FAST NEUTRON DETECTOR WITH A SURFACE-BARRIER VPE …
1
Device fabrication Fig. 5 – Photo of the 2-inch GaAs substrate with the fabricated sensor structures. Chip size = 6 x 6 mm 2 Active area = 5 x 5 mm 2 Fig. 6 – Photo of the packaged GaAs sensor. Fig. 4 – Schematic cross-section of the GaAs sensor. n ++ -GaAs substrate i-GaAs working layer ohmic contact Schottky barrier polyimide n + -GaAs buffer contact pad Structure parameters Schottky contact P t/T iN/Au,0. 1 μm i -layer chloride epitaxy, 42 μm,3 · 10 11 cm - 3 (undoped) n + -buffer chloride epitaxy, 4 . 5 μm,7 · 10 17 cm - 3 ( S doped) n ++ -substrate LEC, 500 μm,2 · 10 18 cm - 3 ( Si doped) Ohmic contact Ni/AuGe/Au A.V. Chernykh1, S.V. Chernykh1, S.I. Didenko1, G.I. Koltsov1 , F.M. Baryshnikov1, G.I. Britvich2, M.Yu. Kostin2, A.P. Chubenko3, Yu.N. Sveshnikov4, N. Burtebayev5 1 – National University of Science and Technology «MISIS», Moscow, Russia 2 – Institute of High Energy Physics, Protvino, Russia 3 – P.N. Lebedev Physical Institute of the Russian Academy of Sciences, Moscow, Russia 4 – «Elma–Malahit» JSC–«Concern Energomera», Moscow, Zelenograd, Russia 5 – Institute of Nuclear Physics, Almaty, Kazakhstan Applications • Fast neutron personal dosimeters for personnel monitoring behind the radiation shield of nuclear facilities, such as reactors, accelerators, neutron generators for medical uses, etc. • Neutron flux measurements of nuclear reactors. • Space and accelerator experiments with high intensities, where high radiation resistance, high time resolution and thermal stability are required. • Proton and alpha-particle contamination monitoring of special equipment (e.g. the monitoring of reprocessed fuel assemblies in the nuclear industry at the temperatures up to 100 o C). Conclusions • Samples of fast neutron detectors based on thin high-purity GaAs epitaxial layers were fabricated. • The active area of the detector is 25 mm 2 . • The fast neutron detection efficiency is 1.04·10 -3 and signal-to-background ratio is 50 at the operating bias. • At zero bias the efficiency of the detector varies lightly and becomes 0.92·10 -3 and signal-to-background ratio becomes 44. • The use of thin epitaxial films as the active region of neutron detectors allows to reduce the contribution of gamma-rays and hence to reduce therecoil proton detection threshold, thereby increasing the fast neutron detection efficiency and the signal-to-background ratio. References 1. D.S. McGregor et al. NIM A 466, 2001, pp.126–141 2. А. Šаgátová-Perdоchová et al. NIM A 591, 2008, pp.98–100 3. А. Šаgátová et al. 2013 JINST 8 C03016 4. C. Crowell, S. Sze. Solid-State Electronics, 9 (1112), 1966, pp.1035–1048 5. G. I. Koltsov et al. Semiconductors, 46 (8), 2012, pp.1066–1071 Acknowledgements The authors are grateful to Evgenia Savchenko and Pavel Ivshin for their assistance in the poster preparation. Introduction Nowadays neutron methods are widely adopted. For some applications detectors based on semiconductor materials are used. Generally, the operation principle of such detectors is based on the detection of secondary particles produced by neutrons in converter material with a semiconductor sensor. Depending on the neutron energy (n,α)-reactions are used for the thermal neutron detection and the recoil proton method is used for the fast neutron detection [1]. Previous works have shown some advantages of using bulk SI GaAs as neutron detectors [2, 3]. This work was aimed on the development of a fast neutron detector using high-purity GaAs layers grown by vapor-phase epitaxy. The using of thin high-purity GaAs epilayers allows to reduce the operating bias of a sensor and the signal-to-background ratio in comparison with bulk SI GaAs material. Characteristics of the epilayers The Ga-AsCl3-H2 VPE process in a vertical reactor was used to grow high-purity GaAs epilayers. In order to reduce the concentration of Group VI impurities (primarily, S and Se) in the growing layers, the temperature of the process was increased and the growth rate was decreased. In order to prevent the introduction of silicon (the main background impurity of Group IV), a small amount of water was added into the gas phase above the gallium source. Fig. 1 – Dependence of the charge carrier concentration in the GaAs epilayers on the concentration of water vapor above the Ga source. Process and layer parameters are given in Table growth rate, μm/hour 9–10 i -layer thickness, μm up to 120 carrier concentration, cm -3 ~ 10 11 μ e · τ e , cm 2 /V 2 · 10 -4 μ p · τ p , cm 2 /V 2 · 10 -5 EL2 concentration, cm -3 ~ 10 13 Ti/Pd/Au and Pt/TiN/Au metallization systems were studied as a Schottky contact. According to the criterion proposed by Crowell and Sze [4] the diffusion theory of rectification was used to describe the charge transport through the fabricated contacts [5]. where I 0 – the reverse metal-semiconductor current, q – the electron charge, V – the voltage drop across the Schottky barrier, m – the ideality factor, k – Boltzmann's constant, T – the absolute temperature, μ n – the electron drift mobility, N C – the effective density of states in the conduction band, S – the contact area, n b – the concentration of electrons in the semiconductor, ε S –the semiconductor permittivity, V bi – the built-in potential, φ B – the Schottky barrier height. Fig. 2 – I–V characteristic of the Pt- contact linearized in the coordinates ln[I/(V bi –V) 1/2 ]=f(V) in the range kT/q<<V<V bi . Fig. 3 – b 0 =f(1/T) dependence for both contacts. ( ) − = 1 exp 0 mkT qV V I I ( ) ( ) − − ⋅ = kT q V V qn S N q V I B bi s b C n ϕ ε µ exp 2 2 1 0 kT b q m 1 = − ⋅ = 0 2 1 2 ln b qn S N q q kT s b C n B ε µ ϕ (1) (2) (3) (4) Comparison table Parameter Ti/Pd/Au Pt/TiN/Au n 1 . 6–1 . 7 1 . 5 φ b , eV 0 . 84 0 . 87 CCE α (5 . 5 MeV ), % at 0 V 50 60 CCE ɣ (14–17 keV ), % at 0 V 59 78 Schottky contact selection Electrical characteristics Fig. 7 – IV-characteristic of the sensor based on high-purity GaAs epilayers. The average dark leakage currents: 1.0 nA at - 50 V and 1.8 nA at -100 V. The best dark leakage currents: 0.7 nA at - 50 V and 1.2 nA at -100 V. Fig. 8 – CV-characteristic of the GaAs sensor (the specific capacitance in the saturation region is 280 pF/cm 2 ). Alpha-particle characterization Measurement conditions: • measuring bench with an 8192-channel ADC; • room temperature; • vacuum (residual pressure 1.33 Pa); • distance between the alpha-source and the detector was 7 cm. Fig. 9 – The typical energy spectrum of alpha-particles from the 238 Pu source measured with the fabricated GaAs sensor. Fig. 10 – Dependence of the charge collection efficiency measured using the 238 Pu-source on the applied bias. Fast neutrons 2. Fast neutron spectral measurements for various converter thicknesses The recoil proton method was used for the detection of fast neutrons. The 241 Am-Be source with the activity A=1.62·10 6 Bq was used as a fast neutron source. Polyethylene was used as a converter layer. Films with a thickness of 80 microns were used to set the required thickness of the converter. The films were placed directly on the package at the distance of 2 mm from the GaAs sensor. Fig. 12 – Spectrum of 241 Am-Be measured for different converter thicknesses (the distance from the source R = 2 cm, the time t = 1000 s, V b = -24 V). Measurement results at V b = -24 V (The detection threshold of recoil protons was selected 1.2 MeV) Fig. 13 – Spectrum of 241 Am-Be (the distance from the source is 11.5 cm) 1. In the range from 1.2 MeV to 4.5 MeV N n =259, N bg =5 for t=1000 sec. 2. For the area of S=5×5 mm 2 the incident flux is neutr./sec 3. Neutron detection efficiency Fig. 14 – Fast neutron detection efficiency for different polyethylene converter thicknesses. 4. Measurements in self-biased mode Fig. 15 – Fast neutron spectrum from the 241 Am-Be source at different biases. The detection efficiency and the signal-to-background ratio are slightly lower than in the case of working bias. ŋ( at 0 V) = 9.2·10 -4 puls./neutr. S/B(at 0 V) = 44.6 3. Measurements of the fast neutron detection efficiency d ( CH 2 ) n , μm N n , puls. N n - N bg , puls. signal/background N n - N bg ,% 80 1569 1512 26 . 5 51 . 4 160 2071 2014 35 . 3 68 . 5 240 2590 2533 44 . 4 86 . 1 480 2946 2889 50 . 7 98 . 2 720 2999 2942 51 . 6 100 . 0 960 2983 2926 51 . 3 99.5 Fig. 11 – Calibration of the measuring bench with the 239 Pu α-source. The source was located close to the detector. 1. The measuring bench and calibration with alpha-particles. The measurements were carried out at room temperature and atmospheric pressure. The detector was connected to an AS01PDA chip developed by JSC "INTEGRAL" (Minsk, Belarus). The AS01PDA chip contains a charge-sensitive amplifier, a pulse shaper and a buffer stage (Gain = 3.5 mV/fC, τ 1/2 = 140 ns). A 1024-channel spectrum analyzer «Angamm» (Dubna, Russia) was also used. 16th iWoRiD I n t e r n a t i o n a l W o r k s h o p o n R a d i a t i o n I m a g i n g D e t e c t o r s FAST NEUTRON DETECTOR WITH A SURFACE-BARRIER VPE GaAs SENSOR 7 . 243 4 2 0 = ⋅ = R S A N π
Transcript of FAST NEUTRON DETECTOR WITH A SURFACE-BARRIER VPE …
Device fabrication
Fig. 5 – Photo of the 2-inch GaAs substrate with the fabricated sensor structures.
Chip size = 6 x 6 mm2
Active area = 5 x 5 mm2
Fig. 6 – Photo of the packaged GaAs sensor.
Fig. 4 – Schematic cross-section of the GaAs sensor.
n++-GaAs substrate
i-GaAs working layer
ohmic contact
Schottky barrierpolyimide
n+-GaAs buffer
contact pad
Structure parameters
Schottky contact P t/T iN/Au, 0.1 µmi -layer chloride epitaxy, 42 µm, 3 · 1011 cm− 3 (undoped)
n+ -bu�er chloride epitaxy, 4 .5 µm, 7 · 1017 cm− 3 (S doped)n++ -substrate LEC, 500 µm, 2 · 1018 cm− 3 (Si doped)Ohmic contact Ni/AuGe/Au
A.V. Chernykh1, S.V. Chernykh1, S.I. Didenko1, G.I. Koltsov1 , F.M. Baryshnikov1,G.I. Britvich2, M.Yu. Kostin2, A.P. Chubenko3, Yu.N. Sveshnikov4, N. Burtebayev5
1 – National University of Science and Technology «MISIS», Moscow, Russia2 – Institute of High Energy Physics, Protvino, Russia3 – P.N. Lebedev Physical Institute of the Russian Academy of Sciences, Moscow, Russia4 – «Elma–Malahit» JSC–«Concern Energomera», Moscow, Zelenograd, Russia5 – Institute of Nuclear Physics, Almaty, Kazakhstan
Applications• Fast neutron personal dosimeters for personnel monitoring behind the radiation shield of nuclear facilities, such as reactors, accelerators, neutron generators for medical uses, etc.• Neutron flux measurements of nuclear reactors.• Space and accelerator experiments with high intensities, where high radiation resistance, high time resolution and thermal stability are required.• Proton and alpha-particle contamination monitoring of special equipment (e.g. the monitoring of reprocessed fuel assemblies in the nuclear industry at the temperatures up to 100 oC).
Conclusions• Samples of fast neutron detectors based on thin high-purity GaAs epitaxial layers were fabricated.• The active area of the detector is 25 mm2.• The fast neutron detection efficiency is 1.04·10-3 and signal-to-background ratio is 50 at the operating bias.• At zero bias the efficiency of the detector varies lightly and becomes 0.92·10-3 and signal-to-background ratio becomes 44.• The use of thin epitaxial films as the active region of neutron detectors allows to reduce the contribution of gamma-rays and hence to reduce therecoil proton detection threshold, thereby increasing the fast neutron detection efficiency and the signal-to-background ratio.
References1. D.S. McGregor et al. NIM A 466, 2001, pp.126–1412. А. Šаgátová-Perdоchová et al. NIM A 591, 2008, pp.98–1003. А. Šаgátová et al. 2013 JINST 8 C030164. C. Crowell, S. Sze. Solid-State Electronics, 9 (1112), 1966, pp.1035–10485. G. I. Koltsov et al. Semiconductors, 46 (8), 2012, pp.1066–1071
AcknowledgementsThe authors are grateful to Evgenia Savchenko and Pavel Ivshin for their assistance in the poster preparation.
IntroductionNowadays neutron methods are widely adopted. For some applications detectors based on semiconductor materials are used. Generally, the operation principle of such detectors is based on the detection of secondary particles produced by neutrons in converter material with a semiconductor sensor. Depending on the neutron energy (n,α)-reactions are used for the thermal neutron detection and the recoil proton method is used for the fast neutron detection [1]. Previous works have shown some advantages of using bulk SI GaAs as neutron detectors [2, 3]. This work was aimed on the development of a fast neutron detector using high-purity GaAs layers grown by vapor-phase epitaxy. The using of thin high-purity GaAs epilayers allows to reduce the operating bias of a sensor and the signal-to-background ratio in comparison with bulk SI GaAs material.
Characteristics of the epilayersThe Ga-AsCl3-H2 VPE process in a vertical reactor was used to grow high-purity GaAs epilayers. In order to reduce the concentration of Group VI impurities (primarily, S and Se) in the growing layers, the temperature of the process was increased and the growth rate was decreased. In order to prevent the introduction of silicon (the main background impurity of Group IV), a small amount of water was added into the gas phase above the gallium source.
Fig. 1 – Dependence of the charge carrier concentration in the GaAs epilayers on the concentration of water vapor above the Ga source.
Process and layer parameters are given in Table
growth rate, µm/hour 9–10i -layer thickness, µm up to 120
carrier concentration, cm− 3 ~ 1011
µe · τe, cm2/V 2 · 10− 4
µp · τp, cm2/V 2 · 10− 5
EL2 concentration, cm− 3 ~ 1013
Ti/Pd/Au and Pt/TiN/Au metallization systems were studied as a Schottky contact.According to the criterion proposed by Crowell and Sze [4] the diffusion theory of rectification was used to describe the charge transport through the fabricated contacts [5].
where I0 – the reverse metal-semiconductor current, q – the electron charge, V – the voltage drop across the Schottky barrier, m – the ideality factor, k – Boltzmann's constant, T – the absolute temperature, µn – the electron drift mobility, NC – the effective density of states in the conduction band, S – the contact area, nb – the concentration of electrons in the semiconductor, εS –the semiconductor permittivity, Vbi – the built-in potential, φB – the Schottky barrier height.
Fig. 2 – I–V characteristic of the Pt- contact linearized in the coordinates ln[I/(Vbi–V)1/2]=f(V) in the range kT/q<<V<Vbi .
Fig. 3 – b0=f(1/T) dependence for both contacts.
( )
−
= 1exp0 mkT
qVVII
( ) ( )
−
−⋅=
kTqVVqnSNqVI B
bis
bCn
ϕε
µ exp2 21
0
kTbqm
1
=
−
⋅= 0
21
2ln bqnSNqq
kT
s
bCnB ε
µϕ
(1)
(2)
(3)
(4)
Comparison tableParameter Ti/Pd/Au Pt/TiN/Au
n 1.6–1.7 1.5φb, eV 0.84 0.87
CCEα (5 .5 MeV ), % at 0 V 50 60CCEɣ (14–17 keV ), % at 0 V 59 78
Schottky contact selection
Electrical characteristics
Fig. 7 – IV-characteristic of the sensor based on high-purity GaAs epilayers.
The average dark leakage currents:1.0 nA at - 50 V and 1.8 nA at -100 V.The best dark leakage currents:0.7 nA at - 50 V and 1.2 nA at -100 V.
Fig. 8 – CV-characteristic of the GaAs sensor (the specific capacitance in the saturation region is 280 pF/cm2).
Alpha-particle characterizationMeasurement conditions:• measuring bench with an 8192-channel ADC;• room temperature;• vacuum (residual pressure 1.33 Pa);• distance between the alpha-source and the detector was 7 cm.
Fig. 9 – The typical energy spectrum of alpha-particles from the 238Pu source measured with the fabricated GaAs sensor.
Fig. 10 – Dependence of the charge collection efficiency measured using the 238Pu-source on the applied bias.
Fast neutrons
2. Fast neutron spectral measurements for various converter thicknesses
The recoil proton method was used for the detection of fast neutrons. The 241Am−Be source with the activity A=1.62·106 Bq was used as a fast neutron source. Polyethylene was used as a converter layer. Films with a thickness of 80 microns were used to set the required thickness of the converter. The films were placed directly on the package at the distance of 2 mm from the GaAs sensor.
Fig. 12 – Spectrum of 241Am-Be measured for different converter thicknesses (the distance from the source R = 2 cm, the time t = 1000 s, Vb = -24 V).
Measurement results at Vb = -24 V (The detection threshold of recoil protons was selected 1.2 MeV)
Fig. 13 – Spectrum of 241Am-Be (the distance from the source is 11.5 cm)
1. In the range from 1.2 MeV to 4.5 MeVNn=259, Nbg=5 for t=1000 sec.
2. For the area of S=5×5 mm2 the incident flux is neutr./sec
3. Neutron detection efficiency
Fig. 14 – Fast neutron detection efficiency for different polyethylene converter thicknesses.
4. Measurements in self-biased mode
Fig. 15 – Fast neutron spectrum from the 241Am-Be source at different biases.
The detection efficiency and the signal-to-background ratio are slightly lower than in the case of working bias.ŋ( at 0 V) = 9.2·10-4 puls./neutr.S/B(at 0 V) = 44.6
3. Measurements of the fast neutron detection efficiency
d(CH2)n , µm N n , puls. N n − N bg, puls. signal/background N n − N bg, %80 1569 1512 26.5 51.4
160 2071 2014 35.3 68.5240 2590 2533 44.4 86.1480 2946 2889 50.7 98.2720 2999 2942 51.6 100.0960 2983 2926 51.3 99.5
Fig. 11 – Calibration of the measuring bench with the 239Pu α-source. The source was located close to the detector.
1. The measuring bench and calibration with alpha-particles.
The measurements were carried out at room temperature and atmospheric pressure. The detector was connected to an AS01PDA chip developed by JSC "INTEGRAL" (Minsk, Belarus). The AS01PDA chip contains a charge-sensitive amplifier, a pulse shaper and a buffer stage (Gain = 3.5 mV/fC, τ1/2= 140 ns). A 1024-channel spectrum analyzer «Angamm» (Dubna, Russia) was also used.
16th iWoRiDInt
ernational Workshop
on Radiation Imaging Detect
ors
FAST NEUTRON DETECTOR WITH A SURFACE-BARRIER VPE GaAs SENSOR
7.2434 20 =⋅
=RSAN
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