Noise behavior of semi-insulating GaAs particle detectors before and after proton irradiation

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NuclearPhysicsB (Proc. Suppl.) 78 (1999) 527-532

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Noise behaviour of semi-insulating GaAs particle detectors before and after proton irradiation U. Biggeri a, C. Canali b, C. Lanzieri c, C. Leroy d, F. Nava e and P. Vanni e aI.N.F.N, and Dipartimento di Energetica, Universith di Firenze, via S.Marta 3, 1-50139 Firenze, Italy bI.N.F.M, and Dipartimento di Scienza dell'Ingegneria, Universith di Modena, via Campi 213/B, I-41100 Modena, Italy CDirezione Ricerche, Alenia S.p.A., via Tiburtina Km 12.400, 1-00131 Roma, Italy d Department of Physics, Universite de Montreal, C.P. 6128, Succursale "Centre-Ville", Montreal (Quebec) H3C 3J7, Canada eI.N.F.N, and Dipartimento di Fisica, Universith di Modena, via Campi 213/A, 1-41100 Modena, Italy As a part of the R&D 8 Collaboration Charge signal and noise were studied in non-irradiated and irradiated Schottky barrier, circular pad detectors of 100 Bm thick made on semi-insulating Gallium Arsenide at Alenia S.p.A., as a function of the reverse bias (Va), the shaping time (1:) and the fluence (f), for minimum ionising electrons yielded by a 1°6Ru source. The detectors have been irradiated with protons (energy 24 GeV) up to a fluence of about 2 x 10 ~4 p/cm 2. A charge signal degradation is observed for irradiated detectors. The charge signals for MIP's at 500 V are reduced from 12900 electrons before irradiation to 6600 electrons after about 2 × 1014 p/cm 2 at a temperature of 7 ° C and with a shaping time of 20 ns, typical of LHC inter-bunch crossing time (25 ns). The charge signal is |bund independent of x tbr full depletion condition (V,, _> 100 V). The measurement of the charge signal as a thnction of V~ shows that the full depletion voltage decreases from 100 V tbr non-irradiated detectors to about 70 V for irradiated ones. The equivalent noise charge extracted from the pedestal width is lbund to scale with ~ according to the predictions when the series noise contribution dominates, while it does not scale with ~ as it should be when the parallel noise contribution, due to the detector leakage current, becomes important.

1. I N T R O D U C T I O N The noise of a detector-preamplifier assembly has a detrimental effect in the spatial resolution, (y, of the microstrip semiconductor detectors. Whatever is the algorithm used to find the centroid of the charge induced on adjacent strips, the (y achieved is, in fact, limited by the signal-to-noise ratio, S/N, according to the expression (~ ~ strip pitch/( s / N ). Since the noise in semiconductor detectors is observed to be a very sensitive parameter to irradiation [ 1,2] and these detectors, proposed for the high energy experiments at CERN large hadron

collider (LHC), will be exposed to the high-particle fluences during 10 years of planned operation [3], predictions for the performances of the detectorpreamplifier system have to take into account the change of the system noise through irradiation. Recently, an accurate analysis of the effect of the bulk damage on the detector performance was done as a function of the non-ionising energy loss (NIEL) of high-energy protons in particle detectors fabricated on semi-insulating (SI), Liquid Encapsulated Czochralski (LEC) grown GaAs. It has been found that the proton irradiated detectors show a correlation between the charge collection efficiency of [3(MIP's)-particles, cce~, and the total

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U Biggeri et al. INudear Physics B (Proc. Suppl.) 78 (1999) 527-532

NIEL, indicating that the reduction in performance of GaAs particle detectors can be ascribed to NIEL [4-6]. Moreover with low injecting ohmic contact (LIOC) GaAs detectors, which have been developed in order to operate at reverse bias voltages, V,,, much higher than the reverse bias, Va, necessary to have a fully active detector without current breakdown [7], the influence of Va, in the range Va > Va, on ccel3 has been emphasised. It has been found, in fact, that cce# increases with increasing V, both in nonirradiated and proton irradiated detectors [6]. However an accurate analysis of the noise on these detectors has not been yet performed and, since the leakage current increases with increasing V,, contributing to enhance the parallel noise component [8], an appropriate operation condition has to be determined for V~, in order to operate at the best signal over noise ratio. In this paper we present the results of a study of the ccel3 and the noise in non-irradiated and proton irradiated LIOC, SI-LEC GaAs detectors taking into account the various constraints imposed by their operation at the LHC like short inter-bunch crossing times (25 ns) and low temperatures (-0 ° C). A qualitative investigation of the various noise components, performed as a function of fluence, temperature and shaping time allowed us to determine the best operation conditions.

detectors by using the same procedure and equipment described in more detail in [1]. The mean value of the signal pulse has been calculated using ~:t = 4.20 eV as the value of energy needed to produce one e-h pair in GaAs and dE/dx = 5.60 MeV/cm as the value of the most probable energy loss for MIP's [10].

3. EXPERIMENTAL RESULTS 3.1. I-V characteristics The shape of the I-V curves obtained at the temperature of 7 ° C for detectors before and after proton irradiation as a function of the reverse bias in the range 100-550 Volts, that is in the full depletion condition, is shown in Fig. I. It is quite evident that the current increases with the irradiation level and that even after the proton irradiation at the highest fluence the detectors can operate up to a very high bias without exhibiting breakdown. S M T 0 5 8 RB; T m = 7 o C; t = 100 ~tm .... --~-#

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2. SAMPLES AND EXPERIMENTAL SET-UP 12

The detectors studied in the present work were made by Alenia [9] on commercially available SI LEC undoped <100> oriented GaAs substrate with n-type resistivity p - 1 0 7 ~ c m , supplied by Sumitomo. The detectors investigated are (100 _+ 6) pin thick with circular Schottky contacts (2 mm in diameter) realised on the front side by Ti/Pt/Au metalization and uniform ohmic contacts realised on the whole back by a surface treatment at room temperature followed by an e-beam deposited Au/Ge/Ni metalization and by a thermal cycle at 430 ° C for 20 s in N2 + H2 (10%) atmosphere. The detectors were irradiated by 24 GeV/c protons at various fluences (up to 16.45x1013 p/cm 2) at 26 ° C in the T7 beam at the CERN-PS. The signal pulse response induced by minimum ionising electrons from a 1°6Ru sourco and the noise were studied for non irradiated and irradiated

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3.2. Equivalent Noise Charge (ENC) A typical pulse height spectrum measured with the particle detector readout described in ref. [1] is shown in Fig. 2. It consists of two peaks, the lower peak, called pedestal, is due to the noise contribution (detectorpreamplifier assembly) while the entries at higher channels are detector signals (MIP's from a l°6Ru source). A Gaussian distribution fitted to the pedestal gave the total equivalent noise charge

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U Biggeri et al./Nuclear Physics B (Proc. Suppl.) 78 (1999) 527-532

(ENC) as the standard deviation of the Gaussian:

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From the data, the equivalent noise charge for non-irradiated and proton irradiated detectors was extracted as a function of the reverse bias, of the temperature, Tin, and the shaping time, z. These values are reported in Fig. 3 a), b) and c) for Tm = 7 ° C and for "c= 100, 70 and 20 ns, respectively. To make it clear, in Fig. 3 b) and c) only the ENC data of the non-irradiated (#4), of the irradiated with the lowest radiation level (#16) and of the most irradiated (#5) detectors are reported. In order to emphasise the dependence of ENC on "c, the experimental values of ENC measured for the most irradiated detectors (#5) are reported in Fig. 4 for the three shaping times as a function of V, together with the amplitudes of the uncertainties.

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3.3. Signal and signal/noise for MIP's To investigate the radiation damage caused by protons on the GaAs detectors, the detector response to minimum ionising particles was measured. Fig. 5 a) shows the mean va]ues of signal pulse heights for MIP's as a function of V,, from several detectors measured at a shaping time of 1 0 0 n s before and

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U Biggeri et al./Nuclear Physics B (Proc. Suppl.) 78 (1999) 527-532

after exposure to different proton fluences. The uncertainties, due to the uncertainty in ~r and in the thickness, t, of the detector, are shown in Figure, only for the non-irradiated detector (#4) for clarity, by vertical segments centered in the mean value of the signal pulse. S M T 0 5 8 RB; T m= 7 °C; t = 1 0 0 p r n 16~ 14

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An accurate study of the main noise components in semiconductor radiation detector-amplifier system, has been perlormed by researchers of the Department of Physics at the Politecnico of Milano [8,13]. They have shown that the square equivalent noise charge can assume the following expression:

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or irradiated detectors for V~,>Vd, iv) in the irradiated detectors the full depletion condition occurs at lower V~, (-70 Volt). The signal pulse heights for MIP's have also been measured with shaping times of 70 and 20 ns. The results are shown for three fluences in Fig. 5 b) and c) respectively in the bias range 100-550 Volt. For all three shaping times, the detectors demonstrate a quite similar signal height values and dependence on the bias voltage, as a consequence of the absence in the charge pulse of a slow component in the afbresaid bias range (11,12). Signal-to-noise ratio, S/N, has been calculated for the three shaping times and for the non-irradiated (#4), the irradiated with the lowest radiation level (#16) and the most irradiated (#5) detectors. The data are shown in Fig. 6 a), b) and c) for "c of 100, 70 and 20 ns respectively as a function of V,. It is observed, for all the three shaping times, that i) S/N increases almost linearly with V, only for z = 20 ns which corresponds to the value where the noise is practically independent of V,, (Fig. 3 c)) and ii) S/N shows a maximum at Va - 300-400 Volt for the z = 100 ns where the noise shows a sharp increment (Fig. 3a)).

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in which, Sj and S 2 a r e constants depending on the shaping time z of the amplifier-shaper circuit, gm is the FET transconductance, o~ is a coefficient ranging from 0.5 to 0.7, Ct,,t is the total input capacitance, IL is the detector leakage current, Rf is the feedback resistor and v is the frequency. T is the absolute temperature and k is Boltzmann's constant. From Eq. (1), where the dependence of ENC 2 on z is purposely stressed, it follows that, if Sj and $2 are of similar magnitude, which is the case for several detector-amplifier configurations [8], the first term, determined by the white component of the

U Biggeri et al./Nudear Physics B (Proc. Suppl.) 78 (1999) 527-532

series generator, dominates at low "c, while the second term, due to the parallel noise, most likely plays the main role at relatively high values of IL. SMT 058 RB; T m =7°C; 24

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reverse bias (550 V) where the parallel noise contribution from the detector leakage current Ie most likely becomes important, the ENC does not scale within the experimental error, with r~ according to eq. (1). The disagreement has been also observed by other authors [12,14] and has been interpreted by assuming that IL does not act as a shot noise source in eq. (1), [15]. Similar results have been obtained for the detectors irradiated at lower proton fluences. The behaviour of the ENC, in the non irradiated and proton irradiated detectors can be then well interpreted by means of eq. (1) only when the series noise contribution becomes dominant with respect to the parallel and l/v noise. 5. CONCLUSIONS

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Several detectors of 100 lam thick, fabricated in Alenia S.p.A. using LEC SI GaAs from SUMITOMO with a low injecting ohmic contact, have been irradiated with high fluences up to about 2 x 1014 p/cm 2 of high energy 24 GeV/c protons. All the detectors were found to be fully functional after irradiation at voltage values (550 V) well above the one required to achieve full depletion (-70 V). The detector performances have been measured at low temperature (7 ° C), which seems to be a constraint in the LHC mode operation [1], since data at 23 ° C have been already reported elsewhere [6]. Typically the detector leakage current increases by a factor of 6 after - 2 x 1014 p/cm 2 at 550V. The change pulse heights for t°~'Ru electrons increase slowly and approximately linearly with the reverse bias voltage, V~,, and it is independently of the shaping time in the range from 20 to 100 ns for V~>100V. At 550 V the pulse height for MIP's corresponds to -12900 electrons before irradiation and after irradiation up to a fluence of ~2×1014 p/cm 2 to -6600 electrons. The detector noise has been studied as a function of Va and over the shaping time range 20 _< "~_< 100 ns, before and alter irradiation. The experimental equivalent noise charge, ENC, extracted from the pedestal width has been interpreted by splitting E N C into three additive contributions, each with a different dependence on z and assuming the l/v noise negligible. One contribution, related to the series noise generator, depends mainly on the square of the total input capacitance through 1/'~and the

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U Biggeri et aL /Nuclear Physics B (Proc. Suppl.) 78 (1999) 527-532

other one, related to the parallel noise generator, depends mainly on the detector dark reverse current, IL, through "c. In the last one IL acts as a shot noise. The main results coming from this analysis are: i) at the lowest bias when the series noise prevails, the ENC scale with 1 ~ and ii) at the highest values of IL for Va = 550 V, the ENC does not scale with as it is predicted by eq. (1) since most likely IL does not act as a shot noise. The behaviour of the signal-to-noise ratio with the shaping time reflects the noise structure: i) S/N increases approximately linearly with increasing Va for z -- 20 ns; ii) S/N shows a maximum around 300 V for "~= 100 ns. Finally, for detectors irradiated at a fluence o f - 2 x 1014 p/cm 2, i.e. 10 years of operation of LHC, S/N -7.3, at 550 V and with a shaping ti:ne of 20 ns corresponding to the LHC inter-bunch time [ 1].

ACKNOWLEDGEMENTS The authors are grateful to Dr. F. Lemeilleur and Dr. B. Dezille from CERN tbr their help with the proton irradiation and wish to thank Prof. G. Bertuccio from Politecnico of Milano for helpful suggestions.

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l l.F. Nava et aI.,IEEE Trans. Nucl. Sci. NS 43 (1996), 1130. 12. W. Braunschweig et al., II Nuovo Cimento 109 A (1996), 1289 13. E. Gatti et al., Nucl. Instr. and Meth. A297 (1990), 467 and references therein. 14. F. Tenbusch et al., Nucl. Phys. B (Proc. Suppl.), 61B (1998), 415. 15. A. van der Ziel: "Noise Sources, Characterisation, Measurements", Prentice Hall, Inc. Englewood Cliffs, N.J. (1970).