Electron paramagnetic resonance study of the HEI4/SI5 center in 4H-SiC
T. Umeda1,a, N. T. Son2,b, J. Isoya1,c, N. Morishita3,d, T. Ohshima3,e, H. Itoh3,fand E. Janzén2,g1

University of Tsukuba, Tsukuba 305-8550, Japan2

Linköping University, S-581 83 Linköping, Sweden

3

Japan Atomic Energy Agency, Takasaki 370-1292, Japan

aumeda@slis.tsukuba.ac.jp, bson@ifm.liu.se, cisoya@slis.tsukuba.ac.jp,
morishita.norio@jaea.go.jp, eohshima.takeshi20@jaea.go.jp, fitoh.hisayoshi@jaea.go.jp,gerj@ifm.liu.sedKeywords: semi-insulating SiC, defect, deep level, EPR.

Abstract. We present new electron-paramagnetic-resonance (EPR) data on the HEI4/SI5 center in
4H-SiC. So far, the SI5 (SI-5) center has been observed only in as-grown SiC substrates; however, wefound that it can be created by electron irradiation to commercial n-type 4H-SiC. The artificiallycreated SI5 center, which we had preliminary called HEI4, was found to be identical with the SI5
center in as-grown SiC. A high-intensity HEI4/SI5 spectrum of irradiated SiC revealed clear
hyperfine structures of 29Si and 13C, which enabled us to identify the origin of this center as a carbon
antisite-vacancy pair in the negative charge state (CSi-VC–). We assessed its electronic levels using
photo-EPR.
Introduction
In high-purity semi-insulating (HPSI) 4H-SiC, special defects labeled SI1 to SI9 (SI-1 to SI-9) [1]were found by means of electron paramagnetic resonance (EPR) spectroscopy [2]. It is believed thatthese defects are related to the compensation of undesired residual carriers in HPSI materials. The SI5center is one of the most abundant and the most stable centers among them. The SI5 center wascharacterized by an electron spin (S) of 1/2, a C3v symmetry, and two hyperfine (HF) structures due to29
Si (nuclear spin I = 1/2, natural abundance = 4.7%) or 13C (I = 1/2, 1.1%) [2]. Its origin was
tentatively assigned to a divacancy (VSi-VC) [2,3]. However, the analysis of SI5’s HF structures [2]have revealed only 12-13% of the wave function for this center (11% on three C atoms and 1.5% on
three Si atoms), which is too small for a divacancy in either the positive or negative charge state with
an electron spin located at dangling bonds of VC-VSi. In this report, we present more complete EPR
data on the SI5 center by taking advantage of the artificial creation of this center by electron
irradiation. The SI5 center in irradiated SiC, which had been called HEI4 in our recent paper [4],revealed complete HF structures of 29Si or 13C, resulting in a new conclusion that this center
originates from a negatively charged carbon antisite-vacancy pair (CSi-VC–). This conclusion was also
confirmed by first principles calculations [5].
Formation of the HEI4/SI5 center by electron irradiation
Substrates used in this study were commercial nitrogen (N)-doped n-type 4H-SiC(0001) wafers(room-temperature carrier concentration = 1017 /cm3). So far the SI5 center has not been reported in
any irradiated SiC and subsequently annealed samples. We found that the temperature during theirradiation is the key factor to yield the SI5 center. If the irradiation was done at room temperature tothe above n-type substrates, the created defects were mostly Si vacancies (the VSi– and Tv2a centers)[6]. If the irradiation temperature was raised above 350°C, both VSi– and Tv2a signals disappeared in
the dark; instead, the HEI4 (SI5) signal appeared in addition to unidentified EPR signals which welabeled HEI5/6 (S = 1/2). Further increasing the irradiation temperature up to 800°C, EPR spectrum
was strongly dominated by the HEI4/SI5 signal. Although we performed a similar experiment onp-type substrates, the formation of the HEI4/SI5 center was not observed. In the following study, wewill analyze the HEI4/SI5 EPR spectra of n-type 4H-SiC irradiated with 2-4×1018 e/cm2 at 800-850°C.
All spectra shown in this paper were measured under photo excitation by a 100-W Halogen lamp.
EPR results and discussion
A typical HEI4/SI5 spectrum and an angular dependence of the resonant fields measured at 100 K areshown in Figs. 1(b) and (a), respectively. The central strong signal, HEI4 [4], is identical with the SI5
signal: in both cases, the same C3v symmetry, g values, and HF structures [HF2’ and HF4’ in Fig.
1(b)] are observed. We have checked that spin-Hamiltonian parameters are in good agreement
between SI5 at 77K [2] and HEI4 at 80 K; g|| = 2.0048 and 2.0049, g⊥ = 2.0030 and 2.0032, A||(HF2’)= 40.7 and 43.2 MHz, A⊥(HF2’) = 31.8 and 34.2 MHz, A||(HF4’) = 11.8 and 11.7, A⊥(HF4’) = 12.7
and 11.8 MHz, respectively. In Fig. 1, new HF structures (HF1’ and HF3’) are clearly observed, thatwere too weak to be detected in HPSI samples. The HF1’ structure shows the largest HF splitting (A||= 8.0 mT, A⊥ = 6.4 mT, axial symmetric around a basal Si-C bond). Its relative intensity to the total
spectrum is approximately 5%, suggesting that the HF1’ splitting originates from one 29Si atom.
Furthermore, we reassign the HF2’ splitting (A|| = 2.2 mT, A⊥ = 1.8 mT, c-axial) due to one 13C atom,
instead of three 13C atoms in the previous assignment [2]. The HF3’ and HF4’ splittings were nearly
isotropic and would be related to outer atoms of the relevant center; hence we focus only on HF1’ andHF2’. The angular dependence of HF1’ indicates that the relevant Si atom is most likely one of three
Si neighbors of a carbon vacancy (VC) [e.g., Si3 atom in the inset of Fig. 1(b)], and the HF1’ 29Si
interaction is caused by a Si dangling bond pointing to the VC. Using the LCAO (linear combinationof atomic orbitals) analysis [4] on HF1’ and HF2’, an electron-spin distribution is estimated to be
18%+5% on one Si atom + one C atom. This result is quite different from the previous one:
11%+1.5% on three C atoms + three Si atoms [2]. Therefore, the divacancy model is not appropriate
and a new model is necessary.
We also found that the HEI4/SI5 spectrum is strongly temperature dependent, as shown in Fig. 2.
As the temperature decreases, the HF1’ lines vanish due to the broadening and the HF2’ splitting
becomes smaller. The HF3’ and HF4’ lines are almost unchanged. But, near 55 K, all HF lines arebroadened and vanish. Further decreasing the temperature, the HEI4/SI5 spectrum shows completelydifferent HF structures (HF1-5) as well as a different symmetry (C1h).
The EPR spectrum and angular dependence of this C1h-symmetric phase are shown in Figs. 3(b)90(a)

Angle (deg)EPR Signal (a.u.)[1120]

HEI1

HEI1
EI5

EI5

[0001]

0100K0deg

HF3',4'x10

HF1'

HF1'
HF2'

Si1,2

Si3

HF2'

VC(b)
HEI4(SI5)
332334

336

EI5
HEI1338

CSi340342

Magnetic Field (mT)Fig. 1. The HEI4/SI5 center (C3v symmetry) measured at 100 K under illumination.

EPR Signal (arb.units)

and (a), respectively. One very
HEI4/SI5
strong HF interaction (HF1) andHF2-5
HF1
HF1
four weak HF splittings (HF2-5)
30K
are observed. The relative intensity
40K
N
Nof HF1 indicates that it originates
50K
from HF interactions of two 29Si
55K
atoms. They can be identified as
HEI1
HEI1
EI5
HEI1
EI5
two Si neighbors of a VC [e.g., Si1,260K
atoms in the inset of Fig. 3(b)]65K
based on the determined HF tensor
70K
of HF1. The atomic model shownin Fig. 3(b) satisfies the observed
75K
C1h symmetry with respect to the80K(11-20) plane. The LCAO analysis
85K
revealed that an electron spin is
HF3',4'strongly localized on the two Si
90K
HF3',4'atoms by 31%+31%. For other HF
95K
structures (HF2-5), we confirmed
100K
by pulsed-ENDOR measurements
HF2'
HF2'
HF1'
HF1'
120K
that they originate from either 29Si
13or C.
150K
HEI1
HEI1
Accordingly, our EPR data
reveal that (1) the HEI4/SI5 center332334336338340342
is not coupled with impurities, (2) itshould involve a VC, (3) it is mostMagnetic Field (mT)likely to a single negative charge
Fig. 2. Temperature dependence of the HEI4/SI5
state because of S=1/2 and its
spectrum measured for B || c.
presence in irradiated n-typesamples, but (4) it should be
different from a simple VC– (theHEI1 center [4]), (5) its wave function distributes mainly on two Si atoms of a VC in thelow-temperature C1h phase, and (6) the HEI4/SI5 center transforms into a C3v configuration at highertemperatures.
To take all these into account, we propose the c-axial negatively charged carbon antisite-vacancy
pair (CSi-VC–) for the origin of the HEI4/SI5 center. A carbon antisite (CSi) is located at the c-axial
neighbor of a VC, as shown in the insets of Figs. 1(b) and 3(b). Naturally, this model is consistent withthe results (1) to (4). The result (5) is also reasonable, because this situation (electron-spin
distribution = 31%+31% on two Si neighbors of VC–) is quite similar to that for a simple VC– (HEI1)[4] where the electron-spin distribution was found to be 30%+30% on two Si neighbors of VC–. The
result (6) can be interpreted by a thermally activated reorientation. A similar phenomenon wasalready observed for EI5 (VC+) and HEI1 (VC–) [4] where the defects exhibited a C1h symmetry at low
temperatures but a C3v symmetry at high temperatures. The thermally activated reorientation rapidly
moves an electron spin among the Si1-3 atoms. As a result, it generates a C3v average state, and also
causes the lifetime broadening effect (< 100 K) and the motional narrowing effect (> 100 K) on theHF1’ 29Si line (see Fig. 2). It is worthy to note that first principles calculations for CSi-VC– in large
super cells could excellently reproduce the observed HF parameters of the HEI4/SI5 center [5].
According to the calculations, the CSi-VC– pair involves a large Jahn-Teller distortion, and the CSi
atom forms a sp2-like structure with a non-bonding orbital. But, this orbital is occupied by twoelectrons and hardly contributes to 13C HF interaction. Therefore, an electron spin is distributed
mainly on the VC side, resulting in the large 29Si HF interactions, as observed in the experiment. The
calculations [5] predicted that the HF2’ 13C interaction is generated from a second nearest neighbor C
atom of VC.

EI6

EPR Signal (a.u.)Angle (deg)

90

HEI1

EI5

EI6

EI6

EI5

HF2,3,4,5

EI5
EI6

Si1,2

HEI1

EI6[1100](a)0HEI1(b)HF1[0001]

HF1VC30K0deg
330

HEI1

Si3_HEI4(SI5)332334

336

HEI1338

CSi340(1120) plain342344

Magnetic Field (mT)Fig. 3. The HEI4/SI5 center (C1h symmetry) measured at 30 K under illumination.
In the last place, we mention a photo-EPR study. In the dark, the HEI4/SI5 signal was detectablebut very weak. It was drastically enhanced by the photo excitation just above 1.1 eV. We found that
this 1.1-eV threshold corresponds to an electron excitation from a defect at EF (the Fermi level) to theconduction band edge (EC) [4]. Namely, EF of our sample is located at EC – 1.1 eV in thermalequilibrium. At this condition, most of CSi-VC were stabilized into an EPR-inactive charge state mostlikely CSi-VC2–. The illumination converted them to CSi-VC–, which caused the enhancement on theHEI4/SI5 signal. Our photo-EPR data suggest that the CSi-VC pairs captured electrons from the N
donors, forming the CSi-VC2– levels at EC – 1.1 eV. By looking the photo excitation data carefully,
there were also weak enhancements on the HEI4/SI5 signal below the 1.1-eV threshold. They were
found at 0.65 eV and 0.9 eV. The theoretical calculations [5] suggest that these thresholds areassociated with the internal excitations of the CSi-VC pair.
Summary
We have studied hyperfine interactions of the HEI4/SI5 EPR center by taking advantage of irradiatedn-type 4H-SiC substrates. The detailed EPR analysis revealed that this center originates fromnegatively charged CSi-VC pairs. They contributed to the compensation of the N donors in theirradiated n-type samples.
References[1] We here use conventional notations such as “SI5” or “HEI1” (alphabets + number), instead of theoriginal labels (“SI-5” or “HEI1”, respectively).
[2] N. T. Son, B. Magnusson, Z. Zolnai, A. Ellison, and E. Janzén: Mat. Sci. Forum Vol. 457-460(2004), p. 437.
[3] U. Gerstmann, E. Rauls, and H. Overhof: Phys. Rev. B Vol. 70 (2004), p. 201204(R).
[4] T. Umeda, Y. Ishitsuka, J. Isoya, N.T. Son, E. Janzén, N. Morishita, T. Ohshima, H. Itoh, and A.
Gali: Phys. Rev. B Vol. 71 (2005), p. 193202.
[5] M. Bockstedte, A. Gali, T. Umeda, N.T. Son, J. Isoya, and E. Janzén: these Proceedings.
[6] N. Mizuochi et al.: Phys. Rev. B Vol. 68 (2003), p. 165206; Phys. Rev. B Vol. 66 (2002), p.
235202.