- Email: [email protected]
Stabilization of lithium-drifted silicon surface-barrier detectors
NUCLEAR
INSTRUMENTS
AND METHODS
37 (1965) 2 o 4 - 2 t 6 ;
© NORTH-HOLLAND
PUBLISHING
CO.
STABILIZATION OF LITHIUM-DRIFlrED SILICON SURFACE-BARRIER DETECTORS A. J. TUZZOLINO, M. A. P E R K I N S and J. K R I S T O F F Laboratory for Astrophysics and Space Research, Enrico Fermi Institute for Nuclear Studies, The University of Chicago, Chicago, Illinois Received 16 July 1965 A simple fabrication technique has been found which yields
measurements of their electrical and :t-particle characteristics taken before, during, a n d after thermal-vacuum testing at 65°C a n d additional testing at 50°C. Degradation in electrical a n d :t-particle characteristics resulting from such testing and aging was found to be negligible over a period of approximately four months. The measurements at 50°C a n d 65°C indicate that the fractional change in the mean energy per ion pair is approximately - 1.3 x 10-4/°C over the temperature range from r o o m temperature to 65°C. The results of these studies are reported.
lithium-drifted silicon surface-barrier detectors which have a low leakage current and which exhibit stable electrical and :t-particle characteristics under conditions of prolonged aging and repeated exposure to thermal-vacuum environments. The detectors will operate at a temperature of 65°C and high vacuum for several days with no evidence of noise instability. The operating characteristics of such detectors have been studied over a period of several months. These studies included 1. Introduction The need for charged-particle detectors which exhibit stable characteristics for prolonged periods of time both under ordinary laboratory conditions and under conditions of high vacuum and elevated temperature is of growing importance in the space sciences. In our laboratory, this need has led to a continuing study of the effects of aging in the laboratory and environmental testing on the electrical and charged-particle characteristics of the lithium-drifted silicon surfacebarrier detector 1' 2). Initially, the sensitive surface of the detectors was allowed to age for one or two days in the ambient atmosphere of the laboratory following fabrication. After this aging, the magnitude of the leakage current for detectors fabricated by identical techniques was found to be extremely variable and all detectors could be put into one of two categories. Detectors in the first category were unusable, having leakage currents of several hundred #A/cm 2. Detectors in the second category were usable and had leakage currents in the range from approximately 2 # A / c m 2 to 10/~A/cm 2. These very large differences in leakage current appeared to be related to the conditions of relative humidity in the laboratory. After exposing detectors to a variety of controlled conditions of humidity, it was found that detectors which were immersed in boiling deionized water for several minutes, immediately after fabrication, exhibited lower leakage currents and much more stable electrical and or-particle characteristics than those that were not given this treatment. A 15-rain boiling-water treatment with the detector completely immersed in the water was adopted as a standard procedure. This paper presents the results of a study of the effects of aging and various thermal-vacuum environments on
the characteristics of surface-barrier detectors which were given the boiling-water treatment. For comparison, similar results are presented for detectors which were not given the boiling-water treatment. The work reported here represents an extension of earlier experiments2), where the effects of aging and environmental testing on detectors prepared by exposure to the ambient atmosphere of the laboratory were studied. The environmental conditions used for detectors to be discussed in the present work were more severe than the conditions used for detectors discussed in earlier work2). 2. Surface-barrier detectors 2.1 DETECTOR FABRICATION
The general techniques used in the fabrication of the surface-barrier (SB) detectors were identical to those described elsewhere i - 4). Single crystals of p-type silicon with electrical resistivities of 1 0 0 o h m . c m and 10130 o h m - c m were used as the starting material. The diffusion and drifting of lithium was carried out as described earlier2), yielding a n-i-p structureS). A SB detector was made from the n-i-p unit by lapping back from the p-side to a depth which was well into the intrinsic region. This lapped face, which exposed the intrinsic region, was then etched and a gold contact evaporated over this face. After mounting the units into insulating rings2), some of the detectors were given the boiling-water treatment. 2.2. ELECTRONIC EQUIPMENT
The amplifier system used for most of the measurements was a transistorized charge-sensitive amplifierpost-amplifier combination designed at this laboratory. This amplifier had an integrating and differentiating time constant of 2/~s. An ORTEC Model 204 charge-
204
LITHIUM-DRIFTED SILICON SURFACE-BARRIER DETECTORS
dumping pulser was used to provide data on electrical resolution and to provide an energy calibration for the system. The pulser was calibrated by making use of a conventional gold-silicon surface-barrier detector 6) and a 21°Po source. The output from the post amplifier was analyzed on an R I D L Model 32-12B 400 channel pulse-height analyzer. Detector currents were monitored with a Keithley Model 410 micro-microammeter. The detector load resistor used throughout the experiment was 105 t2. 2.3. DETECTOR CHARACTERIZATION
The depth of the lithium diffusion and the intrinsic depth was determined by making use of internal conversion electrons from 137Cs and 2°7Bi sources and capacitance measurements, as described earlier'). Such measurements were made periodically during the aging and environmental testing of detectors to determine whether any changes in n- or intrinsic-region depths had occurred. 2.4. THERMAL-VACUUMTEST The aging and thermal-vacuum (t-v) testing procedure consisted of the following: I. detectors were allowed to age at room temperature with no bias applied for approximately six weeks following fabrication. 2. They were then placed in a vacuum chamber and maintained at a temperature: of 65°C and a vacuum of apprOximately 5 x l0 ~5 mm Hg for four days with the bias voltage applied continuously. 3. The detectors were removed from the vacuum chamber and allowed to stand for approximately three weeks with no bias applied. 4. They were then placed in the vacuum chamber and maintained at a temperature of 50°C and a vacuum of 5 x 10 -5 mm Hg for seven days with the bias voltage applied continuously. 5. The detectors were removed from the vacuum chamber and allowed to stand for approximately four weeks with no bias applied. The following detector characteristics were measured: A. current-voltage characteristic; B. n-region depth and intrinsic depth; C. capacitance vs bias voltage; D. electrical resolution vs bias voltage; E. charge-collection efficiency and resolution vs bias voltage for orparticles. Items A - D were measured as a function of time during the aging periods and items A, D and E were measured as a function of time during the environmental tests. Eight SB detectors were studied in this manner. Four of the eight detectors were made from 1000 ohm.cm material and the remaining four from 100 ohm. cm material. The experimental results are presented in the following section.
205
3. Experimental results and discussion Of the eight detectors studied, the four which were made from 1000 ohm. cm silicon were given the boilingwater treatment and the four made from 100 ohm-cm material were not. All eight detectors had sensitive areas of 1.2 cm 2, lithium-diffused depths in the range 125#m to 1501~m and intrinsic depths ranging from 325 #m to 335 #m. The applied bias voltage in each case was 15 V, corresponding to a bulk electric field of ~ 450 V/cm. The charge-collection efficiency (CCE) as measured in this experiment may be written 2) CCE = ~F,
(l)
where r/ is the charge-collection efficiency of the detector alone 7) and F represents the effect of the carrier transit times a) and the amplifier time constant on the output signal amplitudeg). The quantity r/ is defined here as
rl = Qc(T)/QL,
(2)
where QL is the charge liberated at room temperature, and Qc(T)is the charge collected at the temperature T. When the effects of carrier recombination or trapping 7) are considered, the quantity Qc(T) may be written Qc(T) = QL(T)G,
(3)
where QL(T) is the charge liberated at the temperature T and G represents the effects of carrier loss through recombination or trapping. In general, the quantity G is a function of the distribution in space of the ionization density produced by the incident particle, the carrier recombination lifetimes, the carrier trapping lengths, the range of the incident particle, the intrinsic depth and the plasma time7). Thus, from eqs. 0)-(3), ccE
= Q,.(r)rG/Q~.
(4)
At all temperatures and bias voltages used, the transit times a) of the carriers were sufficiently less than the 2 #s amplifier time constant so that the quantity F in eq. (4) was unity. Thus, for all detectors studied,
CCe = Q.L(T)G/QL.
(5)
When the effects of carrier recombination and trapping are negligible, eq. (5) reduces to
CCE = QL(T)/QL = t/c(T),
(6)
where c is the mean energy per ion pair at room temperature *°) and e(T) is the mean energy per ion pair at the temperature 7". In measurements of the CCE for =-particles, it was possible to detect a change in CCE of about 0.1~.
206
A.J. TLrZZOLINO et al. T AB L E 1
8
Age off detector, (d) j
Temp.
(°63
V (v)
1 7 14 21 28 "42 43 44 45 46 49 56 59 63 "64 65 66 67 68 69 70 71 72 f
.(.
I-r
85 91 94 98
,
'I
(%,
i p
E (V/cm)
1
(fig. 1)
I
] I I
t
(fig. 2) (fig. 3) t-v test
450
t-v test
1'5 q 30 45
room temp. " 78
cc)
21Ol
3 5 10 15 20 15 15 15 15
The calibrated pulser set at 5.7 M e V was used to determine the C C E and the electrical noise spread. The contribution to the noise line-width from the amplifier alone was about 44 keV. All a-particle spectra were obtained with uncollimated sources. The experimental results found for the four boilingwater detectors were similar in all respects and will be illustrated for one o f them, hereafter referred to as SB-1. Also, the results found for the four detectors which were not given the boiling-water treatment were similar, and will be illustrated for one o f them, hereafter referred to as SB-2. Because o f the large bulk o f experimental data, the results are presented in table form. To illustrate any differences in behavior o f SB-1
60
CCE
90 c 150 300 450 600, 450 450 450 450
vs bias voltage (fig. 4) (fig. 5) (fig. 6)
and SB-2 more clearly, spectra are Shown corresponding to selected entries in the tables. All comparisons are based u p o n an applied bias o f 15 V ( ~ 450 V/era), unless otherwise stated. The characteristics o f SB-1 and SB-2 before, during, and after the t-v tests are shown in tables 1 and 2 and figs. 1-12. Tables 1 and 2 show that soon after fabrication, detector SB-I had a much lower leakage current than SB-2. Typically, "boiling-water" (B-W) detectors have initial currents ranging from 0.1 P A I c m 2 to 0.2 p A / c m 2, whereas detectors having had no boiling-water treatment (N-B-W) have, when usable, initial currents in the range from 2/~A/cm 2 to 10/~A/cm 2. Also, the initial C C E for SB-2 (99.7~) is somewhat less than for SB-1
LITHIUM-DRIFTED
SILICON
SURFACE-BARRIER
207
DETECTORS
TABLE 2
314 Age of detector (d)
5 13 18
26
Temp. (0 C)
L
I
5.7 MeV
zt0po
Q,A)
(keV)
(keV)
5.0
74 49 49 52 49 43 70 79 94 61 49 52 52
79 68 59 67 68 61 70 92 100 60 60 55 58
room temp.
65
0.8 i
I
15
1" room temp. 50 50 50 50 50
i;.5 room temp. , 74
t 81 87 9O 94
I
V
1.45
[ 65
6
(V)
65 41 44 47 52 ~60 61 62 63 66 67
5
I
1.0 1.5 2.0 3.0 5.0 I0 15 20 15 15 15 18
0.65 0.72 0.63 11.5 15.2 16.2 3.4 0.62 0.77 0.88 3.2 3.2 3.7 4.8 5.1 0.82
pulser
.
CCE (~/.)
99.7 99.'/ 99.6 99.4 99.5 99.2 100.2 100.1 100.1 99.9 100.0 99.6 99.5 .
I
8 E (V/ena)
~
'(fig. 7)
!
" (fig. 8) (fig. 9) " t-v test
450 J
.
t-v test
--
--
46 62
55 625
99.6 77.5
1'5
" 59 56
560 530
81.8 84.3
30 45
56 48 48 48 48 48 51 47 51 57
470 447 355 124 70 62 85 100 120 130
86.4 89.2 95.1 99.3 99.7
0.45
0.48 0.65 0.70 0.77 0.85 0.93 0.99 1.0 1.1 1.1 1.2 1.1
.
7 21Opo
u
60
CCE
90 ~vs 150 bias voltage 300 450 (fig. 10)
99.9
600
99.5 98.8 98.8 98.8
450 450 450 450
(fig. 11) (fig. 12)
Characteristics of detector SB-I (table 1) and SB-2 (table 2), before, during and after thermal-vacuum (t-v) tests. Column 1 gives the detector age in days. Column 2 gives the detector temperature in oC. Columns 3 and 4 give the applied bias in V and th~ teakage current in/~A, respectively. Columns 5 and 6 give the resolutions (fwhm) expressed in keV for a calibrated pulser set at 5.7 MeV and 210po ~t-particles, respectively. Column 7 gives the ~t-partiele CCE and column 8 gives the fields in the intrinsic region. The detector was at room temperature except during the thermal-vacuum tests. The applied bias was 15 V and the electric field 450 V]em during all measurements except those of CCE vs bias voltage. No bias was applied during the time intervals between room temperature measurements. The bias voltage was applied continuously during the thermal-vacuum tests. Test conditions were the same for SB-I and SB-2. (100.0 ~o). I n general, B - W detectors have a higher initial C C E (99.7~o-100.0~o) t h a n N - B - W detectors (99.2~o99.8 ~ ) . N o N - B - W detectors studied were f o u n d to have a n initial C C E o f greater t h a n 9 9 . 8 ~ , whereas m o s t B - W detectors h a d a n initial C C E o f 1 0 0 . 0 ~ . These facts s h o w that, at r o o m temperature, the intial C C E o f N - B - W detectors is given by eq. (5) with G lying in t h e range f r o m 0.992 to 0.998 a n d t h a t for m o s t B - W detectors, a n y initial r e c o m b i n a t i o n or t r a p p i n g losses are n o t measurable. T h e initial response o f SB-1 a n d
SB-2 to 2~°po a-particles is s h o w n in figs. 1 a n d 7. T h e poorer resolution o f SB-2 results f r o m its h i g h leakage current. N o significant low-energy tail is app a r e n t in either ease. Table 1 s h o w s t h a t d u r i n g t h e six-week a g i n g period (no applied bias), t h e current a n d a-resolution o f BS-I r e m a i n e d essentially constant, a n d t h e C C E d u r i n g this time was given by eq. (6), i.e., the C C E was 1 0 0 . 0 ~ . Table 2 s h o w s t h a t d u r i n g this s a m e period (no applied bias) t h e leakage c u r r e n t of SB-2 steadily improved,
208
A.J. TUZZOLINO [
AV
Detector $ 8 - I Age I day Room Temp.
I
I
i
I
Bias 15V Current O.13~A
400
4000 -5,7 MeV Pulser ~
Z Z <[ 300
3000
--
g
Q. ~ 200 Z
2000 --
g
4l
keV"
o
IOO
IOOO
% 170
/ ,:
175 1-76
184 185
CHANNEL
NUMBER
,
J
l,l 190
Fig. 1. Room temperature response of detector SB-1 to 210po =-particles. Detector age was t d. but its CCE was gradually degraded from 99.7% to 99.2%. Figs. 2 and 8 show the response of SB-1 and SB-2 after this aging period. There is still essentially no low energy tail in the 2~°po spectrum for SB-1 (fig. 2), whereas the 2aOpo spectrum for SB-2 (fig. 8) shows a pronounced low energy tail. In general, the leakage current of N-B-W detectors was found to decrease steadily with time, attaining a value o f ~ 0.4/~A/cm 2 after a period of a month or two following fabrication. However, during this same period, they exhibit some degradation in their response to 21°Po ~-particles. This degradation consists Of an overall loss in CCE of from 0.5% to 1.0% and a loss in ~-particle resolution and presumably results from an increase with time in recombination losses near the sensitive surface during aging2). After the six week aging period, the detectors were mounted in a vacuum chamber and maintained at a temperature of 65°C and a pressure of 5 x 10 -s mm Hg for four days. A bias of 15 V was applied continuously during this test. Tables 1 and 2 give the electrical and c~-particle characteristics of SB-I and SB-2 during this test and figs. 3 and 9 show their response to Z~°Po after the first day of the test.
et
al.
At 65°C, all B-W detectors had values of the CCE in the range from 100.4% to 100.6%. The measured CCE of 100.5% for SB-1 (fig. 3) at 65°Cis equal to the average CCE at this temperature for all B-W detectors studied. This average value for the CCE of 100.5 % and the use ofeq. (6), gives a value for the fractional change in the mean energy per ion pair of approximately - 1.3 x 10-4/° C over the temperature range from room temperature to 65°C. This observed decrease in e at 65 ° C is consistent with the behavior of ~ with increasing temperature observed at lower temperatures1°). The accuracy of this measurement (~20%) was not sufficient to justify any comparison of the results 1o) with a calculation of the mean energy per ion pair by ShockleyZ~). The fact that the measured CCE for detector SB-2 at 65°C (fig. 9) was lower than that measured for SB-1 is believed to have resulted from the fact that SB-2 was in a degraded state at the start of the test. At 65°C, all N-B-W detectors had values of the CCE in the range 100.0~o to 100.2%. During the four-day test, the leakage current of all detectors was observed to increase as a function of time as shown in tables 1 and 2. The observed increase in pulser line width and =-resolution during this time resulted from this increase in current. At the end of the four-day test, the leakage current of all detectors studied
1
I
I
I
I
I
I
P°210(100'0%1)
•
I
I
r
I
[
Detector SB-I Age 42 doys Room Temp. Bios 15 V Current O.IO~A
400
5.7 MeV, Pulser
~ 300 xo IX 03
2000
-- I
~ 200 o
~oo
/ i
~'/i 171
I000 -
-
46 heY
:
,
I
1"77 CHANNEL
185
r9a
NUMBER
Fig. 2. Room temperature response of detector SB-1 to 210po ~¢-particles.Detector age was 42 d.
SILICON SURFACE-BARRIER
LITHIUM-DRIFTED
was in the range from 5 #A/cm 2 to 25 #A/cm 2 at 65°C. At the conclusion of the test, the detectors were returned to room temperature while still under vacuum in the chamber. The leakage current of SB-1 and SB-2 at this stage was approximately ten times the value at the start of the test. In addition, the CCE of SB-2 was better at this stage (99.9%) than at the start of the test (99.2%) and a 2t°Po spectrum showed no evidence of any appreciable low-energy tail. Apparently, the degradation in ~t-characteristics of SB-2 which was introduced by the six-week aging period prior to the t-v test was completely removed during the t-v test. The CCE of SB-1 at this stage was still 100.0%. The detectors were removed from the chamber and allowed to age for three days (no applied bias). The leakage currents of SB-1 and SB-2 were found to have decreased during this time to the values they had at the start of the t-v test. Further, both SB-1 and SB-2 had a CCE of 100.0% at this stage. The detectors were then allowed to age for another two weeks (no applied bias). Tables 1 and 2 show that during this time, the characteristics of SB-1 remained constant (CCE of 100.0%), whereas the CCE of SB-2 decreased from 100.0~ to 99.5% at the end of the two week period. This loss of 0 . 5 ~ in the CCE of SB-2 during this two-week period is equal to the loss in CCE it suffered during the ' ' I ' Deleclor S B - I Age 43 days Temp. G5' C. Bias 15 V Current 3.6/.cA
400
'
I
'~"'
'
1
5°7 M( Pulser 150C
(100.5%) p°21° [I
IOOC ~eV
200
500
I0O
~ 170
,
I ,
J
,%,
175 178 184 CHANNEL NUMBER
,!
// i
, ,
,
N t89
Fig, 3, Response o f detector SB-1 to 210Po =-particles d u r i n g t h e r m a l - v a c u u m test. T e m p e r a t u r e was 6 5 ° C a n d pressure was 5 × 10-s m m H g . D e t e c t o r age w a s 43 d.
I
I
209
DETECTORS
I
'
1
i ~"
800C -7OOC6000
,
1
,
)
Detector SB-I Age 78 days Room Temp. Bias 15V Current 0,20 p.A
po~=O(I00.0%)~.
5.7 MeV~ Pulser/
--
o.. 5 0 0 C -
2OOC
C Z4oo0 t)
--
62 keV
150C
3000 -I000
200~ --
/ 5OO
I000 -• ! 165
I
I
,\t, i
/
170 173 181 CHANNEL NUMBER
185
Fig. 4. R o o m temperature response of detector SB-I to 210Po ~t-particles. Detector age was 78 d.
first six week aging period following its fabrication. The detectors were then mounted in the vacuum chamber and maintained at 50°C and a pressure of 5 x 10 -5 mm Hg for seven days, with 15 V bias applied continuously. The leakage current of SB-1 and SB-2 was found to increase gradually with time during this test, similar to what was found during the first t-v test. The CCE for SB-1 at 50°C was found to be 100.3~o, which is consistent with a value of - 1.3 x 10-+/°C for the fractional change in e with temperature which was obtained from the measurement at 65 ° C. The maximum leakage current of SB-1 at 50°C was 2.3 #A. No 2t°Po spectra were taken for SB-2 during this test. Immediately after this test, the characteristics of SB-1 were the same as at the beginning of the test. The CCE for SB-2 was found to be 99.6%, essentially what it was at the start of the one week t-v test. The detectors were allowed to age for one week (no bias applied) and then a measurement of the CCE vs bias voltage was taken. Such measurements are very sensitive to any effects which aging and t-v testing may have on detector properties2). A comparison of the CCE vs bias data given in tables 1 and 2 shows that at each bulk field, the ~roperties of SB-1 are far superior to those of SB-2,
A.J. TUZZOLINO et al.
210 '
'
I
"~'
900( --
('~~P°2'°{99"9%}
8ooc
7000 j
uJ
I
'
F
'
I
Detector SB-I Age 85 days Room Temp. Bias 15 V Current 0.19/z.&
6000
7 MeV Jlser
i 5000 ~ o
2000
!/1
4000 -
3000
I00£
_47 heY
2000
I000
-
/
,~'~"/~
164 165
~
I
170
173 181 185 CHANNEL NUMBER
185
Fig. 5. R o o m temperature response of detector SB-1 to 2Jopo
~t-particles, Detector age was 85 d. particularly at low electric fields. Very similar behavior was found for all B-W and N-B-W detectors studied. The poor CCE and or-resolution of SB-2 at low electric fields is similar to the degradation in these quantities (following t-v testing and aging) suffered by N-B-W detectors Studied earlier2). Figs. 4 and l0 show the response of SB-1 and SB-2 to 21°Po at 15 V bias during the CCE vs bias voltage measurements. The detectors were allowed to age for a week (no applied bias) and then 2 ~Opo spectra were taken, shown in figs. 5 and 11. During this week, fig. 5 indicates that there has been a slight loss in CCE for SB-1 ( ~ 0 . 1 ~o)During this same period, fig. l l shows that the CCE of SB-2 has decreased from 99.7% to 99.5% and that there has been a degradation in or-resolution. In addition, a comparison of figs. 10 and l l shows that SB-2 has begun to develop a low-energy tail in its response to 2 | ° p c . The detectors were then allowed to age for two weeks, with 2 1 ° p o spectra taken during this time. Tables 1 and 2 give the data and the response of SB-1 and SB-2 to 2~°Po at the end of this period is shown in figs. 6 and 12. Fig. 6 shows that there was no change in the characteristics of SB-1 during this time, whereas fig. 12 shows that there was a significant degradation in the ct-characteristics of SB-2 during this same period. At this final stage, detector SB-2 had a CCE of 98.8 %, an or-resolution of 130 keV, a leakage current of 1.1/aA
and a significant low-energy tail in its response to 21°Po ~t-particles. On the other hand, at this same stage, detector SB-1 had a CCE of 99.9~, an or-resolution of 61 keV, a leakage current of 0.18 #A and essentially no low-energy tail in its response to 2 1 ° p o ~t-particles. Tables 1 and 2 and figs. 1-12 show that the overall effects of the aging and testing periods on the properties of SB-1 and SB-2 were quite different. For detector SB-I, the effects produced, at most, an overall loss in CCE of 0.1 ~o (100.0% to 99.9%). There is some uncertainty as to whether this apparent overall loss in the CCE of SB-1 of0.1% is real, since this indicated change is very close to the precision of the measurements. In any case, this apparent loss first occurred after twelve weeks of aging and testing. For SB-2, the effects of identical aging and testing produced a much larger overall loss in CCE of 0.9% (99.7% to 98.8~), and a significant degradation in or-particle resolution. Also, SB-2 began to show a loss in CCE after two weeks of aging in the laboratory, before any t-v testing. The above results show that the boiling-water treatment appears to stabilize the electrical and chargedparticle characteristics of SB-detectors, even under conditions of prolonged aging and t-v testing, at least r l ~ J , J J
9000~-- [
8000
L
7000
I
I
~ I
poZlo (99.9%)~'~
I
Detector SB-I Age 98 days Room Temp. Bias 15V Current 0.18/~A
5.7 MeV Pulser
/
6000 i 2000
u 5000
a. 4 0 0 0
~
s~
z
key
•
o 3000 lO00
tO00
--
165
170 173 181 CHANNEL NUMBER
J~
185 186
Fig. 6. R o o m temperature response of detector SB-1 to 210po or-particles. Detector age was 98 d.
LITHIUM-DRIFTED
SILICON
SURFACE-BARRIER
resulted from changes in the recombination :properties of a region very near the detector surface lanoduced by the environmental testing and subsequenLaging. It was concluded2) that these ~ , :ola~rvcd with 5.3 MeV =-particles, would not t ~ ol~m~able with particles having a greater rangg;-i,.e,, ~awer ionization density near the surface. To verify the validity of this conclusion, the CCE for =-particles having several energies in the range 5.42
over the time intervals considered here. It is interesting to note that Klema t2) has found that a similar boilingwater treatment and subsequent control of humidity to be of extreme importance in producing stable, lownoise, conventional silicon surface-barrier detectors. In earlier studies2) of the effects of aging and t-v testing on the characteristics of N-B-W detectors, it was shown that the observed degradation in response to 2t°po =-particles, such as that exhibited by SB-2, 1
~
,
1
t
I
211
DETECTORS
I
'
5.7 MeV ;
Oeteclor SB-2 300 -- Age 5 days
.(~
Room Temp.
/'~
Bias 15 V
I
PoZI°(99,7%)
1OOO
Pulser/
/
Curren, 5/.LA J \ Zz 200 -I500 ~ z o
I00
I00
169 170 Fig. 7. Room
I00
I
I
~
6o
~Z
50
I
i
I
i
t
'
I
1%
l
'
I
Detector SB-2 Age 38 days
2400 -
Room Temp. Bias 15 V Current 0.65 p.A
2000 ~- /~
-
-r 8 0 c~ a: 7 0
177 1~8 179 185 190 191 192 CHANNEL NUMBER response of detector SB-2 to 2]Opo~¢-particles.Detector a8¢ was 3 d.
temperature
I
i I I I I~_
/I
175
/~,q
''
~ 5 . 7 MeV (Pulser
1600 --
1200--
\-
0 4O
I
'
800
43 key
--
3O
400 --
20 i
I0
~_
164 165 Fig. 8. Room
temperature
170
,Jl
S_~
,\4
,-
172 174 t75 IB4 185 187 189 CHANNEL NUMBER response of detector SB-2 to 210po -particles. Detector age was 38 days.
212
A.J. TUZZOLINO et aL
MeV to 8.78 MeV was measured for a N-B-W SB detector which had shown degradation in its response to 21°Po ~-partieles following aging and t-v testing. This N-B-W detector, hereafter referred to as SB-3, had an intrinsic depth of 200/tin and, at an applied bias of 15 V, had an initial C C E and ~-resolution of 99.7~o and 51 keV, respectively, for =l°Po ~-partieles. After a period of aging and t-v testing, a 21°po spectrum was I
,
J
'
i
~
'
'
taken and the CCE and ~-resolution was found to be 98.0yo and 200keV, respectively. In this degraded state, the response of detector SB-3 to a thorium source emitting =-particles of energies a), 5.42 MeV, b) 5.68 MeV, c) 6.05 MeV, d) 6.28 MeV, e) 6.78 MeV and f) 8.78 MeV, was then measured and is shown in curves a-f of figs. 13-15. The measured CCE and ~-particle resolution is indicated in each curve. The I
m,
J
I
~
Detector SB- 2
pozlof~-"~
12ol-
Age 39 doys
Joo -J w z
t
r
I
J
/,~"k x
moo
5.7 MeV
Current I I.5~A Joo~
•
9(]--
3
70 heY
_
0:
60 --
--
5 0 --
70 ~ keV 500
~ 4o o~ 3oL
/
o;/
;\
.
170 m
DO[
t75
;/
J79
m3 Je4 m5
CHANNEL
NUMBER
19o 191
Fig. 9. R e s p o n s e o f detector SB-2 to 210po at-particles d u r i n g t h e r m a l - v a c u u m test. T e m p e r a t u r e w a s 65 ° C a n d pressure 5 x 10-5 m m H g . D e t e c t o r a g e was 39 d. I
1
I
'
Pov o j 6000
3~'-'
I
l/
I m,
r
,
Detector S B - 2 Age 74 days
(99.7%)
--
I
' "
'
l
I
l
Bias 15V Current 0.99,u.A
Room Temp. 5000
2000
--
--
.A
_ z
5.ZM,V
1600
4 0 0 0 -
~'
70
I_
heV 3000
1200
--
t
o_
~
48
key z
800
2 0 0 0 -
o k) IOOO
4 0 0 --
--
,
'/
i64 '65
T/
,
i
~
I 170
T~L
I
'72 i73 '74 CHANNEL
,~
-, .__ '80
/ p
,
i
"
I "~'l.
I
'85 186 '87
NUMBER
Fig. 10. R o o m temperature response o f detector SB-2 to 210po at-particles. D e t e c t o r a g e was 74 d.
LITHIUM-DRIFTED SILICON SURFACE-BARRIER DETECTORS J
5000
--
~
I
J
J
!
'
p~'O ( 9 9 . 5 % ) ' / ~ ' ~ o t ' ~
,
,
I
'
~'
I
I
I
Detector S B - 2 Age 81 doys Room Temp.
I
t
I
Bios 15V Current I,I/J.A
.,---.,,
4OOO z
z
E
'
213
~.T MeV
J\
1800 3000 1200
o 600 I000
162
i\
/ 165
170 171
%
172
181
18:5
185
NUMBER Fig. 11. Room temperature response of detector SB-2 to zlOpo=-particles. Detector abe was 81 days. CHANNEL
applied bias was 15 V. No calibrated pulser spectra are shown. Curve (a) of fig. 13 shows that the for =particles of lowest energy (5.42 MeV), the CCE is 98.2%, the =-resolution is 143 keV and the spectrum has a significant low-energy tail. This response is essentially identical to the response of SB-3 to 21°po =particles following the t-v testing and aging. However, as the energy of the incident c<-particles increases; i.e., as the ionization density near the sensitive surface decreases, the CCE and =-resolution improve, and the low energy tail becomes less pronounced. Curve (f) of fig. 15 shows that for ~-particles of energy 8.78 MeV, the charge collection is complete and the low-energy tail is absent. Figs. 13-15 illustrate the strong dependence of recombination losses on ionization density 7) and appear to verify the conclusion 2) that for those detectors which show degradation resulting from t-v testing and aging, changes have been produced in the recombination properties near the sensitive surface which are, however, important only for heavily ionizing particles. Measurements of the n-region depths and detector capacitance taken before, during and after the t-v testing and aging indicated that the overall increase in intrinsic depth for all detectors studied was < 5%. During most of the study program described above, no importance had been placed on the resistivity of the silicon used to fabricate the detectors. However, in the latter phases of our study, an apparent dependence of detector zl°Po ~-particle stability on the resistivity of
the silicon used to make the detector was noticed. It was found that detectors which were made from 1000 I
Illll~llllll
I
I
I
I
Detector SB-2 Age 9 4 doys Room Temp. Bias 15 V Current I,I p.A
3000
Po21° ( g e . e % ) /
-
?
2000
5.7 M eV Puls
2
--
I,~9
leE,
170 CHANNEL
181
183
185
NUMBER
Fig. ]2. Room temperature response of detector $B-2 to 210Po
~-l~rticles. I~tec{or age was 94 dws.
214
A.J. "-i~l
600
T U Z Z O L I N O et al.
i i j i i i i j [ i i i i r i i i i , i i i t i i i i r
I r j , ,
Detector SB-5 B i a s 15 V
--
t'~
S
500
98.2%
(o)
(b) 9 8 . 7 %
_/t
i400 Q. 3 0 0
P0 200 t) I00
.,,,
./
/
/
,/"
\
,s
\
i I I I i , I I i I I I I i ",~',"/"I Ii f I I I 250 255 260 265 270 CHANNEL NUMBER
~?~ 245
I I i\,% '-~ -' -' I
::","5 " ' ' Vi30
Fig. 13. R o o m temperature response of detector SB-3 to Th ~¢-particles. Spectrum (a) results from incident ~-particles of energy 5.42 MeV. Spectrum (b) results from incident or-particles of energy 5.68 MeV. The CCE a n d or-resolution is indicated in each curve.
o h m . c m silicon rather than 100 ohm.era silicon exhibited better characteristics. This was found to be true for both B-W and N-B-W detectors.To study this effect in more detail, four SB detectors were fabricated from 1000 ohm" cm silicon. All four detectors had a sensitive i r i i i i I [ f i f i i i [ i i i i I Detector 600
Bios
SB-5
15 V
/f(d)
99.4%
-
area of 1.2 cm 2 and intrinsic depths ranging from 300/~m to 330 #m. They were not given the boiling-water treatment. All data to be discussed was taken at 15 V. These N-B-W detectors had initial currents ranging from 1.2 # A to 1.4 #A, similar to the initial currents of the N-B-W detectors made from 100 ohm.cm material. N o initial 21°Po spectra were taken. They were allowed to age in the laboratory for two months (no
/[f~/ 7OO
I
• z
500
i
z
keY
,oo285
290
•
/ ,L_E v / 2~5
CHANNEL
/
-
200 I00
300
I
AV--
I
t
1
1
I
[
I
I
I
I
[
I
Detector SB-3 Bias 15V
66_ keV
3OO
(c) 9 9 . 2 %
/
[
4OO
6e
300
I
e)99.6%
.t~'(f ) 100.0%
5OO
400
I
305
NUMBER
Fig. ]4. R o o m t e m p e r a t u r e response o f detector SB-3 to "fh =-particles. S p e c t r u m (c) results f r o m incident or-particles of
energy 6.05 MeV. Spectrum (d) results from incident or-particles of energy 6.28 MeV. The CCE a n d =-resolution is indicated in each curve. The O 2 E for spectrum (c) is approximate.
315
; 520
/ 325
\ ^ jf,,,,~.,,,
330 * 420 425 CHANNEL NUMBER
430
Fig. 15. R o o m temperature response of detector SB-3 to Th a-particles. Spectrum (e) results from incident or-particles of energy 6.78 MeV. Spectrum (f) results from incident at-particles of energy 8.78 MeV. The CCE and or-resolution is indicated in each curve.
LITHIUM-DRIFTED
SILICON SURFACE-BARRIER DETECTORS
applied bias). After this aging period, the currents were in the range from 0.2 IrA to 0.3 #A, and all four detectors had a CCE and or-resolution for 21°po orparticles of 9 9 . 7 ~ and 53 keV, respectively. Also, the spectra showed no low-energy tails. The behavior (high CCE and good or-resolution) exhibited by these four detectors following the two month aging period is distinctly different than the behavior of the N-B-W detectors made from 100 o h m . c m material (such as SB-2). Table 2 shows that the N-B-W detectors made from 100 o h m . c m material suffer a loss in CCE of from 0 . 5 ~ to 1.0~o and a loss in or-particle resolution during a much shorter period of time (six weeks) following fabrication. The four "high-resistivity" detectors were then mounted in a vacuum chamber and maintained at 50 ° C and a pressure of 5 x 10- ~ mm Hg for seven days, with the bias voltage applied continuously. During this test, the maximum currents were in the range from 3.9/~A to 4.1 #A and the maximum pulser resolutions were in the range from 54 keV to 60 keV. After this test, the CCE for 21°po was found to be 99.9~o for all four detectors. The detectors were allowed to age for two weeks (no applied bias). After this aging, the CCE was again found to be 99.9 ~o for all four detectors and the spectra still had no low-energy tails. In addition, the leakage currents were essentially equal to their initial values. Thus, no degradation occurred during the twoweek aging period following the t-v test. This behavior is also different than that of N-B-W detectors made from low resistivity material, which show degradation in or-particle characteristics after this same period of time following at t-v test (table 2). Thus, N-B-W detectors made from 1000 o h m . c m silicon appear to have more stable 2~°Po or-particle characteristics than N-B-W detectors made from 100 ohm. cm material, both before and after t-v testing. To further study the effects of detector resistivity, a B-W detector having the same area and intrinsic depth as SB-1 was made from 100 o h m ' e r a material. The detector was put through a sequence of aging and environmental testing similar to that given to SB-1. Although the initial current and CCE of this detector were identical to the initial values for SB-1, it showed some degradation in 2~°Po ~t-particle characteristics during aging, after both environmental tests. Thus B-W detectors, as well as N-B-W detectors, made from 1000ohrn'cm material have more stable 2~°Po ~particle characteristics than detectors made from 100 ohm. cm material. Since the N-B-W detectors made from I000 ohm" cm material were not studied as extensively as the B-W
215
detectors made from 1000 ohm, cm material, it is difficult to decide to what extent the boiling-water treatment was responsible for the long term stability of SB-I and to what extent the use of material of 1000 o h m - c m to make SB-I was responsible. However, the importance of the boiling-water treatment is clearly demonstrated by comparing the initial characteristics of the B-W and N-B-W detectors made from 1000 o h m . c m material. For the B-W detectors, the initial currents were in the range from 0.1 #A/cm 2 to 0.2/~A/cm 2 and the initial CCE was, in most eases, 100.0~. For the N-B-W detectors, the initial currents were approximately ten times greater and only after aging for two months did the currents approach the currents of the B-W detectors. Also, the CCE for the N-B-W detectors was less (99.7~/o) than for the B-W detectors. A comparison of the data for the B-W detector made from 100 ohm.era material with the data of N-B-W detectors made from 100 o h m - c m material (SB-2) shows that the initial leakage currents of the.N'~B-W detectors were approximately ten times greater than the B-W detector and the CCE of the N-B-W detectors (99.2~-99.8%) was less than the CCE of the B-W detector (100.0~o). The above data shows that for given resistivity material, B-W detectors have lower initial leakage current and higher initial CCE than N-B-W detectors. For both B-W and N-B-W detectors, those made from 1000ohm.cm material have more stable 21°po ~particle characteristics than those made from 100 ohm. cm material. 4. Conclusions One feature which was common to the behavior of all SB detectors studied was the absence of any kind of noise breakdown during environmental testing at both 65°C and 50°C. All B-W detectors studied had initial leakage currents in the range from 0.1 pA/cm 2 to 0.2 #A/cm 2, and initial values of CCE in the range 99.7~0 to I00.0~o. All B-W detectors made from 1000 ohm-cm material exhibit stable electrical and 21°po a-particle characteristics under conditions of aging and t-v testing over a period of four months. B-W detectors made from 100 ohm. em material show some degradation in 21°Po or-particle characteristics when aged following t-v testing. N-B-W detectors made from 100 ohm-cm material had initial leakage currents in the range from 2/~A/cm 2 to 10 #A/cm 2 and had initial values of CCE in the range from 9 9 . 2 ~ to 99.8~. N-B-W detectors made from 100 o h m . c m material show a toss in CCE and orresolution for 21°Po or-particles wh¢~ aged ~nder am-
216
A.J. TUZZOLINO et al.
bient laboratory conditions. This degradation occurs both before and after t-v testing. N-B-W detectors made from 1000 ohm-cm silicon show no degradation in a-particle characteristics when aged for two months following fabrication. Subsequent t-v testing and aging over a period of three weeks produces no degradation. For given resistivity material, B-W detectors have lower initial leakage current and higher initial CCE than N-B-W detectors. Both B-W and N-B-W detectors made from 1000ohm.cm silicon have more stable 21°po a-particle characteristics t h a n detectors made from 100 o h m . c m silicon. The degree of degradation in or-particle characteristics suffered by N-B-W detectors made from 100 o h m ' c m material was shown to depend on ionization density. A degraded detector which had a CCE of 98.2,%0 for 5.42 MeV a-particles had a CCE of 100.0% for 8.78 MeV or-particles. The data at 65°C and 50°C for B-W detectors made from 1000ohm:cm silicon indicates that the fractional change in the mean energy per ion pair over the temperature range from room temperature to 65°C is approximately - l . 3 x l 0 - * / ° C . For all detectors studied, the overall increase in intrinsic depth resulting from the environmental testing and aging was < 5 % .
The authors wish to express their thanks to G. Ho for his help in fabricating the detectors, to P. Shen for his help in various aspects o f the testing program and to J. Lamport, Dr. J. Simpson, Dr. A. Turkevich and Dr. C. Fan for their continued encouragement and helpful criticisms. The research reported in this paper was supported by the National Aeronautics and Space Administration under NASA Grant NsG-179-61.
References 1) G. Dearnaley and J. C. Lewis, Nucl. Instr. and Meth. 25 (1964) 237. 2) A. J. Tuzzolino, J. Kristoff and M. A. Perkins, Nucl. Instr. and Meth. 36 (1965) 73. 3) j. L. Blankenshipand C. J. Borkowski, IRE Trans. NucL Sci. NS-9 (1962) 181. 4) j. A. Coleman and J. W. Rodgers, IRE Trans. Nucl. Sci. NS-11 (1964) 213. 5) F. S. Goulding, IEEE Trans. Nucl. Sci. NS-11 (1964) 177. 6) F. R. Shea, Editor, IRE Trans. Nacl. Sci. NS-8 (1961). 7) G. L. Miller and W. M. Gibson, Nucl. Electr. 1 (1962) 477. 8) C. A. J. Ammerlaan,R. F. Rumphorst and L. A. Ch. Koerts, Nucl. Instr. and Meth. 22 (1963) 189. 9) A. B. Gillespie, Signal, Noise and Resolution in Nuclear Counter Amplifiers (Pergamon Press, London, 1953). z0) C. Bussolati, A. Fiorentini and G. Fabri, Phys. Rev. 136 (1964) A1756. i1) W. Shockley,Czech. J. Phys. BI1 (1961) 81. 12) E. D. Klema, Nucl. Instr. and Meth. 26 (1964) 205.
INSTRUMENTS
AND METHODS
37 (1965) 2 o 4 - 2 t 6 ;
© NORTH-HOLLAND
PUBLISHING
CO.
STABILIZATION OF LITHIUM-DRIFlrED SILICON SURFACE-BARRIER DETECTORS A. J. TUZZOLINO, M. A. P E R K I N S and J. K R I S T O F F Laboratory for Astrophysics and Space Research, Enrico Fermi Institute for Nuclear Studies, The University of Chicago, Chicago, Illinois Received 16 July 1965 A simple fabrication technique has been found which yields
measurements of their electrical and :t-particle characteristics taken before, during, a n d after thermal-vacuum testing at 65°C a n d additional testing at 50°C. Degradation in electrical a n d :t-particle characteristics resulting from such testing and aging was found to be negligible over a period of approximately four months. The measurements at 50°C a n d 65°C indicate that the fractional change in the mean energy per ion pair is approximately - 1.3 x 10-4/°C over the temperature range from r o o m temperature to 65°C. The results of these studies are reported.
lithium-drifted silicon surface-barrier detectors which have a low leakage current and which exhibit stable electrical and :t-particle characteristics under conditions of prolonged aging and repeated exposure to thermal-vacuum environments. The detectors will operate at a temperature of 65°C and high vacuum for several days with no evidence of noise instability. The operating characteristics of such detectors have been studied over a period of several months. These studies included 1. Introduction The need for charged-particle detectors which exhibit stable characteristics for prolonged periods of time both under ordinary laboratory conditions and under conditions of high vacuum and elevated temperature is of growing importance in the space sciences. In our laboratory, this need has led to a continuing study of the effects of aging in the laboratory and environmental testing on the electrical and charged-particle characteristics of the lithium-drifted silicon surfacebarrier detector 1' 2). Initially, the sensitive surface of the detectors was allowed to age for one or two days in the ambient atmosphere of the laboratory following fabrication. After this aging, the magnitude of the leakage current for detectors fabricated by identical techniques was found to be extremely variable and all detectors could be put into one of two categories. Detectors in the first category were unusable, having leakage currents of several hundred #A/cm 2. Detectors in the second category were usable and had leakage currents in the range from approximately 2 # A / c m 2 to 10/~A/cm 2. These very large differences in leakage current appeared to be related to the conditions of relative humidity in the laboratory. After exposing detectors to a variety of controlled conditions of humidity, it was found that detectors which were immersed in boiling deionized water for several minutes, immediately after fabrication, exhibited lower leakage currents and much more stable electrical and or-particle characteristics than those that were not given this treatment. A 15-rain boiling-water treatment with the detector completely immersed in the water was adopted as a standard procedure. This paper presents the results of a study of the effects of aging and various thermal-vacuum environments on
the characteristics of surface-barrier detectors which were given the boiling-water treatment. For comparison, similar results are presented for detectors which were not given the boiling-water treatment. The work reported here represents an extension of earlier experiments2), where the effects of aging and environmental testing on detectors prepared by exposure to the ambient atmosphere of the laboratory were studied. The environmental conditions used for detectors to be discussed in the present work were more severe than the conditions used for detectors discussed in earlier work2). 2. Surface-barrier detectors 2.1 DETECTOR FABRICATION
The general techniques used in the fabrication of the surface-barrier (SB) detectors were identical to those described elsewhere i - 4). Single crystals of p-type silicon with electrical resistivities of 1 0 0 o h m . c m and 10130 o h m - c m were used as the starting material. The diffusion and drifting of lithium was carried out as described earlier2), yielding a n-i-p structureS). A SB detector was made from the n-i-p unit by lapping back from the p-side to a depth which was well into the intrinsic region. This lapped face, which exposed the intrinsic region, was then etched and a gold contact evaporated over this face. After mounting the units into insulating rings2), some of the detectors were given the boiling-water treatment. 2.2. ELECTRONIC EQUIPMENT
The amplifier system used for most of the measurements was a transistorized charge-sensitive amplifierpost-amplifier combination designed at this laboratory. This amplifier had an integrating and differentiating time constant of 2/~s. An ORTEC Model 204 charge-
204
LITHIUM-DRIFTED SILICON SURFACE-BARRIER DETECTORS
dumping pulser was used to provide data on electrical resolution and to provide an energy calibration for the system. The pulser was calibrated by making use of a conventional gold-silicon surface-barrier detector 6) and a 21°Po source. The output from the post amplifier was analyzed on an R I D L Model 32-12B 400 channel pulse-height analyzer. Detector currents were monitored with a Keithley Model 410 micro-microammeter. The detector load resistor used throughout the experiment was 105 t2. 2.3. DETECTOR CHARACTERIZATION
The depth of the lithium diffusion and the intrinsic depth was determined by making use of internal conversion electrons from 137Cs and 2°7Bi sources and capacitance measurements, as described earlier'). Such measurements were made periodically during the aging and environmental testing of detectors to determine whether any changes in n- or intrinsic-region depths had occurred. 2.4. THERMAL-VACUUMTEST The aging and thermal-vacuum (t-v) testing procedure consisted of the following: I. detectors were allowed to age at room temperature with no bias applied for approximately six weeks following fabrication. 2. They were then placed in a vacuum chamber and maintained at a temperature: of 65°C and a vacuum of apprOximately 5 x l0 ~5 mm Hg for four days with the bias voltage applied continuously. 3. The detectors were removed from the vacuum chamber and allowed to stand for approximately three weeks with no bias applied. 4. They were then placed in the vacuum chamber and maintained at a temperature of 50°C and a vacuum of 5 x 10 -5 mm Hg for seven days with the bias voltage applied continuously. 5. The detectors were removed from the vacuum chamber and allowed to stand for approximately four weeks with no bias applied. The following detector characteristics were measured: A. current-voltage characteristic; B. n-region depth and intrinsic depth; C. capacitance vs bias voltage; D. electrical resolution vs bias voltage; E. charge-collection efficiency and resolution vs bias voltage for orparticles. Items A - D were measured as a function of time during the aging periods and items A, D and E were measured as a function of time during the environmental tests. Eight SB detectors were studied in this manner. Four of the eight detectors were made from 1000 ohm.cm material and the remaining four from 100 ohm. cm material. The experimental results are presented in the following section.
205
3. Experimental results and discussion Of the eight detectors studied, the four which were made from 1000 ohm. cm silicon were given the boilingwater treatment and the four made from 100 ohm-cm material were not. All eight detectors had sensitive areas of 1.2 cm 2, lithium-diffused depths in the range 125#m to 1501~m and intrinsic depths ranging from 325 #m to 335 #m. The applied bias voltage in each case was 15 V, corresponding to a bulk electric field of ~ 450 V/cm. The charge-collection efficiency (CCE) as measured in this experiment may be written 2) CCE = ~F,
(l)
where r/ is the charge-collection efficiency of the detector alone 7) and F represents the effect of the carrier transit times a) and the amplifier time constant on the output signal amplitudeg). The quantity r/ is defined here as
rl = Qc(T)/QL,
(2)
where QL is the charge liberated at room temperature, and Qc(T)is the charge collected at the temperature T. When the effects of carrier recombination or trapping 7) are considered, the quantity Qc(T) may be written Qc(T) = QL(T)G,
(3)
where QL(T) is the charge liberated at the temperature T and G represents the effects of carrier loss through recombination or trapping. In general, the quantity G is a function of the distribution in space of the ionization density produced by the incident particle, the carrier recombination lifetimes, the carrier trapping lengths, the range of the incident particle, the intrinsic depth and the plasma time7). Thus, from eqs. 0)-(3), ccE
= Q,.(r)rG/Q~.
(4)
At all temperatures and bias voltages used, the transit times a) of the carriers were sufficiently less than the 2 #s amplifier time constant so that the quantity F in eq. (4) was unity. Thus, for all detectors studied,
CCe = Q.L(T)G/QL.
(5)
When the effects of carrier recombination and trapping are negligible, eq. (5) reduces to
CCE = QL(T)/QL = t/c(T),
(6)
where c is the mean energy per ion pair at room temperature *°) and e(T) is the mean energy per ion pair at the temperature 7". In measurements of the CCE for =-particles, it was possible to detect a change in CCE of about 0.1~.
206
A.J. TLrZZOLINO et al. T AB L E 1
8
Age off detector, (d) j
Temp.
(°63
V (v)
1 7 14 21 28 "42 43 44 45 46 49 56 59 63 "64 65 66 67 68 69 70 71 72 f
.(.
I-r
85 91 94 98
,
'I
(%,
i p
E (V/cm)
1
(fig. 1)
I
] I I
t
(fig. 2) (fig. 3) t-v test
450
t-v test
1'5 q 30 45
room temp. " 78
cc)
21Ol
3 5 10 15 20 15 15 15 15
The calibrated pulser set at 5.7 M e V was used to determine the C C E and the electrical noise spread. The contribution to the noise line-width from the amplifier alone was about 44 keV. All a-particle spectra were obtained with uncollimated sources. The experimental results found for the four boilingwater detectors were similar in all respects and will be illustrated for one o f them, hereafter referred to as SB-1. Also, the results found for the four detectors which were not given the boiling-water treatment were similar, and will be illustrated for one o f them, hereafter referred to as SB-2. Because o f the large bulk o f experimental data, the results are presented in table form. To illustrate any differences in behavior o f SB-1
60
CCE
90 c 150 300 450 600, 450 450 450 450
vs bias voltage (fig. 4) (fig. 5) (fig. 6)
and SB-2 more clearly, spectra are Shown corresponding to selected entries in the tables. All comparisons are based u p o n an applied bias o f 15 V ( ~ 450 V/era), unless otherwise stated. The characteristics o f SB-1 and SB-2 before, during, and after the t-v tests are shown in tables 1 and 2 and figs. 1-12. Tables 1 and 2 show that soon after fabrication, detector SB-I had a much lower leakage current than SB-2. Typically, "boiling-water" (B-W) detectors have initial currents ranging from 0.1 P A I c m 2 to 0.2 p A / c m 2, whereas detectors having had no boiling-water treatment (N-B-W) have, when usable, initial currents in the range from 2/~A/cm 2 to 10/~A/cm 2. Also, the initial C C E for SB-2 (99.7~) is somewhat less than for SB-1
LITHIUM-DRIFTED
SILICON
SURFACE-BARRIER
207
DETECTORS
TABLE 2
314 Age of detector (d)
5 13 18
26
Temp. (0 C)
L
I
5.7 MeV
zt0po
Q,A)
(keV)
(keV)
5.0
74 49 49 52 49 43 70 79 94 61 49 52 52
79 68 59 67 68 61 70 92 100 60 60 55 58
room temp.
65
0.8 i
I
15
1" room temp. 50 50 50 50 50
i;.5 room temp. , 74
t 81 87 9O 94
I
V
1.45
[ 65
6
(V)
65 41 44 47 52 ~60 61 62 63 66 67
5
I
1.0 1.5 2.0 3.0 5.0 I0 15 20 15 15 15 18
0.65 0.72 0.63 11.5 15.2 16.2 3.4 0.62 0.77 0.88 3.2 3.2 3.7 4.8 5.1 0.82
pulser
.
CCE (~/.)
99.7 99.'/ 99.6 99.4 99.5 99.2 100.2 100.1 100.1 99.9 100.0 99.6 99.5 .
I
8 E (V/ena)
~
'(fig. 7)
!
" (fig. 8) (fig. 9) " t-v test
450 J
.
t-v test
--
--
46 62
55 625
99.6 77.5
1'5
" 59 56
560 530
81.8 84.3
30 45
56 48 48 48 48 48 51 47 51 57
470 447 355 124 70 62 85 100 120 130
86.4 89.2 95.1 99.3 99.7
0.45
0.48 0.65 0.70 0.77 0.85 0.93 0.99 1.0 1.1 1.1 1.2 1.1
.
7 21Opo
u
60
CCE
90 ~vs 150 bias voltage 300 450 (fig. 10)
99.9
600
99.5 98.8 98.8 98.8
450 450 450 450
(fig. 11) (fig. 12)
Characteristics of detector SB-I (table 1) and SB-2 (table 2), before, during and after thermal-vacuum (t-v) tests. Column 1 gives the detector age in days. Column 2 gives the detector temperature in oC. Columns 3 and 4 give the applied bias in V and th~ teakage current in/~A, respectively. Columns 5 and 6 give the resolutions (fwhm) expressed in keV for a calibrated pulser set at 5.7 MeV and 210po ~t-particles, respectively. Column 7 gives the ~t-partiele CCE and column 8 gives the fields in the intrinsic region. The detector was at room temperature except during the thermal-vacuum tests. The applied bias was 15 V and the electric field 450 V]em during all measurements except those of CCE vs bias voltage. No bias was applied during the time intervals between room temperature measurements. The bias voltage was applied continuously during the thermal-vacuum tests. Test conditions were the same for SB-I and SB-2. (100.0 ~o). I n general, B - W detectors have a higher initial C C E (99.7~o-100.0~o) t h a n N - B - W detectors (99.2~o99.8 ~ ) . N o N - B - W detectors studied were f o u n d to have a n initial C C E o f greater t h a n 9 9 . 8 ~ , whereas m o s t B - W detectors h a d a n initial C C E o f 1 0 0 . 0 ~ . These facts s h o w that, at r o o m temperature, the intial C C E o f N - B - W detectors is given by eq. (5) with G lying in t h e range f r o m 0.992 to 0.998 a n d t h a t for m o s t B - W detectors, a n y initial r e c o m b i n a t i o n or t r a p p i n g losses are n o t measurable. T h e initial response o f SB-1 a n d
SB-2 to 2~°po a-particles is s h o w n in figs. 1 a n d 7. T h e poorer resolution o f SB-2 results f r o m its h i g h leakage current. N o significant low-energy tail is app a r e n t in either ease. Table 1 s h o w s t h a t d u r i n g t h e six-week a g i n g period (no applied bias), t h e current a n d a-resolution o f BS-I r e m a i n e d essentially constant, a n d t h e C C E d u r i n g this time was given by eq. (6), i.e., the C C E was 1 0 0 . 0 ~ . Table 2 s h o w s t h a t d u r i n g this s a m e period (no applied bias) t h e leakage c u r r e n t of SB-2 steadily improved,
208
A.J. TUZZOLINO [
AV
Detector $ 8 - I Age I day Room Temp.
I
I
i
I
Bias 15V Current O.13~A
400
4000 -5,7 MeV Pulser ~
Z Z <[ 300
3000
--
g
Q. ~ 200 Z
2000 --
g
4l
keV"
o
IOO
IOOO
% 170
/ ,:
175 1-76
184 185
CHANNEL
NUMBER
,
J
l,l 190
Fig. 1. Room temperature response of detector SB-1 to 210po =-particles. Detector age was t d. but its CCE was gradually degraded from 99.7% to 99.2%. Figs. 2 and 8 show the response of SB-1 and SB-2 after this aging period. There is still essentially no low energy tail in the 2~°po spectrum for SB-1 (fig. 2), whereas the 2aOpo spectrum for SB-2 (fig. 8) shows a pronounced low energy tail. In general, the leakage current of N-B-W detectors was found to decrease steadily with time, attaining a value o f ~ 0.4/~A/cm 2 after a period of a month or two following fabrication. However, during this same period, they exhibit some degradation in their response to 21°Po ~-particles. This degradation consists Of an overall loss in CCE of from 0.5% to 1.0% and a loss in ~-particle resolution and presumably results from an increase with time in recombination losses near the sensitive surface during aging2). After the six week aging period, the detectors were mounted in a vacuum chamber and maintained at a temperature of 65°C and a pressure of 5 x 10 -s mm Hg for four days. A bias of 15 V was applied continuously during this test. Tables 1 and 2 give the electrical and c~-particle characteristics of SB-I and SB-2 during this test and figs. 3 and 9 show their response to Z~°Po after the first day of the test.
et
al.
At 65°C, all B-W detectors had values of the CCE in the range from 100.4% to 100.6%. The measured CCE of 100.5% for SB-1 (fig. 3) at 65°Cis equal to the average CCE at this temperature for all B-W detectors studied. This average value for the CCE of 100.5 % and the use ofeq. (6), gives a value for the fractional change in the mean energy per ion pair of approximately - 1.3 x 10-4/° C over the temperature range from room temperature to 65°C. This observed decrease in e at 65 ° C is consistent with the behavior of ~ with increasing temperature observed at lower temperatures1°). The accuracy of this measurement (~20%) was not sufficient to justify any comparison of the results 1o) with a calculation of the mean energy per ion pair by ShockleyZ~). The fact that the measured CCE for detector SB-2 at 65°C (fig. 9) was lower than that measured for SB-1 is believed to have resulted from the fact that SB-2 was in a degraded state at the start of the test. At 65°C, all N-B-W detectors had values of the CCE in the range 100.0~o to 100.2%. During the four-day test, the leakage current of all detectors was observed to increase as a function of time as shown in tables 1 and 2. The observed increase in pulser line width and =-resolution during this time resulted from this increase in current. At the end of the four-day test, the leakage current of all detectors studied
1
I
I
I
I
I
I
P°210(100'0%1)
•
I
I
r
I
[
Detector SB-I Age 42 doys Room Temp. Bios 15 V Current O.IO~A
400
5.7 MeV, Pulser
~ 300 xo IX 03
2000
-- I
~ 200 o
~oo
/ i
~'/i 171
I000 -
-
46 heY
:
,
I
1"77 CHANNEL
185
r9a
NUMBER
Fig. 2. Room temperature response of detector SB-1 to 210po ~¢-particles.Detector age was 42 d.
SILICON SURFACE-BARRIER
LITHIUM-DRIFTED
was in the range from 5 #A/cm 2 to 25 #A/cm 2 at 65°C. At the conclusion of the test, the detectors were returned to room temperature while still under vacuum in the chamber. The leakage current of SB-1 and SB-2 at this stage was approximately ten times the value at the start of the test. In addition, the CCE of SB-2 was better at this stage (99.9%) than at the start of the test (99.2%) and a 2t°Po spectrum showed no evidence of any appreciable low-energy tail. Apparently, the degradation in ~t-characteristics of SB-2 which was introduced by the six-week aging period prior to the t-v test was completely removed during the t-v test. The CCE of SB-1 at this stage was still 100.0%. The detectors were removed from the chamber and allowed to age for three days (no applied bias). The leakage currents of SB-1 and SB-2 were found to have decreased during this time to the values they had at the start of the t-v test. Further, both SB-1 and SB-2 had a CCE of 100.0% at this stage. The detectors were then allowed to age for another two weeks (no applied bias). Tables 1 and 2 show that during this time, the characteristics of SB-1 remained constant (CCE of 100.0%), whereas the CCE of SB-2 decreased from 100.0~ to 99.5% at the end of the two week period. This loss of 0 . 5 ~ in the CCE of SB-2 during this two-week period is equal to the loss in CCE it suffered during the ' ' I ' Deleclor S B - I Age 43 days Temp. G5' C. Bias 15 V Current 3.6/.cA
400
'
I
'~"'
'
1
5°7 M( Pulser 150C
(100.5%) p°21° [I
IOOC ~eV
200
500
I0O
~ 170
,
I ,
J
,%,
175 178 184 CHANNEL NUMBER
,!
// i
, ,
,
N t89
Fig, 3, Response o f detector SB-1 to 210Po =-particles d u r i n g t h e r m a l - v a c u u m test. T e m p e r a t u r e was 6 5 ° C a n d pressure was 5 × 10-s m m H g . D e t e c t o r age w a s 43 d.
I
I
209
DETECTORS
I
'
1
i ~"
800C -7OOC6000
,
1
,
)
Detector SB-I Age 78 days Room Temp. Bias 15V Current 0,20 p.A
po~=O(I00.0%)~.
5.7 MeV~ Pulser/
--
o.. 5 0 0 C -
2OOC
C Z4oo0 t)
--
62 keV
150C
3000 -I000
200~ --
/ 5OO
I000 -• ! 165
I
I
,\t, i
/
170 173 181 CHANNEL NUMBER
185
Fig. 4. R o o m temperature response of detector SB-I to 210Po ~t-particles. Detector age was 78 d.
first six week aging period following its fabrication. The detectors were then mounted in the vacuum chamber and maintained at 50°C and a pressure of 5 x 10 -5 mm Hg for seven days, with 15 V bias applied continuously. The leakage current of SB-1 and SB-2 was found to increase gradually with time during this test, similar to what was found during the first t-v test. The CCE for SB-1 at 50°C was found to be 100.3~o, which is consistent with a value of - 1.3 x 10-+/°C for the fractional change in e with temperature which was obtained from the measurement at 65 ° C. The maximum leakage current of SB-1 at 50°C was 2.3 #A. No 2t°Po spectra were taken for SB-2 during this test. Immediately after this test, the characteristics of SB-1 were the same as at the beginning of the test. The CCE for SB-2 was found to be 99.6%, essentially what it was at the start of the one week t-v test. The detectors were allowed to age for one week (no bias applied) and then a measurement of the CCE vs bias voltage was taken. Such measurements are very sensitive to any effects which aging and t-v testing may have on detector properties2). A comparison of the CCE vs bias data given in tables 1 and 2 shows that at each bulk field, the ~roperties of SB-1 are far superior to those of SB-2,
A.J. TUZZOLINO et al.
210 '
'
I
"~'
900( --
('~~P°2'°{99"9%}
8ooc
7000 j
uJ
I
'
F
'
I
Detector SB-I Age 85 days Room Temp. Bias 15 V Current 0.19/z.&
6000
7 MeV Jlser
i 5000 ~ o
2000
!/1
4000 -
3000
I00£
_47 heY
2000
I000
-
/
,~'~"/~
164 165
~
I
170
173 181 185 CHANNEL NUMBER
185
Fig. 5. R o o m temperature response of detector SB-1 to 2Jopo
~t-particles, Detector age was 85 d. particularly at low electric fields. Very similar behavior was found for all B-W and N-B-W detectors studied. The poor CCE and or-resolution of SB-2 at low electric fields is similar to the degradation in these quantities (following t-v testing and aging) suffered by N-B-W detectors Studied earlier2). Figs. 4 and l0 show the response of SB-1 and SB-2 to 21°Po at 15 V bias during the CCE vs bias voltage measurements. The detectors were allowed to age for a week (no applied bias) and then 2 ~Opo spectra were taken, shown in figs. 5 and 11. During this week, fig. 5 indicates that there has been a slight loss in CCE for SB-1 ( ~ 0 . 1 ~o)During this same period, fig. l l shows that the CCE of SB-2 has decreased from 99.7% to 99.5% and that there has been a degradation in or-resolution. In addition, a comparison of figs. 10 and l l shows that SB-2 has begun to develop a low-energy tail in its response to 2 | ° p c . The detectors were then allowed to age for two weeks, with 2 1 ° p o spectra taken during this time. Tables 1 and 2 give the data and the response of SB-1 and SB-2 to 2~°Po at the end of this period is shown in figs. 6 and 12. Fig. 6 shows that there was no change in the characteristics of SB-1 during this time, whereas fig. 12 shows that there was a significant degradation in the ct-characteristics of SB-2 during this same period. At this final stage, detector SB-2 had a CCE of 98.8 %, an or-resolution of 130 keV, a leakage current of 1.1/aA
and a significant low-energy tail in its response to 21°Po ~t-particles. On the other hand, at this same stage, detector SB-1 had a CCE of 99.9~, an or-resolution of 61 keV, a leakage current of 0.18 #A and essentially no low-energy tail in its response to 2 1 ° p o ~t-particles. Tables 1 and 2 and figs. 1-12 show that the overall effects of the aging and testing periods on the properties of SB-1 and SB-2 were quite different. For detector SB-I, the effects produced, at most, an overall loss in CCE of 0.1 ~o (100.0% to 99.9%). There is some uncertainty as to whether this apparent overall loss in the CCE of SB-1 of0.1% is real, since this indicated change is very close to the precision of the measurements. In any case, this apparent loss first occurred after twelve weeks of aging and testing. For SB-2, the effects of identical aging and testing produced a much larger overall loss in CCE of 0.9% (99.7% to 98.8~), and a significant degradation in or-particle resolution. Also, SB-2 began to show a loss in CCE after two weeks of aging in the laboratory, before any t-v testing. The above results show that the boiling-water treatment appears to stabilize the electrical and chargedparticle characteristics of SB-detectors, even under conditions of prolonged aging and t-v testing, at least r l ~ J , J J
9000~-- [
8000
L
7000
I
I
~ I
poZlo (99.9%)~'~
I
Detector SB-I Age 98 days Room Temp. Bias 15V Current 0.18/~A
5.7 MeV Pulser
/
6000 i 2000
u 5000
a. 4 0 0 0
~
s~
z
key
•
o 3000 lO00
tO00
--
165
170 173 181 CHANNEL NUMBER
J~
185 186
Fig. 6. R o o m temperature response of detector SB-1 to 210po or-particles. Detector age was 98 d.
LITHIUM-DRIFTED
SILICON
SURFACE-BARRIER
resulted from changes in the recombination :properties of a region very near the detector surface lanoduced by the environmental testing and subsequenLaging. It was concluded2) that these ~ , :ola~rvcd with 5.3 MeV =-particles, would not t ~ ol~m~able with particles having a greater rangg;-i,.e,, ~awer ionization density near the surface. To verify the validity of this conclusion, the CCE for =-particles having several energies in the range 5.42
over the time intervals considered here. It is interesting to note that Klema t2) has found that a similar boilingwater treatment and subsequent control of humidity to be of extreme importance in producing stable, lownoise, conventional silicon surface-barrier detectors. In earlier studies2) of the effects of aging and t-v testing on the characteristics of N-B-W detectors, it was shown that the observed degradation in response to 2t°po =-particles, such as that exhibited by SB-2, 1
~
,
1
t
I
211
DETECTORS
I
'
5.7 MeV ;
Oeteclor SB-2 300 -- Age 5 days
.(~
Room Temp.
/'~
Bias 15 V
I
PoZI°(99,7%)
1OOO
Pulser/
/
Curren, 5/.LA J \ Zz 200 -I500 ~ z o
I00
I00
169 170 Fig. 7. Room
I00
I
I
~
6o
~Z
50
I
i
I
i
t
'
I
1%
l
'
I
Detector SB-2 Age 38 days
2400 -
Room Temp. Bias 15 V Current 0.65 p.A
2000 ~- /~
-
-r 8 0 c~ a: 7 0
177 1~8 179 185 190 191 192 CHANNEL NUMBER response of detector SB-2 to 2]Opo~¢-particles.Detector a8¢ was 3 d.
temperature
I
i I I I I~_
/I
175
/~,q
''
~ 5 . 7 MeV (Pulser
1600 --
1200--
\-
0 4O
I
'
800
43 key
--
3O
400 --
20 i
I0
~_
164 165 Fig. 8. Room
temperature
170
,Jl
S_~
,\4
,-
172 174 t75 IB4 185 187 189 CHANNEL NUMBER response of detector SB-2 to 210po -particles. Detector age was 38 days.
212
A.J. TUZZOLINO et aL
MeV to 8.78 MeV was measured for a N-B-W SB detector which had shown degradation in its response to 21°Po ~-partieles following aging and t-v testing. This N-B-W detector, hereafter referred to as SB-3, had an intrinsic depth of 200/tin and, at an applied bias of 15 V, had an initial C C E and ~-resolution of 99.7~o and 51 keV, respectively, for =l°Po ~-partieles. After a period of aging and t-v testing, a 21°po spectrum was I
,
J
'
i
~
'
'
taken and the CCE and ~-resolution was found to be 98.0yo and 200keV, respectively. In this degraded state, the response of detector SB-3 to a thorium source emitting =-particles of energies a), 5.42 MeV, b) 5.68 MeV, c) 6.05 MeV, d) 6.28 MeV, e) 6.78 MeV and f) 8.78 MeV, was then measured and is shown in curves a-f of figs. 13-15. The measured CCE and ~-particle resolution is indicated in each curve. The I
m,
J
I
~
Detector SB- 2
pozlof~-"~
12ol-
Age 39 doys
Joo -J w z
t
r
I
J
/,~"k x
moo
5.7 MeV
Current I I.5~A Joo~
•
9(]--
3
70 heY
_
0:
60 --
--
5 0 --
70 ~ keV 500
~ 4o o~ 3oL
/
o;/
;\
.
170 m
DO[
t75
;/
J79
m3 Je4 m5
CHANNEL
NUMBER
19o 191
Fig. 9. R e s p o n s e o f detector SB-2 to 210po at-particles d u r i n g t h e r m a l - v a c u u m test. T e m p e r a t u r e w a s 65 ° C a n d pressure 5 x 10-5 m m H g . D e t e c t o r a g e was 39 d. I
1
I
'
Pov o j 6000
3~'-'
I
l/
I m,
r
,
Detector S B - 2 Age 74 days
(99.7%)
--
I
' "
'
l
I
l
Bias 15V Current 0.99,u.A
Room Temp. 5000
2000
--
--
.A
_ z
5.ZM,V
1600
4 0 0 0 -
~'
70
I_
heV 3000
1200
--
t
o_
~
48
key z
800
2 0 0 0 -
o k) IOOO
4 0 0 --
--
,
'/
i64 '65
T/
,
i
~
I 170
T~L
I
'72 i73 '74 CHANNEL
,~
-, .__ '80
/ p
,
i
"
I "~'l.
I
'85 186 '87
NUMBER
Fig. 10. R o o m temperature response o f detector SB-2 to 210po at-particles. D e t e c t o r a g e was 74 d.
LITHIUM-DRIFTED SILICON SURFACE-BARRIER DETECTORS J
5000
--
~
I
J
J
!
'
p~'O ( 9 9 . 5 % ) ' / ~ ' ~ o t ' ~
,
,
I
'
~'
I
I
I
Detector S B - 2 Age 81 doys Room Temp.
I
t
I
Bios 15V Current I,I/J.A
.,---.,,
4OOO z
z
E
'
213
~.T MeV
J\
1800 3000 1200
o 600 I000
162
i\
/ 165
170 171
%
172
181
18:5
185
NUMBER Fig. 11. Room temperature response of detector SB-2 to zlOpo=-particles. Detector abe was 81 days. CHANNEL
applied bias was 15 V. No calibrated pulser spectra are shown. Curve (a) of fig. 13 shows that the for =particles of lowest energy (5.42 MeV), the CCE is 98.2%, the =-resolution is 143 keV and the spectrum has a significant low-energy tail. This response is essentially identical to the response of SB-3 to 21°po =particles following the t-v testing and aging. However, as the energy of the incident c<-particles increases; i.e., as the ionization density near the sensitive surface decreases, the CCE and =-resolution improve, and the low energy tail becomes less pronounced. Curve (f) of fig. 15 shows that for ~-particles of energy 8.78 MeV, the charge collection is complete and the low-energy tail is absent. Figs. 13-15 illustrate the strong dependence of recombination losses on ionization density 7) and appear to verify the conclusion 2) that for those detectors which show degradation resulting from t-v testing and aging, changes have been produced in the recombination properties near the sensitive surface which are, however, important only for heavily ionizing particles. Measurements of the n-region depths and detector capacitance taken before, during and after the t-v testing and aging indicated that the overall increase in intrinsic depth for all detectors studied was < 5%. During most of the study program described above, no importance had been placed on the resistivity of the silicon used to fabricate the detectors. However, in the latter phases of our study, an apparent dependence of detector zl°Po ~-particle stability on the resistivity of
the silicon used to make the detector was noticed. It was found that detectors which were made from 1000 I
Illll~llllll
I
I
I
I
Detector SB-2 Age 9 4 doys Room Temp. Bias 15 V Current I,I p.A
3000
Po21° ( g e . e % ) /
-
?
2000
5.7 M eV Puls
2
--
I,~9
leE,
170 CHANNEL
181
183
185
NUMBER
Fig. ]2. Room temperature response of detector $B-2 to 210Po
~-l~rticles. I~tec{or age was 94 dws.
214
A.J. "-i~l
600
T U Z Z O L I N O et al.
i i j i i i i j [ i i i i r i i i i , i i i t i i i i r
I r j , ,
Detector SB-5 B i a s 15 V
--
t'~
S
500
98.2%
(o)
(b) 9 8 . 7 %
_/t
i400 Q. 3 0 0
P0 200 t) I00
.,,,
./
/
/
,/"
\
,s
\
i I I I i , I I i I I I I i ",~',"/"I Ii f I I I 250 255 260 265 270 CHANNEL NUMBER
~?~ 245
I I i\,% '-~ -' -' I
::","5 " ' ' Vi30
Fig. 13. R o o m temperature response of detector SB-3 to Th ~¢-particles. Spectrum (a) results from incident ~-particles of energy 5.42 MeV. Spectrum (b) results from incident or-particles of energy 5.68 MeV. The CCE a n d or-resolution is indicated in each curve.
o h m . c m silicon rather than 100 ohm.era silicon exhibited better characteristics. This was found to be true for both B-W and N-B-W detectors.To study this effect in more detail, four SB detectors were fabricated from 1000 ohm" cm silicon. All four detectors had a sensitive i r i i i i I [ f i f i i i [ i i i i I Detector 600
Bios
SB-5
15 V
/f(d)
99.4%
-
area of 1.2 cm 2 and intrinsic depths ranging from 300/~m to 330 #m. They were not given the boiling-water treatment. All data to be discussed was taken at 15 V. These N-B-W detectors had initial currents ranging from 1.2 # A to 1.4 #A, similar to the initial currents of the N-B-W detectors made from 100 ohm.cm material. N o initial 21°Po spectra were taken. They were allowed to age in the laboratory for two months (no
/[f~/ 7OO
I
• z
500
i
z
keY
,oo285
290
•
/ ,L_E v / 2~5
CHANNEL
/
-
200 I00
300
I
AV--
I
t
1
1
I
[
I
I
I
I
[
I
Detector SB-3 Bias 15V
66_ keV
3OO
(c) 9 9 . 2 %
/
[
4OO
6e
300
I
e)99.6%
.t~'(f ) 100.0%
5OO
400
I
305
NUMBER
Fig. ]4. R o o m t e m p e r a t u r e response o f detector SB-3 to "fh =-particles. S p e c t r u m (c) results f r o m incident or-particles of
energy 6.05 MeV. Spectrum (d) results from incident or-particles of energy 6.28 MeV. The CCE a n d =-resolution is indicated in each curve. The O 2 E for spectrum (c) is approximate.
315
; 520
/ 325
\ ^ jf,,,,~.,,,
330 * 420 425 CHANNEL NUMBER
430
Fig. 15. R o o m temperature response of detector SB-3 to Th a-particles. Spectrum (e) results from incident or-particles of energy 6.78 MeV. Spectrum (f) results from incident at-particles of energy 8.78 MeV. The CCE and or-resolution is indicated in each curve.
LITHIUM-DRIFTED
SILICON SURFACE-BARRIER DETECTORS
applied bias). After this aging period, the currents were in the range from 0.2 IrA to 0.3 #A, and all four detectors had a CCE and or-resolution for 21°po orparticles of 9 9 . 7 ~ and 53 keV, respectively. Also, the spectra showed no low-energy tails. The behavior (high CCE and good or-resolution) exhibited by these four detectors following the two month aging period is distinctly different than the behavior of the N-B-W detectors made from 100 o h m . c m material (such as SB-2). Table 2 shows that the N-B-W detectors made from 100 o h m . c m material suffer a loss in CCE of from 0 . 5 ~ to 1.0~o and a loss in or-particle resolution during a much shorter period of time (six weeks) following fabrication. The four "high-resistivity" detectors were then mounted in a vacuum chamber and maintained at 50 ° C and a pressure of 5 x 10- ~ mm Hg for seven days, with the bias voltage applied continuously. During this test, the maximum currents were in the range from 3.9/~A to 4.1 #A and the maximum pulser resolutions were in the range from 54 keV to 60 keV. After this test, the CCE for 21°po was found to be 99.9~o for all four detectors. The detectors were allowed to age for two weeks (no applied bias). After this aging, the CCE was again found to be 99.9 ~o for all four detectors and the spectra still had no low-energy tails. In addition, the leakage currents were essentially equal to their initial values. Thus, no degradation occurred during the twoweek aging period following the t-v test. This behavior is also different than that of N-B-W detectors made from low resistivity material, which show degradation in or-particle characteristics after this same period of time following at t-v test (table 2). Thus, N-B-W detectors made from 1000 o h m . c m silicon appear to have more stable 2~°Po or-particle characteristics than N-B-W detectors made from 100 ohm. cm material, both before and after t-v testing. To further study the effects of detector resistivity, a B-W detector having the same area and intrinsic depth as SB-1 was made from 100 o h m ' e r a material. The detector was put through a sequence of aging and environmental testing similar to that given to SB-1. Although the initial current and CCE of this detector were identical to the initial values for SB-1, it showed some degradation in 2~°Po ~t-particle characteristics during aging, after both environmental tests. Thus B-W detectors, as well as N-B-W detectors, made from 1000ohrn'cm material have more stable 2~°Po ~particle characteristics than detectors made from 100 ohm. cm material. Since the N-B-W detectors made from I000 ohm" cm material were not studied as extensively as the B-W
215
detectors made from 1000 ohm, cm material, it is difficult to decide to what extent the boiling-water treatment was responsible for the long term stability of SB-I and to what extent the use of material of 1000 o h m - c m to make SB-I was responsible. However, the importance of the boiling-water treatment is clearly demonstrated by comparing the initial characteristics of the B-W and N-B-W detectors made from 1000 o h m . c m material. For the B-W detectors, the initial currents were in the range from 0.1 #A/cm 2 to 0.2/~A/cm 2 and the initial CCE was, in most eases, 100.0~. For the N-B-W detectors, the initial currents were approximately ten times greater and only after aging for two months did the currents approach the currents of the B-W detectors. Also, the CCE for the N-B-W detectors was less (99.7~/o) than for the B-W detectors. A comparison of the data for the B-W detector made from 100 ohm.era material with the data of N-B-W detectors made from 100 o h m - c m material (SB-2) shows that the initial leakage currents of the.N'~B-W detectors were approximately ten times greater than the B-W detector and the CCE of the N-B-W detectors (99.2~-99.8%) was less than the CCE of the B-W detector (100.0~o). The above data shows that for given resistivity material, B-W detectors have lower initial leakage current and higher initial CCE than N-B-W detectors. For both B-W and N-B-W detectors, those made from 1000ohm.cm material have more stable 21°po ~particle characteristics than those made from 100 ohm. cm material. 4. Conclusions One feature which was common to the behavior of all SB detectors studied was the absence of any kind of noise breakdown during environmental testing at both 65°C and 50°C. All B-W detectors studied had initial leakage currents in the range from 0.1 pA/cm 2 to 0.2 #A/cm 2, and initial values of CCE in the range 99.7~0 to I00.0~o. All B-W detectors made from 1000 ohm-cm material exhibit stable electrical and 21°po a-particle characteristics under conditions of aging and t-v testing over a period of four months. B-W detectors made from 100 ohm. em material show some degradation in 21°Po or-particle characteristics when aged following t-v testing. N-B-W detectors made from 100 ohm-cm material had initial leakage currents in the range from 2/~A/cm 2 to 10 #A/cm 2 and had initial values of CCE in the range from 9 9 . 2 ~ to 99.8~. N-B-W detectors made from 100 o h m . c m material show a toss in CCE and orresolution for 21°Po or-particles wh¢~ aged ~nder am-
216
A.J. TUZZOLINO et al.
bient laboratory conditions. This degradation occurs both before and after t-v testing. N-B-W detectors made from 1000 ohm-cm silicon show no degradation in a-particle characteristics when aged for two months following fabrication. Subsequent t-v testing and aging over a period of three weeks produces no degradation. For given resistivity material, B-W detectors have lower initial leakage current and higher initial CCE than N-B-W detectors. Both B-W and N-B-W detectors made from 1000ohm.cm silicon have more stable 21°po a-particle characteristics t h a n detectors made from 100 o h m . c m silicon. The degree of degradation in or-particle characteristics suffered by N-B-W detectors made from 100 o h m ' c m material was shown to depend on ionization density. A degraded detector which had a CCE of 98.2,%0 for 5.42 MeV a-particles had a CCE of 100.0% for 8.78 MeV or-particles. The data at 65°C and 50°C for B-W detectors made from 1000ohm:cm silicon indicates that the fractional change in the mean energy per ion pair over the temperature range from room temperature to 65°C is approximately - l . 3 x l 0 - * / ° C . For all detectors studied, the overall increase in intrinsic depth resulting from the environmental testing and aging was < 5 % .
The authors wish to express their thanks to G. Ho for his help in fabricating the detectors, to P. Shen for his help in various aspects o f the testing program and to J. Lamport, Dr. J. Simpson, Dr. A. Turkevich and Dr. C. Fan for their continued encouragement and helpful criticisms. The research reported in this paper was supported by the National Aeronautics and Space Administration under NASA Grant NsG-179-61.
References 1) G. Dearnaley and J. C. Lewis, Nucl. Instr. and Meth. 25 (1964) 237. 2) A. J. Tuzzolino, J. Kristoff and M. A. Perkins, Nucl. Instr. and Meth. 36 (1965) 73. 3) j. L. Blankenshipand C. J. Borkowski, IRE Trans. NucL Sci. NS-9 (1962) 181. 4) j. A. Coleman and J. W. Rodgers, IRE Trans. Nucl. Sci. NS-11 (1964) 213. 5) F. S. Goulding, IEEE Trans. Nucl. Sci. NS-11 (1964) 177. 6) F. R. Shea, Editor, IRE Trans. Nacl. Sci. NS-8 (1961). 7) G. L. Miller and W. M. Gibson, Nucl. Electr. 1 (1962) 477. 8) C. A. J. Ammerlaan,R. F. Rumphorst and L. A. Ch. Koerts, Nucl. Instr. and Meth. 22 (1963) 189. 9) A. B. Gillespie, Signal, Noise and Resolution in Nuclear Counter Amplifiers (Pergamon Press, London, 1953). z0) C. Bussolati, A. Fiorentini and G. Fabri, Phys. Rev. 136 (1964) A1756. i1) W. Shockley,Czech. J. Phys. BI1 (1961) 81. 12) E. D. Klema, Nucl. Instr. and Meth. 26 (1964) 205.