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The equivalence of displacement damage in silicon bipolar junction transistors
Nuclear Instruments and Methods in Physics Research A 677 (2012) 61–66
Contents lists available at SciVerse ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
The equivalence of displacement damage in silicon bipolar junction transistors Chaoming Liu a, Xingji Li a,n, Hongbin Geng a, Erming Rui a, Lixin Guo a, Jianqun Yang a, Liyi Xiao b a b
School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, China Microelectronics Center, Harbin Institute of Technology, Harbin 150001, China
a r t i c l e i n f o
abstract
Article history: Received 14 December 2011 Received in revised form 12 February 2012 Accepted 12 February 2012 Available online 6 March 2012
The current gain degradation in silicon bipolar junction transistors (BJTs) is examined under the irradiation with heavy ions. To characterize the radiation damage of the BJTs, the ionizing dose Di and displacement dose Dd verse the chip depth in the BJTs have been calculated for heavy ions. Based on the irradiation testing and calculation results, an approach to evaluate the equivalence of displacement damage in silicon BJTs is given, which could optimize the non-ionizing energy loss (NIEL) methodology and normalize the displacement damage caused by heavy ions. & 2012 Elsevier B.V. All rights reserved.
Keywords: Displacement equivalence Bipolar junction transistors Radiation effects Heavy ions Gain degradation
1. Introduction Bipolar junction transistors (BJTs) are generally known as devices operating in radiation rich environments, especially in spacecraft, due to their current drive capability, linearity and excellent matching characteristics. In space, the radiation-induced gain degradation is a concern aspect for characterization of the BJTs, and is the primary cause for parametric shifts and functional failures. It is useful to understand the radiation response of these devices to find better design strategies before employing them for specific applications [1–5]. BJTs are sensitive to both ionizing and displacement effects induced by electrons, protons, neutrons and heavy ions [6–8]. Generally, heavy ions could induce more displacement damage in BJTs at same energy and fluence. Investigations using heavy ions are valuable to understand the displacement mechanism of the BJTs. The Messenger–Spratt equation [9], used to characterize the current gain degradation of bipolar devices generated by displacement damage, can be expressed as the linear change in the reciprocal of current gain with irradiation fluence, which can be expressed as follows:
Dð1=bÞ ¼ K F
n
Corresponding author. Tel.: þ86 451 86414445; fax: þ 86 451 86415168. E-mail addresses: [email protected], [email protected] (X. Li).
0168-9002/$ - see front matter & 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2012.02.045
ð1Þ
where the change in the reciprocal of the gain variation (D(1/b)) is defined as the value after irradiation minus the initial one, that is D(1/b)¼1/b 1/bpre-rad; K the damage factor; F the incident particle fluence. It was proposed that the ratio of Dd/(Dd þDi) could be used as a criterion to show the displacement damage capacity of incident particles, where Di and Dd is the ionizing dose and displacement dose in the chip or device, respectively [10–12]. The higher the ratio of Dd/(Dd þ Di), the bigger the displacement damage capacity of the incident particles. The Dd/(Dd þDi) ratios are dependent on the types and energy of incident particles as well as the device nature. Generally, it is required to choose the incident particles with higher ratios of Dd/(Dd þDi) to perform irradiation testing, if the displacement damage is a major concern for the BJTs. When the irradiation particles induce displacement damage to BJTs, the Messenger–Spratt equation becomes suitable to characterize the experimental data. A more convenient technique would use the NIEL methodology to determine the damage factor K in the Messenger–Spratt equation and to characterize the damage equivalence of various heavy ions [8]. Using various heavy ions and neutrons as incident particles on silicon BJTs, Summers et al. [1] show that the damage factor K calculated from the Messenger–Spratt equation scales with the non-ionizing energy loss (NIEL). However, some experimental results show that the NIEL methodology has some limitations [8,13,14]. The displacement damage to BJTs induced by various particles with the same NIEL may be different, which is incompatible with the NIEL methodology. As mentioned in reference [15], the NIEL
C. Liu et al. / Nuclear Instruments and Methods in Physics Research A 677 (2012) 61–66
methodology could be used for light ions where the ions lose very little energy in the sensitive part of the device. However, there is a deviation when the energy losses by incident particles vary significantly in sensitive region of the BJTs. Therefore, it is required to find a way to establish the equivalence of displacement damage for various heavy ions. The purpose of this paper is to develop a new method to assess the displacement damage equivalence in Si BJTs caused by various heavy ions. If the displacement damage could be normalized, the effects of various heavy ions may be characterized in a simple and economic way, without performing detailed time-consuming irradiations for each type of heavy ion.
Change in the reciprocal of current gain, Δ(1/)
62
10-1
10-2
Devices: 3DG112 Br 20 MeV Si 35 MeV C 25 MeV
10-3
10-4
2. Experimental details
108
109
1010
1011 2
3. Results and discussion Displacement damage on bipolar junction transistors increases the number of defects in the silicon bulk, and the lifetime of the minority-carriers in the Si bulk is decreased. Consequently, the base current is increased and the common-emitter current gain (b) is degraded. Figs. 1–3 show the change in the reciprocal gain (D(1/b)) at a given base-emitter voltage of 0.65 V as a function of fluence under 25 MeV C, 35 MeV Si and 20 MeV Br ions irradiation for 3DG112, 3DG130 and 3CG130 bipolar junction transistors, respectively. Fig. 4 shows the change in the reciprocal gain (D(1/b)) at a given base-emitter voltage of 0.65 V as a function of fluence under 25 MeV C, 10–40 MeV Si, and 40 MeV Cl ions irradiation for 3DK4B BJTs. It is clear that D(1/b) increases linearly with the increasing ions fluence, and the relationship between the variations of the reciprocal of current gain and the heavy ions fluence is suitably characterized by the Messenger–Spratt equation. Based on the experimental data from Figs. 1–3, it is apparent that the change in the reciprocal of current gain (D(1/b)) undergoes the largest variations under the Br ions irradiation and
Fluence of heavy ions (ions/cm )
Change in the reciprocal of current gain, Δ(1/ )
Fig. 1. Change in the reciprocal of current gain as a function of fluence for 3DG112 NPN transistors irradiated by various heavy ions.
10-1
10-2 Devices: 3DG130 Br 20 MeV Si 35 MeV C 25 MeV
-3
10
107
108
109
1010
1011
2
Fluence of heavy ions (ions/cm ) Fig. 2. Change in the reciprocal of current gain as a function of fluence for 3DG130 NPN transistors irradiated by various heavy ions.
Change in the reciprocal of current gain, Δ(1/)
Four types of bipolar junction transistors (3DG112, 3DG130, 3CG130, and 3DK4B) were used as samples in this study. 3DG112 and 3DG130 are NPN high frequency and low power transistors. 3CG130 is PNP high frequency and low power transistor. 3DK4B is NPN low power switching transistor. 25 MeV carbon (C), 35 MeV silicon (Si) and 20 MeV bromine (Br) ions were chosen to test the 3DG112, 3DG130 and 3CG130 transistors in this investigation. 25 MeV (C), 10–40 MeV (Si), and 40 MeV chlorine (Cl) ions are chosen to test the 3DK4B transistors. The heavy ions irradiations were performed using the EN Tandem Accelerator in the State Key Laboratory of Nuclear Physics and Technology, Peking University, China. Beam areas were 20 150 mm2 for heavy ions. The irradiations were carried out in an evacuated chamber with a specially designed Faraday cup, which is used to measure the heavy ions beam current. From these measurements, the flux and fluences were determined for the irradiation experiments. The samples were mounted inside a package with removable upper lid for irradiation. All terminals of the samples were grounded during all particle irradiations. Electrical parameters of the NPN BJTs were measured in-situ using a semiconductor characterization system: Keithley 4200-SCS. The turn-over time between irradiation and device measurements is approximately less than 5 s. The irradiations and measurements were performed at room temperature. For the in-situ measurement, a matrix board switching system located outside of the irradiation chamber is used as a naster control panel, which is designed with very high insulation resistance so that very low current can be generated.
100
10-1
Devices: 3CG130 Br 20 MeV Si 35 MeV C 25 MeV
10-2
108
109 Fluence of heavy ions
1010 (ions/cm2)
Fig. 3. Change in the reciprocal of current gain as a function of fluence for 3CG130 PNP transistors irradiated by various heavy ions.
least under the C ions irradiation at a given fluence for 3DG112, 3DG130 and 3CG130 BJTs. As shown in Fig. 4, D(1/b) exhibits the largest variations under the 10 MeV Si ions irradiation and the smallest under 25 MeV C ions irradiation for 3DK4B BJTs.
100
Device: 3DG130 Di and Dd per fluence (Gy/ion)
Change in the reciprocal of current gain, Δ(1/)
C. Liu et al. / Nuclear Instruments and Methods in Physics Research A 677 (2012) 61–66
Devices: 3DK4B 10-1
C 25 MeV Si 10 MeV Si 24 MeV Si 40 MeV Cl 40 MeV
10-2
10-3
108
109
1010
Fluence of heavy ions
10
-5
63
Dd for Br 20 MeV
Di for Br 20 MeV
Dd for Si 35 MeV
Di for Si 35 MeV
Dd for C 25 MeV
Di for C 25 MeV
10-7 Base region 10-9
1011 (ions/cm2)
0
1 2 3 Depth in the chip of device (μm)
Fig. 4. Change in the reciprocal of current gain as a function of fluence for 3DK4B NPN transistors irradiated by various heavy ions.
4
5
Fig. 6. Total ionizing absorbed dose Di and displacement absorbed dose Dd as a function of the depth in the chip of 3DG130 NPN transistor.
10-3
10-3 Di for Br 20 MeV
Dd for Si 35 MeV
Di for Si 35 MeV
Dd for C 25 MeV
Di for C 25 MeV
Device: 3CG130 Di and Dd per fluence (Gy/ion)
10-5
Dd for Br 20 MeV
10-7 Base region 10-9
10-11
10-5
0
1 2 Depth in the chip of device (μm)
The total ionizing absorbed dose Di and displacement absorbed dose Dd were calculated to analyze the radiation damage to BJTs induced by various incident particles. These parameters can be calculated using the following equations Di ðtÞ ¼ 1:6 1010 LETðtÞ F
ð2Þ
Dd ðtÞ ¼ 1:6 1010 NIELðtÞ F
ð3Þ
where Di(t) and Dd(t) are the ionizing dose and the displacement dose, as a function of depth in chip of the NPN BJTs (in Gy); t is the depth in the device chips, (in mm); 1.6 10 10 is the unit conversion parameter (in Gy g/MeV); LET(t) and NIEL(t) are the ionizing and the displacement energy loss as a function of depth in the chip, which are calculated by the stopping power and range of ion in matter (SRIM) code (in MeV cm2/g). The sensitive region of BJTs is the entire base region, the region from the Si/SiO2 interface to the base bottom (include the emitter region, emitterbase space charge region and base region). The calculated results of Di and Dd for 3DG112, 3DG130, 3CG130, and 3DK4B BJTs are shown in Figs. 5–8, respectively. From the results in Figs. 5–7, Di and Dd for 20 MeV Br ions vary gradually in the sensitive region of the 3DG112, 3DG130 and 3CG130 BJTs, while the Di and Dd for
Di for Br 20 MeV
Dd for Si 35 MeV
Di for Si 35 MeV
Dd for C 25 MeV
Di for C 25 MeV
Base region 10-9
3
Fig. 5. Total ionizing absorbed dose Di and displacement absorbed dose Dd as a function of the depth in the chip of 3DG112 NPN transistor.
Dd for Br 20 MeV
10-7
10-11
0
1
2 3 4 5 Depth in the chip of device (μm)
6
7
Fig. 7. Total ionizing absorbed dose Di and displacement absorbed dose Dd as a function of the depth in the chip of 3CG130 PNP transistor.
Di and Dd per fluence (Gy/ion)
Di and Dd per fluence (Gy/ion)
Device: 3DG112
10-6 Base region 10-8
10-10 Device: 3DK4B 10-12
0
Dd for C 25 MeV
Di for C 25 MeV
Dd for Si 10 MeV
Di for Si 10 MeV
Dd for Si 24 MeV
Di for Si 24 MeV
Dd for Si 40 MeV
Di for Si 40 MeV
Dd for Cl 40 MeV
Di for Cl 40 MeV
1 2 3 Depth in the chip of device (μm)
4
Fig. 8. Total ionizing absorbed dose Di and displacement absorbed dose Dd as a function of the depth in the chip of 3DK4B NPN transistor.
25 MeV C and 35 MeV Si ions exhibits very small variations. The 10 MeV Si ions vary gradually in entire base region of the 3DK4B, as shown in Fig. 8.
Change in the reciprocal of current gain, Δ(1/)
10-1
10-2
10-1
10-2 Devices: 3DG130 Br 20 MeV Si 35 MeV C 25 MeV
10-3
10-1
100 101 Displacement absorbed dose, Dd (Gy)
102
Fig. 10. Change in the reciprocal of current gain as a function of Dd for the 3DG130 NPN transistors irradiated by various heavy ions.
100 Change in the reciprocal of current gain, Δ(1/)
As pointed out in [10–12], for a given BJTs, the average ratio of Dd/(Dd þDi) in the sensitive region could be related to the displacement capability for the incident particles. The incident particle with the higher ratio of Dd/(Dd þDi) could induce more displacement damage than the particle with lower ratio for the same BJT. If the ratio is high enough, the displacement damage induced by the incident particles could dominate in the BJTs, and the change in the reciprocal of current gain with fluence will be followed by the Messenger–Spratt equation very well. The average ratios of Dd/(Dd þDi) for 25 MeV C, 35 MeV Si and 20 MeV Br ions are 3.9 10 4, 8.8 10 4 and 0.016, respectively. The ratios are much larger than the value of 2.63 10 4 for 3 MeV protons in the entire base region of 3DG112 BJTs [12]. Therefore, it results likely that 25 MeV C, 35 MeV Si and 20 MeV Br ions may primarily induce displacement damage to 3DG112 BJTs. Based on the results in Fig. 1, it is obvious that the experimental results follow the Messenger–Spratt equation very well. This phenomenon further proves that the average ratio of Dd/(Dd þDi) in the sensitive region of the BJTs is a good symbol to describe the displacement capability for the incident particles. In order to examine the displacement damage behavior of BJTs, it is required to choose the incident particles having the Dd/(Dd þDi) high enough. It is known that the NIEL methodology is often used to characterized the expected particle-induced response of a device in a displacement radiation environment, but it is necessary to appreciate the associated underlying assumptions and limitations in order to use them effectively [1,8,14,15]. If the energy losses of incident particles could not keep steady in the sensitive region of BJTs, the NIEL methodology will not give a good result to normalize the change in the reciprocal of current gain vs. displacement dose caused by various particles. In particular, deviations at very low particle energies are expected [16–18]. Figs. 9–12, show the change in the reciprocal of current gain as a function of displacement absorbed dose Dd for the 3DG112, 3DG130, 3CG130 and 3DK4B transistors irradiated by various heavy ions, respectively. It is clear that the NIEL methodology is not suitable for characterizing the displacement damage induced by 20 MeV Br and 10 MeV Si ions, because of the energy loss induced by 20 MeV Br and 10 MeV Si ions exhibit obvious variations of in base region, as shown in Figs. 5–8. Therefore, it is valuable to provide an alternative approach. As shown in reference [19], the ionizing effect could affect the displacement damage to some extent in the BJTs. Such effects induced by various incident particles are different, since the different particles have different capacities to induce the
Change in the reciprocal of current gain, Δ(1/)
C. Liu et al. / Nuclear Instruments and Methods in Physics Research A 677 (2012) 61–66
10-1
Devices: 3CG130 10-2
Br 20 MeV Si 35 MeV C 25 MeV 10-1
100 101 Displacement absorbed dose, Dd (Gy)
102
Fig. 11. Change in the reciprocal of current gain as a function of Dd for the 3CG130 PNP transistors irradiated by various heavy ions.
100 Change in the reciprocal of current gain, Δ(1/)
64
10-1 Devices: 3DK4B C 25 MeV Si 10 MeV Si 24 MeV Si 40 MeV Cl 40 MeV
10-2
10-3
100
101 102 Displacement absorbed dose, Dd (Gy)
Devices: 3DG112
10-3
Fig. 12. Change in the reciprocal of current gain as a function of Dd for the 3DK4B NPN transistors irradiated by various heavy ions.
Br 20 MeV Si 35 MeV C 25 MeV
10-4 0.1
1 10 Displacement absorbed dose, Dd (Gy)
100
Fig. 9. Change in the reciprocal of current gain as a function of Dd for the 3DG112 NPN transistors irradiated by various heavy ions.
displacement damage with respect to ionization damage. The ionizing damage could cause interface traps and net positive charges in the oxide overlying the emitter–base junction. The interface traps and net positive charges give the complex influence on the displacement damage. There may be two different
where Kd is a constant, the normalized displacement damage factor depending on the device type; log(Di/Dd) is an factor to characterize the effect of the ionization on displacement effect in the sensitive region of BJTs caused by heavy ions, which is related to the types and energy of particles as well as the device nature; D0 d is the normalized displacement dose, which equals log(Di/Dd)Dd. The value of Di and Dd in Eq. (4) is the average value in the entire base region of BJTs, respectively. Fig. 13 shows the change in the reciprocal of current gain as a function of the normalized displacement dose Dd0 for the 3DG112 transistors irradiated by various heavy ions. It is clear that the experimental
Change in the reciprocal of current gain, Δ(1/)
100
10-1
10-2
-3
10
10-3
10-4
10-1
100 101 102 Normalized displacement dose, D'd (Gy)
Fig. 13. Change in the reciprocal of current gain as a function of the normalized displacement dose Dd0 for the 3DG112 NPN transistors irradiated by various heavy ions.
100 101 102 Normalized displacement dose, D'd (Gy)
Change in the reciprocal of current gain, Δ(1/)
100
10-1
Devices: 3CG130 -2
10
Br 20 MeV Si 35 MeV C 20 MeV 100 101 Normalized displacement dose, D'd (Gy)
102
Fig. 15. Change in the reciprocal of current gain as a function of the normalized displacement dose Dd0 for the 3CG130 PNP transistors irradiated by various heavy ions.
100
10-1
10-2
10-3 100
103
Devices: 3DG130 Br 20 MeV Si 35 MeV C 20 MeV
Fig. 14. Change in the reciprocal of current gain as a function of the normalized displacement dose Dd0 for the 3DG130 NPN transistors irradiated by various heavy ions.
10-2 Devices: 3DG112 Br 20 MeV Si 35 MeV C 20 MeV
65
10-1
10-1
Change in the reciprocal of current gain, Δ(1/)
physical mechanisms for the interaction between the ionizing and displacement damage. The first is about positive oxide charge. As shown in reference [19], for NPN devices, the positive oxide charge enhances bulk recombination current by lowering the majority carrier density near the base surface and reducing the difference in subsurface, bulk carrier densities. However, positive oxide charge suppresses bulk recombination current in PNP BJTs by increasing the majority carrier density near the base surface and widening the difference in subsurface, bulk carrier densities. The positive oxide charge could enhance the displacement damage in NPN devices and reduce the displacement damage in PNP devices. The second one is due to interface states. As mentioned in reference [20],for NPN devices, the interface states are positive, and reduce the difference in subsurface, bulk carrier densities. Meanwhile, the interface states are negative in PNP devices, and reduce the difference in subsurface, bulk carrier densities similarly. Therefore, the interface states enhance the displacement damage in both NPN and PNP devices. As mentioned above, the interface traps and the net positive charges result in different influence on the displacement damage. The dominating influence factor, interface traps or net positive charges, on the displacement damage is still not clear, and needs to be further investigated in the future. Therefore, it is required to incorporate a factor that account for the effect of the ionization on displacement damage induced by various particles for the BJTs. log(Di/Dd) is a factor that accounts for the effect of the ionization on displacement damage induced by various particles for the BJTs. The factor is related to the types and energy of incident particles as well as the device characteristics. The Messenger–Spratt equation could be converted to the following expression Dð1=bÞ ¼ K d log Di =Dd Dd ¼ K d D0d ð4Þ
Change in the reciprocal of current gain, Δ(1/)
C. Liu et al. / Nuclear Instruments and Methods in Physics Research A 677 (2012) 61–66
Devices: 3DK4B C 25 MeV Si 10 MeV Si 24 MeV Si 40 MeV Cl 40 MeV 101 102 Normalized displacement dose, D'd (Gy)
103
Fig. 16. Change in the reciprocal of current gain as a function of the normalized displacement dose Dd0 for the 3DK4B NPN transistors irradiated by various heavy ions.
data caused by 25 MeV C, 35 MeV Si and 20 MeV Br ions can be normalized in one line. Similarly, the change in the reciprocal of current gain as a function of Dd0 for the 3DG130, 3CG130 and
66
C. Liu et al. / Nuclear Instruments and Methods in Physics Research A 677 (2012) 61–66
3DK4B transistors irradiated by various heavy ions is shown in Figs. 14–16, respectively, the experimental data of which are normalized likewise. This equivalent methodology will be very useful to evaluate the equivalence of the displacement damage caused by various heavy ions in the Si bipolar junction transistors, without performing detailed time-consuming irradiations for all the heavy ions. In addition, it is easier to predict the end-of-life performance of the BJTs irradiated by heavy ions, based on the linear relationship given by the Eq. (4).
4. Conclusions It is important to normalize the displacement damage in BJTs for various heavy ions, since the displacement effect is the major mechanism of the degradation of current gain. The changes in the reciprocal current gain (D(1/b)) of bipolar junction transistors were examined under the exposure of various heavy ions. From the experimental results, it is clear that when the energy loss of incident particles varies gradually in sensitive region, the NIEL methodology alone is not an effective criterion to normalize the displacement damage for various heavy ions. Alternatively, it is shown that the NIEL term corrected by a log(Di/Dd)Dd can fit well the degradation of the current gain with displacement dose for various heavy ions, where Di is the ionizing dose and Dd is the displacement dose in the sensitive region of BJTs, so that the effects of various heavy ions can be characterized in a simple and economic way without performing detailed time-consuming irradiations for each heavy ion.
Acknowledgments This work was supported by the Fundamental Research Funds for the Central Universities (Grant no. HIT.KLOF.2010003) and
the National Basis Research Foundation of China under Grant no. 51320. References [1] G.P. Summers, E.A. Burke, C.J. Dale, E.A. Wolicki, P.W. Marshall, M.A. Gehlhausen, IEEE Transactions on Nuclear Science NS-34 (1987) 1134. [2] K.V. Madhu, R. Kumar, M. Ravindra, R. Damle, Solid-State Electronics 52 (2008) 1237. [3] S.R. Kulkarni, M. Ravindra, G.R. Joshi, R. Damle, Nuclear Instruments and Methods in Physics Research B 251 (2006) 157. [4] A.P. Gnana Prakash, S.C. Ke, K. Siddappa, Nuclear Instruments and Methods in Physics Research B 215 (2004) 457. [5] A.H. Johnston, B.G. Rax, IEEE Transactions on Nuclear Science NS-53 (2006) 1779. [6] A. Holmes-Siedle, L. Adams, Handbook of Radiation Effects, second ed., Oxford University Press, Oxford, 2002. [7] C. Claeys, E. Simoen, Radiation Effects in Advanced Semiconductor Materials and Devices, Springer-Verlag, Berlin, 2002. [8] J.R. Srour, C.J. Marshall, P.W. Marshall, IEEE Transactions on Nuclear Science NS-50 (2003) 653. [9] G.C. Messenger, M.S. Ash, The Effects of Radiation on Electronic Systems, 2nd edition, Van Nostrand Reinhold, New York, 1992. [10] X.J. Li, H.B. Geng, C.M. Liu, Z.M. Zhao, M.J. Lan, D.Z. Yang, S.Y. He, IEEE Transactions on Nuclear Science NS-57 (2010) 831. [11] X.J. Li, H.B. Geng, M.J. Lan, D.Z. Yang, S.Y. He, C.M. Liu, Chinese Physics B 19 (2010) 066103. [12] X.J. Li, H.B. Geng, M.J. Lan, C.M. Liu, D.Z. Yang, S.Y. He, Physica B 405 (2010) 1489. [13] J.H. Warner, S.R. Messenger, R.J. Walters, G.P. Summers, IEEE Transactions on Nuclear Science NS-52 (2005) 2678. [14] S.R. Messenger, E.A. Burke, G.P. Summers, R.J. Walters, IEEE Transactions on Nuclear Science NS-51 (2004) 3201. [15] E. Bielejec, G. Vizkelethy, N.R. Kolb, D.B. King, B.L. Doyle, IEEE Transactions on Nuclear Science NS-53 (2006) 3681. [16] C.J. Dale, P.W. Marshall, E.A. Burke, G.P. Summers, E.A. Wolicki, IEEE Transactions on Nuclear Science NS-35 (1988) 1208. [17] G.P. Summers, E.A. Burke, P. Shapiro, S.R. Messenger, R.J. Walters, IEEE Transactions on Nuclear Science NS-40 (1993) 1372. [18] H.J. Barnaby, S.K. Smith, R.D. Scrhimpf, D.M. Fleetwood, R.L. Pease, IEEE Transactions on Nuclear Science NS-49 (2002) 2643. [19] V.S. Pershenkov, K.A. Chumakov, A.Y. Nikiforov, A.I. Chumakov, V.N. Ulimov, A.A. Romanenko, In: Proceedings of the IEEE 9th Conference on Radiation and Its Effects on Components and Systems, 1 2007.
Contents lists available at SciVerse ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
The equivalence of displacement damage in silicon bipolar junction transistors Chaoming Liu a, Xingji Li a,n, Hongbin Geng a, Erming Rui a, Lixin Guo a, Jianqun Yang a, Liyi Xiao b a b
School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, China Microelectronics Center, Harbin Institute of Technology, Harbin 150001, China
a r t i c l e i n f o
abstract
Article history: Received 14 December 2011 Received in revised form 12 February 2012 Accepted 12 February 2012 Available online 6 March 2012
The current gain degradation in silicon bipolar junction transistors (BJTs) is examined under the irradiation with heavy ions. To characterize the radiation damage of the BJTs, the ionizing dose Di and displacement dose Dd verse the chip depth in the BJTs have been calculated for heavy ions. Based on the irradiation testing and calculation results, an approach to evaluate the equivalence of displacement damage in silicon BJTs is given, which could optimize the non-ionizing energy loss (NIEL) methodology and normalize the displacement damage caused by heavy ions. & 2012 Elsevier B.V. All rights reserved.
Keywords: Displacement equivalence Bipolar junction transistors Radiation effects Heavy ions Gain degradation
1. Introduction Bipolar junction transistors (BJTs) are generally known as devices operating in radiation rich environments, especially in spacecraft, due to their current drive capability, linearity and excellent matching characteristics. In space, the radiation-induced gain degradation is a concern aspect for characterization of the BJTs, and is the primary cause for parametric shifts and functional failures. It is useful to understand the radiation response of these devices to find better design strategies before employing them for specific applications [1–5]. BJTs are sensitive to both ionizing and displacement effects induced by electrons, protons, neutrons and heavy ions [6–8]. Generally, heavy ions could induce more displacement damage in BJTs at same energy and fluence. Investigations using heavy ions are valuable to understand the displacement mechanism of the BJTs. The Messenger–Spratt equation [9], used to characterize the current gain degradation of bipolar devices generated by displacement damage, can be expressed as the linear change in the reciprocal of current gain with irradiation fluence, which can be expressed as follows:
Dð1=bÞ ¼ K F
n
Corresponding author. Tel.: þ86 451 86414445; fax: þ 86 451 86415168. E-mail addresses: [email protected], [email protected] (X. Li).
0168-9002/$ - see front matter & 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2012.02.045
ð1Þ
where the change in the reciprocal of the gain variation (D(1/b)) is defined as the value after irradiation minus the initial one, that is D(1/b)¼1/b 1/bpre-rad; K the damage factor; F the incident particle fluence. It was proposed that the ratio of Dd/(Dd þDi) could be used as a criterion to show the displacement damage capacity of incident particles, where Di and Dd is the ionizing dose and displacement dose in the chip or device, respectively [10–12]. The higher the ratio of Dd/(Dd þ Di), the bigger the displacement damage capacity of the incident particles. The Dd/(Dd þDi) ratios are dependent on the types and energy of incident particles as well as the device nature. Generally, it is required to choose the incident particles with higher ratios of Dd/(Dd þDi) to perform irradiation testing, if the displacement damage is a major concern for the BJTs. When the irradiation particles induce displacement damage to BJTs, the Messenger–Spratt equation becomes suitable to characterize the experimental data. A more convenient technique would use the NIEL methodology to determine the damage factor K in the Messenger–Spratt equation and to characterize the damage equivalence of various heavy ions [8]. Using various heavy ions and neutrons as incident particles on silicon BJTs, Summers et al. [1] show that the damage factor K calculated from the Messenger–Spratt equation scales with the non-ionizing energy loss (NIEL). However, some experimental results show that the NIEL methodology has some limitations [8,13,14]. The displacement damage to BJTs induced by various particles with the same NIEL may be different, which is incompatible with the NIEL methodology. As mentioned in reference [15], the NIEL
C. Liu et al. / Nuclear Instruments and Methods in Physics Research A 677 (2012) 61–66
methodology could be used for light ions where the ions lose very little energy in the sensitive part of the device. However, there is a deviation when the energy losses by incident particles vary significantly in sensitive region of the BJTs. Therefore, it is required to find a way to establish the equivalence of displacement damage for various heavy ions. The purpose of this paper is to develop a new method to assess the displacement damage equivalence in Si BJTs caused by various heavy ions. If the displacement damage could be normalized, the effects of various heavy ions may be characterized in a simple and economic way, without performing detailed time-consuming irradiations for each type of heavy ion.
Change in the reciprocal of current gain, Δ(1/)
62
10-1
10-2
Devices: 3DG112 Br 20 MeV Si 35 MeV C 25 MeV
10-3
10-4
2. Experimental details
108
109
1010
1011 2
3. Results and discussion Displacement damage on bipolar junction transistors increases the number of defects in the silicon bulk, and the lifetime of the minority-carriers in the Si bulk is decreased. Consequently, the base current is increased and the common-emitter current gain (b) is degraded. Figs. 1–3 show the change in the reciprocal gain (D(1/b)) at a given base-emitter voltage of 0.65 V as a function of fluence under 25 MeV C, 35 MeV Si and 20 MeV Br ions irradiation for 3DG112, 3DG130 and 3CG130 bipolar junction transistors, respectively. Fig. 4 shows the change in the reciprocal gain (D(1/b)) at a given base-emitter voltage of 0.65 V as a function of fluence under 25 MeV C, 10–40 MeV Si, and 40 MeV Cl ions irradiation for 3DK4B BJTs. It is clear that D(1/b) increases linearly with the increasing ions fluence, and the relationship between the variations of the reciprocal of current gain and the heavy ions fluence is suitably characterized by the Messenger–Spratt equation. Based on the experimental data from Figs. 1–3, it is apparent that the change in the reciprocal of current gain (D(1/b)) undergoes the largest variations under the Br ions irradiation and
Fluence of heavy ions (ions/cm )
Change in the reciprocal of current gain, Δ(1/ )
Fig. 1. Change in the reciprocal of current gain as a function of fluence for 3DG112 NPN transistors irradiated by various heavy ions.
10-1
10-2 Devices: 3DG130 Br 20 MeV Si 35 MeV C 25 MeV
-3
10
107
108
109
1010
1011
2
Fluence of heavy ions (ions/cm ) Fig. 2. Change in the reciprocal of current gain as a function of fluence for 3DG130 NPN transistors irradiated by various heavy ions.
Change in the reciprocal of current gain, Δ(1/)
Four types of bipolar junction transistors (3DG112, 3DG130, 3CG130, and 3DK4B) were used as samples in this study. 3DG112 and 3DG130 are NPN high frequency and low power transistors. 3CG130 is PNP high frequency and low power transistor. 3DK4B is NPN low power switching transistor. 25 MeV carbon (C), 35 MeV silicon (Si) and 20 MeV bromine (Br) ions were chosen to test the 3DG112, 3DG130 and 3CG130 transistors in this investigation. 25 MeV (C), 10–40 MeV (Si), and 40 MeV chlorine (Cl) ions are chosen to test the 3DK4B transistors. The heavy ions irradiations were performed using the EN Tandem Accelerator in the State Key Laboratory of Nuclear Physics and Technology, Peking University, China. Beam areas were 20 150 mm2 for heavy ions. The irradiations were carried out in an evacuated chamber with a specially designed Faraday cup, which is used to measure the heavy ions beam current. From these measurements, the flux and fluences were determined for the irradiation experiments. The samples were mounted inside a package with removable upper lid for irradiation. All terminals of the samples were grounded during all particle irradiations. Electrical parameters of the NPN BJTs were measured in-situ using a semiconductor characterization system: Keithley 4200-SCS. The turn-over time between irradiation and device measurements is approximately less than 5 s. The irradiations and measurements were performed at room temperature. For the in-situ measurement, a matrix board switching system located outside of the irradiation chamber is used as a naster control panel, which is designed with very high insulation resistance so that very low current can be generated.
100
10-1
Devices: 3CG130 Br 20 MeV Si 35 MeV C 25 MeV
10-2
108
109 Fluence of heavy ions
1010 (ions/cm2)
Fig. 3. Change in the reciprocal of current gain as a function of fluence for 3CG130 PNP transistors irradiated by various heavy ions.
least under the C ions irradiation at a given fluence for 3DG112, 3DG130 and 3CG130 BJTs. As shown in Fig. 4, D(1/b) exhibits the largest variations under the 10 MeV Si ions irradiation and the smallest under 25 MeV C ions irradiation for 3DK4B BJTs.
100
Device: 3DG130 Di and Dd per fluence (Gy/ion)
Change in the reciprocal of current gain, Δ(1/)
C. Liu et al. / Nuclear Instruments and Methods in Physics Research A 677 (2012) 61–66
Devices: 3DK4B 10-1
C 25 MeV Si 10 MeV Si 24 MeV Si 40 MeV Cl 40 MeV
10-2
10-3
108
109
1010
Fluence of heavy ions
10
-5
63
Dd for Br 20 MeV
Di for Br 20 MeV
Dd for Si 35 MeV
Di for Si 35 MeV
Dd for C 25 MeV
Di for C 25 MeV
10-7 Base region 10-9
1011 (ions/cm2)
0
1 2 3 Depth in the chip of device (μm)
Fig. 4. Change in the reciprocal of current gain as a function of fluence for 3DK4B NPN transistors irradiated by various heavy ions.
4
5
Fig. 6. Total ionizing absorbed dose Di and displacement absorbed dose Dd as a function of the depth in the chip of 3DG130 NPN transistor.
10-3
10-3 Di for Br 20 MeV
Dd for Si 35 MeV
Di for Si 35 MeV
Dd for C 25 MeV
Di for C 25 MeV
Device: 3CG130 Di and Dd per fluence (Gy/ion)
10-5
Dd for Br 20 MeV
10-7 Base region 10-9
10-11
10-5
0
1 2 Depth in the chip of device (μm)
The total ionizing absorbed dose Di and displacement absorbed dose Dd were calculated to analyze the radiation damage to BJTs induced by various incident particles. These parameters can be calculated using the following equations Di ðtÞ ¼ 1:6 1010 LETðtÞ F
ð2Þ
Dd ðtÞ ¼ 1:6 1010 NIELðtÞ F
ð3Þ
where Di(t) and Dd(t) are the ionizing dose and the displacement dose, as a function of depth in chip of the NPN BJTs (in Gy); t is the depth in the device chips, (in mm); 1.6 10 10 is the unit conversion parameter (in Gy g/MeV); LET(t) and NIEL(t) are the ionizing and the displacement energy loss as a function of depth in the chip, which are calculated by the stopping power and range of ion in matter (SRIM) code (in MeV cm2/g). The sensitive region of BJTs is the entire base region, the region from the Si/SiO2 interface to the base bottom (include the emitter region, emitterbase space charge region and base region). The calculated results of Di and Dd for 3DG112, 3DG130, 3CG130, and 3DK4B BJTs are shown in Figs. 5–8, respectively. From the results in Figs. 5–7, Di and Dd for 20 MeV Br ions vary gradually in the sensitive region of the 3DG112, 3DG130 and 3CG130 BJTs, while the Di and Dd for
Di for Br 20 MeV
Dd for Si 35 MeV
Di for Si 35 MeV
Dd for C 25 MeV
Di for C 25 MeV
Base region 10-9
3
Fig. 5. Total ionizing absorbed dose Di and displacement absorbed dose Dd as a function of the depth in the chip of 3DG112 NPN transistor.
Dd for Br 20 MeV
10-7
10-11
0
1
2 3 4 5 Depth in the chip of device (μm)
6
7
Fig. 7. Total ionizing absorbed dose Di and displacement absorbed dose Dd as a function of the depth in the chip of 3CG130 PNP transistor.
Di and Dd per fluence (Gy/ion)
Di and Dd per fluence (Gy/ion)
Device: 3DG112
10-6 Base region 10-8
10-10 Device: 3DK4B 10-12
0
Dd for C 25 MeV
Di for C 25 MeV
Dd for Si 10 MeV
Di for Si 10 MeV
Dd for Si 24 MeV
Di for Si 24 MeV
Dd for Si 40 MeV
Di for Si 40 MeV
Dd for Cl 40 MeV
Di for Cl 40 MeV
1 2 3 Depth in the chip of device (μm)
4
Fig. 8. Total ionizing absorbed dose Di and displacement absorbed dose Dd as a function of the depth in the chip of 3DK4B NPN transistor.
25 MeV C and 35 MeV Si ions exhibits very small variations. The 10 MeV Si ions vary gradually in entire base region of the 3DK4B, as shown in Fig. 8.
Change in the reciprocal of current gain, Δ(1/)
10-1
10-2
10-1
10-2 Devices: 3DG130 Br 20 MeV Si 35 MeV C 25 MeV
10-3
10-1
100 101 Displacement absorbed dose, Dd (Gy)
102
Fig. 10. Change in the reciprocal of current gain as a function of Dd for the 3DG130 NPN transistors irradiated by various heavy ions.
100 Change in the reciprocal of current gain, Δ(1/)
As pointed out in [10–12], for a given BJTs, the average ratio of Dd/(Dd þDi) in the sensitive region could be related to the displacement capability for the incident particles. The incident particle with the higher ratio of Dd/(Dd þDi) could induce more displacement damage than the particle with lower ratio for the same BJT. If the ratio is high enough, the displacement damage induced by the incident particles could dominate in the BJTs, and the change in the reciprocal of current gain with fluence will be followed by the Messenger–Spratt equation very well. The average ratios of Dd/(Dd þDi) for 25 MeV C, 35 MeV Si and 20 MeV Br ions are 3.9 10 4, 8.8 10 4 and 0.016, respectively. The ratios are much larger than the value of 2.63 10 4 for 3 MeV protons in the entire base region of 3DG112 BJTs [12]. Therefore, it results likely that 25 MeV C, 35 MeV Si and 20 MeV Br ions may primarily induce displacement damage to 3DG112 BJTs. Based on the results in Fig. 1, it is obvious that the experimental results follow the Messenger–Spratt equation very well. This phenomenon further proves that the average ratio of Dd/(Dd þDi) in the sensitive region of the BJTs is a good symbol to describe the displacement capability for the incident particles. In order to examine the displacement damage behavior of BJTs, it is required to choose the incident particles having the Dd/(Dd þDi) high enough. It is known that the NIEL methodology is often used to characterized the expected particle-induced response of a device in a displacement radiation environment, but it is necessary to appreciate the associated underlying assumptions and limitations in order to use them effectively [1,8,14,15]. If the energy losses of incident particles could not keep steady in the sensitive region of BJTs, the NIEL methodology will not give a good result to normalize the change in the reciprocal of current gain vs. displacement dose caused by various particles. In particular, deviations at very low particle energies are expected [16–18]. Figs. 9–12, show the change in the reciprocal of current gain as a function of displacement absorbed dose Dd for the 3DG112, 3DG130, 3CG130 and 3DK4B transistors irradiated by various heavy ions, respectively. It is clear that the NIEL methodology is not suitable for characterizing the displacement damage induced by 20 MeV Br and 10 MeV Si ions, because of the energy loss induced by 20 MeV Br and 10 MeV Si ions exhibit obvious variations of in base region, as shown in Figs. 5–8. Therefore, it is valuable to provide an alternative approach. As shown in reference [19], the ionizing effect could affect the displacement damage to some extent in the BJTs. Such effects induced by various incident particles are different, since the different particles have different capacities to induce the
Change in the reciprocal of current gain, Δ(1/)
C. Liu et al. / Nuclear Instruments and Methods in Physics Research A 677 (2012) 61–66
10-1
Devices: 3CG130 10-2
Br 20 MeV Si 35 MeV C 25 MeV 10-1
100 101 Displacement absorbed dose, Dd (Gy)
102
Fig. 11. Change in the reciprocal of current gain as a function of Dd for the 3CG130 PNP transistors irradiated by various heavy ions.
100 Change in the reciprocal of current gain, Δ(1/)
64
10-1 Devices: 3DK4B C 25 MeV Si 10 MeV Si 24 MeV Si 40 MeV Cl 40 MeV
10-2
10-3
100
101 102 Displacement absorbed dose, Dd (Gy)
Devices: 3DG112
10-3
Fig. 12. Change in the reciprocal of current gain as a function of Dd for the 3DK4B NPN transistors irradiated by various heavy ions.
Br 20 MeV Si 35 MeV C 25 MeV
10-4 0.1
1 10 Displacement absorbed dose, Dd (Gy)
100
Fig. 9. Change in the reciprocal of current gain as a function of Dd for the 3DG112 NPN transistors irradiated by various heavy ions.
displacement damage with respect to ionization damage. The ionizing damage could cause interface traps and net positive charges in the oxide overlying the emitter–base junction. The interface traps and net positive charges give the complex influence on the displacement damage. There may be two different
where Kd is a constant, the normalized displacement damage factor depending on the device type; log(Di/Dd) is an factor to characterize the effect of the ionization on displacement effect in the sensitive region of BJTs caused by heavy ions, which is related to the types and energy of particles as well as the device nature; D0 d is the normalized displacement dose, which equals log(Di/Dd)Dd. The value of Di and Dd in Eq. (4) is the average value in the entire base region of BJTs, respectively. Fig. 13 shows the change in the reciprocal of current gain as a function of the normalized displacement dose Dd0 for the 3DG112 transistors irradiated by various heavy ions. It is clear that the experimental
Change in the reciprocal of current gain, Δ(1/)
100
10-1
10-2
-3
10
10-3
10-4
10-1
100 101 102 Normalized displacement dose, D'd (Gy)
Fig. 13. Change in the reciprocal of current gain as a function of the normalized displacement dose Dd0 for the 3DG112 NPN transistors irradiated by various heavy ions.
100 101 102 Normalized displacement dose, D'd (Gy)
Change in the reciprocal of current gain, Δ(1/)
100
10-1
Devices: 3CG130 -2
10
Br 20 MeV Si 35 MeV C 20 MeV 100 101 Normalized displacement dose, D'd (Gy)
102
Fig. 15. Change in the reciprocal of current gain as a function of the normalized displacement dose Dd0 for the 3CG130 PNP transistors irradiated by various heavy ions.
100
10-1
10-2
10-3 100
103
Devices: 3DG130 Br 20 MeV Si 35 MeV C 20 MeV
Fig. 14. Change in the reciprocal of current gain as a function of the normalized displacement dose Dd0 for the 3DG130 NPN transistors irradiated by various heavy ions.
10-2 Devices: 3DG112 Br 20 MeV Si 35 MeV C 20 MeV
65
10-1
10-1
Change in the reciprocal of current gain, Δ(1/)
physical mechanisms for the interaction between the ionizing and displacement damage. The first is about positive oxide charge. As shown in reference [19], for NPN devices, the positive oxide charge enhances bulk recombination current by lowering the majority carrier density near the base surface and reducing the difference in subsurface, bulk carrier densities. However, positive oxide charge suppresses bulk recombination current in PNP BJTs by increasing the majority carrier density near the base surface and widening the difference in subsurface, bulk carrier densities. The positive oxide charge could enhance the displacement damage in NPN devices and reduce the displacement damage in PNP devices. The second one is due to interface states. As mentioned in reference [20],for NPN devices, the interface states are positive, and reduce the difference in subsurface, bulk carrier densities. Meanwhile, the interface states are negative in PNP devices, and reduce the difference in subsurface, bulk carrier densities similarly. Therefore, the interface states enhance the displacement damage in both NPN and PNP devices. As mentioned above, the interface traps and the net positive charges result in different influence on the displacement damage. The dominating influence factor, interface traps or net positive charges, on the displacement damage is still not clear, and needs to be further investigated in the future. Therefore, it is required to incorporate a factor that account for the effect of the ionization on displacement damage induced by various particles for the BJTs. log(Di/Dd) is a factor that accounts for the effect of the ionization on displacement damage induced by various particles for the BJTs. The factor is related to the types and energy of incident particles as well as the device characteristics. The Messenger–Spratt equation could be converted to the following expression Dð1=bÞ ¼ K d log Di =Dd Dd ¼ K d D0d ð4Þ
Change in the reciprocal of current gain, Δ(1/)
C. Liu et al. / Nuclear Instruments and Methods in Physics Research A 677 (2012) 61–66
Devices: 3DK4B C 25 MeV Si 10 MeV Si 24 MeV Si 40 MeV Cl 40 MeV 101 102 Normalized displacement dose, D'd (Gy)
103
Fig. 16. Change in the reciprocal of current gain as a function of the normalized displacement dose Dd0 for the 3DK4B NPN transistors irradiated by various heavy ions.
data caused by 25 MeV C, 35 MeV Si and 20 MeV Br ions can be normalized in one line. Similarly, the change in the reciprocal of current gain as a function of Dd0 for the 3DG130, 3CG130 and
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3DK4B transistors irradiated by various heavy ions is shown in Figs. 14–16, respectively, the experimental data of which are normalized likewise. This equivalent methodology will be very useful to evaluate the equivalence of the displacement damage caused by various heavy ions in the Si bipolar junction transistors, without performing detailed time-consuming irradiations for all the heavy ions. In addition, it is easier to predict the end-of-life performance of the BJTs irradiated by heavy ions, based on the linear relationship given by the Eq. (4).
4. Conclusions It is important to normalize the displacement damage in BJTs for various heavy ions, since the displacement effect is the major mechanism of the degradation of current gain. The changes in the reciprocal current gain (D(1/b)) of bipolar junction transistors were examined under the exposure of various heavy ions. From the experimental results, it is clear that when the energy loss of incident particles varies gradually in sensitive region, the NIEL methodology alone is not an effective criterion to normalize the displacement damage for various heavy ions. Alternatively, it is shown that the NIEL term corrected by a log(Di/Dd)Dd can fit well the degradation of the current gain with displacement dose for various heavy ions, where Di is the ionizing dose and Dd is the displacement dose in the sensitive region of BJTs, so that the effects of various heavy ions can be characterized in a simple and economic way without performing detailed time-consuming irradiations for each heavy ion.
Acknowledgments This work was supported by the Fundamental Research Funds for the Central Universities (Grant no. HIT.KLOF.2010003) and
the National Basis Research Foundation of China under Grant no. 51320. References [1] G.P. Summers, E.A. Burke, C.J. Dale, E.A. Wolicki, P.W. Marshall, M.A. Gehlhausen, IEEE Transactions on Nuclear Science NS-34 (1987) 1134. [2] K.V. Madhu, R. Kumar, M. Ravindra, R. Damle, Solid-State Electronics 52 (2008) 1237. [3] S.R. Kulkarni, M. Ravindra, G.R. Joshi, R. Damle, Nuclear Instruments and Methods in Physics Research B 251 (2006) 157. [4] A.P. Gnana Prakash, S.C. Ke, K. Siddappa, Nuclear Instruments and Methods in Physics Research B 215 (2004) 457. [5] A.H. Johnston, B.G. Rax, IEEE Transactions on Nuclear Science NS-53 (2006) 1779. [6] A. Holmes-Siedle, L. Adams, Handbook of Radiation Effects, second ed., Oxford University Press, Oxford, 2002. [7] C. Claeys, E. Simoen, Radiation Effects in Advanced Semiconductor Materials and Devices, Springer-Verlag, Berlin, 2002. [8] J.R. Srour, C.J. Marshall, P.W. Marshall, IEEE Transactions on Nuclear Science NS-50 (2003) 653. [9] G.C. Messenger, M.S. Ash, The Effects of Radiation on Electronic Systems, 2nd edition, Van Nostrand Reinhold, New York, 1992. [10] X.J. Li, H.B. Geng, C.M. Liu, Z.M. Zhao, M.J. Lan, D.Z. Yang, S.Y. He, IEEE Transactions on Nuclear Science NS-57 (2010) 831. [11] X.J. Li, H.B. Geng, M.J. Lan, D.Z. Yang, S.Y. He, C.M. Liu, Chinese Physics B 19 (2010) 066103. [12] X.J. Li, H.B. Geng, M.J. Lan, C.M. Liu, D.Z. Yang, S.Y. He, Physica B 405 (2010) 1489. [13] J.H. Warner, S.R. Messenger, R.J. Walters, G.P. Summers, IEEE Transactions on Nuclear Science NS-52 (2005) 2678. [14] S.R. Messenger, E.A. Burke, G.P. Summers, R.J. Walters, IEEE Transactions on Nuclear Science NS-51 (2004) 3201. [15] E. Bielejec, G. Vizkelethy, N.R. Kolb, D.B. King, B.L. Doyle, IEEE Transactions on Nuclear Science NS-53 (2006) 3681. [16] C.J. Dale, P.W. Marshall, E.A. Burke, G.P. Summers, E.A. Wolicki, IEEE Transactions on Nuclear Science NS-35 (1988) 1208. [17] G.P. Summers, E.A. Burke, P. Shapiro, S.R. Messenger, R.J. Walters, IEEE Transactions on Nuclear Science NS-40 (1993) 1372. [18] H.J. Barnaby, S.K. Smith, R.D. Scrhimpf, D.M. Fleetwood, R.L. Pease, IEEE Transactions on Nuclear Science NS-49 (2002) 2643. [19] V.S. Pershenkov, K.A. Chumakov, A.Y. Nikiforov, A.I. Chumakov, V.N. Ulimov, A.A. Romanenko, In: Proceedings of the IEEE 9th Conference on Radiation and Its Effects on Components and Systems, 1 2007.