- Email: [email protected]
crystalline silicon diode for neutron detection application
Solid-State Electronics 78 (2012) 156–158
Contents lists available at SciVerse ScienceDirect
Solid-State Electronics journal homepage: www.elsevier.com/locate/sse
Hot wire chemical vapor deposited boron carbide thin film/crystalline silicon diode for neutron detection application Pradip Chaudhari a, Arvind Singh b, Anita Topkar b, Rajiv Dusane a,⇑ a
Semiconductor Thin Films and Plasma Processing Laboratory, Department of Metallurgical Engineering and Materials Science, Indian Institute of Technology Bombay, Powai, Mumbai 400 076, India b Electronic Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085, India
a r t i c l e
i n f o
Article history: Available online 29 June 2012 The review of this paper was arranged by A. Zaslavsky Keywords: Boron carbide (BC) HWCVD PECVD SIMS Ortho-carborane (o-C2B10H12) Neutron detector
a b s t r a c t Boron carbide films have been deposited on n-type crystalline silicon substrate by hot wire chemical vapor deposition technique using ortho-carborane precursor. The deposited films have been characterized by secondary ion mass spectrometry for their isotopic study which shows the presence of 10B in the film making them suitable for thermal neutron absorber application. These films have a p-type semiconducting nature and hence can be employed to form a p-n junction with n-type silicon. We have demonstrated for the first time the formation of a p-n junction with films deposited by hot wire chemical vapor deposition technique. The current–voltage characteristics of the diode have been measured and the characteristics show low leakage current (nA) in the reverse bias condition. From capacitance–voltage study, it is observed that the depletion width is of the order of few microns which is sufficient for detection of alpha particle released by 10B in the n, a reaction. The results of this study indicate that hot wire chemical vapor deposited boron carbide/crystalline silicon diode could be used as a compact device for thermal neutron detection. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Boron carbide (BC) has been used in several applications due to its high hardness and wear-resistance, high melting point, low density, good thermal and electrical properties, and very good resistance to chemical agents. Boron carbide has become an important candidate in high technology applications like fast-breeders, light weight armor application as its neutron absorption cross-section area is very high [1]. It is considered as a strong candidate for compact semiconductor based neutron detection devices [1,2]. Chemical vapor deposition (CVD) is a chemical process widely used in the semiconductor industry to produce thin films of high-purity and high-performance. Out of many types of CVD processes, Plasma Enhanced Chemical Vapor Deposition (PECVD) has been extensively used in semiconductor industry [3,4]. However, this process could cause damage to the surface of the substrate or an underlying device due to the energetic ions or electrons. To avoid these problems, Hot Wire Chemical Vapor Deposition (HWCVD) technique has been used [5]. Boron carbide thin films reported in this study have been deposited by HWCVD technique using ortho-carborane (o-C2B10H12) as a precursor. Properties of the HWCVD BC film have been indepen⇑ Corresponding author. Tel.: +91 22 2576 7633; fax: +91 22 2572 6975. E-mail address: [email protected] (R. Dusane). 0038-1101/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.sse.2012.05.032
dently studied before incorporating these in a HWCVD BC/crystalline silicon (c-Si) p-n junction device. Fig. 1 shows the schematic of o-C2B10H12 molecule. o-carborane is a polyhedral cluster of boron, carbon and hydrogen atoms. The 12 H skeletal atoms in a carborane molecule form an icosahedron by holding 30 electron–deficient covalent bonds [6]. There are three basic isomers of carborane depending on the relative position of carbon atom such as (A) ortho-carborane (1,2-dicarba-closo-decaborane(12)) shown in Fig. 1, (B) meta–carborane (1,7-dicarba-closo-decaborane(12)) and (C) para-carborane (1,12-dicarba-closo-decaborane(12)) [6]. It is reported that the BC film deposited using ortho-carborane and meta-carborane act as a p-type and n-type semiconducting layer respectively [7]. If a p-type BC film is deposited on n-type crystalline silicon, a p-n diode could be formed. The BC layer which acts as a p-type semiconductor contains 10B, so it can be also used as a neutron converter layer to realize a neutron detection device. The following reaction shows how neutron will interact with 10B in the BC film: ( 10
B þ 1n !
7 7
Li ð0:84 MeVÞ þ 4 Heð1:47 MeVÞ þ cð0:48 MeVÞðQ -value ¼ 2:31 MeVÞ Lið1:02 MeVÞ þ 4 Heð1:78 MeVÞðQ -value ¼ 2:8 MeVÞ6%
Alpha and lithium particles released during this reaction will produce charge carriers in the depletion region of the p-n junction device which is operated under reverse bias condition [8].
P. Chaudhari et al. / Solid-State Electronics 78 (2012) 156–158
157
ments. After formation of metal contacts the devices were annealed in nitrogen environment at 300 °C. Fig. 2 shows cross sectional view of HWCVD BC/c-Si device and its front and back side photographs after aluminum metallization. 3. Result and discussion
Fig. 1. Schematic of ortho-carborane (o-C2B10H12) molecule.
2. Experimental In HWCVD, a precursor is required in the gaseous state so that it easily interacts with the hot filament and dissociates into required species. Here, a naturally occurring solid o-carborane is used as a source precursor, which is sublimated at 70–90 °C in a bubbler assembly designed especially for sublimation of o-carborane. The o-carborane vapor is carried into the reactor by Ar gas through heated tubing so as not to allow any condensation. The BC films of thickness 0.2 lm were deposited at 100 mTorr and substrate temperature between 200–300 °C. The filament (tantalum) temperature is in the range of 1300–2000 °C while the distance between the filament and the substrate is 5 cm. The experimental setup and other details are reported in Ref. [2]. Isotopic study of the films was carried out using secondary ion mass spectrometry (SIMS) (Model: PHI TRIFT V nano TOF). Metal contacts were made on to the front and back side of the fabricated device using physical vapor deposition tool to enable electrical characterization such as current–voltage (I–V) and capacitance–voltage (C–V) measure-
Aluminum Boron carbide
Chemical structure characterization using Infrared spectroscopy unambiguously reveals the presence of prominent peak of B–C bond at 1100 cm1 [2]. X-ray diffraction shows the formation of multiphase BC films on the substrate [2]. SIMS measurements were also carried out for the chemical composition. Fig. 3 shows the depth profile of BC film by SIMS. It is observed that these films are homogenous throughout the thickness and contain all elements which are present in precursor such as H, 11B, 10B, and C. In addition we see the presence of O in these films which could originate during the deposition. It should be mentioned that deposition is done with only a rotary pump and not a turbo molecular pump and rotary combination. The presence of 10B in the films makes it useful as a conversion layer for neutron detection because 10 B has a large capture cross section for thermal neutrons (3840 barns). The semiconductor based radiation detectors are operated in reverse bias mode so as to create a depletion layer which enables collection of charge generated by the incoming radiation. In order to see if the BC/c-Si device acts a p-n diode, reverse as well as forward I–V characteristics have been measured. Fig. 4 shows the forward and reverse I–V characteristics of BC/c-Si heterostructure diode and confirms the formation of p-n junction. Inset of Fig. 4 depicts that the reverse leakage current is 42 nA at 30 V. A diode with such low leakage current could be suitable as a radiation detector. For any radiation detector, low leakage currents are required to reduce the noise of the detector and enhance the signal to noise ratio. With low leakage current and high breakdown voltage (<30 V), higher bias can be applied in order to get good charge collection without substantial increase in the noise level. Several such diodes with contact area of 5 mm2 were fabricated by using HWCVD and subjected to neutron detection. When neutrons interact with boron, they will release two secondary particles, namely 7Li with 0.84 MeV and 4He with 1.47 MeV energy. The range of 4He particle and 7Li particle in BC film is about 3.6 lm and 1.6 lm respectively [9]. As the thickness of the BC film is only 0.2 lm, it may be considered that there is negligible energy loss in boron carbide (BC) film. Assuming that, both charged particles are depositing their energy in the depletion region formed in Si
Si <111>
Aluminum
Fig. 2. Cross-section view of HWCVD BC/c-Si device and its front and back side photographs after metallization.
Fig. 3. Depth profile of HWCVD BC film deposited on Si substrate.
158
P. Chaudhari et al. / Solid-State Electronics 78 (2012) 156–158
Fig. 4. Current–Voltage characteristics of HWCVD BC/c-Si p-n diode of area 5 mm2, inset graph shows the reverse leakage current in the range of nA.
device has been carried out. The C–V plot indicates that the depletion layer thickness increases with reverse bias voltage resulting in decrease of capacitance (Fig. 5). The 1/C2 vs V plot is shown in the inset of Fig. 5 which is observed for a one sided abrupt junction [11]. The range of 4He particles (1.47 MeV) which are released by boron-neutron interaction in the 0.2 lm BC layer is 3.6 lm in BC and 5 lm in silicon [9]. If the charge deposition at entrance window i.e. BC is negligible, the alpha particles generated by n, a reaction will lose almost whole energy in the depletion region provided the width of depletion layer is more than range of alpha particles, i.e. 5 lm. The n-silicon wafer used for the fabrication of diode has a resistivity of 1000–1400 X cm. Assuming a single sided abrupt junction approximation for the BC/n-Si diode, the depletion width has been estimated using C–V characteristics [12]. The required depletion width for the BC/c-Si in silicon is attained at 2.33 Volt. The diode is able to withstand much higher reverse bias without degradation in reverse leakage current. The electrical characteristics of the diode therefore indicate that the diode could be used as a neutron detector with BC layer acting as a converter. 4. Conclusion Boron carbide/c-Si p-n diode has been reported for the first time using a HWCVD deposited thin film. The presence of 10B in the HWCVD BC film has been confirmed by SIMS measurements. I–V and C–V study shows the formation of diode with good characteristics required for radiation detector. The study of performance of this device as a neutron detector is in progress. Acknowledgment This work is funded by Board of Research in Nuclear Science, DAE, India. The SIMS was done at SAIF, IIT Bombay. References
Fig. 5. C–V and (1/C2) vs V (in inset) characteristics HWCVD BC/c-Si p-n diode of area 5 mm2.
substrate, about 104–105 electron–hole pairs will be generated [10]. Electrons and holes move under the influence of electric field and are separated as per their polarity. The charge collected is converted to a pulse by using front end electronics [10]. In a semiconductor detector, electric field exists across the depletion region of the p-n junction. This is the region where the charged particles generated due to ionizing radiation are separated before recombination and eventually collected at the electrodes. To find the width of this region, the C–V measurement of the BC/c-Si
[1] Thevenot F. J Eur Ceram Soc 1990;6:205–25. [2] Chaudhari P, Meshram N, Singh A, Topkar A, Dusane R. Thin Solid Films 2011;519:4561–4. [3] Adenwalla S, Welsch P, Harken A, Brand JI, Sezer A, Robertson BW. Appl Phys Lett 2001;79:4357. [4] Lee S, Mazurowski J, Ramseyer G, Dowben PA. J Appl Phys 1992;72:4925–33. [5] Dusane RO. Thin Solid Films 2008;516:779. [6] Hiura H, Kanayama T. J Mol Struct 2005;735–736:367–74. [7] Caruso AN, Billa RB, Balaz S, Brand JI, Dowben PA. J Phys: Condens Matter 2004;16:L139–46. [8] Knoll GF. Radiation and detection measurement. 2nd ed. Republic of Singapore: John Wiley & Sons; 1989. [9] Day E, Diaz MJ, Adenwalla S. J Phy D: Appl Phys 2006;39:2920–4. [10] Tsoulfanidis N. Measurement and detection of radiation. USA: Hemisphere Publishing Co.; 1983. [11] Sze SM. Physics of semiconductor devices. 2nd ed. India: Wiley Eastern Ltd.; 1987. [12] Hong N, Mullins J, Foreman K, Adenwalla S. J Phys D: Appl Phys 2010;43:275101.
Contents lists available at SciVerse ScienceDirect
Solid-State Electronics journal homepage: www.elsevier.com/locate/sse
Hot wire chemical vapor deposited boron carbide thin film/crystalline silicon diode for neutron detection application Pradip Chaudhari a, Arvind Singh b, Anita Topkar b, Rajiv Dusane a,⇑ a
Semiconductor Thin Films and Plasma Processing Laboratory, Department of Metallurgical Engineering and Materials Science, Indian Institute of Technology Bombay, Powai, Mumbai 400 076, India b Electronic Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085, India
a r t i c l e
i n f o
Article history: Available online 29 June 2012 The review of this paper was arranged by A. Zaslavsky Keywords: Boron carbide (BC) HWCVD PECVD SIMS Ortho-carborane (o-C2B10H12) Neutron detector
a b s t r a c t Boron carbide films have been deposited on n-type crystalline silicon substrate by hot wire chemical vapor deposition technique using ortho-carborane precursor. The deposited films have been characterized by secondary ion mass spectrometry for their isotopic study which shows the presence of 10B in the film making them suitable for thermal neutron absorber application. These films have a p-type semiconducting nature and hence can be employed to form a p-n junction with n-type silicon. We have demonstrated for the first time the formation of a p-n junction with films deposited by hot wire chemical vapor deposition technique. The current–voltage characteristics of the diode have been measured and the characteristics show low leakage current (nA) in the reverse bias condition. From capacitance–voltage study, it is observed that the depletion width is of the order of few microns which is sufficient for detection of alpha particle released by 10B in the n, a reaction. The results of this study indicate that hot wire chemical vapor deposited boron carbide/crystalline silicon diode could be used as a compact device for thermal neutron detection. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Boron carbide (BC) has been used in several applications due to its high hardness and wear-resistance, high melting point, low density, good thermal and electrical properties, and very good resistance to chemical agents. Boron carbide has become an important candidate in high technology applications like fast-breeders, light weight armor application as its neutron absorption cross-section area is very high [1]. It is considered as a strong candidate for compact semiconductor based neutron detection devices [1,2]. Chemical vapor deposition (CVD) is a chemical process widely used in the semiconductor industry to produce thin films of high-purity and high-performance. Out of many types of CVD processes, Plasma Enhanced Chemical Vapor Deposition (PECVD) has been extensively used in semiconductor industry [3,4]. However, this process could cause damage to the surface of the substrate or an underlying device due to the energetic ions or electrons. To avoid these problems, Hot Wire Chemical Vapor Deposition (HWCVD) technique has been used [5]. Boron carbide thin films reported in this study have been deposited by HWCVD technique using ortho-carborane (o-C2B10H12) as a precursor. Properties of the HWCVD BC film have been indepen⇑ Corresponding author. Tel.: +91 22 2576 7633; fax: +91 22 2572 6975. E-mail address: [email protected] (R. Dusane). 0038-1101/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.sse.2012.05.032
dently studied before incorporating these in a HWCVD BC/crystalline silicon (c-Si) p-n junction device. Fig. 1 shows the schematic of o-C2B10H12 molecule. o-carborane is a polyhedral cluster of boron, carbon and hydrogen atoms. The 12 H skeletal atoms in a carborane molecule form an icosahedron by holding 30 electron–deficient covalent bonds [6]. There are three basic isomers of carborane depending on the relative position of carbon atom such as (A) ortho-carborane (1,2-dicarba-closo-decaborane(12)) shown in Fig. 1, (B) meta–carborane (1,7-dicarba-closo-decaborane(12)) and (C) para-carborane (1,12-dicarba-closo-decaborane(12)) [6]. It is reported that the BC film deposited using ortho-carborane and meta-carborane act as a p-type and n-type semiconducting layer respectively [7]. If a p-type BC film is deposited on n-type crystalline silicon, a p-n diode could be formed. The BC layer which acts as a p-type semiconductor contains 10B, so it can be also used as a neutron converter layer to realize a neutron detection device. The following reaction shows how neutron will interact with 10B in the BC film: ( 10
B þ 1n !
7 7
Li ð0:84 MeVÞ þ 4 Heð1:47 MeVÞ þ cð0:48 MeVÞðQ -value ¼ 2:31 MeVÞ Lið1:02 MeVÞ þ 4 Heð1:78 MeVÞðQ -value ¼ 2:8 MeVÞ6%
Alpha and lithium particles released during this reaction will produce charge carriers in the depletion region of the p-n junction device which is operated under reverse bias condition [8].
P. Chaudhari et al. / Solid-State Electronics 78 (2012) 156–158
157
ments. After formation of metal contacts the devices were annealed in nitrogen environment at 300 °C. Fig. 2 shows cross sectional view of HWCVD BC/c-Si device and its front and back side photographs after aluminum metallization. 3. Result and discussion
Fig. 1. Schematic of ortho-carborane (o-C2B10H12) molecule.
2. Experimental In HWCVD, a precursor is required in the gaseous state so that it easily interacts with the hot filament and dissociates into required species. Here, a naturally occurring solid o-carborane is used as a source precursor, which is sublimated at 70–90 °C in a bubbler assembly designed especially for sublimation of o-carborane. The o-carborane vapor is carried into the reactor by Ar gas through heated tubing so as not to allow any condensation. The BC films of thickness 0.2 lm were deposited at 100 mTorr and substrate temperature between 200–300 °C. The filament (tantalum) temperature is in the range of 1300–2000 °C while the distance between the filament and the substrate is 5 cm. The experimental setup and other details are reported in Ref. [2]. Isotopic study of the films was carried out using secondary ion mass spectrometry (SIMS) (Model: PHI TRIFT V nano TOF). Metal contacts were made on to the front and back side of the fabricated device using physical vapor deposition tool to enable electrical characterization such as current–voltage (I–V) and capacitance–voltage (C–V) measure-
Aluminum Boron carbide
Chemical structure characterization using Infrared spectroscopy unambiguously reveals the presence of prominent peak of B–C bond at 1100 cm1 [2]. X-ray diffraction shows the formation of multiphase BC films on the substrate [2]. SIMS measurements were also carried out for the chemical composition. Fig. 3 shows the depth profile of BC film by SIMS. It is observed that these films are homogenous throughout the thickness and contain all elements which are present in precursor such as H, 11B, 10B, and C. In addition we see the presence of O in these films which could originate during the deposition. It should be mentioned that deposition is done with only a rotary pump and not a turbo molecular pump and rotary combination. The presence of 10B in the films makes it useful as a conversion layer for neutron detection because 10 B has a large capture cross section for thermal neutrons (3840 barns). The semiconductor based radiation detectors are operated in reverse bias mode so as to create a depletion layer which enables collection of charge generated by the incoming radiation. In order to see if the BC/c-Si device acts a p-n diode, reverse as well as forward I–V characteristics have been measured. Fig. 4 shows the forward and reverse I–V characteristics of BC/c-Si heterostructure diode and confirms the formation of p-n junction. Inset of Fig. 4 depicts that the reverse leakage current is 42 nA at 30 V. A diode with such low leakage current could be suitable as a radiation detector. For any radiation detector, low leakage currents are required to reduce the noise of the detector and enhance the signal to noise ratio. With low leakage current and high breakdown voltage (<30 V), higher bias can be applied in order to get good charge collection without substantial increase in the noise level. Several such diodes with contact area of 5 mm2 were fabricated by using HWCVD and subjected to neutron detection. When neutrons interact with boron, they will release two secondary particles, namely 7Li with 0.84 MeV and 4He with 1.47 MeV energy. The range of 4He particle and 7Li particle in BC film is about 3.6 lm and 1.6 lm respectively [9]. As the thickness of the BC film is only 0.2 lm, it may be considered that there is negligible energy loss in boron carbide (BC) film. Assuming that, both charged particles are depositing their energy in the depletion region formed in Si
Si <111>
Aluminum
Fig. 2. Cross-section view of HWCVD BC/c-Si device and its front and back side photographs after metallization.
Fig. 3. Depth profile of HWCVD BC film deposited on Si substrate.
158
P. Chaudhari et al. / Solid-State Electronics 78 (2012) 156–158
Fig. 4. Current–Voltage characteristics of HWCVD BC/c-Si p-n diode of area 5 mm2, inset graph shows the reverse leakage current in the range of nA.
device has been carried out. The C–V plot indicates that the depletion layer thickness increases with reverse bias voltage resulting in decrease of capacitance (Fig. 5). The 1/C2 vs V plot is shown in the inset of Fig. 5 which is observed for a one sided abrupt junction [11]. The range of 4He particles (1.47 MeV) which are released by boron-neutron interaction in the 0.2 lm BC layer is 3.6 lm in BC and 5 lm in silicon [9]. If the charge deposition at entrance window i.e. BC is negligible, the alpha particles generated by n, a reaction will lose almost whole energy in the depletion region provided the width of depletion layer is more than range of alpha particles, i.e. 5 lm. The n-silicon wafer used for the fabrication of diode has a resistivity of 1000–1400 X cm. Assuming a single sided abrupt junction approximation for the BC/n-Si diode, the depletion width has been estimated using C–V characteristics [12]. The required depletion width for the BC/c-Si in silicon is attained at 2.33 Volt. The diode is able to withstand much higher reverse bias without degradation in reverse leakage current. The electrical characteristics of the diode therefore indicate that the diode could be used as a neutron detector with BC layer acting as a converter. 4. Conclusion Boron carbide/c-Si p-n diode has been reported for the first time using a HWCVD deposited thin film. The presence of 10B in the HWCVD BC film has been confirmed by SIMS measurements. I–V and C–V study shows the formation of diode with good characteristics required for radiation detector. The study of performance of this device as a neutron detector is in progress. Acknowledgment This work is funded by Board of Research in Nuclear Science, DAE, India. The SIMS was done at SAIF, IIT Bombay. References
Fig. 5. C–V and (1/C2) vs V (in inset) characteristics HWCVD BC/c-Si p-n diode of area 5 mm2.
substrate, about 104–105 electron–hole pairs will be generated [10]. Electrons and holes move under the influence of electric field and are separated as per their polarity. The charge collected is converted to a pulse by using front end electronics [10]. In a semiconductor detector, electric field exists across the depletion region of the p-n junction. This is the region where the charged particles generated due to ionizing radiation are separated before recombination and eventually collected at the electrodes. To find the width of this region, the C–V measurement of the BC/c-Si
[1] Thevenot F. J Eur Ceram Soc 1990;6:205–25. [2] Chaudhari P, Meshram N, Singh A, Topkar A, Dusane R. Thin Solid Films 2011;519:4561–4. [3] Adenwalla S, Welsch P, Harken A, Brand JI, Sezer A, Robertson BW. Appl Phys Lett 2001;79:4357. [4] Lee S, Mazurowski J, Ramseyer G, Dowben PA. J Appl Phys 1992;72:4925–33. [5] Dusane RO. Thin Solid Films 2008;516:779. [6] Hiura H, Kanayama T. J Mol Struct 2005;735–736:367–74. [7] Caruso AN, Billa RB, Balaz S, Brand JI, Dowben PA. J Phys: Condens Matter 2004;16:L139–46. [8] Knoll GF. Radiation and detection measurement. 2nd ed. Republic of Singapore: John Wiley & Sons; 1989. [9] Day E, Diaz MJ, Adenwalla S. J Phy D: Appl Phys 2006;39:2920–4. [10] Tsoulfanidis N. Measurement and detection of radiation. USA: Hemisphere Publishing Co.; 1983. [11] Sze SM. Physics of semiconductor devices. 2nd ed. India: Wiley Eastern Ltd.; 1987. [12] Hong N, Mullins J, Foreman K, Adenwalla S. J Phys D: Appl Phys 2010;43:275101.