- Email: [email protected]
Paramagnetic defects in silicon irradiated with 40 MeV As ions
Nuclear Instruments and Me! hods in Physics (Research B80/81 (1993) 620-623 North-Holland
Btiam Intoncttons wtth NlattartalsiAtoms
Paramagnetic defects in silicon irradiated with 40 MeV As ions AN . Dvurecherskii, A .A. Karancivich and AN . Rybitl Institute of Semiconductor Phydcs, 63(X190 Novosibirsk, Russian Federation
R. Griitzschel
Research Center Rossendorf, Germany The structure and the depth distribution of paramagnetic defects in n- and p-type silicon irradiated with 40 MeV As ions at doses in the range 1-7x10 ° cm -2 were investigated by the use of the EPR technique . The tetravacancies (spectrum P3) and W-like centers (g = 2 .0055) were found to dominate after such an irradiation. The depth distribution of the observed defects is characterized by a sharp peak due to the W-like ccntt :rs concentration near the end of this high-energy ion range (Rp =10 Wm) and a relatively broad peak of the tetravacancies concentration at a depth of d = 5 wm. The W-like centers linewidth was found to be dependent on the magnetic field orientation, irradiation dose and the depth of these centers. All the obtained data are analyzed in terms of the production and overlap of damaged regions along the ion track and the influence of ion-stimulated self-annealing of defects at the subsurface region (d=0-5 y.m), where the electronic stopping power (dE/dR), is too high. 1. Introduction High-energy ion implantation has attracted a growing attention because of its favorable applicability to device fabrication and various fundamental physical problems which arise at such an intensive radiation treatment . Among the various applications of McV-energy ion implantation, the ion beam synthesis of deep buried insulating [1] or metallic [2] layers -,i w,aiconductors (as one of the possible ways to three-dimensional integration), the "well engineering" as a useful method of improving many device characteristics in ULSI [3], and so on, are of increasing importance . From the physical point of view, the problem of the nature of radiation defects in the lattice when the electronic stopping power is higher than the nuclear one ((dE/dR)e > (dE/dR) ), the nonlinear kinetics of defect accumulation at such a high level of Frenkel pair generation, and so on, are of great significance. Only a small part of works among the growing number of publications concerning MeV-energy ion implantation is devoted to the problem of the atomic structure and depth distribution of point defects which are introduced into Si under such an irradiation . At the same time these point defects often influence the main electrical properties of the irradiated semiconductors . Therefore investigations of radiation damage in MeV-energy ion implanted silicon using the EPR technique (which is known to be the most powerful tool in point defect structure studies) are very impor . tant. An especial interest presents the case of MeV-en-
ergy implantation of heavy ions, when the electronic stopping power often reaches values > 10 keV/nm. ^nc of the most t,ommonly used ions in semiconductor technology is arsenic. In the present work the results of EPR investigations of the defect structure and the depth distribution of paramagnetic centers in silicon irradiated by 40 MeV ions are described . 2 . Experiment 40 MeV Asp * ions have been implanted into high resistivity n- and p-type (111) silicon wafers in the dose range 1-7 x 10 14 cm -2 (ion current j = 40-80 nA/ cm 2 ) at room temperature under nonchanneling conditions . A standard Varian X-band EPR spectrometer (klystron power = 600 mW) was used to record EPR absoipiion derivative spectra in the temperature range 77-300 K. Surface stripping for depth profile analysis was carried out by polishing with diamond paste. The thickness of the stripped layer was determined by weighing the sample before and after the polishing. Control experiments show that the EPR signal from the defects introduced into the subsurface region by polishing does not lead to distortion of the EPR spectra from the radiation defects because of its low intensity. The Varian concentration standard sample was used to measure the number of defects. Defect concentration depth profiles were obtained by numerical derivation of the preliminary smoothed experimental
0168-583X/93/$06.00 0 1993 - Elsevier Science Publishers B.V. All rights reserved
A. V. Duureckenskii et al. / Paramagnetic dtfects in Si irradiated svith As
621
curves of the remaining number of defects at each stage of the surface stripping . The surface conductivity ,r, was me-sured in the irradiated samples after annealing at M= 45G°C during one hour. The depth profile of the volume c,,nductivity was determined in the same way as for the defect profiling. 3. Experimental results The EPR spectrum observed in the investigated samples for two values of the microwave power is shown in fig. 1 . Independent of the sample type, irradiation dose and sample temperature this signal consists of two components : well resolved sharp lines superposed on a more intensive broad line. These well resolved lines are readily assigned to neutral planar tetravacancies (I3 -spectrum) [4] . The broad line is close (by its g-factor) to the spectrum of amorphous silicon (so called VV-center line with g = 2 .0055 i5]) but has an anisotropic linewidth . So . we labeled these defects as VV-like centers. In fig. 2a, the remaining P3 and W-like center density determined by the EPR measurements at each stage of the surface stripping are plotted against the removed depth for the case of = 10 14 cm -2 40 MeV As implantation . The defect concentration profiles are shown i .t fig. 2b . These profiles are characterized by a broad peak at 5 Wm for tetravacancies and a relatively sharp peak of the VV-like center concentration at the end of this ion range (RP = 10 wm). The conductivity
4
0
0
0 y m a
MAGNETIC FIELD (Gauss) Fig . 1 . EPR spectrum, Gbserved in silicon irradiated with 40 MeV As ions (dose = 7 x 10 14 cm -2 ) for iwo values of the microwave power. The measurement tempe°ature T = 300 K, R II (111 > .
m)
12
CONDUCTIVITY DISTRIBUTIONS
20.
.~ 1 .5
Is.
0
â
10 .
1 .0
0 .5
F
8
N
2.0
.o
w
DEPTH (
' 1
E 0
s .o -°
T, -300 K
0.0
8 12 m) Fig . 2. Remaining defect density determined by the EPR measurements at each stage of the surface stripping vs removed depth for =10 14 cm -2 40 MeV As impiautation (a); concentration profiles of tetravacancies and W-like centers (b); depth distribution of the surface (T,) and volume (Q ) conductivity of the irradiated samples after annealing (c). 0
4
DEPTH (H
profiie in the irradiated sample after annealing is presented in fig . 2c . The W-like center linewidth was found to depend on the implantation dose, orientation of the magnetic field and the depth of these centers (fig . 3). For maximum dose the linewidth coincides with this value for the W-centers in amorphous Si (AHPP = 5 G) and does not depend on the orientation . When the dose decreases the linevidth increases and displays anisotropic properties (fig . 3a). A maximum value of Illb. SEMICONDUCTOR MODIFICATION (b)
62 2
A.V. Drarerhenskii ei at / Paramagnetic defects in Si irradiated with As
0
7 0
(degrees) d (, m) Fig. 3. VV-like center linewidth dependence on the magnetic field orientation for different doses (a) and on the stripped silicon layer thickness for the dose of = 10 14 cm -2 (b) .
the VV-like center linewidth is observed at the orientation B IÎ (I 11) . A narrowing of the VV-like center line after successive removal of the sample surface was observed (fig. 3b). 4. Discussioa
All the obtained results demonstrate the complex character of the physical processes which determh .e the efficiency of defect formation along the high-energy ion track. At the depth range 6-10 Wrn the defect distribution ctir- a.P nunlitatively described by the model suggested in ref.. [6], where the paramagnetic center distribution in silicon was investigated after 3 MeV Si and P ion implantation. In this work the authors supposed that the high-energy ion introduces into the lattice some damaged regions (DR), the concentra,ion of which, NDR, is determined by the nuclear stopping power value (dE/d R), In one separate DR the material is believed to beep the crystalline structure, and only point defects (such as tetravacancies) are induced. In the regions of two or more overlapping DR, the semiconductor is dramatically disordered and VV-like centers (in ref. [6] the authors labeled these as indefinite point defect or IPD) are observed. The growth of the volume occupied by overlapping DR leads to a growth of the VV-like center density and a
decrease of the tetravacancy concentration near the end of the As ion track. On the other hand, near the surface (0-6 p .m) it seems impossible to explain the considerable increase of the tetravacancy concentration and the practically constant density of VV-like centers in terms of the model of ref. [6], which predicts the opposite : a faster increase of the VV-like center ? ,in - entration compared to that of the tetravacancies (Nvv - Np;) . Therefore, to explain the peculiarities of the defect formation in different sections of the MeV-energy ion track, one may take into account some additional physical factors, such as a high level of electronic stopping power (in our case (dE/d R), = 6 keV/nm) . When the electronic stopping power value exceeds I keV/nm one may expect the effects of ion-stimulated self-annealing of defects [7]. Therefore, the decrease of the tetravacancy concentration at the depth range form 6 Wm towards the surface is probably caused by the ion-stimulated self-annealing . The efficiency of this effect decreases with depth in accordance with the electronic stopping power decrease . One of the possible mechanisms of self-annealing is concerned with the process of the electronic subsystem energy relaxation into the nuclear subsystem of the crystal in the vicinity of defect-forming atoms [8] . Such a relaxation leads to a local heating of the lattice and, as a result, to destruction of the defect complex . This
A. V. Drurecherts&ti et al. / Paramagnetic defects tn Si irradtated tvtth As
mechanism seems to be actual in our case because of the relatively low thermal stability of tetravacancies (the anneal temperature of these defects is = 170°C). Besides damaged regions with crystalline structure (which are characterized by the P3-spectrum) and amorphous regions (which are responsible for the observed VV centers at the highest doses of implantation) the irradiated silicon contains a considerable number of t-gions which are characterized by an intermediate disordering. These regions are responsible for the observed VV-like centers in the experimental spectrum. It is common use to consider the observed VV centers in amorphous silicon, observed in the EPR spectrum, as a result of the superposition of signals front the :silicon dangling bonds [9]. The VV center signal is isotropic, and a high concentration of these defects leads to the so called "exchange" narrowing of the EPR line to a value of = 5 G. The observed linewidth orientation dependence of VV-like centers indicates that the regions whey^, these defects are tocalized are nor amorphous in the general sense, i .e . there exists some orientation influence of the surrounding crystal matrix. In the cr~tstalline silicon the EPR spectrum of the Si dangling bonds (which are typically present in come kinds of point defects) is characterized by an axially symmetric g-tensor (gii I (I11)) and involves a series of shairp lines . The distance between these lines depends on the magnetic field orientation and is maximum at B II (111) and minimum at B II (100) . Exactly the same behavior was observed for the VV-like center linewidth (fig. 3a) . Therefore we conclude that the anisotropic signal of VV-like centers is formed as a result of the superposition of the signals from Si dangling bonds, which are localized in highly damaged regions, but keep their orientation along the (1 11) directions of the crystalline matrix . The spread of the surrounding crystal constants for different dangling bonds (and consequently the spread of its g-factors) and strong exchange interaction causes the total EPR spectrum to have a practically isotropic g-factor and an oricntation-dependent linewidth. The VV-Like center linewidth dependence on the
62 3
depth (fig . 3b) indicates that the material disordering in the damaged regions grows with depth . This fact is supposed to be a result of the multiple overlap of DR as the depth increases. Another possible reason of the observed depth dependence of the VV-like center linewidth is concerned with the ion-stimulated self-annealing of these defects. 5 . Conclusions 1) The dominant paramagnetic defects in Si after 40 -cV As ion irradiation are tetravacancies (spectrum P31 and VV-like centers . 2) The paramagnetic defect distribution is well described in terms of creating and overlapping of damaged regions in the depth range 5-10 wm. A relatively low tetravacancy concentration near the surface is supposed to he caused by the effects of ion-stimulated self-annealine of these defects. 3) In Si irradiated at WI-energy a considerable number of damaged regions are characterized by an intermediate disordering (not crystalline and not amorphous) . The crystal constants are randomly changed from point to point in these regions, but the Si dangling bonds keep their orientation along the (111) direction of the crystalline matrix . iKererences [.] :..: EVI,it, ri ..: . Avpi. Phys. Leu. 50 (1987) 19 . [21 J .K .N . Lindner, T . Klassen and E.H . te Kaat, Nuel . tnstr. and Meth . B5,;/60 (1991) 655 . [3) K. Tsukamoto, S . Komori, T . Kuroi and Y. Akasaka, ibid ., p. 584. [41 K.L . Brower, Phys . Rev. B4 (1971) 1968. [51 N.N . Gerasimenko, A .V. Dvurechenskii and L.S. Smirnov, Fiz. Tekhn. Poluprov. 6 (1972) 1111 . [6] Y. Yajima, N. N?tsuaki. K. Yokogawa and S. Nishimatsu, Nuel. Instr. and Meth. (1991) 607. 171 A.M . Zaitsev, Poverkh . Fiz. Khim. Mekh . 10 (1991) 5 . [8] A.R. Urmanov, Phys . Status Solidi B166 (1991) 9. [9) S.I . Rembeza, Paramagnetic resonance in semiconductors (Metallurgia, Moscow, 1988) in Russian .
Illb. SEMICONDI ICTOR MODIFICATION (b)
Btiam Intoncttons wtth NlattartalsiAtoms
Paramagnetic defects in silicon irradiated with 40 MeV As ions AN . Dvurecherskii, A .A. Karancivich and AN . Rybitl Institute of Semiconductor Phydcs, 63(X190 Novosibirsk, Russian Federation
R. Griitzschel
Research Center Rossendorf, Germany The structure and the depth distribution of paramagnetic defects in n- and p-type silicon irradiated with 40 MeV As ions at doses in the range 1-7x10 ° cm -2 were investigated by the use of the EPR technique . The tetravacancies (spectrum P3) and W-like centers (g = 2 .0055) were found to dominate after such an irradiation. The depth distribution of the observed defects is characterized by a sharp peak due to the W-like ccntt :rs concentration near the end of this high-energy ion range (Rp =10 Wm) and a relatively broad peak of the tetravacancies concentration at a depth of d = 5 wm. The W-like centers linewidth was found to be dependent on the magnetic field orientation, irradiation dose and the depth of these centers. All the obtained data are analyzed in terms of the production and overlap of damaged regions along the ion track and the influence of ion-stimulated self-annealing of defects at the subsurface region (d=0-5 y.m), where the electronic stopping power (dE/dR), is too high. 1. Introduction High-energy ion implantation has attracted a growing attention because of its favorable applicability to device fabrication and various fundamental physical problems which arise at such an intensive radiation treatment . Among the various applications of McV-energy ion implantation, the ion beam synthesis of deep buried insulating [1] or metallic [2] layers -,i w,aiconductors (as one of the possible ways to three-dimensional integration), the "well engineering" as a useful method of improving many device characteristics in ULSI [3], and so on, are of increasing importance . From the physical point of view, the problem of the nature of radiation defects in the lattice when the electronic stopping power is higher than the nuclear one ((dE/dR)e > (dE/dR) ), the nonlinear kinetics of defect accumulation at such a high level of Frenkel pair generation, and so on, are of great significance. Only a small part of works among the growing number of publications concerning MeV-energy ion implantation is devoted to the problem of the atomic structure and depth distribution of point defects which are introduced into Si under such an irradiation . At the same time these point defects often influence the main electrical properties of the irradiated semiconductors . Therefore investigations of radiation damage in MeV-energy ion implanted silicon using the EPR technique (which is known to be the most powerful tool in point defect structure studies) are very impor . tant. An especial interest presents the case of MeV-en-
ergy implantation of heavy ions, when the electronic stopping power often reaches values > 10 keV/nm. ^nc of the most t,ommonly used ions in semiconductor technology is arsenic. In the present work the results of EPR investigations of the defect structure and the depth distribution of paramagnetic centers in silicon irradiated by 40 MeV ions are described . 2 . Experiment 40 MeV Asp * ions have been implanted into high resistivity n- and p-type (111) silicon wafers in the dose range 1-7 x 10 14 cm -2 (ion current j = 40-80 nA/ cm 2 ) at room temperature under nonchanneling conditions . A standard Varian X-band EPR spectrometer (klystron power = 600 mW) was used to record EPR absoipiion derivative spectra in the temperature range 77-300 K. Surface stripping for depth profile analysis was carried out by polishing with diamond paste. The thickness of the stripped layer was determined by weighing the sample before and after the polishing. Control experiments show that the EPR signal from the defects introduced into the subsurface region by polishing does not lead to distortion of the EPR spectra from the radiation defects because of its low intensity. The Varian concentration standard sample was used to measure the number of defects. Defect concentration depth profiles were obtained by numerical derivation of the preliminary smoothed experimental
0168-583X/93/$06.00 0 1993 - Elsevier Science Publishers B.V. All rights reserved
A. V. Duureckenskii et al. / Paramagnetic dtfects in Si irradiated svith As
621
curves of the remaining number of defects at each stage of the surface stripping . The surface conductivity ,r, was me-sured in the irradiated samples after annealing at M= 45G°C during one hour. The depth profile of the volume c,,nductivity was determined in the same way as for the defect profiling. 3. Experimental results The EPR spectrum observed in the investigated samples for two values of the microwave power is shown in fig. 1 . Independent of the sample type, irradiation dose and sample temperature this signal consists of two components : well resolved sharp lines superposed on a more intensive broad line. These well resolved lines are readily assigned to neutral planar tetravacancies (I3 -spectrum) [4] . The broad line is close (by its g-factor) to the spectrum of amorphous silicon (so called VV-center line with g = 2 .0055 i5]) but has an anisotropic linewidth . So . we labeled these defects as VV-like centers. In fig. 2a, the remaining P3 and W-like center density determined by the EPR measurements at each stage of the surface stripping are plotted against the removed depth for the case of = 10 14 cm -2 40 MeV As implantation . The defect concentration profiles are shown i .t fig. 2b . These profiles are characterized by a broad peak at 5 Wm for tetravacancies and a relatively sharp peak of the VV-like center concentration at the end of this ion range (RP = 10 wm). The conductivity
4
0
0
0 y m a
MAGNETIC FIELD (Gauss) Fig . 1 . EPR spectrum, Gbserved in silicon irradiated with 40 MeV As ions (dose = 7 x 10 14 cm -2 ) for iwo values of the microwave power. The measurement tempe°ature T = 300 K, R II (111 > .
m)
12
CONDUCTIVITY DISTRIBUTIONS
20.
.~ 1 .5
Is.
0
â
10 .
1 .0
0 .5
F
8
N
2.0
.o
w
DEPTH (
' 1
E 0
s .o -°
T, -300 K
0.0
8 12 m) Fig . 2. Remaining defect density determined by the EPR measurements at each stage of the surface stripping vs removed depth for =10 14 cm -2 40 MeV As impiautation (a); concentration profiles of tetravacancies and W-like centers (b); depth distribution of the surface (T,) and volume (Q ) conductivity of the irradiated samples after annealing (c). 0
4
DEPTH (H
profiie in the irradiated sample after annealing is presented in fig . 2c . The W-like center linewidth was found to depend on the implantation dose, orientation of the magnetic field and the depth of these centers (fig . 3). For maximum dose the linewidth coincides with this value for the W-centers in amorphous Si (AHPP = 5 G) and does not depend on the orientation . When the dose decreases the linevidth increases and displays anisotropic properties (fig . 3a). A maximum value of Illb. SEMICONDUCTOR MODIFICATION (b)
62 2
A.V. Drarerhenskii ei at / Paramagnetic defects in Si irradiated with As
0
7 0
(degrees) d (, m) Fig. 3. VV-like center linewidth dependence on the magnetic field orientation for different doses (a) and on the stripped silicon layer thickness for the dose of = 10 14 cm -2 (b) .
the VV-like center linewidth is observed at the orientation B IÎ (I 11) . A narrowing of the VV-like center line after successive removal of the sample surface was observed (fig. 3b). 4. Discussioa
All the obtained results demonstrate the complex character of the physical processes which determh .e the efficiency of defect formation along the high-energy ion track. At the depth range 6-10 Wrn the defect distribution ctir- a.P nunlitatively described by the model suggested in ref.. [6], where the paramagnetic center distribution in silicon was investigated after 3 MeV Si and P ion implantation. In this work the authors supposed that the high-energy ion introduces into the lattice some damaged regions (DR), the concentra,ion of which, NDR, is determined by the nuclear stopping power value (dE/d R), In one separate DR the material is believed to beep the crystalline structure, and only point defects (such as tetravacancies) are induced. In the regions of two or more overlapping DR, the semiconductor is dramatically disordered and VV-like centers (in ref. [6] the authors labeled these as indefinite point defect or IPD) are observed. The growth of the volume occupied by overlapping DR leads to a growth of the VV-like center density and a
decrease of the tetravacancy concentration near the end of the As ion track. On the other hand, near the surface (0-6 p .m) it seems impossible to explain the considerable increase of the tetravacancy concentration and the practically constant density of VV-like centers in terms of the model of ref. [6], which predicts the opposite : a faster increase of the VV-like center ? ,in - entration compared to that of the tetravacancies (Nvv - Np;) . Therefore, to explain the peculiarities of the defect formation in different sections of the MeV-energy ion track, one may take into account some additional physical factors, such as a high level of electronic stopping power (in our case (dE/d R), = 6 keV/nm) . When the electronic stopping power value exceeds I keV/nm one may expect the effects of ion-stimulated self-annealing of defects [7]. Therefore, the decrease of the tetravacancy concentration at the depth range form 6 Wm towards the surface is probably caused by the ion-stimulated self-annealing . The efficiency of this effect decreases with depth in accordance with the electronic stopping power decrease . One of the possible mechanisms of self-annealing is concerned with the process of the electronic subsystem energy relaxation into the nuclear subsystem of the crystal in the vicinity of defect-forming atoms [8] . Such a relaxation leads to a local heating of the lattice and, as a result, to destruction of the defect complex . This
A. V. Drurecherts&ti et al. / Paramagnetic defects tn Si irradtated tvtth As
mechanism seems to be actual in our case because of the relatively low thermal stability of tetravacancies (the anneal temperature of these defects is = 170°C). Besides damaged regions with crystalline structure (which are characterized by the P3-spectrum) and amorphous regions (which are responsible for the observed VV centers at the highest doses of implantation) the irradiated silicon contains a considerable number of t-gions which are characterized by an intermediate disordering. These regions are responsible for the observed VV-like centers in the experimental spectrum. It is common use to consider the observed VV centers in amorphous silicon, observed in the EPR spectrum, as a result of the superposition of signals front the :silicon dangling bonds [9]. The VV center signal is isotropic, and a high concentration of these defects leads to the so called "exchange" narrowing of the EPR line to a value of = 5 G. The observed linewidth orientation dependence of VV-like centers indicates that the regions whey^, these defects are tocalized are nor amorphous in the general sense, i .e . there exists some orientation influence of the surrounding crystal matrix. In the cr~tstalline silicon the EPR spectrum of the Si dangling bonds (which are typically present in come kinds of point defects) is characterized by an axially symmetric g-tensor (gii I (I11)) and involves a series of shairp lines . The distance between these lines depends on the magnetic field orientation and is maximum at B II (111) and minimum at B II (100) . Exactly the same behavior was observed for the VV-like center linewidth (fig. 3a) . Therefore we conclude that the anisotropic signal of VV-like centers is formed as a result of the superposition of the signals from Si dangling bonds, which are localized in highly damaged regions, but keep their orientation along the (1 11) directions of the crystalline matrix . The spread of the surrounding crystal constants for different dangling bonds (and consequently the spread of its g-factors) and strong exchange interaction causes the total EPR spectrum to have a practically isotropic g-factor and an oricntation-dependent linewidth. The VV-Like center linewidth dependence on the
62 3
depth (fig . 3b) indicates that the material disordering in the damaged regions grows with depth . This fact is supposed to be a result of the multiple overlap of DR as the depth increases. Another possible reason of the observed depth dependence of the VV-like center linewidth is concerned with the ion-stimulated self-annealing of these defects. 5 . Conclusions 1) The dominant paramagnetic defects in Si after 40 -cV As ion irradiation are tetravacancies (spectrum P31 and VV-like centers . 2) The paramagnetic defect distribution is well described in terms of creating and overlapping of damaged regions in the depth range 5-10 wm. A relatively low tetravacancy concentration near the surface is supposed to he caused by the effects of ion-stimulated self-annealine of these defects. 3) In Si irradiated at WI-energy a considerable number of damaged regions are characterized by an intermediate disordering (not crystalline and not amorphous) . The crystal constants are randomly changed from point to point in these regions, but the Si dangling bonds keep their orientation along the (111) direction of the crystalline matrix . iKererences [.] :..: EVI,it, ri ..: . Avpi. Phys. Leu. 50 (1987) 19 . [21 J .K .N . Lindner, T . Klassen and E.H . te Kaat, Nuel . tnstr. and Meth . B5,;/60 (1991) 655 . [3) K. Tsukamoto, S . Komori, T . Kuroi and Y. Akasaka, ibid ., p. 584. [41 K.L . Brower, Phys . Rev. B4 (1971) 1968. [51 N.N . Gerasimenko, A .V. Dvurechenskii and L.S. Smirnov, Fiz. Tekhn. Poluprov. 6 (1972) 1111 . [6] Y. Yajima, N. N?tsuaki. K. Yokogawa and S. Nishimatsu, Nuel. Instr. and Meth. (1991) 607. 171 A.M . Zaitsev, Poverkh . Fiz. Khim. Mekh . 10 (1991) 5 . [8] A.R. Urmanov, Phys . Status Solidi B166 (1991) 9. [9) S.I . Rembeza, Paramagnetic resonance in semiconductors (Metallurgia, Moscow, 1988) in Russian .
Illb. SEMICONDI ICTOR MODIFICATION (b)