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Irradiation effects of 50 MeV lithium ions on silicon diodes
Nuclear Instruments and Methods in Physics Research B 156 (1999) 90±94
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Irradiation eects of 50 MeV lithium ions on silicon diodes P. Sathyavathi, V.N. Bhoraskar
*
Department of Physics, University of Pune, Pune, 411007, India
Abstract Silicon diodes (p+n), of turn-o time 5000 ns, were irradiated either from the n-side or p-side with 50 MeV lithium ions. The displacement type defects could be induced in both the n and p-regions of the diode, since the projected range of the 50 MeV lithium ions was greater than the length of the diode (240 lm). A few diodes were also irradiated with 35 MeV lithium ions to induce defects only in the n-region. The ion ¯uence was varied in the range of 1010 ±1012 ions/cm2 . In the case of 50 MeV lithium ions, the trade-o between the turn-o time, trr , and the forward voltage drop, Vf , exhibited by the diodes irradiated from the n-side (trr 200 ns, Vf 1.10 V), were superior to those irradiated from the p-side (trr 1000 ns Vf 1.30 V). For these ions the nuclear energy loss in diodes varied from 53 to 120 eV/lm. The results therefore indicate that even this low order of energy loss is adequate to reduce signi®cantly the lifetime of minority carriers in silicon, since the turn-o time of the diodes could be reduced to 200 ns. A comparison of the results obtained with 50 MeV lithium ions with those of 35 MeV lithium ions, reveals that the defects induced in the p-region also contribute signi®cantly towards reducing the turn-o time of the diodes. Ó 1999 Elsevier Science B.V. All rights reserved. PACS: 61.80 jh; 61.82fk Keywords: Defects; Lithium ions; Diodes; Turn-o time
1. Introduction It is known that the LET for heavy ions in silicon is greater than that of electrons of the same energy. However, not much work has been reported so far in the ®eld of modi®cation of semiconductor device parameters using heavy ions. Theoretical model (TRIM) depicts that a heavy ions displaces atoms of the medium from the
* Corresponding author. Tel.: +91-0212-356061; fax: +910212-353899; e-mail: [email protected]
lattice through the process of nuclear collisions; of which the cross section increases on decreasing the ion energy. In this work, it is shown that this process of energy deposits is very important in controlling the turn-o time of silicon diodes. To decide suitability of a diode in a fast switching circuits, the important parameters which are taken in to accounts are the turn-o time, trr and the forward voltage drop, Vf . For special types of switching applications, the required low values of trr are achieved by reducing the life-time of minority carriers in silicon. For this purpose, the gold diusion [1] technique is commonly used, since the
0168-583X/99/$ ± see front matter Ó 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 9 9 ) 0 0 2 7 7 - 3
P. Sathyavathi, V.N. Bhoraskar / Nucl. Instr. and Meth. in Phys. Res. B 156 (1999) 90±94
minority carrier life-time varies inversely with the number of the diused gold atoms. Even though low values of turn-o time can be obtained by increasing the gold concentration, however, this method has a major disadvantage that the reverse current can exceed the acceptable limit. The alternative method of reducing the life-time of minority carriers is to irradiate silicon diodes with MeV-energy electrons. Since the range of these electrons is much greater than the total length of a p±n junction silicon diode (240 lm), the lattice atoms are displaced throughout the volume of the diode. As a result, the decrement in the turn-o time is followed by an undesirable increase in the forward-voltage drop Vf [2]. This is mainly because of the fact that the induced defects of certain regions in the silicon diode make a very small contribution in terms of reducing the turn-o time but a substantial one in enhancing the voltage drop. In the case of heavy ions the number of lattice atoms displaced along its path, increases on decreasing the ion energy, which is a case just opposite of that of electrons. Furthermore, for a high energy heavy ion (E > 30 MeV), the number of lattice atoms displaced at the end of the ion trajectory may be orders of magnitude higher than that displaced at the beginning of the trajectory. By making a proper choice of ion type and energy, a large numbers of atoms can be displaced from this lattice positions in the region of interest to obtained desirable changes in the switching characteristics of p+n junction diodes [3]. The present paper reports a study on the irradiation eects of 50 MeV energy lithium ions on silicon diodes. The results of the study revealed that an appreciable number of the displacement type defects can be induced in silicon even if the nuclear energy loss varies only from 85 to 120 eV/lm. 2. Experimental p+n-junction silicon diodes (1 kV, 1 A), specially manufactured without adopting the gold diusion process, were used for this work; a schematic diagram of the same is shown in Fig. 1 (a). For electrical contacts, a thin coating of nickel
91
Fig. 1. The switching characteristics of a typical diode (a) before and (b) after irradiation with 50 MeV lithium ions.
was provided on both the n and p sides of each diode. About 200 such diodes were chosen and their parameters, such a turn-o time, storage time, forward-voltage drop and reverse current were measured. Using the 15 UD Pelletron of the Nuclear Science Center, New Delhi (India), about 60 such diodes were irradiated with 50 MeV lithium ions. Dierent values of ion ¯uence, in the range of 1010 ±1012 ions/cm2 , were used, and at each ¯uence about 10 diodes from the n-side and 10 diodes from the p-side were irradiated. The range [3] of 50 MeV lithium ions in silicon is 309 lm, whereas the thickness of the silicon diode was 240 lm. In the present case, therefore, all the lithium ions could pass through the diode. In this way each lithium ion had energy 20 MeV while leaving the diode. For comparison 20 diodes were irradiated from the n-side with 35 MeV lithium ions. Since the range of 35 MeV lithium ions in silicon is 178 lm, they could not reach the p-region. All the parameters of the lithium ion irradiated diodes such as forward current, reverse current, storage time and turn-o time was measured. Fig. 1(b) shows schematic diagrams of switching characteristics of a typical diode before and after irradiation with 50 MeV energy lithium ions. 3. Results and discussion For a few diodes, the variation in the turn-o time with the ¯uence of 50 MeV lithium ion is shown in Fig. 2. The leakage current in the lithium
92
P. Sathyavathi, V.N. Bhoraskar / Nucl. Instr. and Meth. in Phys. Res. B 156 (1999) 90±94
Fig. 2. The variations in the turn-o time with the ¯uence of 50 MeV lithium ions for the diodes irradiated from the (a) p-side and (b) n-side.
Fig. 3. The trade-o between the turn-o time and the forward voltage drop for the diodes irradiated with 50 MeV lithium ions from the (a) p-side and (b) n-side.
ion irradiated diodes having trr 200 ns, was less than 2 lA at 1 kV, and as such no increment in the reverse current was observed. From the Fig. 2 it is interesting to note that, for equal ¯uences of 50 MeV lithium ions, the decrement in the turn-o time in the diodes irradiated from the n-side is greater than that in the diodes irradiated from the p-side. Similarly, the undesirable increment in the forward-voltage drop, Vf , is relatively smaller in the diodes irradiated from the n-side as compared to that of the diodes irradiated from the p-side. For these diodes the trade-o between the turn-o time and forward voltage drop is shown in Fig. 3. The observed dierence in these two trade-o plots can be explained on the basis of the defects induced by heavy ions in the n-region and in the pregion of the diodes. The lithium ions of 50 MeV energy lose energy in silicon [4] through the process of nuclear collision at a rate of 53 eV/lm initially and therefore the number of lattice atoms displaced along the in the region close to the contact is very small. The rate of energy loss through the nuclear collisions gradually increases as the lithium ions penetrates deeper in the n-region and becomes 80 eV/lm near the junction. The lithium ions with 30 MeV energy enter the pregion and after depositing energy in the p-region, it leaves the diode with 20 MeV energy. However, the energy deposited by lithium ions in silicon through electronic loss is very large, being 95
keV/lm at 50 MeV and 184 keV/lm at 20 MeV. As shown in Fig. 4 for the 50 MeV lithium ion, the energy loss per unit length increases steadily with the depth of penetration in silicon. As a result if a lithium ion enters the diode from the n-side, the number of lattice atoms displaced in the p-region is much greater than that in the n-region with the lithium ion enter diode through the p-region, the number of the lattice atoms displaced at the end of the n-side is greater than that displaced in the p-region. In the case of n-side irradiation, the electron energy loss and the nuclear energy loss
Fig. 4. The energy deposited through nuclear energy loss by 50 MeV lithium ions at dierent locations in a p±n junction diode (a) n-side irradiation and (b) p-side irradiation.
P. Sathyavathi, V.N. Bhoraskar / Nucl. Instr. and Meth. in Phys. Res. B 156 (1999) 90±94
near the junction are, respectively, 135 keV/lm and 80 eV/lm in the case of n-side irradiation and 99 keV/lm and 57 eV/lm, respectively, in the case of p-side irradiation. The defects produced near the junction in the n-side contribute signi®cantly towards reducing the turn-o time of the diode. This is mainly because in the n-region the concentration of the minority carriers is maximum near the junction, and it decreases with the distance from the junction towards the end of the nregion [5]. The induced defects are therefore effective in reducing the turn-o time of the diode, if located near the junction. In the case of n-side irradiation the number of defects produced near the junction is greater than that produced in the case of p-side irradiation. Furthermore, for a given density of the induced defects, the increase in the resistivity of the n-region is greater than that of the p-side, because the doping concentration of the pside is much higher than that of the n-side. The resistivity of the n-side, therefore, mainly decides the total voltage drop across the diode. In the present work the increase in the forward-voltage drop across the diode irradiated from the p-side is greater than that of the diode irradiated from the n-side and therefore these results are consistent with the theory. In the case of p-side irradiation the appreciable decrease in the turn-o time further indicates that a large fraction of the injected minority carriers (i.e. holes), injected from p-side into n-region must be diusing up to the end of the n-side. These results indicate therefore that the diusion length of holes in the n-side is comparable with the length of the n-region. The variation in the vacancy density with the distance in the n-region of diodes irradiated from the n-side with 35 MeV lithium ions and 50 MeV lithium ions are shown by curves (a) and (b), respectively in Fig. 5. It is interesting to note that even though the defect density near the junction in the diodes irradiated with 50 MeV lithium ions is almost two orders of magnitude smaller than that of the diodes irradiated with 35 MeV lithium ions however in both the cases the turn-o time has decreased by the same magnitudes from 5000 to 200 ns. The range of 35 MeV lithium ions is 178 lm, and therefore these ions could not penetrate into the p-region and hence the observed decre-
93
Fig. 5. The variations in the vacancy-density with the distance from the end of the n-region in the diodes with lithium ions of energy (a) 35 MeV and (b) 50 MeV.
ment in the trr is due to the defects produced in the n-region only. These results therefore lead to a conclusion that in the case of 50 MeV lithium ion irradiation the defects induced in the p-region of the diodes has contributed signi®cantly towards reducing the turn-o time of the diodes. In the case of 50 MeV lithium ion irradiation, the nuclear energy loss per unit length is very low being 53 and 120 eV/lm at the beginning and at the end of the trajectory in silicon diode, however the number of defects induced throughout the volume of the diode was sucient to reduce the lifetime of minority carriers appreciably in both the p and n-regions. These results therefore indicate that a large number of defects can be induced in silicon even at these low values of nuclear energy loss. Acknowledgements For this work the Pelletron facilities of Nuclear Science Center, New Delhi were used and the cooperation extended by the concerned scienti®c sta is gratefully acknowledged. References [1] B.J. Baliga, E. Sun, IEEE Trans. Electron Devices ED-24 (1977) 685.
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P. Sathyavathi, V.N. Bhoraskar / Nucl. Instr. and Meth. in Phys. Res. B 156 (1999) 90±94
[2] P.S. Bhave, S.T. Chavan, V.N. Bhoraskar, Nucl. Instr. and Meth. B 95 (1995) 334. [3] P.S. Bhave, V.N. Bhoraskar, Nucl. Instr. and Meth. B 127/ 128 (1997) 383.
[4] J.P. Biersack, J.F. Ziegler, TRIM-96 (C), ISBN-0-08021603-X. [5] B.G. Streetman, Solid State Electronic Devices, 2nd ed., Prentice-Hall of India, 1986, p. 179.
www.elsevier.nl/locate/nimb
Irradiation eects of 50 MeV lithium ions on silicon diodes P. Sathyavathi, V.N. Bhoraskar
*
Department of Physics, University of Pune, Pune, 411007, India
Abstract Silicon diodes (p+n), of turn-o time 5000 ns, were irradiated either from the n-side or p-side with 50 MeV lithium ions. The displacement type defects could be induced in both the n and p-regions of the diode, since the projected range of the 50 MeV lithium ions was greater than the length of the diode (240 lm). A few diodes were also irradiated with 35 MeV lithium ions to induce defects only in the n-region. The ion ¯uence was varied in the range of 1010 ±1012 ions/cm2 . In the case of 50 MeV lithium ions, the trade-o between the turn-o time, trr , and the forward voltage drop, Vf , exhibited by the diodes irradiated from the n-side (trr 200 ns, Vf 1.10 V), were superior to those irradiated from the p-side (trr 1000 ns Vf 1.30 V). For these ions the nuclear energy loss in diodes varied from 53 to 120 eV/lm. The results therefore indicate that even this low order of energy loss is adequate to reduce signi®cantly the lifetime of minority carriers in silicon, since the turn-o time of the diodes could be reduced to 200 ns. A comparison of the results obtained with 50 MeV lithium ions with those of 35 MeV lithium ions, reveals that the defects induced in the p-region also contribute signi®cantly towards reducing the turn-o time of the diodes. Ó 1999 Elsevier Science B.V. All rights reserved. PACS: 61.80 jh; 61.82fk Keywords: Defects; Lithium ions; Diodes; Turn-o time
1. Introduction It is known that the LET for heavy ions in silicon is greater than that of electrons of the same energy. However, not much work has been reported so far in the ®eld of modi®cation of semiconductor device parameters using heavy ions. Theoretical model (TRIM) depicts that a heavy ions displaces atoms of the medium from the
* Corresponding author. Tel.: +91-0212-356061; fax: +910212-353899; e-mail: [email protected]
lattice through the process of nuclear collisions; of which the cross section increases on decreasing the ion energy. In this work, it is shown that this process of energy deposits is very important in controlling the turn-o time of silicon diodes. To decide suitability of a diode in a fast switching circuits, the important parameters which are taken in to accounts are the turn-o time, trr and the forward voltage drop, Vf . For special types of switching applications, the required low values of trr are achieved by reducing the life-time of minority carriers in silicon. For this purpose, the gold diusion [1] technique is commonly used, since the
0168-583X/99/$ ± see front matter Ó 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 9 9 ) 0 0 2 7 7 - 3
P. Sathyavathi, V.N. Bhoraskar / Nucl. Instr. and Meth. in Phys. Res. B 156 (1999) 90±94
minority carrier life-time varies inversely with the number of the diused gold atoms. Even though low values of turn-o time can be obtained by increasing the gold concentration, however, this method has a major disadvantage that the reverse current can exceed the acceptable limit. The alternative method of reducing the life-time of minority carriers is to irradiate silicon diodes with MeV-energy electrons. Since the range of these electrons is much greater than the total length of a p±n junction silicon diode (240 lm), the lattice atoms are displaced throughout the volume of the diode. As a result, the decrement in the turn-o time is followed by an undesirable increase in the forward-voltage drop Vf [2]. This is mainly because of the fact that the induced defects of certain regions in the silicon diode make a very small contribution in terms of reducing the turn-o time but a substantial one in enhancing the voltage drop. In the case of heavy ions the number of lattice atoms displaced along its path, increases on decreasing the ion energy, which is a case just opposite of that of electrons. Furthermore, for a high energy heavy ion (E > 30 MeV), the number of lattice atoms displaced at the end of the ion trajectory may be orders of magnitude higher than that displaced at the beginning of the trajectory. By making a proper choice of ion type and energy, a large numbers of atoms can be displaced from this lattice positions in the region of interest to obtained desirable changes in the switching characteristics of p+n junction diodes [3]. The present paper reports a study on the irradiation eects of 50 MeV energy lithium ions on silicon diodes. The results of the study revealed that an appreciable number of the displacement type defects can be induced in silicon even if the nuclear energy loss varies only from 85 to 120 eV/lm. 2. Experimental p+n-junction silicon diodes (1 kV, 1 A), specially manufactured without adopting the gold diusion process, were used for this work; a schematic diagram of the same is shown in Fig. 1 (a). For electrical contacts, a thin coating of nickel
91
Fig. 1. The switching characteristics of a typical diode (a) before and (b) after irradiation with 50 MeV lithium ions.
was provided on both the n and p sides of each diode. About 200 such diodes were chosen and their parameters, such a turn-o time, storage time, forward-voltage drop and reverse current were measured. Using the 15 UD Pelletron of the Nuclear Science Center, New Delhi (India), about 60 such diodes were irradiated with 50 MeV lithium ions. Dierent values of ion ¯uence, in the range of 1010 ±1012 ions/cm2 , were used, and at each ¯uence about 10 diodes from the n-side and 10 diodes from the p-side were irradiated. The range [3] of 50 MeV lithium ions in silicon is 309 lm, whereas the thickness of the silicon diode was 240 lm. In the present case, therefore, all the lithium ions could pass through the diode. In this way each lithium ion had energy 20 MeV while leaving the diode. For comparison 20 diodes were irradiated from the n-side with 35 MeV lithium ions. Since the range of 35 MeV lithium ions in silicon is 178 lm, they could not reach the p-region. All the parameters of the lithium ion irradiated diodes such as forward current, reverse current, storage time and turn-o time was measured. Fig. 1(b) shows schematic diagrams of switching characteristics of a typical diode before and after irradiation with 50 MeV energy lithium ions. 3. Results and discussion For a few diodes, the variation in the turn-o time with the ¯uence of 50 MeV lithium ion is shown in Fig. 2. The leakage current in the lithium
92
P. Sathyavathi, V.N. Bhoraskar / Nucl. Instr. and Meth. in Phys. Res. B 156 (1999) 90±94
Fig. 2. The variations in the turn-o time with the ¯uence of 50 MeV lithium ions for the diodes irradiated from the (a) p-side and (b) n-side.
Fig. 3. The trade-o between the turn-o time and the forward voltage drop for the diodes irradiated with 50 MeV lithium ions from the (a) p-side and (b) n-side.
ion irradiated diodes having trr 200 ns, was less than 2 lA at 1 kV, and as such no increment in the reverse current was observed. From the Fig. 2 it is interesting to note that, for equal ¯uences of 50 MeV lithium ions, the decrement in the turn-o time in the diodes irradiated from the n-side is greater than that in the diodes irradiated from the p-side. Similarly, the undesirable increment in the forward-voltage drop, Vf , is relatively smaller in the diodes irradiated from the n-side as compared to that of the diodes irradiated from the p-side. For these diodes the trade-o between the turn-o time and forward voltage drop is shown in Fig. 3. The observed dierence in these two trade-o plots can be explained on the basis of the defects induced by heavy ions in the n-region and in the pregion of the diodes. The lithium ions of 50 MeV energy lose energy in silicon [4] through the process of nuclear collision at a rate of 53 eV/lm initially and therefore the number of lattice atoms displaced along the in the region close to the contact is very small. The rate of energy loss through the nuclear collisions gradually increases as the lithium ions penetrates deeper in the n-region and becomes 80 eV/lm near the junction. The lithium ions with 30 MeV energy enter the pregion and after depositing energy in the p-region, it leaves the diode with 20 MeV energy. However, the energy deposited by lithium ions in silicon through electronic loss is very large, being 95
keV/lm at 50 MeV and 184 keV/lm at 20 MeV. As shown in Fig. 4 for the 50 MeV lithium ion, the energy loss per unit length increases steadily with the depth of penetration in silicon. As a result if a lithium ion enters the diode from the n-side, the number of lattice atoms displaced in the p-region is much greater than that in the n-region with the lithium ion enter diode through the p-region, the number of the lattice atoms displaced at the end of the n-side is greater than that displaced in the p-region. In the case of n-side irradiation, the electron energy loss and the nuclear energy loss
Fig. 4. The energy deposited through nuclear energy loss by 50 MeV lithium ions at dierent locations in a p±n junction diode (a) n-side irradiation and (b) p-side irradiation.
P. Sathyavathi, V.N. Bhoraskar / Nucl. Instr. and Meth. in Phys. Res. B 156 (1999) 90±94
near the junction are, respectively, 135 keV/lm and 80 eV/lm in the case of n-side irradiation and 99 keV/lm and 57 eV/lm, respectively, in the case of p-side irradiation. The defects produced near the junction in the n-side contribute signi®cantly towards reducing the turn-o time of the diode. This is mainly because in the n-region the concentration of the minority carriers is maximum near the junction, and it decreases with the distance from the junction towards the end of the nregion [5]. The induced defects are therefore effective in reducing the turn-o time of the diode, if located near the junction. In the case of n-side irradiation the number of defects produced near the junction is greater than that produced in the case of p-side irradiation. Furthermore, for a given density of the induced defects, the increase in the resistivity of the n-region is greater than that of the p-side, because the doping concentration of the pside is much higher than that of the n-side. The resistivity of the n-side, therefore, mainly decides the total voltage drop across the diode. In the present work the increase in the forward-voltage drop across the diode irradiated from the p-side is greater than that of the diode irradiated from the n-side and therefore these results are consistent with the theory. In the case of p-side irradiation the appreciable decrease in the turn-o time further indicates that a large fraction of the injected minority carriers (i.e. holes), injected from p-side into n-region must be diusing up to the end of the n-side. These results indicate therefore that the diusion length of holes in the n-side is comparable with the length of the n-region. The variation in the vacancy density with the distance in the n-region of diodes irradiated from the n-side with 35 MeV lithium ions and 50 MeV lithium ions are shown by curves (a) and (b), respectively in Fig. 5. It is interesting to note that even though the defect density near the junction in the diodes irradiated with 50 MeV lithium ions is almost two orders of magnitude smaller than that of the diodes irradiated with 35 MeV lithium ions however in both the cases the turn-o time has decreased by the same magnitudes from 5000 to 200 ns. The range of 35 MeV lithium ions is 178 lm, and therefore these ions could not penetrate into the p-region and hence the observed decre-
93
Fig. 5. The variations in the vacancy-density with the distance from the end of the n-region in the diodes with lithium ions of energy (a) 35 MeV and (b) 50 MeV.
ment in the trr is due to the defects produced in the n-region only. These results therefore lead to a conclusion that in the case of 50 MeV lithium ion irradiation the defects induced in the p-region of the diodes has contributed signi®cantly towards reducing the turn-o time of the diodes. In the case of 50 MeV lithium ion irradiation, the nuclear energy loss per unit length is very low being 53 and 120 eV/lm at the beginning and at the end of the trajectory in silicon diode, however the number of defects induced throughout the volume of the diode was sucient to reduce the lifetime of minority carriers appreciably in both the p and n-regions. These results therefore indicate that a large number of defects can be induced in silicon even at these low values of nuclear energy loss. Acknowledgements For this work the Pelletron facilities of Nuclear Science Center, New Delhi were used and the cooperation extended by the concerned scienti®c sta is gratefully acknowledged. References [1] B.J. Baliga, E. Sun, IEEE Trans. Electron Devices ED-24 (1977) 685.
94
P. Sathyavathi, V.N. Bhoraskar / Nucl. Instr. and Meth. in Phys. Res. B 156 (1999) 90±94
[2] P.S. Bhave, S.T. Chavan, V.N. Bhoraskar, Nucl. Instr. and Meth. B 95 (1995) 334. [3] P.S. Bhave, V.N. Bhoraskar, Nucl. Instr. and Meth. B 127/ 128 (1997) 383.
[4] J.P. Biersack, J.F. Ziegler, TRIM-96 (C), ISBN-0-08021603-X. [5] B.G. Streetman, Solid State Electronic Devices, 2nd ed., Prentice-Hall of India, 1986, p. 179.