22 October 2009 FCAL workshop, Geneve 1

Polarization effects in the radiation damaged

scCVD Diamond detectors

Sergej Schuwalow, DESY Zeuthen

On behalf of FCAL collaboration

22 October 2009 FCAL workshop, Geneve 2

Diamond properties

Density 3.52 g cm-3

Dielectric constant 5.7 Breakdown field 107 V cm-1

Resistivity >1011 Ω cm Band gap 5.5 eV Electron mobility 1800 (4500) cm2 V-1 s-1

Hole mobility 1200 (3800) cm2 V-1 s-1

Energy to create e-h pair 13.1 eV Average signal created 36 e μm-1

*High purity single crystal CVD diamond

22 October 2009 FCAL workshop, Geneve 3

MIP Response of scCVD Diamond

Diamond

90Sr

Scint.

PM1

PM2

discr

discr

PA delay

&

AD

C

90Sr Source

Sensor box & Preamplifier

Trigger box

ADC spectrum

22 October 2009 FCAL workshop, Geneve 4

‘Ideal’ crystal charge collection

Charge collection efficiency depends on E

Full chargecollection

HV=0

Recombination

HV≠0

Charge collection

+

-

CCE = Qcollected/Qproduced CCD = CCE*d

d

22 October 2009 FCAL workshop, Geneve 5

Radiation damaged crystal

Radiation causes local damages of the lattice structure

These local damages (traps) are able to capture free charge carriers and release them after some time

Assumptions we are using:

Trap density is uniform (bulk radiation damage)

Traps are created independently (linearity vs dose)

electrons

holes

Ionization

22 October 2009 FCAL workshop, Geneve 6

Irradiation of scCVD Diamond

After 5 MGy dose diamond detector is operational

CCD is decreasing with the absorbed dose

Generation of trapping centers due to irradiation

Traps release? CCDcurrent < CCDMIP? Too high ‘missing charge’

~Natoms in the sample Pure trapping mechanism

is contradictory Recombination is

important Polarization?

‘missing charge’

DALINAC, TU-Darmstadt June 2007

CCD from 90Sr setup

CCD from Isens

22 October 2009 FCAL workshop, Geneve 7

Irradiation of scCVD Diamond

No annealing! 1.5 years, a lot of

tests with 90Sr Source, UV-light, several TSC measurements

After 10 MGy absorbed dose MIP signal is still detectable

Leakage current is very small ~pA

Continued in December 2008

Jun 2007 Data

Dec 2008 Data

22 October 2009 FCAL workshop, Geneve 8

Polarization origin

Uniform generation of e-h pairs Asymmetry is introduced by the

applied electric field Specific free charge carrier Specific free charge carrier

density is largest near detector density is largest near detector edgesedges

Asymmetric trap filling according Asymmetric trap filling according to charge carrier densityto charge carrier density

Space charge creation in the bulk Space charge creation in the bulk of the detectorof the detector

Compensation of the external Compensation of the external field by space chargefield by space charge

PolarizationPolarization

+ -Eext

22 October 2009 FCAL workshop, Geneve 9

Polarization origin

Uniform generation of e-h pairs Asymmetry is introduced by the

applied electric field Specific free charge carrier

density is largest near detector edges

Asymmetric trap filling according Asymmetric trap filling according to charge carrier densityto charge carrier density

Space charge creation in the bulk Space charge creation in the bulk of the detectorof the detector

Compensation of the external Compensation of the external field by space chargefield by space charge

PolarizationPolarization

+ -Eext

22 October 2009 FCAL workshop, Geneve 10

Polarization origin

Uniform generation of e-h pairs Asymmetry is introduced by the

applied electric field Specific free charge carrier

density is largest near detector edges

Asymmetric trap filling according Asymmetric trap filling according to charge carrier densityto charge carrier density

Space charge creation in the bulk Space charge creation in the bulk of the detectorof the detector

Compensation of the external Compensation of the external field by space chargefield by space charge

PolarizationPolarization

+ -Eext

22 October 2009 FCAL workshop, Geneve 11

Polarization origin

Uniform generation of e-h pairs Asymmetry is introduced by the

applied electric field Specific free charge carrier

density is largest near detector edges

Asymmetric trap filling according to charge carrier density

Space charge creation in the bulk Space charge creation in the bulk of the detectorof the detector

Compensation of the external Compensation of the external field by space chargefield by space charge

PolarizationPolarization

+ -Eext

22 October 2009 FCAL workshop, Geneve 12

Uniform generation of e-h pairs Asymmetry is introduced by the

applied electric field Specific free charge carrier

density is largest near detector edges

Asymmetric trap filling according to charge carrier density

Space charge creation in the bulk of the detector

Compensation of the external field by space charge

Polarization

Epol

+ -Eext

Polarization origin

E

E00 depth

22 October 2009 FCAL workshop, Geneve 13

Model: 340 μm scCVD diamond after 5 MGy CCD time dependence

time

Space charge Charge collection distance

Electric fieldExpected Signal shape

Neg Pos

Initial field

Low field,recombination

Effective charge collection regions

22 October 2009 FCAL workshop, Geneve 14

Model: 340 μm scCVD diamond after 5 MGy CCD time dependence

time

Space charge Charge collection distance

Electric fieldExpected Signal shape

Neg Pos

Zero field

E

22 October 2009 FCAL workshop, Geneve 15

Model: 340 μm scCVD diamond after 5 MGy CCD time dependence

time

Space charge Charge collection distance

Electric fieldExpected Signal shape

Neg Pos

To be confirmed!

22 October 2009 FCAL workshop, Geneve 16

Long-term signal evolution

Try to minimize an influence of the measurement onto the filled trap distribution

Use the source only for short CCD evaluation runs

Polarization is seen even after 1 month after the initial pumping – long living traps, possibility to fill all of them!

t0 = 35 days

22 October 2009 FCAL workshop, Geneve 17

Damaged Sensor under 90Sr Source: CCD vs time

Illuminate by UV-light to free all traps

Apply HV and source 90Sr

-HV

22 October 2009 FCAL workshop, Geneve 18

Damaged Sensor under 90Sr Source: CCD vs time

90Sr

±HV

Illuminate by UV-light to free all traps

Apply HV and source

22 October 2009 FCAL workshop, Geneve 19

Damaged Sensor under 90Sr Source: CCD vs time

90Sr

±HV

Illuminate by UV-light to free all traps

Apply HV and source

22 October 2009 FCAL workshop, Geneve 20

Damaged Sensor under 90Sr Source: CCD vs time

90Sr

±HV

Illuminate by UV-light to free all traps

Apply HV and source

Sh

ort

liv

ing

tra

ps

22 October 2009 FCAL workshop, Geneve 21

Beam Pumping Test

Trigger Box Linear Drive

90Sr Source

Collimator

Faraday Cup

Detector+Preamp Box

Collimator

Beam

Use intensive beam to fill up short living traps

Move (remotely) detector/preamp box to the low intensity 90Sr line

Measure signal evolution with time since beam-off

22 October 2009 FCAL workshop, Geneve 22

Beam Pumping Test

Clear indication to the presence of fast decaying traps. Additional polarization due to shallow defects filling

5 MGy10 MGy

Dose rate ~ 100 x highest dose rate @ ILC detector

22 October 2009 FCAL workshop, Geneve 23

TSC measurements

At least 3 levels are visible:

trap1 trap2 trap3

Ec-ET

[eV]

1.144

+0.002

0.851

+0.002

0.746

+0.006

nT0

[1014cm-3]

5.7 1.5 0.2

After 5 MGy

Trap concentration ~ 1015 cm-3 (still 8 orders of magnitude less than normal atom density)

22 October 2009 FCAL workshop, Geneve 24

Summary

The performance of scCVD Diamond sensor was studied as a function of absorbed dose up to 10 MGy

Strong polarization effects are observed in the radiation damaged scCVD Diamond detector

Polarizaton significantly decreases the detector charge collection efficiency in addition to pure trapping mechanism

A simple model is developed in order to understand and describe observed phenomena

Method of routinely switching bias HV polarity is proposed to suppress bulk polarization of long-living traps

Beam pumping tests indicate that short-living traps are responsible for the residual detector inefficiency

22 October 2009 FCAL workshop, Geneve 25

Thank you