paul sellin, radiation imaging group charge drift in partially-depleted epitaxial gaas detectors...
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![Page 1: Paul Sellin, Radiation Imaging Group Charge Drift in partially-depleted epitaxial GaAs detectors P.J. Sellin, H. El-Abbassi, S. Rath Department of Physics](https://reader030.vdocument.in/reader030/viewer/2022032800/56649d265503460f949fd2a3/html5/thumbnails/1.jpg)
Paul Sellin, Radiation Imaging Group
Charge Drift in partially-depleted epitaxial GaAs detectors
P.J. Sellin, H. El-Abbassi, S. RathDepartment of Physics
University of Surrey, Guildford, UK
J.C. BourgoinLMDH, Université Pierre et Marie Curie, Paris, France
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Paul Sellin, Radiation Imaging Group
Overview
Chemical reaction growth of thick epitaxial GaAs layers
Depletion thickness and residual impurity concentration
Performance of partially depleted detectors
C-V measurements of impurity concentration at low temperature
Optical probing of charge transport using a focussed laser
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Paul Sellin, Radiation Imaging Group
Potential challenges for epitaxial GaAs
Strengths of epitaxial GaAs: intermediate photon detection efficiency between Si and
CZT/CdTe metal-semiconductor contacts and device physics are well
understood epitaxial GaAs has low concentrations of native EL2 defect source of highly uniform whole wafer material, compatible with
flip-chip bonding and monolithic electronics
Existing problems: even high purity epitaxial is compensated due to residual
impurities- does not exhibit intrinsic carrier concentrations depletion thickness is severely limited charge carrier lifetimes are reduced
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Paul Sellin, Radiation Imaging Group
Chemical Reaction growth of thick epitaxial GaAs
Epitaxial GaAs material studied in this work was grown by a Chemical Reaction Method by Jacques Bourgoin (Paris).
• An undoped GaAs wafer is used as the material source, which is decomposed in the presence of high temperature high pressure water vapour to produce volatile species.
•Typically, growth rates of <10 m/hr are used to achieve EL2 concentrations of ~1013 cm-3
L. El Mir, et al, “Compound semiconductor growth by chemical reaction”, Current Topics in Crystal Growth Research 5 (1999) 131-139.
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Paul Sellin, Radiation Imaging Group
Whole wafer photoluminescence mapping
GaAs material uniformity is characterised using room temperature photo-luminescence mapping - a contact-less, whole wafer technique:
A 25 mW 633 nm HeNe laser is focussed to ~50 m on the wafer
the wafer is mounted on an XY stage, and scanned
PL intensity maps at peak the band edge emission wavelength (870 nm) are acquired
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Paul Sellin, Radiation Imaging Group
PL maps of GaAs
Photoluminescence mapping clearly shows the uniformity of epitaxial GaAs compared to semi-insulating VGF material:
H. Samic et al., NIM A 487 (2002) 107-112.
Epitaxial GaAs Bulk GaAs
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Paul Sellin, Radiation Imaging Group
Calculated depletion thickness
This material is nominally 1-5 x 1014 cm-3- corresponds to a 10-20 m depletion thickness @ 30V, and 15-30 m @ 80V
Width of GaAs Space Charge Region vs Reverse Bias Voltage
Reverse bias voltage (V)
0 50 100 150 200
SC
R w
idth
( m
)
0
50
100
150
200
250
0
50
100
150
200
250
N = 5x1012 cm-3
N = 1x1013 cm-3
N = 5x1013 cm-3
N = 1x1014 cm-3
N = 5x1014 cm-3
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Paul Sellin, Radiation Imaging Group
-particle spectra taken with an applied bias of 30V
Channel no.
0 500 1000 1500 2000 2500
Cou
nts
1
10
100
1000
220C-540CV = 30VV = 80V
Alpha particle spectra
5.48 MeV alpha particles are irradiated through the Schottky (cathode) contact - range in GaAs ~20m.
A peltier cooler controlled the device temperature in the range +25°C to -55°C. Shaping time = 0.5 s.
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Paul Sellin, Radiation Imaging Group
Alpha particle pulse shapes
Alpha particle pulses at room temperature:
preamplifier
shaping amplifier
time base = 1s per division
slow component
fast component
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Paul Sellin, Radiation Imaging Group
Alpha particle tracks
An un-collimated alpha particle source produces a characteristic ‘double peak’ pulse height spectrum if the depletion thickness is shallower than the particle
range:
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Paul Sellin, Radiation Imaging Group
59.5 keV gamma spectra
Depth-dependent CCE produces poorly resolved gamma spectra:
Channel no.
200 400 600 800 1000
log
co
unts
1
10
100
1000
Energy (keV)
0 10 20 30 40 50 60
-15V-30V -50V-70V -90V
T = -50°C
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Paul Sellin, Radiation Imaging Group
Temperature dependent CV analysis
Allows the doping density ND to be extracted from the gradient of 1/C2 vs V :
dVCdqN
rD )1(
22
0
Voltage(V)
0 5 10 15 20
1/C
2 (F-2
)
0.0
2.0e+20
4.0e+20
6.0e+20
8.0e+20
1.0e+21
1.2e+21
1.4e+21
1.6e+21
220C
100C
-20C
-120C
-210C
-420C
-520C
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Paul Sellin, Radiation Imaging Group
Depletion Thickness vs Bias Voltage
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Paul Sellin, Radiation Imaging Group
Impurity Densities
The CV analysis confirm the shallow depletion thicknesses achieved in these devices, and correspond to impurity densities of ~3 x 1013 cm-3 in
sample S16 at low temperature:
Sample Area (mm2) ND (cm-3) Depletionthickness (m)
V = 30V V = 80VT=22C S16 7.1 1.3 x 1014 18 30
S17 3.8 4.3 x 1014 10 16
T=-54C S16 7.1 3.1 x 1013 37 60S17 3.8 2.1 x 1014 14 23
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Paul Sellin, Radiation Imaging Group
Focussed IR laser scans
Probe the variation in pulse shape as a function of position from the Schottky contact, and
temperature
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Paul Sellin, Radiation Imaging Group
Scanning optical bench
850nm laser300ns pulse
XY scanning table
cryostat
imaging camera
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Paul Sellin, Radiation Imaging Group
Laser pulse shapes
T=273K, 20V
At 60m from cathode:
no slow component to signal
At 180m from cathode:
charge drift times are ~350s
IR laser spot appears to have significant beam waist
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Paul Sellin, Radiation Imaging Group
Laser pulse shapes (2)
T=223K, V=90V
At 60m from cathode:
no slow component to signal
At 180m from cathode:
charge drift times are ~350s
IR laser spot appears to have significant beam waist
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Paul Sellin, Radiation Imaging Group
T=273K, V=20V
Position from Schottky (m)
0 50 100 150 200 250
Am
plitu
de
0.0
0.2
0.4
0.6
0.8
1.0
Sig
nal R
iset
ime
(s)
0
100
200
300
400
500
T=223K, V=90V
Position from Schottky (m)
0 20 40 60 80 100 120 140 160 180 200 220
Am
plitu
de
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
Sig
nal R
iset
ime
(s)
0.0
0.5
1.0
1.5
2.0
Pulse risetime and amplitude vs bias
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Paul Sellin, Radiation Imaging Group
Interaction close to the anode - inside depletion region
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Paul Sellin, Radiation Imaging Group
Interaction close to n+ substrate - in low field region
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Paul Sellin, Radiation Imaging Group
Temperature dependent pulse shapes (1)
Laser pulses 60m from Schottky
Time (s)
-100 0 100 200 300 400 500
Am
plitu
de
-3.0
-2.5
-2.0
-1.5
-1.0
-0.5
0.0
273K
223K
248K
248K
198K
223K
V = 60V
V = 20V
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Paul Sellin, Radiation Imaging Group
Temperature dependent pulse shapes (2)
Laser pulses 180m from Schottky
Time (s)
-100 0 100 200 300 400 500
Am
plitu
de
-1.0
-0.8
-0.6
-0.4
-0.2
0.0
273K
223K
248K
248K198K
223K
V = 60V
V = 20V
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Paul Sellin, Radiation Imaging Group
Conclusions
The epitaxial GaAs layers studied showed excellent uniformity, and a residual impurity concentration of 1-5 x 1014cm-3
Long electron lifetimes > 300 s were observed in the low field regions - confirms the very low EL2 concentration
Lateral laser scans show: good charge transport in the shallow depleted region long-lived components to the pulse shapes when irradiated close to
n+ substrate - consistent with slow electron diffusion towards the substrate
significant penetration of the depletion region when cooled to -50°C
Future work: further lateral scanning is required with focussed lasers and high
resolution proton microbeams to quantify these phenomena further modest reductions in impurity concentration will produce
significant performance improvements
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Paul Sellin, Radiation Imaging Group
Acknowledgements
This work was partially funded by the UK’s Engineering and Physics Science Research Council