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Trapping induced Neff and electrical field transformation at different temperatures in neutron irradiated high resistivity silicon detectors
Jj&
Nuclear Instruments
and Methods in Physics Research A 360 (199.5) 4X-462
_-
g$ l!iB ELSEVIER
NUCLEAR INSTRUMENTS 8 METHODS IN PHYSICS RESEARCH SectIon A
Trapping induced N,ff and electrical field transformation at different temperatures in neutron irradiated high resistivity silicon detectors * V. Eremin ‘, Z. Li *, I. Iljashenko
’
BrookhaL’en National Laboratory. Upton, NY 11973-5000. USA
Abstract The trapping of both non-equilibrium electrons and holes by neutron induced deep levels in high resistivity silicon planar detectors have been observed. In the experiments transient current and charge techniques, with short laser light pulse excitation. have been applied in the temperature range of 77-300 K. Light pulse illumination of the front (p’) and back (n+) contacts of the detectors showed effective trapping and detrapping for electrons and holes. At temperatures lower than 200 K, the detrapping becomes inefficient, and the additional charge of trapped non-equilibrium carriers in the space charge region (SCR) of the detectors leads to dramatic transformations of the effective space charge concentration N,, and electric field. The current and charge pulses transformation data can be explained in terms of extraction of electric field to the contact opposite to the illuminated one.
1. Introduction This work, carried out under a joint program of the collaboration between BNL and PTI, presents the novel results of recent investigations on radiation induced degradations of silicon planar detectors. In the field of neutron radiation problems of silicon detectors, the majority of the efforts has been directed to such applied aspects as the transformation of current-voltage and capacitance-voltage characteristics and the increase of the detector full depletion voltage V, and its annealing with neutron fluence at working temperatures of - 270-300 K (0°C to room temperature (RT)) [l-3], and charge collection efficiency at RT [4]. At the same time, phenomena such as trapping as well as detrapping of carriers from neutron induced deep levels at various temperatures have not been investigated so completely. For practical purposes, cooling has been applied to the detectors to low the leakage current and to freeze the V, reverse annealing [3]. At low temperature conditions. however, the trapping and detrapping of deep levels may cause unpredictable modifications of detector electrical properties such as the effective concentration of ionized charges in space charge region (N,,,),
” This research was supported in part by the U.S. Department of Energy: Contract No. DE-ACOZ-76CH00016, and by the National Research Council: Grant #LI-CASTY3. * Corresponding author ’ Permanent address: A.F. Ioffe Physico-Technical Institute of Academy of Sciences of Russia, St. Petersburg, Russia. 016%9002/95/$09.50
which would in turn modify the electrical field distribution and therefore affect the detector charge collection property. In this work, the study of kinetics of non-equilibrium carriers transport in neutron irradiated detectors is presented. The transport phenomena are investigated, using a laser transient current technique (TCT), in the temperature range of 110-310 K. It will be shown that the field transformation has a specific regularity and may be detected at temperatures up to RT at high fluences (> 10’” n/cm’).
2. Samples and experimental
Planar p+-n-n+ detectors produced in BNL from 5 kR cm n-type silicon wafers (thickness 3.50 pm, area 3 X 3 mm*) have been irradiated by 1 MeV neutrons to fluences in the range of 10’3-1014 n(cm’. The sample was assembled on the copper table with a heater and temperature control thermocouple, and the whole assembly was put inside a metal chamber. The chamber was covered by a glass bell protecting the chamber from direct contact with the liquid nitrogen surrounding it. The sample was AC coupled with high frequency oscilloscope TDS-445 (Textronix) by thin coaxial cable. Non-equilibrium carries were generated by a GaAs laser with a wavelength of - O.E$m and a light pulse duration of - 1 ns. The diameter of the laser illumination spot on the detector is - 1 mm. The laser was pumped by nanoseconds current pulses from a pulse generator through a current amplifier. This set-up allows one to analyze detector waveforms of
0 1995 Elsevier Science B.V. All rights reserved
SSDI 0168-9002(95)00112-3
technique
V. Eremin et al. /Nucl.
Instr. and Meth. in Phys. Res. A 360 fl99.5) 458-462
current pulses with amplitudes as small as _ 1 mV, that corresponds to the generation of about 2 X lo6 electronhole pairs. The schematic details of the TCI set-up can be found in Ref. [5]. TCT measurements of current pulses have been made at different temperatures starting from 320 K down to 100 K, with equal time intervals between the measurements to ensure the same quasi-steady state condition. The current shapes were stored on diskettes and later treated by special software.
Tkk m
250MWS
459
1.7SSM AElJL i-__f_----f-_-_.--____
._.._ ___._,
3. Experimental results In this section we present and analyze the temperature dependence of current shapes for non-irradiated detectors and for two detectors irradiated by two different fluences near and after SCR inversion. For non-irradiated detectors, the transformation of current pulses correspond to the well known phenomenon of increasing free carrier mobilities
Tak m
m
2.00mV
I
,o.ons
4
Tek m
~SONSIS
1.78SM Acqs ,_--___--_-)-__-_-______________i
Single Seq S.POGSII ET ~--+~_---_---~ 300K
210K
:.
M 400115 Ch2 /
w m
Tek =Slngle
m
4.**nw
1X02:58
S.*Onr
2.
17OK
40.0ns
b)
Fig. 2. The transformation of current pulses with temperatures for a detector (#296-A3, A = 0.25 cm*, d = 6.50 wrn) irradiated to 2.3 X 10” n/cm’. The laser light was on the p’ side (electron current) and the detector was reverse biased at V = 50 V. (a) electron current and (b) hole current.
Seq S.WGSIs ET ~__~-_+---~
2.OOrn”
1.00mV
14:44:42
lO.Onr
b)
Fig. 1. Electron (a) and hole (b) current pulses for a non-i~diated detector.
with decreasing temperature. The pulse widths, which are equal to the charge collection times and are inversely proportions to the carrier mobilities, decrease by a factor of three for electrons and a factor of four for holes when the detector was cooled from RT to 110 K, as shown in Fig. 1. For a sample irradiated to a low fluence (2.3 x 101’ n/cm”, 15 months after radiation), the electron current shapes (laser illumination on the front side or pc contact) at a bias of 50 V and at temperatures from RT to 170 K are shown in Fig. 2a. This detector has been found to fully deplete at a voltage of less than 15 V corresponding to a N,, of less than 4.65 X 10’“/cm3. Let us consider the tr~sfo~ation of current shapes with temperature shown in Fig 2. At temperatures below
XI. RADIATION DAMAGE
V. Eremin et al. /Nucl. Instr. and Meth. in Phys. Res. A 360 (1995) 458-462
460
300 K but above 220 K, the pulses conserve their forms in general. The main changes were related to mobility increase with the cooling of the detectors, the same as for the non-irradiated detectors. Dramatic transformations of current shapes begin to take place at T E 210 K. As we follow the sequences of current shapes in Fig. 2a, the slope of the top of current shape is negative at RT and starts to increase with cooling. It is practically flat at T = 210 K and becomes positive at lower temperatures, indicating that the maximum electrical field is now shifting from the p+ contact (before the transformation) to the nf contact. This electrical field transformation is a result of the change of the sign of N,, in the SCR from positive to negative. Fig. 2b shows the detector hole current shapes (laser illumination on the back side or n+ contact) at the same bias and temperature conditions as shown in Fig. 2a. Again the trend of carrier mobility increase with temperature is the same as in the electron current case. The same transforma-
nnm Single seq
nlc
2.soGws i-
ET [-__]
...I
il
II
\
tion trend of the current shape top has been observed. However, since holes drift from the back contact towards the front contact, the maximum field stays at the front contact, as compared to shifting from front contact to back contact in the case of electron current shape. In the hole current case, N,, is positive and increases with decreasing temperature. Note here that, in both cases, the sign of the N,, after the transformation is the same as that of the moving free carriers. The same regularities are seen in Fig. 3, where the current pulses at various temperatures for a detector irradiated beyond the inversion point have been plotted. Another important effect associated with current shape transformation is the degradation of the area of the current pulse at temperatures lower than 185 K. This degradation of area correlates with the appearance of a long tail on the current pulses (Figs. 2 and 3). It needs to be pointed out, that a slow component of the current pulse is often observed in the current and charge responses of partially depleted p+-n-n+ detectors in cases when the conductivity of the neutral base is low enough to contribute appreciably to the ohmic current [5-71. The time constant, however, of this ohmic current is on the order of ps for temperatures higher than 260 K, and it increases with the reciprocal temperature. So, at T - 180 K, its value must be much larger than the duration of the observed slow component (in the order of tens of ns).
I 4. Discussions
a,_ m
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,,,,,, l.OOrn"
TeU~Single
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,;
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zoom:
1.0x "en
_-_
i,
0
28Jlm ,994 l5:08:11
_____._..___/
Let us consider the case of laser illumination on the p+- contact. Decreasing the temperature leads to a change in the balance between the rates of trapping and detrapping of the drifting electrons. As the trapping time constant TV increases with decreasing temperature in the form: 1
2.0x "Or2
” = u u,(T)N,
’
(I)
where u is the trapping cross section of the deep level, c+(T) is the thermal velocity and N, is the concentration of the deep level, the detrapping time constant 7, is more strongly dependent on the temperature:
m
1.00rn”
10.0”s
12:.01:17
Fig. 3. The transformation of current pulses with temperatures for a detector (#296-BlO) irradiated to 7.8 X lOI n/cm’. The laser light was on the n+ side (hole current) and the detector was reverse biased at V = 160 V. (a) Electron current; Rl: 300 K, R2: 195 K, R3: 170 K, R4: 150 K, and (b) hole current; Rl: 300 K, R2: 185 K, R3: 170 K, R4: 150 K.
and it increases exponentially with cooling. In the equation, Nc is the density of states in the conduction (Nv for the valence band), E, is the activation energy for detrapping, and k is the Boltzmann constant. Therefore the steady-state balance of trapped and detrapped charges is shifting more and more towards the trapping with decreasing temperature. In other words, the trapped charges are frozen in deep levels at low temperatures. In the case of electron drifting in the SCR, the
461
V. Eremin et al. / Nucl. Instr. and Meth. in Phys. Res. A 360 (1995) 458-462
electron trapping process may lead to conversion from the initial positive space charge into negative. Accordingly, the maximum of the electrical field shifts from the p+ contact to the n+ contact in the case before inversion (Fig. 2a) and increases near the n+ contact in the case beyond inversion (Fig. 3a). Similarly, in the case of hole drifting in the SCR, trapped holes would make the space charge concentration more positive, which leads to an even higher electrical field maximum at the p+ in the case before inversion (Fig. 2b) and shifting of the field maximum from the n+ contact to the p+ contact in the case beyond inversion (Fig. 3b). Note that, in both cases, the electrical field minima are on the side of the illuminated contacts after the transformation. Obviously in both cases after the transformation, the absolute values of the space charge concentration N,, may be larger than the initial one and the junction is on the side opposite to the laser generated free carries. For a given bias enough to deplete the detector before the transformation, the detector may become partially depleted due to the N,, increase. This would leave only part of the distribution of generated free carriers (i.e. the tail) in the drifting electrical field, leading to the decrease of the current pulse amplitude and the area of the current pulse fast component. The generated free carriers in the neutral region (no field region) may still contribute to the current pulse by means of diffusion to the border of the SCR leading to the appearance of the current pulse tail. The average duration of this diffusion process can be estimated to be rdirr = A2/D,
M
m
1.00nl”
where Q, is the charge collected in the fast component, q. is the total generated charge, and h is the absorption length of the laser light in the detector. It is clear that the rate of the degradations of the current pulse amplitude and area is determined not only by the properties of deep levels (N,, u, E,), but also by the free carrier concentration in the SCR and the bias. The free carriers can be supplied by the bulk leakage current (thermally generated carriers) and current of laser generated carriers (externally generated carriers). At low temperatures (T - 200 K), the leakage current for detectors irradi-
02”~
3 ~~~
,994
13:30:15
Fig. 4. The transformation
of current pulses with temperatures for a detector (#436-85) irradiated to 3.1 X 10” n/cm’. The laser light was on the n+ side (hole current). Rl: 305 K; R2: 280 K; R3: 260 K: R4: 240 K.
ated to a few times of lOI n/cm’ does not exceed the value - 10-” A [6] and the free carriers concentration is:
%.L = - ~
IL
e 166 cm-‘,
epEA
where A is the detector area. At the same time, the concentration of laser generated carriers averaged in time at our experimental conditions (the frequency of light pulses fc 100 Hz; each pulse generates - n, = lo7 pairs, light spot on the sample has area A _ 1 mm’, and the absorption length A is about 20 km) can be estimated to be
(3)
where D is the diffusion coefficient of free carriers and A is the thickness of the neutral region. At T - 200 K and A - 10 km, the diffusion time is about 1.5 ns for electrons and 40 ns for holes, in agreement with the observed tails shown in Figs. 2 and 3. As for the decrease of the area of the fast component of the current pulse (or the drifting current), it can estimated as follows:
5.00ns’Ch3/
5.00nr
1 nagen - n,,tdrf~
= 10h cme3.
The main factor of N,, transformation at low temperatures is therefore laser (or externally) generated carriers. At high fluences ( > 1014 n/cm’), the detector leakage current may be high enough at RT to be a dominant part of the total generated carrier concentration n,. As we have discussed in the previous section, n, plays an important role in the field transformation and one may expect some near RT field irregularities for detectors irradiated to high neutron fluences. Fig. 4 shows the electron current shapes (p’ side illumination) of a detector irradiated to 3.1 X 1014 n/cm’. Two current peaks with about equal sizes appear in the RT current shape, a clear indication of high electrical fields on both sides of the detector, or junctions on both sides similar with the earlier observations of Li et al. [8]. The leakage current of the detector is very high at 305 K, about 875 t.r,A, and goes down quickly with decreasing temperature. As the temperature decreases, the high field on the nt side also decreases and disappears at T = 260 K (I, = 5.3 ~_LA).It seems that the trapping of thermally generated carriers in the leakage is the main cause of this field irregularity. The preliminary model is that a) the
XI. RADIATION
DAMAGE
462
V. Eremin et al. / Nucl. Instr. and Meth. in Phys. Res. A 360 (1995) 458-362
detector leakage current supplies more carriers than externally generated carries at temperatures near RT; and b) since all electrons have to go through the n+ contact and all holes have to go through the p+ contact and not vice versa, more electrons would be trapped near the nf contact and therefore make the region more negative in space charge (or more “p” type in SCR near n+ contact) and more holes would be trapped near the pf contact and make it more positive in space charge (or more “n” type near p+ contact). We will perform more detailed studies of this effect and the results will be published elsewhere. We have pointed out here the applied aspect of the findings reported and described above. Although operation of detectors at temperatures less than 200 K is not of current interest for high energy physics, the same field transformations observed at low temperature for fluences N 10’” n/cm’ also appear near room temperature for fluences > 10’” n/cm’. These field transformations may affect the real, near RT (0°C to RT) applications of the silicon
detectors
in the future
large
collider.
5. Conclusions A model has been proposed which is in qualitative agreement with the observed irregularities such as the field (or N,,) transformations and the current pulse amplitude and area degradation at low temperatures. These irregularities have been attributed to the steady state balance of trapping and detrapping of deep levels. At low temperatures, the dominant process is trapping, which not only causes the well known effect of charge collection loss, but also affects the net concentration of the space charge N,,,. More investigations are planed and the results and details of the modeling will be published elsewhere.
Acknowledgments The authors are grateful to N. Strokan of PTI for his interest in and support of this work. The authors would also like to thank E. Verbitskaya and A. Ivanov of PTI, H.W. Kraner of BNL, and G. Lindstroem and E. Fretwurst of the University of Hamburg for helpful discussions.
References
[II R. Wunstorf,
M. Benkert, N. Claussen. N. Croitoru. E. Fretwurst, G. Lindstrom and T. Schulz, Nucl. Instr. and Meth. A 315 (1992) 149. El K. Gill. G. Hall. s. Roe. S. Sotthibandhu, R. Whcadon. P. Giubellino and L. Ramello, Nucl. Instr. and Meth. A 322 (1992) 177. [31 H.J. Ziock et al., IEEE Trans. Nucl. Sci. NS-40 (4) (1903) 344. [41 H.W. Kraner. Z. Li and E. Fretwurst. Nucl. Instr. and Meth. A. 326 (1993)350. [51 V. Ercmin and Z. Li. Determination of the Fermi level position for neutron irradiated high resistivity silicon detectors and materials using the transient charge technique (TChT), BNL60072, IEEE Trans. Nucl. Sci. 41 (1994) 1907. [61 2. Li, V. Eremin, N. Strokan and E. Verbitskaya. Trans. Nucl. Sci. Ns-40 (4) (1993) 367. 171 V. Eremin. N. Strokan, and E. Verbitskaya and Z. Li. The development of transient current and charge techniques for measurement of effective concentration of ionized space charges (N,,,) of ppn junction detector, BNL-60156, to be submitted to the Nucl. Instr. and Meth. [81 Z. Li and H.W. Kraner, Proc. 3rd Workshop on Radiation-induccd and/or Process-related Electrically Active Defects in Semiconductor~lnsulator Systems, Research Triangle Park, NC, Sept. 10-13. 19Yl. p. 59, J. Electron. Mater. 21 (7) (1992) 701.
Nuclear Instruments
and Methods in Physics Research A 360 (199.5) 4X-462
_-
g$ l!iB ELSEVIER
NUCLEAR INSTRUMENTS 8 METHODS IN PHYSICS RESEARCH SectIon A
Trapping induced N,ff and electrical field transformation at different temperatures in neutron irradiated high resistivity silicon detectors * V. Eremin ‘, Z. Li *, I. Iljashenko
’
BrookhaL’en National Laboratory. Upton, NY 11973-5000. USA
Abstract The trapping of both non-equilibrium electrons and holes by neutron induced deep levels in high resistivity silicon planar detectors have been observed. In the experiments transient current and charge techniques, with short laser light pulse excitation. have been applied in the temperature range of 77-300 K. Light pulse illumination of the front (p’) and back (n+) contacts of the detectors showed effective trapping and detrapping for electrons and holes. At temperatures lower than 200 K, the detrapping becomes inefficient, and the additional charge of trapped non-equilibrium carriers in the space charge region (SCR) of the detectors leads to dramatic transformations of the effective space charge concentration N,, and electric field. The current and charge pulses transformation data can be explained in terms of extraction of electric field to the contact opposite to the illuminated one.
1. Introduction This work, carried out under a joint program of the collaboration between BNL and PTI, presents the novel results of recent investigations on radiation induced degradations of silicon planar detectors. In the field of neutron radiation problems of silicon detectors, the majority of the efforts has been directed to such applied aspects as the transformation of current-voltage and capacitance-voltage characteristics and the increase of the detector full depletion voltage V, and its annealing with neutron fluence at working temperatures of - 270-300 K (0°C to room temperature (RT)) [l-3], and charge collection efficiency at RT [4]. At the same time, phenomena such as trapping as well as detrapping of carriers from neutron induced deep levels at various temperatures have not been investigated so completely. For practical purposes, cooling has been applied to the detectors to low the leakage current and to freeze the V, reverse annealing [3]. At low temperature conditions. however, the trapping and detrapping of deep levels may cause unpredictable modifications of detector electrical properties such as the effective concentration of ionized charges in space charge region (N,,,),
” This research was supported in part by the U.S. Department of Energy: Contract No. DE-ACOZ-76CH00016, and by the National Research Council: Grant #LI-CASTY3. * Corresponding author ’ Permanent address: A.F. Ioffe Physico-Technical Institute of Academy of Sciences of Russia, St. Petersburg, Russia. 016%9002/95/$09.50
which would in turn modify the electrical field distribution and therefore affect the detector charge collection property. In this work, the study of kinetics of non-equilibrium carriers transport in neutron irradiated detectors is presented. The transport phenomena are investigated, using a laser transient current technique (TCT), in the temperature range of 110-310 K. It will be shown that the field transformation has a specific regularity and may be detected at temperatures up to RT at high fluences (> 10’” n/cm’).
2. Samples and experimental
Planar p+-n-n+ detectors produced in BNL from 5 kR cm n-type silicon wafers (thickness 3.50 pm, area 3 X 3 mm*) have been irradiated by 1 MeV neutrons to fluences in the range of 10’3-1014 n(cm’. The sample was assembled on the copper table with a heater and temperature control thermocouple, and the whole assembly was put inside a metal chamber. The chamber was covered by a glass bell protecting the chamber from direct contact with the liquid nitrogen surrounding it. The sample was AC coupled with high frequency oscilloscope TDS-445 (Textronix) by thin coaxial cable. Non-equilibrium carries were generated by a GaAs laser with a wavelength of - O.E$m and a light pulse duration of - 1 ns. The diameter of the laser illumination spot on the detector is - 1 mm. The laser was pumped by nanoseconds current pulses from a pulse generator through a current amplifier. This set-up allows one to analyze detector waveforms of
0 1995 Elsevier Science B.V. All rights reserved
SSDI 0168-9002(95)00112-3
technique
V. Eremin et al. /Nucl.
Instr. and Meth. in Phys. Res. A 360 fl99.5) 458-462
current pulses with amplitudes as small as _ 1 mV, that corresponds to the generation of about 2 X lo6 electronhole pairs. The schematic details of the TCI set-up can be found in Ref. [5]. TCT measurements of current pulses have been made at different temperatures starting from 320 K down to 100 K, with equal time intervals between the measurements to ensure the same quasi-steady state condition. The current shapes were stored on diskettes and later treated by special software.
Tkk m
250MWS
459
1.7SSM AElJL i-__f_----f-_-_.--____
._.._ ___._,
3. Experimental results In this section we present and analyze the temperature dependence of current shapes for non-irradiated detectors and for two detectors irradiated by two different fluences near and after SCR inversion. For non-irradiated detectors, the transformation of current pulses correspond to the well known phenomenon of increasing free carrier mobilities
Tak m
m
2.00mV
I
,o.ons
4
Tek m
~SONSIS
1.78SM Acqs ,_--___--_-)-__-_-______________i
Single Seq S.POGSII ET ~--+~_---_---~ 300K
210K
:.
M 400115 Ch2 /
w m
Tek =Slngle
m
4.**nw
1X02:58
S.*Onr
2.
17OK
40.0ns
b)
Fig. 2. The transformation of current pulses with temperatures for a detector (#296-A3, A = 0.25 cm*, d = 6.50 wrn) irradiated to 2.3 X 10” n/cm’. The laser light was on the p’ side (electron current) and the detector was reverse biased at V = 50 V. (a) electron current and (b) hole current.
Seq S.WGSIs ET ~__~-_+---~
2.OOrn”
1.00mV
14:44:42
lO.Onr
b)
Fig. 1. Electron (a) and hole (b) current pulses for a non-i~diated detector.
with decreasing temperature. The pulse widths, which are equal to the charge collection times and are inversely proportions to the carrier mobilities, decrease by a factor of three for electrons and a factor of four for holes when the detector was cooled from RT to 110 K, as shown in Fig. 1. For a sample irradiated to a low fluence (2.3 x 101’ n/cm”, 15 months after radiation), the electron current shapes (laser illumination on the front side or pc contact) at a bias of 50 V and at temperatures from RT to 170 K are shown in Fig. 2a. This detector has been found to fully deplete at a voltage of less than 15 V corresponding to a N,, of less than 4.65 X 10’“/cm3. Let us consider the tr~sfo~ation of current shapes with temperature shown in Fig 2. At temperatures below
XI. RADIATION DAMAGE
V. Eremin et al. /Nucl. Instr. and Meth. in Phys. Res. A 360 (1995) 458-462
460
300 K but above 220 K, the pulses conserve their forms in general. The main changes were related to mobility increase with the cooling of the detectors, the same as for the non-irradiated detectors. Dramatic transformations of current shapes begin to take place at T E 210 K. As we follow the sequences of current shapes in Fig. 2a, the slope of the top of current shape is negative at RT and starts to increase with cooling. It is practically flat at T = 210 K and becomes positive at lower temperatures, indicating that the maximum electrical field is now shifting from the p+ contact (before the transformation) to the nf contact. This electrical field transformation is a result of the change of the sign of N,, in the SCR from positive to negative. Fig. 2b shows the detector hole current shapes (laser illumination on the back side or n+ contact) at the same bias and temperature conditions as shown in Fig. 2a. Again the trend of carrier mobility increase with temperature is the same as in the electron current case. The same transforma-
nnm Single seq
nlc
2.soGws i-
ET [-__]
...I
il
II
\
tion trend of the current shape top has been observed. However, since holes drift from the back contact towards the front contact, the maximum field stays at the front contact, as compared to shifting from front contact to back contact in the case of electron current shape. In the hole current case, N,, is positive and increases with decreasing temperature. Note here that, in both cases, the sign of the N,, after the transformation is the same as that of the moving free carriers. The same regularities are seen in Fig. 3, where the current pulses at various temperatures for a detector irradiated beyond the inversion point have been plotted. Another important effect associated with current shape transformation is the degradation of the area of the current pulse at temperatures lower than 185 K. This degradation of area correlates with the appearance of a long tail on the current pulses (Figs. 2 and 3). It needs to be pointed out, that a slow component of the current pulse is often observed in the current and charge responses of partially depleted p+-n-n+ detectors in cases when the conductivity of the neutral base is low enough to contribute appreciably to the ohmic current [5-71. The time constant, however, of this ohmic current is on the order of ps for temperatures higher than 260 K, and it increases with the reciprocal temperature. So, at T - 180 K, its value must be much larger than the duration of the observed slow component (in the order of tens of ns).
I 4. Discussions
a,_ m
-I-T-----,,
,,,,,, l.OOrn"
TeU~Single
Seq Mfl
,;
M lO.OIlS'Ch2'J
,o.ons 2.50E.l~ ET ,----[-_q
zoom:
1.0x "en
_-_
i,
0
28Jlm ,994 l5:08:11
_____._..___/
Let us consider the case of laser illumination on the p+- contact. Decreasing the temperature leads to a change in the balance between the rates of trapping and detrapping of the drifting electrons. As the trapping time constant TV increases with decreasing temperature in the form: 1
2.0x "Or2
” = u u,(T)N,
’
(I)
where u is the trapping cross section of the deep level, c+(T) is the thermal velocity and N, is the concentration of the deep level, the detrapping time constant 7, is more strongly dependent on the temperature:
m
1.00rn”
10.0”s
12:.01:17
Fig. 3. The transformation of current pulses with temperatures for a detector (#296-BlO) irradiated to 7.8 X lOI n/cm’. The laser light was on the n+ side (hole current) and the detector was reverse biased at V = 160 V. (a) Electron current; Rl: 300 K, R2: 195 K, R3: 170 K, R4: 150 K, and (b) hole current; Rl: 300 K, R2: 185 K, R3: 170 K, R4: 150 K.
and it increases exponentially with cooling. In the equation, Nc is the density of states in the conduction (Nv for the valence band), E, is the activation energy for detrapping, and k is the Boltzmann constant. Therefore the steady-state balance of trapped and detrapped charges is shifting more and more towards the trapping with decreasing temperature. In other words, the trapped charges are frozen in deep levels at low temperatures. In the case of electron drifting in the SCR, the
461
V. Eremin et al. / Nucl. Instr. and Meth. in Phys. Res. A 360 (1995) 458-462
electron trapping process may lead to conversion from the initial positive space charge into negative. Accordingly, the maximum of the electrical field shifts from the p+ contact to the n+ contact in the case before inversion (Fig. 2a) and increases near the n+ contact in the case beyond inversion (Fig. 3a). Similarly, in the case of hole drifting in the SCR, trapped holes would make the space charge concentration more positive, which leads to an even higher electrical field maximum at the p+ in the case before inversion (Fig. 2b) and shifting of the field maximum from the n+ contact to the p+ contact in the case beyond inversion (Fig. 3b). Note that, in both cases, the electrical field minima are on the side of the illuminated contacts after the transformation. Obviously in both cases after the transformation, the absolute values of the space charge concentration N,, may be larger than the initial one and the junction is on the side opposite to the laser generated free carries. For a given bias enough to deplete the detector before the transformation, the detector may become partially depleted due to the N,, increase. This would leave only part of the distribution of generated free carriers (i.e. the tail) in the drifting electrical field, leading to the decrease of the current pulse amplitude and the area of the current pulse fast component. The generated free carriers in the neutral region (no field region) may still contribute to the current pulse by means of diffusion to the border of the SCR leading to the appearance of the current pulse tail. The average duration of this diffusion process can be estimated to be rdirr = A2/D,
M
m
1.00nl”
where Q, is the charge collected in the fast component, q. is the total generated charge, and h is the absorption length of the laser light in the detector. It is clear that the rate of the degradations of the current pulse amplitude and area is determined not only by the properties of deep levels (N,, u, E,), but also by the free carrier concentration in the SCR and the bias. The free carriers can be supplied by the bulk leakage current (thermally generated carriers) and current of laser generated carriers (externally generated carriers). At low temperatures (T - 200 K), the leakage current for detectors irradi-
02”~
3 ~~~
,994
13:30:15
Fig. 4. The transformation
of current pulses with temperatures for a detector (#436-85) irradiated to 3.1 X 10” n/cm’. The laser light was on the n+ side (hole current). Rl: 305 K; R2: 280 K; R3: 260 K: R4: 240 K.
ated to a few times of lOI n/cm’ does not exceed the value - 10-” A [6] and the free carriers concentration is:
%.L = - ~
IL
e 166 cm-‘,
epEA
where A is the detector area. At the same time, the concentration of laser generated carriers averaged in time at our experimental conditions (the frequency of light pulses fc 100 Hz; each pulse generates - n, = lo7 pairs, light spot on the sample has area A _ 1 mm’, and the absorption length A is about 20 km) can be estimated to be
(3)
where D is the diffusion coefficient of free carriers and A is the thickness of the neutral region. At T - 200 K and A - 10 km, the diffusion time is about 1.5 ns for electrons and 40 ns for holes, in agreement with the observed tails shown in Figs. 2 and 3. As for the decrease of the area of the fast component of the current pulse (or the drifting current), it can estimated as follows:
5.00ns’Ch3/
5.00nr
1 nagen - n,,tdrf~
= 10h cme3.
The main factor of N,, transformation at low temperatures is therefore laser (or externally) generated carriers. At high fluences ( > 1014 n/cm’), the detector leakage current may be high enough at RT to be a dominant part of the total generated carrier concentration n,. As we have discussed in the previous section, n, plays an important role in the field transformation and one may expect some near RT field irregularities for detectors irradiated to high neutron fluences. Fig. 4 shows the electron current shapes (p’ side illumination) of a detector irradiated to 3.1 X 1014 n/cm’. Two current peaks with about equal sizes appear in the RT current shape, a clear indication of high electrical fields on both sides of the detector, or junctions on both sides similar with the earlier observations of Li et al. [8]. The leakage current of the detector is very high at 305 K, about 875 t.r,A, and goes down quickly with decreasing temperature. As the temperature decreases, the high field on the nt side also decreases and disappears at T = 260 K (I, = 5.3 ~_LA).It seems that the trapping of thermally generated carriers in the leakage is the main cause of this field irregularity. The preliminary model is that a) the
XI. RADIATION
DAMAGE
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detector leakage current supplies more carriers than externally generated carries at temperatures near RT; and b) since all electrons have to go through the n+ contact and all holes have to go through the p+ contact and not vice versa, more electrons would be trapped near the nf contact and therefore make the region more negative in space charge (or more “p” type in SCR near n+ contact) and more holes would be trapped near the pf contact and make it more positive in space charge (or more “n” type near p+ contact). We will perform more detailed studies of this effect and the results will be published elsewhere. We have pointed out here the applied aspect of the findings reported and described above. Although operation of detectors at temperatures less than 200 K is not of current interest for high energy physics, the same field transformations observed at low temperature for fluences N 10’” n/cm’ also appear near room temperature for fluences > 10’” n/cm’. These field transformations may affect the real, near RT (0°C to RT) applications of the silicon
detectors
in the future
large
collider.
5. Conclusions A model has been proposed which is in qualitative agreement with the observed irregularities such as the field (or N,,) transformations and the current pulse amplitude and area degradation at low temperatures. These irregularities have been attributed to the steady state balance of trapping and detrapping of deep levels. At low temperatures, the dominant process is trapping, which not only causes the well known effect of charge collection loss, but also affects the net concentration of the space charge N,,,. More investigations are planed and the results and details of the modeling will be published elsewhere.
Acknowledgments The authors are grateful to N. Strokan of PTI for his interest in and support of this work. The authors would also like to thank E. Verbitskaya and A. Ivanov of PTI, H.W. Kraner of BNL, and G. Lindstroem and E. Fretwurst of the University of Hamburg for helpful discussions.
References
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