Time structure of charge signals and noise studies of GaAs detectors irradiated by neutrons and protons

IL NUOVO CIMENTO VOL. 109 A, N. 9 Settembre 1996

Time structure of charge signals and noise studies of GaAs detectors irradiated by neutrons and protons (*)

W. BRAUNSCHWEIG, Z. H. CHU, W. KARPINSKI, TH. KUBICKI, K. L0]3ELSMEYER C. RENTE, O. SYBEN, F. TENBUSCH, M. TOPOROWSKY, B. WITTMER and W. J. XIAO

L Physikalisches Institut, R W T H Aachen - Sommerfeldstrafle 28, 52056 Aachen, Germany

(ricevuto il 18 Dicembre 1995; approvato il 22 Maggio 1996)

Summary. - - Semiconductor detectors processed in Aachen using SI GaAs from different manufacturers have been irradiated with neutrons (peak energy - 1 MeV) up to 4.0.10 la n/cm e and protons (energy 24 GeV) up to 8.2.1018 p/cm 2. All detectors work well after the exposure. The leakage current density at 200 V of detectors made of AXT material increases by a factor of four after the highest neutron fluence and a factor of three after the maximal proton fluence. For the FEW-LC material the leakage current decreases significantly after irradiation. No significant difference can be observed between biased and non-biased detectors during the exposure to neutrons. The equivalent noise charge (ENC) calculated from the noise density spectra agrees well with that extracted from the pedestal width for the different neutron and proton fluences. Before irradiation the charge signals for minimum ionizing particles (MIPs) increase with the peaking time in the range of 40 ns to 2 ~s, while this dependence cannot be observed after the exposure to neutrons or protons. The reason for this behaviour is the different time structure of the charge signals before and after irradiation. For AXT material the charge signals for MIPs correspond to 7100 electrons at 200 V before irradiation for a peaking time of 40 ns, while signals of 7200 electrons after 4.0.10 t4 n/cm 2 and of 4300 electrons after 8.2" 1013 p/cm 2 are obtained. For FEW-LC material the signals at 200 V are reduced from 15700 electrons before irradiation to 6000 electrons after 8.2.1013 p/cm 2 for this peaking time.

PACS 29.40.Wk - Solid-state detectors. PACS 07.77.Ka - Charged-particle beam sources and detectors. PACS 01.30.Cc - Conference proceedings.

1. - I n t r o d u c t i o n

The radiation resistance of GaAs detectors is the key issue regarding their use as tracking detectors at the Large Hadron Collider (LHC)[1]. Our previous

(*) Paper presented a t the II International Conference on Large-Scale Applications and Radiation Hardness of Semiconductor Detectors, 28-30 June 1995, Florence, Italy.

1289

1290 w. BRAUNSCHWEIG~ Z. H. CHU, W. KARPINSKI, ETC.

TABLE I. - Parameters of the wafers irradiated by neutrons.

Parameter Wafer 19 Wafer 41

material GaAs (AXT) GaAs (FEW) thickness 300 ~m 250 ~m diameter 50.8 mm 50.8 mm diode | 3 mm (5 • 5) mm 2 contacts NiCr/Au NiCr/Au

Flux (cm -2) # Flux (cm -2) #

Irradiation fluences 6.2.1013 20 - - - - 2 . 2 . 1 0 1 4 23 - - - - 4.0" 1014 20 4.0' 1014 4

TABLE II. - Parameters of the wafers irradiated by protons.

Parameter Wafer 51 Wafer 57 Wafer 58

material GaAs (FEW-LC) GaAs (AXT) GaAs (FEW-LC) thickness 250 ~tm 300 ~tm 250 ~tm diameter 50.8 mm 50.8 mm 50.8 mm structure (3 • 3) mm 2 (3 • 3) mm 2 (3 • 3) mm 2 contacts Ti/Pt/Au Ti/Pt/Au Ti/Pt/Au

Flux (p/cm 2) # Flux (p/cm 2) # Flux (p/cm 2) #

Irradiation 3.6.1018 1 3.6-1013 1 3.6.1018 1 fluences 8.2.1013 1 8.2.1018 1 8.2.10 TM 1

investigations have shown tha t the GaAs detectors could survive the exposure to 1 MGy electrons (3 MeV)[2] or 1 MGy photons dose (6~ source)[3].

Fo r fur ther radiation tes ts we have processed five wafers in Aachen with two Schot tky contacts[4] using AXT[5], F re ibe rge r Normal Carbon (FEW) and Fre ibe rge r Low Carbon (FEW-LC) ' mater ial [6]. Two wafers were i r radiated with neutrons at Rutherford Appleton Labora to ry in the IS IS neutron facility [7] for total fluences up to 4.0.1014 n / c m 2. The pa rame te r s for the neutron irradiation(1) are shown in table I (2).

Two diodes of each of the other three wafers were exposed to protons (24 GeV energy) at C E R N in the T7 beam for fluences up to 8.2.1013 p / c m 2. The corresponding pa rame te r s (s) are shown in table II .

(1) The estimated error of the neutron fluence is _+ 25%. (2) In tables I and II, # denotes the number of diodes. (8) The estimated error of the proton fluence is _ 25%.

TIME STRUCTURE OF CHARGE SIGNALS AND NOISE STUDIES ETC. 1291

2 . - I - V c h a r a c t e r i s t i c s and n o i s e s t u d i e s

Figure 1 shows the mean values of the leakage current density of the detectors of wafers 19 and 41, before and after the exposure to neutrons, as a function of the reverse bias voltage. Before irradiation the I -V curves show the typical diode behaviour with a saturation value less than 1.8 ~ / c m 2. The leakage current densities increase with the neutron fluence. For the AXT material the increase amounts to a factor of four at 200 V after 4.0- 1014 n/cm e, corresponding to a damage constant of 1.17.10-19 A/cm. Above 200V for AXT material (150V for FEW material) the leakage current densities increase more rapidly (and approximately linearly) with the reverse bias voltage. The onset of this effect is correlated with the saturation of the charge signals for minimum ionizing particles (MIPs) (see next section).

During the exposure to neutrons five diodes (AXT) for each fluence were reversely biased with 200 V. Figure 2 shows the I -V characteristics of the biased and non-biased detectors. No significant difference can be observed between them.

The noise of the detectors has been carefully studied before and after irradiation. The noise density spectra of the detectors are shown in fig. 3 for a reverse bias voltage of 200 V. The flat noise density in the high-frequency domain ( > 3. l0 s Hz) is dominated by the white-noise contribution. This white noise increases after the irradiation with neutrons, while the probe irradiated at 6.2.10 *a n/cm 2 has the maximal noise density value. Taking the frequency band (3.10a-105 Hz) of the shaper (Ortec 572) into account, one can calculate the ENC [8,9] with

D n ~ p S (1) ENC - ,

q

-1

.... - 2

~-3 .< " ~ - 4

v

.~-5 ~ - 6

~ - 7

o - 8

- 9

**** �9

*********~

**** A"*

~A A

9

-10 -300 -250 -200 -150 -100

bias voltage(V)

I r r l l l r r t l q l l l l l l l l l l l l l J I T l l l l --50 0

Fig. 1. - I-V characteristics of the detectors at room temperature before and after neutron irradiation. AXT: W19 (300 ~m), �9 before irradiation, �9 6.2.10 la n/cm 2, * 2.2.1014 n/cm 2, �9 4.0.1014 n/cm2; FEW: W41 (250 ~tm), [] before irradiation, �9 4.0-1014 n/cm 2.

1292 w. BRAUNSCHWEIG, Z. H. CHU, W. KARPINSKI, ETC.

0

-1

~ - 2

- 8

- - 9 _

-I0- , I , , , , I , , r,I,,LLI . . . . I . . . . , . . . . I , I 1 [ 1 1 1 1 1 1 L L

-300 -250 -200 -150 -100 -50 0 bias voltage (V)

Fig. 2. - I-V characteristics of the detectors biased with 200 V during the neutron irradiation and those not biased (22~ �9 6.2.101Sn/cm 2, biased, �9 6.2-101~n/cm 2, not biased, [] 2.2.1014n/cm 2, biased, �9 2.2.1014n/cm 2, not biased, A 4.0-1014n/cm 2, biased, �9 4.0.1014 n/cm 2, not biased.

where Dn, vp = 2.2 ~ts, and S -- 0.47 are the mean noise density taken from the noise density spectra in this frequency band of the shaper, the peaking time, and the noise integral of the shaper, respectively. The calculated ENC and that extracted from the

~10 -n -4

~10 -12

10 -13

101

non-irradiated

i

6.2.1013 n /cm 2

~" ~hi~' 2.2.1014 n /cm 2

. ~ ~ . 4.0" 1014 n / cm 2

4.0-1014 n /cm 2 ' ~

'

I I | l l l i d I l t i l l l l i l I i I t t l l l i l i i l t i ,

10 2 l 0 s 10 4 10

frequency (Hz)

Fig. 3. - Noise density spectra of the detectors of wafer 19 (AXT) before and after exposure to neutrons.

T I M E S T R U C T U R E O F C H A R G E S I G N A L S A N D N O I S E S T U D I E S E T C . 1293

4.0

3.5

3.0

2.5 c o

O

v 2.0

Z 1.5

1.0

0.5

0.0 , I . . . . I . . . . I ~ , i , I . . . . I . . . .

0 1 2 3 4 irradiation fluence (1014n/cm 2)

Fig. 4. - The ENC of the detectors (AXT) at 2.2 ~s peaking time as a function of the neutron fluence. �9 Extracted from the pedestal width, o calculated from the noise density.

pedestal width are shown in fig. 4. Very good agreement is found between the data and the calculation. On the other hand, we observe that the ENC does not scale with h/~ at the large peaking time of 2.2 ~s as simply expected, where I is the leakage current.

The I-V curves of the diodes of wafer 57 (AXT) before and after the exposure to protons are shown in fig. 5, demonstrating a qualitatively similar behaviour as for the neutron irradiation: The current densities increase after the irradiation; above 200 V the increase of the currents with reverse-bias voltage is more rapid than in the lower voltage region. The damage constant a = 6.2.10 -19 A/cm at 200 V is larger than that of the neutron irradiation.

As shown in fig. 6, the detectors made of FEW Low Carbon (FEW-LC) material have a higher current density before irradiation (J - 6 ~ / c m 2 at 200 V) than those made of AXT and FEW Normal Carbon material (see fig. 1 and 5). The faster increase of the current densities for bias voltages above 200V following irradiation, is observed for all materials. However for the FEW-LC material the leakage current densities decrease with the increasing proton fluence (4).

The noise density spectra of the detectors have also been measured before and after proton irradiation. As seen in fig. 7, the noise density is reduced in the whole frequency range (10-105 Hz), corresponding to the reduction of the ENC shown in fig. 8. The values calculated from the noise density spectra again agree well with the data.

(4) This behaviour for the FEW-LC material is also observed for neutron and pion irradiation.

1294 W . B R A U N S C H W E I G ~ Z . H . C H U ~ W . K A R P I N S K I ~ E T C .

- 1

~9

"~" - - 3

~ '~ - - 4

~ - 5 CD

~.z - - 6

~ - 7 C9

- 8

- 9 :

- 1 0 -400

, , , [ . . . . i . . . . i . . . . i . . . . i . . . . ] , , , k l r l i i

-300 -200 - 1 0 0 0 b ias vo l tage (V)

Fig. 5. - I-V characteristics of the detectors of wafer 57 (AXT) at room temperature before and after proton irradiation. �9 Before irradiation, �9 after 3.6.1018 p/cm 2, * after 8.2.101~ p/cm 2.

0

- 2

4-4

- 6

- 8

- 9

- 1 0 -400

O

2

g g

g ~ , 1 . . . . I . . . . I . . . . J . . . . I . . . . i . . . . f . . . .

-300 -200 - 1 0 0 0 b ias vo l tage (V)

Fig. 6. - I-V characteristics of the detectors from FEW-LC material at room tempera ture before and after proton irradiation. �9 Before irradiation, �9 after 3.6.1018 p /cm 2, * after 8.2.10 is p /cm 2.


0~

! r 09 '5

1 0 - 1 1 ~ i r r a d i a t e d

�9 �9 1 3 2

10 -12

1 0 -13

1 0 1 1 0 2 1 0 3 1 0 4 1 0 5

frequency (Hz)

Fig. 7. - Noise density spectra of the detectors from FEW-LC material before and after proton irradiation.

4.0,

3.5

3.0

2.5 S

2.0

Z 1.5

1.0

0.5

0.0 -2

i i 1 , , r I r ~ I r f i i l l , I r , E

0 2 4 6 8 irradiation fluence (1013 p / c m 2)

10

Fig. 8. - The ENC of the detectors from FEW-LC material at 2.2 ps peaking time as a function of the proton fluence. �9 Extracted from the pedestal width, �9 calculated from the noise density.


3. - S i g n a l for M I P s a n d t h e i r t i m e r e s p o n s e

To investigate the radiation damage caused by neutrons and protons on the GaAs detectors we measured the detector response to minimum ionizing particles (electrons from a 9~ source)[10]. All detectors were found to work well after the exposures.

Figure 9 shows the mean values of signal pulse heights for MIPs from several detectors measured at a peaking time of 2.2 ~s before and after exposure to different neutron fluences. Before irradiation we observe the typical approximately linear dependence of the signals on the reverse bias voltage [4]. For the three fluences, the detectors demonstrate a similar signal dependence on the bias voltage: following an approximately linear increase with the bias voltage, the signal pulse height fa t tens off or even decreases with increasing voltage. The voltage value at which the flattening off occurs coincides with the change of the slope in the I-V curves (see fig. 1). At 200V a pulse height of about 7200 electrons is obtained for MIPs after 4.0- 1014 n/era 2.

The signal pulse heights for MIPs have also been measured with peaking times of 40 ns and 100 ns. Figure 10 shows a spectrum for electrons from a 9~ source taken with a peaking time of 100 ns at 200 V after the exposure to 4.0.1014 n/cm ~. A signal-to- noise ratio of 10 : 1 was achieved. The pulse heights, measured for the AXT material with the three different peaking times at 200 V, are shown in fig. 11 as a function of the neutron fluence. We observe that the signals for all three peaking times are similar after irradiation, while for the non-irradiated probes the signals (7100 electrons) at 100 ns and 40 ns peaking times are 30% smaller than the signals obtained with a peaking time of 2.2 ~s. For a bias voltage of 200 V the signal is largest (13 800 electrons) after an irradiation of 6.2.1018 n/cm 2 decreasing to 7200 electrons for the

25

15

.~ 1 0 ~Q

2O

@

�9 �9

~r

"k

~r "k

. . . . I . . . . [ . . . . I . . . . I ~ , i T t l l ~ F i T 1 1 1 ~ i l t

0 100 200 300 400 reverse bias voltage (V)

Fig 9. - Mean value of signals for 9~ electrons (peaking time 2.2 ~s) as a function of reverse-bias voltage before and after neutron irradiation with different fluences for wafer 19. �9 Before irradiation, �9 6.2.1013 n/cm 2, * 2.2.1014 n/cm 2, �9 4.0.1014 n/cm 2.

T I M E S T R U C T U R E OF C H A R G E S I G N A L S A N D N O I S E S T U D I E S ETCo 1297

8

300

200

100

, , I , . ~ - 5 5 10 15 20 25 30

s igna l - to -no i se

Fig. 10. - Spectrum for 9~ electrons after 4.0.1014 n/cm 2 with 100 ns peaking time. S /N = 10.

25.0

22.5

20.0

17.5

15.0 F-t "~ 12.5

~ 1 0 . 0

7.5

5.0

2A

0.(

E �9

W

, I , I I z I I k , I I I r f I I . . . . [ ' , '

0 1 2 3 4 5

i r r a d i a t i o n f luence (1014 n / cm 2

Fig. 11. - Signals for 9~ electrons, measured with different peaking times, as a function of the neutron fluence for wafer 19 (AXT). �9 Peaking time 2.2 ~s, [] peaking time 100 ns, �9 peaking t ime 40 ns, at 200 V.

1298 W. BRAUNSCHWEIG, Z. H. CHU, W. KARPINSKI, ETC.

-0.060

-0.062

-0.064 >

-0.066

-0.068

-0.070

-0.072 -2

4.0.1014 n/cm 2

. . . . . . . . :2: ~

6.2.1013 n/cm 2

, I , I , I , I , I ,

0 2 4 6 8 xlO -6

time (s)

Fig. 12. - Time structure of charge pulse for 9~ electrons before and after exposure to neutrons for wafer 19 at 200 V.

highest fiuence of 4.0.1014 n/cm 2. The signal height variation with peaking time is a consequence of the time structure of the charge pulse as can been seen from fig. 12. For the non-irradiated probe the charge pulse has a large slow component, while this slow component nearly disappears after irradiation (a).

For the irradiation with protons, fig. 13 and 14 show the variation of the charge signals for MIPs with the proton fluence (3.6.1018 and 8.2.101~ p/cm 2) and with the bias voltage for FEW-LC and AXT material, respectively. Before irradiation we see again the typical approximately linear behaviour of the charge pulse with the bias voltage for both materials. After the exposure the signal heights are reduced and the saturation behaviour of the charge signals at high reverse bias voltages is again observed. After 8.2-1018 p/cm 2 signal heights of 6000 electrons for FEW-LC and 4300 electrons for AXT are measured at 200 V.

Figure 15 shows the pulse heights for MIPs at 200V, measured with peaking times of 40 ns and 2.2 ~s, as a function of the proton fiuence for FEW-LC material. For this material the signals decrease from 15 700 electrons before irradiation to about 6000 electrons after 8.2.1018 p/cm 2 for 40 ns peaking time. A similar dependence of the signals on the peaking time is observed as for the neutron irradiation (see fig. 11). Namely, for the non-irradiated diodes, the charge signal increases with the peaking time between 40ns and 2~s, while after irradiation the charge signals are independent of the peaking time in this range. Again this behaviour can be explained by the different time structure of the charge pulse before and after irradiation as shown in fig. 16.

(5) A systematic investigation for peaking times ranging from 40 ns to 2.2 ~s and bias voltages between 100 V and 300 V will be given in a subsequent publication.


25

20

15 %

10

5

0

0

o Q

o

, , , I . . . . ] . . . . I . . . . I . . . . ] , ~ 1

100 200 300 400 500 600 reverse bias voltage (V)

Fig. 13. - Charge signals for 9~ electrons, measured with a peaking time of 2.2 ~ts vs . reverse bias voltage before and after proton irradiation for wafer 58 (FEW-LC). �9 Before irradiation, �9 after 3.6.1023 p/cm 2 , * after 8.2.1018 p/cm 2 .

25

20

% 15

.~ lO

0

0

0

0

I i ~

O 0 �9 ~-k ~ ~ "~ ~ "k ,k

"k

100 200 300 400 500 60q

reverse bias voltage (V)

Fig. 14. - Same as fig. 13 but for wafer 57 (AXT).


25

20

15

10

i

0

0

0

0 2 4 6 8 IO

proton f luence (101ap/cm 2)

Fig. 15. - Signals for 9~ electrons, measured with 40 ns [o) and 2.2 ~ts ( .) peaking times, as a function of the proton fluence for wafer 58.

-0 .318

-0 .320

> -0 .322 qJ

~ -0 .324

-0 .326

-0 .328 - 2

8.2.10 la p / c m 2

3.6.1013 p / c m 2

I , I . , . . I , I , I

0 2 4 6 8 x10 -6 t ime (s)

Fig. 16. - Time structure of charge pulse for n~ electrons before and after proton irradiation for wafer 58 at 200 V.


4 . - C o n c l u s i o n

More than 60 detectors fabricated in Aachen using SI GaAs from AXT and F E W were irradiated with neutron fluences of 6.2.1018 n/cm 2, 2.2"1014 n/cm 2 and 4.0.1014 n/cm 2 (the peak neutron energy is - 1 MeV). Two diodes made of AXT and four diodes made of FEW-LC were exposed to protons (24 GeV) at 3.6.1018 p/cm 2 and 8.2.10 is p/cm 2. All detectors were found to be fully functional after irradiation, leading to a typical ratio S/N = 10 for 9~ electrons (MIPs) with 100 ns peaking time at the highest radiation level. Biasing with 200 V during the exposure to neutrons made no significant effect.

For the AXT material the leakage current density increases by a factor of four after 4.0.1014 n/cm 2 and by a factor of three after 8.2-1018 p/cm 2 at 200V. The corresponding damage constants are 1.17.10-19 A/cm and 6.2.10-19 A/cm, respectively. For the FEW-LC material the current density decreases significantly with increasing proton fluence.

The detector noise before and after irradiation has been studied. The equivalent noise charge extracted from the pedestal width is in good agreement with the value calculated from the noise density spectra.

Before irradiation the charge pulse heights for 9~ electrons increase approximately linearly with the reverse-bias voltage, while after the exposure to neutrons or protons the signal heights saturate at high voltages. The pulse heights increase with the peaking time in the range from 40 ns to 2.2 ~s before irradiation, while they become independent of the peaking time after the exposure to neutrons or protons. For AXT material at 200 V and 40 ns peaking time the signal pulse heights for MIPs correspond to 7100 electrons before irradiation. After 4.0.1014 n/cm 2 7200 electrons are obtained and after 8.2-10 is p/cm 2 4300 electrons are obtained. For the FEW-LC material the signal heights at 200V are reduced from 15700 electrons before irradiation to about 6000 electrons after 8.2.1018 p/cm 2 for a peaking time of 40 ns. The pulse form from non-irradiated detectors has a large slow component, which is strongly reduced after irradiation with neutrons or protons.

Further investigations of the radiation damage of GaAs detectors by pions, neutrons and protons will be undertaken in the coming months.

This work was supported by the German Bundesministerium ftir Bildung und Forschung under contract number 056ACllP.

R E F E R E N C E S

[1] Technical Proposal ofCMS CoUaboration, CERN/LHCC 94-38, LHCC/P1, 15 December 1994, pp. 114-117.

[2] KARPINSKI W. et aL, Nucl. Instrum. Methods A, 323 (1992) 635. [3] BRAUNSCHWEIG W. et al., Nucl. Inst,'tim. Methods A, 372 (1996) 111.


[4] KUBICKI TH. et al., Development of GaAs detectors at RWTH Aachen, in Proceedings of the I International Conference on Large Scale Applications and Radiation Hardness of Semiconductor Detectors, edited by A. BALDINI and E. FOCARDI (SIF, Bologna) 1994.

[5] American Xtal Technology SI GaAs wafer product, 6780 Sierra Court, Dublin, California 94568.

[6] Freiberger Elektronikwerkstoffe GmbH product, 09584 Freiberg/Saxony, Germany. [7] EDWARDS M. et al., The radiation hardness test facility at RAL, in Proceedings of Large

Hadron Collider Workshop, Aachen, Germany, 4-9 October 1990, CERN 90-10, Vol. III, p. 584.

[8] VAN DER ZIEL A., Noise Sources, Characterization, Measurements, Prentice-HaU Information and System Science Series (Prentice-Hall, Inc., New Jersey) 1970.

[9] LUBELSMEYER K et al., Nucl. Instrum. Methods A, 350 (1994) 530. [10] KUBICI(I TH., Au]bau einer Koinzidenzanordnung zur Messung der Ladungsausbeute von

GaAs-Dioden, RWTH Aachen internal report, PITHA 93/17.

Documents

Time structure of charge signals and noise studies of GaAs detectors irradiated by neutrons and protons