GaAs radiation imaging detectors with an active layer thickness up to 1 mm.
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GaAs radiation imaging detectors with an active layer thickness up to 1 mm. D.L.Budnitsky, O.B.Koretskaya , V.A. Novikov, L.S.Okaevich A.I.Potapov , O.P.Tolbanov , A.V.Tyazhev . Siberian Physical Technical Institute , Russia, Tomsk Fax: +7-3822-233034, E-mail: [email protected]
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GaAs radiation imaging detectors with an active layer thickness up to 1 mm. D.L.Budnitsky, O.B.Koretskaya , V.A. Novikov, L.S.Okaevich A.I.Potapov , O.P.Tolbanov , A.V.Tyazhev . Siberian Physical Technical Institute , Russia, Tomsk Fax: +7-3822-233034, E-mail: [email protected]. Outline. - PowerPoint PPT Presentation
Transcript of GaAs radiation imaging detectors with an active layer thickness up to 1 mm.
GaAs radiation imaging detectors with an active layer thickness up to 1 mm.
D.L.Budnitsky, O.B.Koretskaya , V.A. Novikov, L.S.Okaevich A.I.Potapov , O.P.Tolbanov , A.V.Tyazhev .
Siberian Physical Technical Institute , Russia, Tomsk
Fax: +7-3822-233034, E-mail: [email protected]
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Outline
• Introduction
• Experimental data
Electric field distribution in GaAs detectors based on GaAs:EL2
Resistivity distribution in GaAs:Cr slices
Electrophysical characteristics of high resistivity GaAs
I-V characteristics of GaAs:Cr
CCE dependencies on bias voltage of detectors based on GaAs:Cr
• Conclusion
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The electric field profile of a LEC SI-GaAs as shown in [1, 2]
Distance from p+ contact, m
Ele
ctri
c fi
eld,
V/
m
Ele
ctri
c fi
eld,
kV
/cm
depth, m
[1] – k. Berwick et al., Proc. Semiconductors for room-temperature radiation detector applications,San Francisco, CA, USA, 12-16 April 1993, MRS Symp. Proc., vol. 302
[2] - A. Cola et al./ Nuc.Instr. And Meth. In Phys. A395 (1997) 98-100
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The calculated electric field and electrostatic potential profiles as shown in [2]
depth, m
pote
ntia
l, V
electric field, kV/cm
4
3
2
1
Reverse bias: 1- 20V, 2- 40V,
3 - 60V, 4 – 80V
The active layer thickness has penetration rate 1 m/V
acti
ve th
ickn
ess,
m
applied voltage, V
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The amplitude spectrum of the LEC SI-GaAs detector with current oscillations
The presence of current oscillations makes difficult the detection
of the desired signal in the amplitude spectrum
0 100 200 300 4000
10
20
30
40
50
60
70
2
1
Cou
nts
Channel
1 – with -radiation2 - without -radiation
U= 300V, 241Am source
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Experimental setup for electric field distribution profiling based on Franz-Keldysh effect.
13 42
I R890-910 nm
1 – DLM (diffraction lattice monochromator)
2- detector sample, 3 – optical system,
4 – IR-camera (charge-coupled device)
Ubias
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Electric field strength oscillations in LEC SI-GaAS
We have measured samples made in different firms.
The analysis of the results shows that in all structures
fabricated by means of LEC SI-GaAs technology, a non-
uniform (х) distribution and electric field strength
oscillations are observed.
LEC SI-GaAs (positive)
LEC SI-GaAS (negative)
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Electric field distribution (1) (EL2 COMPENSATED GaAs LAYERS )
GaAs:EL2
t1
t2
t3
d
Spatial distribution F function and light transmission (T) through detector thickness
under bias voltage 250 V in various time instants t1, t2 t3 (t1 t2 < t3)
F=1-T
The main disadvantages of the LEC SI-GaAs
•The low value of the electron life time n (0.2-1 ns). It results in the decrease
of the electron drift length and, consequently, in low values of the electron
component of the charge collection efficiency.
•The maximum value of the electric field penetration depth up to 500m that
limits the sensitive layer thickness of the LEC SI-GaAs structures and provide
non-uniform electric field distribution, (х), through the detector thickness.
•Current oscillations are formed in the external circuit at a rather low average
value of the electric field strength in the detector 1kV/cm.
•Electric field increase of the capture cross section of EL2+ centers.
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Advantages of Cr impurity as compared to the EL2 centers for detector material production
Deep acceptor
Deep donor
-
+
• small value of the electron capture cross section
and absence of the field increase of the capture
cross section on the electric field intensity
• absence of current oscillations
• possibility to reach uniform high electric field distribution through whole the detector with the thickness up to 1 mm
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Technological cycle of manufacturing GaAs compensated with Cr
n-GaAs
GaAs:Cr with up to 109 *cm
12
Resistivity distribution in the slice thickness
0 200 400 600 800 1000
105
106
107
108
109
surface
detector thickness, d
, oh
m*c
m
slice thickness, m0 200 400 600 800 1000 1200
105
106
107
108
109
, o
hm*c
m
slice thickness, m
The experimental values of the resistivity are (0.2-1)109cm, which are
more than an order higher as compared to the resistivity of structures on the basis
of LEC SI-GaAs.
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Electrophysical characteristics of high resistivity GaAs
Material
о
(10 -9/*cм)
no
(10 5 cm –3)
po
(10 5 cm –3)
Ln
(cm)
GaAs
EL2
6-9 70-100 4-6 0.03 - 0.05
GaAs
Cr
0.6-1.1 2-3 120-200 0.07 – 0.2
The hole concentration in GaAs:Cr exceeds the concentration of electrons. The
difference changes from 10 to 100 times depending on conditions of the diffusion
process and the initial material characteristics.
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Current-voltage characteristics
(DIFFUSION CROMIUM COMPENSATED GaAs LAYERS ) •High resistivity causes a
transformation of the structure
of a barrier type to the
structures of a resistor type.
•The structure current-
voltage characteristic is linear.
The current density value at
operating voltage does not
exceed 10-6 A/cm2.
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Electric field distribution (2)
(DIFFUSION CROMIUM COMPENSATED GaAs LAYERS ) The most important distinction of our structures, as compared to the traditional LEC SI-
GaAs, is the uniform electric field distribution and the absence of current oscillations.
Spatial distribution of the F function and light transmission (T) in detector thickness d , F=1-T
GaAs:Cr500 V
1000 V
d
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Amplitude spectra of the GaAs:Cr detector for variousenergies of the gamma radiation
100 200 300 400 500 600 700
20
40
60
80
100
3
2 1
Detector thickness d=780 m
1 - Ey=60 keV, 241Am source
2 - Ey=122 keV, 57Co source
3 - Ey=140 keV, 99mTc source
CC
E, %
U, V
0 200 400 600 8000
40
80
120
160
Detector thickness d=780 m
1 - Ey=60 keV, 241Am source
2 - Ey=122 keV, 57Co source
3 - Ey=140 keV, 99mTc source
Bias voltage U= 600 V
3
2
1
Cou
nts
Channel number
There is the 70-80 % of CCE in a wide range of gamma-quantum energies
(E = 60, 122, 140 keV ) for the bias about 600V and the detector thickness of 780m.
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0 100 200 300 4000
200
400
600
800
3
21
E = 60 keV (Am241)
1 - d = 1200 m, U = 800 V2 - d = 830 m, U = 600 V3 - d = 715 m, U = 500 V
Cou
nts
channel number
800 1000 120060
70
80
90
800 V
600 V
500 V
CC
E, %
detector thickness, d
Amplitude spectra for GaAs:Cr detectors with differentactive layer thicknesses. E 60 keV (241Am source)
It should be noted that value of CCE declines with increase of the detector thickness.Nevertheless, CCE is about 70 % for the detector with an active layer thickness
d=1.2 mm when bias voltage is 1000V.
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Conclusion
• The technology of high temperature Cr doping of n-type GaAs allows to
produce the high resistive GaAs layers with resistivity up to 109 *cm and
thickness up to 1 mm.
• The detector structures based on GaAs:Cr have more uniform electric field
distribution as compared to the detector structures based on GaAs:EL2 in a
wide range of the applied bias voltage.
• The detector structures based on GaAs:Cr have applicable values of the CCE
in a wide range of the gamma quantum energies (E = 60, 122, 140 keV) and
can be used in the production of pixel detector.
• We suppose to apply the 3” wafers to produce detector material in the
nearest future.
