Radiation Effects in Silicon Carbide
S.Kanazawa1, M.Okada2, I.Kimura3
1
Department of Nuclear Engineering, Kyoto University, Yoshida, Sakyo, Kyoto, 606-8501, Japan
2
Research Reactor Institute, Kyoto University, Kumatori, Osaka, 590-0494, Japan
3
Institute of Nuclear Safety System, Inc., Sata, Mihama, Fukui, 919-1205, Japan
Abstract. Radiation effects in p- and n-type 4H-SiC as well as 6H-SiC irradiated with neutrons and electrons have been
investigated by electron spin resonance (ESR), optical absorption and electric property measurements with annealing experiment.
The dopant spectrum observed at LNT before irradiation disappeared after irradiation, while many centers arising from
radiation-induced defects were observed in ESR spectra. The angular dependences and isochronal annealing behaviors of these
centers are also described. On the basis of the above results, we discuss the structural models for these defects. In this study,
irradiation temperature dependence of formation efficiency of defects induced in neutron-irradiated SiC is examined by ESR and
optical absorption measurements. The defect formation efficiencies of silicon vacancy center in n-type SiC increase with the
irradiation temperature ,while those in p-type SiC decrease slightly. Electrical properties of n-type SiC single crystals irradiated
with neutrons have been also investigated by resistivity and Hall effect measurements. In the neutron irradiated SiC, the carrier
mobility rises with increasing of the carrier density.
type SiC single crystals irradiated with neutrons are
investigated by resistivity and Hall effect
measurements.
INTRODUCTION
Single crystalline silicon carbide (SiC) is expected
to be an excellent material by way of electronic
devices acting under severe environment such as
intense ionizing radiation field with high temperature.
For the fabrication of high-power semiconductor
devices, SiC wafers must have excellent homogeneity
in resistivity. The neutron transmutation doping (NTD)
technique[1] can be applicable for producing n-type
SiC wafers as well as n-type Si wafers. During the
NTD process, however, radiation damage in SiC
crystal causes serious problems such as carrier
trapping. The NTD technique requires significant
knowledge on radiation effects and the annealing
behaviors of the effects. Radiation effects in SiC
irradiated with radiation (electron, neutron, proton,)
have been studied by electron spin resonance (ESR) ,
deep level transient spectroscopy (DLTS), photoluminescence (PL), optical absorption and electric
properties measurements. However, the understanding
of radiation effects in SiC is still rather poor.
In this study, radiation effects in p- and n-type 4HSiC as well as 6H-SiC irradiated with neutrons and
higher energy electrons have been examined by ESR,
optical absorption and electric property measurements
with annealing experiments. Irradiation temperature
dependence of formation efficiency of defects induced
in neutron-irradiated SiC is studied by ESR and optical
absorption measurements. Electrical properties of n-
EXPERIMENTAL
The nitrogen-doped n-type SiC single crystals used
in this study were manufactured by Nippon Steel
Corporation. And the p-type SiC samples were
obtained from Cree, Inc. Ohmic contacts were made
on each n-type SiC wafer by evaporating Ni dots
followed by annealing at 1,100˚C for 30 minutes in Ar
atmosphere. The irradiations were performed at the
low-temperature irradiation loop facility (LTL) and the
electron linac (LINAC) of the Kyoto University
Research Reactor Institute (KURRI). The neutron
fluence was monitored by the activation method with
Ni foils. Isochronal annealing was performed at
intervals of 100K in the temperature range between
373K and 1773K for 5 minutes in helium atmosphere.
ESR spectra were measured at room temperature (RT)
and liquid nitrogen temperature (LNT) with an X-band
(9GHz) microwave incident on a TE110 cylindrical
cavity using the JEOL JES-TE200. Optical absorption
spectra were measured at RT and LNT with a
Shimadzu
UV-3100
spectrophotometer.
The
measurements of resistivity and Hall effect in SiC
were carried out at RT by the Van der Pauw method.
CP680, Application of Accelerators in Research and Industry: 17th Int'l. Conference, edited by J. L. Duggan and I. L. Morgan
© 2003 American Institute of Physics 0-7354-0149-7/03/$20.00
881
2000
1000
ESR Signal Intensity iarb. j
ESR Signal Intensity iarb. j
K1
(a)
K1
K1
K1
K1
0
K6
K5
-2000
333
(b)
K1
K2
K3
K3
K4
K4
0
K2
-1000
300
334
Magnetic Field(mT)
350
Magnetic Field(mT)
FIGURE 1. ESR spectra observed at (a) RT and (b) LNT for the neutron-irradiated n-type 6H-SiC.
is larger. On the other hand, the donor level energy, ED,
of the 4H-SiC is smaller than that of the 6H-SiC. From
the above, the experimental fact, whose the decreasing
rate of the nitrogen donor in the 4H-SiC is larger than
that in the 6H-SiC, can be explained well.
RESULTS AND DISCUSSION
(1) Dopant Signal
In the as-grown n-type SiC samples, the signals of
the nitrogen donor are detected in the ESR and the
optical absorption spectra. In the p-type samples, the
accepter signal is also observed in the ESR spectrum
while not in optical one. These carrier signals decrease
with increasing irradiation doses in the irradiated
samples and then the electric resistance increases with
those. The change of the carrier concentration, which
results from irradiation, influences the Fermi-level by
the existence of the radiation-induced defects. It can be
also thought that the change of the Fermi- level
influences a stability of the energy levels of the
irradiation defects, which are introduced into the
forbidden band. Consequently, their electrical
properties make a great change.
The ESR signal of the nitrogen donor, which is
observed in hexagonal SiC, consists of a cubic-site and
a hexagonal-site. In the irradiated samples, the
decreasing rate of the nitrogen donor of the hexagonalsite is larger than that of the cubic-site, and when it is
compared with the 4H-SiC and the 6H-SiC, the former
(2) Defect Formation
Figure 1 shows typical ESR spectra for the neutronirradiated n-type 6H-SiC obtained at (a)RT and b)LNT.
In the ESR signals, six centers named K1, K2, K3, K4,
K5 and K6 or distinct lines are observed at LNT. The
K1, K2,K5 and K6 are also observed at RT, but the K3
and K4 are not observed[2]. The characteristics of the
ESR centers found in neutron irradiated SiC are
summarized in the Table 1.
In reactor-irradiated SiC samples, a broad absorption
band at 780nm is observed [8]. For both the 780nm
absorption band and the ESR K1 center, the annealing
behavior is very similar between 400K and 1200K.
Therefore, the origin of the 780nm band was attributed
to a defect center consisting of single Si vacancy (Vsitype center)[8].
Figure 2 shows the ESR spectra at (a) RT and (b)
LNT for electron(22MeV)-irradiated p-type 4H-SiC
with 1018e/cm2. The ESR signals termed K1(Vsi), K12,
2000
(a)
K1(Vsi)
ESR Signal Intensity iarb. j
ESR Signal Intensity iarb. j
2000
K12
1000
0
-1000
-2000
(b)
330
331
K1(Vsi)+K13
K11
1000
0
-1000
-2000
329
K14
330
Magnetic Field(mT)
Magnetic Field(mT)
FIGURE 2. ESR spectra observed at (a) RT and (b) LNT for 22MeV electron-irradiated p-type 4H-SiC.
882
331
TABLE 1. The characteristics of the ESR centers in the irradiated SiC.
Center Spin:S g-value
Polytype D-value
Average distance Ta
(10-2cm-1)
of two spins(Å) (˚C)
K1
3/2
2.0032
4H,6H
800
K2
1
2.0032
K3
1
2.0013
K4
1
// 2.0050
┴2.0083
// 2.0029
4H
6H
4H
6H
4H
0.232
0.426
4.39
5.58
3.60
10.04
8.49
3.90
3.60
4.16
6H
4.02
4.01
┴ 2.0073
K5
1/2
200
1,100
References
Vsi-
T1(3C)[3],
NA[4]
Vsi-Vsi? Vsi0 [5]
T6[6]
Vsi-I?
EI3[7]
NC[4]
Vsi-Vc?
NB[4]
1,300
// 2.0028
┴ 2.0045
K6
800
Model
1,300
// 2.0042
┴ 2.0042
Ta: annealing temperature.
early stage of the irradiation. The K14 center with an
effective spin S=1/2 exhibits g-values of g//=2.0035,
g┴=2.0051. The intensities of these defects change
when the irradiation advances, and then the intensities
of these ESR centers decrease. It is necessary to
consider the influence on defect generation by
electronic excitation under irradiation.
In the electron-irradiated n-type SiC, the K18
center (g//=2.0028, g┴=2.0031 at RT) exists mainly in
an early stage of irradiation below 2x1017 e/cm2, and
during continued irradiation the K19 (g//=2.0034,
g┴=2.0026), the K20 (g//=2.0032, g┴=2.0034) and the
K21 (g//=2.0028, g┴=2.0025) centers are observed at
LNT. By the progressive increase of the irradiation,
the K15 (g//=2.0022, g┴=2.0015) and the K13 centers
in the irradiated p-type samples are mainly in the early
stage of irradiation (<1017 e/cm2), and then the K1,
K12 and K3 centers are observed in samples irradiated
nearly 6x1017 e/cm2. Continuously, the K4, K2, K5
and K6 centers are produced successively above its
dose. As mentioned above, it can be thought that the
change of the Fermi-level, which is caused by
irradiation, is related with the production of these
various centers.
K11,K1+K13 and K14 are observed[9]. The ESR
spectra for the irradiated p-type 6H-SiC were similar
to that for the irradiated p-type 4H-SiC. The K11
signal is similar to the T5 center in proton-irradiated
3C-SiC[10] and the PB center in electron-irradiated
6H-SiC[11]. By Itoh et al [10], this center was
attributed to the singly positive charged carbon
vacancy. Recently, it has been proposed that the defect
model of the K11 center consists of complex defect
which combines with a carbon vacancy and a
hydrogen atom, (Vc+H)0[7]. However, this model is
not complete because the HF signal due to nuclear spin
in hydrogen is not observed. On the other hand, the
stability of the singly positive charged state of the
carbon vacancy (Vc+) has been examined theoretically
by the first principle calculation. From the calculation,
there are two results as follows; 1) it is stable [12] and
2) unstable [13]. Also, the other workers have been
recently reported that the defect centers (EI5[14] and
Ky2 [15] centers) which are different from the K11
center is the Vc+. As their experimental results show,
there is difference about an angular dependence and
temperature range of a measurement, though it has HF
character where the EI5 and Ky2 centers are compared.
From the above results, it is not clear which is right
more, the Vc+ or (Vc+H)0 as a defect center model of
the K11 center at present.
The K12 center observed at RT appears at lower
irradiation dose. The K12 center with an effective spin
S=1/2 exhibits g-values of g//=2.0021, g┴=2.0032. The
K1(Vsi) + K13 centers, which are overlapped and
observed at LNT, appears remarkably at lower dose.
Moreover, the K14 center observed at LNT is a
radiation-induced defect, which is produced in the
. (3)Irradiation Temperature Dependence
of Defect Formation Efficiency
The ESR signal intensities (it is proportional to the
defect formation efficiency) of the K1 and K11 center
are shown as a function of the irradiation temperature
in Fig. 3 and Fig. 4, respectively. As shown in Fig.3,
the defect formation efficiencies of K1 center in n-type
883
ESR Signal Intensity(arb.)
ESR Signal Intensity(arb.)
1.8
1.6
1.4
1.2
1
0.8
4H -n
6H -n
4H -p
6H -p
6H -n
4H -p
0.6
0.4
0.2
type
type
type
type
type
type
1.8
1.6
4H -p type
1.4
6H -p type
1.2
4H -p type
1
0.8
0.6
0.4
0.2
0
0
0
50
100
150
200
250
300
350
0
400
50
100
150
200
250
300
Irradiation T em perature(K )
Irradiation T em perature(K )
FIGURE 4. ESR signal intensites of K11 center as a
function of irradiation temperature.
FIGURE 3. ESR signal intensities of K1 center as a
function of irradiation temperature.
SiC increase with the irradiation temperature ,while
these in p-type SiC decrease slightly. The similar
result has been obtained by optical absorption
measurement [16]. As seen in Fig.4, the defect
formation efficiencies of the K11 center in p-type SiC
decrease with the irradiation temperature.
REFERENCES
1.
(4) Electrical Properties
2.
As we reported previously, the change rate of the
mobility rises with increasing of the initial carrier
density, while that of the electrical resistivity decreases.
On the other hand, the change rate of the carrier
density does not change in the samples after irradiation
[17]. After neutron irradiation, the carrier mobility
rises with increasing carrier density, though the
mobility before the irradiation is decreased with
increasing of the carrier density.
From these facts, it can be concluded that the
electrical properties of neutron-irradiated n-type SiC
are influenced by the existence of a large concentration
donor [17].
3.
4.
5.
6.
7.
8.
9.
10.
ACKNOWLEDGMENTS
11.
The authors would like to thank Professor
T.Yoshiie and Dr.Q.Xu of KURRI for their assistance
in operation of the low temperature irradiation facility
and Professor K.Kobayashi, Dr. S.Yamamoto and
Mr.K.Takami of KURRI for their assistance in
operation of the LINAC and Professor H.Hase and Dr.
T.Warashina of KURRI for their guidance in the ESR
measurements. This work was done under the Visiting
Researchers Program of the KURRI. The present
work was supported by the Grant-in-Aid for Scientific
Research, No. 12680509, from the Ministry of
Education, Science and Culture of Japan.
12.
13.
14.
15.
16.
17.
884
Tamura,S., Kimoto,T., Matsunami,H., Okada,M.,
Kanazawa,S., Kimura,I., Mater. Sci. Forum 338-342,
849-852(2000).
Kanazawa,S., Kimura,I., Okada,M., Nozaki,T.,
Kanno,I., Ishihara,S., and Watanabe,M., Mater. Sci.
Forum 338-342, 825-828(2000).
Itoh,H., Hayakawa,N., Nashiyama,I., and Sakuma,E.,
J.Appl.Phys. 66,4529-4531(1989).
A.Kawasuso,
H.Itoh,
D.Cha
and
S.Okada:
Mater.Sci.Forum 264-268, 611-614 (1998).
von Bardelben,H.J., Cantin,J.L., and Vicridge,I.,
Battistg,G., Phys. Rev. B62, 10126-10134(2000).
Itoh,I., Hayakawa,N., Nashiyama,I.,et. al., phys. stat. sol.
(a) 162, 173-198(1997).
Son, N.T., Chen, W.M., Lindstrom,J.L., Monemar, B.,
Janzen, E., Mater Sci. Eng. B61-62, 202-206(1999).
Okada,M.,
Kimura,I.,
Nakata,T.,
Watanabe,M.,
Kanazawa,S., Kanno,I., Kamitani,K., Atobe,K., and
Nakagawa,M., Inst.Phys.Conf.Ser.142, 469 -472(1996).
Kanazawa,S., Okada,M., Nozaki,T., Kimura,I.,Kanno,I.,
Ishihara,S., Mater.Sci.Forum 389-393, 521-524(2002).
Itoh,H., Yoshikawa,M., Nashiyama,I., Misawa,S.,
Okumura,H., Yoshida,S., J.Electro.Mater.21, 707-710
(1992).
Cha,D., Itoh,H., Morishita,N., Kawashiso,A., Oshima,T.,
Watanabe,Y., Ko,J., Lee,K., and Nashiyama,I.,
Mater.Sci.Forum 264-268, 615-618 (1998).
Torpo,L., Marlo,M., Staab, T.E.M., and Nieminen,R.M.,
J. Phys. Condens. Matter 13, 6203-6231(2001).
Zywietz,A., Furthmuller,J., and Bechstedt,F., Phys. Rev.
B59, 15166-15180(1999).
Son, N.T., Hai, P.N., Janzen,E., Phys. Rev. B63, 2012011-4(2001).
Bratus,V.Ya., Petrenko,T.T., von Bardelben,H.J.,
Kalinina,E.V., Hallen,H., Appl. Surf. Sci. 184, 229236(2001)
Okada, M., Atobe,K., Nakagawa,M., Kanazawa,S.,
Kanno,I., Kimura,I., Nucl. Instru. Meth. B166-167, 339403(2000).
Kanazawa,S.,
Okada,M.,
Nozaki,T.,
Kimura,I.,
Ishihara,S., Mater.Sci.Forum 389-393, 517-520(2002).