Materials Science Forum Vols. 600-603 (2009) pp 279-284
online at http://www.scientific.net© (2009) Trans Tech Publications, Switzerland

EPR Identification of Defects and Impurities in SiC: To be decisive
J. Isoya1,a, T. Umeda1, N. Mizuochi1, N.T. Son2, E. Janzén2 and T. Ohshima3
1)

University of Tsukuba,Tsukuba 305-8550, Japan, 2)Department of Physics, Chemistry and Biology,
Linköping University, SE-581 83 Linköping, Sweden
3)
Japan Atomic Energy Agency, Takasaki 370-1292, Japanaisoya@slis.tsukuba.ac.jp

Keywords: SiC, EPR, ENDOR, defects, impurities

Abstract. In EPR (electron paramagnetic resonance) identification of point defects, hyperfine (HF)
interaction is decisive information not only for chemical identity but also for the local geometry andthe electronic state. In some intrinsic defects in SiC, the wave function of the unpaired electron
extends quite unevenly among major atoms comprising the defects. In such a case, the determinationof the number of equivalent atoms and the chemical identity (Si or C) of those atoms even with weak
HF splitting are useful to compare with HF parameters obtained theoretically. For vacancy-related
defects of relatively deep levels, the sum of the spin densities on the nearest-neighbor shell is found to
be 60-68%.
Introduction. Among new semiconductor materials with which electronic devices having
performances and functions not feasible with silicon are exploited, SiC is promising for high-power,
high-temperature, and radiation-tolerant devices. Physical properties of SiC crystals are strongly
affected by point defects even with a low concentration. While most of point defects spoil the device
properties, doping of shallow donors and acceptors is essential in fabrication of devices and moreover,semi-insulating (SI) substrates are attained by utilizing intrinsic defects. To solve problems such as
obtaining high-quality substrates and epilayers and improving efficiency of ion implantation doping,understanding of behaviors of point-defects is critically important. In structure determination of point
defects in silicon, the most successful technique proved to be EPR. In SiC of a complexity comingfrom compound semiconductors, EPR identification of point defects is often not straightforward,
however, is likely to be equally successful as in silicon, with careful measurements and with thebenefit of theoretical HF parameters which are nowadays provided by first principles calculations.
Keys to EPR identification. In EPR spectra of point defects in SiC, several sets (i=1,2, ··) of 29Si
and/or 13C hyperfine (HF) lines are expected to appear as satellite lines of the strong primary lines.
The angular dependences of the line positions of i-th hyperfine satellite lines are described by the
spin-Hamiltonian
H = [βeS·g·B + S·D·S ++ S·Aimp·Iimp] + S·Ai·Ii −gnβnIi·B.

(1)

The line positions of the primary lines are determined by g, by D in the case of S≥1, and by Aimp in thecase of impurities with magnetic nuclei such as 14N (I=1, 99.6%) and 31P (I=1/2, 100%). The low
abundances of the isotopes with non-zero nuclear spin (29Si, I=1/2, 4.7%, 13C, I=1/2, 1.1%), limiting
severely the intensity of the HF satellite lines, bring a good resolution with the narrow line width(∆Bpp∼0.03mT) for both primary and satellite lines. For impurities, while chemical identities are
obtained by HF of the primary lines (Aimp), the 29Si/13C satellite lines are useful in determining thesublattice (Si or C) occupied [1, 2]. In the case of intrinsic defects, the primary lines are useful in
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Silicon Carbide and Related Materials 2007

distinguishing EPR spectra from different defects. Until microscopic structure model is established,
EPR spectra are referred tentatively by using labels (n=1,2,3, ···) like SIn, EIn, HEIn for those whichwere found in SI substrates, in electron-irradiated samples, and after electron irradiation at hightemperatures, respectively, and like Pn, Tn, ·· by the initials indicating the research group. For
building up the structure model of intrinsic defects, HF satellite lines are most informative. From the
HF tensor Ai determined from the angular dependence of the 29Si/13C HF lines of the i-th Si/C atomcomprising a point defect, the orbital parameters of the i-th atom such as the spin density ηi2, the sp
hybrid ratio βi2/αi2, and the direction of the pi-orbital with respect to the crystal axes are obtained. Here,
we use a LCAO description for the wave function of the unpaired electron
Ψ= Σi ηi ( αiϕsi + βiϕpi),(2)where αi2+βi2=1 andϕsi, ϕpi are 2s, 2p for i-th carbon or 3s, 3p for i-th silicon. The tensor Ai is uniaxialwith the principal values A// (Aiso+2b) and A⊥ (Aiso−b) or is nearly uniaxial. From the isotropic part Aiso
and the anisotropic part b, the orbital parameters are estimated (ηi2αi2=Aiso/A0, ηi2βi2=b/b0), where weuse (134.77mT, 3.83mT) and (-163.93mT, -4.075mT) for (A0, b0) of 13C and that of 29Si, respectively[3]. For atoms with a small spin density, dipolar field from the spin density on other atom(s) might bealso taken into account as a source of the anisotropic part of HF.
In structure determination of an intrinsic defect, obtaining the orbital parameters from all major
atoms comprising the defect such as four nearest-neighbors in a vacancy and six nearest-neighbors in
a divacancy is crucially important. As in the cases of divacancy [VSiVC]0, the unpaired electron might
be localized mostly on a part of the major component atoms [4]. For some of the major-component
atoms which exhibit weak HF splitting, although only the number of equivalent atoms and ηi2αi2 are
obtained, these information is still important to compare with the HF parameters estimated fromtheoretical studies. To extract weak HF interaction or small anisotropy of HF interaction which ishidden underneath the line broadening of conventional cw(continuous wave)-EPR, we apply pulsed
ENDOR (electron nuclear double resonance) and ESEEM (electron spin echo envelope modulation)[1, 2,5]. These methods obtaining the ENDOR frequencies (vN=|MS·Aeff/2−gNβNB|) are also useful toidentify the nucleus (29Si or 13C) giving rise to the resolved HF satellite lines [6]. After identifying thenucleus (29Si or 13C), the number of equivalent atoms is determined for each of HF satellite lines fromthe intensity relative to the primary line.
Spin multiplicity. The determination of the spin multiplicity (S) is important to determine thecharge state. For S≥1, zero-field splitting (D) splits the primary lines into 2S lines. However, for VSi−
with the absence of the zero-field splitting, S=3/2 was determined by ENDOR [7]. The spin
multiplicity (S=3/2) of TV2a was determined by nutation method of pulsed technique [8] and by pulsed
ENDOR [9]. Thus, TV2a belongs to a family of VSi−.
Separation of overlapping spectra. We use both cw-EPR and pulsed EPR. To enhance spectral
resolution, one way is to increase the difference in the field positions of the primary lines withdifferent g values by utilizing high frequencies such as Q-band (35GHz) and W-band (240GHz).
3.0

(a)
2.5

echo intensity

Fig.1 Echo-detected EPR of(a)3C-SiC:N, 20K (b)4H-SiC:N(h),10K and (c) 4H-SiC:N(k), 40K(B//[0001] for 4H-SiC).
Resolution compatible to that ofcw-EPR is obtainable. Moreover, by
utilizing the difference in T1, N(h)and N(k) spectra are separately
observed.

(b)

0.5

(c)

2.0

6

0
1
x

1.5
1.0

0.0
3400
340

3420
342

3440
344

3460
346
(mT)

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281

EI5
EI6
8

3.0

(a)

6
6

(a)

2.5
2.0
6

0
1
x 4

(b)

(b)

0
1
x 1.5

1.0
2

(c)

(c)

0.5

0

0.0
3425

3430

343

3435

3440

344

3445

3450

345

3455

(mT)

3350
335

3400
340

3450
345

3500
350
(mT)

Fig. 2 Separation of EI5/EI6 spectra by echo-detected EPR. Primary lines (left panel) and 29Si HF
lines (right panel) are shown, respectively. (a) 10K, τ=1µs, repetition delay (r.d.)=8ms, (b) 10K,
τ=20µs, r.d.=8ms, (c) 5K, τ=1.2µs, r.d.=1ms.
However, sometimes, HF lines overlap with primary lines of other centers in the high frequency
measurements. For such a case, X-band frequency (10GHz) gives better angular-dependence data.
In our X-band pulsed EPR measurements, a field-swept spectrum (echo-detected EPR) is obtained bymeasuring the intensity of 2-pulse echo (90°−τ−180°−τ−echo) as a function of the magnetic field. In
echo-detected EPR, while using a strong microwave pulse (~1mT), the resolution similar to cw-EPR
is obtainable by using a selective detection (See, Figs.1, 2). In 4H-SiC, VC(k)+ undergoes thermally
activated reorientation among symmetry-lowered configurations of C1h symmetry [5,6]. In the lowtemperature spectrum of VC(k)+, both phase memory time TM (the time constant of 2-pulse echo
decay) and the spin-lattice relaxation time T1 (the time required for Mz to recover to thermal
equilibrium value) is much shorter than those of VC(h)+. Fig. 2a shows the standard spectra using theecho-detected EPR. With using a large τ, echo-detected EPR spectrum with VC(k)+ suppressedconsiderably is obtained (Fig.2b). With using a short delay in repeating the pulse sequence inaccumulation, echo-detected EPR spectrum with VC(h)+ suppressed is obtained (Fig.2c).
Production and thermal behavior. The production and thermal behaviors useful in
identification of defects are monitored by strong primary lines by which defects are distinguished. To
track the angular dependence of the 29Si/13C HF lines which does lead to decisive structure
determination, it is desirable to produce the intrinsic defect of interest selectively at sufficientconcentration. For intrinsic defects, we took a sort of defect engineering approach by using various
combinations of the temperature (40-800°C) of 3MeV electron irradiation, the total dose
(1×1017-1×1019 e/cm2), the heat-treatment (400-1200°C) after irradiation, and 4H-SiC samples(3×10×1.5mm) of both n- and p-type of different dopant concentrations.
Symmetry. In 4H-SiC, point defects preserving the C3v symmetry of the lattice has one
magnetically distinguishable site with both g and D (S≥1) having axial symmetry. The line positionsof primary lines depend on the angle between B and the c-axis. When symmetry is lowered to C1h, thepresence of symmetry-related sites which have same structure except different orientations splits theprimary lines upon rotating from the c-axis. Utilizing the orientational degeneracy ofsymmetry-related sites, EPR parameters for C1h symmetry are obtained by a rotation of the crystal
about one axis ([11-20] or [1-100]), as in the case of C3v symmetry. In VSi−and Tv2a which have theC3V symmetry, among the four nearest-neighbor carbons, one located along the c-axis has C3v
symmetry while the 13C hyperfine lines of basal carbon atoms split into four lines (1:1:2:2) in therotation around the [11-20] axis (There are two orientations of tetrahedron of four carbon atoms)[8,10]. From the pattern of the angular dependence and the number of equivalent sites at special
directions, the symmetry is determined.

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Hexagonal(h) and cubic(k) sites. SiC is unique in abundant polytypes. For intrinsic defects, wemostly work on 4H-SiC, in which both silicon and carbon atoms have two inequivalent lattice sites,
hexagonal (h) and quasi-cubic (k) sites. It is desirable to list up a set of EPR spectra originating fromthe two inequivalent sites. Defects corresponding to the two different sites are expected to havedifferent configurations which are distinguishable by EPR, however, to have a similar production andannealing behaviors. Observation of four configurations (hh, kk, hk, kh) is a basis of both assignments,
P6(C3v, S=1) / P7(C1h, S=1) to divacancy [VSiVC]0 [4] and HEI9 / HEI10 to positively charged carbon
antisite-vacancy pair CSiVC+ [11]. With the development of first principles calculations, HF
interactions are used to examine a defect model. Calculated HF interactions are helpful to assign thesite (h or k) [1]. A remarkable difference in the Jahn-Teller distortion between VC+(h) and VC+(k) was
first pointed out by theoretical studies [12,13] and EI6/EI5 spectra correspond to VC+(h)/VC+(k) [5,6].

80

60
Angle (deg)40

Fig.3. Angular dependence of29
Si hyperfine lines of VC+(k) at
5K by echo-detected EPR(B⊥[[1-100]). Note that
disturbing signals from othercenters are considerably
suppressed.

20
500
400
300
200
100
0

0

3400
340

3420
3440
3460
344
Magnetic field (Gauss)VSi− (h)[VSi VC]0(hh)VC+(h)[CSi VC+ ](hh)（ ）3480mT
348

VC+(k)

VC − (h)Fig.4 Schematic view of the distribution of spin densities among the nearest-neighbors ofvacancy-related defects in 4H-SiC

Materials Science Forum Vols. 600-603

283

Table I. Spin densities (η2shell-I: sum of ηi2 of nearest-neighbors), sp-hybrid ratio (βi2/αi2), and angle(θi) between the direction of pi-orbital and the c-axis determined by EPR for vacancy-related
centers in 4H-SiC
EPR center anddefect model
EI6
VC+(h)EI5
VC+(k)

symmetryof center
150K
C3v
5K
C3v
150K
C3v
5K
C1h

P6b
[VSiVC]0(hh)

77 K
C3v

HEI1
VC−(h)60K
C1h

VSi−(I)
VSi−(h)VSi− (II)VSi−(k)TV2adistorted VSi−HEI9a
CSiVC+(hh)SI5CSiVC−

295 K
C3v

160K
C3v
295 K
C3v
30K
C1h

atoms

ηi2

η2shell-I

βi2/αi2

θi

Ref.

SiI(1)
SiI(2, 3, 4)
SiI(1 )
SiI(2, 3, 4)
SiI(1)
SiI(2, 3, 4)
SiI(1)
SiI(2)
SiI(3, 4)
CI(2, 3, 4)
SiI(2, 3, 4)
SiII(1, 2, 3)
SiII(4-9)
SiI(1)
SiI(2)

(%)
33.4
9.3
47.3
6.9
19.6
15.1
19.9
14.7
16.5
20.0
0.07
0.26
0.20
24.1
36.6

(%)
61.3

5.3
5.5
5.3
5.9
5.3
5.0
5.4
5.4
5.1
9.7

0°
102.6°
0°97.7°
0°109.2°
7.7°
121.5°
103.2°
117°

[5]

60.7

3.6
3.6

7°
101°

[14]

CI(1)
CI(2, 3, 4)
CI(1)
CI(2, 3, 4)
CI(1)
CI(2, 3, 4)
CSi

15.9
15.9
16.1
16.2
15.4
15.6
55.2

63.6

0°
110.0°
0°109.2°
0°107.5°
0°

[10]

―

11.3
11.2
12.7
11.9
10.7
12.2
16.4

[11]

Si1,2

30.3

―

3.5

111°

[15]

68.0
64.9
67.6

60.2

64.7
62.2

[6]
[5]
[6]

[4]

[8]

In VSi− (S=3/2), the isotropic g, the isotropic 29Si HF interaction of next-nearest-neighbors and theabsence of the zero field splitting were ascribed to local tetrahedral arrangement. The angulardependence of the 13C HF lines of the nearest-neighbors revealed the C3v symmetry and resolved two
spectra VSi−(I) and VSi−(II) arising from the inequivalent lattice sites [10]. The difference in the spin
density between the axial and basal carbon atoms and the deviation of the 2p-orbital direction of thebasal carbon from the tetrahedral angle were detected. Assignment, VSi−(I)/VSi−(II) to VSi−(h)/VSi−(k),is based on the signal intensity ratio of the corresponding centers in 6H-SiC.
Comparison among vacancy-related defects.
The orbital parameters obtained for vacancy-related centers in 4H-SiC are listed in Table I (SeeFig.4). We note that the sum of the spin densities of the nearest-neighbors η2shell−I is 60−68%,
regardless of vacancy site (Si/C or h/k). The sp hybrid ratio (βi2/αi2) varies from 3.5 to 16.4. The larger
value of βi2/αi2 is likely to be caused by dominant breathing relaxation. It should be noted that(η2shell−I, βi2/αi2) are estimated to be (83%, 6.9), (82%, 9.0), (55%, 2.5) and (58%, 5.5), respectively,
for V−(Td, S=3/2), [VV]0 ( C2h, S=1) in diamond [16, 17] and V− (C2v, S=1/2), V+ (D2d, S=1/2) insilicon [18, 19].

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Silicon Carbide and Related Materials 2007

Acknowledgements.
Here we review our works from the experimentalist side. Our identification of point defects has beenachieved with collaboration with theoretical groups, A. Gali (Budapest University of Technology andEconomics) and M. Bockstedte (Universität Erlangen-Nürunberg).
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