and 共hk兲 configurations 共Table I兲. However,
the two a⬘ levels can be slightly mixed in the off-axis configurations. As a result, the spin density also distributes oneither the Si1 or Si2,3 atoms of VC, as illustrated in Fig. 4共b兲.
The calculations also predict that three second-neighbor Si
atoms of the CSi atom have HF constants similar to those of
Si1–3. A total of four or five 29Si HF interactions will bemeasurable, which can account for a number of the observed
29
Si HF lines in Fig. 2. For the 共kh兲 center, we speculate thatthe spin density on Si1 can turn the g储 principal axis toward
the direction of the Si1 atom. Therefore, we assign HEI10a
共␪ = 145° for g储兲 to the 共kh兲 configuration and HEI10b 共␪
= 115° 兲 to the 共hk兲 configuration. For these cases, the agreement between the calculated and measured HF tensors of CSi
atom is not as good as for the on-axis case, but we still
believe it to be reasonable. We think that the orbital mixingof the CSi atom and the silicon DBs may cause larger error in
the LDA calculation than in the on-axis configurations,where this orbital mixing is absent.
When the Fermi level EF is positioned above EV + 1.2⬃ 1.3 eV as the a1 or a⬘ level of CSiVC being singly occupied, the divacancy may be negatively charged and also
cause a spin-1 / 2 EPR center. Note that its 共0兩–兲 level was
predicted to be about EV + 1.4 eV.12 We have also checked
this possibility. The HF parameters of a negatively charged
divacancy were already calculated by Gerstmann et al.16
They reported that, as with CSiVC+, VSiVC− 共in 3C-SiC兲 generates the single largest 13C HF interaction. However, thecalculated 13C HF constants of A储共zz兲 = 155 MHz 共5.53 mT兲
and A⬜共xx,yy兲 = 63 MHz 共2.24 mT兲 do not fit our experimental
values 共see Table I兲. In particular, the 2p orbital density
关=共A储 − A⬜兲 / 3 / A2p兴 共Ref. 20兲 on the main 13C atom is estimated to be only 29% for VSiVC− as against 52%–54%
共38%–48%兲 in our experiments 共calculation兲. Therefore, we
can exclude the VSiVC− model as an explanation of the origin
of HEI9 / 10.
IV. THERMAL STABILITY AND ELECTRONIC LEVEL OF
CARBON AV PAIRSFirst, we compared the thermal stability of the carbon AV
pairs with that of other coexisting defects in irradiated SiC.
In thermal equilibrium, all irradiated p-type samples showeddominant EPR signals of CSiVC+ 共HEI9 / 10兲 and VC+
共EI5 / 6兲. As is plotted in Fig. 5共a兲, their concentrations werequite comparable; e.g., after 4 ⫻ 1018 e / cm2 irradiation, 3.2⫻ 1017 and 2.5⫻ 1017 cm−3 for CSiVC+ and VC+, respectively.
This implies that the formation energy or kinetics is similar

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PHYSICAL REVIEW B 75, 245202 共2007兲

IDENTIFICATION OF POSITIVELY CHARGED CARBON…

(a)3

Concentration (10 /cm )

5

(b)

4

17

HEI9/10 (CSiVC)

3
2
1

EI5/6 (VC)

boron
acceptor

P6/7 (VSiVC)

unknown0
0 2 4 6 8 1018

2Dose (10 e/cm )

800

1000

1200
o

Anneal Temperature ( C)

FIG. 5. 共Color online兲 Concentrations of HEI9 / 10 centers andother coexisting defects in electron-irradiated p-type 4H-SiC: 共a兲electron irradiation performed at around 800 ° C, and 共b兲 a sample
after 4 ⫻ 1018 e / cm2 irradiation was subjected to isochronal annealing. HEI9 / 10 共CSiVC+兲, EI5 / 6 共VC+兲, and unknown signals wereobserved at room temperature, whereas neutral boron acceptor 共Bk0兲
and P6 / 7 共VSiVC0兲 signals were measured at 30 K. Concentrations
were estimated in thermal equilibrium 共in dark兲, except for P6 / 7
center where its concentration was nominally estimated under
100 W halogen lamp illumination.

for both defects. The VSiVC0 共P6 / P7兲 signal, on the other
hand, was absent in thermal equilibrium but weakly observed
under illumination at low temperatures 共nominally 0.1⫻ 1017 cm−3兲.
Next, an irradiated sample 共electron dose= 4⫻ 1018 e / cm2兲 was subjected to isochronal annealing in Ar
ambient for 30 min. The result is shown in Fig. 5共b兲. The
CSiVC+ and VC+ defects exhibited a very similar behavior;
they were almost unchanged after 1000 ° C annealing, but
95% of them were annealed out at 1100 ° C. The VSiVC0
signal also disappeared at this stage. We simultaneously observed the recovery of boron acceptors and the generation of
weak unknown signals. Note that the above annealed-out
temperature for CSiVC, VC, and VSiVC was lower than thatobserved
for
other
types
of
samples
共1200– 1500 ° C兲.4,12–14,17 This can be ascribed to a different
EF position and different abundances of defects among thesamples. In any case, we infer that CSiVC+ pairs have a thermal stability comparable to that of VC+ or VSiVC0.
The finding that CSiVC+ and VC+ coexisted but VSiVC0 did
not appear in thermal equilibrium means that electronic levels of CSiVC+ and VC+ are located close to each other, and at
this energy position, VSiVC defects should not be neutral. We
speculate that VSiVC defects are most probably negatively

1 A. Zywietz, J. Furthmüller, and F. Bechstedt, Phys. Rev. B

59,
15166 共1999兲.
2 M. Bockstedte, A. Mattausch, and O. Pankratov, Phys. Rev. B
68, 205201 共2003兲.
3
N. Mizuochi, S. Yamasaki, H. Takizawa, N. Morishita, T. Ohshima, H. Itoh, T. Umeda, and J. Isoya, Phys. Rev. B 72, 235208

charged, as discussed earlier, and tentatively speculate that
the EF position higher than EV + 1.4 eV is required for thiscase.12 On the other hand, for VC, two photoinduced EPR
data were reported for p- and n-type 4H-SiC.9,27 In bothtypes, a rapid increase in the EI5 EPR signal 共VC+兲 was detected with approximately 1.8 eV optical excitation. This increase was caused by the conversion of VC− or VC0 into VC+,
which was in turn caused by excitation of electron共s兲 from
VC to the conduction band.9,27 Therefore, VC+ defect levels
must be generated in EC − 1.8 eV 共EV + 1.5 eV兲 and lower. On
the basis of these considerations, we speculate that CSiVC+
defect levels are distributed at around EV + 1.4⬃ 1.5 eV,
which is a range similar to that for VC+. This speculation
does not conflict with the calculated a1 or a⬘ levels for
CSiVC+ 共EV + 1.2⬃ 1.3 eV兲.
V. SUMMARY

We have identified the missing positively charged carbon
AV pairs 共CSiVC+兲 in electron-irradiated p-type 4H-SiC bymeans of EPR and ab initio supercell calculation. Theory
predicted that such pairs should be thermally stable in thep-type region.2,14,15 The reason they have not been observedin previous studies was simply that a strong microwave saturation effect hindered their EPR signals under optimum conditions for conventional EPR experiments. We have found
four EPR centers, HEI9a / b 共S = 1 / 2, C3v symmetry兲 and
HEI10a / b 共S = 1 / 2, C1h symmetry兲, which have been successfully assigned to CSiVC+共hh兲 / 共kk兲 and CSiVC+共kh兲 / 共hk兲,
respectively, on the basis of EPR and theoretical analyses of
their 13C hyperfine interactions. Their wave functions werestrongly localized on each CSi atom, originating from the a1
or a⬘ character of their paramagnetic electronic levels 共calculated at about EV + 1.2⬃ 1.3 eV兲. An isochronal annealing
study suggested that the thermal stability of CSiVC+ is as highas that of VC+. It was also suggested that electronic levels ofboth CSiVC+ and VC+ are located at around EV + 1.4⬃ 1.5 eV, whereas VSiVC should not be neutral in this range.
Furthermore, in previous works,9,14 electronic levels of
CSiVC− and VC− were found in the same range of EC− 1.1 eV and deeper. On the whole, the CSiVC defects arequite similar to the VC defects, meaning they can coexist in
SiC.
ACKNOWLEDGMENT

Support of Grant No. NIIF-1090 from the National Information Infrastructure Development Program in Hungary isappreciated.

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