- Email: [email protected]
On the behaviour of high resistivity Si surface barrier detectors
NUCLEAR INSTRUMENTS
AND M E T H O D S 33
(1965) 2 9 3 - 2 9 7 ; ~ N O R T H - H O L L A N D P U B L I S H I N G CO,
O N THE B E H A V I O U R OF H I G H RESISTIVITY Si S U R F A C E BARRIER D E T E C T O R S C. BUSSOLATI
and
M. BERTOLACC1NI and S. COVA
Istituto Eli Fisica Sperimentale, Politecnico, Milano, Italy
Gruppo di Ricerca 1.N.F.N. Pofitecnico di Milano, Italy
Received 7 October 1964 An investigation has been performed on the behaviour of high resistivity (21 k,Q.cm) silicon surface barrier detectors in view of their application to electron spectroscopy. Our detectors can develop depletion layers 1.3 mm thick and have been used to detect Cs 137 and Sn ll3 internal conversion electrons. Some anomalous effects, consisting in poor energy resolution and dis-
1. Introduction In recent years semiconductor junction detectors have been widely used. in radiation energy measurements and are now employed with a large variety of ionizing radiations ranging from heavy ions and fission fragments to electrons and 7-raysl'2). One of the main problems connected with the detection of electrons is to obtain sufficiently thick depletion layers. In silicon surface barrier detectors3), the thickness of the depletion layer w (p) depends on the resistivity p (f2. cm) and the bias voltage V(V) as: w = 0.53 (p- V)~. Therefore it seems convenient to use high resistivity silicon. We used detectors made at CISE Labs. of Milano out of 21 kf2.cm silicon and having an useful area of 155 mm2; depletion layers of 1.3 mm can be obtained with a bias of 325 V. On the other hand large inhomogeneities in resistivity may be expected in 21 kf2.cm silicon owing to its low donor concentration and perhaps to the presence of compensating impurities. As a matter of fact resistivity measurements performed on the silicon slices from which detectors are made, gave different results from point to point. The present investigation was carried out to test the behaviour of these detectors and to detect possible spurious effects connected with the inhomogeneities quoted above. 2. Electron spectroscopy The electronic chain (fig. 1) consists in a charge preamplifier4), a linear voltage amplifier, a main amplifier including an R C - R C shaping network and a multichannel pulse-height analyzer. The same R C - R C shaping network, having equal integrating and differentiating time constants of about 1.2/~sec, has been used throughout the whole investigation. The electrical noise of the chain s) having at the input only the parasitic capacitance (-~ 25 pF), is equivalent to 550 electrical charges (fwhm-~ 5 keV). The whole
tortion of the spectra, have been observed cooling the detectors (T ~ 260 ° K), with low bias voltage. These effects are probably due to inhomogeneities of Si that may cause nonuniform trapping of charge carriers in the detector; they can be reduced by increasing the bias and by collimating the beam of ionizing particles so as to work with only a small portion of the surface of the detector.
3_
Fig. 1. Block scheme of electronic chain.
chain, tested observing, with a 1.8 kO-cm Si surface barrier detector, the spectrum of Am 24~ a-particles, proved to work correctly. To test the behaviour of 21 kf2.cm detectors we have used them to detect the internal conversion electrons of CS 137 and Sn 1~3. Spectra of these electron sources have been taken both at room temperature and at a lower one (260 ° K) in the aim of obtaining, by a reduction of the leakage current in the detector, a better energy resolution. The spectrum of CS 137 reported in fig. 2 is obtained at room temperature. In fig. 3 a spectrum recorded at 260°K shows that, while the electrical noise has been effectively reduced (as indicated by the artificial line superimposed), the fwhm of the electron line is abnormally large and the shape of the line itself is distorted. In the next paragraphs we shall deal with these anomalous effects. Here it shall suffice to observe that they can be reduced both by increasing the bias voltage of the detectors and by collimating the impinging beam of electrons on a small area of the detector surface. Operating in this way we obtained the spectra of fig. 4 for CS 137 and of fig. 5 for Sn 113. 3. Anomalous effects We thought it interesting to investigate the dependence of the anomalous effects shown in figs. 3 and 4, besides from temperature, also from other experimental parameters like detector voltage (that is electric field in the depletion layer) and size and position of the region of the detector's surface hit by the radiation
293
294
c.
BUSSOLATI
et al. Coonfs
Counh
K
Artificial
~210/B/7 ~Cs137 ]295'K ]200V inv ~TSOOnA Area i155mm 2 Oseful area ]OOmm 2 Detector Source Ternp Bias
2000
i Artificial
e~ /
I ~
2000 : 210/B/7
Detector Source Temp
Cs 137
: 260 ° K i200V ~ ~3.5nA
Bi~s I inv 150C
Area Useful
f
•
: I S S mm2 : 1 0 0 mm 2
area
1500
-]
13key
,=
Q
.f
12.6 key
I000
~"--12
I000
/
l 0
60
\.
t
i
I
80
90 Chanoel
I10
number
beam (the useful area). In this research we have used, as a source of electrons, Sn 113 instead o f Cs ~37, because, owing to the lower energy (365 and 389 keV), we can operate the detector with lower bias. We studied the behaviour o f the K conversion electron line o f Sn 113 varying the experimental parameters; therefore from here on we shall make reference to this K line shortly as electron line. The anomalies consist both in a broadening orthe lines and in their distortion from the gaussian shape; therefore to describe the phenomena under investigation, it appears worthwhile to introduce two parameters:
z =
!
L*M
I 100
I "D 80 Channel
Fig. 3. Spectrum
°~l 90
I00
number
of Cs137takenat 260°K.
The values o f table 1 have all been obtained with the same detector (210/B/7). Using other detectors analogous phenomena, with the same dependence from the experimental parameters, have been observed. T o get more information the investigation has been extended to c~-particles. With the c(-particles of A m T M phenomena of the same type of those observed with electrons, showing the same dependence from experimental parameters, are even more marked. At low temperatures (T = 260°K) even with well collimated TABLE 1 Temperature
fwhm of Sn 113 K line fwhm o f artificial line
(°K) (figs. 2 and 3)
295
number o f pulses distorting the gaussian shape total number o f pulses under the peak
Varying the experimental parameters, the results o f table 1 have been obtained.
I 70
I 60
50
Fig. 2. Spectrum of Cs 137 electrons taken at room temperature.
(7=
/
500
70
keV
•
"
"
C
7
keV
!i [
Electric field
i
Useful area
(kV/cm) (figs. 6 and 7)
,
(ram 2) (figs. 8 and 9)
2 6 0 , ,__[Emaj=2"6[Emax=3"7
100
20
1.56 0.24
1.05 0.05
!
~r r
1.03 0.07
[i 1.72 I 0.25
1.94 0.25
l I
1.44 0.21
HIGH
Si
RESISTIVITY
SURFACE
Counts ' 2 5 O0
BARRIER
295
DETECTORS
Counts Detector
: 210/B/4
Source
: Cs 137
Temp
:273eK
BIas
:170 V :150 mm2
Area
(It, ( 3 6 5 keY)
Useful oreo : 2 0 m m 2
1500
Detector Source Temp Bias Area ~UsefuI areQ
~210/B/7 :$n n3 273oK :320V " rSOmm2 2©ram 2
I
2000
K ( 6 2 5 key}
1500 IOOO ~ 8 . 5
i
I000
500
•
old., 60
70
9 keY
6oo h
80 Channel
L*M ( 3 8 9 keV)
;!
,j'
L',M ( 6 5 6 keY)
....
90
keY
I 20
::
I00
number
Fig. 4. Spectrum of Cs 137taken at 273 °K with a well collimated beam. beams (Us. area = 3 mm 2) and at room temperature (295 ° K) with less collimated beams (Us. area = 20 mm 2) we observed multiple peaks. Increasing the bias voltage, the multiple peaks turn into one distorted line that, further increasing the bias, becomes more and more symmetric. The surface of the detector has been scanned with a well collimated beam of e-particles. This experiment has not shown any systematic dependence of the phenomena from the position. ~r and T are strongly but irregularly dependent on the position; yet, if one does not take into account anomalous pulses, lines taken at different points on the detector surface correspond to the same mean collected charge (their peak points fall at the same abscissae on the axis of collected charge). No systematic correlation can therefore be stated between geometrical position and charge collection efficiency: it looks as if the observed phenomena were due to nonuniformities on a microscopic scale.
I 40
I SO
80 Channel
I00 number
Fig. 5. Spectrum of Sn113 electrons taken at 273 °K with a well collimated beam.
4. Discussion From the experimental picture given above we may draw some conclusions about the nature of the phenomena investigated. When an ionizing particle enters a semiconductor detector it loses energy generating hole-electron pairs. It is a measurement of this charge that allows to perform a measurement of the particle energy. As a matter of fact the current pulse released by a detector has a slow component. These pulse tails decrease for increasing reverse bias voltage and temperature of the detector. Since, due to short time constants of the main amplifier (introduced to get low noise), the integration time of the electronic chain is of the order of 1/~sec, a part of the charge released is lost in our measurements. As already observed by other workers this slow component can be explained on the basis of two different mechanisms: a) charge carriers may be captured (and successively released) by trapping centers6);
296
c. BUSSOLATI et al.
JC'
Counts
Counts
Artificial
2500 ~210/B/7 :Sn 113 ~265 ° K ~200V ~ : IOnA :lSSmm2 ~100 rnm2
Oetect°rK / i
K
Artificial
7
Source Temp
Bias
/. 2000
Detector
Source
1210/B/7 ~Srl H3
Tamp
1265OK
Bias I inv
I inv Area Useful area
:roov ~:lOnA
Area Useful area
200C
U55mm 2
~lOOmm2
/
1500
1500
1000
11.3 k e y
i
L
I 60
L* M
./
/
/
g
500
./
/
J u
.J I
I 80
I
I I00 Channet
keV
I000
~-
L+M
.oo f
-e~9.4
I
2 2 keV
~"
/ I
--
4--
13.5 keY ~ ~ - - -
I
I 120
t 140
:./\
__
number
SO
0
70
I
I
I
1
80
90
I00
IlO
Channel
I
I
[
1
t20 130 140 150 number
Fig. 6. Spectrum of Sn 1~3 taken with a detector bias of 100 V.
Fig. 7. Spectrum of SnU3 taken with a detector bias of 200 V.
b) the electric charge released must charge, in addition to the capacitance associated with the depletion layer, also another stray capacitance due to a not perfectly ohmic back contactV). Actually we observed, at the output of the charge preamplifier, that a small fraction of the pulses have an anomalously large delayed current component: these pulses give rise to distortion o f the lines. Nonuniformities of the detector should cause the charge released in different points to be collected with different efficiency. Processes, by which the observed phenomena could be explained on the basis of anyone of the two mechanism proposed above, can be easily hypothesized. Surely the back-contact of our detectors is not perfectly ohmic, owing to the high resistivity of silicon. On the other hand, to see if trapping is present in our detectors, we have performed the following experiment. The same detector, in identical experimental arrangements, has been used to detect s-particles and electrons. From the knowledge of the energy of the particle and
the amplitude of the output pulses one can calculate the apparent mean energy required to generate a pair of carriers (~;)6,7). By successive measurements with different bias voltage of the detector we have made a plot of s versus the reverse of the electric field both for s-particles and electrons (fig. 10). The different slope of the two plots suggests that the charge collection efficiency is different for the two particles, that is ionization density dependent. Such an effect cannot be accounted for by any equivalent circuit mechanism and therefore trapping must be present in our detectors. It seems that the hypothesis of trapping can more easily explain our experimental picture. In fact, while the detectors' behaviour with temperature, electric field and size of utilized surface (reported in table 1) does not allow to make a choice between the two hypothesis, an interpretation of the results obtained scanning the surface of the detector on the basis of trapping appears more convincing because the other mechanism would require too large differences in equivalent circuit from
HIGH RESISTIVITY
Si S U R F A C E
Counts
BARRIER
297
DETECTORS
Counts
Arttticial
Artificial f
1
Detector Source Temp
210/B/7 iSn 113 :265OK
~'000
2000
Defector Source Tamp
1210/B/7 : Sn 113 :265 o K
Bias
; 150
V
I mY r~ : I0 nA Area : 155mm z Useful area : 20ram 2
Area :tSSmm 2 Useful area IOOmm z
.! 1500 1500
i
IO00
!
IO.l keV
/-
/
L÷ M
,q
•
i
•,. I
6O
[
t
80
I
I
I
I00 Channel
I ~¢'1
~20
-I
10.5 k e y tOkeV----=
500
..,z I
~~1~-~
L÷M
/. /.
500
--
IO00
o°
1 16
~,~ I
I 48
32
M_., 64
,J
80
96
112
Channel
number
128
I
140
number
Fig. 9. Spectrum of Sn 1~3 taken utilizing an area of 20 mm 2.
Fig. 8. Spectrum of Sn 113 taken utilizing an area of 100 mm 2.
a point to a very near one. In conclusion the anomalous pulses seem due to different trapping from point to point of the detector caused both by differences in electric field and number of trapping centers. Moreover the observed phenomena look very similar to multiple peaks already observed by other workers.
E Detector
210/B/8
300 oK
3.8
=~
~
. E l ec trons -
The authors wish to thank Prof. E. Baldinger for useful discussion and Prof. A. Bisi and Prof. E. Gatti for helpful suggestions and assistance. References ])J. W. Mayer, I.R.E. Trans. Nucl. Sci. NS-9, no. 3 (1962) 124. 2) D. A. Bromley, I.R.E. Trans. Nucl. Sci. NS-9, no. 3 (1962) 135. 3) j. L. Blankenship, Nat. Acad. of Sci. Nat. Res. Council Publo 871 (1961) 43. 4) C. Cottini, E. Gatti, G. Giannelli and G. Rossi, Nuovo Cimento 3 (1956) 473. 5) E. Fairstein, I.R.E. Trans. Nuc[. Sci. NS-8, no. 1 (1961) 129. 6) G. Fabri, E. Gatti and V. Svclto, Phys. Rev. 131 (1963) 134. 7) E. Baldinger, J. Gutman and G. MatiIe, Z. Ang. Math. und Phys. 1 (1964) 90.
/ 5.6
J~
/ ,
,
~
] 0.05
~
I 0.10
~
~ I
~/V+Vo Fig. 10. e(eV/pair) as a function of (V+ V0)-½ for c~-particles and electrons.
AND M E T H O D S 33
(1965) 2 9 3 - 2 9 7 ; ~ N O R T H - H O L L A N D P U B L I S H I N G CO,
O N THE B E H A V I O U R OF H I G H RESISTIVITY Si S U R F A C E BARRIER D E T E C T O R S C. BUSSOLATI
and
M. BERTOLACC1NI and S. COVA
Istituto Eli Fisica Sperimentale, Politecnico, Milano, Italy
Gruppo di Ricerca 1.N.F.N. Pofitecnico di Milano, Italy
Received 7 October 1964 An investigation has been performed on the behaviour of high resistivity (21 k,Q.cm) silicon surface barrier detectors in view of their application to electron spectroscopy. Our detectors can develop depletion layers 1.3 mm thick and have been used to detect Cs 137 and Sn ll3 internal conversion electrons. Some anomalous effects, consisting in poor energy resolution and dis-
1. Introduction In recent years semiconductor junction detectors have been widely used. in radiation energy measurements and are now employed with a large variety of ionizing radiations ranging from heavy ions and fission fragments to electrons and 7-raysl'2). One of the main problems connected with the detection of electrons is to obtain sufficiently thick depletion layers. In silicon surface barrier detectors3), the thickness of the depletion layer w (p) depends on the resistivity p (f2. cm) and the bias voltage V(V) as: w = 0.53 (p- V)~. Therefore it seems convenient to use high resistivity silicon. We used detectors made at CISE Labs. of Milano out of 21 kf2.cm silicon and having an useful area of 155 mm2; depletion layers of 1.3 mm can be obtained with a bias of 325 V. On the other hand large inhomogeneities in resistivity may be expected in 21 kf2.cm silicon owing to its low donor concentration and perhaps to the presence of compensating impurities. As a matter of fact resistivity measurements performed on the silicon slices from which detectors are made, gave different results from point to point. The present investigation was carried out to test the behaviour of these detectors and to detect possible spurious effects connected with the inhomogeneities quoted above. 2. Electron spectroscopy The electronic chain (fig. 1) consists in a charge preamplifier4), a linear voltage amplifier, a main amplifier including an R C - R C shaping network and a multichannel pulse-height analyzer. The same R C - R C shaping network, having equal integrating and differentiating time constants of about 1.2/~sec, has been used throughout the whole investigation. The electrical noise of the chain s) having at the input only the parasitic capacitance (-~ 25 pF), is equivalent to 550 electrical charges (fwhm-~ 5 keV). The whole
tortion of the spectra, have been observed cooling the detectors (T ~ 260 ° K), with low bias voltage. These effects are probably due to inhomogeneities of Si that may cause nonuniform trapping of charge carriers in the detector; they can be reduced by increasing the bias and by collimating the beam of ionizing particles so as to work with only a small portion of the surface of the detector.
3_
Fig. 1. Block scheme of electronic chain.
chain, tested observing, with a 1.8 kO-cm Si surface barrier detector, the spectrum of Am 24~ a-particles, proved to work correctly. To test the behaviour of 21 kf2.cm detectors we have used them to detect the internal conversion electrons of CS 137 and Sn 1~3. Spectra of these electron sources have been taken both at room temperature and at a lower one (260 ° K) in the aim of obtaining, by a reduction of the leakage current in the detector, a better energy resolution. The spectrum of CS 137 reported in fig. 2 is obtained at room temperature. In fig. 3 a spectrum recorded at 260°K shows that, while the electrical noise has been effectively reduced (as indicated by the artificial line superimposed), the fwhm of the electron line is abnormally large and the shape of the line itself is distorted. In the next paragraphs we shall deal with these anomalous effects. Here it shall suffice to observe that they can be reduced both by increasing the bias voltage of the detectors and by collimating the impinging beam of electrons on a small area of the detector surface. Operating in this way we obtained the spectra of fig. 4 for CS 137 and of fig. 5 for Sn 113. 3. Anomalous effects We thought it interesting to investigate the dependence of the anomalous effects shown in figs. 3 and 4, besides from temperature, also from other experimental parameters like detector voltage (that is electric field in the depletion layer) and size and position of the region of the detector's surface hit by the radiation
293
294
c.
BUSSOLATI
et al. Coonfs
Counh
K
Artificial
~210/B/7 ~Cs137 ]295'K ]200V inv ~TSOOnA Area i155mm 2 Oseful area ]OOmm 2 Detector Source Ternp Bias
2000
i Artificial
e~ /
I ~
2000 : 210/B/7
Detector Source Temp
Cs 137
: 260 ° K i200V ~ ~3.5nA
Bi~s I inv 150C
Area Useful
f
•
: I S S mm2 : 1 0 0 mm 2
area
1500
-]
13key
,=
Q
.f
12.6 key
I000
~"--12
I000
/
l 0
60
\.
t
i
I
80
90 Chanoel
I10
number
beam (the useful area). In this research we have used, as a source of electrons, Sn 113 instead o f Cs ~37, because, owing to the lower energy (365 and 389 keV), we can operate the detector with lower bias. We studied the behaviour o f the K conversion electron line o f Sn 113 varying the experimental parameters; therefore from here on we shall make reference to this K line shortly as electron line. The anomalies consist both in a broadening orthe lines and in their distortion from the gaussian shape; therefore to describe the phenomena under investigation, it appears worthwhile to introduce two parameters:
z =
!
L*M
I 100
I "D 80 Channel
Fig. 3. Spectrum
°~l 90
I00
number
of Cs137takenat 260°K.
The values o f table 1 have all been obtained with the same detector (210/B/7). Using other detectors analogous phenomena, with the same dependence from the experimental parameters, have been observed. T o get more information the investigation has been extended to c~-particles. With the c(-particles of A m T M phenomena of the same type of those observed with electrons, showing the same dependence from experimental parameters, are even more marked. At low temperatures (T = 260°K) even with well collimated TABLE 1 Temperature
fwhm of Sn 113 K line fwhm o f artificial line
(°K) (figs. 2 and 3)
295
number o f pulses distorting the gaussian shape total number o f pulses under the peak
Varying the experimental parameters, the results o f table 1 have been obtained.
I 70
I 60
50
Fig. 2. Spectrum of Cs 137 electrons taken at room temperature.
(7=
/
500
70
keV
•
"
"
C
7
keV
!i [
Electric field
i
Useful area
(kV/cm) (figs. 6 and 7)
,
(ram 2) (figs. 8 and 9)
2 6 0 , ,__[Emaj=2"6[Emax=3"7
100
20
1.56 0.24
1.05 0.05
!
~r r
1.03 0.07
[i 1.72 I 0.25
1.94 0.25
l I
1.44 0.21
HIGH
Si
RESISTIVITY
SURFACE
Counts ' 2 5 O0
BARRIER
295
DETECTORS
Counts Detector
: 210/B/4
Source
: Cs 137
Temp
:273eK
BIas
:170 V :150 mm2
Area
(It, ( 3 6 5 keY)
Useful oreo : 2 0 m m 2
1500
Detector Source Temp Bias Area ~UsefuI areQ
~210/B/7 :$n n3 273oK :320V " rSOmm2 2©ram 2
I
2000
K ( 6 2 5 key}
1500 IOOO ~ 8 . 5
i
I000
500
•
old., 60
70
9 keY
6oo h
80 Channel
L*M ( 3 8 9 keV)
;!
,j'
L',M ( 6 5 6 keY)
....
90
keY
I 20
::
I00
number
Fig. 4. Spectrum of Cs 137taken at 273 °K with a well collimated beam. beams (Us. area = 3 mm 2) and at room temperature (295 ° K) with less collimated beams (Us. area = 20 mm 2) we observed multiple peaks. Increasing the bias voltage, the multiple peaks turn into one distorted line that, further increasing the bias, becomes more and more symmetric. The surface of the detector has been scanned with a well collimated beam of e-particles. This experiment has not shown any systematic dependence of the phenomena from the position. ~r and T are strongly but irregularly dependent on the position; yet, if one does not take into account anomalous pulses, lines taken at different points on the detector surface correspond to the same mean collected charge (their peak points fall at the same abscissae on the axis of collected charge). No systematic correlation can therefore be stated between geometrical position and charge collection efficiency: it looks as if the observed phenomena were due to nonuniformities on a microscopic scale.
I 40
I SO
80 Channel
I00 number
Fig. 5. Spectrum of Sn113 electrons taken at 273 °K with a well collimated beam.
4. Discussion From the experimental picture given above we may draw some conclusions about the nature of the phenomena investigated. When an ionizing particle enters a semiconductor detector it loses energy generating hole-electron pairs. It is a measurement of this charge that allows to perform a measurement of the particle energy. As a matter of fact the current pulse released by a detector has a slow component. These pulse tails decrease for increasing reverse bias voltage and temperature of the detector. Since, due to short time constants of the main amplifier (introduced to get low noise), the integration time of the electronic chain is of the order of 1/~sec, a part of the charge released is lost in our measurements. As already observed by other workers this slow component can be explained on the basis of two different mechanisms: a) charge carriers may be captured (and successively released) by trapping centers6);
296
c. BUSSOLATI et al.
JC'
Counts
Counts
Artificial
2500 ~210/B/7 :Sn 113 ~265 ° K ~200V ~ : IOnA :lSSmm2 ~100 rnm2
Oetect°rK / i
K
Artificial
7
Source Temp
Bias
/. 2000
Detector
Source
1210/B/7 ~Srl H3
Tamp
1265OK
Bias I inv
I inv Area Useful area
:roov ~:lOnA
Area Useful area
200C
U55mm 2
~lOOmm2
/
1500
1500
1000
11.3 k e y
i
L
I 60
L* M
./
/
/
g
500
./
/
J u
.J I
I 80
I
I I00 Channet
keV
I000
~-
L+M
.oo f
-e~9.4
I
2 2 keV
~"
/ I
--
4--
13.5 keY ~ ~ - - -
I
I 120
t 140
:./\
__
number
SO
0
70
I
I
I
1
80
90
I00
IlO
Channel
I
I
[
1
t20 130 140 150 number
Fig. 6. Spectrum of Sn 1~3 taken with a detector bias of 100 V.
Fig. 7. Spectrum of SnU3 taken with a detector bias of 200 V.
b) the electric charge released must charge, in addition to the capacitance associated with the depletion layer, also another stray capacitance due to a not perfectly ohmic back contactV). Actually we observed, at the output of the charge preamplifier, that a small fraction of the pulses have an anomalously large delayed current component: these pulses give rise to distortion o f the lines. Nonuniformities of the detector should cause the charge released in different points to be collected with different efficiency. Processes, by which the observed phenomena could be explained on the basis of anyone of the two mechanism proposed above, can be easily hypothesized. Surely the back-contact of our detectors is not perfectly ohmic, owing to the high resistivity of silicon. On the other hand, to see if trapping is present in our detectors, we have performed the following experiment. The same detector, in identical experimental arrangements, has been used to detect s-particles and electrons. From the knowledge of the energy of the particle and
the amplitude of the output pulses one can calculate the apparent mean energy required to generate a pair of carriers (~;)6,7). By successive measurements with different bias voltage of the detector we have made a plot of s versus the reverse of the electric field both for s-particles and electrons (fig. 10). The different slope of the two plots suggests that the charge collection efficiency is different for the two particles, that is ionization density dependent. Such an effect cannot be accounted for by any equivalent circuit mechanism and therefore trapping must be present in our detectors. It seems that the hypothesis of trapping can more easily explain our experimental picture. In fact, while the detectors' behaviour with temperature, electric field and size of utilized surface (reported in table 1) does not allow to make a choice between the two hypothesis, an interpretation of the results obtained scanning the surface of the detector on the basis of trapping appears more convincing because the other mechanism would require too large differences in equivalent circuit from
HIGH RESISTIVITY
Si S U R F A C E
Counts
BARRIER
297
DETECTORS
Counts
Arttticial
Artificial f
1
Detector Source Temp
210/B/7 iSn 113 :265OK
~'000
2000
Defector Source Tamp
1210/B/7 : Sn 113 :265 o K
Bias
; 150
V
I mY r~ : I0 nA Area : 155mm z Useful area : 20ram 2
Area :tSSmm 2 Useful area IOOmm z
.! 1500 1500
i
IO00
!
IO.l keV
/-
/
L÷ M
,q
•
i
•,. I
6O
[
t
80
I
I
I
I00 Channel
I ~¢'1
~20
-I
10.5 k e y tOkeV----=
500
..,z I
~~1~-~
L÷M
/. /.
500
--
IO00
o°
1 16
~,~ I
I 48
32
M_., 64
,J
80
96
112
Channel
number
128
I
140
number
Fig. 9. Spectrum of Sn 1~3 taken utilizing an area of 20 mm 2.
Fig. 8. Spectrum of Sn 113 taken utilizing an area of 100 mm 2.
a point to a very near one. In conclusion the anomalous pulses seem due to different trapping from point to point of the detector caused both by differences in electric field and number of trapping centers. Moreover the observed phenomena look very similar to multiple peaks already observed by other workers.
E Detector
210/B/8
300 oK
3.8
=~
~
. E l ec trons -
The authors wish to thank Prof. E. Baldinger for useful discussion and Prof. A. Bisi and Prof. E. Gatti for helpful suggestions and assistance. References ])J. W. Mayer, I.R.E. Trans. Nucl. Sci. NS-9, no. 3 (1962) 124. 2) D. A. Bromley, I.R.E. Trans. Nucl. Sci. NS-9, no. 3 (1962) 135. 3) j. L. Blankenship, Nat. Acad. of Sci. Nat. Res. Council Publo 871 (1961) 43. 4) C. Cottini, E. Gatti, G. Giannelli and G. Rossi, Nuovo Cimento 3 (1956) 473. 5) E. Fairstein, I.R.E. Trans. Nuc[. Sci. NS-8, no. 1 (1961) 129. 6) G. Fabri, E. Gatti and V. Svclto, Phys. Rev. 131 (1963) 134. 7) E. Baldinger, J. Gutman and G. MatiIe, Z. Ang. Math. und Phys. 1 (1964) 90.
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