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Surface barrier detectors made of ultra-high purity p-type silicon crystals
Nuclear Instruments and Methods 196 (1982) 137-141 North-Holland Publishing Company
137
S U R F A C E BARRIER D E T E C T O R S M A D E OF U L T R A - H I G H P U R I T Y P - T Y P E SILICON CRYSTALS F. S H I R A I S H I a n d Y. T A K A M I Institute for Atornic Energy, Rikkyo University, Yokosuka, Kanagawa 240-01, Japan
Surface barrier detectors have been fabricated with new high purity p-type silicon crystals whose resistivity is greater than 80 kflcm, and their charge collection behavior has been extensively investigated. The thickness of the detectors ranges from 3.1 to 13.8 ram. It was found that the depletion layers extend from both faces, front and rear, at room temperature, but that they extend from the front aluminum face only at liquid nitrogen temperature. Totally depleted thick detectors of excellent characteristics can easily be fabricated with these silicon crystals. The fabrication procedure and charge trapping effect will also be discussed in the paper.
1. Introduction Ultra-high purity p-type silicon crystals * with resistivity higher than 80 k~2cm become available from raw materials which are produced by decomposition of mono-silane purified by means of the molecular sieve method to the grade of phosphorus impurities of less than 0.01 ppb. Usually, surface barrier detectors are produced by using n-type silicon crystals. In regard to depletion layer thickness for a given bias, this p-type silicon will be equivalent to the n-type silicon with a resistivity of 30 kf~cm which corresponds to the highest resistivity of available n-type silicon. Also the uniformity of resistivity in a crystal is better for the p-type than for the n-type, because boron impurities distribute more uniformly in a crystal than donor impurities. The p-type silicon, in contrast to n-type silicon, is applicable to the fabrication of thick detectors. It also seems useful that surface barrier detectors with a wide depletion depth will be produced by this new p-type silicon, comparable to that of the Si(Li) detector, because it will not require troublesome thermal diffusion of lithium and also offer a contact with a very thin dead layer. In producing the surface barrier contact with the p-type silicon wafer, surface cleaning by means of the conventional etching process is not sufficient alone, because this process causes a strong inversion of the surface layer and this results in an increase of leakage current. Many authors have reported various preparation procedures. In our preliminary experiments, it was
* Supplied by Komatsu Electronic Metals Co. 0029-554X/82/0000-0000/$02.75 © 1982 North-Holland
found that p-type silicon surface barrier detectors are fabricated successfully by immersing the wafer in K2Cr20 v solution after the etching. This surface treatment is controllable and effective for reducing leakage currents as small as that of the Si(Li) detector. In this paper, the preparation procedure of the detector using the new ultra-high purity p-type silicon and the performance for alpha particle and electron detection will be described. Next, the charge collection behavior investigated by alpha particles and depletion layer formation of these detectors will be discussed.
2. Preparation of detectors Three thick p-type silicon surface barrier detectors were fabricated. The thicknesses of these detectors were 3.1, 7.4 and 13.8 mm, respectively. The wafers had a diameter of 25 mm and a resistivity of about 80 kflcm. After etching, the wafers were immersed in a HFsolution for 5 min, then rinsed with de-ionized water. After this process the wafer has a strong inversion layer. Next, the wafers were dipped in a solution of 1% KzCr20 7 with 5% HzSO 4 for 30 s, and were rinsed well with de-ionized water, then dried in a desiccator. As the K2CrzOv-solution turns the silicon surface into p-type, one can control the surface condition by this process. For example, a shorter dipping time in the solution will give a larger leakage current and a longer dipping time will result in a lower breakdown voltage. The electrodes of the front and the rear surfaces of the detector were produced by vacuum evaporation of aluminum and gold, respectively. The electrodes had an area of 1.0 cm2: No edge protection of the electrodes was applied and the wafers were held between two teflon disks. III. SOLID STATE DETECTORS
138
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3. Experimental results and discussion
10-8F.
.
.
.
.
.
.
.
.7.1. Leakage current The leakage currents as a function of bias voltage at room temperature are shown in Fig. 1. The two lower curves show the leakage currents of thin detectors fabricated by the same method. The leakage currents are relatively flat and clearly depend on the detector thickness. The leakage currents at 77 K are shown in fig. 2. Steep increases are seen when the detectors are fully depleted. The rear contact may be responsible for this sharp rise.
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3.2. Charge collection The pulse-height saturation curves were obtained by alpha particles from 241Am, injected to both front and rear faces at room and liquid nitrogen temperatures• Fig. 3 shows the pulse height saturation curves of the detector with a thickness of 3.1 mm. The detector is totally depleted at about 400V. The two solid lines indicating charge collection at room temperature show no significant difference between the front and the rear face injection of alpha particles. The dashed lines, indicating charge collection from the front aluminum face injection of alpha particles at liquid nitrogen temperature, become more efficient than the rear gold injection at low biases. The same tendency is observed for the other two detectors. In the pulse-height saturation curves for aluminum face injection, the small deviation from the 100% saturation value comes from tailing of the pulse-height distribution. This is due to the hole trapping effect• Fig. 4 shows the pulse-height distribution of three different alpha sources, obtained for front and rear face
Fig. 2. Leakage currents at 77 K.
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REVERSE BIAS fV) Fig. 3. Charge collection of a 3.1 mm detector; RT and CT indicate temperature of room and 77 K, respectively. AI and Au indicate alpha particle injection faces.
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Fig. 1, Leakage currents at room temperature. Figures indicate detector thickness in ram.
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Fig. 4. Alpha particle spectra of 241Am,obtained using a 3.1 mm detector at 77 K.
F. Shiraishi, 17. Takami / P-type silicon c~stals
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CH. NO
Fig. 5. Conversion electron spectrum of 2°7Bi, obtained using a 3.1 mm detector at 77 K.
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a injection• A resolution of 14 keV fwhm was obtained at liquid nitrogen temperature. Fig. 5 shows the spectrum of conversion electrons from 2°7Bi. A resolution of 3.3 keV fwhm was obtained at liquid nitrogen temperature. As shown in fig. 6, the detector with a thickness of 7.4 mm shows a saturated pulse-height for alpha particles injected at the rear gold face, but the pulse-height for the aluminum front face does not attain 100% saturation value. This degradation is attributed to the hole trapping effect. Fig. 7 shows the pulse-height saturation behavior of this detector. The multiple peaks are seen for aluminum face injection of alpha particles even at high biases. This indicates that the hole trapping is due to centers with a deep level. Such a trapping effect was confirmed by the pulse shape of the preamplifier output. The two pulse-height distributions for injection of alpha particles at both faces are compared in fig. 7. As shown in fig. 8, in the detector with a thickness of 13.8 mm the charge collection of alpha particles injected to the rear face is also observed at room temperature. It
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Fig. 7, (a) Alpha particle spectra injected to A1 face at several detector biases. (b) Alpha particle spectra injected to Au and AI faces respectively•
does not attain 100% saturation value even at a reverse bias of 3 kV, and the charge collection is not observed at liquid nitrogen temperature. Figs. 9 and 10 show the
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Fig. 6. Charge collection of a 7.4 mm detector.
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Fig. 8. Charge collection of a 13.8 mm detector. III. SOLID STATE DETECTORS
140
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pulse-height saturation behavior of this detector at room temperature, for the aluminum front and for the gold rear face, respectively. The saturation curves of the three detectors for the front face injection of alpha particles are shown in fig. I1. The same charge collection is now observed for given biases, independent of the detector thickness. The charge collection is much improved at low temperature, as shown by the dashed lines.
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Fig. I 1. Charge collection of three detectors obtained by alpha particles injected at the A1 face. Solid and dashed lines indicate room temperature and 77 K, respectively.
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The saturation curves of the three detectors for the rear face injection of alpha particles are shown in fig. 12. At low temperature, quite contrary to the front aluminum face injection, the saturation curves shift towards higher voltage. From this figure, if one assumes that the depletion layer extends only from the aluminum side, and that the depletion depth is determined by the square root of oV, the resistivity value O must be much higher than that of intrinsic silicon. Therefore, the assumption that the depletion layer extends only from the aluminum face is not correct, and the layers extend from both surfaces of aluminum and gold at room temperature. This result can be explained qualitatively by the differences of the work functions between aluminum and silicon, for which the Fermi level is located near the middle of the band gap, and between silicon and gold. Furthermore, it is also explained by a similar consideration that the depletion layer does not extend from the gold face at liquid nitrogen temperature, because the shift of the Fermi level towards the valence band due to lowering temperature does not result in barrier formation on the gold face of the silicon.
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CH. NO. Fig. I0. Spectra of alpha particles injected at the Au face of a 13.8 mm detector.
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f
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100 REVERSE BIAS (v)
l
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Fig. 12. Charge collection of three detectors obtained by alpha particles injected at the Au face.
F. Shiraishi, Y. Takami / P-type silicon cr),stals
4. Conclusion Thick p-type silicon surface barrier detectors with practically no dead layer can be easily fabricated by this method. Until now, good totally depleted detectors with
141
4 mm thickness have been made. It was experimentally confirmed that the depletion layer of the ultra-high purity silicon extends from both faces of the silicon crystal. This indicates that these silicon crystals behave almost like intrinsic silicon.
III. SOLID STATE DETECTORS
137
S U R F A C E BARRIER D E T E C T O R S M A D E OF U L T R A - H I G H P U R I T Y P - T Y P E SILICON CRYSTALS F. S H I R A I S H I a n d Y. T A K A M I Institute for Atornic Energy, Rikkyo University, Yokosuka, Kanagawa 240-01, Japan
Surface barrier detectors have been fabricated with new high purity p-type silicon crystals whose resistivity is greater than 80 kflcm, and their charge collection behavior has been extensively investigated. The thickness of the detectors ranges from 3.1 to 13.8 ram. It was found that the depletion layers extend from both faces, front and rear, at room temperature, but that they extend from the front aluminum face only at liquid nitrogen temperature. Totally depleted thick detectors of excellent characteristics can easily be fabricated with these silicon crystals. The fabrication procedure and charge trapping effect will also be discussed in the paper.
1. Introduction Ultra-high purity p-type silicon crystals * with resistivity higher than 80 k~2cm become available from raw materials which are produced by decomposition of mono-silane purified by means of the molecular sieve method to the grade of phosphorus impurities of less than 0.01 ppb. Usually, surface barrier detectors are produced by using n-type silicon crystals. In regard to depletion layer thickness for a given bias, this p-type silicon will be equivalent to the n-type silicon with a resistivity of 30 kf~cm which corresponds to the highest resistivity of available n-type silicon. Also the uniformity of resistivity in a crystal is better for the p-type than for the n-type, because boron impurities distribute more uniformly in a crystal than donor impurities. The p-type silicon, in contrast to n-type silicon, is applicable to the fabrication of thick detectors. It also seems useful that surface barrier detectors with a wide depletion depth will be produced by this new p-type silicon, comparable to that of the Si(Li) detector, because it will not require troublesome thermal diffusion of lithium and also offer a contact with a very thin dead layer. In producing the surface barrier contact with the p-type silicon wafer, surface cleaning by means of the conventional etching process is not sufficient alone, because this process causes a strong inversion of the surface layer and this results in an increase of leakage current. Many authors have reported various preparation procedures. In our preliminary experiments, it was
* Supplied by Komatsu Electronic Metals Co. 0029-554X/82/0000-0000/$02.75 © 1982 North-Holland
found that p-type silicon surface barrier detectors are fabricated successfully by immersing the wafer in K2Cr20 v solution after the etching. This surface treatment is controllable and effective for reducing leakage currents as small as that of the Si(Li) detector. In this paper, the preparation procedure of the detector using the new ultra-high purity p-type silicon and the performance for alpha particle and electron detection will be described. Next, the charge collection behavior investigated by alpha particles and depletion layer formation of these detectors will be discussed.
2. Preparation of detectors Three thick p-type silicon surface barrier detectors were fabricated. The thicknesses of these detectors were 3.1, 7.4 and 13.8 mm, respectively. The wafers had a diameter of 25 mm and a resistivity of about 80 kflcm. After etching, the wafers were immersed in a HFsolution for 5 min, then rinsed with de-ionized water. After this process the wafer has a strong inversion layer. Next, the wafers were dipped in a solution of 1% KzCr20 7 with 5% HzSO 4 for 30 s, and were rinsed well with de-ionized water, then dried in a desiccator. As the K2CrzOv-solution turns the silicon surface into p-type, one can control the surface condition by this process. For example, a shorter dipping time in the solution will give a larger leakage current and a longer dipping time will result in a lower breakdown voltage. The electrodes of the front and the rear surfaces of the detector were produced by vacuum evaporation of aluminum and gold, respectively. The electrodes had an area of 1.0 cm2: No edge protection of the electrodes was applied and the wafers were held between two teflon disks. III. SOLID STATE DETECTORS
138
t:: Stll#'alshl,
}'. 7"~lka##ll !" lJ-ll7)~ , w/i('otl CIT,~'lai.Y
3. Experimental results and discussion
10-8F.
.
.
.
.
.
.
.
.7.1. Leakage current The leakage currents as a function of bias voltage at room temperature are shown in Fig. 1. The two lower curves show the leakage currents of thin detectors fabricated by the same method. The leakage currents are relatively flat and clearly depend on the detector thickness. The leakage currents at 77 K are shown in fig. 2. Steep increases are seen when the detectors are fully depleted. The rear contact may be responsible for this sharp rise.
.
10-9
z -I0
T
,
•
UJ
,
.,.':
Y'
//'/
.,
•
1t
'
10-12 10 100 REVERSE BIAS IV)
3.2. Charge collection The pulse-height saturation curves were obtained by alpha particles from 241Am, injected to both front and rear faces at room and liquid nitrogen temperatures• Fig. 3 shows the pulse height saturation curves of the detector with a thickness of 3.1 mm. The detector is totally depleted at about 400V. The two solid lines indicating charge collection at room temperature show no significant difference between the front and the rear face injection of alpha particles. The dashed lines, indicating charge collection from the front aluminum face injection of alpha particles at liquid nitrogen temperature, become more efficient than the rear gold injection at low biases. The same tendency is observed for the other two detectors. In the pulse-height saturation curves for aluminum face injection, the small deviation from the 100% saturation value comes from tailing of the pulse-height distribution. This is due to the hole trapping effect• Fig. 4 shows the pulse-height distribution of three different alpha sources, obtained for front and rear face
Fig. 2. Leakage currents at 77 K.
~i00~3_2
ol
--[
,oo
/
:.i:-:~"-----
,ooo
REVERSE BIAS fV) Fig. 3. Charge collection of a 3.1 mm detector; RT and CT indicate temperature of room and 77 K, respectively. AI and Au indicate alpha particle injection faces.
Am-241
Cm-244
i'
)
Au i'! /
I_-
Io
Pu-239
'°It;
1000
,
1 "3 . 8 J yI t ~ ' J
p ! . /'/14 KeY
,"'. .O] i ~ - -
I
I I0 I00 REVESE BIASIV}
lOOO
Fig. 1, Leakage currents at room temperature. Figures indicate detector thickness in ram.
,"
650
iti t +
..
700 CH.NO.
Fig. 4. Alpha particle spectra of 241Am,obtained using a 3.1 mm detector at 77 K.
F. Shiraishi, 17. Takami / P-type silicon c~stals
Ek976 KeY
139
Bi-20'7
T.P
Am-241 d
400V
T.P
¢.... i~v,,,~.l ev
2.2 KeV I ~ e L10/.,,8KeVFWHM i r
tlii .
.
.
.
.
.
.
.
.
:~,;-
.
.
• .
%.
Z
.."". F_~O60KeV."
1900
2000
...................
0.-- ......
~.....~:...-~.`.:: ~.~......--....-~..:.....-.:....`~.~....'.-.-:-`....~.......^.-:.` """""'"". ."%r": ... '""
-:
CH. NO
Fig. 5. Conversion electron spectrum of 2°7Bi, obtained using a 3.1 mm detector at 77 K.
:
).--
c.Pl -2 / (7./,,mm)
]
I
' .
.',s: /, ..;~J :t.~j ,~-
.
.
.
.
lO0V
;~',':'E'-.'..~-~'tr~:,... -.y
a injection• A resolution of 14 keV fwhm was obtained at liquid nitrogen temperature. Fig. 5 shows the spectrum of conversion electrons from 2°7Bi. A resolution of 3.3 keV fwhm was obtained at liquid nitrogen temperature. As shown in fig. 6, the detector with a thickness of 7.4 mm shows a saturated pulse-height for alpha particles injected at the rear gold face, but the pulse-height for the aluminum front face does not attain 100% saturation value. This degradation is attributed to the hole trapping effect. Fig. 7 shows the pulse-height saturation behavior of this detector. The multiple peaks are seen for aluminum face injection of alpha particles even at high biases. This indicates that the hole trapping is due to centers with a deep level. Such a trapping effect was confirmed by the pulse shape of the preamplifier output. The two pulse-height distributions for injection of alpha particles at both faces are compared in fig. 7. As shown in fig. 8, in the detector with a thickness of 13.8 mm the charge collection of alpha particles injected to the rear face is also observed at room temperature. It
20a/
...........
O
"::':':"""
30Ch/
,50V
........ CH.NQ
_i~., .~1.0 Kv
;I
._AL
ot
A.~q& I.P
Am-2/4 d
1.0 KV
~
}
T.P
i
[
b
CHNQ
Fig. 7, (a) Alpha particle spectra injected to A1 face at several detector biases. (b) Alpha particle spectra injected to Au and AI faces respectively•
does not attain 100% saturation value even at a reverse bias of 3 kV, and the charge collection is not observed at liquid nitrogen temperature. Figs. 9 and 10 show the
~100
3-2
(13.~jrnrn}
,¢i~f"i-i-/~-*'---~-*I
....
b~
a_ 50
RT-AI ,-..~'~- 7 C T - A / / ~ / '/ RT-Au
10
sc
CT-Au
IO0 REVERSE BIAS (V)
hO00
Fig. 6. Charge collection of a 7.4 mm detector.
/
_,,.z_/~RI-AI
10
I00
REVERSE BIAS (V)
lO00
Fig. 8. Charge collection of a 13.8 mm detector. III. SOLID STATE DETECTORS
140
t:~ 5,'t~irat~'hc ). Taka,,m ; /'.(lT~e ~Atc
Am-241 c~ [Bi-207 !:1 {'
•
i ~'" fi "
C~:= __-
" col
,; ! ::_":
"
Zl
,
. . . . . .
;
][IP " 500V
,',
," .~
13.4V
...................... 1V
pulse-height saturation behavior of this detector at room temperature, for the aluminum front and for the gold rear face, respectively. The saturation curves of the three detectors for the front face injection of alpha particles are shown in fig. I1. The same charge collection is now observed for given biases, independent of the detector thickness. The charge collection is much improved at low temperature, as shown by the dashed lines.
Am-2.41 d, Au ;, I P - i
i: !(t
Z ~)l~I
i
.....,,::7'v'.
~ l'~i
....j . ~7/ / o3 ~
a cu
/ I
, I, I /--'q'/,f°- (
o~ o
.--I r
~
i -:-
N
]
"-~ * " /,
.--U 7Z..'" y,~,
j
I0
' t
i
r i
L ......
k___l_._ i 000 REVERSE BIAS (VJ
J
1 O0
Fig. I 1. Charge collection of three detectors obtained by alpha particles injected at the A1 face. Solid and dashed lines indicate room temperature and 77 K, respectively.
.:
CH. NO Fig. 9. Spectra of alpha particles injected at the AI face of a 13.8 mm detector.
~ i [ 'k+,.
,
50V
:'!s
iRi-207_)_
©
i
......
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. . . .
L ......... ---. _ _ :
I :\:!~/
~ ~-IENTRA~[_
,:,
1370V
~'i 880 V
The saturation curves of the three detectors for the rear face injection of alpha particles are shown in fig. 12. At low temperature, quite contrary to the front aluminum face injection, the saturation curves shift towards higher voltage. From this figure, if one assumes that the depletion layer extends only from the aluminum side, and that the depletion depth is determined by the square root of oV, the resistivity value O must be much higher than that of intrinsic silicon. Therefore, the assumption that the depletion layer extends only from the aluminum face is not correct, and the layers extend from both surfaces of aluminum and gold at room temperature. This result can be explained qualitatively by the differences of the work functions between aluminum and silicon, for which the Fermi level is located near the middle of the band gap, and between silicon and gold. Furthermore, it is also explained by a similar consideration that the depletion layer does not extend from the gold face at liquid nitrogen temperature, because the shift of the Fermi level towards the valence band due to lowering temperature does not result in barrier formation on the gold face of the silicon.
~ZlOC
Au- E N T R A N C E _ _
! /,"'/ (
t..!J
4
sc
N
I~7.~,#i !
!' 410V i'
CH. NO. Fig. I0. Spectra of alpha particles injected at the Au face of a 13.8 mm detector.
i
f
13.8 . . . .
lO
100 REVERSE BIAS (v)
l
1003
Fig. 12. Charge collection of three detectors obtained by alpha particles injected at the Au face.
F. Shiraishi, Y. Takami / P-type silicon cr),stals
4. Conclusion Thick p-type silicon surface barrier detectors with practically no dead layer can be easily fabricated by this method. Until now, good totally depleted detectors with
141
4 mm thickness have been made. It was experimentally confirmed that the depletion layer of the ultra-high purity silicon extends from both faces of the silicon crystal. This indicates that these silicon crystals behave almost like intrinsic silicon.
III. SOLID STATE DETECTORS