Transcript Document

Radiation hardness of GaAs Sensors
.
K. Afanaciev, Ch. Grah, A. Ignatenko,
W. Lange, W. Lohmann, M. Ohlerich
20-Jul-15
FCAL collaboration meeting. Krakow.
Very Forward Region of the ILC Detector
BeamCal
Interaction
point
EM calorimeter with sandwich structure:
30 layers of 1 X0
3.5mm W and 0.3mm sensor
Angular coverage from 5 mrad to 28 mrad
Moliére radius RM ≈ 1cm
Segmentation between 0.5 and 0.8 x RM
• The purpose of the instrumentation of the very forward region is:
– Hermeticity: increase the coverage to polar angles > 5mrad
– Fast beam diagnostics
20-Jul-15
FCAL collaboration meeting. Krakow.
The Challenges for BeamCal
Creation of beamstrahlung at the ILC
e-
e+
Bethe-Heitler process
e+e- pairs from beamstrahlung are
deflected into the BeamCal
≈ 1 MGy/y
15000 e+e- per BX => Edep  10 – 20 TeV
~ 5 MGy per year strongly dependent on
the beam and magnetic field configuration
=> radiation hard sensors
Detect signatures of single high energetic
particles on top of the background.
≈ 5 MGy/y
=> high dynamic range/linearity
For 1 layer, per cell
20-Jul-15
FCAL collaboration meeting. Krakow.
Irradiation facility
Superconducting DArmstadt LINear ACcelerator
Technical University of Darmstadt
V.Drugakov
2X0
Energy spectrum of
shower particles in BeamCal
Irradiation up to several MGy using the injector line of the S-DALINAC:
10 ± 0.015 MeV and beam currents from 2 to 100 nA
corresponding to doses about 10 to 600 kGy/h
20-Jul-15
FCAL collaboration meeting. Krakow.
Irradiation facility
20-Jul-15
FCAL collaboration meeting. Krakow.
The material
Ga
Gallium arsenide (GaAs)
.
As





20-Jul-15
Compound semiconductor, direct bandgap
Two sublattices of face centered cubic lattice
(zinc-blende type)
GaAs
Density
5.32 g/cm3
Pair creation E
4.3 eV/pair
Band gap
1.42 eV
Electron mobility 8500 cm2/Vs
Hole mobility
400 cm2/Vs
Dielectric const. 12.85
Radiation length 2.3 cm
Ave. Edep/100 m
(by 10 MeV e-)
69.7 keV
Ave. pairs/100 m 13000
Structure
p-n or insul.
Si
2.33
3.6
1.14
1350
450
11.9
9.4
Diamond
3.51
13
5.47
1800
1200
5.7
18.8
53.3
9200
p-n
34.3
3600
insul.
FCAL collaboration meeting. Krakow.
The material
Supplied by FCAL group at JINR
Produced by Siberian Institute of Technology, Tomsk
Sample is semi-insulating GaAs doped by Sn
(shallow donor) and compensated by Cr (deep
acceptor). This is done to compensate electron
trapping centers and provide i-type conductivity.
Sample works as a solid state ionisation chamber
Structure provided by metallisation (similar to diamond)
500 m thick detector is divided into 87 5x5 mm pads
and mounted on a 0.5mm PCB with fanout
Metallisation is V (30 nm) + Au (1 m)
2 samples
20-Jul-15
FCAL collaboration meeting. Krakow.
Methodology. Irradiation
•Irradiation under bias voltage
•Monitoring of beam and sample currents, sample temperature
Beam
exit window
of beam line
collimator (IColl)
sensor box (Is, Ts, HV)
20-Jul-15
Faraday cup (IFC, TFC)
FCAL collaboration meeting. Krakow.
Methodology. CCD Setup
Sr90
sample
Sr90 source
Preamplifier
Scint
.
PA
PM1
PM2
discr
discr
delay
&
ADC
Gate
Sensor box
Trigger box
Typical spectrum of GaAs sensor
20-Jul-15
FCAL collaboration meeting. Krakow.
Characterization: I-V and C-V
Constant pad capacity
no dependence on V =>
no structure
Pad capacity about 12 pF
Zeitkonstante Spannung
Almost linear IV characteristics => resistor
capacitance [F]
Rpad  500 MOhm
1.30E-011
r8p5
r6p4
r3p3
r11p6
1.20E-011
Pad parameters homegenious
0
20
40
60
Stepnummer [Dealy = 2000ms]
20-Jul-15
FCAL collaboration meeting. Krakow.
80
100
Characterization: signal
S8 pad4
ring 4
Clear separation of peaks from Sr90 source
Quite homogeneous response
over different pads
Saturation of signal @ about 200V bias
S8 pad4
ring 6
Collection efficiency  60%
20-Jul-15
FCAL collaboration meeting. Krakow.
GaAs. Irradiation
GaAs2 before irradiation
Samples 7&8 (GaAs1, GaAs2)
pad4, ring6 @ 200V
Absorbed doses 0.85 and 1.5 MGy
GaAs2 after irradiation
20-Jul-15
FCAL collaboration meeting. Krakow.
GaAs. Irradiation results
CCD
GaAs1 pad4 ring6, 0.85 MGy
IV
CCE dropped sharply after irradiation but signal is visible
Signal increases with bias voltage
20-Jul-15
FCAL collaboration meeting. Krakow.
GaAs. Irradiation results
CCD
GaAs2 pad4 ring6, 1.5 MGy
IV
Results: CCE dropped to about 6% from 60% (by ~ 90%)
but signal is still visible for absorbed dose of about 1.5 MGy
Dark current increased  2 times (from 0.4 to 1 A @ 200V)
20-Jul-15
FCAL collaboration meeting. Krakow.
GaAs. Neighbor pads
GaAs2 pad5 ring6
Double peak - result of a partial exposure of neighbor pads
pad size 5x5 mm vs. collimator size 10x10 mm
Spectra measured a few weeks after the irradiation =>
no defect diffusion on this timescale at room temperature.
20-Jul-15
FCAL collaboration meeting. Krakow.
Conclusion
• Signal still visible up to absorbed doses of 1.5 MGy
• Good homogeneity over the sensor area
• Predictable behavior under irradiation
• Signal could still be increased with increasing bias voltage
• New samples with higher radiation hardness are received by
Zeuthen => new beamtest is needed.
• Irradiated samples are returned to manufacturer for the
radiation damage investigation.
• At the moment GaAs proved to have radiation hardness
close to the requirements of the BeamCal, but an improvement
is still needed.
• Could be considered as one of the main candidates
for the BeamCal prototype.
20-Jul-15
FCAL collaboration meeting. Krakow.
20-Jul-15
FCAL collaboration meeting. Krakow.