- Email: [email protected]
p-Si structure
Vacuum 58 (2000) 308}314
A neutron detector based on an ITO/p-Si structure夽 D. Sueva *, S.S. Georgiev, N. Nedev, A. Toneva, N. Chikov Faculty of Physics, Soxa University, 5 James Boucheir Boulevard, Soxa 1164, Bulgaria Institute of Solid State Physics, Bulgarian Academy of Sciences, 72 Tzarigradsko Chaussee Boulevard, Soxa 1784, Bulgaria Central Laboratory of Solar Energy and New Energy Sources, Bulgarian Academy of Sciences, 72 Tzarigradsko Chaussee Boulevard, Soxa 1784, Bulgaria Institute for Nuclear Reasearch and Nuclear Energy, Bulgarian Academy of Sciences, 72 Tzarigradsko Chaussee Boulevard, Soxa 1784, Bulgaria
Abstract Neutron detectors based on ITO/p-Si structure are fabricated and investigated. The ITO layer is deposited on p-type (100) Si with resistivity 60 ) cm by DC reactive magnetron sputtering of a 90% In}10% Sn target at 4503C substrate temperature. The low temperature of the deposition process retains the large lifetime of the minority carriers in the Si wafer. The degenerately doped ITO layer functions as a metal. Due to contact potential di!erence between Si and ITO, an energy barrier for holes is formed at the interface. It is shown that using relatively low-resistivity silicon (50}100 ) cm), the requirements for neutron detection can be satis"ed and that the ITO/p-Si structure with LiF (80%Li enrichment) deposited directly on the ITO layer can be used for neutron detection. The relatively small depletion region width makes the detector insensitive to the background c-"eld. This is an advantage compared to the conventional high-resistivity Si detectors. 2000 Elsevier Science Ltd. All rights reserved. Keywords: ITO; Schottky contact; Silicon neutron detectors; Mixed gamma-neutron radiation
1. Introduction Silicon detectors, based on p}n junction or Schottky contact, are widely used for neutron registration. For this purpose, (n,a) reactions can be used [1]. In the case of p}n Si detectors 夽
Paper presented at the 11th International School on Vacuum, Electron and Ion Technologies, 20}25 September 1999, Varna, Bulgaria. * Corresponding author. Fax: #359-2-9625-276. E-mail address: [email protected]"a.bg (D. Sueva). 0042-207X/00/$ - see front matter 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 0 4 2 - 2 0 7 X ( 0 0 ) 0 0 1 8 3 - 4
D. Sueva et al. / Vacuum 58 (2000) 308}314
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a conversion layer can be deposited directly on the active detector surface, but this type of detectors are fabricated at high temperature leading to defect generation in the silicon bulk. Schottky contact (surface barrier) detectors, usually Au/n-Si, have much thinner insensitive layer and are fabricated at a temperature of about 4003C. Since the thickness of the Au layer is only several nanometers due to the destruction of the layer wholeness its characteristics deteriorate with time. ITO layers are widely used in optoelectronics because of their low resistivity (of the order of 10\ ) cm) and high transparency (&90%). They can be deposited by evaporation [2,3], sputtering [4,5], etc. An additional advantage of these layers is that they allow formation of a Schottky contact to p-Si. The energy barrier of such contacts is relatively high (&0.8 eV). On the basis of ITO/p-Si structure, solar cells with 12% e$ciency are fabricated [6]. In [7], the same type of structure is investigated as a position-sensitive detector. In this work the ITO/p-Si structure is investigated as a neutron detector. The use of p-type Si is desirable because the minority carriers in p-Si (electrons) have higher mobility than the minority carriers in n-type Si (holes). This di!erence in the minority carrier mobility is the reason for the higher radiation hardness of the p-type silicon. The neutron detection is carried out by using a LiF converting layer (the Li(n, a)H reaction). 2. Experimental The ITO layers were deposited on p-type (100) Si with resistivity 60 ) cm by DC reactive magnetron sputtering of a 90% In}10% Sn target in pure O ambient. The working gas pressure during the deposition was 10\ Pa, the cathode voltage was 350 V and the substrate temperature was 4503C. The deposition rate at the above conditions was 13 nm/min. The low temperature of the deposition process retains the large lifetime of the minority carriers in the silicon. Before the ITO deposition the Si wafers were cleaned in HF acid using the standard procedure. The thickness and the refractive index of the deposited ITO layers were measured ellipsometrically. The thickness was in the range 80}90 nm and the refractive index was 2.2. The back Al contact was deposited by vacuum thermal evaporation and annealed at 4003C for 1 h in a N ambient. Since the ITO (90% In O #10% SnO ) layer is degenerately doped it has high conductivity and plays the role of a Schottky contact to Si. Because of its glass-like properties it acts as a passivating layer, on which the LiF converting layer (80% Li enrichment) is directly deposited by vacuum thermal evaporation at 10\ Pa (melting point 8453C). A Mo boat was used for the LiF evaporation. The C}< measurements were performed by 1 MHz digital LCR meter (Hewlett-Packard 4271B). The I}< measurements were carried out using a Hewlett-Packard HP 4140B. The active area of the detectors is 4 mm. The Pu}Be source used emits 4.5;10n/(4ps) and is combined with an 8 cm thick para$n moderator. The c-rays dose rate is 1.4 Gy/s at 10 cm from the neutron source. The spectroscopy measurements were performed with a charge preampli"er G-2 type designed and manufactured by us. It has low noise 50 mV/2000 pF and high steepness of the input (25 mA/V). The rest of the electronics is standard: linear ampli"er 542-ORTEC type and multichannel analyser SA 40B-INTERTECHNIC. All spectroscopic measurements are performed at room temperature and atmospheric pressure.
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Fig. 1. Energy band diagram of the ITO/p-Si structure.
3. Results and discussion 3.1. Physical model Fig. 1 shows the energy band diagram of the ITO/p-Si structure [8]. The native silicon dioxide (SiO ) between Si and ITO has a tunnelling thickness. The degenerately doped ITO layer functions as a metal. Due to contact potential di!erence between Si and ITO an energy barrier U for holes is formed at the interface. When an a-particle emitted from the converter reaches the junction depletion region a large number of electron}hole pairs are generated (about few hundred thousands). This leads to a strong increase of the nonequilibrium carrier concentration in a local region around the a-particle path. These carriers screen the electric "eld and a great number of them recombine. The rest of the carriers are separated by the junction electric "eld and form an output signal. That is why it is important that the intensity of the electric "eld E in the depletion region must be su$ciently high. This intensity E should ensure a saturation velocity V of the generated carriers in the depletion 1 region (for silicon V is 10 m/s). 1 Silicon with resistivity of the order of 50}100 ) cm allows to obtain a strong electric "eld at a relatively low bias voltage. On the other hand, the Si substrate must have high enough resistivity in order to ensure the required depletion region width (w) necessary for full-energy absorption. Taking into account the relations for the bias voltage U "E .(w/2) and for the depletion region 1 width [9] w"(2eU /qN ) (here E is the electrical "eld at the Si/ITO interface, E "2E, the e is 1 the Si permittivity, N is the acceptor concentration and E is the average value of the electric "eld in the depletion region) we obtain for N :N "eE /qw. From here one may estimate the Si wafer 1 resistivity at previously chosen w and E. In [10] using `burrieda layer the authors also limit the depletion region width and modify the vertical electric "eld distribution of a silicon radiation detector. This method is very perspective but in our case the high-energy ion implantation (10 MeV, 10 cm\ B or P ions) is avoided.
D. Sueva et al. / Vacuum 58 (2000) 308}314
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The Li(n, a)H reaction yield is H with E"2.73 MeV and He with E"2.05 MeV. The range of the 2.73 MeV charged particle in Si is 15 lm [11]. In accordance with the above formula, at w"15 lm and E"10 V/cm, one obtains for N , N "4.4;10 cm\ (q"100 ) cm). Thus for obtaining a su$ciently high average electric "eld at a depletion region width of about 15 lm one can use relatively low-resistivity (100 ) cm) silicon. Such resistivity is much smaller than the resistivity of the conventional silicon detectors (about 1000}2000 ) cm). The relatively small depletion region width (15 lm) gives an additional advantage for e!ective separation of the a-particle spectrum from the c-rays one. 3.2. Electrical tests The experimental I(<) dependence is shown in Fig. 2. In the forward direction, near zero voltage, the I(<) dependence can be described by the ideal diode equation: J"J [exp(qV/kT)!1]. J is determined from the intersection point between the extrapolated I(<) curve and the y-axis. If the value 120 A cm\K\ for the Richardson constant is used [9], one obtains that the e!ective potential barrier height at the ITO/p-Si interface is 0.78 eV. This value is typical for this type of structure [8]. The reverse current at 20 V is about 15 lA. The breakdown voltage of the structure is approximately 30 V. The dependence (1/C) versus bias voltage V is plotted in Fig. 3. The dependence is a straight line which shows that the ITO layer can be considered as a Schottky to the silicon. The slope of the straight line (d(1/C)/dV ) is 6;10 F\ cm V\ which corresponds to an N value of 2;10 cm\ (60 ) cm resistivity). The extrapolated straight line intersects the x-axis at V "0.9 V. This value has a meaning of a contact potential di!erence between ITO and Si. 3.3. Spectroscopic tests The energy spectrum of the a-particles emitted from the Pu}Am source measured at U "20 V for t"10 min is shown in Fig. 4. The spectrum is obtained with LiF layer on the active
Fig. 2. Experimental I}< characteristic of ITO/p-Si structure. The ITO area is 4 mm.
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Fig. 3. The dependence (1/C) versus bias voltage V . The slope of the straight line (d(1/C)/dV ) is 6;10 F\ cm V\ which corresponds to an N value of 2;10 cm\ (60 ) cm resistivity).
Fig. 4. Experimentally measured energy spectrum of the a-particles emitted from the Pu}Am source (U "20 V, distance source/detector"10 cm, t"10 min). The spectrum is obtained with LiF layer on the active detector surface.
detector surface. The a-spectrum is shifted to lower energy compared with the spectrum measured without LiF layer which shows that the LiF converter increases the thickness of the insensitive layer (0.8 nm ITO#4.4 lm LiF). This energy shift gives a possibility to calculate the LiF layer thickness. The energy shift for E"5.486 MeV a-particles is 570 keV which corresponds to a LiF thickness of 1.1 mg/cm or 4.4 lm (q "2.64 g cm\). The three peaks of the a-particles (with * $ energies 5.486, 5.442 and 5.42 MeV) are observed as one at 5.4 MeV since the energy resolution of the detectors is about 175 keV. But in our case, it is more important to separate the a-spectrum from the c-background. The neutron spectrum of the Pu}Be radiation source measured at U "20 V for t"60 min by the ITO/p-Si detector with LiF converter is shown in Fig. 5. The detector}source distance is 10 cm.
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Fig. 5. The neutron spectrum of the Pu}Be radiation source measured at U "20 V, distance source/detector"10 cm, for t"60 min by the ITO/p-Si detector with LiF converter layer.
At a reverse bias of 20 V the depletion region width (13 lm) is approximately su$cient for the full energy absorption of the a-particles created by the Li(n,a)H reaction. The relatively small depletion region width makes the detector insensitive to the background c-"eld. Therefore, this detector may be used for neutron detection in the presence of intensive c-"eld. This is an advantage compared to the conventional high-resistivity Si detectors.
4. Conclusion The ITO layer can be considered as a Schottky contact to the p-Si with an e!ective barrier height of 0.78 eV. The ITO layer allows deposition of an additional LiF converter layer over the active area of the detector. This structure, with LiF, may be used for detection of neutrons in the presence of c-"eld. Using relatively low-resistivity silicon (100 ) cm) the requirements for neutron detection can be satis"ed } su$ciently high electric "eld and width of the depletion region which ensures full energy absorption of the a-particles, generated by the Li(n,a)H reaction.
Acknowledgements This work was supported in part by the Bulgarian National Fund `Scienti"c Investigationsa under Contract F-631.
References [1] Sueva D, Chikov N, Amov B, Kalinkova N. Nucl Instr and Meth B 1989;35:95}9. [2] Jeong JI, Moon JH, Hong JH, Kang JS, Lee YP. Appl Phys Lett 1994;64:1215}7.
314 [3] [4] [5] [6] [7] [8] [9] [10] [11]
D. Sueva et al. / Vacuum 58 (2000) 308}314 Pan CA, Ma TP. Appl Phys Lett 1980;37:163}5. Bellingham JR, Phillips WA, Adkins CJ. J Phys Condens Matter 1990;2:6207}10. Kasiviswanathan S, Rangarajan G. J Appl Phys 1994;75:2572}7. Kobayashy H, Ishida T, Nakato Y, Mori H. J Appl Phys 1995;78:3991}3. Georgiev SS, Sueva D, Nedev N. J Phys Condens Matter 1997;9:4995}5001. Shewchun J, Burk D, Spitzer M. IEEE Trans Electron Devices 1980;ED-27:705}6. Sze SM. Physics of semiconductor devices, 2nd ed. New York: Wiley, 1981 (Chapter 5). Borany J von, Schmidt B, GroK tzschel R. Nucl Instr and Meth A 1996;377:514}20. Knoll GF. Radiation detection and measurement, 2nd ed. New York: Wiley, 1988 (Chapter 2).
A neutron detector based on an ITO/p-Si structure夽 D. Sueva *, S.S. Georgiev, N. Nedev, A. Toneva, N. Chikov Faculty of Physics, Soxa University, 5 James Boucheir Boulevard, Soxa 1164, Bulgaria Institute of Solid State Physics, Bulgarian Academy of Sciences, 72 Tzarigradsko Chaussee Boulevard, Soxa 1784, Bulgaria Central Laboratory of Solar Energy and New Energy Sources, Bulgarian Academy of Sciences, 72 Tzarigradsko Chaussee Boulevard, Soxa 1784, Bulgaria Institute for Nuclear Reasearch and Nuclear Energy, Bulgarian Academy of Sciences, 72 Tzarigradsko Chaussee Boulevard, Soxa 1784, Bulgaria
Abstract Neutron detectors based on ITO/p-Si structure are fabricated and investigated. The ITO layer is deposited on p-type (100) Si with resistivity 60 ) cm by DC reactive magnetron sputtering of a 90% In}10% Sn target at 4503C substrate temperature. The low temperature of the deposition process retains the large lifetime of the minority carriers in the Si wafer. The degenerately doped ITO layer functions as a metal. Due to contact potential di!erence between Si and ITO, an energy barrier for holes is formed at the interface. It is shown that using relatively low-resistivity silicon (50}100 ) cm), the requirements for neutron detection can be satis"ed and that the ITO/p-Si structure with LiF (80%Li enrichment) deposited directly on the ITO layer can be used for neutron detection. The relatively small depletion region width makes the detector insensitive to the background c-"eld. This is an advantage compared to the conventional high-resistivity Si detectors. 2000 Elsevier Science Ltd. All rights reserved. Keywords: ITO; Schottky contact; Silicon neutron detectors; Mixed gamma-neutron radiation
1. Introduction Silicon detectors, based on p}n junction or Schottky contact, are widely used for neutron registration. For this purpose, (n,a) reactions can be used [1]. In the case of p}n Si detectors 夽
Paper presented at the 11th International School on Vacuum, Electron and Ion Technologies, 20}25 September 1999, Varna, Bulgaria. * Corresponding author. Fax: #359-2-9625-276. E-mail address: [email protected]"a.bg (D. Sueva). 0042-207X/00/$ - see front matter 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 0 4 2 - 2 0 7 X ( 0 0 ) 0 0 1 8 3 - 4
D. Sueva et al. / Vacuum 58 (2000) 308}314
309
a conversion layer can be deposited directly on the active detector surface, but this type of detectors are fabricated at high temperature leading to defect generation in the silicon bulk. Schottky contact (surface barrier) detectors, usually Au/n-Si, have much thinner insensitive layer and are fabricated at a temperature of about 4003C. Since the thickness of the Au layer is only several nanometers due to the destruction of the layer wholeness its characteristics deteriorate with time. ITO layers are widely used in optoelectronics because of their low resistivity (of the order of 10\ ) cm) and high transparency (&90%). They can be deposited by evaporation [2,3], sputtering [4,5], etc. An additional advantage of these layers is that they allow formation of a Schottky contact to p-Si. The energy barrier of such contacts is relatively high (&0.8 eV). On the basis of ITO/p-Si structure, solar cells with 12% e$ciency are fabricated [6]. In [7], the same type of structure is investigated as a position-sensitive detector. In this work the ITO/p-Si structure is investigated as a neutron detector. The use of p-type Si is desirable because the minority carriers in p-Si (electrons) have higher mobility than the minority carriers in n-type Si (holes). This di!erence in the minority carrier mobility is the reason for the higher radiation hardness of the p-type silicon. The neutron detection is carried out by using a LiF converting layer (the Li(n, a)H reaction). 2. Experimental The ITO layers were deposited on p-type (100) Si with resistivity 60 ) cm by DC reactive magnetron sputtering of a 90% In}10% Sn target in pure O ambient. The working gas pressure during the deposition was 10\ Pa, the cathode voltage was 350 V and the substrate temperature was 4503C. The deposition rate at the above conditions was 13 nm/min. The low temperature of the deposition process retains the large lifetime of the minority carriers in the silicon. Before the ITO deposition the Si wafers were cleaned in HF acid using the standard procedure. The thickness and the refractive index of the deposited ITO layers were measured ellipsometrically. The thickness was in the range 80}90 nm and the refractive index was 2.2. The back Al contact was deposited by vacuum thermal evaporation and annealed at 4003C for 1 h in a N ambient. Since the ITO (90% In O #10% SnO ) layer is degenerately doped it has high conductivity and plays the role of a Schottky contact to Si. Because of its glass-like properties it acts as a passivating layer, on which the LiF converting layer (80% Li enrichment) is directly deposited by vacuum thermal evaporation at 10\ Pa (melting point 8453C). A Mo boat was used for the LiF evaporation. The C}< measurements were performed by 1 MHz digital LCR meter (Hewlett-Packard 4271B). The I}< measurements were carried out using a Hewlett-Packard HP 4140B. The active area of the detectors is 4 mm. The Pu}Be source used emits 4.5;10n/(4ps) and is combined with an 8 cm thick para$n moderator. The c-rays dose rate is 1.4 Gy/s at 10 cm from the neutron source. The spectroscopy measurements were performed with a charge preampli"er G-2 type designed and manufactured by us. It has low noise 50 mV/2000 pF and high steepness of the input (25 mA/V). The rest of the electronics is standard: linear ampli"er 542-ORTEC type and multichannel analyser SA 40B-INTERTECHNIC. All spectroscopic measurements are performed at room temperature and atmospheric pressure.
310
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Fig. 1. Energy band diagram of the ITO/p-Si structure.
3. Results and discussion 3.1. Physical model Fig. 1 shows the energy band diagram of the ITO/p-Si structure [8]. The native silicon dioxide (SiO ) between Si and ITO has a tunnelling thickness. The degenerately doped ITO layer functions as a metal. Due to contact potential di!erence between Si and ITO an energy barrier U for holes is formed at the interface. When an a-particle emitted from the converter reaches the junction depletion region a large number of electron}hole pairs are generated (about few hundred thousands). This leads to a strong increase of the nonequilibrium carrier concentration in a local region around the a-particle path. These carriers screen the electric "eld and a great number of them recombine. The rest of the carriers are separated by the junction electric "eld and form an output signal. That is why it is important that the intensity of the electric "eld E in the depletion region must be su$ciently high. This intensity E should ensure a saturation velocity V of the generated carriers in the depletion 1 region (for silicon V is 10 m/s). 1 Silicon with resistivity of the order of 50}100 ) cm allows to obtain a strong electric "eld at a relatively low bias voltage. On the other hand, the Si substrate must have high enough resistivity in order to ensure the required depletion region width (w) necessary for full-energy absorption. Taking into account the relations for the bias voltage U "E .(w/2) and for the depletion region 1 width [9] w"(2eU /qN ) (here E is the electrical "eld at the Si/ITO interface, E "2E, the e is 1 the Si permittivity, N is the acceptor concentration and E is the average value of the electric "eld in the depletion region) we obtain for N :N "eE /qw. From here one may estimate the Si wafer 1 resistivity at previously chosen w and E. In [10] using `burrieda layer the authors also limit the depletion region width and modify the vertical electric "eld distribution of a silicon radiation detector. This method is very perspective but in our case the high-energy ion implantation (10 MeV, 10 cm\ B or P ions) is avoided.
D. Sueva et al. / Vacuum 58 (2000) 308}314
311
The Li(n, a)H reaction yield is H with E"2.73 MeV and He with E"2.05 MeV. The range of the 2.73 MeV charged particle in Si is 15 lm [11]. In accordance with the above formula, at w"15 lm and E"10 V/cm, one obtains for N , N "4.4;10 cm\ (q"100 ) cm). Thus for obtaining a su$ciently high average electric "eld at a depletion region width of about 15 lm one can use relatively low-resistivity (100 ) cm) silicon. Such resistivity is much smaller than the resistivity of the conventional silicon detectors (about 1000}2000 ) cm). The relatively small depletion region width (15 lm) gives an additional advantage for e!ective separation of the a-particle spectrum from the c-rays one. 3.2. Electrical tests The experimental I(<) dependence is shown in Fig. 2. In the forward direction, near zero voltage, the I(<) dependence can be described by the ideal diode equation: J"J [exp(qV/kT)!1]. J is determined from the intersection point between the extrapolated I(<) curve and the y-axis. If the value 120 A cm\K\ for the Richardson constant is used [9], one obtains that the e!ective potential barrier height at the ITO/p-Si interface is 0.78 eV. This value is typical for this type of structure [8]. The reverse current at 20 V is about 15 lA. The breakdown voltage of the structure is approximately 30 V. The dependence (1/C) versus bias voltage V is plotted in Fig. 3. The dependence is a straight line which shows that the ITO layer can be considered as a Schottky to the silicon. The slope of the straight line (d(1/C)/dV ) is 6;10 F\ cm V\ which corresponds to an N value of 2;10 cm\ (60 ) cm resistivity). The extrapolated straight line intersects the x-axis at V "0.9 V. This value has a meaning of a contact potential di!erence between ITO and Si. 3.3. Spectroscopic tests The energy spectrum of the a-particles emitted from the Pu}Am source measured at U "20 V for t"10 min is shown in Fig. 4. The spectrum is obtained with LiF layer on the active
Fig. 2. Experimental I}< characteristic of ITO/p-Si structure. The ITO area is 4 mm.
312
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Fig. 3. The dependence (1/C) versus bias voltage V . The slope of the straight line (d(1/C)/dV ) is 6;10 F\ cm V\ which corresponds to an N value of 2;10 cm\ (60 ) cm resistivity).
Fig. 4. Experimentally measured energy spectrum of the a-particles emitted from the Pu}Am source (U "20 V, distance source/detector"10 cm, t"10 min). The spectrum is obtained with LiF layer on the active detector surface.
detector surface. The a-spectrum is shifted to lower energy compared with the spectrum measured without LiF layer which shows that the LiF converter increases the thickness of the insensitive layer (0.8 nm ITO#4.4 lm LiF). This energy shift gives a possibility to calculate the LiF layer thickness. The energy shift for E"5.486 MeV a-particles is 570 keV which corresponds to a LiF thickness of 1.1 mg/cm or 4.4 lm (q "2.64 g cm\). The three peaks of the a-particles (with * $ energies 5.486, 5.442 and 5.42 MeV) are observed as one at 5.4 MeV since the energy resolution of the detectors is about 175 keV. But in our case, it is more important to separate the a-spectrum from the c-background. The neutron spectrum of the Pu}Be radiation source measured at U "20 V for t"60 min by the ITO/p-Si detector with LiF converter is shown in Fig. 5. The detector}source distance is 10 cm.
D. Sueva et al. / Vacuum 58 (2000) 308}314
313
Fig. 5. The neutron spectrum of the Pu}Be radiation source measured at U "20 V, distance source/detector"10 cm, for t"60 min by the ITO/p-Si detector with LiF converter layer.
At a reverse bias of 20 V the depletion region width (13 lm) is approximately su$cient for the full energy absorption of the a-particles created by the Li(n,a)H reaction. The relatively small depletion region width makes the detector insensitive to the background c-"eld. Therefore, this detector may be used for neutron detection in the presence of intensive c-"eld. This is an advantage compared to the conventional high-resistivity Si detectors.
4. Conclusion The ITO layer can be considered as a Schottky contact to the p-Si with an e!ective barrier height of 0.78 eV. The ITO layer allows deposition of an additional LiF converter layer over the active area of the detector. This structure, with LiF, may be used for detection of neutrons in the presence of c-"eld. Using relatively low-resistivity silicon (100 ) cm) the requirements for neutron detection can be satis"ed } su$ciently high electric "eld and width of the depletion region which ensures full energy absorption of the a-particles, generated by the Li(n,a)H reaction.
Acknowledgements This work was supported in part by the Bulgarian National Fund `Scienti"c Investigationsa under Contract F-631.
References [1] Sueva D, Chikov N, Amov B, Kalinkova N. Nucl Instr and Meth B 1989;35:95}9. [2] Jeong JI, Moon JH, Hong JH, Kang JS, Lee YP. Appl Phys Lett 1994;64:1215}7.
314 [3] [4] [5] [6] [7] [8] [9] [10] [11]
D. Sueva et al. / Vacuum 58 (2000) 308}314 Pan CA, Ma TP. Appl Phys Lett 1980;37:163}5. Bellingham JR, Phillips WA, Adkins CJ. J Phys Condens Matter 1990;2:6207}10. Kasiviswanathan S, Rangarajan G. J Appl Phys 1994;75:2572}7. Kobayashy H, Ishida T, Nakato Y, Mori H. J Appl Phys 1995;78:3991}3. Georgiev SS, Sueva D, Nedev N. J Phys Condens Matter 1997;9:4995}5001. Shewchun J, Burk D, Spitzer M. IEEE Trans Electron Devices 1980;ED-27:705}6. Sze SM. Physics of semiconductor devices, 2nd ed. New York: Wiley, 1981 (Chapter 5). Borany J von, Schmidt B, GroK tzschel R. Nucl Instr and Meth A 1996;377:514}20. Knoll GF. Radiation detection and measurement, 2nd ed. New York: Wiley, 1988 (Chapter 2).