Radiation hardness of GaAs Sensors

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Radiation hardness of GaAs Sensors
.

K. Afanaciev, Ch. Grah, A. Ignatenko, W.
Lange, W. Lohmann, M. Ohlerich

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Very Forward Region of the ILC Detector
Interaction point
BeamCal

• 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 gt 5mrad
• Fast beam diagnostics

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The Challenges for BeamCal
Creation of beamstrahlung at the ILC
e-
e
Interaction
Bethe-Heitler process
ee- pairs from beamstrahlung are deflected into
the BeamCal 15000 ee- per BX gt Edep ? 10
20 TeV 5 MGy per year strongly dependent on
the beam and magnetic field configuration gt
radiation hard sensors Detect signatures of
single high energetic particles on top of the
background. gt high dynamic range/linearity
1 MGy/y
5 MGy/y
For 1 layer, per cell
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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
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Irradiation facility
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The material
Ga

• Gallium arsenide (GaAs)
• Compound semiconductor, direct bandgap
• Two sublattices of face centered cubic lattice
• (zinc-blende type)

.
As
GaAs Si Diamond Density 5.32 g/cm3
2.33 3.51 ? Pair creation E 4.3 eV/pair
3.6 13 ? Band gap 1.42 eV 1.14 5.47 ?
Electron mobility 8500 cm2/Vs 1350 1800 Hole
mobility 400 cm2/Vs 450 1200 ? Dielectric
const. 12.85 11.9 5.7 ? Radiation
length 2.3 cm 9.4 18.8 Ave. Edep/100
?m (by 10 MeV e-) 69.7 keV 53.3 34.3
Ave. pairs/100 ?m 13000 9200
3600 Structure p-n or insul. p-n insul.

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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

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Methodology. Irradiation

• Irradiation under bias voltage
• Monitoring of beam and sample currents, sample
  temperature

Beam
exit window of beam line
collimator (IColl)
Faraday cup (IFC, TFC)
sensor box (Is, Ts, HV)
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Methodology. CCD Setup
Sr90
sample
ADC
delay
Sr90 source
Scint.
discr
PM1

Gate
discr
PM2
Preamplifier
Sensor box
Trigger box
Typical spectrum of GaAs sensor
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Characterization I-V and C-V
Constant pad capacity no dependence on V gt no
structure
Pad capacity about 12 pF
Almost linear IV characteristics gt resistor
Rpad ? 500 MOhm
Pad parameters homegenious
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Characterization signal
S8 pad4 ring 4
Clear separation of peaks from Sr90 source
S8 pad4 ring 6
Quite homogeneous response over different pads
Saturation of signal _at_ about 200V bias
Collection efficiency ? 60
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GaAs. Irradiation
GaAs2 before irradiation
Samples 78 (GaAs1, GaAs2) pad4, ring6 _at_ 200V
Absorbed doses 0.85 and 1.5 MGy
GaAs2 after irradiation
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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
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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 _at_ 200V)
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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 gt no defect diffusion on this
timescale at room temperature.
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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 gt 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.

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