Electron paramagnetic resonance study of carbon antisite-vacancy pairin p-type 4H-SiC
T. Umeda1,a, N. Morishita2,b, T. Ohshima2,c, H. Itoh2,d, and J. Isoya1,e1

Graduate School for Library, Information and Media Studies, University of Tsukuba, Tsukuba
305-8550, Japan2
Japan Atomic Energy Agency, Takasaki 370-1292, Japanaumeda@slis.tsukuba.ac.jp, bmorishita.norio@jaea.go.jp, cohshima.takeshi20@jaea.go.jp,
ditoh.hisayoshi@jaea.go.jp, eisoya@slis.tsukuba.ac.jp

Keywords: antisite-vacancy pair, electron paramagnetic resonance.

Abstract. Carbon antisite-vacancy pair (CSiVC) is a fundamental defect in SiC, and is theoretically
predicted to be very stable in p-type materials; however, this pair was found only in the from of anegatively charged state (i.e., the SI5 center = CSiVC−) in n-type and semi-insulating 4H-SiC, and yet,its presence has not been shown in p-type SiC. In this report, we present the first EPR observation on
positively charged CSiVC pairs in p-type 4H-SiC. By carefully examining p-type samples after
electron irradiation, we found a pair of new defects with C3v and C1h symmetries. They correspond to“c-axial” pairs (C3v) and “basal” pairs (C1h) of CSiVC+, respectively. The positively charged pairs arecharacterized by a strong 13C hyperfine interaction due to a dangling bond on a carbon antisite (CSi),
which is successfully resolved for the c-axial pairs.
Introduction
Antisite-vacancy (AV) pairs are one of the fundamental defects in compound semiconductors,
because they are metastable/stable counterparts of monovacancies. In SiC, silicon vacancy [Fig. 1(a)]can be converted into carbon AV pairs [Fig. 1(b)]. Recently, their negatively charged state (CSiVC−)
was assigned to the origin of the SI5 center in n-type and semi-insulating (SI) 4H-SiC [1]. In thesesubstrates, the carbon AV pair plays an important role on the SI property of SiC [1]. This pair shouldbe also important in p-type substrates, because theoretical studies [2,3] predicted that the pair is morestable in p-type SiC (i.e., in the form of a positively charged state) rather than in n-type SiC. However,c-axialCSi109°basal
CSi[0001]e levelfrom Si DBs
a levelfrom C DB(CSi)c-axial pair(a) VSiECbasal pair(b) CSiVC

EC
electronspin
S=1/2EVEV(c) CSiVC−(d) CSiVC+Fig. 1. Basic features of carbon AV pairs (CSiVC) in 4H-SiC. Atomic models of (a) siliconvacancy (VSi) and (b) its counterpart, CSiVC pair. They can be transformed into each other. As
shown in (b), the carbon AV pair exhibits two different orientations and has a set of three silicon
dangling bonds (DBs) and one carbon DB. These DBs generate two gap levels as shown for (c)negative charge state or (d) positive charge state (in the valence band, another a level isgenerated and is doubly occupied) [1,4]. Depedening on the charge state, an electron spin has aquite different nature.

contrary to the theoretical predictions, stable positively charged AV pairs have not been found yet. In
this report, we present the first experimental data on positively charged carbon AV pairs (CSiVC+) inp-type 4H-SiC using electron paramagnetic resonance (EPR). The pair possesses four dangling bonds(DBs) of either Si or C [Fig. 1(b)], which generate two electronic levels (a and e) in the gap [1,4]. In
its negative charge state [Fig. 1(c)], an electron spin (S = 1/2) occupies the doubly-degenerate e level
mainly due to Si DBs, resulting in strong hyperfine (HF) interactions due to 29Si isotopes (nuclearspin I = 1/2, natural abundance = 4.7%) [1]. On the contrary, in its positive charge state, an electronspin should be strongly localized on a CSi atom, generating a strong HF interaction due to a 13C
isotope (I = 1/2, natural abundance = 1.1%). We here show this 13C HF signature for c-axial carbon
AV pairs [c.f., the left hand side one of Fig. 1(b)].
Experiment
We prepared a series of irradiated p-type 4H-SiC samples. Boron doped p-type 4H-SiC substrates(room temperature carrier concentration = 1015 /cm3) were irradiated by 3-MeV electrons at around
800 ºC with a dose of 2×1018 e/cm2 or higher. With the same irradition to n-type 4H-SiC samples [1],carbon AV pairs (the SI5 center) were dominantly formed, so that we expected a similar situation in
the p-type samples. EPR spectra were measured by a Bruker E500 X-band spectrometer. Most ofmeasurements were done at room temperatures where EPR intensities were considerably reduced ascompared to those at low temperatures. To gain sufficient signal intensities, we used 1.5-mm thicksamples. All measuremtns shown here were carried out in the dark (in the thermal equilibrium).
Results and discussion

EPR signal (arb.units)

Figure 2(a) shows a typical EPR spectrum of our p-type samples measured at 150 K, where thewell-known EI5/EI6 EPR signals (from positively charged carbon vacancies, VC+) are dominant, asreported previously in Ref. [5]. Examining the same samples carefully at room temperature with avery low microwave power (0.1 μW), three new EPR signals were detected as shown in Fig. 2(b). We
labeled them “HEI9” (there are two minor types, “a” and “b”, as is seen in the figure, and we call a setof them “HEI9”), “HEI10”, and “HEI11”. These new signals were easily saturated even by a low
microwave power, and disappeared under a normal power of 1 mW [see dashed line in Fig. 2(b),where only the EI5/6 signal is observed]. Due to this saturation effect, it was difficult to detect them at
low temperatures (< 200 K), because longer spin relaxation times at lower temperatures make thesignal saturation being much easier. With this reason, they might have not been found so far. But,
they were formed as easily as for VC+ in p-type irradiated SiC: e.g., spin densities were 3×1017 /cm3for VC+ (EI5/6) and 3×1017 /cm3 for the new centers (HEI9/10/11) after 2×1018 e/cm2 irradiation. Thus,
we speculate that they are also major defects in many p-type 4H-SiC samples and probably in

EI5/6 (VC )(b) 297K
0.0001mW

334

Fig. 2. EPR spectraof irradiated p-type
4H-SiC measured
at 150 and 297 K
using microwave
powers of 1 mW or
0.1 μW. Verticalscales are differentfor (a) and (b).

+(a) 150K1mW

335

HEI11336

HEI9a/101mW(EI5/6)

HEI9b

337

338

Magnetic field (mT)

339

340

Magnetic field (mT)semi-insulating substrates. The338.0
coexistence of VC+ and the new(a) HEI9(C3v)(c) HEI11(C1h)(b) HEI10(C1h)centers in the dark indicates that
their electronic levels are located337.5
at similar positions in the gap.
HEI9b
We believe that they are related
to positively charged carbon AV
337.0
pairs (CSiVC+), which will be
HEI9a
supported by the following EPR
analyses.
Angular dependences of the336.5
central lines of HEI9/10/11 are090 090 090
shown in Figs. 3(a) to (c), which
Angle from the c axis (degrees)
reveal C3v symmetry for HEI9Fig. 3. Angular maps of EPR spectra for (a) HEI9a/b, (b)and C1h symmetry for HEI10 andHEI10, and (c) HEI11. Magnetic field was rotated fromHEI11. Their g tensors were[0001] to [-1100]. Symbols express experimental data (larger
determined as summarized inand draker symbols represent larger peaks) and solid lines
Table I. They show simialr
are fitting results.
principal g values: g|| (gZZ) ≈2.002 and g⊥ (gXX and gYY) ≈2.003-2.004. The main (||)
principal g axis is parallel to the c axis for HEI9, while for HEI11 the axis is aligned nearly with abasal bond (θ = 109°). For HEI10, we found an intermediate situation between the former and thelatter cases. From these results we speculate that a pair of the C3v center (HEI9a/b) and the C1h center(HEI10 and/or HEI11) corresponds to a common “pair”-type defect. For instance, it is well known
that divacancies (VSiVC) exhibit a pair of P6(C3v) and P7(C1h) EPR spectra, which originate fromc-axial pair (C3v) and basal pair (C1h) of VSiVC, respectively [6]. By analogy, we assign HEI9(C3v) toc-axial CSiVC+ pairs. This assignment can be confirmed by their HF interactions. It is also reasonable
to consider that their minor types (a and b) correspond to two inequivalent configurations (kk and hh[6]) in 4H-SiC.
The HEI9a/b centers show strong HF interactions with nuclear spin(s) of I = 1/2, as shown in Fig.
4. For HEI9b, an intensity ratio for the HF lines b to the main line was estimated to be 2%, whichsuggests the contribution of one 13C atom (1.1%) rather than one 29Si atom (4.7%) (Note that thecontribution of two nuclei is inconsistent with the observed C3v symmetry). In addition, if weassigned one 29Si atom to the origin of the HF lines a (see Fig. 4), the main line of HEI9a would takeonly 4% of the observed main line of “HEI9a/10”, which is quite inconsistent with the observed
intensity ratio of HEI9a:HEI10 in EPR spectra at other magnetic-field angles. Accordingly, weconclude that the observed HF interactions of HEI9a/b are due to one 13C atom. Table I shows theprincipal values of the 13C HF tensor (A⊥ and A||). By using the standard isotropic and anisotropic HF
constants of 13C (Aiso = 134.77 mT and Aaniso = 3.83 mT), we find that an electron spin of HEI9a(HEI9b) is distributed on a c-axial carbon DB by 55 (58) % [2s-orbital density = 3 (4) % and
2p-orbital density = 52 (54) %]. Therefore, the HEI9 centers are strongly localized on a c-axial C
Table 1. Spin-Hamiltonian parameters for the new EPR centers in p-type 4H-SiC. θ expresses theangle between the main principal g axis and the c axis.
Centersymmetry

gXX

gYY

gZZ (g||)θ (deg.)A⊥ (mT)

A|| (mT)

HEI9a
HEI9b
HEI10
HEI11

C3v
C3v
C1h
C1h

2.00408
2.00380
2.003482.003992.00408
2.00380
2.00258
2.00345

2.002282.00195
2.00226
2.00263

00145
116

2.3
3.7

8.3
9.9Angle (deg)90

EPR signal (a.u.)

0abmain lines
HEI9a/10
HEI11

HF linesx20295K0deg

333

HF lines
abx20

HEI9b334

335

336

337338339

340

341

342

Magnetic field (mT)Fig. 4. Angular map of 13C hyperfine interactions for HEI9a/b. Magnetic field was rotated from[0001] to [-1100]. EPR spectrum shown here was measured using a microwave power of 0.1
μW. Solid lines in the angular map indicate fitting results.
atom (i.e., c-axial CSi atom in Fig. 1), identifying them as c-axial CSiVC pairs in the positive charge
state [c.f., Fig. 1(d)]. We should note that this positive charge state does not involve Jahn-Teller
instability and hence C3v symmetry is observed.
The basal pairs of CSiVC+ will correspond to HEI10 and/or HEI11. For this case, there should bealso two inequivalent configurations, kh and hk [6]. Likewise the c-axial pairs, we should observe astrong 13C HF interaction due to a basal CSi atom [c.f., Fig. 1(b)]. In the angular map of Fig. 4, one
finds unidentified HF lines, a part of which may originate from the basal CSiVC+ pairs.
Conclusion
Based on careful EPR analyses on irradiated p-type 4H-SiC, we have revealed for the first time stable
positively charged carbon AV pairs (CSiVC+). The HEI9a/b centers with C3v symmetry were assigned
to c-axial pairs of CSiVC+. We also found two C1h-symmetric centers (HEI10 and HEI11) similar toHEI9, which supposedly corresponds to basal pairs of CSiVC+.
Acknowledgements. We thank Dr. Adam Gali for intensive discussions from a theoretical view
point.
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