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Reduction of plasma-induced damage by electron beam excited plasma CVD
Solar Energy Materials & Solar Cells 65 (2001) 185}191
Reduction of plasma-induced damage by electron beam excited plasma CVD K. Okitsu *, M. Imaizumi , K. Yamaguchi , A. Khan , M. Yamaguchi , M. Ban, M. Tokai, K. Kawamura Toyota Technological Institute, 2-12-1 Hisakata, Tempaku-ku, Nagoya 468-8511, Japan Kawasaki Heavy Industries, Ltd., 118 Futatsuzuka, Noda, Chiba 278-8585, Japan Chubu Electric Power Co. Inc, 20-1 Kitasekiyama, Ohdaka-cho, Midori-ku, Nagoya 459-8522, Japan
Abstract The electron beam excited plasma (EBEP-) CVD has succeeded in making nano-crystaline "lms. On the other hand, the existence of the plasma-induced damage by EBEP-CVD has been con"rmed using the hydrogen plasma by measuring the photoluminescence (PL). After plasma exposure, broad band peak appears in the region of 1.0}0.78 eV (1.2}1.6 lm), and intensity of bound exciton peak with energy of 1.093 eV, which is measured and the non-irradiated silicon has been decreased. The same experiment was also performed with RF plasma and the peak appeared not only for EBEP but also for conventional RF plasma. The damage peak tends to disappear over 4203C of the substrate temperature. The damage recovery analysis has been done in relation to the annealing temperature of the substrate after the plasma exposure. Exciton peak has been increased by increasing the temperature especially at 3503C. Furthermore, plasma-induced peak intensity has been decreased at temperature higher than 5003C. Similar peak has been observed in the samples irradiated with high-energy protons. Therefore, positive ions in the plasma are thought to be the source of the damage of the silicon. The origin of the plasma-induced defect in Si is also considered. According to these results, the electric potential of substrate was controlled in order to avoid collision with positive ions in the plasma. When it was set to zero, plasma-induced peaks did not appear. 2001 Elsevier Science B.V. All rights reserved. Keywords: EBEP-CVD; Photoluminescence; Substrate temperature; Annealing; Proton irradiation
* Corresponding author. Tel.: #81-52-809-1877; fax : #81-52-809-1879. E-mail address: [email protected] (K. Okitsu). 0927-0248/01/$ - see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 7 - 0 2 4 8 ( 0 0 ) 0 0 0 9 4 - 5
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1. Introduction Recently, high-density plasma utilizing systems are widely investigated especially for making polycrystalline silicon layer [1]. Electron beam excited plasma (EBEP-) CVD is one of such kinds of deposition system, in which the plasma is generated by a well-controlled electron beam [2}4]. The gas molecule has generally a very large cross-section for ionization on about 100 eV compared to around a few eV, which is used for the conventional P-CVD. As reported at the latest conference [5], we have succeeded in growing crystalline "lms just from SiH without hydrogen-dilution at low deposition temperature of 4003C or less. EBEP-CVD also has another attractive feature that the plasma is formed independent of the electric or magnetic "eld in the reactive chamber. Therefore, we can avoid acceleration of ions to the substrate by electric and magnetic power. Therefore, it is expected to grow high-quality silicon "lms with much lower level of damage. Then plasma-induced damage by EBEP-CVD was con"rmed and it was reduced by the control of the potential in this study.
2. Experimental In the EBEP-CVD, plasma is generated by the collision of electrons accelerated up to 100 eV. Source gas was SiH . Four-inch boron-doped silicon (1 0 0) wafers are used as substrates for deposition. Nano-crystalline "lms were deposited in various conditions. The damage is thought to be caused by either electrons or ions in plasma. In order to avoid the complication in separating the in#uence of the plasma damage and characteristics of deposited "lm, e!ects by exposure of silicon wafers to hydrogen plasma were investigated. That means H was used for source gas instead of SiH . The conditions of EBEP for this study were uni"ed for standard values: Electron acceleration voltage"100 V, discharge current"15 A, hydrogen #ow rate"5.0 sccm, pressure of the chamber"3.5 Pa. Plasma exposure time was usually set to 60 min. Photoluminescence (PL) mainly evaluated the plasma-induced damages. For the application of CVD, substrate temperature (¹ ) during plasma exposure was varied from RT to 4203C. Subsequently, in order to evaluate thermal recovery properties from the plasma-induced damage, sample annealing was carried out for 30 min from 2003C to 6003C by slow degrees at 503C step. Here 2003C is expected to be by plasma without heating. In order to compare with conventional plasma, exposure to hydrogen plasma was also examined with RF plasma. The results were compared with samples irradiated by high-energy electrons and protons in order to identify the origin of the damage, for which deep level transient spectroscopy (DLTS) measurements were done [6]. Then the control of the substrate's electric potential was carried out.
K. Okitsu et al. / Solar Energy Materials & Solar Cells 65 (2001) 185}191
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3. Results and discussion Since damaged layer is expected to be thin, samples were analyzed by PL mainly. Fig. 1 shows typical PL spectrum of the hydrogen plasma exposed sample. Same peaks were observed after EBEP-CVD growth but apparently di!erent to the spectrum of amorphous silicon "lm deposited by EBEP-CVD. Sharp peak at 1.134 lm (1.093 eV) originates from bound exciton, it is also measured in non-irradiated silicon. It is made weaker before exposure to plasma while broad band peak appears after plasma exposure in the region of 1.2}1.6 lm (1.0}0.78 eV) [7]. In order to con"rm the existence of plasma-induced damage on conventional plasma, the same experiment was done for RF plasma with hydrogen. Just the same phenomena were observed as in the conventional sample. Secondary, e!ects of the substrate temperature concerning the application of CVD on plasma damage induction were studied. Because damage was expected to be relaxed when the temperature is high, the substrate temperature was varied from RT to 4203C, which is typically used for our EBEP-CVD. Damage-induced peak which spread broadly in the region from 1.2 to 1.6 lm (energy are 1.0 and 0.78 eV, respectively) appears from RT to 3403C, although peak intensity changes drastically, as shown in Fig. 2. The intensity of the damage peak was decreased with increase in the temperature, and disappeared at ¹ of over 4203C. On the contrary, the intensity of the crystalline silicon peak increased with increase in ¹ . As a result, it is considered that the crystallinity of the wafer would not be disturbed by the plasma when the temperature is higher than about 4003C, otherwise some method to avoid the plasma damage is necessary.
Fig. 1. PL spectrum of the plasma-exposed silicon. Sharp peak at 1134 nm was also measured in non-irradiated silicon wafer, and it is originated from bound exciton. After plasma exposure, broad band peak was generated from 1.2 to 1.6 lm.
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Fig. 2. Changes in PL intensities of damage-peak (indicated `broada) and exciton-peak as a function of the substrate temperature.
Fig. 3. PL intensities after annealing. Some recovery of the crystallinity was observed, especially with annealing temperature of over 5003C.
From the result of substrate temperature, thermal e!ect is expected to invalidate the plasma-induced damage. Therefore, to get the recovering method from damage and to analyze the origins of plasma-induced defects, sample annealing was carried out. It was repeated for 30 min each from 2003C to 6003C at steps of 503C. The results are shown in Fig. 3. Some recovery was observed over 3503C at the "rst stage [8]. The intensity of the sharp crystalline peak increases gradually after annealing. Subsequently, damage-induced peak tends to disappear at temperatures higher than 5003C. The di!erence of damage recovery temperature of the exposed time and annealing is considered to originate from the di!erence of the particle having kinetic energy and thermal energy or having only thermal energy.
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Fig. 4. PL spectrum of the silicon irradiated by 10 MeV protons (3;10 protons/cm).
The results caused by plasma were compared with results for the samples irradiated with high-energy electrons and protons. Although the intensity of the damage (broad band) peak was about 10 times larger than that of plasma-damaged sample, similar broad peak was observed in proton-irradiated sample with 10 MeV (Fig. 4). Di!erence of the intensity would be caused by the thickness of damaged layer [9}11]. From the coincidence of the spectra, we conclude that positive ions in the plasma would cause damage to the silicon. The proton irradiated sample was measured by DLTS and Ev#0.36 eV defect center strongly appeared [6]. Furthermore, this was decreased after annealing over 3003C. It was similar to the PL spectrum of the plasma damage, then it is considered that these kind of damages are both originated from Ci}Oi or V}O}B (Bi}Oi) [6]. According to this study, e!ects of the electric potential of the substrate on plasma damage have been examined. The substrate has ever been electrically #oating and negatively charged naturally. When we measure the electrical potential of the substrate, it was about !100 V. It was considered that the potential is originated from the #ow of electrons and ions in the plasma. Then, potential control technique was introduced to EBEP system. In this respect, EBEP is superior in practice, because it does not require any conditions for the substrate and is everywhere in the reactive chamber. The potential was varied from !150 to 0. When it was set to zero in order to avoid the damage by collision with positively charged ions, the broad band peak induced by plasma did not appear and crystalline peak still existed on the PL measurement as shown in Fig. 5.
4. Conclusion Plasma-induced damage by EBEP-CVD was observed in the PL spectrum because of the sensitivity. Its existence was con"rmed by the exposure to hydrogen plasma in
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Fig. 5. PL spectrum after the introduction of electric potential control. When the potential is zero, plasma damage related peak did not appear and exciton's peak still existed.
order to separate the property of the grown "lm. After the exposure, broad band peak was generated in 1.2}1.6 lm region (1.0}0.78 eV). And sharp peak intensity at 1.134 lm (1.093 eV), which is originated from bound exciton in silicon, has decreased. Considering the apparatus for CVD of silicon, substrate temperature was varied from RT to 4203C. Increasing the temperature, exciton's peak was increased, and plasma-induced peak did not appear at 4203C. To analyze the thermal property, damaged samples were annealed. Consequently, (1) exciton peak was enlarged at 3503C especially, and (2) broad band peak in 1.2}1.6 lm disappeared over 5003C. By comparing with the results for the sample irradiated by protons, we concluded that the damage is caused by the collision of the positive ions with the silicon substrate. The origin of plasma-induced defects in Si is considered to be Ci}Oi or V}O}B (Bi}Oi). According to this study, electrical potential of the substrate was changed from negative value to zero, and then we succeeded in avoiding the plasma damage in PL spectrum. Therefore, in order to reduce the e!ect of the plasma-induced damage, electrical potential control technique is important for EBEP-CVD.
Acknowledgements This work was partially supported by the Japan Ministry of Education as part of the studies of the `Private University High-Tech Research Center Programa at TTI.
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References [1] [2] [3] [4]
[5] [6] [7] [8] [9] [10] [11]
M. Rostalsky, T. Kunze, N. Linke, J. Muller, IEEE P.V. Specialists Conference 1997 (1997) 743. T. Hara, M. Hamagaki, A. Sanda, Y. Aoyagi, S. Namba, J. Vac. Sci. Technol. B5 (1987) 366. T. Ohgo, T. Hara, M. Hamagaki, K. Ishii, M. Otsuka, J. Appl. Phys. 70 (1991) 4050. T. Ito, I. Konomi, K. Okitsu, M. Imaizumi, K. Yamaguchi, M. Yamaguchi, T. Hara, M. Ban, M. Tokai, K. Kawamura, Second World Conference and Exhibition on PV Solar Energy Conversion, 1998. M. Imaizumi, T. Ito, K. Okitsu, K. Yamaguchi, M. Yamaguchi, T. Hara, Proceedings of the Second World Conference on Photovoltaic Solar Energy Conversion, WIP, 1998, pp. 1758}1761. A. Khan, M. Yamaguchi, S.J. Taylor, T. Hisamatsu, S. Matsuda, Jpn. J. Appl. Phys. 38 (1998) 2679. S. Nitta, K. Shimakawa, Y. Tsutsumi, T. Endo, K. Morigaki, in: B.L.H. Wilson (Ed.), Physics of Semiconductors, Institute of Physics. Conf. Series No. 43, 1978, p. 1151. M. Kastner, Phys. Rev. Lett. 28 (1972) 355. G.P. Summers, G.J. Shaw, M.A. Xapsos, B.D. Weaver, G.P. Summers, IEEE Trans. Nucl. Sci. 40 (1993) 1300. G.P. Summers, E.A. Burke, C.J. Dale, E.A. Wolicki, P.W. Marshall, M.A. Gehlhausen, IEEE Trans. Nucl. Sci. 34 (1987) 1134. G.P. Summers, E.A. Burke, M.A. Xapsos, C.J. Dale, P.W. Marshall, E.L. Petersen, IEEE Trans. Nucl. Sci. 35 (1988) 1221.
Reduction of plasma-induced damage by electron beam excited plasma CVD K. Okitsu *, M. Imaizumi , K. Yamaguchi , A. Khan , M. Yamaguchi , M. Ban, M. Tokai, K. Kawamura Toyota Technological Institute, 2-12-1 Hisakata, Tempaku-ku, Nagoya 468-8511, Japan Kawasaki Heavy Industries, Ltd., 118 Futatsuzuka, Noda, Chiba 278-8585, Japan Chubu Electric Power Co. Inc, 20-1 Kitasekiyama, Ohdaka-cho, Midori-ku, Nagoya 459-8522, Japan
Abstract The electron beam excited plasma (EBEP-) CVD has succeeded in making nano-crystaline "lms. On the other hand, the existence of the plasma-induced damage by EBEP-CVD has been con"rmed using the hydrogen plasma by measuring the photoluminescence (PL). After plasma exposure, broad band peak appears in the region of 1.0}0.78 eV (1.2}1.6 lm), and intensity of bound exciton peak with energy of 1.093 eV, which is measured and the non-irradiated silicon has been decreased. The same experiment was also performed with RF plasma and the peak appeared not only for EBEP but also for conventional RF plasma. The damage peak tends to disappear over 4203C of the substrate temperature. The damage recovery analysis has been done in relation to the annealing temperature of the substrate after the plasma exposure. Exciton peak has been increased by increasing the temperature especially at 3503C. Furthermore, plasma-induced peak intensity has been decreased at temperature higher than 5003C. Similar peak has been observed in the samples irradiated with high-energy protons. Therefore, positive ions in the plasma are thought to be the source of the damage of the silicon. The origin of the plasma-induced defect in Si is also considered. According to these results, the electric potential of substrate was controlled in order to avoid collision with positive ions in the plasma. When it was set to zero, plasma-induced peaks did not appear. 2001 Elsevier Science B.V. All rights reserved. Keywords: EBEP-CVD; Photoluminescence; Substrate temperature; Annealing; Proton irradiation
* Corresponding author. Tel.: #81-52-809-1877; fax : #81-52-809-1879. E-mail address: [email protected] (K. Okitsu). 0927-0248/01/$ - see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 7 - 0 2 4 8 ( 0 0 ) 0 0 0 9 4 - 5
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K. Okitsu et al. / Solar Energy Materials & Solar Cells 65 (2001) 185}191
1. Introduction Recently, high-density plasma utilizing systems are widely investigated especially for making polycrystalline silicon layer [1]. Electron beam excited plasma (EBEP-) CVD is one of such kinds of deposition system, in which the plasma is generated by a well-controlled electron beam [2}4]. The gas molecule has generally a very large cross-section for ionization on about 100 eV compared to around a few eV, which is used for the conventional P-CVD. As reported at the latest conference [5], we have succeeded in growing crystalline "lms just from SiH without hydrogen-dilution at low deposition temperature of 4003C or less. EBEP-CVD also has another attractive feature that the plasma is formed independent of the electric or magnetic "eld in the reactive chamber. Therefore, we can avoid acceleration of ions to the substrate by electric and magnetic power. Therefore, it is expected to grow high-quality silicon "lms with much lower level of damage. Then plasma-induced damage by EBEP-CVD was con"rmed and it was reduced by the control of the potential in this study.
2. Experimental In the EBEP-CVD, plasma is generated by the collision of electrons accelerated up to 100 eV. Source gas was SiH . Four-inch boron-doped silicon (1 0 0) wafers are used as substrates for deposition. Nano-crystalline "lms were deposited in various conditions. The damage is thought to be caused by either electrons or ions in plasma. In order to avoid the complication in separating the in#uence of the plasma damage and characteristics of deposited "lm, e!ects by exposure of silicon wafers to hydrogen plasma were investigated. That means H was used for source gas instead of SiH . The conditions of EBEP for this study were uni"ed for standard values: Electron acceleration voltage"100 V, discharge current"15 A, hydrogen #ow rate"5.0 sccm, pressure of the chamber"3.5 Pa. Plasma exposure time was usually set to 60 min. Photoluminescence (PL) mainly evaluated the plasma-induced damages. For the application of CVD, substrate temperature (¹ ) during plasma exposure was varied from RT to 4203C. Subsequently, in order to evaluate thermal recovery properties from the plasma-induced damage, sample annealing was carried out for 30 min from 2003C to 6003C by slow degrees at 503C step. Here 2003C is expected to be by plasma without heating. In order to compare with conventional plasma, exposure to hydrogen plasma was also examined with RF plasma. The results were compared with samples irradiated by high-energy electrons and protons in order to identify the origin of the damage, for which deep level transient spectroscopy (DLTS) measurements were done [6]. Then the control of the substrate's electric potential was carried out.
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3. Results and discussion Since damaged layer is expected to be thin, samples were analyzed by PL mainly. Fig. 1 shows typical PL spectrum of the hydrogen plasma exposed sample. Same peaks were observed after EBEP-CVD growth but apparently di!erent to the spectrum of amorphous silicon "lm deposited by EBEP-CVD. Sharp peak at 1.134 lm (1.093 eV) originates from bound exciton, it is also measured in non-irradiated silicon. It is made weaker before exposure to plasma while broad band peak appears after plasma exposure in the region of 1.2}1.6 lm (1.0}0.78 eV) [7]. In order to con"rm the existence of plasma-induced damage on conventional plasma, the same experiment was done for RF plasma with hydrogen. Just the same phenomena were observed as in the conventional sample. Secondary, e!ects of the substrate temperature concerning the application of CVD on plasma damage induction were studied. Because damage was expected to be relaxed when the temperature is high, the substrate temperature was varied from RT to 4203C, which is typically used for our EBEP-CVD. Damage-induced peak which spread broadly in the region from 1.2 to 1.6 lm (energy are 1.0 and 0.78 eV, respectively) appears from RT to 3403C, although peak intensity changes drastically, as shown in Fig. 2. The intensity of the damage peak was decreased with increase in the temperature, and disappeared at ¹ of over 4203C. On the contrary, the intensity of the crystalline silicon peak increased with increase in ¹ . As a result, it is considered that the crystallinity of the wafer would not be disturbed by the plasma when the temperature is higher than about 4003C, otherwise some method to avoid the plasma damage is necessary.
Fig. 1. PL spectrum of the plasma-exposed silicon. Sharp peak at 1134 nm was also measured in non-irradiated silicon wafer, and it is originated from bound exciton. After plasma exposure, broad band peak was generated from 1.2 to 1.6 lm.
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Fig. 2. Changes in PL intensities of damage-peak (indicated `broada) and exciton-peak as a function of the substrate temperature.
Fig. 3. PL intensities after annealing. Some recovery of the crystallinity was observed, especially with annealing temperature of over 5003C.
From the result of substrate temperature, thermal e!ect is expected to invalidate the plasma-induced damage. Therefore, to get the recovering method from damage and to analyze the origins of plasma-induced defects, sample annealing was carried out. It was repeated for 30 min each from 2003C to 6003C at steps of 503C. The results are shown in Fig. 3. Some recovery was observed over 3503C at the "rst stage [8]. The intensity of the sharp crystalline peak increases gradually after annealing. Subsequently, damage-induced peak tends to disappear at temperatures higher than 5003C. The di!erence of damage recovery temperature of the exposed time and annealing is considered to originate from the di!erence of the particle having kinetic energy and thermal energy or having only thermal energy.
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189
Fig. 4. PL spectrum of the silicon irradiated by 10 MeV protons (3;10 protons/cm).
The results caused by plasma were compared with results for the samples irradiated with high-energy electrons and protons. Although the intensity of the damage (broad band) peak was about 10 times larger than that of plasma-damaged sample, similar broad peak was observed in proton-irradiated sample with 10 MeV (Fig. 4). Di!erence of the intensity would be caused by the thickness of damaged layer [9}11]. From the coincidence of the spectra, we conclude that positive ions in the plasma would cause damage to the silicon. The proton irradiated sample was measured by DLTS and Ev#0.36 eV defect center strongly appeared [6]. Furthermore, this was decreased after annealing over 3003C. It was similar to the PL spectrum of the plasma damage, then it is considered that these kind of damages are both originated from Ci}Oi or V}O}B (Bi}Oi) [6]. According to this study, e!ects of the electric potential of the substrate on plasma damage have been examined. The substrate has ever been electrically #oating and negatively charged naturally. When we measure the electrical potential of the substrate, it was about !100 V. It was considered that the potential is originated from the #ow of electrons and ions in the plasma. Then, potential control technique was introduced to EBEP system. In this respect, EBEP is superior in practice, because it does not require any conditions for the substrate and is everywhere in the reactive chamber. The potential was varied from !150 to 0. When it was set to zero in order to avoid the damage by collision with positively charged ions, the broad band peak induced by plasma did not appear and crystalline peak still existed on the PL measurement as shown in Fig. 5.
4. Conclusion Plasma-induced damage by EBEP-CVD was observed in the PL spectrum because of the sensitivity. Its existence was con"rmed by the exposure to hydrogen plasma in
190
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Fig. 5. PL spectrum after the introduction of electric potential control. When the potential is zero, plasma damage related peak did not appear and exciton's peak still existed.
order to separate the property of the grown "lm. After the exposure, broad band peak was generated in 1.2}1.6 lm region (1.0}0.78 eV). And sharp peak intensity at 1.134 lm (1.093 eV), which is originated from bound exciton in silicon, has decreased. Considering the apparatus for CVD of silicon, substrate temperature was varied from RT to 4203C. Increasing the temperature, exciton's peak was increased, and plasma-induced peak did not appear at 4203C. To analyze the thermal property, damaged samples were annealed. Consequently, (1) exciton peak was enlarged at 3503C especially, and (2) broad band peak in 1.2}1.6 lm disappeared over 5003C. By comparing with the results for the sample irradiated by protons, we concluded that the damage is caused by the collision of the positive ions with the silicon substrate. The origin of plasma-induced defects in Si is considered to be Ci}Oi or V}O}B (Bi}Oi). According to this study, electrical potential of the substrate was changed from negative value to zero, and then we succeeded in avoiding the plasma damage in PL spectrum. Therefore, in order to reduce the e!ect of the plasma-induced damage, electrical potential control technique is important for EBEP-CVD.
Acknowledgements This work was partially supported by the Japan Ministry of Education as part of the studies of the `Private University High-Tech Research Center Programa at TTI.
K. Okitsu et al. / Solar Energy Materials & Solar Cells 65 (2001) 185}191
191
References [1] [2] [3] [4]
[5] [6] [7] [8] [9] [10] [11]
M. Rostalsky, T. Kunze, N. Linke, J. Muller, IEEE P.V. Specialists Conference 1997 (1997) 743. T. Hara, M. Hamagaki, A. Sanda, Y. Aoyagi, S. Namba, J. Vac. Sci. Technol. B5 (1987) 366. T. Ohgo, T. Hara, M. Hamagaki, K. Ishii, M. Otsuka, J. Appl. Phys. 70 (1991) 4050. T. Ito, I. Konomi, K. Okitsu, M. Imaizumi, K. Yamaguchi, M. Yamaguchi, T. Hara, M. Ban, M. Tokai, K. Kawamura, Second World Conference and Exhibition on PV Solar Energy Conversion, 1998. M. Imaizumi, T. Ito, K. Okitsu, K. Yamaguchi, M. Yamaguchi, T. Hara, Proceedings of the Second World Conference on Photovoltaic Solar Energy Conversion, WIP, 1998, pp. 1758}1761. A. Khan, M. Yamaguchi, S.J. Taylor, T. Hisamatsu, S. Matsuda, Jpn. J. Appl. Phys. 38 (1998) 2679. S. Nitta, K. Shimakawa, Y. Tsutsumi, T. Endo, K. Morigaki, in: B.L.H. Wilson (Ed.), Physics of Semiconductors, Institute of Physics. Conf. Series No. 43, 1978, p. 1151. M. Kastner, Phys. Rev. Lett. 28 (1972) 355. G.P. Summers, G.J. Shaw, M.A. Xapsos, B.D. Weaver, G.P. Summers, IEEE Trans. Nucl. Sci. 40 (1993) 1300. G.P. Summers, E.A. Burke, C.J. Dale, E.A. Wolicki, P.W. Marshall, M.A. Gehlhausen, IEEE Trans. Nucl. Sci. 34 (1987) 1134. G.P. Summers, E.A. Burke, M.A. Xapsos, C.J. Dale, P.W. Marshall, E.L. Petersen, IEEE Trans. Nucl. Sci. 35 (1988) 1221.