Materials Science Forum Vols. 527-529 (2006) pp. 527-530
online at http://www.scientific.net© (2006) Trans Tech Publications, Switzerland

Divacancy model for P6/P7 centers in 4H- and 6H-SiC
N.T. Son1,a, T. Umeda2, J. Isoya2, A. Gali3, M. Bockstedte4, B. Magnusson1,5,A. Ellison5, N. Morishita6, T. Ohshima6, H. Itoh6, and E. Janzén1
1

Department of Physics, Chemistry and Biology, Linköping University, 581 83 Linköping, Sweden
2
Graduate School of Library, Information and Media Studies, Research Center for Knowledge
Communities, University of Tsukuba, Tsukuba 305-8550, Japan3
Department of Atomic Physics, Budapest University of Technology and Economics, Hungary
4Universität Erlangen-Nürnberg, Erlangen, Germany, and Departamento de Física de Materiales,
Universidad del País Vasco, E-20018 San Sebastián, Spain
5
Norstel AB, Ramshällsvägen 15, SE-602 38 Norrköping, Sweden
6
Japan Atomic Energy Research Institute, Takasaki 370-1292, Japanaemail: son@ifm.liu.se
Keywords: divacancies, electron paramagnetic resonance, ab initio calculations.

Abstract. Electron paramagnetic resonance (EPR) studies of the P6/P7 centers in 4H- and 6H-SiC
are reported. The obtained principal values of the hyperfine tensors of C and Si neighbors are ingood agreement with the values of the neutral divacancy (VCVSi0) calculated by ab initio supercell
calculations. The results suggest that the P6/P7 centers, which were previously assigned to thephoto-excited triplet states of the carbon vacancy-carbon antisite pairs in the double positive charge
state (VCCSi2+), are related to the triplet ground states of the C3v/C1h configurations of VCVSi0.
Introduction
The P6/P7 spectra were first observed in heat treated n-type 6H-SiC substrates by electron
paramagnetic resonance (EPR) under photo-excitation [1]. Based on their symmetry (C3v and C1hfor the P6 and P7 centers, respectively), the spectra were suggested to be related to the excited stateof the C3v/C1h configurations of the divacancy [1], i.e. the close pair of the carbon and siliconvacancy. The P6/P7 spectra were also detected in as-grown, n-type 6H-SiC by optical detection ofmagnetic resonance (ODMR) [2]. In a study using magnetic circular dichroism of the absorption(MCDA), MCDA-detected EPR and ab initio calculations [3], the center was instead assigned to theexcited triplet state of the carbon vacancy-antisite pairs in the doubly positively charged state,
VCCSi2+. The center was also found to be a common defect in high-purity semi-insulating (HPSI)
4H- and 6H-SiC substrates [4,5]. The formation of the center was suggested to be due to themigration of a nearest C neighbor into the silicon vacancy (VSi) [3]. The process (VSi → VCCSi) istheoretically predicted to have a low migration barrier (~1.7 eV [3] and ~2.5 eV [6]), and cantherefore be a dominating process. It is common in other semiconductors that the single vacancies
interact with each other to form the divacancies. For SiC, so far there is no evidence that the process
VSi + VC → VCVSi is an important one and that the divacancy is a common defect. The divacancy in
SiC is predicted to be a very stable defect [6-8], but it has not so far been experimentally identified.
In a previous EPR study of HPSI SiC substrates [5], a very stable center, SI-5, was assigned tothe divacancy based on the observed hyperfine (hf) interactions. This assignment was later
supported by calculations [8]. However, in recent EPR studies [9] the symmetry lowering of SI-5
from C3v to C1h and additional large hf interactions with 29Si were observed, that invalidated thismodel. Recent experiments [9] and ab initio supercell calculations [10] suggest SI-5 to be thecarbon vacancy-carbon antisite pair in the negative charge state, VCCSi–.
In this work, we present new EPR data on the P6/P7 centers in 4H- and 6H-SiC including the hfinteractions with C and Si neighbor atoms. Comparing the EPR data with the parameters obtainedby ab initio supercell calculations of the neutral divacancy, the P6/P7 centers are attributed to theground triplet states of the neutral divacancy in the axial/non-axial configurations, respectively.
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Silicon Carbide and Related Materials 2005

ExperimentStarting materials used in the study are N-doped n-type (concentration ~1×1017 cm–3), Al-doped ptype (~1×1018 cm–3) and HPSI 4H-SiC wafers. In HPSI samples, the concentration of N is ~4×1015
cm–3 [11]. The irradiation by 3 MeV electrons was performed at room temperature with a dose of
2×1018 cm–2. For some n-type samples, the irradiation was performed at 850 °C with doses of
2×1018 cm–2 and 1×1019 cm–2. EPR measurements were performed on Bruker ER200D and E580 Xband spectrometers. For light illumination, a Xenon lamp (150 W) was used in combination with aJobin-Yvon 0.25 m grating monochromator and/or different optical filters.
Results and discussion

EPR Intensity (linear scale)

The P6/P7 spectra can be detected after irradiation but are very weak. The signals reach themaximum after annealing at ~850 °C. In irradiated p-type 4H-SiC the spectra can only be detectedunder illumination with light of photon energies ≥1.1 eV. In heavily irradiated (1×1019 cm–2) n-typesamples, the spectra can be detected in completely dark for the whole temperature range 4-293 K.
Figure 1 shows the P6/P7 spectra observed at 8 K in dark.
Using on-axis samples we obtained more accurate fine structure parameters D and E for the P7
type center compared to that in our previous work using off-axis samples [12]. We name the P6
spectra according to the labels used in Ref. 13 and labelled the corresponding C1h spectra as P7b andP7’b. The g-value for P6/P7 centers is 2.003; in 4H-SiC the axially symmetric D and anisotropic E
values of the fine structure parameter (in unit of 10–4 cm-1) deduced from the fits to the angulardependences are: D(P6b) = D(P7b) = 447, D(P6’b) = 436, D(P7’b) = 408, E(P7b) = 90 and E(P7’b)= 10. The angle between the principal axis of the fine structure tensor and the c axis for the P7b andP7’b centers is 70.5° and 71°, respectively. In Ref. 13 the P6 and P6’(also P6’’) family spectra were
attributed to VCCSi2+ and VCCSi(SiCCSi) complex defects, respectively. In our samples, we did notobserve the P6’c and P6’’ spectra.
Figures 2(a) and 2(b) show the detailed hf structure of the low-field lines P6b and P6’b in HPSI
4H- and 6H-SiC, respectively. From the intensity ratio between the two outer hf lines and the mainlines of ~3.3-3.4 %, which is approximately the natural abundance of three 13C nuclei (I=1/2,1.11%), these hf lines are assigned to the hf interaction with three nearest C neighbors (labeled CI).
Using a field modulation amplitude of 0.01
mT, the two inner hf structures can be well4H-SiC
9.641 GHz
resolved for the P6’b line in 4H-SiC [Fig.
T= 8 K, in dark2(a)]. Their intensity ratios agree well with
P7'b29the interaction with three and six Si nuclei(I=1/2, 4.7%). Fig. 3 shows the C and Si
P7'bP7bneighbors of a divacancy in axial symmetry.
P6bP6'bP7b
The simulation of the P6b and P6’b lines
P6'bP6cand their hf structures includes the hfP6cinteractions with following atoms: (i) 3 CIx5for the outer hf, (ii) 3 SiIIa (on the bond alongthe c axis) and 6 SiIIb (in the basal plane) forP6b
the two inner hf, (iii) 3 nearest SiI neighborsof VC for the unresolved shoulders. As canbe seen in ×10 scale spectra in Figs. 2(a) and300
320
340
360
3802(b), the simulation fits perfectly theMagnetic Field (mT)observed spectra, not only the intensity ofthe hf lines but also their detailed Fig. 1: EPR spectra of the P6/P7 centers insuperhyperfine structures. The CI hf structure electron-irradiated and annealed n-type 4H-SiC
was also detected at some other directions measured for B‫׀׀‬c at 8 K in dark.
and the CI hf tensors of the P6b and P6’b

Materials Science Forum Vols. 527-529

EPR Intensity (linear scale)

4H-SiC
9.47508 GHz
P6b

P6'b

T= 77 K
B c

3 SiI

6H-SiC

6 SiIIb

3 CI

T=77 K
B c

P6b

9.4714 GHz
experimentexperiment
3 CI529

P6'b3 CI

6 SiIIb
3 CI

3 SiIIa

3 SiIIax10x10
simulation
289

290

3 SiI
291x10x10(a)simulation

292x10289290(b)
291

292

Magnetic Field (mT)Fig.2: Part of the P6b and P6’b spectra in electron-irradiated and annealed (850 °C) HPSI (a)4H- and (b) 6H-SiC recorded at 77 K for B‫׀׀‬c showing hf structures of 3 nearest CI and 3 SiIIaand 6 SiIIb in the second neighbor shell of VSi. The simulations using the hf parameters andline width obtained from experiments are plotted for comparison. Detailed structures of hflines can be seen in ×10 scale inserts of experimental and simulated spectra.
centers in 4H-SiC could be determined. Their principal values are summarized in Table 1. Theangle between the principal z-axis of the CI hf tensor of P6b and P6’b centers in 4H-SiC is 73°.
For the P7b and P7’b centers in both 4H-and 6H-SiC, the hf structure due to the interaction withthree CI atoms was also detected at some angles close to 70° with respect to the c axis where thespectra are well separated from others. Within the experimental error, the inner hf interactions ofP6b and P6’b are isotropic. Our observation of the P6/P7 signals at low temperatures in dark
confirms that the centers are related to the triplet ground state but not the photo-excited state aspreviously suggested [1,3]. The observed hf interactions with the nearest and second neighbors
suggest that these centers may be the divacancy in the neutral charge state, VCVSi0.
The divacancy model for the P6/P7 centers is supported by recent ab initio supercell calculationsof the neutral divacancy in 4H-SiC by Gali et al [11]. As can be seen in Table 1, the calculated hf
parameters for Si and C neighbors of the divacancy are in good agreement with the values obtainedby EPR. Comparing the calculated hf parameters of three CI neighbors of VCVSi0 in the hexagonalTable 1. The principal values (in MHz) of the hf tensors of C and Si neighbors determined by
EPR for the P6b and P6’b centers. The calculated values for the neutral divacancy of C3v
symmetry at the hexagonal (hh) and cubic (kk) sites in 4H-SiC from Ref. 11 are given forcomparison. The angle between the principal z-axis of the A tensor and the c axis is 73° for all thecenters. The number of equivalent atoms is shown in parentheses.
CenterCI (×3)
SiIIa (×3)
SiIIb (×6)
SiI (×3)(VSiVC)0 (hh) [11]P6b(VSiVC)0 (kk) [11]P6’bAxx

Ayy

Azz

Axx

AyyAzz

Axx

Ayy

Azz

Axx

Ayy

Azz

53
129350129311012935599356
99411698547
1310345
13
103

104
13
10349
10
10
149
1010
1110
992

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Silicon Carbide and Related Materials 2005

hexagonal (hh) and cubic-cubic (kk) configurations
with the experimental values of the P6b and P6’bcenters in 4H-SiC, we assign P6b and P6’b to theaxial (C3v) configuration of the neutral divacancy atthe hexagonal and cubic lattice site, respectively. The
calculated (+/0) and (0/–) levels of the neutral
divacancy are at ~0.5 eV and ~1.4 eV above thevalence band, respectively [11]. The neutral charge
state with S=1 is the ground state of the divacancy
when the Fermi-level is in this range.
Conclusion

Fig. 3: Carbon and silicon neighbors of adivacancy in axial symmetry.

Based on the EPR observation and ab initio supercell
calculations we identified the P6/P7 centers in 4Hand 6H-SiC to be related to the ground triplet state of the neutral divacancy, VCVSi0, in theaxial/non-axial (C3v/C1h) configurations and assigned the P6b and P6’b axial centers to the C3v
configuration at the hexagonal and cubic site, respectively. The spin density is found to be locatedmainly on three nearest C neighbors of VSi (~59-60%) whereas it is negligible on the nearest Si
neighbors of VC. The vacancy model for the P6/P7 centers also implies that the interaction between
VSi and VC to form divacancies is significant and the divacancy is a common defect in SiC.
Acknowledgements. Support from the Swedish Foundation for Strategic Research program
SiCMAT, the Hungarian OTKA F-038357, the Swedish National Infrastructure for Computing grant
SNIC 011/04-8, the Swedish Foundation for International Cooperation in Research and Higher
Education, and the German DFG (SiC-Research group and BO1851/2-1) is acknowledged.
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