paul sellin, radiation imaging group charge drift in partially-depleted epitaxial gaas detectors...
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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
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
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
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.
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
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
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
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.
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
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:
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
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
Paul Sellin, Radiation Imaging Group
Depletion Thickness vs Bias Voltage
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
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
Paul Sellin, Radiation Imaging Group
Scanning optical bench
850nm laser300ns pulse
XY scanning table
cryostat
imaging camera
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
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
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
Paul Sellin, Radiation Imaging Group
Interaction close to the anode - inside depletion region
Paul Sellin, Radiation Imaging Group
Interaction close to n+ substrate - in low field region
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
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
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
Paul Sellin, Radiation Imaging Group
Acknowledgements
This work was partially funded by the UK’s Engineering and Physics Science Research Council