- Email: [email protected]
Irradiation effects on thin epitaxial silicon detectors
ARTICLE IN PRESS
Nuclear Instruments and Methods in Physics Research A 568 (2006) 61–65 www.elsevier.com/locate/nima
Irradiation effects on thin epitaxial silicon detectors$ V. Khomenkova,, D. Biselloa, M. Bruzzib, A. Candeloria, A. Litovchenkoa, C. Piemontec, R. Randoa, F. Ravottid, N. Zorzic a Dipartimento di Fisica, Universita` di Padova and INFN Sezione di Padova, Via Marzolo 8, I-35131, Padova, Italy Dipartimento di Energetica, Universita` di Firenze and INFN Sezione di Firenze, Via S. Marta 3, I-50139, Firenze, Italy c ITC-irst, Microsystems Division, Via Sommarive 18, I-38050, Povo (Trento), Italy d CERN, Geneve 23, CH-1211, Switzerland
b
Available online 28 June 2006
Abstract Radiation hardness of silicon detectors based on thin epitaxial layer on Czochralski (CZ) substrate for the LHC upgrade (Super-LHC) was studied. No type inversion was observed after irradiation by 24 GeV/c protons up to the fluence of 1016 p/cm2 due to overcompensating donor generation. After long-term annealing (corresponding to 500 days at room temperature) proton irradiated devices show a decrease of the effective doping concentration and then undergo type inversion. Measurements confirm that thin epitaxial devices on CZ substrate could be used for innermost layers of vertex detectors in future experiments at the Super-LHC. r 2006 Elsevier B.V. All rights reserved. PACS: 29.40.Gx; 29.40.Wk; 61.82.Fk Keywords: Radiation damage; Silicon detector; Epitaxial layer; 24 GeV/c protons; Super-LHC
1. Introduction The planned Large Hadron Collider upgrade to the Super-LHC supposes an increase of expected fast hadron fluences up to 1016 cm2 for the innermost detector layers [1]. Present detectors manufactured on oxygen-enriched silicon substrates can survive up to 1015 1-MeV equivalent neutrons/cm2 [2]. At so high fluences radiation-induced traps strongly reduce the carrier effective drift length and the produced signal does not increase linearly with the sensor thickness. Instead, thin devices have advantages of lower depletion voltage and leakage current and allow to use highly -doped, low-resistivity materials. Obviously, this means capacitance increase which can be compensated by smaller sensor size, and signal decrease which demands improved electronics. In recent studies, the superior radiation tolerance of silicon (Si) detectors based on thin epitaxial layers on $ This study was performed in the framework of the CERN RD50 Collaboration and founded by the INFN SMART project. Corresponding author. Tel.: +39 049 827 7215; fax: +39 049 827 7237. E-mail address: [email protected] (V. Khomenkov).
0168-9002/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2006.05.202
Czochralski (CZ) substrate and irradiated by fast hadrons (24 GeV/c protons and fast nuclear reactor neutrons) [3] and 58 MeV Li ions [4] has been demonstrated. It has been shown that irradiation by fast hadrons and Li ions causes donor generation and thus leads to the compensation of radiation-induced damage. In the present paper we study the behaviour of thin epitaxial detectors on CZ substrate after irradiation with 24 GeV/c protons up to 1016 p/cm2 and their long-term annealing characteristics, showing that such devices can be a viable solution for the experiments at the Super-LHC. 2. Tested devices, their irradiation and electrical characterization The starting material was an epitaxial silicon layer (50 mm thick with nominal resistivity of 50 O cm) grown by ITME (Warsaw, Poland) on a 100 mm-diameter o1 1 14 n-type Czochralski (Cz) silicon wafers (300 mm thick with nominal resistivity of 0.02 O cm). The CZ substrate is the backside n+ ohmic contact, which is not depleted due to the high Sb doping level and acts also as a mechanical
ARTICLE IN PRESS V. Khomenkov et al. / Nuclear Instruments and Methods in Physics Research A 568 (2006) 61–65
support for the epitaxial layer which is the active volume of the detector. During the epitaxial growth, oxygen diffuses from the CZ substrate (OE1018 cm3) into the epitaxial layer (OE1017 cm3): high oxygen concentration in the epitaxial layer improves the detector radiation hardness due to donor activation during irradiation, which compensates the radiation-induced deep acceptors, as recently reported in Ref. [5]. The detector fabrication was performed at ITC-irst (Trento, Italy) in the framework of the INFN-SMART Project. The process is based on the technology developed at ITC-irst for realizing AC-coupled microstrip detectors on high-resistivity silicon substrates: (1) a 1 mm field-oxide is thermally grown at the beginning of the process; (2) boron implantations are performed through a screen oxide; (3) integrate coupling capacitors feature a recessed structure with a stacked dielectric insulator based on SiO2–Si3N4 layers. Both AC-coupled microstrip detectors and diode test structures were processed on the same wafer [6] but in this study we have focused on the investigation of diodes with guard rings to improve the breakdown voltage characteristics of the devices. The tested structures were square diodes with active area 13.7 mm2 surrounded by 1 large and 10 small guard rings. Devices were electrically characterized at room temperature by capacitance–voltage (C–V) measurements in serial mode at a frequency of 10 kHz and by current–voltage (I–V) measurements, in order to determine the full depletion voltage and the corresponding current density. During these measurements, the detector active area and the first guard ring were separately grounded to prevent edge effects, and the reverse bias was applied to the backside contact. The full depletion voltage Vdep is obtained by the intersection point of two linear fits before and after the kink of the C–V curve plotted in logarithmic scale. This value is used also for the determination of the effective doping concentration Neff: jN eff j ¼ 2V dep =ðqd 2 Þ,
(1)
where e is the silicon dielectric constant, q the electron charge, and d the active layer thickness. Eq. (1) explains the expedience of using thin detectors: decreasing the active-layer thickness makes it possible to increase significantly the effective doping concentration, while the full-depletion voltage still remains moderate. The leakage current volume density JD is determined from the I–V curve as a current at the depletion voltage point, divided by the active volume of the detector. Since this value strongly depends on temperature T, it is rescaled to the accepted standard temperature of 20 1C (293 K) using the well-known dependence. Eg J D ðTÞ / T 2 exp (2) kB T where Eg ¼ 1.12 eV is the forbidden gap in silicon, kB the Boltzmann constant and T the temperature in Kelvin.
The samples were irradiated by 24 GeV/c protons at the CERN PS facility [7]. Seven diodes were irradiated at different fluence values covering the range from 1.5 1015 up to 1016 p/cm2, and two diodes were irradiated the same highest fluence to check the reliability of the test results. After irradiation and between annealing cycles all samples were stored at 20 1C in order to freeze radiation-induced defects by decreasing their mobility.
3. Experimental results and discussion To study behaviour of the irradiated detectors during long-term operation cumulative annealing steps with electrical characterization after each step were performed from 4, 8,16, y up to 8192 min. The temperature of annealing was 80 1C to accelerate the process comparing to the room temperature (i.e., 20 1C). Annealing for 1 min at 80 1C corresponds approximately to 7400 min at room temperature [8].
3.1. Leakage current and depletion voltage The history of radiation damage study shows that leakage current density linearly increases with the particle fluence F: JD ¼ aF. The a parameter, called leakage current density increase rate, scales with the Non Ionizing Energy Loss (NIEL) of the impinging hadron radiation, independently from the substrate silicon characteristics and/or device processing [9]. Ratio ki ¼ ai/a(1 MeVn) for i-type radiation is called ‘‘Hardness Factor’’ and used to compare radiation damage from different irradiation. Fig. 1 shows the dependence of leakage current density on the proton fluence. As was expected it is linear both after irradiation and at different stages of isothermal annealing. Using hardness factor 0.62 obtained for 24 GeV/c protons [10], we can calculate a value scaled to 1 MeV α (10-17 A/cm)
0.40 after irradiation, 8 min@80°C, 128 min@80°C, 8192 min@80°C,
0.35 JD at Vdep, 20°C (A/cm3)
62
0.30
3.27+0.36 2.74+0.39 1.55+0.12 0.98+0.17
0.25 0.20 0.15 0.10 0.05 0.00
0
2
4
6
8
10
Fluence (1015 p/cm2) Fig. 1. Leakage current density as a function of proton fluence and a values after irradiation and at different annealing stages.
ARTICLE IN PRESS V. Khomenkov et al. / Nuclear Instruments and Methods in Physics Research A 568 (2006) 61–65
400 20
300
15
200
10
150 100
3
2 α0 = (2.29±0.07)×10-17 A/cm
5
α1 = (1.24±0.08)×10-17 A/cm
1
τ1 = (19.6±2.5) min α2 = (1.48±0.10)×10-18 A/cm
50 0
0 0
2
4
6
8
0
10
Fluence (1015 p/cm2) Fig. 2. Depletion voltage (effective doping concentration) as a function of proton fluence after irradiation and at different annealing stages.
400 350
(3)
where annealing time t is in minutes and t2 is set to 1 min.We successfully used this expression for fitting. The results are given in Fig. 3. 3.3. Annealing of effective doping concentration The evolution of depletion voltage with annealing time is shown in Fig. 4. As clearly seen, for all samples Vdep initially increases, and then decreases after 8 min annealing. Such a behaviour is typical for not inverted n-type detectors. Then the values for the sample irradiated by the lowest fluence of 2.2 1015 p/cm2 continue decreasing, while for all other samples they reach deep minima (as earlier as higher the fluence is) and become increasing
103
104
Annealing time (min)
Fluence (1015 p/cm2): 2.2 8.3 4.0 10.1 6.1 10.1
20
Vdep (V)
15
250 200
10
150 100
5
50 after irradiation
The evolution of the reverse current damage constant a with the annealing time is shown in Fig. 3. It continuously decreases with time due to the annealing of radiationinduced defects responsible for the current generation. For a wide spectra of radiations (hadrons [9], Li ions [11], and high-energy electrons [12]) a(t) dependence can be fitted by:
102
300
0
3.2. Annealing of a
101
25 450
aeq(8 min at 80 1C) ¼ (4.4270.63) 1017 A/cm. The dependence of depletion voltage on the equivalent fluence is shown in Fig. 2. First of all, it should be noted that the depletion voltage after irradiation does not exceed 220 V even for the highest irradiation fluence, hence, the detectors still remain operable. Secondly, Vdep decreases and then increases almost linearly, with the minimal values about 90 V. As is shown by the annealing study, the irradiation does not result in space charge sign inversion (SCSI) of detectors.
100
Fig. 3. Damage constant a as a function of annealing time at 80 1C and its fit.
neutron equivalent fluence, corresponding to the end of short-term annealing (see Fig. 4):
aðtÞ ¼ a0 þ a1 expðt=t1 Þ a2 lnðt=t2 Þ
after irradiation
⏐Neff⏐ (1013 cm-3)
Vdep (V)
250
α (10-17 A/cm)
350
Fluence (1015 p/cm2) 1.5 2.2 4.0 6.1 8.3 10.1 Average Fitting curve
4
⏐Neff⏐ (1013 cm-3)
after irradiation 8 min@80°C 128 min@80°C 8192 min@80°C
63
0 100
101
102
103
104
Annealing time (min)
Fig. 4. Depletion voltage as a function of annealing time at 80 1C for different proton fluences.
again. Such a behaviour is the evidence that the space charge sign inversion occurs during long-term annealing. Taking all mentioned into account we plot in Fig. 5 differential effective doping concentration DNeff(t) ¼ Neff0– Neff(t) as a function of annealing time. This dependence was successfully fitted with the expression [8] DN eff ðtÞ ¼ N eff;0 N eff ðtÞ ¼ N C þ N 1 exp t=t1 1 þ N 2 1 exp t=t2 þ N 3 1 1 þ t=t3 ð4Þ The results of fitting are given in the Table 1. In the Eq. (4) Neff,0 is effective doping concentration before irradiation, NC a stable radiation damage component, the next three terms are initial annealing effect and the first and second order reverse annealing processes, with corresponding effective concentrations N and time constants t. The time
ARTICLE IN PRESS V. Khomenkov et al. / Nuclear Instruments and Methods in Physics Research A 568 (2006) 61–65
64
constant for the first order reverse annealing should be independent on fluence, while for the second order process it depends on initial concentration of defects taking part in the reaction. Hence, t1 and t2 are chosen common for all samples, and t3 is varying. The dependence on fluence of concentration parameters used for fitting in Eq.(4) is shown in Fig. 6. As one can see, it is almost linear for all parameters. The results of linear interpolation Ni ¼ ai+biF are given in Table 2. From all of these parameters the most interesting are a and b for NC, i.e. stable damage. In Ref [8] the dependence of stable damage on fluence is described as N C ðFÞ ¼ N C0 ½1 expðcFÞ þ gc F
leakage current and full depletion voltage increase. They can survive after irradiation by fast protons up to the fluence 1016 cm2 because of high initial oxygen concentration. Donor generation during irradiation prevails, thus keeping positive space charge and providing moderate changes in depletion voltage. Long-term annealing test at 80 1C has shown that radiation-induced donors annealing and following acceptors generation leads to continuous decrease of effective doping concentration and even to inversion of space charge. Thus, a properly chosen irradiation scenario, in particular, storage temperature during beam-off time, might allow to keep appropriate operation voltage. Charge collection efficiency was not measured in this work. However, recent measurements of
(5)
where the first term corresponds to incomplete donor removal and the second one to radiation induced defect generation. Constant gC (defect generation rate, bC in our notation) is negative, i.e. we observe that donor generation during irradiation prevails over the acceptor generation.
30 NC
25
N (1013 cm-3)
N1
4. Conclusions Silicon detectors based on thin epitaxial layer have again proven their superior radiation tolerance in terms of
20
N2
15
N3
10 5 0 -5
25
∆Neff (1013 cm-3)
20
2.2×1015 p/cm2
-10
4.0×1015 p/cm2
-15 0
6.1×1015 p/cm2
2
15
10.1×1015 p/cm2
6
8
10
Fig. 6. Dependence of concentration fitting parameters on proton fluence and its linear interpolation.
10.1×1015 p/cm2
10
4
Fluence (1015 p/cm2)
8.3×1015 p/cm2
5 Table 2 Fitting parameters for effective doping concentration in epitaxial silicon diodes irradiated by 24 GeV/c protons
0 -5 after irradiation
100
101 102 Annealing time (min)
103
104
Fig. 5. Evolution of differential effective doping concentration with annealing time at 80 1C for different proton fluences and fitting curves.
Parameter
a (1013 cm3)
b (102 cm1)
NC N1 N2 N3
2.5170.36 1.9371.01 2.3071.50 0.9771.37
1.3270.05 0.4870.14 2.4870.20 1.6770.18
Table 1 Fitting parameters for effective doping concentration in epitaxial silicon diodes irradiated by 24 GeV/c protons F (1015 p/cm2) 13
3
Neff,0 (10 cm ) NC (1013 cm3) N1 (1013 cm3) N2 (1013 cm3) N3 (1013 cm3) t1 (min) t2 (min) t3 (103 min)
2.2
4.0
6.1
8.3
10.1
10.1
7.7 0.570.3 1.970.5 2.670.7 4.170.4 4.970.5 6873 1.270.6
7.5 3.270.4 4.570.6 6.571.3 9.170.9
7.2 6.870.4 5.770.5 14.570.6 10.770.4
7.2 9.170.5 6.870.6 19.970.6 13.170.7
7.2 10.070.5 6.270.6 21.970.6 19.170.8
7.2 10.070.5 6.170.6 21.770.6 17.870.8
0.470.1
1.270.2
2.970.6
3.170.5
3.170.5
ARTICLE IN PRESS V. Khomenkov et al. / Nuclear Instruments and Methods in Physics Research A 568 (2006) 61–65
similar devices with minimal ionizing electrons [13] and a particles [14] show that CCE remains at 70–80% after irradiation with 24 GeV/c protons up to F ¼ 1016 p/cm2. All these make epitaxial detectors very promising for use in the innermost layers of vertex detectors in experiments at the Super-LHC. References [1] F. Gianotti, Physics potential and experimental challenges of the LHC luminosity upgrade, hep-ph/0204087, April 2002. [2] G. Lindstrom, M. Ahmed, S. Albergo, et al., Nucl. Instr. and Meth. A 466 (2001) 308. [3] G. Kramberger, D. Contarato, E. Fretwurst, et al., Nucl. Instr. and Methods. A 515 (2003) 665. [4] A. Candelori, A. Schramm, D. Bisello, et al., IEEE Trans. Nucl. Sci. NS-51 (2004) 1766.
65
[5] I. Pintilie, M. Buda, E. Fretwurst, et al., Nucl. Instr. and Meth. A 552 (2005) 56. [6] M. Bruzzi, D. Bisello, L. Borello, et al., Nucl. Instr. and Meth. A 552 (2005) 20. [7] http://irradiation.web.cern.ch/irradiation/ [8] M. Moll, Ph.D. thesis, University of Hamburg, 1999; DESYTHESIS-1999-040, December 1999. [9] M. Moll, E. Fretwurst, G. Lindstro¨m, Nucl. Instr. and Meth. A 426 (1999) 87. [10] M. Moll, E. Fretwurst, M. Kuhnke, G. Lindstroem, Nucl. Instr. and Meth. B 186 (2002) 100. [11] A. Candelori, D. Bisello, G.F. Dalla Betta, et al., IEEE Trans. Nucl. Sci. NS-51 (2004) 2865. [12] S. Dittongo, L. Bosisio, M. Ciacchi, et al., IEEE Trans. Nucl. Sci. NS-51 (2004) 2794. [13] G. Kramberger, V. Cindro, I. Dolenc, et al., Nucl. Instr. and Meth. A 554 (2005) 212. [14] G. Lindstrom, E. Fretwurst, F. Honniger, et al., Nucl. Instr. and Meth. A 556 (2006) 451.
Nuclear Instruments and Methods in Physics Research A 568 (2006) 61–65 www.elsevier.com/locate/nima
Irradiation effects on thin epitaxial silicon detectors$ V. Khomenkova,, D. Biselloa, M. Bruzzib, A. Candeloria, A. Litovchenkoa, C. Piemontec, R. Randoa, F. Ravottid, N. Zorzic a Dipartimento di Fisica, Universita` di Padova and INFN Sezione di Padova, Via Marzolo 8, I-35131, Padova, Italy Dipartimento di Energetica, Universita` di Firenze and INFN Sezione di Firenze, Via S. Marta 3, I-50139, Firenze, Italy c ITC-irst, Microsystems Division, Via Sommarive 18, I-38050, Povo (Trento), Italy d CERN, Geneve 23, CH-1211, Switzerland
b
Available online 28 June 2006
Abstract Radiation hardness of silicon detectors based on thin epitaxial layer on Czochralski (CZ) substrate for the LHC upgrade (Super-LHC) was studied. No type inversion was observed after irradiation by 24 GeV/c protons up to the fluence of 1016 p/cm2 due to overcompensating donor generation. After long-term annealing (corresponding to 500 days at room temperature) proton irradiated devices show a decrease of the effective doping concentration and then undergo type inversion. Measurements confirm that thin epitaxial devices on CZ substrate could be used for innermost layers of vertex detectors in future experiments at the Super-LHC. r 2006 Elsevier B.V. All rights reserved. PACS: 29.40.Gx; 29.40.Wk; 61.82.Fk Keywords: Radiation damage; Silicon detector; Epitaxial layer; 24 GeV/c protons; Super-LHC
1. Introduction The planned Large Hadron Collider upgrade to the Super-LHC supposes an increase of expected fast hadron fluences up to 1016 cm2 for the innermost detector layers [1]. Present detectors manufactured on oxygen-enriched silicon substrates can survive up to 1015 1-MeV equivalent neutrons/cm2 [2]. At so high fluences radiation-induced traps strongly reduce the carrier effective drift length and the produced signal does not increase linearly with the sensor thickness. Instead, thin devices have advantages of lower depletion voltage and leakage current and allow to use highly -doped, low-resistivity materials. Obviously, this means capacitance increase which can be compensated by smaller sensor size, and signal decrease which demands improved electronics. In recent studies, the superior radiation tolerance of silicon (Si) detectors based on thin epitaxial layers on $ This study was performed in the framework of the CERN RD50 Collaboration and founded by the INFN SMART project. Corresponding author. Tel.: +39 049 827 7215; fax: +39 049 827 7237. E-mail address: [email protected] (V. Khomenkov).
0168-9002/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2006.05.202
Czochralski (CZ) substrate and irradiated by fast hadrons (24 GeV/c protons and fast nuclear reactor neutrons) [3] and 58 MeV Li ions [4] has been demonstrated. It has been shown that irradiation by fast hadrons and Li ions causes donor generation and thus leads to the compensation of radiation-induced damage. In the present paper we study the behaviour of thin epitaxial detectors on CZ substrate after irradiation with 24 GeV/c protons up to 1016 p/cm2 and their long-term annealing characteristics, showing that such devices can be a viable solution for the experiments at the Super-LHC. 2. Tested devices, their irradiation and electrical characterization The starting material was an epitaxial silicon layer (50 mm thick with nominal resistivity of 50 O cm) grown by ITME (Warsaw, Poland) on a 100 mm-diameter o1 1 14 n-type Czochralski (Cz) silicon wafers (300 mm thick with nominal resistivity of 0.02 O cm). The CZ substrate is the backside n+ ohmic contact, which is not depleted due to the high Sb doping level and acts also as a mechanical
ARTICLE IN PRESS V. Khomenkov et al. / Nuclear Instruments and Methods in Physics Research A 568 (2006) 61–65
support for the epitaxial layer which is the active volume of the detector. During the epitaxial growth, oxygen diffuses from the CZ substrate (OE1018 cm3) into the epitaxial layer (OE1017 cm3): high oxygen concentration in the epitaxial layer improves the detector radiation hardness due to donor activation during irradiation, which compensates the radiation-induced deep acceptors, as recently reported in Ref. [5]. The detector fabrication was performed at ITC-irst (Trento, Italy) in the framework of the INFN-SMART Project. The process is based on the technology developed at ITC-irst for realizing AC-coupled microstrip detectors on high-resistivity silicon substrates: (1) a 1 mm field-oxide is thermally grown at the beginning of the process; (2) boron implantations are performed through a screen oxide; (3) integrate coupling capacitors feature a recessed structure with a stacked dielectric insulator based on SiO2–Si3N4 layers. Both AC-coupled microstrip detectors and diode test structures were processed on the same wafer [6] but in this study we have focused on the investigation of diodes with guard rings to improve the breakdown voltage characteristics of the devices. The tested structures were square diodes with active area 13.7 mm2 surrounded by 1 large and 10 small guard rings. Devices were electrically characterized at room temperature by capacitance–voltage (C–V) measurements in serial mode at a frequency of 10 kHz and by current–voltage (I–V) measurements, in order to determine the full depletion voltage and the corresponding current density. During these measurements, the detector active area and the first guard ring were separately grounded to prevent edge effects, and the reverse bias was applied to the backside contact. The full depletion voltage Vdep is obtained by the intersection point of two linear fits before and after the kink of the C–V curve plotted in logarithmic scale. This value is used also for the determination of the effective doping concentration Neff: jN eff j ¼ 2V dep =ðqd 2 Þ,
(1)
where e is the silicon dielectric constant, q the electron charge, and d the active layer thickness. Eq. (1) explains the expedience of using thin detectors: decreasing the active-layer thickness makes it possible to increase significantly the effective doping concentration, while the full-depletion voltage still remains moderate. The leakage current volume density JD is determined from the I–V curve as a current at the depletion voltage point, divided by the active volume of the detector. Since this value strongly depends on temperature T, it is rescaled to the accepted standard temperature of 20 1C (293 K) using the well-known dependence. Eg J D ðTÞ / T 2 exp (2) kB T where Eg ¼ 1.12 eV is the forbidden gap in silicon, kB the Boltzmann constant and T the temperature in Kelvin.
The samples were irradiated by 24 GeV/c protons at the CERN PS facility [7]. Seven diodes were irradiated at different fluence values covering the range from 1.5 1015 up to 1016 p/cm2, and two diodes were irradiated the same highest fluence to check the reliability of the test results. After irradiation and between annealing cycles all samples were stored at 20 1C in order to freeze radiation-induced defects by decreasing their mobility.
3. Experimental results and discussion To study behaviour of the irradiated detectors during long-term operation cumulative annealing steps with electrical characterization after each step were performed from 4, 8,16, y up to 8192 min. The temperature of annealing was 80 1C to accelerate the process comparing to the room temperature (i.e., 20 1C). Annealing for 1 min at 80 1C corresponds approximately to 7400 min at room temperature [8].
3.1. Leakage current and depletion voltage The history of radiation damage study shows that leakage current density linearly increases with the particle fluence F: JD ¼ aF. The a parameter, called leakage current density increase rate, scales with the Non Ionizing Energy Loss (NIEL) of the impinging hadron radiation, independently from the substrate silicon characteristics and/or device processing [9]. Ratio ki ¼ ai/a(1 MeVn) for i-type radiation is called ‘‘Hardness Factor’’ and used to compare radiation damage from different irradiation. Fig. 1 shows the dependence of leakage current density on the proton fluence. As was expected it is linear both after irradiation and at different stages of isothermal annealing. Using hardness factor 0.62 obtained for 24 GeV/c protons [10], we can calculate a value scaled to 1 MeV α (10-17 A/cm)
0.40 after irradiation, 8 min@80°C, 128 min@80°C, 8192 min@80°C,
0.35 JD at Vdep, 20°C (A/cm3)
62
0.30
3.27+0.36 2.74+0.39 1.55+0.12 0.98+0.17
0.25 0.20 0.15 0.10 0.05 0.00
0
2
4
6
8
10
Fluence (1015 p/cm2) Fig. 1. Leakage current density as a function of proton fluence and a values after irradiation and at different annealing stages.
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400 20
300
15
200
10
150 100
3
2 α0 = (2.29±0.07)×10-17 A/cm
5
α1 = (1.24±0.08)×10-17 A/cm
1
τ1 = (19.6±2.5) min α2 = (1.48±0.10)×10-18 A/cm
50 0
0 0
2
4
6
8
0
10
Fluence (1015 p/cm2) Fig. 2. Depletion voltage (effective doping concentration) as a function of proton fluence after irradiation and at different annealing stages.
400 350
(3)
where annealing time t is in minutes and t2 is set to 1 min.We successfully used this expression for fitting. The results are given in Fig. 3. 3.3. Annealing of effective doping concentration The evolution of depletion voltage with annealing time is shown in Fig. 4. As clearly seen, for all samples Vdep initially increases, and then decreases after 8 min annealing. Such a behaviour is typical for not inverted n-type detectors. Then the values for the sample irradiated by the lowest fluence of 2.2 1015 p/cm2 continue decreasing, while for all other samples they reach deep minima (as earlier as higher the fluence is) and become increasing
103
104
Annealing time (min)
Fluence (1015 p/cm2): 2.2 8.3 4.0 10.1 6.1 10.1
20
Vdep (V)
15
250 200
10
150 100
5
50 after irradiation
The evolution of the reverse current damage constant a with the annealing time is shown in Fig. 3. It continuously decreases with time due to the annealing of radiationinduced defects responsible for the current generation. For a wide spectra of radiations (hadrons [9], Li ions [11], and high-energy electrons [12]) a(t) dependence can be fitted by:
102
300
0
3.2. Annealing of a
101
25 450
aeq(8 min at 80 1C) ¼ (4.4270.63) 1017 A/cm. The dependence of depletion voltage on the equivalent fluence is shown in Fig. 2. First of all, it should be noted that the depletion voltage after irradiation does not exceed 220 V even for the highest irradiation fluence, hence, the detectors still remain operable. Secondly, Vdep decreases and then increases almost linearly, with the minimal values about 90 V. As is shown by the annealing study, the irradiation does not result in space charge sign inversion (SCSI) of detectors.
100
Fig. 3. Damage constant a as a function of annealing time at 80 1C and its fit.
neutron equivalent fluence, corresponding to the end of short-term annealing (see Fig. 4):
aðtÞ ¼ a0 þ a1 expðt=t1 Þ a2 lnðt=t2 Þ
after irradiation
⏐Neff⏐ (1013 cm-3)
Vdep (V)
250
α (10-17 A/cm)
350
Fluence (1015 p/cm2) 1.5 2.2 4.0 6.1 8.3 10.1 Average Fitting curve
4
⏐Neff⏐ (1013 cm-3)
after irradiation 8 min@80°C 128 min@80°C 8192 min@80°C
63
0 100
101
102
103
104
Annealing time (min)
Fig. 4. Depletion voltage as a function of annealing time at 80 1C for different proton fluences.
again. Such a behaviour is the evidence that the space charge sign inversion occurs during long-term annealing. Taking all mentioned into account we plot in Fig. 5 differential effective doping concentration DNeff(t) ¼ Neff0– Neff(t) as a function of annealing time. This dependence was successfully fitted with the expression [8] DN eff ðtÞ ¼ N eff;0 N eff ðtÞ ¼ N C þ N 1 exp t=t1 1 þ N 2 1 exp t=t2 þ N 3 1 1 þ t=t3 ð4Þ The results of fitting are given in the Table 1. In the Eq. (4) Neff,0 is effective doping concentration before irradiation, NC a stable radiation damage component, the next three terms are initial annealing effect and the first and second order reverse annealing processes, with corresponding effective concentrations N and time constants t. The time
ARTICLE IN PRESS V. Khomenkov et al. / Nuclear Instruments and Methods in Physics Research A 568 (2006) 61–65
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constant for the first order reverse annealing should be independent on fluence, while for the second order process it depends on initial concentration of defects taking part in the reaction. Hence, t1 and t2 are chosen common for all samples, and t3 is varying. The dependence on fluence of concentration parameters used for fitting in Eq.(4) is shown in Fig. 6. As one can see, it is almost linear for all parameters. The results of linear interpolation Ni ¼ ai+biF are given in Table 2. From all of these parameters the most interesting are a and b for NC, i.e. stable damage. In Ref [8] the dependence of stable damage on fluence is described as N C ðFÞ ¼ N C0 ½1 expðcFÞ þ gc F
leakage current and full depletion voltage increase. They can survive after irradiation by fast protons up to the fluence 1016 cm2 because of high initial oxygen concentration. Donor generation during irradiation prevails, thus keeping positive space charge and providing moderate changes in depletion voltage. Long-term annealing test at 80 1C has shown that radiation-induced donors annealing and following acceptors generation leads to continuous decrease of effective doping concentration and even to inversion of space charge. Thus, a properly chosen irradiation scenario, in particular, storage temperature during beam-off time, might allow to keep appropriate operation voltage. Charge collection efficiency was not measured in this work. However, recent measurements of
(5)
where the first term corresponds to incomplete donor removal and the second one to radiation induced defect generation. Constant gC (defect generation rate, bC in our notation) is negative, i.e. we observe that donor generation during irradiation prevails over the acceptor generation.
30 NC
25
N (1013 cm-3)
N1
4. Conclusions Silicon detectors based on thin epitaxial layer have again proven their superior radiation tolerance in terms of
20
N2
15
N3
10 5 0 -5
25
∆Neff (1013 cm-3)
20
2.2×1015 p/cm2
-10
4.0×1015 p/cm2
-15 0
6.1×1015 p/cm2
2
15
10.1×1015 p/cm2
6
8
10
Fig. 6. Dependence of concentration fitting parameters on proton fluence and its linear interpolation.
10.1×1015 p/cm2
10
4
Fluence (1015 p/cm2)
8.3×1015 p/cm2
5 Table 2 Fitting parameters for effective doping concentration in epitaxial silicon diodes irradiated by 24 GeV/c protons
0 -5 after irradiation
100
101 102 Annealing time (min)
103
104
Fig. 5. Evolution of differential effective doping concentration with annealing time at 80 1C for different proton fluences and fitting curves.
Parameter
a (1013 cm3)
b (102 cm1)
NC N1 N2 N3
2.5170.36 1.9371.01 2.3071.50 0.9771.37
1.3270.05 0.4870.14 2.4870.20 1.6770.18
Table 1 Fitting parameters for effective doping concentration in epitaxial silicon diodes irradiated by 24 GeV/c protons F (1015 p/cm2) 13
3
Neff,0 (10 cm ) NC (1013 cm3) N1 (1013 cm3) N2 (1013 cm3) N3 (1013 cm3) t1 (min) t2 (min) t3 (103 min)
2.2
4.0
6.1
8.3
10.1
10.1
7.7 0.570.3 1.970.5 2.670.7 4.170.4 4.970.5 6873 1.270.6
7.5 3.270.4 4.570.6 6.571.3 9.170.9
7.2 6.870.4 5.770.5 14.570.6 10.770.4
7.2 9.170.5 6.870.6 19.970.6 13.170.7
7.2 10.070.5 6.270.6 21.970.6 19.170.8
7.2 10.070.5 6.170.6 21.770.6 17.870.8
0.470.1
1.270.2
2.970.6
3.170.5
3.170.5
ARTICLE IN PRESS V. Khomenkov et al. / Nuclear Instruments and Methods in Physics Research A 568 (2006) 61–65
similar devices with minimal ionizing electrons [13] and a particles [14] show that CCE remains at 70–80% after irradiation with 24 GeV/c protons up to F ¼ 1016 p/cm2. All these make epitaxial detectors very promising for use in the innermost layers of vertex detectors in experiments at the Super-LHC. References [1] F. Gianotti, Physics potential and experimental challenges of the LHC luminosity upgrade, hep-ph/0204087, April 2002. [2] G. Lindstrom, M. Ahmed, S. Albergo, et al., Nucl. Instr. and Meth. A 466 (2001) 308. [3] G. Kramberger, D. Contarato, E. Fretwurst, et al., Nucl. Instr. and Methods. A 515 (2003) 665. [4] A. Candelori, A. Schramm, D. Bisello, et al., IEEE Trans. Nucl. Sci. NS-51 (2004) 1766.
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[5] I. Pintilie, M. Buda, E. Fretwurst, et al., Nucl. Instr. and Meth. A 552 (2005) 56. [6] M. Bruzzi, D. Bisello, L. Borello, et al., Nucl. Instr. and Meth. A 552 (2005) 20. [7] http://irradiation.web.cern.ch/irradiation/ [8] M. Moll, Ph.D. thesis, University of Hamburg, 1999; DESYTHESIS-1999-040, December 1999. [9] M. Moll, E. Fretwurst, G. Lindstro¨m, Nucl. Instr. and Meth. A 426 (1999) 87. [10] M. Moll, E. Fretwurst, M. Kuhnke, G. Lindstroem, Nucl. Instr. and Meth. B 186 (2002) 100. [11] A. Candelori, D. Bisello, G.F. Dalla Betta, et al., IEEE Trans. Nucl. Sci. NS-51 (2004) 2865. [12] S. Dittongo, L. Bosisio, M. Ciacchi, et al., IEEE Trans. Nucl. Sci. NS-51 (2004) 2794. [13] G. Kramberger, V. Cindro, I. Dolenc, et al., Nucl. Instr. and Meth. A 554 (2005) 212. [14] G. Lindstrom, E. Fretwurst, F. Honniger, et al., Nucl. Instr. and Meth. A 556 (2006) 451.