Materials Science Forum Vols. 600-603 (2009) pp 409-412
online at http://www.scientific.net© (2009) Trans Tech Publications, Switzerland

Photo-EPR Study of Vacancy-type Defects in Irradiated n-type 4H-SiC
T. Umeda1,a, N. Morishita2,b, T. Ohshima2,c, H. Itoh2,d, and J. Isoya1,e1

Graduate School of Library, Information and Media Studies, University of Tsukuba, Tsukuba,
305-8550, Japan2

Japan Atomic Energy Agency, Takasaki, 370-1292, Japanabc

umeda@slis.tsukuba.ac.jp, morishita.norio@jaea.go.jp, ohshima.takeshi20@jaea.go.jp,ditoh.hisayoshi@jaea.go.jp, eisoya@slis.tsukuba.ac.jp

Keywords: photo-EPR, energy level, carbon vacancy, silicon vacancy, divacancy, carbon
antisite-vacancy pair, 4H-SiC.

Abstract. We report photo-induced electron paramagnetic resonance (photo-EPR) data for irradiatedn-type 4H-SiC. Energy levels and associated photo-induced transitions are discussed for siliconvacancy (VSi), carbon vacancy (VC), carbon antisite-vacancy pair (CSiVC), and divacancy (VSiVC).
Introduction
In SiC, vacancy-type defects can be thermally stable and be abundant, having a significant influenceon the properties of this material. So far, in 4H-SiC, combined studies of electron paramagnetic
resonance (EPR) and first principles calculation have identified four fundamental types of such
defects, VSi [1,2], VC [3,4], CSiVC [5,6], and VSiVC [7]. These defects play an important role inhigh-purity semi-insulating 4H-SiC [8]. Furthermore, they are believed to be linked to fundamental
defect levels studied by other techniques such as photoluminescence (PL) and deep level transient
spectroscopy. However, relationships between the defects and the defect levels are still unclear. Thephoto-EPR technique [4,5,8-10] is useful for revealing such relationships. Therefore, using thistechnique, we planed to study a series of irradiated 4H-SiC samples that included all or most of theabove defects. In such samples, we can obtain photo-EPR data on coexisting defects simultaneously,
and can compare one’s photo response with others under the same condition, which expectedly give
us a hint for unraveling a puzzle of the defect levels.
Photo-EPR experimentsPhoto EPR setup is simply consisted of a Bruker Bio-Spin E500 X-band spectrometer and two
compact monochromator units (Shimadzu Corp.) combined with a 150-W Xenon lamp. The two
monochromators were operated with different sets of concave blazed holographic gratings.
Combining them with a set of low-pass optical filters, we can sweep the photon energy from 0.5 to 5.6
eV. A photon energy resolution was estimated to be 0.03 eV or lower, which was primarily
determined by an optical slit of a monochromator. The slit width was 1.00 mm and 0.50 mm for lower(<1.8 eV) and higher (≥1.8 eV) energy ranges, respectively. The monochromatic light was illuminatedto a sample through an optical fiber guide (transparent above approximately 0.5 eV). First, wemeasured dark EPR spectra after keeping a sample in the darkness for days, and then recorded thespectra for each photon energy (increment step = 0.05 eV) under a steady state after illumination.
Results and discussions
In this study, we present photo-EPR results for three irradiated n-type 4H-SiC samples, which we callA, B, and C hereafter. They were prepared by high-temperature electron irradiations (3 MeV, 1×1018
All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of the
publisher: Trans Tech Publications Ltd, Switzerland, www.ttp.net. (ID: 133.51.28.135-19/08/08,09:08:42)

410

Silicon Carbide and Related Materials 2007

e/cm2) to nitrogen-doped commercial substrates (room temperature carrier density = 1017 /cm3) at 350ºC for A, 565 ºC for B, and 800 ºC for C. With lower temperatures (i.e., for the sample A), more VSi
defects (VSi– and TV2a) but less CSiVC defects (SI5) were produced after the irradiation. Figure 1 shows
typical EPR spectra in the dark (thermal equilibrium) and under photo excitation.
In the dark, the HEI5/6 centers (electron spin S = 1/2, C1h symmetry) [5] were commonly observedin the three samples as well as other n-type ones. Other centers (HEI1 [4], HEI2, HEI3, SI5 [5], etc.)
also appeared with increasing the irradiation temperature. The HEI5/6 centers were very common andmight be related to one of the known vacancy-type defects introduced above. They showed similar
sets of four 29Si hyperfine (HF) satellites, implying that they include Si nearest neighbors of VC. For aconclusive identification, however, full analyses of HF tensors should be necessary. On the other hand,the HEI2 and HEI3 centers (S = 1/2) are believed to be complexes of intrinsic defects, because theirformations were enhanced by higher temperatures and higher electron doses.
With photo excitation, the known vacancy-type defects such as VSi– [1], the TV2a center (VSi–) [2],the HEI1 center (VC–) [4], the EI5 center (VC+) [3], the SI5 center (CSiVC–) [5], and the P6/7 centers(VSiVC0) [7] were strongly observed, as is seen in Fig. 1. The photo responses (EPR signal intensities
versus photon energy) of these vacancy-type defects as well as HEI5/6 are plotted in Fig. 2. In all
examined n-type samples, photo-induced changes started weakly at 0.80-0.85 eV and thendramatically at 1.00-1.10 eV. The latter “main” threshold will correspond to a primary transition that
an electron is excited from a defect level at the Fermi level (EF) to the conduction band edge (EC). At
such a threshold, many defects will change their charge states simultaneously. Therefore, wespeculate that EF positions in our samples are EC – 1.0-1.1 eV. On the other hand, the weak thresholdat 0.80-0.85 eV might correspond to photo-induced transitions via intermediate states (e.g., excitedstates of defects) at around EC – 0.2 eV. A recent photo-EPR study also reported the presence of suchintermediate transitions via energy levels at EC – 0.2 eV [8]. The other defects, HEI2 and HEI3, wereless sensitive to the photo excitation, and hence no data are shown in Fig. 2. Finally, the mainthreshold energies for each defect were determined as summarized in Table I. In below, we attempt tointerpret the present results in comparison with previous experimental and theoretical studies.
VSi: VSi– and TV2a exhibited the almost same threshold at 1.05-1.10 eV (Table I). Since both centers
are single negative charge states [2], the observed threshold will correspond to a transition from VSi3–×5

HEI1
EI5
SI5–VSi(A) 350°C e-irradiation

SI5TV2aEPR signal intensity (a.u.)

EI5(hf)

TV2aEI5(hf)

HEI6
HEI5
EI5
HEI5
HEI6

dark
2.50eV(B) 565°C e-irradiation(C) 800°C e-irradiation
HEI3

TV2aHEI2
HEI1

P7

P6

300
333

334

335

336
337
Magnetic field (mT)

338

320

340
339

Fig. 1. EPR spectra of irradiated n-type 4H-SiC samples A, B, and C at 60 K (microwave of 9.428
GHz and 0.2 µW, field modulation of 100 kHz and 0.05-mT width, magnetic field // c axis).
Assignments of the observed centers have been checked by angular dependence of EPR spectra.

EPR signal intensity (a.u.)

Materials Science Forum Vols. 600-603

1000

10010(A) 350°C e-irr.

(B) 565°C e-irr.

1.10
1.05

1.05
1.00

HEI1

HEI5
0.85

HEI6

SI5

2.00
EI5(hf)

2.0

1.05
0.85
0.80

2.05
HEI1

0.85
0.80

HEI5
HEI6 EI5

HEI1
2.05

1.20
1.15 P6c

EI5
EI5(hf)

TV2aSI5

P6b/b'P6b/b'
P7
3.0

SI5

HEI5
HEI6

1.10

noise level
1.0(C) 800°C e-irr.

–EI5TV2a1VSi411

1.0
2.0
Photon energy (eV)

3.0

1.0

2.0

3.0Fig. 2. Photo-EPR data on vacancy-type defects in irradiated n-type 4H-SiC samples A, B, and C
at 60 K. First data point of each trace (below 0.5 eV) was measured in the dark. “(hf)” means
hyperfine satellite lines.
or VSi2– to VSi–. Since both -3 and -2 charge states are paramagnetic (Table I), we should detect either
state by EPR in the dark. Its detection is a future problem. Previously, first principles calculations
predicted the ionization levels of VSi at 0.84-0.92 eV for (-3/-2) and 1.50-1.55 eV for (-2/-1) below EC[11]. The observed threshold energy (1.05-1.10 eV) seems to be enough for converting VSi3– into VSi2–
but insufficient for VSi2– into VSi– by a single excitation. Namely, we need an intermediate level of VSi
between EC and the (-2/-1) level which are separated by about 1.1 eV. The observed threshold energy
will correspond to this intra-vacancy transition energy. On the other hand, VSi exhibits two sharp
V1/V2 PL lines (1.438 and 1.352 eV, respectively) owing to two discrete levels of VSi (one level must
belong to VSi–) in the band gap [12]. Probably, the photo-EPR data can be correlated with such levels,
which also remains a future problem.
VC: We could detect VC– from 1.00-1.05 eV and VC+ from 2.00-2.05 eV. The first threshold(1.00-1.05 eV) is almost identical to our previous result (1.08 eV) [4], and we can conclude again that
VC2– are stable when EF = EC – 1.0-1.1 eV. For VC, theoretical calculations predicted the ionizationlevels at 1.15-1.21 eV for (-2/-1) and at 1.03 eV for (-1/0) below EC [11] and a negative-U nature[11,13]. Our observation can be consistent with such negative-U model that VC2– is more stable thanVC–. In other irradiated n-type samples we had prepared, VC2– were always majority, which is also
reasonable supposing the negative-U model. For VC+, we obtained threshold energies of 2.00-2.05 eV,
which are larger than previous results (1.8 eV) [4,9]. To account for this difference, we have toassume the presence of a structural relaxation at least larger than 0.2 eV.
CSiVC: The present data are identical to those of our photo-EPR work on a different n-type sample[5]. Likewise VC, this defect is stabilized into CSiVC2– in the dark. We also found that CSiVC2– werealways majority in other n-type samples. However, theories did not expect the negative-U behaviorfor CSiVC2– [5,11]. So, we alternatively propose that the (-2/-1) level of CSiVC is accidentally very
close to the (-2/-1) level of VC, and then CSiVC2– could accompany VC2–. It should be also mentionedthat no positive charge states (CSiVC+, the HEI9/10 centers [6]) were detectable in a higher energy
range, in spite of a careful optimization for the detection.
VSiVC: No VSiVC0 signals were detectable in the dark, because VSiVC2– are most stable according toa theoretical prediction that the ionization level at EC – 1.42-1.50 eV for (-2/-1) [14]. The observed
threshold energies in B (1.05-1.10 eV) are insufficient for exciting an electron from VSiVC2– to EC.
Moreover, these energies are quite close to 1.10-1.14 eV of the sharp PL lines corresponding to P6/7
[10]. Thus, the observed thresholds will correspond to an intra-vacancy transition between lower andupper levels of VSiVC in the band gap. The observed threshold in C (1.20 eV) is slightly larger, and we
do not understand the reason yet.

412

Silicon Carbide and Related Materials 2007defectVSiVC

CSiVC

VSiVC

charge-3-2-1-2-10+1-2-10+1-2-10

electronspin (S)a)
1/2
1
3/2
0
1/2
0
1/20
1/2
0
1/2
0
1/2
1EPR center
Not identified
Not identified
VSi– [1]TV2a [2]–HEI1 [4]–EI5/6 [3]–SI5 [5]–
HEI9/10 [6]–Not identified
P6 [7]
P7 [7]

sample A

observed threshold energy (eV)sample B sample C
previous works

1.10
1.10×
1.05××1.09b) [12]

1.05

1.00

1.05

1.08 [4]

2.00

2.05

2.051.80c) [9], 1.8 [4]

1.05

1.00

1.00

1.1 [5,8]

×××~1.9d) [6]

××

1.05
1.10

1.20×1.10b) [10]1.11, 1.14b) [10]Table I. Summary of photo-EPR study and comparison with previous works. a) cited from a theory[13] and experiments [1-7]. b) measured by both PL and EPR; c) measured with respect to valenceband edge (EV); d) deduced relative to VC+.
References[1] T. Wimbauer, B. K. Meyer, A. Hofstaetter, A. Scharmann, H. Overhof, Phys. Rev. B Vol. 56(1997), p.7384.
[2] N. Mizuochi, S. Yamasaki, H. Takizawa, N. Morishita, T. Ohshima, H. Itoh, T. Umeda, J. Isoya,
Phys. Rev. B Vol. 72 (2005), p.235208.
[3] T. Umeda, J. Isoya, N. Morishita, T. Ohshima, T. Kamiya, A. Gali, P. Deák, N. T. Son, E. Janzén,
Phys. Rev. B Vol. 70 (2004), p.235212.
[4] T. Umeda, Y. Ishitsuka, J. Isoya, N. T. Son, E. Janzén, N. Morishita, T. Ohshima, H. Itoh, A. Gali,
Phys. Rev. B Vol. 71 (2005), p.193202.
[5] T. Umeda, N. T. Son, J. Isoya, E. Janzén, T. Ohshima, N. Morishita, H. Itoh, A. Gali, M.
Bockstedte, Phys. Rev. Lett. Vol. 96 (2006), p.145501.
[6] T. Umeda, J. Isoya, T. Ohshima, N. Morishita, H. Itoh, A. Gali, Phys. Rev. B Vol. 75 (2007),p.245202.
[7] N. T. Son, P. Carlsson, J. ul Hassan, E. Janzén, T. Umeda, J. Isoya, A. Gali, M. Bockstedte, N.
Morishita, T. Ohshima, H. Itoh, Phys. Rev. Lett. Vol. 96 (2006), p.055501.
[8] N. T. Son, P. Carlsson, J. ul Hassan, B. Magnusson, E. Janzén, Phys. Rev. B Vol. 75 (2007),p.155204.
[9] N. T. Son, B. Magnusson, E. Janzén, Appl. Phys. Lett. Vol. 81 (2002), p.3945.
[10] W. E. Carlos, E. R. Glaser, B. V. Shanabrook, Physica B Vol. 340-342 (2003), p.151.
[11] M. Bockstedte, A. Mattausch, O. Pankratov, Phys. Rev. B Vol. 68 (2003), p.205201.
[12] E. Sörman, N. T. Son, W. M. Chen, O. Kordina, C. Hallin, E. Janzén, Phys. Rev. B Vol. 61(2000), p.2613.
[13] A. Zywietz, J. Furthmüller, F. Bechstedte, Phys. Rev. B Vol. 59, (1999), p.15166.
[14] L. Torpo, T. E. M. Staab, R. M. Nieminen, Phys. Rev. B Vol. 65 (2002), p.085202.