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Radiation defects induced by 20 MeV electrons in MOS structures
ARTICLE IN PRESS
Vacuum 76 (2004) 307–310 www.elsevier.com/locate/vacuum
Radiation defects induced by 20 MeV electrons in MOS structures S. Kaschievaa,, Zh. Todorovab, S.N. Dmitrievc a
Bulgarian Academy of Sciences, Institute of Solid State Physics, 72 Tzarigradsko Chaussee, 1784 Sofia, Bulgaria b Bulgarian Academy of Sciences, Institute of Electronics, 72 Tzarigradsko Chaussee, 1784 Sofia, Bulgaria c Joint Institute on Nuclear Research, Dubna, Moscow region 141980, Russia
Abstract The influence of high-energy electron irradiation on the electron states at Si–SiO2 interface of n- and p-type MOS structures was studied by thermally stimulated current (TSC) method. All Si wafers were oxidised at 1000 1C in dry oxygen to oxide thickness of 22 nm. Aluminum gate electrodes were then created by photolithography technique and the samples were irradiated with 20 MeV electrons at a fluence of 8 1012 cm 2 s 1 for—60 or 120 s. The activation energy and the concentration of the traps are evaluated. It is shown that the main peaks in the irradiated n- and p-type MOS structures correspond to the vacancy-phosphorus or vacancy-boron complexes, respectively, that is the generated defects can be related to the main impurities in the Si substrate. r 2004 Elsevier Ltd. All rights reserved. Keywords: Electron irradiation; MOS structures; Si–SiO2 interface; Interface states
1. Introduction The multilayer character of MOS structures and the presence of oxide/semiconductor interface make them rather sensitive to irradiation. Most of the radiation-induced damage is located at or near the Si–SiO2 interface. Extensive studies have been completed and have shown that most kinds of radiation have two primary effects on MOS structures: positive charge accumulation in the Corresponding author. Fax:+359-2-9753632.
E-mail address: [email protected] (S. Kaschieva).
oxide and creation of new electron states at the Si–SiO2 interface. In our previous work the influence of highenergy electron irradiation on the interface states of Si–SiO2 structures with various oxide thickness has been investigated [1–4]. It was found that 11–12 MeV electrons create a new spectrum of interface states in the silicon band gap. The parameters of these states such as their energy and capture cross sections have been estimated [1,2]. In this paper, a comparison of the interface states generated by 20 MeV electrons in n- and
0042-207X/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2004.07.034
ARTICLE IN PRESS 308
S. Kaschieva et al. / Vacuum 76 (2004) 307–310
p-type MOS structures with equal oxide thickness has been carried out. The activation energy and the concentration of the radiation-induced traps are assessed using the thermally stimulated current (TSC) method. It is demonstrated that most of the created radiation defects can be related to the main impurities in the Si substrate, vacancy-phosphorus or vacancy-boron complexes for MOS structures based on n- and p-type Si wafers, respectively.
curves are taken by a calibrated electrometer as a function of the temperature. The latter is measured using a thermocouple placed near the sample. The initial rise plot method and the Grossweiner’s techniques [5] are applied to the TSC curves to evaluate the parameters of the defect centers associated with each of the observed peaks. The energy positions of the traps resulting from both methods appear to be in a good agreement.
2. Experimental
3. Results and discussion
The MOS capacitors used in this study were fabricated on n-type (4.7 O cm) and p-type (6.4 O cm) Si substrates cut along the o1004 crystallographic plane. The samples were oxidized at 1000 1C in dry oxygen in order to grow oxide thickness of 22 nm. This thickness was measured by ellipsometry. Aluminum gate electrodes were created by photolithography on the top of the oxide layer. The back side of the silicon wafers was coated with a thin layer of Al that served as ohmic contact. The obtained MOS capacitors were irradiated by 20 MeV electrons for 60 or 120 s. The irradiation with a flux of about 8 1012 cm 2 s 1 electron was carried out in Microtron MT-25 in Flerov Laboratory of Nuclear Reactions at the Joint Institute of Nuclear Research (FLNR, JINR) in Dubna, Russia. The irradiation was performed under vacuum of about 1 10 2 Pa. The distance between the Microtron window and the samples was 150 mm. The irradiation was carried out from the gate side of the MOS samples. The electron energy is high enough to penetrate all way through the samples. No bias was applied to the capacitors during the irradiation. In order to characterize the induced defect centers thermally stimulated current characteristics before and after the irradiation were measured. Prior to the measurements the MOS structures are cooled down in dark to about 80 K with accumulation voltage applied. When the filling of the radiation-induced traps at the Si–SiO2 interface is completed, the voltage is turned off and the system is brought to nonequilibrium depletion state. The sample temperature is then raised at a constant rate in the dark and the TSC
The density of interface states induced by 20 MeV electron irradiation of MOS structures is well demonstrated in the TSC curves presented in Figs. 1 and 2. They illustrate typical curves measured for MOS samples based on n- and p-type Si wafers, respectively. In both cases, TSC signal could not be registered before the irradiation. Curves labeled 1 and 2 refer to irradiation for 60 and 120 s, respectively. One can easily observe the appearance of new states at the Si–SiO2 interface, indicating the generation of new discrete energy levels in the silicon forbidden gap. The first
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0 100
150
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T [K] Fig. 1. TSC curves of MOS structures based on n-type /100S Si wafers (4.7 O cm) following the irradiation with 20 MeV electrons for 60 s (curve 1) and 120 s (curve 2).
ARTICLE IN PRESS S. Kaschieva et al. / Vacuum 76 (2004) 307–310
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200 250 T [K]
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Fig. 2. TSC curves of MOS structures prepared on p-type /100S Si wafers (6.4 O cm) after irradiation with 20 MeV electrons for 60 s (curve 1) and 120 s (curve 2).
60 s electron irradiation is enough to start observing four peaks in the TSC spectra, which means that four different kinds of defects at the Si–SiO2 interface have been created. Although the number of peaks in the TSC spectra for both the groups of irradiated samples are equal, their spectra differ (for instance curves 1 in Figs. 1 and 2). While the TSC spectra of n-type samples (Fig. 1) spans from about 80 to 270 K, for p-type MOS samples the spectra is extended towards higher temperatures of around 330 K. For n-type of samples (Fig. 1) the activation energy of these states is evaluated as Ec-0.21; Ec0.26; Ec-0.32 and Ec-0.40 eV. The first energy level can be regarded as interface state associated with vacancy-oxygen defect and the second one as the acceptor level of di-vacancy. The third Ec-0.32 eV level is still not completely understood, but it can be correlated with high-order defects [6]. The last peak, at Ec-0.40 eV can be related with defects like vacancy-phosphorus. It can be seen from curve 2 that this type of defects contributes almost half of the generated traps. An increase in the height and the area incorporating all peaks for longer electron-irradiation times is also observed from curve 2, indicating increase in the concentration of
309
the radiation induced traps at the Si–SiO2 interface with dose. As mentioned above, the TSC spectra of p-type MOS samples (Fig. 2) also reveal four peaks but in a broader temperature interval. From a comparison of the corresponding curves in Figs. 1 and 2 it is obvious that, in this case 20 MeV electron irradiation creates lower (by about 40%) defect concentration at the Si–SiO2 interface. Since some of the peaks overlap, they can be separated by ‘‘thermal’’ and ‘‘voltage cleaning’’ of the TSC spectra. The first peak is located at 115 K and it is connected with the levels Ev+0.17 eV, associated with the free oxygen present at the Si–SiO2 interface [7]. This level is a mirror image of A-centers observed in electronirradiated n- MOS samples [2]. The other peaks in the spectra in Fig. 2 located at about 150, 210 and 300 K are clearly revealed after the electron irradiation (curve 2). The energy positions of the peaks in the Si band gap are Ev+0.24, Ev +0.32 and Ev+0.45 eV. The first two were observed earlier in our previous work [2]. They can be related with di-vacancies and Si atoms recoiled from the Si–SiO2 interface, that migrated into the lattice. The last peak attributed to a level at energy of Ev+0.45 eV is associated with boron-vacancy defect complex. The generation of this kind of defects increase with rising the boron concentration and reduction of the oxygen one in the Si substrate [8]. Curves 1 and 2 also indicate that the main defects created by high-electron irradiation are of V–B type and that their concentration increases intensively with the irradiation dose. Although the boron concentration in p-type Si substrate of about 3 1015 cm 3 was twice the phosphorus concentration in n-type Si substrates, defect generation in n-type MOS samples by high-energy electron irradiation was much more significant. The fact that most part of the radiation induced defects at the Si–SiO2 interface of MOS in this study is related to the main impurities in the Si substrate can be explained if one takes into account that high-energy electron irradiation generates vacancies. The latter tend to form complexes with the basic impurities in the substrate such as phosphorus in n-Si and boron in p-Si wafers.
ARTICLE IN PRESS 310
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4. Conclusions
References
Two important conclusions can be drawn from our results: (1) the kind of radiation-induced interface traps and their concentration depend on the type of Si wafers used to create MOS structures; and (2) the total concentration of electron irradiationinduced defects at the Si–SiO2 interface of MOS structures is larger in the case of n-type Si wafer.
[1] Kaschieva S. Nucl Instrum Methods B 1994;93:274–6. [2] Stefanov K, Kaschieva S, Karpuzov D. Vacuum 1998;51: 235–7. [3] Kaschieva S, Alexanrdova S. Nucl Instrum Methods 2001;174:324–8. [4] Kaschieva S, Dmitriev SN, Angelov Hr. Nucl Instrum Methods B 2003;206:452–6. [5] Nicholas K, Woods J. Br J Appl Phys 1964;15:783–98. [6] Shinoda K, Ohta E. Appl Phys Lett 1992;61:2691–8. [7] Konozenko D, Semeniuk K. Radiation Effect in Si. Kiev: Naukova Dumka; 1974 p. 72. [8] Vavilov V, Plotnikov A, Tkachev V. Fiz Tv Tela 1982;4:3446–9 (in Russian, English translation in Soviet Physics: Solid State).
Acknowledgements The authors are grateful to Prof. D. Karpuzov for critical discussion of this manuscript.
Vacuum 76 (2004) 307–310 www.elsevier.com/locate/vacuum
Radiation defects induced by 20 MeV electrons in MOS structures S. Kaschievaa,, Zh. Todorovab, S.N. Dmitrievc a
Bulgarian Academy of Sciences, Institute of Solid State Physics, 72 Tzarigradsko Chaussee, 1784 Sofia, Bulgaria b Bulgarian Academy of Sciences, Institute of Electronics, 72 Tzarigradsko Chaussee, 1784 Sofia, Bulgaria c Joint Institute on Nuclear Research, Dubna, Moscow region 141980, Russia
Abstract The influence of high-energy electron irradiation on the electron states at Si–SiO2 interface of n- and p-type MOS structures was studied by thermally stimulated current (TSC) method. All Si wafers were oxidised at 1000 1C in dry oxygen to oxide thickness of 22 nm. Aluminum gate electrodes were then created by photolithography technique and the samples were irradiated with 20 MeV electrons at a fluence of 8 1012 cm 2 s 1 for—60 or 120 s. The activation energy and the concentration of the traps are evaluated. It is shown that the main peaks in the irradiated n- and p-type MOS structures correspond to the vacancy-phosphorus or vacancy-boron complexes, respectively, that is the generated defects can be related to the main impurities in the Si substrate. r 2004 Elsevier Ltd. All rights reserved. Keywords: Electron irradiation; MOS structures; Si–SiO2 interface; Interface states
1. Introduction The multilayer character of MOS structures and the presence of oxide/semiconductor interface make them rather sensitive to irradiation. Most of the radiation-induced damage is located at or near the Si–SiO2 interface. Extensive studies have been completed and have shown that most kinds of radiation have two primary effects on MOS structures: positive charge accumulation in the Corresponding author. Fax:+359-2-9753632.
E-mail address: [email protected] (S. Kaschieva).
oxide and creation of new electron states at the Si–SiO2 interface. In our previous work the influence of highenergy electron irradiation on the interface states of Si–SiO2 structures with various oxide thickness has been investigated [1–4]. It was found that 11–12 MeV electrons create a new spectrum of interface states in the silicon band gap. The parameters of these states such as their energy and capture cross sections have been estimated [1,2]. In this paper, a comparison of the interface states generated by 20 MeV electrons in n- and
0042-207X/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2004.07.034
ARTICLE IN PRESS 308
S. Kaschieva et al. / Vacuum 76 (2004) 307–310
p-type MOS structures with equal oxide thickness has been carried out. The activation energy and the concentration of the radiation-induced traps are assessed using the thermally stimulated current (TSC) method. It is demonstrated that most of the created radiation defects can be related to the main impurities in the Si substrate, vacancy-phosphorus or vacancy-boron complexes for MOS structures based on n- and p-type Si wafers, respectively.
curves are taken by a calibrated electrometer as a function of the temperature. The latter is measured using a thermocouple placed near the sample. The initial rise plot method and the Grossweiner’s techniques [5] are applied to the TSC curves to evaluate the parameters of the defect centers associated with each of the observed peaks. The energy positions of the traps resulting from both methods appear to be in a good agreement.
2. Experimental
3. Results and discussion
The MOS capacitors used in this study were fabricated on n-type (4.7 O cm) and p-type (6.4 O cm) Si substrates cut along the o1004 crystallographic plane. The samples were oxidized at 1000 1C in dry oxygen in order to grow oxide thickness of 22 nm. This thickness was measured by ellipsometry. Aluminum gate electrodes were created by photolithography on the top of the oxide layer. The back side of the silicon wafers was coated with a thin layer of Al that served as ohmic contact. The obtained MOS capacitors were irradiated by 20 MeV electrons for 60 or 120 s. The irradiation with a flux of about 8 1012 cm 2 s 1 electron was carried out in Microtron MT-25 in Flerov Laboratory of Nuclear Reactions at the Joint Institute of Nuclear Research (FLNR, JINR) in Dubna, Russia. The irradiation was performed under vacuum of about 1 10 2 Pa. The distance between the Microtron window and the samples was 150 mm. The irradiation was carried out from the gate side of the MOS samples. The electron energy is high enough to penetrate all way through the samples. No bias was applied to the capacitors during the irradiation. In order to characterize the induced defect centers thermally stimulated current characteristics before and after the irradiation were measured. Prior to the measurements the MOS structures are cooled down in dark to about 80 K with accumulation voltage applied. When the filling of the radiation-induced traps at the Si–SiO2 interface is completed, the voltage is turned off and the system is brought to nonequilibrium depletion state. The sample temperature is then raised at a constant rate in the dark and the TSC
The density of interface states induced by 20 MeV electron irradiation of MOS structures is well demonstrated in the TSC curves presented in Figs. 1 and 2. They illustrate typical curves measured for MOS samples based on n- and p-type Si wafers, respectively. In both cases, TSC signal could not be registered before the irradiation. Curves labeled 1 and 2 refer to irradiation for 60 and 120 s, respectively. One can easily observe the appearance of new states at the Si–SiO2 interface, indicating the generation of new discrete energy levels in the silicon forbidden gap. The first
10
2 1
I x 10-11 [ A ]
8
6
4
2
0 100
150
200
250
300
T [K] Fig. 1. TSC curves of MOS structures based on n-type /100S Si wafers (4.7 O cm) following the irradiation with 20 MeV electrons for 60 s (curve 1) and 120 s (curve 2).
ARTICLE IN PRESS S. Kaschieva et al. / Vacuum 76 (2004) 307–310
10
I x 10-11 [ A ]
8
2 1
6
4
2
0 100
150
200 250 T [K]
300
350
Fig. 2. TSC curves of MOS structures prepared on p-type /100S Si wafers (6.4 O cm) after irradiation with 20 MeV electrons for 60 s (curve 1) and 120 s (curve 2).
60 s electron irradiation is enough to start observing four peaks in the TSC spectra, which means that four different kinds of defects at the Si–SiO2 interface have been created. Although the number of peaks in the TSC spectra for both the groups of irradiated samples are equal, their spectra differ (for instance curves 1 in Figs. 1 and 2). While the TSC spectra of n-type samples (Fig. 1) spans from about 80 to 270 K, for p-type MOS samples the spectra is extended towards higher temperatures of around 330 K. For n-type of samples (Fig. 1) the activation energy of these states is evaluated as Ec-0.21; Ec0.26; Ec-0.32 and Ec-0.40 eV. The first energy level can be regarded as interface state associated with vacancy-oxygen defect and the second one as the acceptor level of di-vacancy. The third Ec-0.32 eV level is still not completely understood, but it can be correlated with high-order defects [6]. The last peak, at Ec-0.40 eV can be related with defects like vacancy-phosphorus. It can be seen from curve 2 that this type of defects contributes almost half of the generated traps. An increase in the height and the area incorporating all peaks for longer electron-irradiation times is also observed from curve 2, indicating increase in the concentration of
309
the radiation induced traps at the Si–SiO2 interface with dose. As mentioned above, the TSC spectra of p-type MOS samples (Fig. 2) also reveal four peaks but in a broader temperature interval. From a comparison of the corresponding curves in Figs. 1 and 2 it is obvious that, in this case 20 MeV electron irradiation creates lower (by about 40%) defect concentration at the Si–SiO2 interface. Since some of the peaks overlap, they can be separated by ‘‘thermal’’ and ‘‘voltage cleaning’’ of the TSC spectra. The first peak is located at 115 K and it is connected with the levels Ev+0.17 eV, associated with the free oxygen present at the Si–SiO2 interface [7]. This level is a mirror image of A-centers observed in electronirradiated n- MOS samples [2]. The other peaks in the spectra in Fig. 2 located at about 150, 210 and 300 K are clearly revealed after the electron irradiation (curve 2). The energy positions of the peaks in the Si band gap are Ev+0.24, Ev +0.32 and Ev+0.45 eV. The first two were observed earlier in our previous work [2]. They can be related with di-vacancies and Si atoms recoiled from the Si–SiO2 interface, that migrated into the lattice. The last peak attributed to a level at energy of Ev+0.45 eV is associated with boron-vacancy defect complex. The generation of this kind of defects increase with rising the boron concentration and reduction of the oxygen one in the Si substrate [8]. Curves 1 and 2 also indicate that the main defects created by high-electron irradiation are of V–B type and that their concentration increases intensively with the irradiation dose. Although the boron concentration in p-type Si substrate of about 3 1015 cm 3 was twice the phosphorus concentration in n-type Si substrates, defect generation in n-type MOS samples by high-energy electron irradiation was much more significant. The fact that most part of the radiation induced defects at the Si–SiO2 interface of MOS in this study is related to the main impurities in the Si substrate can be explained if one takes into account that high-energy electron irradiation generates vacancies. The latter tend to form complexes with the basic impurities in the substrate such as phosphorus in n-Si and boron in p-Si wafers.
ARTICLE IN PRESS 310
S. Kaschieva et al. / Vacuum 76 (2004) 307–310
4. Conclusions
References
Two important conclusions can be drawn from our results: (1) the kind of radiation-induced interface traps and their concentration depend on the type of Si wafers used to create MOS structures; and (2) the total concentration of electron irradiationinduced defects at the Si–SiO2 interface of MOS structures is larger in the case of n-type Si wafer.
[1] Kaschieva S. Nucl Instrum Methods B 1994;93:274–6. [2] Stefanov K, Kaschieva S, Karpuzov D. Vacuum 1998;51: 235–7. [3] Kaschieva S, Alexanrdova S. Nucl Instrum Methods 2001;174:324–8. [4] Kaschieva S, Dmitriev SN, Angelov Hr. Nucl Instrum Methods B 2003;206:452–6. [5] Nicholas K, Woods J. Br J Appl Phys 1964;15:783–98. [6] Shinoda K, Ohta E. Appl Phys Lett 1992;61:2691–8. [7] Konozenko D, Semeniuk K. Radiation Effect in Si. Kiev: Naukova Dumka; 1974 p. 72. [8] Vavilov V, Plotnikov A, Tkachev V. Fiz Tv Tela 1982;4:3446–9 (in Russian, English translation in Soviet Physics: Solid State).
Acknowledgements The authors are grateful to Prof. D. Karpuzov for critical discussion of this manuscript.