PHYSICAL REVIEW LETTERSPRL 96, 145501 (2006)

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Identification of the Carbon Antisite-Vacancy Pair in 4H-SiC
T. Umeda,1 N. T. Son,2 J. Isoya,1 E. Janzén,2 T. Ohshima,3 N. Morishita,3 H. Itoh,3 A. Gali,4 and M. Bockstedte5,6
1

University of Tsukuba, Tsukuba 305-8550, Japan
Linköping University, S-581 83 Linköping, Sweden
3
Japan Atomic Energy Agency, Takasaki 370-1292, Japan
4
Budapest University of Technology and Economics, Budafoki út 8., H-1111, Budapest, Hungary
5
Universität Erlangen-Nürnberg, Staudtstrasse 7/B2 D-91058 Erlangen, Germany
6
Universidad del Paı́s Vasco, E-20018 San Sebastián, Spain(Received 24 August 2005; published 10 April 2006)
2

The metastability of vacancies was theoretically predicted for several compound semiconductors
alongside their transformation into the antisite-vacancy pair counterpart; however, no experiment todate has unambiguously confirmed the existence of antisite-vacancy pairs. Using electron paramagnetic
resonance and first principles calculations we identify the SI5 center as the carbon antisite-vacancy pair in
the negative charge state (CSi VC ) in 4H-SiC. We suggest that this defect is a strong carrier-compensating
center in n-type or high-purity semi-insulating SiC.
DOI: 10.1103/PhysRevLett.96.145501

PACS numbers: 61.72.Ji, 71.15.m, 76.30.Mi

Vacancies are one of the most simple and fundamental
point defects in crystals. However, in a compound material,
the vacancies are not simple defects, as is generally expected. For instance, in an AB compound material, during
the diffusion of the A vacancy one of its nearest neighbors,a B atom, can move into the vacant lattice site forming apair of a B antisite and a B vacancy. The antisite-vacancy(AV) pairs are the counterpart of the isolated vacancies incompound materials, and can be energetically stable ormetastable defects with respect to the vacancies. AV complexes in III-V semiconductors have already been studied.
Theoretical studies [1–3] predicted that the arsenic AV
complex is more stable than the Ga vacancy in p-type
GaAs. A recent paper [4], however, has shown that noneof the detected electron paramagnetic resonance (EPR)
centers can be associated with the AV complex in GaAs.
Also, theoretical calculations [5] have indicated that thenitrogen AV complex can appear in p-type GaN but noexperimental evidence has supported this assertion so far.
In these materials, the stability order of the cation vacancies and their anion AV counterparts depends on the Fermi
level (EF ): in p-type material, the anion AV complex ismore stable than the cation vacancy, whereas in n-typematerial the cation vacancy is more stable. For a certain EF
position both configurations are equally stable. Mutualtransformations of AV complex and the cation vacancy
can then be induced, e.g., when irradiation induced compensation centers anneal out and change the EF position. In
this sense, the cation vacancy and the anion AV counterparts form a bistable defect pair. In silicon carbide (SiC),
carbon is more electronegative than silicon;, i.e., the Si
vacancy is the cation vacancy. As for the III-V semiconductors, a similar bistability was theoretically predicted
between the carbon AV complex (CSi VC ) and the isolated
Si vacancy depending on EF [6,7]: in n-type SiC, the lattervacancy is more stable, whereas in p-type, semi-insulating(SI) materials, the former AV pair is more stable. The
0031-9007=06=96(14)=145501(4)$23.00

P6=P7 EPR centers were associated with the excited triplet
states of CSi VC2 based on experiments and calculations for
SiC [8,9]. Recent EPR measurements have shown, however, that the P6=P7 centers originate from the triplet
ground states of the neutral divacancy [10]. To the bestof our knowledge, no experimental evidence unambiguously shows the existence of an AV complex as a fundamental defect in a compound semiconductor.
Group III-V semiconductors are widely used in optoelectronic devices. SiC is a promising wide-gap semiconductor for high-power, high-frequency, and hightemperature electronics. The SiC research is rapidly expanding thanks to the availability of high-quality SiC
single crystals [11]. Recently, high-purity semi-insulating(HPSI) SiC grown by physical vapor transport [12] or hightemperature chemical vapor deposition [13] has become
available. As for semi-insulating GaAs [14] or p-type GaN[15], deep electronic levels of native defects are believed tobe responsible for the SI properties of HPSI SiC materials.
Yet, in contrast to the situation in GaAs or GaN, where
either the EL2 center (arsenic antisite) or vacancy-dopant
complexes facilitate the SI properties [1,14], the responsible native defects in HPSI SiC have yet to be identified[16 –18]. Hence, it is an important task to identify thedefects that could account for the SI properties. The SI5
EPR center was first detected in some n-type and HPSI
4H-SiC substrates in a series of other EPR centers, labeled
SI1-SI9 [16]. In this Letter, we identify the SI5 EPR centeras the negatively charged carbon AV complex (CSi VC ) in
4H-SiC by EPR and first principles theory. We also showthat this complex is an important compensating center in
HPSI SiC samples with acceptor levels at around 1.1 eV
below the conduction band edge.
The SI5 EPR center with C3v symmetry was observed in
as-grown 4H-SiC substrates at 77 K [16]. Its concentrationin these HPSI samples was insufficient to resolve all thehyperfine (HF) lines in detail. Therefore, we enhanced the145501-1© 2006 The American Physical Societyconcentration of SI5 centers in n-type SiC samples byelectron irradiation of 3 MeV [19]. Our starting substrates
were commercial n-type 4H-SiC (N concentration
1017 cm3 ). EPR spectra were measured by Bruker
X-band EPR or pulsed electron nuclear double resonance(ENDOR) spectrometers, with or without illumination by a
100 W halogen lamp with a filter (transparent above
0.6 eV) or by a 150 W xenon lamp with a monochromator.
The uncertainty of the photon energy induced by the wideopened slits of the monochromator is about 0.06 eV [20].
We first report a characteristic temperature dependencyof the SI5 EPR signal: at low temperatures [LT spectrum at
30 K, Fig. 1(a)], the signal has C1h (low) symmetry while athigh temperatures [HT spectrum at 100 K, Fig. 1(b)], it
shows C3v (higher) symmetry. The transformation between
the LT and HT spectra occurs at 50 K. This behavior of
the SI5 center was also found in the as-grown HPSI
samples in Refs. [10,16]. Similar transitions were also
found for HEI1 (VC ) [20] and EI5 (VC ) [21] EPR centers.
By analogy, we conclude that the HT (C3v ) spectrum of theSI5 center corresponds to a thermal average of the threesymmetrically equivalent C1h configurations. In the LT
spectrum [Fig. 1(a)], one strong HF splitting (HF1) and
four weak HF splittings (HF2-5) due to nuclear spins I 
1=2 were observed. To identify the sources of HF2-5, we
performed pulsed-ENDOR measurements for the magnetic
field B k c 0001 with the Mims sequence [21]. The
ENDOR signals were observed at 37.5 MHz for 13 C (I 
1=2, natural abundance 1.1%), 26.1 MHz for 29 Si (I  1=2,4.7%), 16.1 MHz for 13 C, and 11.9 MHz for 29 Si, which
agreed with the observed HF splittings of HF2-5 (37.2,
26.3, 15.4, and 10.4 MHz, respectively). For HF1, its
intensity ratio to the central line (0:09=1) suggests that
HF1 originates from two 29 Si nuclei (29 Si 2). The
wave function of the LT SI5 center is strongly localizedon these two Si atoms. In the HT configuration [Fig. 1(b)],four HF structures (HF10 -40 ) with I  1=2 were detected.
Only the high-intensity spectrum of Fig. 1 enabled us todetect the HF lines of HF10 and HF30 . Led by the intensity
ratios of HF10 and HF30 to the central line (0:050:1 and
0:014:1, respectively), we assigned HF10 and HF30 to theinteraction with one 29 Si and one 13 C nucleus, respectively.
We also correct the previous assignment of HF20 (13 C 3)[16]: 13 C 1 or 13 C 2 are more likely, as its intensity
ratio to the central line is estimated to be 0:018:1. The
model of the negatively charged divacancy for this center
proposed in Refs. [9,16] is obsolete as it only accounts for
the HF20 and HF40 lines detected in the HPSI SiC samples
and not for the new additional HF10 and HF30 lines. The
divacancy model further predicted that the largest HF
splitting would originate from one 13 C atom at low temperatures [9] and not from two 29 Si atoms (HF1) now
detected. Hence, we need a new model for the SI5 center.
Table I summarizes the spin-Hamiltonian parameters forboth SI5-LT and SI5-HT configurations, which were determined by the angular-pattern simulations shown in
Figs. 2(a) and 2(b). The HF parameters of the SI5-LT
configuration and temperature dependence of the SI5 center are very similar to those of the EPR center HEI1 (VC )[20]. Our working model thus became the axial configuration of the negatively charged carbon AV complex(CSi VC ). The theoretical analysis below demonstrates this.
We carried out first principles calculations on the CSi VC
defect and its HF interaction based on the density functional theory and the local spin density approximations(LSDAs). The complexes were represented by large supercells including up to 288 atoms. Firstly the atomic structure
was obtained using the pseudopotential method as outlined

343

341

HF1

342
338

EI6
EI5

HF5

332

HF3' HF4'[1120]y

EPR Signal (arb.units)

FIG. 1 (color online). SI5 EPR spectra in irradiated 4H-SiC(dose: 2 1018 e=cm2 ) measured at (a) 30 K and (b) 100 K for
B k c, 9.452 GHz, under illumination by a 100 W halogen lamp.
Other weaker EPR signals of EI5=6, HEI1, HEI5=6, and N
donors are also indicated.

336

HF2

HF2'

334

HF1

331

342

HEI1

SI5

333

HF1'

HEI1

(x) 900 (z)Angle (degrees)(c) CSiVC -LT (C1h)

Si1,2
(HF1)
340

HEI6
HEI5

HF3

111°

x10

336
338
Magnetic Field (mT)

HF2'

HF3

z
[0001]

334

HEI1

HF1'

340
338

HF2

336

(a) 30K

332(b) SI5-HT(C3v), 100K(a) SI5-LT (C1h), 30K

0 (z)(b) 100K

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PHYSICAL REVIEW LETTERS

Magnetic Field (mT)

PRL 96, 145501 (2006)

VC(y) 90(d) CSiVC -HT (C1h)

CI
(HF2)

109°
unpairedelectronorbitalVC

paired electron
orbitalCSi

CII(HF2’)

Si3(HF1’)[1100]xCSi

FIG. 2 (color online). Angular maps of the SI5 center in the(a) LT and (b) HT configurations, measured for B rotating in and (b) 1100
(a) 1120
planes. The solid lines were calculatedusing the parameters in Table I and Refs. [20] (HEI1) and [21](EI5=6). Respective atomic models are shown in (c) and (d).

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PHYSICAL REVIEW LETTERSPRL 96, 145501 (2006)

TABLE I. Spin-Hamiltonian parameters of the SI5 center andtheoretical HF tensors for CSi VC in the LT and HT configurations. The spin-Hamiltonian is defined in Ref. [20], and g is a gtensor and Ai is a HF tensor for an atom i. Principal values of
A are given in mT (1 mT  28:02 MHz). Calculated values
refer to the cubic and (in parentheses) hexagonal complex.
The main ( k ) principal axes are expressed in the conventionalpolar angles of  and  of the coordinate system in Fig. 2. Labels
and notation are given in the text and in Fig. 2.
Z (k);  ( )EPR
SI5-LT (C1h ) 30 Kg2.003 72 2.002 59 2.005 34
10.04
12.99
HF1, A29 Si 2 10.16
1.3
1.3
1.8
HF2, A13 C 2
0.9
1.1
HF3, A29 Si 2 0.9

10; 180111; 51

X (?)

Y (?)

SI5-HT (C3v ) 100 Kg2.003 39 2.003 39 2.004 84
0
6.38
6.38
8.02
109
HF10 ,A29 Si 1
1.77
2.22
0
HF20 , A13 C 1a 1.77
TheoryCSi VC -LT (C1h )
10.8
10.9
13.8
108; 52
ASi1;2 (11.2)(11.3)(14.2)
(108)
0.1
0.1
0.1
ACSi 
(0.2)
(0.2)
(0.2)
1.3 (1.4) 1.3 (1.4) 1.7 (1.7)
ACI 2
CSi VC -HT (C1h )
7.2 (7.5) 7.6 (7.9) 8.6 (8.9)
ASi1-3  avg.
0.9 (1.3) 0.9 (1.3) 1.3 (1.7)
ACII a
We used 13 C 1 as the origin of HF20 , as the calculation
predicted this case rather than 13 C 2.

in Ref. [7] and then the HF tensors were calculated with the
all-electron projector augmentation wave method [20,21].
Calculations were performed with and without a scissorsoperator (cf. Ref. [7] and references therein) to assess theeffect of the LSDA band gap failure on the calculated HF
tensors. The values obtained with and without the correction agree to within their accuracy. In 4H-SiC, there are
two inequivalent substitutional sites, called cubic (k) andhexagonal (h) lattice sites. In the cubic (hexagonal) AV
complex aligned with the c axis (axial complexes), thevacancy and antisite occupy neighboring kh sites. The
calculated HF tensors of these two AV complexes are
summarized in Table I. The negatively charged axial
CSi VC complexes undergo a Jahn-Teller distortion, due tothe occupation of the degenerate e level by one electron
within the band gap. As shown in Fig. 2, two distinct
configurations were obtained with the major part of thespin density being located either at Si1;2 [LT configuration,
Fig. 2(c)] or at Si3 [HT configuration, Fig. 2(d)]. Bothconfigurations exhibit C1h symmetry consistent with theexperiment. The LT configuration was found to be energetically favored (by 0.02 eV) over the HT configuration,verifying the low-temperature HF signature. Moreover, theweek ending
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calculated HF tensors of Si1;2 for the k and h sites are ingood agreement with the experimental ones (HF1 inTable I) regarding both the principal values and axes.
The next largest HF interactions were found on the firstC neighbors of Si3 [CI in Fig. 2(c)], which also agree withthe experimental values for HF2 (Table I). Additional
measurable HF interactions were found on the first neighbor C and second neighbor Si atoms of Si1;2 atoms, whichmight account for the not fully resolved lines HF3-5 of the
EPR spectrum. According to our calculations, the other HF
interactions are weaker, in particular, for CSi (see Table I).
Its dangling bond mainly contributes to the doubly occupied localized a level below the paramagnetic e level. For
the HT configuration, the calculations predicted that an
unpaired electron is localized on one Si atom (Si3 ), similar
to the HT spectrum. Again, the calculated HF tensor of CSi
is negligible. The HF20 interaction observed for SI5-HT
most likely arises from the CII ligand of Si3 [see Fig. 2(d)].
The C3v symmetry should originate from a motional average over the three different orientations of the HT configuration. We simulated this average by taking the spatial
average of the main HF tensors of symmetrically equivalent atoms (Si1-3 ). The calculated principal values of theaverage Si tensor are shown in Table I as ‘‘ASi1-3 avg.’’
Within the limits of this approximation, these values reproduce well the experimental observation at 100 K. For
both the LT and HT configurations, the difference between
the calculated HF tensors of the cubic and hexagonal
complexes is below the expected accuracy. Qualitatively,
the two complexes are distinguishable by the HF tensors of
the third and fourth neighbor shells. These could not be
resolved experimentally, and it is likely that both complexes contributed to the SI5 spectrum. The above experimental and theoretical arguments consistently explain boththe LT and HT spectra, and we therefore identify the SI5
center as the axial CSi VC complex.
In the present study we observe only the axial complex,
but the basal complex should also exist, like for VSi VC(P6=P7 centers) [8,10]. This phenomenon occurs for the
EPR centers which are sensitive to light illumination. For
example, it was reported that in some irradiated n-type SiC
samples, the P6 (c-axial complex) signal was present but
the P7 (basal complex) signal was completely absent [22].
As for HPSI SiC [16], the SI5 signal in the irradiatedsamples was strongly enhanced by illumination. Our
photo-EPR measurements on an irradiated sample (dose:
4 1018 e=cm2 ) suggested that its EF is located at about
1.1 eV below the conduction band edge (EC ) [20]. In thesame sample, we observed a significant photo enhancement of the SI5 signal just above 1:1 0:06 eV. Thus, themajority of the CSi VC complexes exist in a nonparamagnetic doubly negatively charged state (CSi VC2 ). This isconsistent with the previous photo-EPR data on HPSIand n-type SiC [16] that indicated a dominance of theSI5 level (singly negatively charged level) in the range
from EC  1:2 eV to EC  1:5 eV. First principles calcu-

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PHYSICAL REVIEW LETTERS

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3

Spin Density (10 /cm )PRL 96, 145501 (2006)
30
20
10
0
8

(a)

(b)

6
4
2

0
start

10

17

10

18

10
2

Dose (e/cm )19 800

1000

1200
o

Anneal Temp. ( C)

FIG. 3 (color online). Spin densities of defects and N donors in(a) electron radiation experiment (using six samples with different doses) and in (b) isochronal annealing experiment (using asingle sample). At 70 K, N-donor and HEI5=6 signals were
measured in the dark, whereas SI5, P6, and VC signals were
measured under illumination by a 100 W halogen lamp. Data for
P6 are nominal, as its triplet spin system could have a nonequilibrium population by the photo excitation.

lations also support this view: the ionization levels (0j)
and (j2) as calculated in the largest cell (288 atoms)
amount to EC  1:0 eV and EC  0:9 eV, respectively.
Note that the precision of these values is limited due tothe finite size of the supercells and errors of the exchangecorrelation functional. Yet, energy increment between the1 and 2 charged states is relevant, and both the levels
can compensate the residual carriers. Accordingly, EF is
pinned at EC  1:1 eV or lower due to CSi VC in SiC.
To examine the role of the SI5 center in the carrier
compensation, we carried out step-by-step irradiation andsubsequent isochronal annealing experiments of n-type
SiC samples as shown in Fig. 3. In the nonirradiatedsubstrate, the EPR spectrum was dominated by the signalof the neutral N donors at the k site. This donor signal
rapidly decreased with increasing electron dose, whereas
the SI5 center, the P6 center, the unidentified HEI5=6
centers, and the C-vacancy centers (HEI1 and EI5) wereformed [Fig. 3(a)]. However, the spin densities of VC andHEI5=6 were lower than the total density of N donors.
Figure 3(b) plots the spin density for an isochronal annealed (30 min in Ar ambient) n-type sample after the 11018 -e=cm2 irradiation. The SI5 signal started to decrease
after annealing at 1100 C alongside with the recovery of
the N donor signal. Both observations suggest a correlation
between the SI5 center and the compensation of the N
donors. In contrast to our irradiated samples, HPSI substrates exhibited the SI5 signal even after annealing at
1600 C [10,16]. Its higher thermal stability in the HPSI
samples could be due to a lower EF position and thedifferent abundances of other defects interfering with theannealing of SI5. Theoretical studies [7] predicted that theavailable annealing paths and their activation energies
strongly depend on EF , which should result in a lower
thermal stability of SI5 at a higher EF in the gap. Recent
positron annihilation studies suggested the presence ofweek ending
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vacancy aggregates in HPSI SiC samples [23]. According
to theory, such aggregates can dissociate and be a source ofisolated vacancies and AV complex at high annealing
temperatures [7,9,24]. Thus, vacancy defects as well as
the SI5 center (carbon AV complexes) play an importantrole in the SI property of SiC.
In conclusion, our EPR analysis and first principles
calculations identify the SI5 center in 4H-SiC with thefundamental AV complex in the negative charge state(CSi VC ). In n-type SiC this defect is singly or doubly
negatively charged due to the compensation by residual
carriers and plays an important role in facilitating the SI
property in some HPSI SiC substrates.
Support from the Swedish Foundation for Strategic
Research program SiCMAT, the Swedish National Infrastructure for Computing grant No. SNIC 007/05-6 and the
Hungarian OTKA grant No. F-038357 is acknowledged.
M. B. acknowledges financial support by the Deutsche
Forschungsgemeinschaft (BO1851/2-1 and SiC-researchgroup). T. U. thanks Y. Ishitsuka for his helpful support.

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