GaAs:Cr detector testing for FCAL calorimeter at planned linear

accelerator ILC

V. Elkin, U.Kruchonak

XVI научная конференция молодых учёных и специалистов ОМУС 2012

The International Linear Collider ILC

Parameters of the ILC:•e+e- accelerator, sc cavities, gradient 31.5 MV/m => 30km long•CMS energy: 200 to 500 GeV (possible upgrade to 1 TeV)•One interaction region, beam crossing angle of 14mrad and two detectors („push-pull“ scenario) •Peak luminosity: 2 x 1034 cm-2s-1

•typical beam size: (h x v) 650 nm x 5.7nm & beam intensity 2 x 1010 e+e-

~ 30 km

10.10.2007 C.Grah: Radhard Sensors for BeamCal 3

Very Forward Region of the ILC Detectors

• R&D of the detectors in the forward region is done by the FCAL Collaboration.• Precise (LumiCal) and fast (BeamCal) luminosity measurement • Hermeticity (electron detection at low polar angles)• Mask for the inner detectors• Not shown here: GamCal, a beamstrahlung photon detector at about 180m post-IP.

LumiCal

TPC

ECAL

HCAL

BeamCal

10.10.2007 C.Grah: Radhard Sensors for BeamCal 4

The Beam Calorimeter - BeamCal

Interaction point

Compact EM calorimeter with sandwich structure: 30 layers of 1 X0

o 3.5mm W absorber and 0.3mm radiation hard sensor Angular coverage from 5mrad to 28 mrad

(6.0 > |η| > 4.3) Moliére radius RM ≈ 1cm Segmentation between 0.5 and 0.8 x RM

BeamCalLDC

~10cm

~12cm

Space for electronics

The Challenges for BeamCal

e+e- pairs from beamstrahlung are deflected into the BeamCal

15000 e+e- per BX

=> 10 – 20 TeV total energy dep. ~ 10 MGy per year strongly dependent on the beam and magnetic field configuration

=> radiation hard sensors Detect the signature of single high energetic particles on top of the background.

=> high dynamic range/linearity

e- e+e-

e-

γ

e-

γ

e+e.g. Breit-Wheeler process

GaAs:Cr Sensor Plane

Produced by the Siberian Institute of Technology, Tomsk semi-insulating GaAs doped by Sn (shallow donor) compensated by Cr (deep acceptor): to compensate electron trapping centers EL2+ and provide i-type conductivity.

8

Advantages of Cr impurity as compared to the EL2 centers for detector material production

Deep acceptor

Deep donor

-

+

• small value of the electron capture cross section and absence of the field increase of the capture cross section on the electric field intensity

• absence of current oscillations

• possibility to reach uniform high electric field distribution through whole the detector with the thickness up to 1 mm

9

Electrophysical characteristics of high resistivity GaAs

Material о

 

(10 -9/*cм)

no

 (10 5 cm –3)

po

 (10 5 cm –3)

Ln

(cm)

GaAsEL2

6-9 70-100 4-6 0.03 - 0.05

GaAsCr

0.6-1.1 2-3 120-200 0.07 – 0.2

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.

• 11 sector size GaAs sensors• Thickness - 500 μm • Metallization -1 μm Ni• 12 rings• 64 pads from 18 to 42 mm2

• Surrounded by 120 μm width guard ring

GaAs:Cr Sensor Plane

№ Detector thickness,um

collected charge,

e

metallization,Type of metallization

pixels Guard ring

1 AG-84 №13 498 38450 Ni Ni

2 AG 84 № 26 502 36490 Ni Ni

3 AG 84 № 7 490 39750 Ni Ni

4 AG 84 № 19 495 38520 Ni Ni

5 AG 84 № 21 492 36560 Ni Ni

6 AG 84 № 32 502 38720 Ni Ni

7 AG 84 № 39 495 36450 Ni Ni

8 AG 84 № 41 487 37700 Ni Ni

9 AG 84 № 29 487 38420 Ni Ni

10 AG 84 № 28 500 38590 Ni Ni

11 AG 221 №25 492 41570 Ni Ni

GaAs:Cr Sensor producer data

Circuit of capacitance measurement

Bias

Adap

ter

BP

Gua

rd R

ing

L

H

Bias

Rin

g

LCR

Met

er

Typical C-V , measured between pad and backplane for different rates 100Hz -100 kHz

•Capacitance decrease with frequency

•Chosen frequency 10kHz

Pads capacitance distribution for 8 sensors

Pads capacitance distribution

A

DC-

PAD

s

BP

Gua

rd R

ing

Circuit for I/V pad measurement

Typical IV measurement for single pad is symmetric and linear in the range +/-500V

Temperature dependence of the resistance

Measured from -10 C to 50 CThe range from 65 GΩ*cm to 0.12 G Ω*cm

Measurement of the barrier height on border of metal−semiinsulating gallium arsenide

total current through the device

Reff - effective resistance of the entire device

In or Isat - Saturation current - Limiting current from metal to semiconductor

[1]

S- Cross-sectional area, A*n = 8.16 A*cm-2 *K-2 - Richardson constant for GaAs

T - Temperature of device, q - electron charge, k - Boltzmann constant

- barrier height on border of metal−semiinsulating gallium arsenide

I-V in range +/-10VParameterization of the IV curve by 4 parameters:

• Isaturation-• Isaturation+• Resestivity-• Resestivity+

Average resistivity and barrier height of AG84N19

Average resistivity and barrier height of 11 sensors № Detector thickness,

umResistivity, Ohm*cm Darrier height, volts

ρ+ ρ- φ+ φ-

1 AG-84 №13 498 Not processed Not processed Not processed Not processed

2 AG 84 № 26 502 1.20E9 1.10E9 0.75 0.74

3AG 84 № 7

490 2.25E9 2.18E9 0.78 0.79

4AG 84 № 19

495 3.00E9 2.79E9 0.78 0.78

5 AG 84 № 21 492 2.42E9 2.37E9 0.76 0.76

6AG 84 № 32

502 3.20E9 3.04E9 0.78 0.77

7 AG 84 № 39 495 3.71E9 3.48E9 0.79 0.76

8 AG 84 № 41 487 4.07E9 3.84E9 0.76 0.75

9AG 84 № 29

487 3.67E9 3.23E9 0.78 0.81

10 AG 84 № 28 500 2.09E9 1.92E9 0.76 0.77

11AG 221 №25

492 2.49E9 2.42E9 0.79 0.81

Unusual padsSome pads or guard rings have untypical behavior.For now we’ve found 6 sensors with guard ring and one pad: that

may be related to metallization defects or internal structure injury. It appears on the crystal boundary in all cases.

Unusual pad, unusual guardring and normal pad C-V

Growth up to 110pF at 100 V

Similar behavior with guard ring

Break through while measuring with guard ring

Typical C < 12pF at 10kHz

Unusual pad I-V

Growth up for positive Vbias

Normal behavior in negative area

Similar behavior with guard ring

Unusual padsNo Pad No Breakdown voltage, V

AsGa84N7 Guard ring -330

AsGa84N7 Ring12-Pad1 -310

AsGa84N13 Guard ring -160; +200

AsGa84N13 Ring12-Pad6 -280

AsGa84N21 Guard ring +30

AsGa84N21 Ring1-Pad4 +50

AsGa84N26 Guard ring -110; +230

AsGa84N26 Ring12-Pad1 -210

AsGa84N28 Guard ring -300

AsGa84N28 Ring12-Pad6 +330

AsGa84N41 Guard ring +110

AsGa84N41 Ring12-Pad6 +110

Thank youfor attention!

Forward region of the ILC

Beam calorimeter (BeamCal) - monitor the beam parameters at the interaction point; adjacent to the beampipe.

Luminosity detector (LumiCal) – covers larger polar angles; luminometer of the detector.

The gamma detector (GamCal) – together with BeamCal, measures beamstrahlung photons, which are very collinear to the beam.

Изучение характеристик детекторов на основе GaAs компенсированного

хромом для будущего калориметра FCAL линейного ускорителя ILC

V. Elkin, U.Kruchonak

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

GaAs grown by Liquid EncapsulatedCzochralski (LEC).doped by Te or Sn (shallow donor)to fill EL2+ trapping centers.Compensated by Cr (deep acceptor) tohigh-ohmic intrinsic material.Compensation is temperature controlled

Semi-insulating - no p-n junction

Signal charge transport mainly by electrons

Structure provided by metallisation (similar to diamond)

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 μm 69.7 keV (by 10 MeV e-) Ave. pairs/100 μm 13000 Structure p-n or insul