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Identification of the microscopic structure of new hot carrier damage centers in short channel MOSFETs
MIGlqOIIL1EICTIOI~IIC IINGIIIIIIII~Irl ELSEVIER
Microelectronic Engineering 36 (1997) 271-274
Identification o f the M i c r o s c o p i c Structure o f N e w H o t Carrier D a m a g e C e n t e r s in Short Channel MOSFETs C. A. Billman a, P. M. Lenahana, and W. Weberb a The Pennsylvania State University University Park, PA 16802 b Siemens Corporate Research and Development, ZTME2 D-81739 Munich Germany We show, for the first time, that E' centers can be generated in hot hole stressing of short channel metal oxide silicon field effect transistors (MOSFETs). Prior to this study only Pb centers had been directly linked to this stressing phenomena. convincingly, that the Pbo center g tensor indicates 1. I n t r o d u c t i o n a silicon back bonded to three other silicons (Si3-Si • ) at the Si/SiO2 boundary t4. Furthermore, Hot carrier effects in short channel more recent observations of the Pbo 29Si hyperfine MOSFETs involve the creation of interface states tensor6'15 show that the Pbo center's unpaired at the Si/SiO2 boundary as well as the capture of electron is strongly localized in a very high p charge within the oxide 14. Although hot carder character (~85%-90% p, ~.10%-15% s) phenomena have been extensively studied by wavefunction, hybridization expected of the quite sophisticated "electronic" measurements TM Caplan/Poindexter model and hybridization very an "atomic scale" understanding of these similar, almost (within experimental error) phenomena remains incomplete, identical, to that of the noncontroversial To date, only one atomic scale defect has (111)Si/SiO2 Pb, which is unambiguously been identified in hot carder stressing, the Pb Si3~-Si. 16. The 29Si hyperfine results, analysis, center 5'6. The Pb center is an interface trap defect and conclusions6'j5 were recently confirmed in a with two broad levels in the silicon band gap, a study of porous silicon 17. Indeed, the very recent donor level in the lower part of the gap, an porous silicon results for the Pbo hyperfine tensor acceptor level in the upper part of the gap79. In are, within experimental error, virtually identical an n-channel MOSFET, it is the Pb acceptor level to those reported earlier by Gabrys et al6. for hot in the upper part of the gap which would be hole stressed short channel MOSFETs. important. The peaks of the two levels are Therefore, although the interface Pbo Si3--Sio separated by -0.6eV. Since a strong Pb spin model is not universally accepted H~3, we believe dependent recombination (SDR) signal is the evidence in support of it is overwhelming. observed from levels at or very near mid-gap, the Pb densities of states must extend to or (likely) 2. Experimental Results and overlap at mid gap j°. Discussion The structure of Pb centers on the technologically important (100)Si/SiO2 interface In our study we show that, in addition to remain controversial ~J-~3. However, a very strong the Pb centers, E' centers can also be generated in case can be made that the Pbo center is an unpaired electron on a silicon back bonded to three other hot hole stressing. The E' center structure is less silicons (Si3---Si') at the (100)Si/SiO2 boundary, controversial. Many studies indicate that E' The Pbo center appears to be the important Pb centers are the dominant deep hole trap in high variant on the (100)Si/SiO2 boundary5"6. Many quality SiO2 gate oxides 8"9'18"23. However, prior to this work, their role in hot carrier damage had years ago, Caplan e t aL 14 argued, rather never been directly demonstrated. Although E' 0167-9317/97/$17.00 © Elsevier Science B.V. All rights reserved. PII: S0167-9317(97)00062-2
272
C.A.
Billman
et al./Microelectronic
centers are usually found to be large capture cross section hole trap sites, under some circumstances, E' centers can also appear as neutral or compensated defects 24'2s. They may also behave as near Si/SiOz interface switching traps 26. The most commonly observed E' variant, sometimes called E'y, is a hole trapped in an oxygen vacancys'9'18"23. A less stable E' variant called E'~ or EP is also sometimes observed 27-3°. The E'~ / EP center structure is not known in detail, but since it reacts with molecular hydrogen in almost exactly the same way as E'~ and since its spin lattice relaxation time is similar to E'y one can conclude that it is closely related to the E'~ centers 31'32. Quite recent work by Zhang and Leisure 33 supports this view and strongly suggests that the center involves a hole trapped at two oxygen vacancies in extremely close proximity. In our study we observe the two defects (Pb and E') in short channel MOSFETs in which hot holes were injected into the oxide. We have identified these centers in short n-channel (l~tm) devices stressed with a high drain voltage (VD ~ 7 volts) and a low gate voltage (V~ ~ 1 volt). The defects were detected using the ultra sensitive electron spin resonance technique called spin dependent recombination (SDR). In SDR measurements, a stressed MOSFET is configured as a gate controlled diode 5'6'1°. The diode current is measured with the source/drain to substrate slightly forward biased, and the gate voltage varied to maximize the recombination current in the near drain region of the Si/SiO2 interface. The fact that recombination events are spin dependent is exploited in SDR. The MOSFET is simultaneously exposed to microwave irradiation and a slowly varying magnetic field; this induces magnetic resonance at the trap sites when the magnetic resonance condition is met. The magnetic resonance then induces changes in the recombination current ~°. These may be caused by spin resonance events at an interface trap or via spin diffusion with other nearby (<2nm)oxide defects, The results of hot carder stressing on the gated diode recombination currents are shown in figure 1. This figure shows a single peak in the
Engineering
36
(1997)
271-274
recombination current of the unstressed device, however; after stressing, a two peaked curve is observed. The larger peak is shifted-1.5 volts from that of the pre-stressed curve, which indicates a large generation of interface traps as welt as the presence of the positive charge in the oxide above the interface in the near drain oxide. ~ 4.5 - 4¢ ~ 3.5 ~ 3i post 0 2.5 [ ~ 1,5 ~ 1i pre ~ 0.5 1 ~, o ~. . . . . . . ,. . . . . . . . . . . . . . . . . . . . . ~ . . . . . +. . . . . . . . -5
-4
-3
-2
-1
0
Gate Volt~e (volts)
1
Figure1. Souroa/drainto subslrateoutrentofa before(pre)and af~r(post)~thaestressir~. The observed SDR spectrum (figure 2) shows two signals. The trace was taken with the applied magnetic field perpendicular to the (100) Si/SiO2 surface. At this orientation, the g = 2.006 signal corresponds to that of the previously observed Pbo center. The second signal at g = 2.0004 demostrates the presence of unpaired spins on oxygen deficient silicon atoms in SiO2, E' centers. o0
~ ~,
=•
~
g=2.006 ~...... ~ ....
3370
3375
~.................
3380
3385
3390
~. . . . . . . . . .
3395
M ~ F ~ d (Gauss)
3400
-~ . . . .
3405
~gure2" SDRspeclmofp~oentersandld'centersofashort daanne4M(2SFE]alter I"~-hole injedJon. As mentioned previously, E' centers have been identified in many earlier conventional ESR studies as dominating deep hole traps in high quality gate oxides. The E' SDR signal is consistent with what one would expect for a trapped hole near the Si/SiO2 boundary. The Pbo signal is consistent with what we would expect for an interface trap defect. It is thus likely that we are observing the structural nature of both
C.A. Billman et al./Microelectronic Engineering 36 (1997) 271-274
oxide charge trapping and the hot cartier induced interface trap formation process. However, a second explanation for the E' spectra cannot be ruled out. Hole trapping very near the Si/SiO2 boundary can create "switching traps" in MOS devices subjected to ionizing radiation26. These traps can be E' centers close to the Si/SiO2 boundary26. The switching trap can, over a period of hours, exchange charge carriers with the silicon substrate. An E' site very close to the Si/SiO2 boundary could presumably exchange charge quite rapidly and thus might serve as a hot hole injection induced interface state defect.
6 J.W.Gabrys, P.M.Lenahan, and W.Weber, Microelectronic Eng., 22, 273 (1993). 7 P.M.Lenahan and P.V.Dressendorfer, Appl. Phys. Lea., 41, 542 (1982). s P.M.Lenahan and P.V.Dressendorfer, Appl. Phys. Lett., 44, 96 (1984). 9 P.M.Lenahan and P.V.Dressendorfer, J. Appl. Phys., 55, 3495 (1984). 10 P.M.Lenahan and M.A.Jupina, Colloids and Surfaces, 42, 191 (1990). 1~ J.H.Stathis and L.Dori, Appl. Phys. Lett., 5_..88, 1641 (1991). ~2 E.Cartier, J.H.Stathis, and D.A.Buchanan, Appl. Phys. Lea., 63, 1510 (1993). J3 J.H.Stathis and E.Cartier, Phys. Rev. Lett., 72,
3. C o n c l u s i o n
2745 (1994). 14 P.J.Caplan, E.H.Poindexter, B.E.Deal, and R.R.Razoak, J. Appl. Phys., 50, 5847 (1979). 15 M.A.Jupina and P.M.Lenahan, IEEE Trans. Nucl. Sci., 37, 1650 (1990). 16 K.L.Brower, Appl. Phys. Lett., 43, 1111 (1983). ~7 J.L.Cantin, M.Schoisswohi, H.J.von Barteleben, V.Morazzani, J.J.Ganem, and I.Trimaille, in The Physics and Chemistry of SiO2 and Si-SiO2 Interface III, H. Z. Massoud, E. H. Poindexter, and C. R. Helms eds, The Electrochemical Society Pennington, NJ (1996) p28. 18 P.M.Lenahan and P.V.Dressenderfer, IEEE Trans. Nucl. Sci., 29, 1459 (1982). 19 P.M.Lenahan and P.V.Dressenderfer, IEEE Trans. Nucl. Sci., 30, 4602 (1983). 20 H.Miki, M.Noguchi, K.Yokogawa, B.Kim, K.Asada, and T.Sugano, IEEE Trans. Elec. Dev., 35, 2245 (1988). 21 T.Takahashi, B.B.Trplett, K.Yokogawa, and T.Sugano, Appl. Phys. Lett., 26, 1334 (1987). 22 L.Lipkin, L.Rowan, A.Reisman, and C.K.Williams, J. Electrochem Soc., 13__88,2050 (1991). 23 K.Awazu, K.Watanabe, and K.Kawazoe, J. Appl. Phys., 2054 (1993). 24 J.F.Conley Jr, P.M.Lenahan, and P.Roitman, IEEE Trans. Nucl. Sci., 38, 1247 (1991). 25 W.L.Warren, P.M.Lenahan, B.Robinson, and J.H.Stathis, Appl. Phys. Lett., 53,482 (1988).
In conclusion, we report the first observation of E' center generation in hot hole stressing of n-channel MOSFETs. E' centers are unambiguously oxygen deficient silicons within the oxide. They are almost always generated by holes in high quality thermally grown oxides and are usually associated with holes trapped at oxygen vacancies, 4. A c k n o w l e d g m e n t This work has been supported by the National Aeronautics and Space Administration (USA) through a subcontract with the Center for Applied Radiation Research of Prairie View A & M University.
T.H.Ning, P.W.Cook, R.H.Dennard, C.M.Osbum, S.E.Schuster, and H.N.Yu, IEEE Trans., Ed 26, 346 (1979). 2 G.Groseneken, H.E.Maes, N.Beltran, and R.F.Dekeersmaecker, IEEE Trans., Ed 31, 42 (1984). 3 P.Heremans, H.E.Maes, and N.Saks, IEEE Elec. Dev. Lett., 7, 428 (1986). 4 W.Weber and M.Brox, Microelectronic Eng., 1__99,453 (1992). 5 J.T.Krick, P.M.Lenahan, and G.J.Dunn, Appl. Phys. Lett., 5__99,3437 (1991).
273
274
C.A, Billman et aL / Mieroelectronic Engineering 36 (1997) 271-274
26 J.F.Conley Jr, P.M.Lenahan, A.J.Lelis, and T.R.Oldham, Appl. Phys. Lea., (1995). 27 J.F.Conley Jr, P.M.Lenahan, and P.Roitman, Appl. Phys. Lett., 60, 2889 (1992). 2s J.F.Conley Jr, P.M.Lenahan, and P.Roitman, IEEE Trans. Nucl. Sci., 39, 2114 (1992). 29 K.Vanheusden and A.Stesmans, Appl. Phys. Lett., 62, 2405 (1993). 30 W.L,Warren, M.R.Shaneyfelt, J.R.Schwank, D.M.Fleetwood, P.S.Winokur, R.A.B.Devine,
W.P.Mazara, and J.B.McKitterick, IEEE Trans. Nucl. Sci., 4_9.0,1755 (1993). 31 J.F.Conley and P.M.Lenahan, IEEE Trans. Nucl. Sci., 42, 1740 (1995). 32 J.F.Conley and P.M.Lenahan, Microelectronic Eng., 28, 35 (1995). 33 L.Zhang and R.G.Leisure, J. Appl. Phys., 80, 3744 (1996).
Microelectronic Engineering 36 (1997) 271-274
Identification o f the M i c r o s c o p i c Structure o f N e w H o t Carrier D a m a g e C e n t e r s in Short Channel MOSFETs C. A. Billman a, P. M. Lenahana, and W. Weberb a The Pennsylvania State University University Park, PA 16802 b Siemens Corporate Research and Development, ZTME2 D-81739 Munich Germany We show, for the first time, that E' centers can be generated in hot hole stressing of short channel metal oxide silicon field effect transistors (MOSFETs). Prior to this study only Pb centers had been directly linked to this stressing phenomena. convincingly, that the Pbo center g tensor indicates 1. I n t r o d u c t i o n a silicon back bonded to three other silicons (Si3-Si • ) at the Si/SiO2 boundary t4. Furthermore, Hot carrier effects in short channel more recent observations of the Pbo 29Si hyperfine MOSFETs involve the creation of interface states tensor6'15 show that the Pbo center's unpaired at the Si/SiO2 boundary as well as the capture of electron is strongly localized in a very high p charge within the oxide 14. Although hot carder character (~85%-90% p, ~.10%-15% s) phenomena have been extensively studied by wavefunction, hybridization expected of the quite sophisticated "electronic" measurements TM Caplan/Poindexter model and hybridization very an "atomic scale" understanding of these similar, almost (within experimental error) phenomena remains incomplete, identical, to that of the noncontroversial To date, only one atomic scale defect has (111)Si/SiO2 Pb, which is unambiguously been identified in hot carder stressing, the Pb Si3~-Si. 16. The 29Si hyperfine results, analysis, center 5'6. The Pb center is an interface trap defect and conclusions6'j5 were recently confirmed in a with two broad levels in the silicon band gap, a study of porous silicon 17. Indeed, the very recent donor level in the lower part of the gap, an porous silicon results for the Pbo hyperfine tensor acceptor level in the upper part of the gap79. In are, within experimental error, virtually identical an n-channel MOSFET, it is the Pb acceptor level to those reported earlier by Gabrys et al6. for hot in the upper part of the gap which would be hole stressed short channel MOSFETs. important. The peaks of the two levels are Therefore, although the interface Pbo Si3--Sio separated by -0.6eV. Since a strong Pb spin model is not universally accepted H~3, we believe dependent recombination (SDR) signal is the evidence in support of it is overwhelming. observed from levels at or very near mid-gap, the Pb densities of states must extend to or (likely) 2. Experimental Results and overlap at mid gap j°. Discussion The structure of Pb centers on the technologically important (100)Si/SiO2 interface In our study we show that, in addition to remain controversial ~J-~3. However, a very strong the Pb centers, E' centers can also be generated in case can be made that the Pbo center is an unpaired electron on a silicon back bonded to three other hot hole stressing. The E' center structure is less silicons (Si3---Si') at the (100)Si/SiO2 boundary, controversial. Many studies indicate that E' The Pbo center appears to be the important Pb centers are the dominant deep hole trap in high variant on the (100)Si/SiO2 boundary5"6. Many quality SiO2 gate oxides 8"9'18"23. However, prior to this work, their role in hot carrier damage had years ago, Caplan e t aL 14 argued, rather never been directly demonstrated. Although E' 0167-9317/97/$17.00 © Elsevier Science B.V. All rights reserved. PII: S0167-9317(97)00062-2
272
C.A.
Billman
et al./Microelectronic
centers are usually found to be large capture cross section hole trap sites, under some circumstances, E' centers can also appear as neutral or compensated defects 24'2s. They may also behave as near Si/SiOz interface switching traps 26. The most commonly observed E' variant, sometimes called E'y, is a hole trapped in an oxygen vacancys'9'18"23. A less stable E' variant called E'~ or EP is also sometimes observed 27-3°. The E'~ / EP center structure is not known in detail, but since it reacts with molecular hydrogen in almost exactly the same way as E'~ and since its spin lattice relaxation time is similar to E'y one can conclude that it is closely related to the E'~ centers 31'32. Quite recent work by Zhang and Leisure 33 supports this view and strongly suggests that the center involves a hole trapped at two oxygen vacancies in extremely close proximity. In our study we observe the two defects (Pb and E') in short channel MOSFETs in which hot holes were injected into the oxide. We have identified these centers in short n-channel (l~tm) devices stressed with a high drain voltage (VD ~ 7 volts) and a low gate voltage (V~ ~ 1 volt). The defects were detected using the ultra sensitive electron spin resonance technique called spin dependent recombination (SDR). In SDR measurements, a stressed MOSFET is configured as a gate controlled diode 5'6'1°. The diode current is measured with the source/drain to substrate slightly forward biased, and the gate voltage varied to maximize the recombination current in the near drain region of the Si/SiO2 interface. The fact that recombination events are spin dependent is exploited in SDR. The MOSFET is simultaneously exposed to microwave irradiation and a slowly varying magnetic field; this induces magnetic resonance at the trap sites when the magnetic resonance condition is met. The magnetic resonance then induces changes in the recombination current ~°. These may be caused by spin resonance events at an interface trap or via spin diffusion with other nearby (<2nm)oxide defects, The results of hot carder stressing on the gated diode recombination currents are shown in figure 1. This figure shows a single peak in the
Engineering
36
(1997)
271-274
recombination current of the unstressed device, however; after stressing, a two peaked curve is observed. The larger peak is shifted-1.5 volts from that of the pre-stressed curve, which indicates a large generation of interface traps as welt as the presence of the positive charge in the oxide above the interface in the near drain oxide. ~ 4.5 - 4¢ ~ 3.5 ~ 3i post 0 2.5 [ ~ 1,5 ~ 1i pre ~ 0.5 1 ~, o ~. . . . . . . ,. . . . . . . . . . . . . . . . . . . . . ~ . . . . . +. . . . . . . . -5
-4
-3
-2
-1
0
Gate Volt~e (volts)
1
Figure1. Souroa/drainto subslrateoutrentofa before(pre)and af~r(post)~thaestressir~. The observed SDR spectrum (figure 2) shows two signals. The trace was taken with the applied magnetic field perpendicular to the (100) Si/SiO2 surface. At this orientation, the g = 2.006 signal corresponds to that of the previously observed Pbo center. The second signal at g = 2.0004 demostrates the presence of unpaired spins on oxygen deficient silicon atoms in SiO2, E' centers. o0
~ ~,
=•
~
g=2.006 ~...... ~ ....
3370
3375
~.................
3380
3385
3390
~. . . . . . . . . .
3395
M ~ F ~ d (Gauss)
3400
-~ . . . .
3405
~gure2" SDRspeclmofp~oentersandld'centersofashort daanne4M(2SFE]alter I"~-hole injedJon. As mentioned previously, E' centers have been identified in many earlier conventional ESR studies as dominating deep hole traps in high quality gate oxides. The E' SDR signal is consistent with what one would expect for a trapped hole near the Si/SiO2 boundary. The Pbo signal is consistent with what we would expect for an interface trap defect. It is thus likely that we are observing the structural nature of both
C.A. Billman et al./Microelectronic Engineering 36 (1997) 271-274
oxide charge trapping and the hot cartier induced interface trap formation process. However, a second explanation for the E' spectra cannot be ruled out. Hole trapping very near the Si/SiO2 boundary can create "switching traps" in MOS devices subjected to ionizing radiation26. These traps can be E' centers close to the Si/SiO2 boundary26. The switching trap can, over a period of hours, exchange charge carriers with the silicon substrate. An E' site very close to the Si/SiO2 boundary could presumably exchange charge quite rapidly and thus might serve as a hot hole injection induced interface state defect.
6 J.W.Gabrys, P.M.Lenahan, and W.Weber, Microelectronic Eng., 22, 273 (1993). 7 P.M.Lenahan and P.V.Dressendorfer, Appl. Phys. Lea., 41, 542 (1982). s P.M.Lenahan and P.V.Dressendorfer, Appl. Phys. Lett., 44, 96 (1984). 9 P.M.Lenahan and P.V.Dressendorfer, J. Appl. Phys., 55, 3495 (1984). 10 P.M.Lenahan and M.A.Jupina, Colloids and Surfaces, 42, 191 (1990). 1~ J.H.Stathis and L.Dori, Appl. Phys. Lett., 5_..88, 1641 (1991). ~2 E.Cartier, J.H.Stathis, and D.A.Buchanan, Appl. Phys. Lea., 63, 1510 (1993). J3 J.H.Stathis and E.Cartier, Phys. Rev. Lett., 72,
3. C o n c l u s i o n
2745 (1994). 14 P.J.Caplan, E.H.Poindexter, B.E.Deal, and R.R.Razoak, J. Appl. Phys., 50, 5847 (1979). 15 M.A.Jupina and P.M.Lenahan, IEEE Trans. Nucl. Sci., 37, 1650 (1990). 16 K.L.Brower, Appl. Phys. Lett., 43, 1111 (1983). ~7 J.L.Cantin, M.Schoisswohi, H.J.von Barteleben, V.Morazzani, J.J.Ganem, and I.Trimaille, in The Physics and Chemistry of SiO2 and Si-SiO2 Interface III, H. Z. Massoud, E. H. Poindexter, and C. R. Helms eds, The Electrochemical Society Pennington, NJ (1996) p28. 18 P.M.Lenahan and P.V.Dressenderfer, IEEE Trans. Nucl. Sci., 29, 1459 (1982). 19 P.M.Lenahan and P.V.Dressenderfer, IEEE Trans. Nucl. Sci., 30, 4602 (1983). 20 H.Miki, M.Noguchi, K.Yokogawa, B.Kim, K.Asada, and T.Sugano, IEEE Trans. Elec. Dev., 35, 2245 (1988). 21 T.Takahashi, B.B.Trplett, K.Yokogawa, and T.Sugano, Appl. Phys. Lett., 26, 1334 (1987). 22 L.Lipkin, L.Rowan, A.Reisman, and C.K.Williams, J. Electrochem Soc., 13__88,2050 (1991). 23 K.Awazu, K.Watanabe, and K.Kawazoe, J. Appl. Phys., 2054 (1993). 24 J.F.Conley Jr, P.M.Lenahan, and P.Roitman, IEEE Trans. Nucl. Sci., 38, 1247 (1991). 25 W.L.Warren, P.M.Lenahan, B.Robinson, and J.H.Stathis, Appl. Phys. Lett., 53,482 (1988).
In conclusion, we report the first observation of E' center generation in hot hole stressing of n-channel MOSFETs. E' centers are unambiguously oxygen deficient silicons within the oxide. They are almost always generated by holes in high quality thermally grown oxides and are usually associated with holes trapped at oxygen vacancies, 4. A c k n o w l e d g m e n t This work has been supported by the National Aeronautics and Space Administration (USA) through a subcontract with the Center for Applied Radiation Research of Prairie View A & M University.
T.H.Ning, P.W.Cook, R.H.Dennard, C.M.Osbum, S.E.Schuster, and H.N.Yu, IEEE Trans., Ed 26, 346 (1979). 2 G.Groseneken, H.E.Maes, N.Beltran, and R.F.Dekeersmaecker, IEEE Trans., Ed 31, 42 (1984). 3 P.Heremans, H.E.Maes, and N.Saks, IEEE Elec. Dev. Lett., 7, 428 (1986). 4 W.Weber and M.Brox, Microelectronic Eng., 1__99,453 (1992). 5 J.T.Krick, P.M.Lenahan, and G.J.Dunn, Appl. Phys. Lett., 5__99,3437 (1991).
273
274
C.A, Billman et aL / Mieroelectronic Engineering 36 (1997) 271-274
26 J.F.Conley Jr, P.M.Lenahan, A.J.Lelis, and T.R.Oldham, Appl. Phys. Lea., (1995). 27 J.F.Conley Jr, P.M.Lenahan, and P.Roitman, Appl. Phys. Lett., 60, 2889 (1992). 2s J.F.Conley Jr, P.M.Lenahan, and P.Roitman, IEEE Trans. Nucl. Sci., 39, 2114 (1992). 29 K.Vanheusden and A.Stesmans, Appl. Phys. Lett., 62, 2405 (1993). 30 W.L,Warren, M.R.Shaneyfelt, J.R.Schwank, D.M.Fleetwood, P.S.Winokur, R.A.B.Devine,
W.P.Mazara, and J.B.McKitterick, IEEE Trans. Nucl. Sci., 4_9.0,1755 (1993). 31 J.F.Conley and P.M.Lenahan, IEEE Trans. Nucl. Sci., 42, 1740 (1995). 32 J.F.Conley and P.M.Lenahan, Microelectronic Eng., 28, 35 (1995). 33 L.Zhang and R.G.Leisure, J. Appl. Phys., 80, 3744 (1996).