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Multivacancy clusters in silicon carbide
ARTICLE IN PRESS
Physica B 340–342 (2003) 128–131
Multivacancy clusters in silicon carbide I.V. Ilyina, M.V. Muzafarovaa, E.N. Mokhova, P.G. Baranova,*, S.B. Orlinskiib, J. Schmidtb a
A.F.Ioffe Physico-Technical Institute, Russian Academy of Sciences, St. Petersburg 194021, Russia b Huygens laboratory, Leiden University, Leiden, The Netherlands
Abstract Five new centers with the electron spins S ¼ 12 and 1 and axial symmetry along c-axis were shown by EPR to arise in SiC after heavy neutron irradiation and following high-temperature annealing at 1500 C. The striking feature of all the centers discovered was a strong hyperfine interaction with a great number of equivalent host Si (C) atoms in SiC lattice. It can be assumed that these defects have high-symmetry structure which suggests a high stability and consists in multivacancy clusters. In addition, antisite Si (C) atoms could be included. There is a good probability that some of the new centers could be related to famous D1 center. r 2003 Elsevier B.V. All rights reserved. PACS: 61.72.Ji; 61.80.Fe; 61.82.Fk; 76.30.v Keywords: Silicon carbide; EPR; Vacancies; Clusters
The ionic implantation and neutron transmutation produce unwanted damages in the SiC lattice that are difficult to repair. Some of the defects can be removed by thermal annealing but others persist even at very high temperatures and these defects can be detrimental to device performance. The D1 center is the best known example of the latter type of defect in SiC which persists even at 1500 C [1]. Therefore the investigation of the radiation defects in SiC is of great importance also in view of the potential application of SiC-based devices for operation in radiation surroundings. EPR is known [2] to be important experimental tool for the identification and study of the defects *Corresponding author. Fax: +7-812-247-1017. E-mail address: [email protected] (P.G. Baranov).
in semiconductors. Transmission electron microscopy [3] and positron annihilation spectroscopy studies [4] show that a wide range of clusters and vacancy aggregates are created by damage-inducing treatments in SiC. As far as we know, EPR was not used for investigation of the aggregate centers in SiC. Five new centers having the electron spins S ¼ 12 and 1 have been discovered by EPR in the Lelygrown 6H-SiC crystal after heavy neutron irradiation with dose of 1021 cm2 and following hightemperature annealing at 1500 C. All the centers have the axial symmetry along the c-axis. Along with a very high-temperature stability, the striking feature of all the centers discovered was a strong well-resolved hyperfine (hf) interaction of unpaired electron with a great number (up to 12) of equivalent host atoms in SiC lattice. Such results
0921-4526/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2003.09.053
ARTICLE IN PRESS I.V. Ilyin et al. / Physica B 340–342 (2003) 128–131
6H-SiC X-band B||c
9Si mvc-1a x7 300 K
EPR intensity (orb. un.)
x7
exp. sim. * mvc-1b 60 K
9Si
327
328 329 330 Magnetic field (mT)
Fig. 1. The experimental and simulated (dashed line) EPR spectra of mvc-1a and mvc-1b centers. The signal of quartz is indicated by star in all figures.
129
give ground to believe that these EPR spectra belong to high-temperature stable multi-vacancy clusters (mvc’s) which are very stable, for instance, in Si. This finding provides the basis for labeling of all the new centers as ‘‘mvc’’. Fig. 1 shows the EPR spectra observed in 6HSiC. Two different spectra have been observed at low and at high temperatures. The spectrum observed at 300 K was labeled as mvc-1a and the spectrum observed at 65 K was labeled as mvc-1b. In both spectra, there is a strong central line plus weaker satellites symmetrically disposed on either side. The mvc-1a and mvc-1b seem to be high and low-temperature states of the same defect. Upon temperature lowering, the mvc-1a center disappears at about 100 K and an mvc-1a signal arises, which has maximum at 65 K. The simulated spectra are indicated by dashed line. The hf structure parameters presented in Table 1 were used for simulation. High-frequency EPR experiments at W-band (95 GHz) reveal a very small g-factor anisotropy for mvc-1a center, gjj g> D8 105 : Fig. 2 shows the angular dependence of the EPR spectra observed for mvc-1b center. The external magnetic field is rotated in ð112% 0Þ plane. The
Table 1 Parameters of the spin Hamiltonian for mvc’s Label
Spin D
Temp. (K)
g-Factor
A(29Si), A(13C) mT
mvc-1a
1/2
300 (200–300)
g ¼ 2:0048
mvc-1b
1/2
60 (40–100)
g ¼ 2:0047
mvc-2
1 180.2 (104 cm1) 1 276.3 (104 cm1)
65 (25–300)
g ¼ 2:002
300 (200–300)
g ¼ 2:002
1/2
20 (10–120)
gjj ¼ 2:0041 g> ¼ 2:0076
Ajj ¼ 0:65 (9Si) Ajj ¼ 2:7? (1C) Ajj ¼ 1:85 (9Si) ðA> Þ0 ¼ 2:0 ðA> Þ00 ¼ 1:6 Ajj ¼ 0:7 (11Si) Ajj ¼ 2:2 (1Si or 4C) Ajj ¼ 2:8 (2C) Ajj ¼ 0:86 (12Si) Ajj ¼ 2:8? (4C or 1Si) Ajj ¼ 0:9 (3Si or 12C)
mvc-3
mvc-4
ðA> Þ0 ¼ 0:75 ðA> Þ00 ¼ 0:38? Ajj and A> means the values measured for Bjjc and B>c; respectively. Temperature range of the signal observation is shown in brackets.
ARTICLE IN PRESS I.V. Ilyin et al. / Physica B 340–342 (2003) 128–131
130
* B || c 15o 25o 35o 40o 50o 60o 75o 90o
326
328 330 Magnetic field (mT)
Fig. 2. The angular dependence of the EPR spectra observed for mvc-1b center. Bars indicate the positions of the lines for Bjjc and arrows indicate the splitting of hf components for B>c:
positions of the lines for the mvc-1b center are indicated for two orientations Bjjc (bars) and B>c (arrows). The signal is characterized by almost isotropic g factor (Table 1). The hf interaction of mvc-1a and mvc-1b centers with nine Si atoms have a small anisotropy about 10% which causes splitting of hf satellites in the orientation B>c (Fig. 2). Two centers with S ¼ 1 labeled as mvc-2 and -3 have been observed. The mvc-2 signal could be observed in the temperature range 20–300 K with maximum intensity at 45 K while mvc-3 signal has maximum intensity at 300 K and could be observed down to 200 K. The parameters of these centers presented in Table 1. Fig. 3 shows the experimental EPR spectra of high-field line of mvc-2 and -3 centers in 6H-SiC for Bjjc. A dashed line indicates the simulated spectra using hf parameters presented in Table 1. Fig. 4 shows the angular dependence of the EPR spectra observed at X-band at 100 K in 6H-SiC under band-gap optical excitation. The EPR signal labeled as mvc-4 and indicated by vertical bars for two orientations Bjjc and B>c arises after irradiation with UV light. The simulation (dashed
EPR intensity (orb. un.)
mvc-1b
EPR intensity (orb. un.)
EPR intensity (orb. un.)
6H-SiC X-band 60 K
exp. sim.
exp. sim.
Fig. 3. Experimental and simulated EPR spectra of high-field line of mvc-2 center and mvc-3.
line) was made for two orientations by using the hf interaction with three equivalent Si atoms (0.9 mT) (or 12 equivalent C atoms), eight Si atoms (0.2 mT) for the orientation Bjjc; and the hf interaction with three equivalent Si atoms (0.75 mT) (or 12 equivalent C atoms), eight Si atoms (0.16 mT) for the orientation B>c: The signal of quartz is indicated by the star. It can be assumed that all the centers discovered have high-symmetry structure which suggests a high stability. The structure, where the vacancies form a part of the tetrahedral VSi3VC is considered as a microscopic model of the mvc-1a and b centers. It is argued that the unusual temperature variation in the hf structure arises from a redistribution of unpaired electron which for low-temperature center (mvc-1b) is mainly
ARTICLE IN PRESS I.V. Ilyin et al. / Physica B 340–342 (2003) 128–131 6H-SiC X-band 100 K UV light
mvc-4 3Si(12C)
00
B *
EPR intensity (orb. un.)
0
15
300 450 600
131
some vacancy ring structure with axial symmetry along the c-axis. In addition, antisites Si (C) atoms could be included. The microscopic model of the photo active mvc-4 center as a tetrahedral VC–4VSi is discussed in the case the hf interaction with 12C atoms. It should be noted that there is a good probability that some of new centers could be related to D1 :
Acknowledgements
750 B⊥c
900 3Si(12C) 329
330 Magnetic field (T)
331
Fig. 4. Angular dependence of the EPR spectra of mvc-4 centers observed under band-gap optical excitation. The simulation (dashed line) was made for orientations Bjjc and B>c:
localized (up to 50%) at nine Si atoms and for high-temperature center (mvc-1a) a part of unpaired electron is localized at C atom. The triplet centers having hf interaction with 11, 12 equivalent Si atoms seem to be symmetrical multi-vacancy clusters where the vacancies create
The authors wish to thank Prof. S.G. Konnikov for useful discussions. This work was supported by RFBR under Grant No. 03-02-17645 and Program of RAS Spin-dependent effects in solids and Spintronics.
References [1] A. Gali, et al., Phys. Rev. B 67 (2003) 155203 and references therein. [2] G. Watkins, Deep centers in semiconductors, in: S.T. Pantelides (Ed.), Gordon and Breach, New York, 1986, p. 147. [3] A.A. Sitnikova, E.N. Mokhov, E.I. Radovanova, Phys. Status. Solidi. A 135 (1993) K45. [4] A.I. Girka, et al., Sov. Phys. JETP (1990) 70; L. Henry, et al., Phys. Rev. B 67 (2003) 115210.
Physica B 340–342 (2003) 128–131
Multivacancy clusters in silicon carbide I.V. Ilyina, M.V. Muzafarovaa, E.N. Mokhova, P.G. Baranova,*, S.B. Orlinskiib, J. Schmidtb a
A.F.Ioffe Physico-Technical Institute, Russian Academy of Sciences, St. Petersburg 194021, Russia b Huygens laboratory, Leiden University, Leiden, The Netherlands
Abstract Five new centers with the electron spins S ¼ 12 and 1 and axial symmetry along c-axis were shown by EPR to arise in SiC after heavy neutron irradiation and following high-temperature annealing at 1500 C. The striking feature of all the centers discovered was a strong hyperfine interaction with a great number of equivalent host Si (C) atoms in SiC lattice. It can be assumed that these defects have high-symmetry structure which suggests a high stability and consists in multivacancy clusters. In addition, antisite Si (C) atoms could be included. There is a good probability that some of the new centers could be related to famous D1 center. r 2003 Elsevier B.V. All rights reserved. PACS: 61.72.Ji; 61.80.Fe; 61.82.Fk; 76.30.v Keywords: Silicon carbide; EPR; Vacancies; Clusters
The ionic implantation and neutron transmutation produce unwanted damages in the SiC lattice that are difficult to repair. Some of the defects can be removed by thermal annealing but others persist even at very high temperatures and these defects can be detrimental to device performance. The D1 center is the best known example of the latter type of defect in SiC which persists even at 1500 C [1]. Therefore the investigation of the radiation defects in SiC is of great importance also in view of the potential application of SiC-based devices for operation in radiation surroundings. EPR is known [2] to be important experimental tool for the identification and study of the defects *Corresponding author. Fax: +7-812-247-1017. E-mail address: [email protected] (P.G. Baranov).
in semiconductors. Transmission electron microscopy [3] and positron annihilation spectroscopy studies [4] show that a wide range of clusters and vacancy aggregates are created by damage-inducing treatments in SiC. As far as we know, EPR was not used for investigation of the aggregate centers in SiC. Five new centers having the electron spins S ¼ 12 and 1 have been discovered by EPR in the Lelygrown 6H-SiC crystal after heavy neutron irradiation with dose of 1021 cm2 and following hightemperature annealing at 1500 C. All the centers have the axial symmetry along the c-axis. Along with a very high-temperature stability, the striking feature of all the centers discovered was a strong well-resolved hyperfine (hf) interaction of unpaired electron with a great number (up to 12) of equivalent host atoms in SiC lattice. Such results
0921-4526/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2003.09.053
ARTICLE IN PRESS I.V. Ilyin et al. / Physica B 340–342 (2003) 128–131
6H-SiC X-band B||c
9Si mvc-1a x7 300 K
EPR intensity (orb. un.)
x7
exp. sim. * mvc-1b 60 K
9Si
327
328 329 330 Magnetic field (mT)
Fig. 1. The experimental and simulated (dashed line) EPR spectra of mvc-1a and mvc-1b centers. The signal of quartz is indicated by star in all figures.
129
give ground to believe that these EPR spectra belong to high-temperature stable multi-vacancy clusters (mvc’s) which are very stable, for instance, in Si. This finding provides the basis for labeling of all the new centers as ‘‘mvc’’. Fig. 1 shows the EPR spectra observed in 6HSiC. Two different spectra have been observed at low and at high temperatures. The spectrum observed at 300 K was labeled as mvc-1a and the spectrum observed at 65 K was labeled as mvc-1b. In both spectra, there is a strong central line plus weaker satellites symmetrically disposed on either side. The mvc-1a and mvc-1b seem to be high and low-temperature states of the same defect. Upon temperature lowering, the mvc-1a center disappears at about 100 K and an mvc-1a signal arises, which has maximum at 65 K. The simulated spectra are indicated by dashed line. The hf structure parameters presented in Table 1 were used for simulation. High-frequency EPR experiments at W-band (95 GHz) reveal a very small g-factor anisotropy for mvc-1a center, gjj g> D8 105 : Fig. 2 shows the angular dependence of the EPR spectra observed for mvc-1b center. The external magnetic field is rotated in ð112% 0Þ plane. The
Table 1 Parameters of the spin Hamiltonian for mvc’s Label
Spin D
Temp. (K)
g-Factor
A(29Si), A(13C) mT
mvc-1a
1/2
300 (200–300)
g ¼ 2:0048
mvc-1b
1/2
60 (40–100)
g ¼ 2:0047
mvc-2
1 180.2 (104 cm1) 1 276.3 (104 cm1)
65 (25–300)
g ¼ 2:002
300 (200–300)
g ¼ 2:002
1/2
20 (10–120)
gjj ¼ 2:0041 g> ¼ 2:0076
Ajj ¼ 0:65 (9Si) Ajj ¼ 2:7? (1C) Ajj ¼ 1:85 (9Si) ðA> Þ0 ¼ 2:0 ðA> Þ00 ¼ 1:6 Ajj ¼ 0:7 (11Si) Ajj ¼ 2:2 (1Si or 4C) Ajj ¼ 2:8 (2C) Ajj ¼ 0:86 (12Si) Ajj ¼ 2:8? (4C or 1Si) Ajj ¼ 0:9 (3Si or 12C)
mvc-3
mvc-4
ðA> Þ0 ¼ 0:75 ðA> Þ00 ¼ 0:38? Ajj and A> means the values measured for Bjjc and B>c; respectively. Temperature range of the signal observation is shown in brackets.
ARTICLE IN PRESS I.V. Ilyin et al. / Physica B 340–342 (2003) 128–131
130
* B || c 15o 25o 35o 40o 50o 60o 75o 90o
326
328 330 Magnetic field (mT)
Fig. 2. The angular dependence of the EPR spectra observed for mvc-1b center. Bars indicate the positions of the lines for Bjjc and arrows indicate the splitting of hf components for B>c:
positions of the lines for the mvc-1b center are indicated for two orientations Bjjc (bars) and B>c (arrows). The signal is characterized by almost isotropic g factor (Table 1). The hf interaction of mvc-1a and mvc-1b centers with nine Si atoms have a small anisotropy about 10% which causes splitting of hf satellites in the orientation B>c (Fig. 2). Two centers with S ¼ 1 labeled as mvc-2 and -3 have been observed. The mvc-2 signal could be observed in the temperature range 20–300 K with maximum intensity at 45 K while mvc-3 signal has maximum intensity at 300 K and could be observed down to 200 K. The parameters of these centers presented in Table 1. Fig. 3 shows the experimental EPR spectra of high-field line of mvc-2 and -3 centers in 6H-SiC for Bjjc. A dashed line indicates the simulated spectra using hf parameters presented in Table 1. Fig. 4 shows the angular dependence of the EPR spectra observed at X-band at 100 K in 6H-SiC under band-gap optical excitation. The EPR signal labeled as mvc-4 and indicated by vertical bars for two orientations Bjjc and B>c arises after irradiation with UV light. The simulation (dashed
EPR intensity (orb. un.)
mvc-1b
EPR intensity (orb. un.)
EPR intensity (orb. un.)
6H-SiC X-band 60 K
exp. sim.
exp. sim.
Fig. 3. Experimental and simulated EPR spectra of high-field line of mvc-2 center and mvc-3.
line) was made for two orientations by using the hf interaction with three equivalent Si atoms (0.9 mT) (or 12 equivalent C atoms), eight Si atoms (0.2 mT) for the orientation Bjjc; and the hf interaction with three equivalent Si atoms (0.75 mT) (or 12 equivalent C atoms), eight Si atoms (0.16 mT) for the orientation B>c: The signal of quartz is indicated by the star. It can be assumed that all the centers discovered have high-symmetry structure which suggests a high stability. The structure, where the vacancies form a part of the tetrahedral VSi3VC is considered as a microscopic model of the mvc-1a and b centers. It is argued that the unusual temperature variation in the hf structure arises from a redistribution of unpaired electron which for low-temperature center (mvc-1b) is mainly
ARTICLE IN PRESS I.V. Ilyin et al. / Physica B 340–342 (2003) 128–131 6H-SiC X-band 100 K UV light
mvc-4 3Si(12C)
00
B *
EPR intensity (orb. un.)
0
15
300 450 600
131
some vacancy ring structure with axial symmetry along the c-axis. In addition, antisites Si (C) atoms could be included. The microscopic model of the photo active mvc-4 center as a tetrahedral VC–4VSi is discussed in the case the hf interaction with 12C atoms. It should be noted that there is a good probability that some of new centers could be related to D1 :
Acknowledgements
750 B⊥c
900 3Si(12C) 329
330 Magnetic field (T)
331
Fig. 4. Angular dependence of the EPR spectra of mvc-4 centers observed under band-gap optical excitation. The simulation (dashed line) was made for orientations Bjjc and B>c:
localized (up to 50%) at nine Si atoms and for high-temperature center (mvc-1a) a part of unpaired electron is localized at C atom. The triplet centers having hf interaction with 11, 12 equivalent Si atoms seem to be symmetrical multi-vacancy clusters where the vacancies create
The authors wish to thank Prof. S.G. Konnikov for useful discussions. This work was supported by RFBR under Grant No. 03-02-17645 and Program of RAS Spin-dependent effects in solids and Spintronics.
References [1] A. Gali, et al., Phys. Rev. B 67 (2003) 155203 and references therein. [2] G. Watkins, Deep centers in semiconductors, in: S.T. Pantelides (Ed.), Gordon and Breach, New York, 1986, p. 147. [3] A.A. Sitnikova, E.N. Mokhov, E.I. Radovanova, Phys. Status. Solidi. A 135 (1993) K45. [4] A.I. Girka, et al., Sov. Phys. JETP (1990) 70; L. Henry, et al., Phys. Rev. B 67 (2003) 115210.