Available online at Nuclear Instruments and Methods in Physics Research B 266 (2008)

Available online at www.sciencedirect.com Nuclear Instruments and Methods in Physics Research B 266 (2008) 1713 1718 NIM B Beam Interactions with Materials & Atoms www.elsevier.com/locate/nimb Effect of 50 MeV Li 3+ ion irradiation on electrical characteristics of high speed NPN power transistor C.M. Dinesh a, *, Ramani a, M.C. Radhakrishna a, R.N. Dutt b, S.A. Khan b, D. Kanjilal b a Department of Physics, Bangalore University, Bangalore 560 056, India b Inter University Accelerator Center, Aruna Asaf Ali Marg, New Delhi 110 067, India Received 24 September 2007; received in revised form 23 February 2008 Available online 4 March 2008 Abstract Silicon NPN overlay RF power high speed commercial bipolar junction transistors (BJTs) find applications in military, space and communication equipments. Here we report the effect of 50 MeV Li 3+ ion irradiation in the fluence range 1 10 11 1.8 10 12 ions cm 2 on NPN power transistor. The range (R), electronic energy loss (S e ), nuclear energy loss (S n ), total ionizing dose (TID) and total displacement damage (D d ) in the silicon target are calculated from TRIM Monte Carlo Code. Output resistance is 3.568 10 4 X for unirradiated device and it increases to 6 10 7 X as the fluence is increased from 1 10 11 to 1.8 10 12 ions cm 2. The capacitance of the emitter-base junction of the transistor decreases and dielectric loss of the emitter-base junction increases with increase in ion fluence. The built in voltage of the unirradiated sample is 0.5 V and it shifts to 0.4 V after irradiation at fluence of 1.8 10 12 ions cm 2 and the corresponding doping density reduced to 5.758 10 16 cm 3. The charge carrier removal rate varies linearly with the increase in ion fluence. Ó 2008 Published by Elsevier B.V. PACS: 85.30.Pq; 61.80.Lj; 61.80. x; 73.40.Kq; 84.37.+q Keywords: Transistor; Irradiation; Defects; Junction; Capacitance 1. Introduction * Corresponding author. Tel.: +91 80 22961474; fax: +91 80 23219295. E-mail address: cm.dinesh@gamil.com (C.M. Dinesh). There is an increasing interest in the effect of high energy charged particles on the properties of microelectronic devices. NPN transistors are being extensively used in space and radiation rich environments [1 6]. A number of researchers have extensively investigated and reported the radiation effect of high energy proton, neutron, electron and gamma-ray on semiconductor devices [7]. However, only few literature is available on the effect of high energy heavy ion irradiation on bipolar junction transistors (BJT). An indigenously made device has to be characterized for radiation response for different ions at different energies. These investigations are important from the space application and understanding the mechanism of interaction of radiation with the device. BJTs have important applications in analog and mixed signal ICs and bipolar complementary metal oxide semiconductor (BICMOS) circuits because of their current drive capacity, linearity and excellent matching characteristics [2]. The aim of this work is to study the effect of 50 MeV Li 3+ ion irradiation on the electrical behavior of commercial BJTs. An attempt is made to explain the radiationinduced degradation based on linear energy transfer (LET), total ionizing dose (TID), displacement damage (D d ) and its effect on depletion/diffusion capacitance, doping density, built in voltage and dielectric loss. 2. Experimental The device examined here is silicon NPN power transistor (2N3866) manufactured by Bharath Electronics 0168-583X/$ - see front matter Ó 2008 Published by Elsevier B.V. doi:10.1016/j.nimb.2008.02.048

1714 C.M. Dinesh et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 1713 1718 Limited (BEL), India. The device has high RF current handling capability and high power gain. The insulating silicon dioxide (SiO 2 ) has thickness of about 600 700 nm, the thickness of the emitter (n + ) is about 1lm and thickness of the base (p + ) is about 2 lm and intrinsic surface doping is about 5 10 17 1 10 18 cm 3. The n epitaxial layer thickness is 16 lm (silicon h111i, 2 X-cm) and the thickness of n + substrate is 200 lm (silicon h111i, 1 X-cm). The transistor is irradiated by 50 MeV Li 3+ ion beam using 15 UD, 16 MV Tandem accelerator facility [9], available at Inter University Accelerator Centre (IUAC), New Delhi, India. The irradiation is performed in GPSC (general purpose scattering chamber) line having a typical pressure of 8 10 9 torr. The decapped transistor is mounted on a vertical metal ladder enabling direct exposure of chip for the beam, which avoids the energy loss in the protective metal cap. The fluence is varied from 1 10 11 to 1.8 10 12 ions cm 2. To avoid heating effect the beam current (measured by a Faraday cup at a relatively large distance before the actual target assembly) is maintained at 1 pna [1 pna (particle nanoampere) = 6.25 10 8 particles/cm 2 /s]. A beam spot of 2 mm 2 mm area is used to scan over a 10 mm 10 mm area with a magnetic scanner to irradiate the sample uniformly. E V CE characteristics are performed using Keithley 236 source meter together with computer interface before and after the irradiation. The forward voltage applied to the emitter-base junction ) is in the range 1.0 V, which allows to measure collector current 10 9 A< <10 3 A. The E V CE characteristic curve of a transistor is a plot of collector current ( ) versus collector emitter voltage (V CE ) at constant emitter-base voltage (V CE = constant). The of unirradiated and irradiated transistor has been measured by varying V CE from to 5.5 V (step size of 2 V) for different V BE =, 0.5, 1.0 V) and at =50lA. From these characteristic curves, the collector saturation current (sat ) and the corresponding collector emitter saturation voltage (V CEsat ) for various fluences are studied. Gummel plots are acquired by sweeping the base-emitter voltage from 0 V to 1 V in steps of 1 V at constant collector voltage of 5 V. Capacitance and dielectric loss spectra are studied for the transistor before and after irradiation by 50 MeV Li 3+ ion with different fluences using the same setup before. Capacitance and dielectric loss measurements are carried out simultaneously at a frequency of 1 MHz using precision LCR meter (AGILENT 4285A, 75 khz to 30 MHz) instrument with computer interface. The depletion capacitance and diffusion capacitance and the corresponding dielectric loss is recorded for emitter-base junction by varying V BE from 1.5 V to +1.5 V. The slope from plot of (1/C 2 ) versus voltage gives the doping concentration of the emitter-base junction, which is used to calculate carrier density. 3. Results and discussions Gain degradation in irradiated bipolar transistor is a significant problem, for linear integrated circuits. Understanding the mechanisms responsible for radiation-induced gain degradation in bipolar transistors is important in developing appropriate hardness assurance methods. For most bipolar technologies that are used in linear integrated circuits, the total-dose failure mechanism is reduction of the current gain. The lattice damage degrades the electrical characteristics of BJTs by increasing number of trapping, scattering and recombination centers. The trapping centers remove carriers from the conduction process. n and p type silicon gradually changes towards intrinsic material with the increase of radiation exposure [10 12]. 3.1. SRIM-NIEL, TID and D d calculations To understand the degradation in 50 MeV Li 3+ ion irradiated transistor, it is important to analyze the effect of irradiation on device structure and the role of the associated energy loss mechanism. It is well known that when an energetic ion penetrates into any material, it loses energy mainly by two nearly independent processes: (i) elastic collisions with the nuclei known as nuclear energy loss S n, which dominates at an energy of about 1 kev/ amu; and (ii) inelastic collisions of the highly charged projectile ion with the atomic electrons of the matter known as electronic energy loss S e which dominates at an energy of about 1 MeV/amu or more. In the inelastic collision (cross-section 10 16 cm 2 ) the energy is transferred from the projectile to the atoms through excitation and ionization of the surrounding electrons. The amount of electronic energy loss in each collision varies from tens of ev to a few kev per Angstrom [14]. The nuclear energy loss of 50 MeV Li 3+ ion is much smaller than the electronic energy loss (3 order of magnitude, Fig. 1) in a Si-target material due to smaller elastic scattering cross-section. Therefore the maximum energy deposited to the material is expected mainly due to the electronic energy loss during its passage through the Si-material [3]. The device suffers uniform irradiation effects as the projected ion range (310 lm, Fig. 1) is greater than the device thickness (220 lm). The damage caused due to the linear energy transfer [LET = S e + S n 0.408 MeV/(mg/cm 2 )] in the Si target is obtained using TRIM calculations (Table 1) [13]. SRIM includes a comprehensive Monte Carlo program called transport of ions in matter (TRIM), which provides a detailed treatment of ion damage cascades and the ion distribution within the target. This can give us a quantitative estimate of the damage on the layer structure of the transistor. NIEL is the rate at which energy is lost to non-ionizing events (energy per unit length) [8]. NIELðEÞ ¼ N Z p drðh; EÞ T ½h; EŠL½T ðh; EÞŠ dx; A dx h min

C.M. Dinesh et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 1713 1718 1715 electronic / nuclear energy loss [ MeV / (mg / cm 2 )] 10 3 10 30 50 70 90 10 2 10 1 10 0 10-1 10-2 10-3 10-4 310.24 m S e = 0. 408 [ MeV / ( mg / cm 2 ) ] Electronic energy loss of 50 MeV lithium ion in Si target (S e ) Nuclear energy loss of 50 MeV lithium ion in Si target (Sn) S n = 2. 293 E - 4 [ MeV / (mg/cm 2 )] 10 30 50 70 90 Li ion energy (MeV) Fig. 1. Linear energy transfer and range calculations as a function of energy for lithium ion in silicon target. Table 1 TRIM data for 50 MeV Li 3+ ion in silicon target Average damage factor for each ion Total average displacements 1296.8 Total average replacements 99.2 Total average vacancies 1197.6 Total average interstitials 1296.8 where N is Avogadro s number, A is atomic number and h min is scattering angle for which the recoil energy equals the threshold for atomic displacements (for Si threshold energy = 15 ev from SRIM 2003). T is the average recoil energy of the target atoms, L is the term which partitions the energy into ionizing and non-ionizing events, called the Linhard partition function. NIEL is the direct analog of the LET or stopping power for ionization events. The unit of NIEL is typically MeV cm 2 /g (or) MeV/cm. The calculation of NIEL requires information regarding the differential cross-section for atomic displacements dr dx. The value of NIEL from SRIM calculations [8] is found to be 0.7113 MeV cm 2 /g. The fluence dependent TID (measured in rad) and D d are tabulated in the Table 2. TID is calculated using the relation TID = U S, where U is fluence and S is stopping power (S = S e + S n ) and the total displacement damage D d is calculated using the relation D d = U NIEL. Table 2 Fluence dependence total ionizing dose (TID) and displacement damage (D d ) for Si target Ion fluence (ions/ cm 2 ) Total ionizing dose (TID) (rad.) Displacement damage (D d ) (rad.) 1 10 11 6.536 10 5 1.138 10 3 6 10 11 3.919 10 6 6.828 10 3 1.2 10 12 7.838 10 6 1.366 10 4 1.8 10 12 1.176 10 7 2.049 10 4 3.2. I V Measurements Fig. 2 shows the output E V CE characteristic curve for unirradiated and for the 50 MeV Li 3+ ion irradiated transistor with fluence varied from 1 10 11 to 1.8 10 12 ions cm 2. From figure it is clear that the output collector current of the irradiated transistor reduces to almost negligible value at a total fluence of 1.8 10 12 ions cm 2.Itis also observed that there is a decrease in sat with increase in fluence. The decrease in sat is mainly due to the production of Li ion induced displacements, vacancies and interstitials (defects listed in Table 1). These MeV ion induced defects reduce the minority carrier lifetime and are responsible for the increase in series resistance in turn reducing sat and V CEsat (Table 3). Fig. 3 shows the variation of base and collector current as a function of base-emitter voltage (forward Gummel plots) before and after exposure to various fluences of 50 MeV Li 3+ ions. In the earlier work related to irradiation, it is reported that the gain of the transistor decreases due to the change in base current ( ) and the collector current ( ) of the transistor with electron and gamma-ray irradiation [19,20]. But in the case of oxygen ion irradiation is found to decrease significantly along with the decrease in the [4]. High energy ions can produce ionizations in oxides and bulk silicon, which can also produce point and complex defects leading to the light particles like electrons and gamma photons. In the present case with lithium ion irradiation, is found to decrease with increase in ion fluence and decreases initially for fluence of 1 10 11 ions cm 2, it increases with further increase in fluence (Fig. 3). The 50 MeV Li 3+ ions produce defects like vacancies, displacements and divacancies in collector region which reduces the minority carrier lifetime. A decrease in the minority carrier lifetime will be reflected in the degradation of forward current gain of the Collector current, (A) 3.0x10-3 2.5x10-3 2.0x10-3 1.5x10-3 1.0x10-3 5.0x10-4 -5.0x10-4 unirrad 1.0x10 11 ions/cm 2 6.0x10 11 ions/cm 2 1.2x10 12 ions/cm 2 1.8x10 12 ions/cm 2-0.4-0.2 0.2 0.4 0.6 0.8 1.0 V CE (V) Fig. 2. The output collector current characteristic curves at =50lA before and after 50 MeV Li 3+ ion irradiation.

1716 C.M. Dinesh et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 1713 1718 Table 3 Fluence dependence variation of electrical parameters for different biased voltages Ion fluence (ions/cm 2 ) V BE = V V BE = 0.5 V V BE = 1.0 V (sat )A (V Csat )V R f, X (sat )A (V Csat )V R f, X (sat )A (V Csat )V R f, X Unirradiated 2.44 10 4 3.50 1.23 10 4 3.09 10 4 4.40 1.17 10 4 3.82 10 4 5.10 1.14 10 4 1 10 11 5.47 10 7 0.65 5.44 10 5 1.71 10 6 0.80 2.03 10 5 3.01 10 6 0.90 1.43 10 5 6 10 11 1.52 10 7 0.55 2.08 10 6 4.29 10 7 0.65 6.35 10 5 7.93 10 7 0.75 3.70 10 5 1.2 10 12 7.67 10 8 0.50 1.39 10 7 1.09 10 7 0.55 4.04 10 6 1.63 10 7 0.60 2.17 10 6 1.8 10 12 6.59 10 8 0.45 4 10 7 6.91 10 8 0.50 6.49 10 7 7.36 10 8 0.50 1.52 10 7, (A) 10-2 10-3 10-4 10-5 10-6 10-7 10-8 10-9 10-10 10-11 unexposed of 1x10 11 ions/cm 2 of 6x10 11 ions/cm 2 of 1.2x10 12 ions/cm 2 of 1.8x10 12 ions/cm 2 unexposed of 1x10 11 ions/cm 2 of 6x10 11 ions/cm 2 of 1.2x10 12 ions/cm 2 of 1.8x10 12 ions/cm 2 0.1 0.2 0.3 0.4 0.5 0.6 0.7 V BE (V) Fig. 3. Variation of collector current ( ), base current ( ) with V BE in Li 3+ ion irradiated transistor for different fluences (V CE = 5.0 V). Normalizeg gain=(b/b 0 ) 1.0 0.8 0.6 0.4 0.2 normalized gainat V CE = 0.6V 1.0x10 11 6.0x10 11 1.1x10 12 1.6x10 12 Fluence (ions/cm) 2 Fig. 4. Relationship between current gain (b) normalized to pre-irradiation gain (b 0 ) = 0.6 V) value and fluence of ion irradiation. transistor. The variation of gain as a function of fluence is shown in Fig. 4. Table 2 shows fluence dependent total ionizing dose (TID) and displacement damage (D d ). The total ionizing dose leads to the degradation of the transistor, which is due to the increase in recombination of emitter-base region. The TID is mainly dependent on LET of the MeV ion in the target; hence electronic excitations contribution is more compared to nuclear displacements by 3 orders of magnitude (Fig. 1). The displacement damage contribution is less compared to TID by a factor of 2. This may be a considerable factor at higher fluence range for the semiconductor devices. The displacement damage increases the recombination in the neutral base region, which leads to the decrease in collector saturation current. It is found [2] that the number of recombination centers produced in silicon is proportional to the energy going into atomic displacement process. The non-ionizing energy deposition induced damage scaling of the gain variation of bipolar transistor is already observed for neutron and carbon ions [2,15]. The fluence dependent total ionizing dose (TID) and NIEL dependent displacement damage (D d ) versus saturation current and saturation voltage are plotted in Fig. 5. It shows correlation between the calculated defects and measured electrical parameters (sat and V CEsat ). It is also observed that from Fig. 5, the saturation voltage and current abruptly decreased with ion fluence of 1 10 11 ions cm 2 and it remained constant with increase in fluence. The BJTs are found to be very sensitive to the ionizing radiation and transistor gain degradation is the primary cause for the parametric shifts and functional failures. 3.3. Capacitance measurements Fig. 6 shows the plot of capacitance versus emitter-base voltage for 50 MeV Li 3+ ion irradiated and unirradiated transistor. It can be seen that the depletion capacitance is less than the diffusion capacitance. With the increase in fluence the capacitance of the emitter-base region decreases. The irradiation affects more on the diffusion capacitance compared to the depletion capacitance. It is observed that the capacitance is reduced after the irradiation from nanofarad to pico-farad in the emitter-base region of the device. The change in capacitance is attributed to the reduction in the carrier density of the emitter-base region. This reduction in capacitance may be mainly due to the change in series resistance [16 18]. Due to irradiation the shift in the built-in potential is observed, for unirradiated transistor it is found to be 0.437 V and it increased to 0.445 V as the device is irradiated with 1 10 11 ions cm 2. The doping density of the emitter-base region decrease from

C.M. Dinesh et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 1713 1718 1717 NIEL dependent displacement damage, D d (rad) 5.0x10 3 1.0x10 4 1.5x10 4 2.0x10 4 10-4 sat = V) sat = 0.5V) sat = 1.0V) 5.5 5.0 4.5 4.0 sat (A) 10-5 10-6 V CEsat V CEsat V CEsat = V) = 0.5V) = 1.0V) 3.5 3.0 2.5 2.0 V C Esat (V) 1.5 10-7 1.0 0.5 2.0x10 6 4.0x10 6 6.0x10 6 8.0x10 6 1.0x10 7 1.2x10 7 TID (rad) Fig. 5. Variation of output collector saturation current and collector emitter saturation voltage of 50 MeV Li 3+ ion irradiated transistor as a function of D d and TID. junction capacitance (farad) 7.0x10-10 6.0x10-10 5.0x10-10 4.0x10-10 3.0x10-10 2.0x10-10 1.0x10-10 before irradiation 1.0x10 11 ions/cm 2 6.0x10 11 ions/cm 2 1.2x10 12 ions/cm 2 1.8x10 12 ions/cm 2-1.5-1.0-0.5 0.5 1.0 1.5 base-emitter voltage (volts) Fig. 6. Capacitance versus emitter-base voltage of 50 MeV Li 3+ ion irradiated transistor. 7.06 10 16 cm 3 to 5.92 10 16 cm 3 at the fluence 1 10 11 ions cm 2. These parameters play an important role in conduction of the device. The decrease in current and reduction in capacitance due to irradiation is understood on the basis of irradiation-induced disorders and defects like recombination centers, vacancies and interstitials in the emitter-base region. The introduction of compensating defects on irradiation will reduce the conductivity as well as the capacitance. Carrier removal rate can be calculated from the changes in the space charge of the barriers. The barrier capacitance C 0 of unirradiated sample will be reduced to C A after irradiation with dose/fluence U by the introduction of charge defects N U [21]. The carrier removal rate can be estimated from the relation ðc 2 0 C2 A Þ C 2 0 N D U ¼ N U U : Fig. 7 shows the variation of carrier removal rate with fluence (the biasing conditions of the emitter-base junction are as said in experimental session). From the figure the carrier removal rates seems to be small and it is found to be varying linearly with fluence of irradiation. Fig. 8 shows the plot of dielectric loss of the emitter-base region as a function of applied voltage. It can be seen that Carrierremovalrate, (C 0 2 -CA 2 )/C0 2 4 3 2 1 0 6.0x10 11 1.2x10 12 1.8x10 12 Fluence (ions/cm 2 ) ð Fig. 7. Variation of the relation C2 0 C2 AÞ with fluence. N C 2 D = 5.75 0 10 16 cm 3.

1718 C.M. Dinesh et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 1713 1718 dielectric loss 10 3 10 2 10 1 10 0 10-1 10-2 10-3 for unirradiated transistor the dielectric loss increases with the increase in the applied voltage. For irradiated device the curve shows similarity in the dielectric loss. As the fluence increases from 1 10 11 ions cm 2 to 1.8 10 12 ions cm 2 the dielectric loss increases, which indicates the decrease in its conducting property. However, the new peaks (at 5 V, 0.15 V and 0.15 V) show the production of new defects which are responsible for the decrease in its capacitance. 4. Conclusions before irradiation fluence 1.0x10 11 ions/cm 2 fluence 6.0x10 11 ions/cm 2 fluence 1.2x10 12 ions/cm 2 fluence 1.8x10 12 ions/cm 2-1.5-1.0-0.5 0.5 1.0 1.5 emitter-base voltage (volt) Fig. 8. Dielectric loss versus emitter-base voltage of 50 MeV Li 3+ ion irradiated transistor. This work reports the effect of 50 MeV Li 3+ ion irradiation on silicon NPN transistor device with fluence range from 1 10 11 ions cm 2 to 1.8 10 12 ions cm 2. An effort is made successfully to correlate the electrical degradation with non-ionizing energy deposition due to MeV ion irradiation using TRIM Monte Carol Code. Fluence dependent TID and D d is calculated for 50 MeV lithium ion in silicon target. The shift in collector saturation current and collector emitter voltage is mainly due to the total ionizing dose. In addition to these shifts, Li-ion irradiation causes increase in forward resistance of the collector emitter region. The decrease in diffusion capacitance is more pronounced compared to depletion capacitance. The increase in the built-in potential and decrease in doping density are could be due to the production of new defects in the emitter-base region of the transistor. These studies help to improve the device fabrication technology to make radiation hard devices for advanced applications. Acknowledgement This work is carried under UGC-IUAC funded, UFUP project, IUAC, New Delhi. References [1] G.P. Summers, E.A. Burke, C.J. Dale, E.A. Wolicki, P.W. Marshall, M.A. Gehlhausen, IEEE Trans. Nucl. Sci. 34 (6) (1987) 1134. [2] D. Codegoni, A. Colder, et al., Nucl. Instr. and Meth. B 217 (2004) 65. [3] A.P. Gnana Prakash, S.C. Ke, K. Siddappa, Semicond. Sci. Technol. 19 (2004) 1029. [4] A.P. Gnana Prakash, S.C. Ke, K. Siddappa, Nucl. Instr. and Meth. B 215 (2004) 457. [5] K.V. Madhu, S.R. Kulkarni, M. Ravindra, R. Damle, Nucl. Instr. and Meth. B 254 (2007) 98. [6] S.R. Kulkarni, M. Ravindra, G.R. Joshi, R. Damle, Radiat. Eff. Def. Solids 159 (2004) 273. [7] Sanaa A. Kamh, F.A.S. Solman, Nucl. Instr. and Meth. A 564 (1) (2006) 463. [8] S.R. Messenger, E.A. Burke, G.P. Summers, M.A. Xapsos, R.J. Walters, E.M. Jackson, B.D. Weaver, IEEE Trans. Nucl. Sci. 46 (6) (1999) 1595. [9] D. Kanjilal, S. Chopra, M.M. Naraynan, I.S. Iyer, V. Jha, R. Joshi, S.K. Datta, Nucl. Instr. and Meth. A 97 (1993) 328. [10] U. Biggeri, E. Borchi, M. Bruzzi, S. Lazanu, Z. Li, IEEE Trans. Nucl. Sci. 43 (3) (1996) 1599. [11] R.D. Schrimpf, IEEE Trans. Nucl. Sci. 43 (3) (1996). [12] J.E. Drennan, D.J. Hamman, Radiation Effects Design Handbook, NASA Contractor Report, Section 4, Transistors, August 1971. [13] J.F. Ziegler, J.P. Biersack, Stopping and Ranges of Ions in Matter (2003), SRIM.com. [14] D. Kanjilal, Curr. Sci. 80 (12) (2001) 25. [15] A. Colder, N. Croitoru, et al., Nucl. Instr. and Meth. B 179 (2001) 397. [16] P.C. Srivastava, S.P. Pandey, O.P. Sinha, D.K. Avasthi, K. Asokan, Nucl. Instr. and Meth. B 156 (1999) 105. [17] S.J. Taylor, M. Yamaguchi, S. Matsuda, T. Hisamatsu, O. Kawasaki, J. Appl. Phys. 82 (7) (1997) 3239. [18] P.C. Srivastava, S.P. Pandey, D.K. Avasthi, K. Asokan, Vacuum 48 (12) (1997) 965. [19] S.C. Witczak, R.C. Lacoe, M.R. Shaneyfelt, D.C. Mayer, J.R. Schwank, P.S. Winokur, IEEE Trans. Nucl. Sci. 47 (6) (1999) 666. [20] E.W. Enlow, R.L. Pease, W. Combs, R.D. Schrimpf, R.N. Nowlin, IEEE Trans. Nucl. Sci. 38 (6) (1991) 342. [21] S.D. Kouimttzi, J. Phys. C: Solid State Phys. 19 (1986) 2427.