Transcript Amplitude spectra of the GaAs detector for different

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
• 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
2
Electric field, V/m
Electric field, kV/cm
The electric field profile of a LEC SI-GaAs as
shown in [1, 2]
Distance from p+ contact, m
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
3
The calculated electric field and electrostatic
potential profiles as shown in [2]
Reverse bias: 1- 20V, 2- 40V,
3 - 60V, 4 – 80V
potential, V
active thickness, m
applied voltage, V
3
1
2
electric field, kV/cm
4
depth, m
The active layer thickness has penetration rate  1 m/V
4
The amplitude spectrum of the LEC SI-GaAs detector
with current oscillations
70
1 – with -radiation
2 - without -radiation
60
Counts
50
U= 300V, 241Am source
40
30
1
20
10
2
0
0
100
200
300
400
Channel
The presence of current oscillations makes difficult the detection
of the desired signal in the amplitude spectrum
5
Experimental setup for electric field distribution
profiling based on Franz-Keldysh effect.
1
Ubias
2
3
4
IR
890-910 nm
1 – DLM (diffraction lattice monochromator)
2- detector sample, 3 – optical system,
4 – IR-camera (charge-coupled device)
6
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 nonuniform (х) distribution and electric field strength
oscillations are observed.
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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
8
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 500m 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.
Advantages of Cr impurity as compared to the EL2
centers for detector material production
-
Deep acceptor
• absence of current oscillations
• small value of the electron capture cross section
and absence of the field increase of the capture
cross section on the electric field intensity
• possibility to reach uniform high electric field
distribution through whole the detector with the
thickness up to 1 mm
+
Deep donor
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Technological cycle of manufacturing GaAs
compensated with Cr
n-GaAs
GaAs:Cr with 
up to 109 *cm
11
10
9
10
8
10
7
10
6
10
5
detector thickness, d
, ohm*cm
, ohm*cm
Resistivity distribution in the slice thickness
surface
0
200
400
600
slice thickness, m
800
1000
10
9
10
8
10
7
10
6
10
5
0
200
400
600
800
slice thickness, m
1000
1200
The experimental values of the resistivity are (0.2-1)109cm, 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
о
no
po
Ln
(10 -9/*cм)
(10 5 cm –3)
(10 5 cm –3)
(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
Material
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 SIGaAs, is the uniform electric field distribution and the absence of current oscillations.
500 V
GaAs:Cr
d
1000 V
Spatial distribution of the F function and light transmission (T)
in detector thickness d , F=1-T
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Amplitude spectra of the GaAs:Cr detector for various
energies of the gamma radiation
160
100
Detector thickness d=780 m
241
1 - Ey=60 keV, Am source
120
3
2 - Ey=122 keV, Co source
3 - Ey=140 keV,
80
Tc source
Bias voltage U= 600 V
1
2
99m
CCE, %
1
Counts
80
57
60
Detector thickness d=780 m
241
1 - Ey=60 keV, Am source
40
57
40
2 - Ey=122 keV, Co source
2
3 - Ey=140 keV,
20
99m
Tc source
3
0
0
200
400
Channel number
600
800
100
200
300
400
500
600
700
U, V
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 780m.
16
Amplitude spectra for GaAs:Cr detectors with different
active layer thicknesses. E  60 keV (241Am source)
800
241
E = 60 keV (Am )
500 V
1 - d = 1200 m, U = 800 V
2 - d = 830 m, U = 600 V
3 - d = 715 m, U = 500 V
400
CCE, %
Counts
600
90
1
200
80
600 V
70
2
800 V
3
0
60
0
100
200
channel number
300
400
800
1000
1200
detector thickness, d
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.
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