EPR and pulsed ENDOR study of EI6 and related defects in 4H-SiC
T. Umeda1, Y. Ishitsuka1, J. Isoya1, N. Morishita2, T. Ohshima2, T. Kamiya2
1

Research Center for Knowledge Communities, University of Tsukuba, Tsukuba 305-8550, Japan2

Japan Atomic Energy Research Institute, Takasaki 370-1292, Japan

Keywords: SiC, Carbon vacancy, Silicon antisite, EPR, ENDOR

Abstract. The EI5 and EI6 centers are typical intrinsic defects in radiation-damaged andsemi-insulating SiC. So far, the origins of EI5 and EI6 have been identified as positively-charged
carbon vacancies (VC+) and silicon antisites (SiC+), respectively. However, our complete set of 29Si
hyperfine (HF) data changes these identifications. Our EPR data clearly show that both centers canbe well described by VC+ but their locations should be different (cubic sites for EI5 and hexagonalsites for EI6), as recently proposed by other groups. It was also found that both defects have similar
high thermal stability over 1000 ºC. In addition to EI5 and EI6, we found a new thermally-stable
center, labeled HEI1, in n-type radiation-damaged 4H-SiC.
Introduction
Electron or neutron irradiation to SiC has been widely studied to explore a variety of intrinsic
defects (vacancies, antisites, etc.) in this material. These radiation experiments were performed atrelatively low temperatures (4 K or room temperature ~ 450 ºC). From a technological point of view,
defects that have high thermal stability are of crucial importance. Also, the origin of high thermal
stability is of great interest to science. Therefore, we investigate thermally-stable defects in SiC bymeans of electron irradiation at a high temperature (850 ºC) and identify their atomic structures byusing electron paramagnetic resonance (EPR). In our study, 4H polytype was selected because of its
importance for electronic-device applications.
In our high-temperature radiation experiments, only selected (i.e., thermally-stable) defects werefound in a specimen. There were at least three types of thermally-stable defects; EI5, EI6, and a newcenter (we label it “HEI1”). Although the identification of EI5 and EI6 had been published [1,2],
more recent EPR experiment [3] and theoretical calculation [4] suggested an alternative model thatboth EI5 and EI6 are VC+ defects but they are located at cubic and hexagonal sites, respectively [4].
The conclusion has not been obtained yet, because of a lack of HF interaction data. Thus, we herereport complete angular dependence of 29Si HF interactions of EI5 and EI6. Then, we show that EI6
is well compatible with a hexagonal VC+ rather than a simple SiC+. In addition, EPR data of HEI1 arebriefly reported.
Experimental
SiC substrates are commercial 4H-SiC(0001) wafers supplied by Nippon steel corporation.
Dopant concentrations are 1×1015 and 1×1017 cm-3 for the p- and n-type wafers, respectively.
Electron irradiation was performed with an electron energy of 3 MeV and a dose of 4×1018 cm-2.
During the irradiation, substrates were located in a high vacuum and were heated at a constanttemperature of 850 ºC. Then, they were measured by X-band EPR and pulsed-EPR spectrometers(Bruker E500 and E580 systems, respectively). Pulsed-EPR was used to deconvolute HF structuresby means of pulsed electron-nuclear-double-resonance (ENDOR) techniques.

Results and Discussions
EI5 and EI6 in p-type 4H-SiC. Figure 1(b) shows EPR spectrum of a p-type sample measured for B(magnetic field) || c. As is indicated in the figure, EPR spectrum consists mainly of central lines of
EI5 and EI6, and many HF satellites (labeled a to g). Before starting the analyses of this spectrum, weshould mention that EI5 (VC+) was stable over 1000 °C, although its anneal-out temperature wasknown to be much lower (450 ~ 600 °C [1,2,4]) than that of EI6 (> 1000 °C [1]). In our anneal study,
both EI5 and EI6 were fully stable at 1000 °C, started to be annihilated at 1200 °C, and almost
disappeared at 1500 °C. Thus, we have to change our mind with respect to the thermal stability ofEI5.
Figure 1(a) shows an angular-dependence
map when B was rotated from the c axis ([0001]) to the_c-normal direction ([1120]). The HF satellites a, b and c were already reported by Son et al. [1,2].

Angle [deg]90(a)[0001], c axis(e)60Si3Si4Si230

Si10Si1(EI5)

ab11

EPR intensity [a.u.]Si1(EI6)

Si2 Si2 Si5
Si3 Si3 Si6 g
Si4 Si4 Si7(EI5) (EI6) fcd e(b)33

Si5 Si2
Si6 Si3
Si7 Si4(EI6)de3

33

center
EI5+EI6Si2Si3Si4(EI5)c

Si1(EI5)

Si1(EI6)

ba1

13(f)
Si3
Si4

Si2fSi1g330

331

332

333

334

335

336

337

338

339

340

341

342343Magnetic Field [mT]

Si6Si7Si5__dede(c)[1120]
ENDOR intensity [a.u.]

EPR intensity [a.u.](EI6)
satelite a29Si29Si__[1100](EI6)
satellite e13(d)C29Si343344

Magnetic field [mT]

0510152025

RF [MHz]Fig. 1. EPR measuremts of electron-irradiatedp-type 4H-SiC at 150 K. (a) Angular_dependence for B rotation in the (1100) plane. (b) EPR spectrum for B || c. For HF satellites,
their relative intensities and origins are given. (c) High-field-side HF satellite a (EI6) shows
weak HF satellites d and e. (d) Pulsed-ENDOR spectrum for satellite e (EI6) measured at 10 K,
B || c. This spectrum ensures that satellite e originates from a 29Si HF interaction. (e) Atomicmodels for EI5 (cubic carbon vacancy) and (f) for EI6 (hexagonal carbon vacancy).

According to the literatures, satellite a is a doublet 29Si HF structure of EI6 [1], and satellites b and care 29Si HF structures of EI5 [2]. Also Son et al. [2] have shown that satellite b arises from a Si1 siteof VC [see Fig. 1(e)] and satellite c originates from Si2, Si3, and Si4 sites. Our angular-dependence
data for EI5 were consistent with those of Son et al. Therefore, the identification of EI5 isunchanged, and in this paper, our analyses will be focused on the EI6 center.
For other HF satellites (d to g), their angular dependence has not been reported yet. Since thecentral lines of EI5 and EI6 completely overlapped each other, a simple judgment on the center
position of HF satellites was meaningless. Thus, to classify HF satellites d to g into either EI5 or EI6,
we measured weak HF structures of satellite a (EI6), as shown in Fig. 1(c). Satellite a clearly showed
two HF satellites, and their splitting widths coincided exactly with those of d and e. We therefore
concluded that satellites d and e originate from EI6. Also a similar analysis has been performed forsatellite c (EI5). Then, it was found that satellites f and g belong to EI5.
Relative intensities of HF satellites are estimated in Fig. 1(b); for EI5, b : c = 1 : 3; for EI6, a : d :e = 1 : 3 : 3. These values were consistent with those revealed by the recent 240-GHz EPR study ofKonovalov et al. [3]. These intensities give us an information about the number of 29Si (naturalabundance = 4.7 %) or 13C (1.1%) atoms in a defect center. We already know that satellite acorresponds to a single 29Si atom (namely, the fractional intensity of this satellite is close to 4.7 % ofa total intensity of EI6) [1,3]. Therefore, satellites d and e should originate from three 29Si atoms inthe EI6 center. Furthermore, we carried out pulsed-EDNOR measurements on satellite e, andconfirmed that this satellite is a 29Si HF structure [Fig. 1(d)]. Consequently, all of satellites a, d, ande are assigned to 29Si HF satellites.
In the next place, angular-dependence patterns are discussed. The angular pattern of satellite d(EI6) is very similar to that of c (EI5). This means that satellite d originates from Si2, Si3, and Si4sites [Fig. 1(f)], as similarly to the case of EI5. We also found a close similarity of the patterns
between satellites a, e (EI6) and b (EI5). These satellites commonly showed an axially-symmetric29
Si HF interaction around the c axis. In the original model of a simple SiC+, the origins of satellites aand e were ascribed as a SiC site and a Si1 site, respectively [1]. However, this assignment isincompatible with our and Konovalov’s data that satellite e is related to three Si sites. To explain ourresults reasonably, a hexagonal VC+ model, proposed by a theoretical calculation [4], is quite
appropriate. Namely, we consider that satellite a arises from a Si1 site and satellite e does from Si5,Si6, and Si7 sites. The theoretical calculation predicted that 29Si HF interactions of SiC+ are muchsmaller than the observed HF interactions, but those of hexagonal VC+ are relatively closer to theobserved values of EI6 [4]. Moreover, the fact that EI5 and EI6 have a similar thermal stability
supports the present identification.
Accordingly, we conclude that both EI5 and EI6 are VC+ defects and their locations are cubic andhexagonal site, respectively.
HEI1 in n-type 4H-SiC. In n-type radiation-damaged samples, EPR spectrum [Fig. 2(b)] was more
complicated, as compared to p-type samples. However, we can clearly detect a new signal (HEI1, S =1/2), in addition to the EI5 signal. Also very weak EI6 signal was present [see Fig. 2(a)]. The HEI1
signal was detected in the temperature range between 100 and 150 K. At lower temperatures, the EI6
signal became strong, instead of HEI1. The g values of HEI1 were g|| = 2.00399 and g⊥ = 2.00380 at
150 K. The trend of g anisotropy (g|| > g⊥) was opposite of those for EI5 and EI6 (g|| < g⊥), whichsuggests that the HEI1 center corresponds to a negatively-charged state. The HEI1 signal has a HF
satellite, labeled a [Fig. 2(b)]. This HF satellite showed axially symmetry around the c axis and its
intensity was approximately 4.7 % of a total HEI1 signal. Thus, this was quite similar to the Si1 HF
satellites of EI5 and EI6. Based on these experimental data as well as an analogy with EI5 and EI6,
we tentatively speculated that the HEI1 center is a sort of negatively-charged carbon vacancies.
Satellite a might be due to a Si1 site of negative carbon vacancy. The HEI1 signal also showed other
HF satellites b to e, which is evident in Fig. 2(c). The HF splitting widths of b to e were much smallerthan those of Si2 to Si6 of EI5 and EI6. This suggests that the wave function of HEI1 is moredelocalized than that of EI5 and EI6, probably because an unpaired electron would occupy theanti-bonding state of VC. The angular dependence of satellites b to e has not been resolved yet,because of their too small splitting widths. The conclusive identification of HEI1 will be achieved
after the examination of their full angular dependences.

(EI6)
Si1(EI6)
Si160(a)30

EPR intensity [a.u.]0(EI5)
Si1(HEI1)
Si1?

(EI5)
Si2
Si3
Si4

EI5+EI6
centera(EI5)Si2Si3Si4(EI5)
Si1(HEI1)
Si1?

(HEI1)
satellite a(c)EPR intensity [a.u.]Angle [deg]90ed cba341
HEI1center(b)331332b dc e333334335336

342
Magnetic Field [mT]

337338339

340

341342Magnetic Field [mT]Fig. 2. EPR measurement of electron-irradiatedn-type 4H-SiC at 150 K. (a) Angular_dependence for B rotation in the (1120) plane. (b) EPR spectrum for B || c. (c) High-field-side
HF satellite a of HEI1.

Summary
We have studied thermally-stable intrinsic defects in SiC by means of EPR/ENDOR andhigh-temperature (850 ºC) electron-radiation experiments. Typical residual defects were EI5, EI6,and HEI1 (new defect). Although the EI5 and EI6 centers were originally identified as VC+ and SiC+,
respectively, our complete EPR data demonstrated that both EI5 and EI6 are VC+ defects but EI5 islocated at cubic sites and EI6 at hexagonal sites. It was also notable that these carbon vacancies were
thermally stable over 1000 ºC, which is much higher than the anneal-out temperatures known so far(≤ 600 ºC). We also showed EPR data of HEI1 and discussed its origin tentatively.
References[1] N. T. Son, P. N. Hai, and E. Janzén, Phys. Rev. Lett. 87, 045502 (2001).
[2] N. T. Son, P. N. Hai, and E. Janzén, Phys. Rev. B 63, 201201(R) (2001).
[3] V. V. Konovalov, M. E. Zvanut, and J. van Tol, Phys. Rev. B 68, 012102 (2003).
[4] M. Bockstedte, M. Heid, and O. Pankratov, Phys. Rev. B 67, 193102 (2003).
[5] C. C. Ling, C. D. Beling, and S. Fung, Phys. Rev. B 62, 8016 (2000).