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The radiation damage of crystalline silicon PN diode in tritium beta-voltaic battery
Applied Radiation and Isotopes 90 (2014) 165–169
Contents lists available at ScienceDirect
Applied Radiation and Isotopes journal homepage: www.elsevier.com/locate/apradiso
The radiation damage of crystalline silicon PN diode in tritium beta-voltaic battery Yisong Lei, Yuqing Yang, Yebing Liu, Hao Li, Guanquan Wang, Rui Hu, Xiaoling Xiong, Shunzhong Luo n Institute of Nuclear Physics and Chemistry, China Academy of Engineering Physics, Mianyang 621900, China
H I G H L I G H T S
Remarkable degradation of output property is observed in tritium beta-voltaic battery. X-ray emitted from tritium source is the main factor which leads to the degradation. Radiation induced positive charge mainly influences open-circuit voltage and Si–SiO2 interface trap mainly influences short-circuit current. Beta particle has positive effect in eliminating the radiation damage by x-ray.
art ic l e i nf o
a b s t r a c t
Article history: Received 9 November 2013 Received in revised form 15 February 2014 Accepted 24 March 2014 Available online 5 April 2014
A tritium beta-voltaic battery using a crystalline silicon convertor composed of (1 0 0)Si/SiO2/Si3N4 film degrades remarkably with radiation from a high intensity titanium tritide film. Simulation and experiments were carried out to investigate the main factor causing the degradation. The radiation damages mainly comes from the x-ray emitted from the titanium tritide film and beta particle can relieve the damages. The x-ray radiation induced positive charges in the SiO2 film destroying the output property of the PN diode with the induction of an electric field. & 2014 Elsevier Ltd. All rights reserved.
Keywords: Tritium beta-voltaic Radiation damage Geant4 ESR SiO2–Si Soft x-ray
1. Introduction With the booming in MEMS (Micro Electro Mechanical Systems), the optimization of micro energy systems, which are a crucial part, becomes more and more urgent. The beta-voltaic battery, one type of radioactive isotope battery (RIB), which is distinguished by excellent properties in integration with MEMS and specialized properties such as a long service life, high energy density and free of maintenance (Wang et al., 2010), has becomes a focus of this work. Many attempts have been made in design of high-efficiency beta-voltaic batteries through the optimization structures and materials (Chandrashekhar et al., 2006; Lee, 2009; San et al., 2013; Sun et al., 2005). Among energy conversation devices, silicon is one of the most popular materials in beta-voltaic battery due to the mature processing n
Corresponding author. Tel.: þ 86 13700960909; fax: þ 86 816 2496863. E-mail address: [email protected] (S. Luo).
http://dx.doi.org/10.1016/j.apradiso.2014.03.027 0969-8043/& 2014 Elsevier Ltd. All rights reserved.
technology. In terms of radioisotopes, tritium is the most promising radioisotope because it is safe, cheap and commercially available (Liu et al., 2009). Titanium tritide (Ti3H2) is a well-developed tritium source with high stability (Li et al., 2012). In this work, c-Si with a SiO2–Si3N4 passivated surface, and loaded with a titanium tritide film was adopted. In our previous work, both experiment (Liu et al., 2012) and simulation (Li et al., 2012) of tritium beta-voltaics have been studied. In recent studies, remarkable radiation degradation of tritium betavoltaic batteries using a c-Si PN energy conversion device with a high intensity titanium tritide films has been observed. Little research has been carried out on the radiation damage of c-Si PN diode energy conversion devices in tritium beta-voltaic batteries because the energy of the beta particles emitted from tritium is far below the threshold energy of displacement damage in c-Si. In spite of numerous reports (Gwyn, 1969) the focus on low-energy-radiation induced degradation in Metal Oxide Semiconductor (MOS) systems,
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Y. Lei et al. / Applied Radiation and Isotopes 90 (2014) 165–169
the mechanism of the defect formation in the PN diode is still obscure. The energy conversion mechanism of beta-voltaic batteries resembles the photovoltaic battery, but the damage only focuses on displacement damage induced by high energy particles. Investigating the degradation effect in tritium beta-voltaic batteries is necessary. In this work, a Geant4 Monte Carlo simulation and experiment detection were carried out to investigate the degradation phenomenon. There is not only beta but also x-ray emitted from titanium tritide sources applied in beta-voltaic batteries. The beta particles and x-rays generated in the Ti3H2 film are investigated independently in order to find out the main factor causing radiation damage. Simulations of the energy deposition in energy conversion devices and the emissions from the titanium tritide are implemented. Conjunction of the simulation and experiment results, are supported with measurements with X-ray Photoelectron Spectroscopy (XPS) and Electron Spin Resonance (ESR) to find out the variation in the micro-structure of the PN diode after radiation. Based on these results, analysis of a possible theoretic mechanisms or radiation damage and some optimization suggestions are given. The results are important in the design and optimization of new tritium beta-voltaic batteries.
2. Experiments 2.1. Theoretic simulation The structure of our cell is presented in Fig. 1. The simulation model consists of a Ti3H2 layer, an air gap, a Si3N4 layer, a SiO2 layer, and a Si layer. The beta particle is emitted in 4π directions from the Ti3H2 film which also emits x-rays. The air-gap in Fig. 1 is 0.1 mm width when the cell functions regularly. The Geant4.9.2 Monte Carlo transport toolkit was used to simulate the energy deposition of 2 107 beta particles in the converter considering two gap values of 0.1 mm and 5 mm. The energy spectrum of x-ray emitted from the radioactive source was calculated with 1 108 beta particles. The electron and photon physics used in the Geant4 simulation is the Penelope model. 2.2. Radiation experiment and measurement The energy conversion device is a plain (1 0 0)c-Si P þ NN þ diode. The radiation sources is a Ti3H2 film with an intensity of 87G-Bq/cm2 loaded on a round metal slice substrate. All the used
sources are parallel. According to the effective range of beta in air, when the air gap value was set to 5 mm, only x-ray can interact with energy conversion device in this model. When the air gap was 0.1 mm, there are both x-ray and beta particle interactions with the energy conversion device. The leakage currents before and after a long period (approximately 8 weeks) of irradiation were measured. All micro-currents and voltages were detected by KEITHLEY Model 6517B and KEITHLEY 2635 electrometers. After irradiation in each model, the energy conversion devices were annealed at 475 K for 2 h under pure nitrogen gas. Experiments were carried out in clean lab with controllable temperature and humidity. In addition, XPS and ESR of the energy conversion devices were measured to analyze the radiation damage. The XPS is determined by XSAM800 spectrometer and the device for ESR detection is a Bruker EMX-10/12 PLUS.
3. Results 3.1. Simulation Simulation result of the x-ray's energy spectrum emitted from the Ti3H2 film is shown in Fig. 2. The x-ray spectrum is made up of two parts. The first part is the product of the bremsstrahlung and the other is generated by the transition of outer electrons in the inner shell of Ti. Two characteristic peaks are found at 4.45 keV and 4.85 keV, respectively, which correspond to the Kα and Kβ transition of titanium. The energy conversion devices were irradiated under only x-ray or co-irradiated under both x-ray and beta particle. Both gap values were simulated with 2 107 beta particles where 5.745 keV is the average energy of each beta particle. The results of energy deposition simulation are shown in Table 1. Contrasting the beta deposition results of 5 mm to those of a 0.1 mm gap in Table 1, it is reasonable to suppose that the beta energy deposited in energy conversion devices of 5 mm gap is three orders of magnitude less than that of a 0.1 mm gap. Comparing x-ray energy deposition values of the 5 mm gap value with those of 0.1 mm, the deposited x-ray energy values in all films except in the Si3N4 layer in the 0.1 mm gap are always greater than those of the 5 mm gap. It is reasonable to expect a lower degradation with radiation in the 5 mm gap while other parameters are identical. In order to connect the simulation results with titanium tritide used in radiation experiments, absorbed dose was used to evaluate the radiation. Absorbed dose can be calculated by eq. 1. EA ¼
Ed N t n Mc
ð1Þ
In Eq. (1), EA stands for absorbed dose, Ed for energy deposition, N for the activity of our titanium tritide film, t for irradiation time, n for the value of Monte Carlo event and MC for the mass of different solid convertor layer. When t¼ 8 weeks (pure x-ray
Fig. 1. The schematic of plain silicon beta-voltaic loaded with Ti3H2 film.
Fig. 2. The x-ray spectrum emitted from Ti3H2 source layer with 1 108 simulation beta particles.
Y. Lei et al. / Applied Radiation and Isotopes 90 (2014) 165–169
167
Table 1 The energy deposition of 2 107 simulation beta particles in energy conversion devices with two different gap radiations. Radiation model (mm)
Material
Energy deposition by electron (GeV)
Beta's percent of total energy
Energy deposition by x-ray (MeV)
X-ray's percent of total energy
5
Ti3H2 Air Si3N4 SiO2 Silicon Ti3H2 Air Si3N4 SiO2 Silicon
110.3 2.07 6.85E 05 2.26E 05 2.24E 03 110.4 3.64E 01 2.29E 01 7.82E 02 1.388
9.60E 01 1.80E 02 5.97E 07 1.97E 07 1.95E 05 9.61E 01 3.17E 03 2.00E 03 6.81E 04 1.21E 02
10.07 4.18E 01 1.155 1.62E 02 1.206 8.471 5.39E 02 0.358 2.43E 02 2.172
8.76E 05 3.64E 06 1.01E 05 1.41E 07 1.05E 05 7.37E 05 4.70E 07 3.12E 06 2.12E 07 1.89E 05
0.1
Table 2 The absorbed dose of the different layers of converter after 8 weeks irradiation by 87G Bq/cm2 titanium tritide film. Absorbed dose (J/kg) GAP ¼ 5 mm Si3N4 SiO2 Si GAP ¼ 0.1 mm Si3N4 SiO2 Si
β attribution 1.65E4 1.26E4 1.01E2
X-ray attribution 2.78E5 9.03E3 54.02
5.51E7 4.35E7 6.22E4
8.61E4 1.35E4 97.3
Fig. 4. The ISC decreases with time in 0.1 mm gap radiation.
Table 3 The leakage current results of different states of the c-Si PN diode. Leakage current
Before radiation
After long-time radiation
After annealing
0.1 mm Gap 5 mm Gap
102pA 511pA
380pA 11.5nA
132pA 11.3nA
shown in Table 3. AKT I sc V oc ¼ 1n þ1 q I0
Fig. 3. The output properties of our beta-voltaic cell array with 0.1 mm radiation.
irradiation time), energy deposition datum in Table 1 are used, absorbed dose are shown in Table 2.
3.2. Experiment A beta-voltaic cell array was made up by 10 single cells with a source to energy conversation device gap of 0.1 mm, and the I–V curves of the array before and after a long period of irradiation are shown in Fig. 3. The figure reflects the average radiation degradation of the cells. The properties of a single cell for a long period of irradiation were observed. The radiation is a combination of beta and x-rays. The current-time curve is shown in Fig. 4. The short circuit current (ISC) decreases drastically from 1.24 uA to 0.68 uA in 10,000 min while the voltage decreases slowly from 0.274 V to 0.242 V which is relevant to the huge current change according to similar I–V Eq. (2). Leakage currents were also detected, and the results are
ð2Þ
In expression (1), A stands for quality factor and I0 stands for reverse saturation current. According to the results of simulation, irradiation with a 5 mm gap value is approximately as pure x-ray situation. A group of experiments with a 5 mm gap value were carried out, and the output properties of one of them with a long period of radiation are shown in Fig. 5. Fig. 5 shows that VOC consistently decreases and then sharply drops to a value close to 0 V when the radiation dose reaches a certain value. Meanwhile the surface damage is discernible, which is not the case for the devices irradiated with a 0.1 mm gap. Comparing Fig. 4 with Fig. 5, it can be concluded that the output properties of the irradiation with a 5 mm gap degrades more drastically than the case for a 0.1 mm gap, especially in the variation of voltage. The result of the pure x-ray radiation was beyond our expectation from simulation. The leakage current after x-ray radiation was also detected, and the results are shown in Table 2. The devices after radiation were annealed, at 475 K in a pure nitrogen gas atmosphere and the process was sustained for 2 h. After annealing, the ISC, VOC and Leakage current were detected and all the leakage currents in experiments are shown in Table 2.
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It is found that the devices in with 0.1 mm gap can be restored to its initial state while the device in 5 mm cannot. There is a different physical process between pure x-ray radiation and co-radiation with beta and x-rays by the results of different voltage variation and effect of annealing. According to the simulation results in Table 1, it can be surmised that the Si3N4 layer absorbed more x-rays leading to the surface damage. Hence, the XPS technique was utilized to determine the composition and valence state of the Si3N4 layer after radiation. The results of XPS are shown in Fig. 6. The symmetric Si2p and N1s core-level peaks indicate that the Si3N4 layer keeps the same structure after the radiation. From these investigations XPS, we believe that the fundamental reason leading to radiation damage lies in the SiO2 layer. Capacitance–Voltage measurements are sensitive to the charge variation of the SiO2–Si systems, and defects in SiO2 were detected. The results of the C–V measurements show a flat-band voltage shift from 3.30 V to 3.27 V, which indicates increasing positive charge in SiO2 after radiation. Except for positive charge, a dangling bond is one kind of interface trap which is crucial to the interface properties in Si–SiO2 interface. Numerous reports show ESR is a good method for examining dangling bonds. The ESR spectrum is showed in Fig. 7. The peak of g¼ 1.998 is derived from the manifest of the PN diode, which is relevant to majority carrier in doped Si (Afanas'ev, 1998; Fukata et al., 2007; Stesmans et al., 2006). The curves of both radiation models have a peak in g ¼2.003 after long period of irradiation which shows a dangling bond (Si2O Si)̇ in the Si–SiO2 interface (Ren et al., 2010), and its effect on leakage current. The intensity of the defect signal
Fig. 5. The output properties change with time in 5 mm gap radiation.
changed, which is possibly relevant to the distribution of majority carriers in the P-layer of silicon.
4. Discussion Contrasting the results of the two kinds of radiation, the results reveal that the x-ray is the dominant factor for radiation damage in Ti3H2/c-Si battery. The results also revealed that beta particles have a positive effect in the process of formation of radiation damage. According to some reports on MOS structure, ionization effects in SiO2–Si system can induce four defects which are known as interface traps, fixed oxide charge, mobile ionic charge and oxide trapped charge. The phenomenon that the radiation damage of the 0.1 mm gap can be annealed to the initial state demonstrates that the fixed oxide charge is not attributable to radiation damage. The fixed oxide charge only can be removed at temperatures above 1100 K. Mobile ionic charge is mainly derived from metallic ionic such as Na þ and K þ , which were excluded by manufacture. The explanation should lie in the oxide trapped charge in SiO2 induced by radiation or interface trap in Si–SiO2 interface. Existing reports (Devine, 1994) point out that the radiation induced defects in SiO2 are mostly hole traps because the initial defects in SiO2 contain many hole-trap centers such as oxygen vacancy ( Si‥Si ). After capturing the holes by ionization, they have positive charges. These positive charges can interact with the built-in electric field in the P þ NN þ diode. In the condition of pure x-ray radiation,
Fig. 7. ESR spectrum of c-Si PN diode in three different situations.
Fig. 6. The XPS result of Si2p and N1s in the Si3N4 layer.
Y. Lei et al. / Applied Radiation and Isotopes 90 (2014) 165–169
a positive charge layer is formed in the SiO2 layer, and it increases with the radiation dose. These positive charges increase the surface potential of the P-layer and drive the majority-carrier holes to inner positions of the P-layer. The energy band is curved and forms a depletion of majority carrier. A hole-depletion layer in the surface of P-layer can form an opposing electric field that counterbalances the extra field induced by positive charge in the SiO2 layer. The thickness of depletion layer is determined by the dopant concentration and the amount of positive charge in the SiO2 layer. The width of depletion can increase with the amount of positive charge accumulation. When the curving of the surface energy band exceeds the midpoint of the forbidden band, an inversion layer will be formed. Meanwhile, a field induced junction appears and it is connected with the PN diode. This field induced junction is heavily doped and has a low breakdown voltage, because it is an abrupt junction and contains many defects. With the increase of in positive charge, the field induced junction and PN diode will break down in sequence. In this situation, the built-in field of the P þ NN þ diode is destroyed and causes a sharp decrease of VOC. In the circumstance of co-radiation, the betas have the function of eliminating the positive charge in the SiO2 layer, and no obvious degradation of VOC is occurred. From the results of ESR, The intensity of the g¼ 1.998 peak decreases which may correspond to the formation of hole depletion in the P-layer. Both co-radiation and pure x-ray radiation resemble peaks with dangling bonds, and the difference lies in the variation of intensity. The interface defects may correspond to the huge degradation of current. A variety of reports (Gwyn, 1969; Zukotynski, 2002) point out that the dangling bond can be annealed in an atmosphere of 475 K. In the co-radiation, dangling bonds can be annealed and the leakage current is restored. As a result, the ISC and VOC were annealed to the initial value. The leakage currents and ISC cannot be annealed in the pure x-ray irradiation, indicating that the dangling bond is a factor with relevance to the leakage current, but not the most important factor. 5. Conclusion In this paper, we investigated the radiation damage of a tritium beta-voltaic battery, and a remarkable degradation was observed. Contrasting the results of pure x-ray radiation and co-radiation with beta and x-ray, reveals that the x-ray from the Ti3H2 source is the predominant factor resulting in radiation damage of the c-Si PN diode conversation device in the tritium beta-voltaic battery, and beta particle from the tritium has a beneficial effect in
169
resisting the damage from the x-ray. We have shown that the damage mainly occurs in the Si–SiO2 interface, this implies that xrays from the tritium source in the beta-voltaic battery must be decreased to extend the efficient life-time of the device. In addition, it suggests that the Si–SiO2 interface should be avoided in the crystalline silicon PN energy conversation device in tritium beta-voltaic battery.
Acknowledgments The work is financially supported by the Special Project Foundation of China Academy of Engineering Physics (grant no. CAEP 9100204). Authors also thank Mr. Tong Wei for his kind help in ESR measurement and discussion. References Afanas'ev, A.S.a.V.V., 1998. Hydrogen-induced thermal interface degradation in (1 1 1)Si/SiO2 revealed by electron-spin resonance. Appl. Phys. Lett., 72. Chandrashekhar, M.V.S., Thomas, C.I., Li, H., Spencer, M.G., Lal, A., 2006. Demonstration of a 4H SiC betavoltaic cell. Appl. Phys. Lett. 88, 033506. Devine, R.A.B., 1994. The structure of SiO2, its defects and radiation hardness. IEEE Trans. Nucl. Sci., 41. Fukata, N., Chen, J., Sekiguchi, T., Matsushita, S., Oshima, T., et al., 2007. Phosphorus doping and hydrogen passivation of donors and defects in silicon nanowires synthesized by laser ablation. Appl. Phys. Lett. 90, 153117. Gwyn, C.W., 1969. Model for radiation-induced charge trapping and annealing in the oxide layer of MOS devices. J. Appl. Phys. 40, 4886. Lee, S.-k., 2009. Development of nuclear micro-battery with solid tritium source. Appl. Radiat. Isot. 67 (2009), 1234–1238. Li, H., Liu, Y., Hu, R., Yang, Y., Wang, G., Zhong, Z., Luo, S., 2012. Simulations about self-absorption of tritium in titanium tritide and the energy deposition in a silicon Schottky barrier diode. Appl. Radiat. Isot. 70, 2559–2563. Liu, B., Chen, K.P., Kherani, N.P., Zukotynski, S., 2009. Power-scaling performance of a three-dimensional tritium betavoltaic diode. Appl. Phys. Lett. 95, 233112. Liu, Y., Hu, R., Yang, Y., Wang, G., Luo, S., Liu, N., 2012. Investigation on a radiation tolerant betavoltaic battery based on Schottky barrier diode. Appl. Radiat. Isot. 70, 438–441. Ren, H., Cheng, S.L., Nishi, Y., Shohet, J.L., 2010. Effects of vacuum ultraviolet and ultraviolet irradiation on ultrathin hafnium-oxide dielectric layers on (1 0 0)Si as measured with electron-spin resonance. Appl. Phys. Lett. 96, 192904. San, H., Yao, S., Wang, X., Cheng, Z., Chen, X., 2013. Design and simulation of GaN based Schottky betavoltaic nuclear micro-battery. Appl. Radiat. Isot.. Stesmans, A., Somers, P., Afanas'ev, V.V., Claeys, C., Simoen, E., 2006. Inherent density of point defects in thermal tensile strained (1 0 0)Si/SiO2 entities probed by electron spin resonance. Appl. Phys. Lett. 89, 152103. Sun, W., Kherani, N.P., Hirschman, K.D., Gadeken, L.L., Fauchet, P.M., 2005. A threedimensional porous silicon p–n diode for betavoltaics and photovoltaics. Adv. Mater. 17, 1230–1233. Wang, G., Hu, R., Wei, H., Zhang, H., Yang, Y., Xiong, X., Liu, G., Luo, S., 2010. The effect of temperature changes on electrical performance of the betavoltaic cell. Appl. Radiat. Isot. 68, 2214–2217. Zukotynski, S., 2002. Metastability in tritiated amorphous silicon. J. Non-Cryst. Solids 299, 476–481.
Contents lists available at ScienceDirect
Applied Radiation and Isotopes journal homepage: www.elsevier.com/locate/apradiso
The radiation damage of crystalline silicon PN diode in tritium beta-voltaic battery Yisong Lei, Yuqing Yang, Yebing Liu, Hao Li, Guanquan Wang, Rui Hu, Xiaoling Xiong, Shunzhong Luo n Institute of Nuclear Physics and Chemistry, China Academy of Engineering Physics, Mianyang 621900, China
H I G H L I G H T S
Remarkable degradation of output property is observed in tritium beta-voltaic battery. X-ray emitted from tritium source is the main factor which leads to the degradation. Radiation induced positive charge mainly influences open-circuit voltage and Si–SiO2 interface trap mainly influences short-circuit current. Beta particle has positive effect in eliminating the radiation damage by x-ray.
art ic l e i nf o
a b s t r a c t
Article history: Received 9 November 2013 Received in revised form 15 February 2014 Accepted 24 March 2014 Available online 5 April 2014
A tritium beta-voltaic battery using a crystalline silicon convertor composed of (1 0 0)Si/SiO2/Si3N4 film degrades remarkably with radiation from a high intensity titanium tritide film. Simulation and experiments were carried out to investigate the main factor causing the degradation. The radiation damages mainly comes from the x-ray emitted from the titanium tritide film and beta particle can relieve the damages. The x-ray radiation induced positive charges in the SiO2 film destroying the output property of the PN diode with the induction of an electric field. & 2014 Elsevier Ltd. All rights reserved.
Keywords: Tritium beta-voltaic Radiation damage Geant4 ESR SiO2–Si Soft x-ray
1. Introduction With the booming in MEMS (Micro Electro Mechanical Systems), the optimization of micro energy systems, which are a crucial part, becomes more and more urgent. The beta-voltaic battery, one type of radioactive isotope battery (RIB), which is distinguished by excellent properties in integration with MEMS and specialized properties such as a long service life, high energy density and free of maintenance (Wang et al., 2010), has becomes a focus of this work. Many attempts have been made in design of high-efficiency beta-voltaic batteries through the optimization structures and materials (Chandrashekhar et al., 2006; Lee, 2009; San et al., 2013; Sun et al., 2005). Among energy conversation devices, silicon is one of the most popular materials in beta-voltaic battery due to the mature processing n
Corresponding author. Tel.: þ 86 13700960909; fax: þ 86 816 2496863. E-mail address: [email protected] (S. Luo).
http://dx.doi.org/10.1016/j.apradiso.2014.03.027 0969-8043/& 2014 Elsevier Ltd. All rights reserved.
technology. In terms of radioisotopes, tritium is the most promising radioisotope because it is safe, cheap and commercially available (Liu et al., 2009). Titanium tritide (Ti3H2) is a well-developed tritium source with high stability (Li et al., 2012). In this work, c-Si with a SiO2–Si3N4 passivated surface, and loaded with a titanium tritide film was adopted. In our previous work, both experiment (Liu et al., 2012) and simulation (Li et al., 2012) of tritium beta-voltaics have been studied. In recent studies, remarkable radiation degradation of tritium betavoltaic batteries using a c-Si PN energy conversion device with a high intensity titanium tritide films has been observed. Little research has been carried out on the radiation damage of c-Si PN diode energy conversion devices in tritium beta-voltaic batteries because the energy of the beta particles emitted from tritium is far below the threshold energy of displacement damage in c-Si. In spite of numerous reports (Gwyn, 1969) the focus on low-energy-radiation induced degradation in Metal Oxide Semiconductor (MOS) systems,
166
Y. Lei et al. / Applied Radiation and Isotopes 90 (2014) 165–169
the mechanism of the defect formation in the PN diode is still obscure. The energy conversion mechanism of beta-voltaic batteries resembles the photovoltaic battery, but the damage only focuses on displacement damage induced by high energy particles. Investigating the degradation effect in tritium beta-voltaic batteries is necessary. In this work, a Geant4 Monte Carlo simulation and experiment detection were carried out to investigate the degradation phenomenon. There is not only beta but also x-ray emitted from titanium tritide sources applied in beta-voltaic batteries. The beta particles and x-rays generated in the Ti3H2 film are investigated independently in order to find out the main factor causing radiation damage. Simulations of the energy deposition in energy conversion devices and the emissions from the titanium tritide are implemented. Conjunction of the simulation and experiment results, are supported with measurements with X-ray Photoelectron Spectroscopy (XPS) and Electron Spin Resonance (ESR) to find out the variation in the micro-structure of the PN diode after radiation. Based on these results, analysis of a possible theoretic mechanisms or radiation damage and some optimization suggestions are given. The results are important in the design and optimization of new tritium beta-voltaic batteries.
2. Experiments 2.1. Theoretic simulation The structure of our cell is presented in Fig. 1. The simulation model consists of a Ti3H2 layer, an air gap, a Si3N4 layer, a SiO2 layer, and a Si layer. The beta particle is emitted in 4π directions from the Ti3H2 film which also emits x-rays. The air-gap in Fig. 1 is 0.1 mm width when the cell functions regularly. The Geant4.9.2 Monte Carlo transport toolkit was used to simulate the energy deposition of 2 107 beta particles in the converter considering two gap values of 0.1 mm and 5 mm. The energy spectrum of x-ray emitted from the radioactive source was calculated with 1 108 beta particles. The electron and photon physics used in the Geant4 simulation is the Penelope model. 2.2. Radiation experiment and measurement The energy conversion device is a plain (1 0 0)c-Si P þ NN þ diode. The radiation sources is a Ti3H2 film with an intensity of 87G-Bq/cm2 loaded on a round metal slice substrate. All the used
sources are parallel. According to the effective range of beta in air, when the air gap value was set to 5 mm, only x-ray can interact with energy conversion device in this model. When the air gap was 0.1 mm, there are both x-ray and beta particle interactions with the energy conversion device. The leakage currents before and after a long period (approximately 8 weeks) of irradiation were measured. All micro-currents and voltages were detected by KEITHLEY Model 6517B and KEITHLEY 2635 electrometers. After irradiation in each model, the energy conversion devices were annealed at 475 K for 2 h under pure nitrogen gas. Experiments were carried out in clean lab with controllable temperature and humidity. In addition, XPS and ESR of the energy conversion devices were measured to analyze the radiation damage. The XPS is determined by XSAM800 spectrometer and the device for ESR detection is a Bruker EMX-10/12 PLUS.
3. Results 3.1. Simulation Simulation result of the x-ray's energy spectrum emitted from the Ti3H2 film is shown in Fig. 2. The x-ray spectrum is made up of two parts. The first part is the product of the bremsstrahlung and the other is generated by the transition of outer electrons in the inner shell of Ti. Two characteristic peaks are found at 4.45 keV and 4.85 keV, respectively, which correspond to the Kα and Kβ transition of titanium. The energy conversion devices were irradiated under only x-ray or co-irradiated under both x-ray and beta particle. Both gap values were simulated with 2 107 beta particles where 5.745 keV is the average energy of each beta particle. The results of energy deposition simulation are shown in Table 1. Contrasting the beta deposition results of 5 mm to those of a 0.1 mm gap in Table 1, it is reasonable to suppose that the beta energy deposited in energy conversion devices of 5 mm gap is three orders of magnitude less than that of a 0.1 mm gap. Comparing x-ray energy deposition values of the 5 mm gap value with those of 0.1 mm, the deposited x-ray energy values in all films except in the Si3N4 layer in the 0.1 mm gap are always greater than those of the 5 mm gap. It is reasonable to expect a lower degradation with radiation in the 5 mm gap while other parameters are identical. In order to connect the simulation results with titanium tritide used in radiation experiments, absorbed dose was used to evaluate the radiation. Absorbed dose can be calculated by eq. 1. EA ¼
Ed N t n Mc
ð1Þ
In Eq. (1), EA stands for absorbed dose, Ed for energy deposition, N for the activity of our titanium tritide film, t for irradiation time, n for the value of Monte Carlo event and MC for the mass of different solid convertor layer. When t¼ 8 weeks (pure x-ray
Fig. 1. The schematic of plain silicon beta-voltaic loaded with Ti3H2 film.
Fig. 2. The x-ray spectrum emitted from Ti3H2 source layer with 1 108 simulation beta particles.
Y. Lei et al. / Applied Radiation and Isotopes 90 (2014) 165–169
167
Table 1 The energy deposition of 2 107 simulation beta particles in energy conversion devices with two different gap radiations. Radiation model (mm)
Material
Energy deposition by electron (GeV)
Beta's percent of total energy
Energy deposition by x-ray (MeV)
X-ray's percent of total energy
5
Ti3H2 Air Si3N4 SiO2 Silicon Ti3H2 Air Si3N4 SiO2 Silicon
110.3 2.07 6.85E 05 2.26E 05 2.24E 03 110.4 3.64E 01 2.29E 01 7.82E 02 1.388
9.60E 01 1.80E 02 5.97E 07 1.97E 07 1.95E 05 9.61E 01 3.17E 03 2.00E 03 6.81E 04 1.21E 02
10.07 4.18E 01 1.155 1.62E 02 1.206 8.471 5.39E 02 0.358 2.43E 02 2.172
8.76E 05 3.64E 06 1.01E 05 1.41E 07 1.05E 05 7.37E 05 4.70E 07 3.12E 06 2.12E 07 1.89E 05
0.1
Table 2 The absorbed dose of the different layers of converter after 8 weeks irradiation by 87G Bq/cm2 titanium tritide film. Absorbed dose (J/kg) GAP ¼ 5 mm Si3N4 SiO2 Si GAP ¼ 0.1 mm Si3N4 SiO2 Si
β attribution 1.65E4 1.26E4 1.01E2
X-ray attribution 2.78E5 9.03E3 54.02
5.51E7 4.35E7 6.22E4
8.61E4 1.35E4 97.3
Fig. 4. The ISC decreases with time in 0.1 mm gap radiation.
Table 3 The leakage current results of different states of the c-Si PN diode. Leakage current
Before radiation
After long-time radiation
After annealing
0.1 mm Gap 5 mm Gap
102pA 511pA
380pA 11.5nA
132pA 11.3nA
shown in Table 3. AKT I sc V oc ¼ 1n þ1 q I0
Fig. 3. The output properties of our beta-voltaic cell array with 0.1 mm radiation.
irradiation time), energy deposition datum in Table 1 are used, absorbed dose are shown in Table 2.
3.2. Experiment A beta-voltaic cell array was made up by 10 single cells with a source to energy conversation device gap of 0.1 mm, and the I–V curves of the array before and after a long period of irradiation are shown in Fig. 3. The figure reflects the average radiation degradation of the cells. The properties of a single cell for a long period of irradiation were observed. The radiation is a combination of beta and x-rays. The current-time curve is shown in Fig. 4. The short circuit current (ISC) decreases drastically from 1.24 uA to 0.68 uA in 10,000 min while the voltage decreases slowly from 0.274 V to 0.242 V which is relevant to the huge current change according to similar I–V Eq. (2). Leakage currents were also detected, and the results are
ð2Þ
In expression (1), A stands for quality factor and I0 stands for reverse saturation current. According to the results of simulation, irradiation with a 5 mm gap value is approximately as pure x-ray situation. A group of experiments with a 5 mm gap value were carried out, and the output properties of one of them with a long period of radiation are shown in Fig. 5. Fig. 5 shows that VOC consistently decreases and then sharply drops to a value close to 0 V when the radiation dose reaches a certain value. Meanwhile the surface damage is discernible, which is not the case for the devices irradiated with a 0.1 mm gap. Comparing Fig. 4 with Fig. 5, it can be concluded that the output properties of the irradiation with a 5 mm gap degrades more drastically than the case for a 0.1 mm gap, especially in the variation of voltage. The result of the pure x-ray radiation was beyond our expectation from simulation. The leakage current after x-ray radiation was also detected, and the results are shown in Table 2. The devices after radiation were annealed, at 475 K in a pure nitrogen gas atmosphere and the process was sustained for 2 h. After annealing, the ISC, VOC and Leakage current were detected and all the leakage currents in experiments are shown in Table 2.
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It is found that the devices in with 0.1 mm gap can be restored to its initial state while the device in 5 mm cannot. There is a different physical process between pure x-ray radiation and co-radiation with beta and x-rays by the results of different voltage variation and effect of annealing. According to the simulation results in Table 1, it can be surmised that the Si3N4 layer absorbed more x-rays leading to the surface damage. Hence, the XPS technique was utilized to determine the composition and valence state of the Si3N4 layer after radiation. The results of XPS are shown in Fig. 6. The symmetric Si2p and N1s core-level peaks indicate that the Si3N4 layer keeps the same structure after the radiation. From these investigations XPS, we believe that the fundamental reason leading to radiation damage lies in the SiO2 layer. Capacitance–Voltage measurements are sensitive to the charge variation of the SiO2–Si systems, and defects in SiO2 were detected. The results of the C–V measurements show a flat-band voltage shift from 3.30 V to 3.27 V, which indicates increasing positive charge in SiO2 after radiation. Except for positive charge, a dangling bond is one kind of interface trap which is crucial to the interface properties in Si–SiO2 interface. Numerous reports show ESR is a good method for examining dangling bonds. The ESR spectrum is showed in Fig. 7. The peak of g¼ 1.998 is derived from the manifest of the PN diode, which is relevant to majority carrier in doped Si (Afanas'ev, 1998; Fukata et al., 2007; Stesmans et al., 2006). The curves of both radiation models have a peak in g ¼2.003 after long period of irradiation which shows a dangling bond (Si2O Si)̇ in the Si–SiO2 interface (Ren et al., 2010), and its effect on leakage current. The intensity of the defect signal
Fig. 5. The output properties change with time in 5 mm gap radiation.
changed, which is possibly relevant to the distribution of majority carriers in the P-layer of silicon.
4. Discussion Contrasting the results of the two kinds of radiation, the results reveal that the x-ray is the dominant factor for radiation damage in Ti3H2/c-Si battery. The results also revealed that beta particles have a positive effect in the process of formation of radiation damage. According to some reports on MOS structure, ionization effects in SiO2–Si system can induce four defects which are known as interface traps, fixed oxide charge, mobile ionic charge and oxide trapped charge. The phenomenon that the radiation damage of the 0.1 mm gap can be annealed to the initial state demonstrates that the fixed oxide charge is not attributable to radiation damage. The fixed oxide charge only can be removed at temperatures above 1100 K. Mobile ionic charge is mainly derived from metallic ionic such as Na þ and K þ , which were excluded by manufacture. The explanation should lie in the oxide trapped charge in SiO2 induced by radiation or interface trap in Si–SiO2 interface. Existing reports (Devine, 1994) point out that the radiation induced defects in SiO2 are mostly hole traps because the initial defects in SiO2 contain many hole-trap centers such as oxygen vacancy ( Si‥Si ). After capturing the holes by ionization, they have positive charges. These positive charges can interact with the built-in electric field in the P þ NN þ diode. In the condition of pure x-ray radiation,
Fig. 7. ESR spectrum of c-Si PN diode in three different situations.
Fig. 6. The XPS result of Si2p and N1s in the Si3N4 layer.
Y. Lei et al. / Applied Radiation and Isotopes 90 (2014) 165–169
a positive charge layer is formed in the SiO2 layer, and it increases with the radiation dose. These positive charges increase the surface potential of the P-layer and drive the majority-carrier holes to inner positions of the P-layer. The energy band is curved and forms a depletion of majority carrier. A hole-depletion layer in the surface of P-layer can form an opposing electric field that counterbalances the extra field induced by positive charge in the SiO2 layer. The thickness of depletion layer is determined by the dopant concentration and the amount of positive charge in the SiO2 layer. The width of depletion can increase with the amount of positive charge accumulation. When the curving of the surface energy band exceeds the midpoint of the forbidden band, an inversion layer will be formed. Meanwhile, a field induced junction appears and it is connected with the PN diode. This field induced junction is heavily doped and has a low breakdown voltage, because it is an abrupt junction and contains many defects. With the increase of in positive charge, the field induced junction and PN diode will break down in sequence. In this situation, the built-in field of the P þ NN þ diode is destroyed and causes a sharp decrease of VOC. In the circumstance of co-radiation, the betas have the function of eliminating the positive charge in the SiO2 layer, and no obvious degradation of VOC is occurred. From the results of ESR, The intensity of the g¼ 1.998 peak decreases which may correspond to the formation of hole depletion in the P-layer. Both co-radiation and pure x-ray radiation resemble peaks with dangling bonds, and the difference lies in the variation of intensity. The interface defects may correspond to the huge degradation of current. A variety of reports (Gwyn, 1969; Zukotynski, 2002) point out that the dangling bond can be annealed in an atmosphere of 475 K. In the co-radiation, dangling bonds can be annealed and the leakage current is restored. As a result, the ISC and VOC were annealed to the initial value. The leakage currents and ISC cannot be annealed in the pure x-ray irradiation, indicating that the dangling bond is a factor with relevance to the leakage current, but not the most important factor. 5. Conclusion In this paper, we investigated the radiation damage of a tritium beta-voltaic battery, and a remarkable degradation was observed. Contrasting the results of pure x-ray radiation and co-radiation with beta and x-ray, reveals that the x-ray from the Ti3H2 source is the predominant factor resulting in radiation damage of the c-Si PN diode conversation device in the tritium beta-voltaic battery, and beta particle from the tritium has a beneficial effect in
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resisting the damage from the x-ray. We have shown that the damage mainly occurs in the Si–SiO2 interface, this implies that xrays from the tritium source in the beta-voltaic battery must be decreased to extend the efficient life-time of the device. In addition, it suggests that the Si–SiO2 interface should be avoided in the crystalline silicon PN energy conversation device in tritium beta-voltaic battery.
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