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DLTS study of defects in 6H- and 4H-SiC created by proton irradiation
Physica B 308–310 (2001) 641–644
DLTS study of defects in 6H- and 4H-SiC created by proton irradiation D.V. Davydova,*, A.A. Lebedeva, V.V. Kozlovskib, N.S. Savkinaa, A.M. Strel’chuka a
A.F. Ioffe Physicotechnical Institute, Russian Academy of Sciences, Polytekniheskaya 26, St. Petersburg 194021, Russia b St. Petersburg State Technical University, St. Petersburg 194027, Russia
Abstract Experiments on proton irradiation with energy of 150 KeV, 8 MeV and 1 GeV were made. Capacitance–voltage characteristics measured at 650 K showed that 8 MeV and 1 GeV proton irradiation of 6H-SiC leads to an increase of uncompensated donor concentration. However, donor concentration in 6H-SiC remains unaffected after 150 keV proton irradiation. Deep centers were investigated by deep levels transient spectroscopy (DLTS). Results of C–V measurements are interpreted using DLTS data. The results obtained show the possibility of uzing proton irradiation for producing local high-resistance regions in SiC devices not intended for high-temperature applications. r 2001 Elsevier Science B.V. All rights reserved. Keywords: SiC; Proton irradiation; DLTS
1. Introduction
2. Experiment
There is several reports on formation of SiC layers semi-insulating (SI) at room temperature (RT) by means of proton irradiation [1–3]. However, the parameters and concentrations of radiation defects (RD) formed during irradiation have not been studied. In other works [4,5] RD parameters were studied but the compensation appearing in the course of irradiation was not analyzed. The aim of this work was to fabricate SI 6H- and 4HSiC layers by proton irradiation with different energies, compare their electrical properties, and determine the parameters of RDs responsible for the compensation. The samples were studied with capacitance–voltage (C–V) characteristics and deep levels transient spectroscopy (DLTS) methods.
Silicon carbide p+–n structures and epitaxial layers commercially produced by CREE Research, Inc., or fabricated at the Ioffe Institute by sublimation epitaxy (SE) [6] were used. The n-type layer thickness was about 5 mm; that of the p-type layer: about 1 mm; and substrate thickness: about 400 mm. The diameter of Schottky barriers or mesa structures of the diodes was in the range 600–700 mm. The concentration of uncompensated donors in the n-type layer (Nd 2Na ) was (0.8– 4) 1016 cm3, that in the substrate, (3–5) 1018 cm3, and concentration of acceptors in the p-type layer: B5 1018 cm3. Thus, the doping level in the substrate and p-type emitter exceeded by no less than two orders of magnitude that in the n-type base layer. A set of 6H- and 4H-SiC samples (p–n structure and Schottky diodes) were step by step irradiated with 8 MeV protons. Total irradiation dose (D) was changed from 1 1014 cm2 to 2 1016 cm2. Two sequential 1 GeV proton irradiations with D of 3 1014 and 9 1014 cm2 were made on one 6H-SiC SE sample
*Corresponding author. Tel.: +7-812-247-9930; fax: +7812-247-6425. E-mail address: [email protected]ffe.rssi.ru (D.V. Davydov).
0921-4526/01/$ - see front matter r 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 0 1 ) 0 0 7 7 5 - X
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with Schottky diodes. The epilayer was nonuniformly dopedFNd 2Na varied from 5 1014 to 8 1015 cm3 at the surface and at 7 mm depth, respectively. Furthermore, three 6H-SiC SE samples having initial Nd 2Na value of 4 1016 cm3 were irradiated with 150 keV protons with doses of 1011, 1012, and 1013 cm2. All irradiations were carried out at RT. The C–V characteristics were measured on a bridge C–V setup on a frequency of 10 kHz. A study of 8 MeV irradiated samples demonstrated a decline in the Nd 2Na value measured at RT, with Nd 2Na markedly increasing on heating a structure to 650 K. For 6H-SiC the Nd 2Na value measured at 650 K was even higher than that in the initial structures prior to irradiation. With increasing irradiation dose, this difference became more pronounced (Figs. 1 and 2). Irradiation of lightly doped samples with D of 1016 cm2 led to formation of SI layers with specific resistivity of about 109 O cm at RT. Typical C–V characteristics measured after 150 keV proton irradiation are shown on Fig. 3. At high reverse bias the characteristic is similar to initial one; but irradiation resulted in appearance of flat region at low bias voltages where the capacitance does not depend on voltage, and its value corresponds to depleted region width of B1 mm. We attribute this to formation of a SI layer due to the irradiation. The characteristics measured at 100 K shift right along the voltage axis. Increase of the temperature to about 500 K led to recovery of initial C–V characteristics of the diodes. The shape of the characteristics were the same for all the samples (all the irradiation dose values). First 1 GeV irradiation did not noticeably influence on C–V characteristics. However, after the second irradiation (D ¼ 1:2 1015 cm2) RT capacitance became independent on voltage (the epilayer became SI). C–V characteristic measured at temperature of 500 K after second irradiation was linear (Fig. 4); it revealed
Fig. 1. 6H-SiC: Nd 2Na measured at T ¼ 300 K (1) and T ¼ 650 K (2), difference of them (3), and concentration of the center located at Ec 1:22 eV (4) vs. the irradiation dose.
Fig. 2. 4H-SiC: Nd 2Na measured at T ¼ 300 K (1) and T ¼ 650 K (2), difference of them (3), and total concentration of the centers RD1/2, RD3, and RD4 (4) vs. the irradiation dose.
Fig. 3. C–V characteristics of n-6H-SiC epilayer before irradiation (1) and after 150 keV irradiation with dose of 1013 cm3 measured at RT (2), 500 K (3), and 100 K (4).
Fig. 4. C–V characteristics of unirradiated n-6H-SiC epilayer (1) and one measured at temperature of 500 K after 1 GeV proton irradiation (D ¼ 1:2 1015 cm3) (2).
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uniform doping with Nd 2Na of 7 1015 cm3 which exceeds much the initial one. Six electron traps were found in DLTS spectra of ntype 6H-SiC samples irradiated with 8 MeV protons. Most of them were observed in electron-irradiated samples [7] or had parameters close to intrinsic structural defects. Two of the observed centers (with position in the band gap Ec 0:16 0:2 eV and Ec 0:5 eV) were completely annealed out at 500– 650 K. Sample with p–n junction was annealed at 800 K what resulted in small increase of the center Ec 0:7 eV concentration. DLTS investigation of electron traps in n-type 4H-SiC revealed 5 deep centers. Most of these were found in samples implanted with He+ ions [4] or had parameters close to those of intrinsic defects. One of the observed centers (Ec 0:18 eV) was completely eliminated by annealing at 500–650 K. Tables 1 and 2 list ionization energies of the observed centers, their estimated electron capture cross-sections (sn ) and concentrations after irradiation with a dose of 2 1014 cm2. For both of the polytypes studied, no pronounced difference was observed between the spectrum of deep centers formed in CREE epitaxial layers (CVD) and those grown by SE. The introduced RDs were completely annealed out at p2100 K. In 6H-SiC epilayer irradiated with 1 GeV protons with dose of 3 1014 cm2 DLTS revealed only two deep
electron traps: Ec 0:3520:4 eV (E1 =E2 ) and Ec 21:2 eV (R-center) in concentrations of 1 1013 cm3 and 5 1013 cm3, respectively. The R-center introduction rate was equal to 70 cm1 for 8 MeV, and 0.17 cm1 for 1 GeV protons.
3. Discussion and conclusion As shown by the DLTS study, both 8 MeV and 1 GeV proton irradiation creates in n-type 6H-SiC a defect with the highest introduction rate (Ec 1:22 eV) with parameters close to those of the known structural defectFR center [8]. As follows from the parameters of the center located at Ec 1:22 eV, the recharging time (t) for this center is of about two weeks at 300 K. Thus, the charge state of this and deeper lying centers does not change during C–V measurements at RT. At the same time, the t value for the center which is the closest to this level, (Ec 0:8 eV), is 3.3 s. Hence, the center Ec 0:8 eV (and all shallower centers) can be considered completely ionized in C–V measurements at RT. In n-type 4H-SiC, several deep centers with ionization energy in the range of 0.96–1.5 eV are formed. The time constant of charge exchange for the RD1,2 center is 5 103 s at RT. Similarly to the case of 6H-SiC, the charge state of these and deeper centers remains unchanged in the course of RT C–V measurements. At
Table 1 Parameters of RDs observed in 6H-SiC Parameters of observed RDs Ec FE0 (eV) 0.16–0.2 0.36/0.4 0.5 0.7 0.8 1.1–1.22
Identification 2
sn (cm ) 17
6 10 2 1015 5 1015 4 1015 4 1015 2 1015
3
Ng (cm ) 14
3 10 3.3 1015 2.2 1015 1.3 1015 6 1014 2 1016
Tann (K)
Electron irrad. [7]
o650
L1 L3/L4 L6 L7/L8 L9 L10
o650
Intrinsic defects E1 =E2 [4] Z1/Z2 [4] R [8]
Table 2 Parameters of RDs observed in 4H-SiC Parameters of observed RDs Ec FE0 (EV) 0.18 0.63–0.7 0.96 1.0 1.5
Identification
sn (cm2) 15
6 10 5 1015 5 1015 1 1016 2 1013
Ng (cm3) 14
2 10 5 1015 6.3 1015 6.3 1015 5 1015
Tann (K)
Implantation of He+ [4]
o650
P1/P2 Z1 RD1,2 RD3 RD4
Intrinsic defects Z1 [4] 1.1 eV [9]
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the same time, the time t for the Z1 center, the closest to the mentioned above levels, as low as 3.6 102 s at RT. At temperature of 650 K all known electron traps become ionized. Thus, the difference between the Nd 2Na values measured at 300 and 650 K must be equal to the concentration of the center located at Ec 1:22 eV for 6H-SiC and sum of RD1,2, RD3, and RD4 concentrations for 4H-SiC, determined from DLTS spectra. As seen from Figs. 1 and 2, this equality agrees well with experiment. Most pronounced feature of 150 keV proton irradiation is insensitivity of the C–V characteristics to dose of irradiation. This fact together with identity of the C–V characteristics measured at 500 K after irradiation and ones of unirradiated samples means that the introduction rate of acceptor-like hole traps is equal to the one of donor-like electron traps. That is, 150 keV irradiation does not lead to change of charge density in space charge region of Shottky diodes at RT and higher temperatures. At the same time, at 100 K relatively shallow RDs are characterized by high t and remains filled with electrons and, hence, their negative charge causes the shift of C–V characteristics. Using approach described above (8 MeV protons) we estimated the Rcenter concentration. We extrapolate RT C–V characteristic as shown in Fig. 4 (dashed curve) and obtained R-center concentration of 7 1015 cm3. Hence, the Rcenter introduction rate by 150 keV proton irradiation is of about 700 cm1. In conclusion, SI layers of 6H and 4H-SiC were created by proton irradiation with energies of 1 GeV, 8 MeV, 150 keV and 8 MeV, respectively. At the same time, it is shown that 1 GeV and 8 MeV irradiation of n-6H-SiC leads to increase of uncompensated donor concentration. Irradiation with energy of 150 keV leads only to capturing of electrons by
RD levels, but not to change in Nd 2Na value. The introduction rate of the deepest RD in n-6H-SiCFR center was found to be 0.17, 70, and 700 cm1 for 1 GeV, 8 MeV, and 150 keV protons, respectively. In 4H-SiC we obtained decrease of Nd 2Na value after 8 MeV proton irradiation. The obtained results may be applicable for creating local high-resistance SiC regions in technology of devices not intended for operation at high temperature, e.g., radiation detectors.
Acknowledgements This work was partly supported by the grant INTAS9730834. References [1] A.O. Konstantinov, V.N. Kuzmin, L.S. Lebedev, D.P. Litvin, A.G. Ostroumov, V.I. Sankin, V.V. Semenov, Zh. Tekhnich. Phys. 54 (1984) 1622 (in Russian). [2] G.C. Rybicki, J. Appl. Phys. 78 (1995) 2996. [3] R.K. Nadela, M.A. Capano, Appl. Phys. Lett. 70 (1997) 886. [4] T. Dalibor, G. Pensl, H. Matsunami, T. Kimoto, W.J. Choyke, A. Schoner, N. Nordel, Phys. Stat. Sol (A) 162 (1997) 199. [5] W. Puff, P. Mascher, A.G. Balogh, H. Baumann, Mater. Sci. Forum 258–263 (1997) 733. [6] N.S. Savkina, A.A. Lebedev, D.V. Davydov, A.M. Strel’chuk, A.S. Tregubova, M.A. Yagovkina, Mater. Sci. Eng. B 61–62 (1999) 165. [7] V.S. Ballandovich, Sov. Phys. Semicond. 33 (1999) 1188. [8] M.M. Anikin, A.S. Zubrilov, A.A. Lebedev, A.M. Strel’chuk, A.E. Cherenkov, Sov. Phys. Semicond. 25 (1991) 519. [9] W.C. Mitchel, et al., Mater. Sci. Forum 338–342 (2000) 21.
DLTS study of defects in 6H- and 4H-SiC created by proton irradiation D.V. Davydova,*, A.A. Lebedeva, V.V. Kozlovskib, N.S. Savkinaa, A.M. Strel’chuka a
A.F. Ioffe Physicotechnical Institute, Russian Academy of Sciences, Polytekniheskaya 26, St. Petersburg 194021, Russia b St. Petersburg State Technical University, St. Petersburg 194027, Russia
Abstract Experiments on proton irradiation with energy of 150 KeV, 8 MeV and 1 GeV were made. Capacitance–voltage characteristics measured at 650 K showed that 8 MeV and 1 GeV proton irradiation of 6H-SiC leads to an increase of uncompensated donor concentration. However, donor concentration in 6H-SiC remains unaffected after 150 keV proton irradiation. Deep centers were investigated by deep levels transient spectroscopy (DLTS). Results of C–V measurements are interpreted using DLTS data. The results obtained show the possibility of uzing proton irradiation for producing local high-resistance regions in SiC devices not intended for high-temperature applications. r 2001 Elsevier Science B.V. All rights reserved. Keywords: SiC; Proton irradiation; DLTS
1. Introduction
2. Experiment
There is several reports on formation of SiC layers semi-insulating (SI) at room temperature (RT) by means of proton irradiation [1–3]. However, the parameters and concentrations of radiation defects (RD) formed during irradiation have not been studied. In other works [4,5] RD parameters were studied but the compensation appearing in the course of irradiation was not analyzed. The aim of this work was to fabricate SI 6H- and 4HSiC layers by proton irradiation with different energies, compare their electrical properties, and determine the parameters of RDs responsible for the compensation. The samples were studied with capacitance–voltage (C–V) characteristics and deep levels transient spectroscopy (DLTS) methods.
Silicon carbide p+–n structures and epitaxial layers commercially produced by CREE Research, Inc., or fabricated at the Ioffe Institute by sublimation epitaxy (SE) [6] were used. The n-type layer thickness was about 5 mm; that of the p-type layer: about 1 mm; and substrate thickness: about 400 mm. The diameter of Schottky barriers or mesa structures of the diodes was in the range 600–700 mm. The concentration of uncompensated donors in the n-type layer (Nd 2Na ) was (0.8– 4) 1016 cm3, that in the substrate, (3–5) 1018 cm3, and concentration of acceptors in the p-type layer: B5 1018 cm3. Thus, the doping level in the substrate and p-type emitter exceeded by no less than two orders of magnitude that in the n-type base layer. A set of 6H- and 4H-SiC samples (p–n structure and Schottky diodes) were step by step irradiated with 8 MeV protons. Total irradiation dose (D) was changed from 1 1014 cm2 to 2 1016 cm2. Two sequential 1 GeV proton irradiations with D of 3 1014 and 9 1014 cm2 were made on one 6H-SiC SE sample
*Corresponding author. Tel.: +7-812-247-9930; fax: +7812-247-6425. E-mail address: [email protected]ffe.rssi.ru (D.V. Davydov).
0921-4526/01/$ - see front matter r 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 0 1 ) 0 0 7 7 5 - X
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with Schottky diodes. The epilayer was nonuniformly dopedFNd 2Na varied from 5 1014 to 8 1015 cm3 at the surface and at 7 mm depth, respectively. Furthermore, three 6H-SiC SE samples having initial Nd 2Na value of 4 1016 cm3 were irradiated with 150 keV protons with doses of 1011, 1012, and 1013 cm2. All irradiations were carried out at RT. The C–V characteristics were measured on a bridge C–V setup on a frequency of 10 kHz. A study of 8 MeV irradiated samples demonstrated a decline in the Nd 2Na value measured at RT, with Nd 2Na markedly increasing on heating a structure to 650 K. For 6H-SiC the Nd 2Na value measured at 650 K was even higher than that in the initial structures prior to irradiation. With increasing irradiation dose, this difference became more pronounced (Figs. 1 and 2). Irradiation of lightly doped samples with D of 1016 cm2 led to formation of SI layers with specific resistivity of about 109 O cm at RT. Typical C–V characteristics measured after 150 keV proton irradiation are shown on Fig. 3. At high reverse bias the characteristic is similar to initial one; but irradiation resulted in appearance of flat region at low bias voltages where the capacitance does not depend on voltage, and its value corresponds to depleted region width of B1 mm. We attribute this to formation of a SI layer due to the irradiation. The characteristics measured at 100 K shift right along the voltage axis. Increase of the temperature to about 500 K led to recovery of initial C–V characteristics of the diodes. The shape of the characteristics were the same for all the samples (all the irradiation dose values). First 1 GeV irradiation did not noticeably influence on C–V characteristics. However, after the second irradiation (D ¼ 1:2 1015 cm2) RT capacitance became independent on voltage (the epilayer became SI). C–V characteristic measured at temperature of 500 K after second irradiation was linear (Fig. 4); it revealed
Fig. 1. 6H-SiC: Nd 2Na measured at T ¼ 300 K (1) and T ¼ 650 K (2), difference of them (3), and concentration of the center located at Ec 1:22 eV (4) vs. the irradiation dose.
Fig. 2. 4H-SiC: Nd 2Na measured at T ¼ 300 K (1) and T ¼ 650 K (2), difference of them (3), and total concentration of the centers RD1/2, RD3, and RD4 (4) vs. the irradiation dose.
Fig. 3. C–V characteristics of n-6H-SiC epilayer before irradiation (1) and after 150 keV irradiation with dose of 1013 cm3 measured at RT (2), 500 K (3), and 100 K (4).
Fig. 4. C–V characteristics of unirradiated n-6H-SiC epilayer (1) and one measured at temperature of 500 K after 1 GeV proton irradiation (D ¼ 1:2 1015 cm3) (2).
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uniform doping with Nd 2Na of 7 1015 cm3 which exceeds much the initial one. Six electron traps were found in DLTS spectra of ntype 6H-SiC samples irradiated with 8 MeV protons. Most of them were observed in electron-irradiated samples [7] or had parameters close to intrinsic structural defects. Two of the observed centers (with position in the band gap Ec 0:16 0:2 eV and Ec 0:5 eV) were completely annealed out at 500– 650 K. Sample with p–n junction was annealed at 800 K what resulted in small increase of the center Ec 0:7 eV concentration. DLTS investigation of electron traps in n-type 4H-SiC revealed 5 deep centers. Most of these were found in samples implanted with He+ ions [4] or had parameters close to those of intrinsic defects. One of the observed centers (Ec 0:18 eV) was completely eliminated by annealing at 500–650 K. Tables 1 and 2 list ionization energies of the observed centers, their estimated electron capture cross-sections (sn ) and concentrations after irradiation with a dose of 2 1014 cm2. For both of the polytypes studied, no pronounced difference was observed between the spectrum of deep centers formed in CREE epitaxial layers (CVD) and those grown by SE. The introduced RDs were completely annealed out at p2100 K. In 6H-SiC epilayer irradiated with 1 GeV protons with dose of 3 1014 cm2 DLTS revealed only two deep
electron traps: Ec 0:3520:4 eV (E1 =E2 ) and Ec 21:2 eV (R-center) in concentrations of 1 1013 cm3 and 5 1013 cm3, respectively. The R-center introduction rate was equal to 70 cm1 for 8 MeV, and 0.17 cm1 for 1 GeV protons.
3. Discussion and conclusion As shown by the DLTS study, both 8 MeV and 1 GeV proton irradiation creates in n-type 6H-SiC a defect with the highest introduction rate (Ec 1:22 eV) with parameters close to those of the known structural defectFR center [8]. As follows from the parameters of the center located at Ec 1:22 eV, the recharging time (t) for this center is of about two weeks at 300 K. Thus, the charge state of this and deeper lying centers does not change during C–V measurements at RT. At the same time, the t value for the center which is the closest to this level, (Ec 0:8 eV), is 3.3 s. Hence, the center Ec 0:8 eV (and all shallower centers) can be considered completely ionized in C–V measurements at RT. In n-type 4H-SiC, several deep centers with ionization energy in the range of 0.96–1.5 eV are formed. The time constant of charge exchange for the RD1,2 center is 5 103 s at RT. Similarly to the case of 6H-SiC, the charge state of these and deeper centers remains unchanged in the course of RT C–V measurements. At
Table 1 Parameters of RDs observed in 6H-SiC Parameters of observed RDs Ec FE0 (eV) 0.16–0.2 0.36/0.4 0.5 0.7 0.8 1.1–1.22
Identification 2
sn (cm ) 17
6 10 2 1015 5 1015 4 1015 4 1015 2 1015
3
Ng (cm ) 14
3 10 3.3 1015 2.2 1015 1.3 1015 6 1014 2 1016
Tann (K)
Electron irrad. [7]
o650
L1 L3/L4 L6 L7/L8 L9 L10
o650
Intrinsic defects E1 =E2 [4] Z1/Z2 [4] R [8]
Table 2 Parameters of RDs observed in 4H-SiC Parameters of observed RDs Ec FE0 (EV) 0.18 0.63–0.7 0.96 1.0 1.5
Identification
sn (cm2) 15
6 10 5 1015 5 1015 1 1016 2 1013
Ng (cm3) 14
2 10 5 1015 6.3 1015 6.3 1015 5 1015
Tann (K)
Implantation of He+ [4]
o650
P1/P2 Z1 RD1,2 RD3 RD4
Intrinsic defects Z1 [4] 1.1 eV [9]
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the same time, the time t for the Z1 center, the closest to the mentioned above levels, as low as 3.6 102 s at RT. At temperature of 650 K all known electron traps become ionized. Thus, the difference between the Nd 2Na values measured at 300 and 650 K must be equal to the concentration of the center located at Ec 1:22 eV for 6H-SiC and sum of RD1,2, RD3, and RD4 concentrations for 4H-SiC, determined from DLTS spectra. As seen from Figs. 1 and 2, this equality agrees well with experiment. Most pronounced feature of 150 keV proton irradiation is insensitivity of the C–V characteristics to dose of irradiation. This fact together with identity of the C–V characteristics measured at 500 K after irradiation and ones of unirradiated samples means that the introduction rate of acceptor-like hole traps is equal to the one of donor-like electron traps. That is, 150 keV irradiation does not lead to change of charge density in space charge region of Shottky diodes at RT and higher temperatures. At the same time, at 100 K relatively shallow RDs are characterized by high t and remains filled with electrons and, hence, their negative charge causes the shift of C–V characteristics. Using approach described above (8 MeV protons) we estimated the Rcenter concentration. We extrapolate RT C–V characteristic as shown in Fig. 4 (dashed curve) and obtained R-center concentration of 7 1015 cm3. Hence, the Rcenter introduction rate by 150 keV proton irradiation is of about 700 cm1. In conclusion, SI layers of 6H and 4H-SiC were created by proton irradiation with energies of 1 GeV, 8 MeV, 150 keV and 8 MeV, respectively. At the same time, it is shown that 1 GeV and 8 MeV irradiation of n-6H-SiC leads to increase of uncompensated donor concentration. Irradiation with energy of 150 keV leads only to capturing of electrons by
RD levels, but not to change in Nd 2Na value. The introduction rate of the deepest RD in n-6H-SiCFR center was found to be 0.17, 70, and 700 cm1 for 1 GeV, 8 MeV, and 150 keV protons, respectively. In 4H-SiC we obtained decrease of Nd 2Na value after 8 MeV proton irradiation. The obtained results may be applicable for creating local high-resistance SiC regions in technology of devices not intended for operation at high temperature, e.g., radiation detectors.
Acknowledgements This work was partly supported by the grant INTAS9730834. References [1] A.O. Konstantinov, V.N. Kuzmin, L.S. Lebedev, D.P. Litvin, A.G. Ostroumov, V.I. Sankin, V.V. Semenov, Zh. Tekhnich. Phys. 54 (1984) 1622 (in Russian). [2] G.C. Rybicki, J. Appl. Phys. 78 (1995) 2996. [3] R.K. Nadela, M.A. Capano, Appl. Phys. Lett. 70 (1997) 886. [4] T. Dalibor, G. Pensl, H. Matsunami, T. Kimoto, W.J. Choyke, A. Schoner, N. Nordel, Phys. Stat. Sol (A) 162 (1997) 199. [5] W. Puff, P. Mascher, A.G. Balogh, H. Baumann, Mater. Sci. Forum 258–263 (1997) 733. [6] N.S. Savkina, A.A. Lebedev, D.V. Davydov, A.M. Strel’chuk, A.S. Tregubova, M.A. Yagovkina, Mater. Sci. Eng. B 61–62 (1999) 165. [7] V.S. Ballandovich, Sov. Phys. Semicond. 33 (1999) 1188. [8] M.M. Anikin, A.S. Zubrilov, A.A. Lebedev, A.M. Strel’chuk, A.E. Cherenkov, Sov. Phys. Semicond. 25 (1991) 519. [9] W.C. Mitchel, et al., Mater. Sci. Forum 338–342 (2000) 21.